β-amino esters from the reductive ring opening of aziridine-2-carboxylates.

A general study is undertaken to examine the scope of the reductive ring opening of aziridine-2-carboxylates with samarium diiodide. The competition between C-C and C-N bond cleavage is examined as a function of the nature of the N-substituent of the aziridine, the nature of the substituent in the 3-position of the aziridine, and whether the substituent in the 3-position is in a cis or trans relationship with the carboxylate in the 2-position. The desired C-N bond cleavage leads to β-amino esters that are the predominant products for most aziridines with an N-activating group. However, C-C cleavage products are observed with an aryl group in the 3-position; this can be particularly pronounced with cis-aziridines where a nearly equal mixture of the two is observed. Exclusive formation of the C-N cleavage product is observed for all aziridines with the strongly N-activating p-toluene sulfonate group. Similarly high selectivity is observed for the 2-trimethylsilylethyl sulfonate group (SES), which is easier to remove. The utility of these methods is illustrated in the synthesis of protected forms of (R)-β(3)-DOPA and L-DOPA from the same aziridine, the former by SmI2-mediated reductive opening at C-2 and the latter by palladium-mediated reductive opening at C-3.

Introduction

The synthesis of β-amino acids has been a subject of great interest and importance for quite some time[1] but especially because it was discovered[2] that β-peptides derived from β-amino acids have many of the properties of α-peptides but are much more proteolytically stable. There has been a decided uptick in the efforts to develop catalytic asymmetric methods for the synthesis of β-amino acids in the past decade.[3] Our interest in this area follows from our experiences in the development of the catalytic asymmetric synthesis of aziridines.[4] We have found that n class="Chemical">aziridines can be prepared with a high degree of enantio- and diastereoselection by a three-component coupling of an aldehyde, an amine, and ethyl diazoacetate under the aegis of a BOROX catalyst (Scheme 1).[5,6] High yields of aziridine-2-carboxylates can be realized starting with aryl, alkyl, or alkynyl aldehydes with a typical selectivity for the cis isomer of ≥50:1.[7] The enantioselection can depend on the nature of the amine substitutent[7d] or on the nature of the ligand; with the right combination, a minimum of 96% ee can be obtained with aryl, alkynyl, and first, second, and third degree aliphatic aldehydes.[7,8] The diastereoselection for the aziridine can be switched to trans with the use of a diazoacetamide.[9] The purpose of the present work is to explore the reductive opening of cis-aziridine-2-carboxylates, with the goal of directing opening at the C-2 position to provide for an efficient and highly stereoselective catalytic asymmetric route to β-amino esters.

Scheme 1

A number of methods are known for the reductive opening of aziridine-2-carboxylates to give β-n class="Chemical">amino esters.[10,11] The nature of the reducing agent can be quite critical when it comes to aziridines with an aryl group in the 3-position. As illustrated in Scheme 2, such aziridines are prone to reductive opening, resulting in α-amino esters by hydrogenolysis or Lewis or Brønsted acid-mediated reduction.[10,12] However, this proclivity for reduction can be reversed by using electron-transfer reduction methods; because the electron preferentially adds to the carbonyl function, it directs ring opening to the 2-position, resulting in β-amino esters. This has been reported with samarium diiodide[13] and magnesium metal.[14] Interestingly, the use of manganese(0) to reduce 3-arylaziridine-2-carboxamides occurs with opening at the 3-position to give α-amino amides,[15] whereas the same substrates are reduced to β-amino amides by samarium diiodide.[13b] In a non-electron-transfer process, tributyltin hydride is known to open aziridinyl ketones to give β-amino ketones in a radical process.[16] In a distinct and mechanistically different reaction, the addition of silyl anions to 2-aziridinyl ketones leads to β-amino ketones via an addition, Brook rearrangement, and anionic induced-ring opening.[17]

Scheme 2

Results and Discussion

Whereas 3-arylaziridine-2-carboxylates can be reductively opened to β-n class="Chemical">amino esters with either samarium diiodide or magnesium(0), all examples in the literature are with trans isomers of the aziridine.[13,14] Because the BOROX catalyst produces very high selectivities for cis-aziridines, it became imperative to determine if the same regioselectivities observed in the reductive opening of trans-aziridines would translate to cis-aziridines. We also began our studies with Fmoc aziridines because this would be the most desirable N-substituent for the purposes of solid-state synthesis of β-peptides. Fmoc aziridine cis-11a has a phenyl group in the 3-position cis to the ethyl carboxylate, and its reductive ring opening was examined with both magnesium in methanol and samarium diiodide in the presence of N,N-dimethylethanol amine (DMEA). The purpose of DMEA is to sequester the samarium(III) that formed and prevent it from opening the aziridine as a Lewis acid giving α-amino ester product 14a.[13a,13b] The reduction with magnesium did not occur under the reported conditions (−23 °C, Fmoc aziridines were not included in the literature study),[14] and high conversion was realized only after prolonged heating at 55 °C. However, no α-cleavage product 12a, β-cleavage product 14a, or C–C cleavage product 13a was observed in the crude reaction mixture. The products that were formed were not separated and identified. The same result was obtained with two different batches of commercial magnesium powder. The reduction of cis-11a with samarium diiodide was performed under the reported conditions[13a,13b] indicated in Scheme 3, and the result was that both α-cleavage product 12a and C–C cleavage product 13a were formed in substantial amounts.

Scheme 3

The reductive ring opening of cis-11a could be brought to completion with 4 equiv of n class="Chemical">SmI2 and 8 equiv of DMEA in THF in 1 h at 0 °C (Table 1, entry 1). This reaction resulted in the isolation of α-cleavage product 12a in 45% yield and C–C cleavage product 13a in 46% yield.[18] Upon examining the reductive ring opening of the corresponding trans-aziridine trans-11a under the optimized conditions, it became clear that the product distribution greatly depends on the stereochemistry of the aziridine. The cis-aziridine 11a gives a 1:1 mixture of 12 and 13, whereas the trans-aziridine 11a gives a 16.7:1 mixture of 12 and 13 (Table 1, entries 1 and 3, respectively). This was not the case with aziridines bearing an alkyl group in the 3-position. Both the cis and trans isomers of 3-cyclohexyl aziridine 11b gave exclusive opening at the α-position and a highly selective formation of β-amino ester 12b (Table 1, entries 2 and 4, respectively).

Table 1

Reductive Opening of cis- and trans-Fmoc Aziridinesa

entry	aziridine	R	12/13b	% yield 12	% yield 13d
1e	cis-11a	phenyl	1:1	45	46c
2e	cis-11b	cyclohexyl	>99:1	89	<1
3f	trans-11a	phenyl	16.7:1	82d	5
4g	trans-11b	cyclohexyl	>99:1	73d	<1

Unless otherwise specified, all reactions were run with 0.2 mmol of aziridine in THF (0.07 M) with 4 equiv of SmI2 and 8 equiv of DMEA at 0 °C for 1 h and went to completion.

Determined from the 1H NMR spectrum of the crude reaction mixture.

Isolated yield after silica-gel chromatography.

Yield from the 1H NMR spectrum of the crude reaction mixture with an internal standard.

Data previously reported in ref (18).

Reaction with 5.5 equiv of SmI2 and 11 equiv of DMEA.

Reaction with 6 equiv of SmI2 and 12 equiv of DMEA.

Unless otherwise specified, all reactions were run with 0.2 mmol of aziridine in n class="Chemical">THF (0.07 M) with 4 equiv of SmI2 and 8 equiv of DMEA at 0 °C for 1 h and went to completion.

Reaction with 5.5 equiv of SmI2 and 11 equiv of n class="Chemical">DMEA.

Reaction with 6 equiv of SmI2 and 12 equiv of n class="Chemical">DMEA.

In the search for a more general reductive ring-opening method for converting aziridines to β-n class="Chemical">amino esters, a number of different N-protecting groups were examined. The results of this analysis are presented in Table 2. As a carbamate, it was not surprising to find that the profile for the SmI2-mediated reductive ring opening of the Boc-protected aziridines closely matched that for the Fmoc aziridines, with just slightly lower selectivities. The ring opening of phenyl-substituted cis-aziridine was not selective (1.4:1, Table 2, entry 1). Phenyl-substituted trans-aziridine 15a was more selective at 6.7:1 (entry 3)[13e], and both isomers of cyclohexyl-substituted aziridine 15b were highly selective (Table 2, entries 2 and 4). Clearly, the most felicitous N-protecting group, with regard to the selectivity of reductive ring opening by samarium diiodide, is the tosyl group. The profile observed is flat, with >99:1 selectivity for the β-amino ester; cis- and trans-aziridines and phenyl- and cyclohexyl-substituted aziridines were all in very high yields (Table 2, entries 5–8). The SES (trimethylsilyl ethyl sulfonyl) group[19] is an attractive activating group for an amino function because it is easier to remove than tosyl and has excellent selectivity for the β-amino ester 21a, as seen with cis-aziridine 17a (23:1, Table 2, entry 9). The slightly lower selectivity of the SES group, as compared to that of the tosyl group (Table 2, entries 5 and 9, respectively), could be expected for an alkyl sulfonate compared to an aryl sulfonate (vide infra). It was also found that the N-acetyl group is capable of delivering high selectivity for β-amino ester 22a over the C–C cleavage product in the ring opening of cis-phenyl aziridine cis-18a. However, the isolated yield of β-amino ester 22a was only moderate; the reaction occurs with the formation of 13% of the α-amino ester corresponding to 14 in Scheme 3 (Table 2, entry 10). The latter may result from initial electron transfer to the amide carbonyl followed by the ring opening to a benzyl radical (or anion, vide infra).

Table 2

Reductive Opening of cis- and trans-N-Activated Aziridinesa

entry	PG	aziridine	R	A/Bb	% yield Ac	% yield Bc
1	Boc	cis-15a	phenyl	1.4:1	55 (19a)	32 (23a)
2	Boc	cis-15b	cyclohexyl	>99:1	84 (19b)	(23b)
3	Boc	trans-15a	phenyl	6.7:1	85 (19a)	10 (23a)
4	Boc	trans-15b	cyclohexyl	>99:1	84 (19b)d	(23b)
5	Ts	cis-16a	phenyl	>99:1	93 (20a)e	(24a)
6	Ts	cis-16b	cyclohexyl	>99:1	97 (20b)f	(24b)
7	Ts	trans-16a	phenyl	>99:1	88 (20a)	(24a)
8	Ts	trans-16b	cyclohexyl	>99:1	95 (20b)	(24b)
9	SES	cis-17a	phenyl	23:1	84 (21a)g	4 (25a)
10	Ac	cis-17b	phenyl	>99:1	52 (21b)h	(25b)

Unless otherwise specified, all reactions were run with 0.2 mmol of aziridine in THF (0.07 M) with 6 equiv of SmI2 and 12 equiv of DMEA at 0 °C for 1 h and were allowed to go to completion.

Determined from the 1H NMR spectrum of the crude reaction mixture.

Isolated yield after silica-gel chromatography.

Yield from the 1H NMR spectrum of the crude reaction mixture with an internal standard.

Data taken from ref (18).

Reaction with 5 equiv of SmI2 and 10 equiv of DMEA.

A small amount (∼6%) of SES-protected benzylamine was also observed.

Reaction with 4 equiv of SmI2 and 8 equiv of DMEA. The ring-opening product from N–C3 cleavage to give an α-amino ester was obtained in 13% isolated yield.

Reaction with 5 equiv of SmI2 and 10 equiv of n class="Chemical">DMEA.

A small amount (∼6%) of SES-protected n class="Chemical">benzylamine was also observed.

Reaction with 4 equiv of SmI2 and 8 equiv of n class="Chemical">DMEA. The ring-opening product from N–C3 cleavage to give an α-amino ester was obtained in 13% isolated yield.

Because this is the class of aziridines that the n class="Chemical">BOROX catalysts are most efficient at producing, the reductive ring opening of unactivated aziridines by samarium diiodide would be a very useful reaction (Scheme 1). Kumamoto and coworkers examined the samarium diiodide-mediated ring opening of trans-aziridine-2-carboxylates with an aryl group in the 3-position and a benzyl group on the aziridine nitrogen and found that β-amino esters could be obtained only in very low yields.[13d] The corresponding cis-aziridines were not investigated. The present work finds that if samarium diiodide is generated from samarium metal and iodine instead of methylene iodide, it results in the isomerization of trans-aziridine to a mixture of cis- and trans-aziridines. There are no other examples of the reductive ring opening of aziridines bearing an alkyl group on the nitrogen with samarium diiodide in which the aziridines have a carbonyl group in the 2-position and either an aryl or alkyl substituent in the 3-position. On the basis of this, we decided to probe the first examples of the reductive ring opening of unactivated cis-aziridine-2-carboxylates with samarium diiodide (Table 3). The samarium diiodide used in these studies was prepared from samarium metal and methylene iodide, and no isomerization of the aziridine was observed. If the substituent in the 3-position is a phenyl group, then the only product that is observed is the C–C cleavage product. Yields in the range of 69–75% are observed for this product with N–H aziridines, as well as with benzhydryl substituents on the aziridine nitrogen (Table 3, entries 1 and 2). A complete switch in the product distribution is seen with cis-aziridines bearing a cyclohexyl group in the 3-position. Here the β-amino ester is generated to the exclusion of the C–C cleavage product; the yields, however, are quite low (Table 3, entries 3 and 4). These reactions produce a complex mixture from which only β-amino ester 30b, starting cis-aziridine 29b, and benzhydryl amine 34 could be isolated and characterized. The isolation of 34 suggests that β-cyclohexyl ethyl acrylate should also be formed, but it could not be detected in the 1H NMR spectrum of the crude reaction mixture.

Table 3

Reductive Opening of Unactivated cis-Aziridinesa

entry	PG	aziridine	R	A/Bb	% yield Ac	% yield Bc
1	H	cis-27a	phenyl	<1:99		75 (31a)
2	CHPh2	cis-29a	phenyl	<1:99		69 (33b)
3	CHPh2	cis-29b	cyclohexyl	>99:1d	22 (30b)
4	CHPh2	cis-29b	cyclohexyl	>99:1e	22 (30b)

Unless otherwise specified, all reactions used 0.2 mmol of aziridine in THF (0.07 M) with 5 equiv of SmI2 and 10 equiv of DMEA at 0 °C for 40 min and were allowed to go to completion.

Determined from the 1H NMR spectrum of the crude reaction mixture.

Isolated yield after silica-gel chromatography.

Isolated as a 1:1.4 mixture of 30b and cis-29b (22% + 31%). The 1H NMR indicated the formation of amine 34 in 39% yield.

Reaction was run for 2 h at 25 °C. Amine 34 was isolated in 52% yield, and aziridine cis-29b was isolated in 20% yield.

Isolated as a 1:1.4 mixture of 30b and cis-29b (22% + 31%). The n class="Chemical">1H NMR indicated the formation of amine 34 in 39% yield.

Reaction was run for 2 h at 25 °C. Amine 34 was isolated in 52% yield, and n class="Chemical">aziridine cis-29b was isolated in 20% yield.

The reductive ring opening of stereoisomerically pure trisubstituted aziridine 35(20) occurs with loss of stereochemical information and the formation of two diastereomers in a ratio of 4.8:1 (Scheme 4). The major diastereomer was identified as the anti isomer of 36 by chemical correlation to a known compound (Supporting Information). This loss of stereochemistry is to be expected on the basis of the likely mechanism for this reaction (Scheme 5). The ring opening of 35 also occurs with the formation of C–C cleavage product 37 in 30% yield. Note that the distribution between C–N and C–C cleavage products is essentially the same as for cis disubstituted n class="Chemical">N-Boc aziridine cis-15a (Table 2, entry 1).

Scheme 4

Scheme 5

The generally accepted mechanism for the reductive ring opening of aziridines by n class="Chemical">samarium diiodide is illustrated in Scheme 5.[13b] After initial reduction to form ketyl 39 and in the absence of a proton source, this species then undergoes a ring opening to give nitrogen-based radical 40 that, upon further reduction, gives the species 41, containing both a samarium enolate and a samarium amide. The intermediacy of this enolate has been demonstrated, and its utility has been displayed in its alkylation with alkyl halides[13b] and aldol reactions with aldehydes.[13c] Under conditions where a proton source is present, ketyl 39 is thought to be protonated to give neutral radical 43 that, depending on the nature of the aziridine, then undergoes a C–N and/or C–C bond scission. Subsequent reduction of the resulting nitrogen- or carbon-based radicals 44 and 47 and final protonation would provide β-amino ester 46 and/or glycine derivative 49. This mechanistic interpretation accounts for the observations made in the present work. Of all of the examples in Tables 1 and 2, the aziridines with a cyclohexyl group in the 3-position give a much higher selectivity for the β-amino ester over the glycine derivative than do aziridines with a phenyl group in the 3-position. This reflects a higher preference for C–C over C–N cleavage for the phenyl aziridines than for the cyclohexyl aziridines, which can be attributed to the greater stability of a benzyl radical compared to an alkyl radical in intermediate 47. Conversely, the presence of a radical stabilizing group on the nitrogen facilitates the ring opening with C–N bond scission over C–C bond scission. This can be seen in the ring-opening reactions of activated aziridines (Tables 1 and 2) versus unactivated aziridines (Table 3). This is illustrated by the comparison of N-tosyl aziridine cis-16a (Table 2, entry 5) to N-benzhydryl aziridine cis-29a (Table 3, entry 3) and by the comparison of N-tosyl aziridine cis-16a (Table 2, entry 5) to N-Boc aziridine cis-15a (Table 2, entry 1).

The ratio of C–N versus C–C cleavage is a function not only of the radical stabilizing ability of the substituent on the nitrogen and the substituent on the n class="Gene">C-3 position of the aziridine but also of the stereochemistry of the aziridine. The cis-aziridines exhibit a much greater preference for C–C bond cleavage than do the trans-aziridines. N-Boc aziridine trans-15a gives a 6.7:1 mixture of C–N to C–C cleavage products, whereas the corresponding cis-15a gives a much greater propensity for the C–C cleavage product (1.4:1, Table 2, entries 1 and 3, respectively). The same is also true for N-Fmoc aziridines cis-11a and trans-11a (Table 1, entries 1 and 3, respectively). The greater preference for C–C cleavage products with cis-aziridines may be due to a relief in steric interactions between the two cis-substituents in the transition state, allowing the C–C bond to begin to lengthen (Scheme 6). It is intersting that this relief in steric interaction would not be realized in the trans-aziridine. C–C cleavage products have been rarely seen in the reductive ring opening of aziridines involving single electron-transfer processes; this may be due to the fact that cis-aziridines have not been previously evaluated in this reaction. The only example that we are aware of involves an unactivated (N–H) aziridine-2-carboxylate with a trans-phenyl group in the 3-position, which gives a 28:16 split between C–N and C–C cleavage products,[13b] as compared to aziridine cis-27a, which gives exclusively the C–C cleavage product in 75% yield (Table 3, entry 1). From the synthesis point of view, the N-tosyl group is the protecting group of choice for samarium diiodide-mediated reductive ring opening of aziridines if removal of the tosyl group does not cause problems in a later stage. N-Tosyl aziridines are completely selective for the C–N cleavage product (>99:1) for both the cis- and trans-aziridines, with both aryl and alkyl substituents in the 3-position (Table 2, entries 5–8). The SES protecting group can be considered to be an alternative to a tosyl sulfonamide, which is notorious for its potential to be troublesome during deprotection. SES-protected aziridine 17a gave excellent selectivity (23:1, Table 2) for the β-amino ester, and the deprotection of SES is known to proceed under much milder reaction conditions.

Scheme 6

The utility of aziridine-2-carboxylates in the preparation of both α- and β-amino acids is illustrated in Scheme 7, which shows how the same n class="Chemical">aziridine provides access to both l-DOPA and (R)-β3-DOPA. l-DOPA is the biological precursor to the catecholamine neurotransmitters, a treatment of Parkinson’s disease,[21] and a key compound in the formation of marine-adhesive proteins.[22]l-DOPA became the first commercial pharmaceutical agent to be manufactured by a nonproteinaceous asymmetric catalyst, which was acknowledged in the 2001 Nobel Prize in chemistry to William S. Knowles.[23] The isomeric (R)-β3-DOPA has been isolated as an iron(III) complex from a dark-blue-violet mushroom of the species Cortinarius violaceus.[24,25] Both natural products could potentially be obtained from the reductive opening of the same aziridine via controlled reductive ring opening at the C-2 and C-3 positions. Of the many synthesis methods of l-DOPA and (R)-β3-DOPA, no other method can provide access to both isomers from the same asymmetric strategy.[23−25,41]

Scheme 7

Our first approach to the synthesis of l-DOPA and (R)-β3-DOPA began with n class="Chemical">bis-acetoxy aziridine 54, which was prepared in one step in 98% yield and >98.5% ee from aldehyde 50, amine 52, and ethyl diazoacetate 53 by a catalytic, asymmetric, multicomponent aziridination with 5 mol % VAPOL BOROX catalyst (Scheme 8).[5] To set the stage for the regiocomplementary reductive ring opening of aziridine 54, the MEDAM group was cleaved with trifluoracetic acid in anisole and the resulting N–H aziridine was not purified but rather directly protected by TsCl or SESCl to give N-protected aziridines 57 and 58 in 70 and 78% yield, respectively, for the two steps. The samarium diiodide-mediated ring opening of both tosyl aziridine 57 and SES-aziridine 58 gave complex mixtures because of the cleavage of zero, one, or two of the acetoxy groups on the benzene ring. By treating the crude mixtures with acetic anhydride in the presence of triethylamine, β-amino esters 61A and 62A were isolated in good yields, although an increase in the yield of the C–C cleavage product was observed in both cases compared to the corresponding phenyl-substituted aziridines cis-16a and cis-17a (Table 2). This may be due to the electronic effect of the two acetate groups on the benzene ring. Despite the effort that was expended to investigate the removal of the tosyl group following many of the standard procedures, 61A could not be deprotected without decomposition.

Scheme 8

Before exploring of SES-deprotection of 62A, we turned our attention to the n class="Chemical">acetate cleavage problem in the samarium diiodide ring-opening process. This was solved by replacing the acetate groups with the bulky tert-butyl acyloxy (pivaloyl) groups. The catalytic asymmetric multicomponent aziridination worked smoothly with aldehyde 51 and afforded aziridine 55 in 97% yield and 98% ee. Interestingly, the MEDAM group could be removed with trifluoroacetic acid in anisole without the cleavage of the pivaloyl groups and then directly reacting the N–H aziridine with SESCl gave aziridine 59 in 86% yield. The formation of β-amino ester 63A was smoothly achieved (74% yield) from the reductive ring-opening reaction with no detection of cleavage of the pivolyl groups. The first attempt to remove the SES group from the amine function in 63A following a literature procedure[19] involving heating with CsF in DMF at 95 °C resulted in the formation of only ethyl 3,4-dihydroxycinnamate 65 in 65% yield (NMR yield). This is probably due to fluoride-mediated deprotonation at the α-position of the carbonyl, causing an elimination of the SES-amino group. Alternatively, 65 could result from a fluoride-mediated cleavage of the pivaloyl group, followed by a phenoxide-assisted elimination of the SES-amino group and a final rearomatization. A similar outcome was also observed for SES-protected β-amino ester 62A. It has been previously reported that the elimination of the SES group from SES-protected α-amino carbonyl compounds can be a problem during the SES deprotection step.[26] However, there is no example reported for the elimination of the SES-NH2 group from a β-amino ester during SES deprotection. It is known that N-acyl-substituted SES groups are much more readily deprotected than simple SES groups.[26] On the basis of the latter, we carried out the acylation of 63A by treating it with (Boc)2O and subsequently treating the crude mixture with TBAF in THF at 25 °C for 1.5 h to afford the desired β-amino ester 64, the protected form of (R)-β3-DOPA, in 88% isolated yield. There was some cleavage of the pivaloyl groups by TBAF, and thus a workup with pivaloyl chloride gives 64 as a pure compound. No purification was performed during any of the steps in the conversion of 63A to 64. Finally, the reductive ring opening of aziridine 55 with palladium hydroxide-catalyzed hydrogenation in the presence of (Boc)2O resulted in the formation of α-amino ester 56, the protected form of l-DOPA, in 86% isolated yield.

Conclusions

The reductive ring opening of 3-substituted aziridine-2-carboxylates with n class="Chemical">samarium diiodide can be controlled to proceed via C–N bond cleavage to give high yields of β-amino esters. The competing C–C bond cleavage gives rise to glycine derivatives. It is necessary to have an activating group on the aziridine nitrogen to achieve selective C–N bond cleavage. Aziridines with nonactivating nitrogen substituents (hydrogen or benzhydryl) exclusively form glycine derivatives when there is a phenyl group in the 3-position and, when there is a cyclohexyl group in the 3-position, low yields of β-amino esters and other decomposition products. The selectivity between C–N and C–C bond cleavage directly correlates with the electron-withdrawing power of the activating group on the nitrogen. Sulfonyl groups give higher selectivity than carbamate groups, and this is especially noticeable with cis-aziridines that have a phenyl group in the 3-position. The lower selectivity with cis-aziridines is thought to be due to a steric release during the C–C bond cleavage, leading to glycine products. The utility of this methodology is illustrated in the synthesis of a protected form of (R)-β3-DOPA by the reductive opening of aziridine 59 with samarium diiodide to give β-amino ester 63A. Furthermore, this synthesis features the targeting of (R)-β3-DOPA and its regioisomer l-DOPA by ring opening of the same aziridine, 55, the former by reductive opening at the C-2 position and the latter by reductive opening at the C-3 position.

Experimental Section

Tetrahydrofuran (n class="Chemical">THF) and toluene were distilled from sodium under nitrogen. Dichloromethane was distilled from calcium hydride under nitrogen. Hexanes and ethyl acetate were ACS grade and used as purchased. Other reagents were used as purchased. VAPOL was prepared according to literature procedures and was determined to be at least 99% optically pure.[27] The preparation of aziridine esters cis-11a,b,[18]cis-15a,b,[18]cis-16a,b,[18]cis-27a,[18]trans-27a,[28]cis-29a,b,[7a] and 35(20) has been previously reported.

Melting points were determined on a capillary melting-point apparatus and were uncorrected. IR spectra were taken on an FTIR spectrometer. 1H NMR and n class="Chemical">13C NMR spectra were recorded on a 300, 500, or 600 MHz spectrometer in CDCl3, unless otherwise noted. CHCl3 was used as the internal standard for both 1H NMR (δ = 7.24) and 13C NMR (δ = 77.0). HR-MS was performed on a TOF-MS spectrometer. Analytical thin-layer chromatography (TLC) was performed on silica-gel plates with an F-254 indicator. Visualization was by short wave (254 nm) and long wave (365 nm) ultraviolet light, by staining with phosphomolybdic acid in ethanol, or with the aid of iodine vapor. Column chromatography was performed with silica gel 60 (230–450 mesh). Optical rotations were obtained on a polarimeter at a wavelength of 589 nm (sodium D line) using a 1.0 decimeter cell with a total volume of 1.0 mL. Specific rotations are reported in degrees per decimeter at 20 °C.

trans-N-9-Fluorenylmethyl-carbamate-2-carboxyethyl-3-phenylaziridine (11a)

To a 100 mL round-bottomed flask were added racemic trans-2-carboxyethyl-3-phenylaziridine n class="Chemical">27a (0.296 g, 1.5 mmol, 1.0 equiv), NaHCO3 (0.252 g, 3.0 mmol, 2.0 equiv), 30 mL of a mixture of acetone, and H2O (3:1). The mixture was stirred at room temperature for 5 min, and then 9-fluorenylmethylchloroformate (0.388 g, 1.5 mmol, 1.0 equiv) was added. The reaction mixture was then stirred at room temperature for 48 h. The acetone was removed by rotary evaporation, and the aqueous residue was extracted with ethyl acetate (10 mL × 3). The combined organic layer was washed with sat aq NaCl, dried over MgSO4, and concentrated by rotary evaporation to afford a yellow oil. Purification by silica-gel chromatography (column 1: 18 mm × 250 mm, 4:1:1 hexanes/diethyl ether/CH2Cl2 as eluent; column 2: 18 mm × 250 mm, CH2Cl2 as eluent) afforded trans-11a as a colorless oil in 72% isolated yield that solidified upon standing (white solid, mp 93–95 °C). Spectral data for trans-11a: 1H NMR (500 MHz, CDCl3) δ 1.25 (t, 3H, J = 7.2 Hz), 3.20 (d, 1H, J = 2.4 Hz), 3.90 (d, 1H, J = 2.4 Hz), 4.18 (q, 2H, J = 7.2 Hz), 4.24 (t, 1H, J = 7.2 Hz), 4.33 (dd, 1H, J = 10.5, 7.8 Hz), 4.48 (dd, 1H, J = 10.5, 7.2 Hz), 7.25–7.31 (m, 2H), 7.32–7.42 (m, 7H), 7.55–7.61 (m, 2H), 7.74 (d, 2H, J = 7.5 Hz); 13C NMR (150 MHz, CDCl3) δ 14.0, 44.2, 45.1, 46.8, 62.2, 68.8, 119.96, 119.97, 125.2, 125.3, 126.4, 127.05, 127.06, 127.7, 127.8, 128.6, 128.7, 134.9, 141.25, 141.26, 143.6, 143.7, 159.7, 167.3; IR (thin film) 1744 (vs), 1179 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C26H24NO4+, 414.1705; found, 414.1731.

trans-N-9-Fluorenylmethyl-carbamate-cis-2-carboxyethyl-3-cyclohexylaziridine (11b)

The aridizine was prepared according to the procedure described above for n class="Chemical">trans-11a starting with racemic trans-2-carboxyethyl-3-cyclohexylaziridine[29] (296 mg, 1.50 mmol). Purification by silica-gel chromatography (15 mm × 250 mm, 8:1 hexanes/EtOAc as eluent) afforded trans-11b as a colorless oil in 91% isolated yield (572 mg, 1.36 mmol). Spectral data for trans-11b: R = 0.28 (5:1 hexanes/EtOAc); 1H NMR (600 MHz, CDCl3) δ 1.16–1.28 (m, 6H), 1.22 (t, 3H, J = 7.2 Hz), 1.62–1.68 (m, 1H), 1.69–1.78 (m, 3H), 1.81 (d, 1H, J = 12 Hz), 2.66 (dd, 1H, J = 7.2, 2.7 Hz), 2.96 (d, 1H, J = 2.7 Hz), 4.08–4.16 (m, 2H), 4.22 (t, 1H, J = 6.9 Hz), 4.32 (dd, 1H, J = 10.5, 7.5 Hz), 4.46 (dd, 1H, J = 10.2, 7.2 Hz), 7.27–7.31 (m, 2H), 7.38 (t, 2H, J = 7.5 Hz), 7.60 (t, 2H, 1H, J = 7.8 Hz), 7.74 (d, 2H, 1H, J = 7.8 Hz); 13C NMR (150 MHz, CDCl3) δ 14.0, 25.5, 25.6, 26.0, 29.7, 29.9, 39.2, 39.4, 46.9, 49.0, 61.8, 68.3, 119.90, 119.91, 125.1, 125.2, 127.01, 127.03, 127.66, 127.70, 141.27, 141.29, 143.7, 143.8, 160.2, 168.3; IR (thin film) 2928 (s), 2853 (w), 1743 (vs), 1451 (m), 1310 (m), 1177 (s) cm–1; mass spectrum, m/z (% rel intensity) 419 M+ (0.07), 346 (0.15), 178 (100), 165 (62), 84 (34), 49 (56); HRMS (ESI-TOF) m/z: [M + H]+ calcd for C26H30NO4+, 420.217; found, 420.216.

trans-N-Boc-2-carboxyethyl-3-phenylaziridine (15a)

To a flame-dried 25 mL round-bottomed flask filled with argon were added n class="Chemical">trans-2-carboxyethyl-3-phenylaziridine 27a(28) (150 mg, 0.784 mmol) and 5 mL of MeOH followed by the addition of NaHCO3 (197 mg, 0.450 mmol, 3.00 equiv). The flask was put in an ultrasonic bath for 5 min, and then (Boc)2O (428 mg, 1.96 mmol, 2.50 equiv) was added. The mixture was left in the ultrasonic bath for 4 h with a needle in the rubber septum to release the generated CO2 gas. After 4 h, additional portions of both NaHCO3 and (Boc)2O in equal amounts were added. The reaction mixture was then stirred for 4 days. The reaction mixture was filtered through Celite, and the solid residue was washed with Et2O. The cloudy filtrate was filtered again through Celite, and then concentrated by rotary evaporation to afford the crude product as a colorless liquid. Purification by silica-gel chromatography (15 mm × 250 mm, hexanes and then 1:9 ethyl acetate/hexanes as eluent) afforded trans-15a as a colorless oil in 71% isolated yield (163 mg, 0.56 mmol). Spectral data for trans-15a: R = 0.36 (hexanes/EtOAc = 4:1); 1H NMR (300 MHz, CDCl3) δ 1.30 (t, 3H, J = 7.0 Hz), 1.44 (s, 9H), 3.07 (d, 1H, J = 2.6 Hz), 3.79 (d, 1H, J = 2.6 Hz), 4.14–4.36 (m, 2H), 7.24–7.36 (m, 5H); 13C NMR (150 MHz, CDCl3) δ 14.16, 27.88, 44.02, 44.92, 61.82, 82.04, 126.43, 128.27, 128.48, 135.33, 158.29, 167.40. The 1H and 13C NMR data match those previously reported for this compound.[13b]

trans-N-Boc-2-carboxyethyl-3-cyclohexylaziridine (15b)

To a flame-dried 25 mL round-bottomed flask filled with argon were added racemic n class="Chemical">trans-2-carboxyethyl-3-cyclohexylaziridine[29] (198 mg, 1.0 mmol, 1.0 equiv), MeOH (6.5 mL), and NaHCO3 (0.84 g, 10 mmol, 10 equiv). The flask was put in an ultrasonic bath for 5 min, and then (Boc)2O (1.09 g, 5.00 mmol, 5.00 equiv) was added. The mixture was left in the ultrasonic bath for 4 h with a needle in the rubber septum to release the generated CO2 gas, and then it was stirred at room temperature for another 18 h. The reaction mixture was filtered through Celite, and the filter cake was washed with diethyl ether. The cloudy filtrate was filtered again through Celite and then concentrated by rotary evaporation to afford the crude product as a light-yellow liquid. Purification by silica-gel chromatography (15 mm × 250 mm, 45:1 hexanes/EtOAc as eluent until the first fraction came out and then 15:1 hexanes/EtOAc as eluent) afforded trans-15b as a colorless oil in 89% isolated yield (265 mg, 0.89 mmol). Spectral data for trans-15b: R = 0.42 (5:1 hexanes/EtOAc); 1H NMR (500 MHz, CDCl3) δ 1.04–1.25 (m, 6H), 1.28 (t, 3H, J = 7.2 Hz), 1.43 (s, 9H), 1.60–1.65 (m, 1H), 1.66–1.76 (m, 3H), 1.80–1.86 (m, 1H), 2.60 (dd, 1H, J = 6.8, 3.0 Hz), 2.83 (d, 1H, J = 3.0 Hz), 4.11–4.28 (m, 2H); 13C NMR (150 MHz, CDCl3) δ 14.2, 25.5, 25.6, 26.1, 28.0, 29.7, 30.0, 39.3, 39.4, 48.7, 61.5, 81.5, 159.0, 168.4; IR (thin film) 2980 (w), 2930 (s), 2855 (w), 1744 (vs), 1728 (s), 1154 (s) cm–1; mass spectrum, m/z (% rel intensity) 224 (M+ – 73) (5.6), 196 (21), 124 (85), 95 (73), 57 (100); Anal. calcd for C16H27NO4: C, 64.62; H, 9.15; N, 4.71. Found: C, 64.88; H, 9.27; N, 4.72.

trans-N-Tosyl-2-ethoxycarbonyl-3-phenylaziridine (trans-16a)

To a flame-dried 50 mL round-bottomed flask filled with argon were added racemic n class="Chemical">trans-2-carboxyethyl-3-phenylaziridine 27a(28) (0.335g, 1.75 mmol, 1.0 equiv) and CH2Cl2 (14 mL, freshly distilled). The solution was cooled to 0 °C in an ice bath followed by the addition of Et3N (0.7 mL, 5.03 mmol, 2.9 equiv, freshly distilled). After the reaction mixture was stirred for 5 min at 0 °C, TsCl (0.534 g, 2.8 mmol, 1.6 equiv) was added to the mixture at 0 °C. Thereafter, the ice bath was removed and the mixture was stirred at room temperature for 94 h. The reaction was quenched by the addition of 26 mL of sat aq NH4Cl and 5 mL of H2O. The organic layer was separated, and the aqueous layer was extracted with CH2Cl2 (30 mL × 3). The combined organic layer was washed with the following reagents in the indicated sequence: 5 mL of H2O, 10 mL of aq citric acid, 5 mL of H2O, 10 mL of sat aq NaHCO3, and 20 mL of sat aq NaCl. The combined organic layer was dried over MgSO4 and concentrated by rotary evaporation to afford the crude product as an orange oil. Purification by silica-gel chromatography (column 1: 15 mm × 250 mm, 40:9 hexanes/EtOAc as eluent; column 2: 15 mm × 250 mm, 16:4:1 hexanes/CH2Cl2/EtOAc as eluent) afforded trans-16a as a light-yellowish oil in 54% yield (0.326 g, 0.94 mmol). Spectral data for trans-16a: R = 0.24 (4:1 hexanes/EtOAc); 1H NMR (300 MHz, CDCl3) δ 1.32 (t, 3H, J = 7.0 Hz), 2.39 (s, 3H), 3.49 (d, 1H, J = 3.9 Hz), 4.20–4.39 (m, 2H), 4.41 (d, 1H, J = 3.9 Hz), 7.19–7.31 (m, 7H), 7.71–7.80 (m, 2H); 13C NMR (150 MHz, CDCl3) δ 14.0, 21.6, 47.1, 47.7, 62.4, 127.3, 127.5, 128.6, 128.9, 129.5, 132.7, 137.2, 144.2, 165.7. The 1H and 13C NMR data match those previously reported for this compound.[13b]

trans-N-Tosyl-2-ethoxycarbonyl-3-cyclohexylaziridine (16b)

According to the same procedure used for trans-16a, racemic n class="Chemical">trans-2-carboxyethyl-3-cyclohexylaziridine[29] (197 mg, 1.0 mmol, 1.0 equiv) was reacted with tosyl chloride (0.305 g, 1.6 mmol, 1.6 equiv) and Et3N (freshly distilled, 0.42 mL, 3.0 mmol, 3.0 equiv) in 1:1(v/v) CH2Cl2/CHCl3 (8 mL) for 72 h. Purification by silica-gel chromatography (column 1: 15 mm × 250 mm, 10:1 hexanes/EtOAc; column 2: 15 mm × 250 mm, 6:1:16 CH2Cl2/diethyl ether/hexanes as eluent) and recrystallization from hexanes afforded trans-16b as white crystals (mp 71–73 °C) in 70% isolated yield (246 mg, 0.70 mmol). Spectral data for trans-16b: R = 0.26 (5:1 hexanes/EtOAc); 1H NMR (300 MHz, CDCl3) δ 1.00–1.35 (m, 8H), 1.58–1.79 (m, 5H), 1.95 (d, 1H, J = 10.5 Hz), 2.41 (s, 3H), 3.02 (dd, 1H, J = 9.0, 4.2 Hz), 3.18 (d, 1H, J = 4.2 Hz), 4.15 (q, 2H, J = 6.9 Hz), 7.24–7.32 (m, 2H), 7.79–7.83 (m, 2H); 13C NMR (150 MHz, CDCl3) δ 13.9, 21.6, 25.2, 25.4, 25.9, 30.6, 31.1, 37.5, 43.7, 53.7, 61.9, 127.5, 129.5, 137.1, 144.2, 166.7; IR (thin film) 2930 (s), 2855 (m), 1745 (s), 1331 (s), 1101 (s) cm–1; mass spectrum, m/z (% rel intensity) 306 (M+ – 45) (2.3), 278 (3.2), 197 (34), 196 (100), 168 (29), 122 (88), 67 (83); Anal. calcd for C18H25NO4S: C, 61.51; H, 7.17; N, 3.99. found: C, 61.60; H, 7.22; N, 3.96.[30]

cis-Ethyl-(2R,3R)-3-phenyl-1-((2-(trimethylsilyl)ethyl)sulfonyl)aziridine-2-carboxylate (17a)

To a flame-dried 10 mL round-bottomed flask were added 2-(trimethylsilyl)ethanesulfonyl chloride (82.5 μL, 0.639 mmol, 1.28 equiv), n class="Chemical">Et3N (0.7 mL, 5 mmol, 10 equiv), and CH2Cl2 (2 mL, freshly distilled). The mixture was precooled in an ice bath for 5 min. cis-(2R,3R)-2-Carboxyethyl-3-phenylaziridine 27a(18) (95.6 mg, 0.5 mmol, 1 equiv, 98% ee) was dissolved in 0.5 mL of CH2Cl2, and the solution was added dropwise to the 10 mL round-bottomed flask containing 2-(trimethylsilyl)ethanesulfonyl chloride. The ice bath was removed, and the reaction mixture was stirred at room temperature for 45 h. Subsequently, additional portions of 2-(trimethylsilyl)ethanesulfonyl chloride (160 μL, 1.26 mmol, 2.5 equiv) and Et3N (0.7 mL, 5 mmol, 10 equiv) were added to the reaction mixture. After the mixture was stirred for 24 h at room temperature, the reaction was quenched with 2 mL of sat aq NH4Cl and 1 mL of H2O. The organic layer was separated, and the aqueous layer was extracted with CH2Cl2 (4 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated to give a dark-brown oil. Purification by silica-gel chromatography (15 mm × 180 mm, 9:1 hexanes/EtOAc as eluent) afforded cis-17a as a light-yellow oil in 83% yield (0.147 g, 0.413 mmol). Spectral data for cis-17a: R = 0.19 (9:1 hexanes/EtOAc); 1H NMR (500 MHz, CDCl3) δ 0.04 (s, 9H), 1.00 (t, 3H, J = 7.0 Hz), 1.13–1.24 (m, 2H), 3.16–3.26 (m, 2H), 3.67 (d, 1H, J = 7.5 Hz), 3.93–4.05 (m, 2H), 4.07 (d, 1H, J = 7.5 Hz), 7.26–7.34 (m, 3H), 7.37–7.41 (m, 2H); 13C NMR (125 MHz, CDCl3) δ −2.0, 9.7, 13.9, 43.7, 44.3, 49.5, 61.6, 127.5, 128.3, 128.7, 131.4, 164.5; IR (thin film) 2955 (s), 1755 (vs) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C16H26NO4SSi+, 356.1352; found, 356.1351; [α]D20 = −38.0° (c 1.0, CH2Cl2) on 98% ee material (the optical purity was assumed to be unchanged from 27a).

cis-(2R,3R)-Ethyl-1-acetyl-3-phenylaziridine-2-carboxylate (18a)

To a 25 mL flame-dried round-bottomed flask filled with argon were added n class="Chemical">cis-(2R,3R)-2-carboxyethyl-3-phenylaziridine 27a(18) (38 mg, 0.20 mmol), 1 mL CHCl3, Et3N (90 μL, 0.65 mmol), and acetic anhydride (29 μL, 0.31 mmol) at room temperature. The mixture was stirred at room temperature for 4 h, and then the solvent was removed under reduced pressure to give the crude cis-(2R,3R)-18a product as a white solid. Purification by silica-gel chromatography (9 mm × 250 mm, 1:5 EtOAc/hexanes as eluent) gave 18a as a white solid in 100% yield (47 mg, 0.20 mmol). Spectral data for cis-(2R,3R)-18a: R = 0.35 (1:5, EtOAc/hexane); 1H NMR (CDCl3, 300 MHz) δ 0.92 (t, 3H, J = 7.1 Hz), 2.20 (s, 3H), 3.47 (d, 1H, J = 6.6 Hz), 3.81 (d, 1H, J = 6.6 Hz), 3.85–3.97 (m, 2H), 7.24–7.31 (m, 3H), 7.35–7.39 (m, 2H); 13C NMR (CDCl3, 75 MHz) δ 13.7, 23.1, 42.0, 43.5, 61.1, 127.3, 128.0, 128.2, 132.5, 165.6, 181.13; IR (thin film) 1747 (s), 1713 (s) cm–1; mass spectrum, m/z (% rel intensity) 233 M+ (0.09), 191 (14), 146 (11), 117 (100), 91 (18), 79 (10), 55 (12), 43 (44). Anal. calcd for C13H15NO3: C, 66.94; H, 6.48; N, 6.00. Found: C, 66.87; H, 6.85; N, 5.94.

Representative Procedure for the Reductive Ring Opening of Aziridine-2-carboxylates with SmI2 (Illustrated for cis-15a)

To a flame-dried 10 mL round-bottomed flask filled with argon were added Sm (180 mg, 1.20 mmol, 6.00 equiv) and dry n class="Chemical">THF (1.8 mL, freshly distilled). The vacuum adapter on the flask was changed to a rubber septum, and the suspension was purged with nitrogen under the surface of the solution for 5 min by a needle through the septum. Another needle in the septum was used as an outlet for the nitrogen gas. CH2I2 (92.5 μL, 1.15 mmol, 5.7 equiv) was then added to the reaction flask, and the reaction mixture was purged with nitrogen for another 1 min. The outlet needle was removed, and the needle used for nitrogen flow was lifted above the surface of the solution. The reaction mixture was stirred at room temperature for 2 h, resulting in a dark-blue slurry. The slurry was then precooled to 0 °C in an ice bath. Subsequently, to a flame-dried 5 mL round-bottomed flask filled with nitrogen were added cis-(2R,3R)-15a[18] (78% ee, 60 mg, 0.2 mmol, 1.0 equiv), dry THF (1.5 mL, freshly distilled), and N,N-dimethylethanolamine (0.24 mL, 2.4 mmol, 12.0 equiv). The solution was purged with nitrogen under the surface of the solution for 2 min and transferred to the flask containing the SmI2 slurry dropwise via cannula. Vigorous stirring was maintained during the addition of the aziridine to the SmI2 slurry. The 5 mL flask was washed with 0.3 mL of degassed THF, and the rinse was also transferred to the reaction flask containing SmI2. The reaction mixture was stirred at 0 °C for 40 min to 1 h and then quenched by the addition of sat aq NaHCO3 (5 mL) at 0 °C. The organic layer was separated, and the aqueous layer was extracted with EtOAc (5 mL × 4). The combined organic layer was dried with Na2SO4 and filtered. The solvent was removed by rotary evaporation to give a light-yellow solid. The 1H NMR spectrum of the crude reaction mixture showed that it was a mixture of 19a and 23a in a ratio of 1.4:1. Purification by silica-gel chromatography (18 × 250 mm, 1:1:4.6 diethyl ether/hexanes/CH2Cl2 as eluent) afforded (S)-19a as a white solid (mp 75–77 °C) in 55% isolated yield (33.6 mg, 0.11 mmol) and 23a as a colorless oil in 32% isolated yield (19.2 mg, 0.065 mmol). The optical purity of (S)-19a was determined to be 78% ee by HPLC analysis (Chiralcel OD-H column, 98:2 hexanes/iPrOH at 222 nm, flow rate: 1.0 mL/min); retention times of R = 6.24 min (minor enantiomer (R)-19a) and R = 7.08 min (major enantiomer (S)-19a). TLC and spectral data for (S)-19a: R = 0.24 (5:1 hexanes/EtOAc); 1H NMR (CDCl3, 300 MHz) δ 1.14 (t, 3H, J = 7.1 Hz), 1.40 (s, 9H), 2.70–2.90 (m, 2H), 4.05 (q, 2H, J = 7.1 Hz), 5.09 (brs, 1H), 5.46 (brs, 1H), 7.19–7.35 (m, 5H); 13C NMR (CDCl3, 150 MHz) δ 14.0, 28.3, 41.0, 51.2, 60.6, 79.6, 126.1, 127.4, 128.6, 141.2, 155.0, 170.88; [α]D20 = −31.6 (c 1.0, EtOAc) on 78% ee (S)-19a.[31] TLC and spectral data for 23a (mixture of rotamers): R = 0.34 (5:1 hexanes/EtOAc); 1H NMR (CDCl3, 300 MHz) δ 1.219 and 1.224 (2t, 3H, J = 7.1 Hz), 1.447 and 1.452 (2s, 9H), 3.75 and 3.89 (2s, 2H), 4.13 and 4.14 (2q, 2H, J = 7.1 Hz), 4.49 and 4.52 (2s, 2H), 7.18–7.35 (m, 5H); 13C NMR (CDCl3, 150 MHz) δ 14.1, 14.2, 28.2, 28.3, 47.7, 48.1 51.0, 51.5, 60.9, 61.0, 80.4, 80.6, 127.37, 127.43, 127.5, 128.1, 128.5, 137.4, 137.6, 155.6, 155.8, 169.90, 169.94.[32]

Reductive Ring Opening of trans-11a

The reaction was carried out according to the general procedure described above, starting with racemic trans-11a (62.1 mg, 0.15 mmol, 1.0 equiv), n class="Chemical">SmI2 (5.5 equiv), N,N-dimethylethanolamine (0.17 mL, 1.7 mmol, 11.0 equiv), and dry THF (1.5 mL for SmI2 and 1.5 mL for aziridine, freshly distilled) held at 0 °C for 1 h. The 1H NMR spectrum of the crude reaction mixture showed that 12a(18) and 13a(18) were obtained in a ratio of 16.7:1. The NMR yield of 12a was determined to be 82% with the aid of Ph3CH as an internal standard.

Reductive Ring Opening of trans-11b

This reaction was carried out according to the general procedure described above, starting with racemic trans-11b (62.7 mg, 0.15 mmol, 1.0 equiv), n class="Chemical">SmI2 (6.0 equiv), N,N-dimethylethanolamine (0.18 mL, 1.8 mmol, 12.0 equiv), and dry THF (1.5 mL for SmI2 and 1.5 mL for aziridine, freshly distilled) held at 0 °C for 1 h. The NMR yield of 12b(18) was determined to be 73% with the aid of Ph3CH as an internal standard.

Reductive Ring Opening of trans-15a

The reaction was carried out according to the general procedure described above, starting with racemic trans-15a (58.3 mg, 0.2 mmol, 1.0 equiv), n class="Chemical">SmI2 (6.0 equiv), N,N-dimethylethanolamine (0.24 mL, 2.4 mmol, 12.0 equiv), and dry THF (1.8 mL for SmI2 and 1.8 mL for aziridine, freshly distilled) held at 0 °C for 1 h. The 1H NMR spectrum of the crude reaction mixture showed that 19a and 23a were present in a ratio of 6.7:1. Purification by silica-gel chromatography (18 × 250 mm, 1:1:4.6 diethyl ether/hexanes/CH2Cl2 as eluent) afforded 19a as a white solid (mp 75–77 °C) in 85% isolated yield (50.1 mg, 0.17 mmol) and 23a as a colorless oil in 10% isolated yield (5.9 mg, 0.02 mmol). The spectral data for 19a and 23a are the same as those obtained in the reductive ring opening of cis-15a.

Reductive Ring Opening of cis-15b

The general procedure for the reductive ring opening described above was followed, starting with cis-15b, except that 4 equiv of n class="Chemical">SmI2 and 8 equiv of DMEA were used. Starting from cis-15b[18] (38 mg, 0.12 mmol), purification by silica-gel chromatography (18 mm × 150 mm, 1:5 EtOAc/hexanes as eluent) gave 19b as a white solid in 84% isolated yield (30 mg, 0.1 mmol). The 1H NMR spectrum of the crude reaction mixture showed the ratio of 19b/23b > 99:1. TLC and spectral data for 19b: R = 0.25 (1:5 EtOAc/hexane); 1H NMR (CDCl3, 500 MHz) δ 0.83–0.99 (m, 2H), 1.01–1.17 (m, 3H), 1.21 (t, 3H, J = 7.1 Hz), 1.38 (s, 9H), 1.31–1.40 (m, 1H), 1.61–1.75 (m, 5H), 2.38–2.51 (m, 2H), 3.63–3.75 (m, 1H), 4.08 (q, 2H, J = 7.1 Hz), 4.88 (d, 1H, J = 7.1 Hz); 13C NMR (CDCl3, 125 MHz) δ 14.1, 25.98, 26.02, 26.2, 28.3, 28.9, 29.7, 37.0, 41.5, 52.2, 60.4, 79.0, 155.5, 172.0; IR (thin film) 3360 (br, s), 2930 (s), 1719 (vs), 1504 (m), 1173 (s) cm–1. The 1H NMR spectral data match those previously reported for this compound.[33] The spectral data for 23b has been reported.[34]

Reductive Ring Opening of trans-15b

The reaction was carried out according to the general procedure described above, starting with racemic trans-15b (59.5 mg, 0.2 mmol, 1.0 equiv), n class="Chemical">SmI2 (6.0 equiv), N,N-dimethylethanolamine (0.24 mL, 2.4 mmol, 12.0 equiv), and dry THF (1.8 mL for SmI2 and 1.8 mL for aziridine, freshly distilled) held at 0 °C for 1 h. β-Amino ester 19b was obtained in 84% NMR yield with the aid of Ph3CH as an internal standard. The 1H NMR spectrum of the crude reaction mixture showed the ratio of 19b/23b > 99:1.

Reductive Ring Opening of trans-16a

The reaction was carried out according to the general procedure described above, starting with racemic trans-16a (51.9 mg, 0.15 mmol, 1.0 equiv), n class="Chemical">SmI2 (6.0 equiv), N,N-dimethylethanolamine (0.18 mL, 1.8 mmol, 12.0 equiv), and dry THF (1.5 mL for SmI2 and 1.5 mL for aziridine, freshly distilled) held at 0 °C for 1 h. Purification by silica-gel chromatography (18 × 250 mm, 5:1 hexanes/EtOAc as eluent) afforded 20a as a colorless liquid in 88% isolated yield (45.9 mg, 0.132 mmol). Spectral data for 20a: R = 0.25 (1:3 EtOAc/hexanes); 1H NMR (CDCl3, 500 MHz) δ 1.10 (t, 3H, J = 7.2 Hz), 2.35 (s, 3H), 2.72 (dd, 1H, J = 16, 6.0 Hz), 2.81 (dd, 1H, J = 16, 6.0 Hz), 3.94–4.05 (m, 2H), 4.70 (q, 1H, J = 6.7 Hz), 5.67 (d, 1H, J = 7.5 Hz), 7.06–7.11 (m, 2H), 7.12–7.19 (m, 5H), 7.56–7.60 (m, 2H); 13C NMR (CDCl3, 125 MHz) δ 14.0, 21.4, 41.2, 54.3, 60.9, 126.4, 127.1, 127.7, 128.5, 129.4, 137.5, 139.3, 143.2, 170.6. These data match those previously reported for this compound.[13b] The 1H NMR spectrum of the crude reaction mixture showed the ratio of 20a/24a > 99:1.

Reductive Ring Opening of cis-16b

The general procedure for the reductive ring opening described above was followed, starting with aziridine (2R,3R)-16b[18] (82% ee, 53.4 mg, 0.15 mmol, 1.0 equiv), n class="Chemical">SmI2 (5.0 equiv), N,N-dimethylethanolamine (0.15 mL, 1.5 mmol, 10.0 equiv), and dry THF (1.5 mL for SmI2 and 1.5 mL for aziridine, freshly distilled) held at 0 °C for 1 h. Purification by silica-gel chromatography (18 × 250 mm, 5:1 hexanes/EtOAc as eluent) afforded (S)-20b as a colorless oil in 97% isolated yield (52.1 mg, 0.147 mmol). The optical purity of (S)-20b was determined to be 84% ee by HPLC analysis (Chiralcel OD-H column, 98:2 hexanes/iPrOH at 222 nm, flow rate: 1.0 mL/min); retention times: R = 12.73 min (major enantiomer (S)-20b) and R = 16.94 min (minor enantiomer (R)-20b). The 1H NMR spectrum of the crude reaction mixture showed the ratio of 20b/24b > 99:1. Spectral data for (S)-20b: R = 0.17 (4:1 hexanes/EtOAc); 1H NMR (CDCl3, 600 MHz) δ 0.78 (qd, 1H, J = 12.0, 3.2 Hz), 0.87 (qd, 1H, J = 12.0, 3.2), 1.00–1.17 (m, 3H), 1.18 (t, 3H, J = 7.1 Hz), 1.37–1.45 (m, 1H), 1.54–1.61 (m, 2H), 1.63–1.70 (m, 2H), 1.73–1.81 (m, 1H), 2.25 (dd, 1H, J = 16.0, 5.6 Hz), 2.37 (dd, 1H, J = 16.0, 5.6 Hz), 2.39 (s, 3H), 3.26–3.34 (m, 1H), 4.01 (m, 2H), 5.24 (d, 1H, J = 9.3 Hz), 7.26 (d, 2H, J = 8.1 Hz), 7.72 (d, 2H, J = 8.1 Hz); 13C NMR (CDCl3, 150 MHz) δ 14.1, 21.5, 25.87, 25.93, 26.1, 29.1, 29.3, 35.9, 41.2, 55.5, 60.6, 127.0, 129.6, 138.2, 143.2, 171.6; IR (thin film) 3292 (m), 2928 (vs), 2854 (m), 1734 (vs), 1718 (s), 1456 (m), 1324 (s), 1161 (vs) cm–1; mass spectrum, m/z (% rel intensity) 354 (MH+) (6.4), 271 (31), 270 (100), 224 (50), 198 (42), 155 (86), 91 (86), 41 (12); Anal. calcd for C18H27NO4S: C, 61.16; H, 7.70; N, 3.96. Found: C, 61.30; H, 8.12; N, 3.88. [α]D20 = −10 (c 0.4, EtOAc) on 84% ee (S)-20b.[35]

Reductive Ring Opening of trans-16b

The reaction was carried out according to the general procedure described above, starting with racemic trans-16b (53.3 mg, 0.15 mmol, 1.0 equiv), SmI2 (6.0 equiv), n class="Chemical">N,N-dimethylethanolamine (0.18 mL, 1.8 mmol, 12.0 equiv), and dry THF (1.5 mL for SmI2 and 1.5 mL for aziridine, freshly distilled) held at 0 °C for 1 h. Purification by silica-gel chromatography (18 × 250 mm, 5:1 hexanes/EtOAc as eluent) afforded 20b as a colorless oil in 95% isolated yield (51.0 mg, 0.144 mmol). The spectral data of 20b are the same as the product obtained from the reductive ring opening of cis-16b. The 1H NMR spectrum of the crude reaction mixture showed the ratio of 20b/24b > 99:1

Reductive Ring Opening of cis-17a

The general procedure for the reductive ring opening described above was followed, starting with aziridine cis-17a (53.4 mg, 0.15 mmol, 1.0 equiv), n class="Chemical">SmI2 (4.0 equiv), N,N-dimethylethanolamine (0.12 mL, 1.2 mmol, 8.0 equiv), and dry THF (2.0 mL for SmI2 and 1.5 mL for aziridine, freshly distilled) held at 0 °C for 1 h. The 1H NMR spectrum of the crude reaction mixture showed that 21a and 25a were present in a ratio of 23:1. Purification by silica-gel chromatography (18 × 250 mm, 5:1 hexanes/EtOAc as eluent) afforded (S)-21a as a white solid in 84% isolated yield (45.2 mg, 0.126 mmol) and 25a as a colorless oil in 4% isolated yield (2.0 mg, 0.0056 mmol). Spectral data for 21a: white solid; mp 60–61 °C; 1H NMR (500 MHz, CDCl3) δ −0.14 (s, 9H), 0.75 (td, 1H, J = 14, 4.5 Hz), 0.84 (td, 1H, J = 14, 4.0 Hz), 1.17 (t, 3H, J = 7 Hz), 2.52 (td, 1H, J = 14, 4.5 Hz), 2.63 (td, 1H, J = 14, 4.0 Hz), 2.79–2.90 (m, 2H), 4.08 (qd, 2H, J = 7.2, 1.5 Hz), 4.81–4.90 (m, 1H), 5.57 (d, 1H, J = 8 Hz), 7.24–7.30 (m, 1H), 7.30–7.39 (m, 4H); 13C NMR (CDCl3, 125 MHz) δ −2.2, 10.2, 14.0, 41.9, 49.9, 54.5, 61.0, 126.6, 128.2, 128.9, 140.3, 170.5; IR (thin film) 3277 (br, s), 2955 (w), 1736 (s), 1144 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C16H28NO4SiS+, 358.1508; found, 358.1509; [α]D20 = −21.7 (c 0.87, CH2Cl2) on 98% ee (S)-21a. Spectral data for 25a: 1H NMR (500 MHz, CDCl3) δ 0.05 (s, 9H), 1.11–1.16 (m, 2H), 1.24 (t, 3H, J = 7.2 Hz), 3.05–3.10 (m, 2H), 3.90 (s, 2H), 4.16 (q, 2H, J = 7.2 Hz), 4.53 (s, 2H), 7.27–7.36 (m, 5H); 13C NMR (CDCl3, 125 MHz) δ −2.0, 10.3, 14.2, 46.9, 50.0, 51.8, 61.3, 128.1, 128.5, 128.8, 135.5, 169.6; IR (thin film) 2955 (w), 1746 (s), 1333 (s), 1142 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C16H28NO4SiS+, 358.1508; found, 358.1530.

Reductive Ring Opening of cis-18a

The general procedure for the reductive ring opening described above was followed, except that 2.5 equiv of Sml2 and 5 equiv of DMEA were used. Starting from n class="Chemical">cis-18a (47 mg, 0.19 mmol), purification of the product by silica-gel chromatography (18 mm × 300 mm, 1:5 EtOAc/hexanes as eluent) gave 22a as a colorless semisolid in 52% isolated yield (24 mg, 0.1 mmol), along with the α-amino ester resulting from the cleavage of the N–C3 bond as a colorless semisolid in 13% isolated yield (6 mg, 0.025 mmol). TLC and spectral data for 22a: R = 0.15 (1:5 EtOAc/hexane); 1H NMR (CDCl3, 500 MHz) δ 1.12 (t, 3H, J = 7.1 Hz), 1.98 (s, 3H), 2.77 (dd, 1H, J = 15.4, 6.0 Hz), 2.87 (dd, 1H, J = 15.7, 6.0 Hz), 4.04 (q, 2H, J = 7.1 Hz), 5.36–5.43 (m, 1H), 6.60 (d, 1H, J = 8.0 Hz), 7.20–7.32 (m, 5H). The 1H NMR spectral data match those previously reported for this compound.[13b]

Reductive Ring Opening of cis-27a

The general procedure for the reductive ring opening described above was followed, except that 5.0 equiv of Sml2 and 10 equiv of DMEA were used. Starting from n class="Chemical">cis-27a(18) (19 mg, 0.10 mmol), purification of the product by silica-gel chromatography (18 mm × 300 mm, 1:1:4 Et2O/CH2Cl2/hexanes as eluent) gave 31a as a colorless oil in 75% isolated yield (14.4 mg, 0.0746 mmol). Spectral data for 31a: 1H NMR (CDCl3, 300 MHz,) δ 1.25 (t, 3H, J = 7.1 Hz), 3.39 (s, 2H), 3.78 (s, 2H), 4.17 (q, 2H, J = 7.2 Hz), 7.20–7.35 (m, 5H) (N–H proton not located). The 1H NMR spectral data match those previously reported for this compound.[36]

Reductive Ring Opening of cis-29a

The general procedure for the reductive ring opening described above was followed, except that 5.0 equiv of Sml2 and 10 equiv of DMEA were used. Starting from n class="Chemical">cis-29a[7a] (35 mg, 0.10 mmol), purification of the product by silica-gel chromatography (18 mm × 300 mm, 1:1:4 Et2O/CH2Cl2/hexanes as eluent) gave 33a as a colorless oil in 69% isolated yield (25 mg, 0.069 mmol). Spectral data for 33a: 1H NMR (300 MHz, CDCl3) δ 1.17 (t, 3H, J = 7.2 Hz), 3.25 (s, 2H), 3.82 (s, 2H), 4.04 (q, 2H, J = 7.2 Hz), 5.27 (s, 1H), 7.15–7.51 (m, 15H); 13C NMR (75 MHz, CDCl3) δ 14.2, 50.3, 54.9, 59.9, 70.3, 127.0, 127.1, 128.28, 128.33, 128.4, 128.7, 139.2, 142.1, 171.6.

Reductive Ring Opening of cis-29b

The general procedure for the reductive ring opening described above was followed, starting with cis-29b[7a] (44 mg, 0.12 mmol). Purification of the product by n class="Chemical">silica-gel chromatography (18 mm × 300 mm, 1:5 EtOAc/hexanes as eluent) gave a 1:1.4 mixture of β-amino ester 30b (22% NMR yield) and unreacted cis-29b (31% NMR yield). The 1H NMR spectrum of the crude reaction mixture indicated the formation of amine 34 in 39% yield. Extending the reaction time from 40 min to 2 h for the ring-opening reaction of cis-29b (42 mg, 0.12 mmol) at room temperature gave 30b (8.6 mg, 0.024 mmol) in 22% isolated yield and amine 34 in 52% isolated yield. The unreacted cis-29b was isolated with a 20% recovery. Spectral data for 30b: 1H NMR (600 MHz, CDCl3) δ 0.90–1.04 (m, 2H), 1.05–1.23 (m, 2H), 1.20 (t, 3H, J = 7.1 Hz), 1.45–1.54 (m, 1H), 1.60–1.80 (m, 6H), 2.34 (dd, 1H, J = 14.5, 7.0 Hz), 2.47 (dd, 1H, J = 14.5, 5.3 Hz), 2.77 (dt, 1H, J = 7.0, 5.3 Hz), 4.03–4.14 (m, 2H), 4.94 (s, 1H), 7.15–7.19 (m, 2H), 7.23–7.28 (m, 4H), 7.35–7.41 (m, 4H) (N–H proton not located); 13C NMR (150 MHz, CDCl3) δ 14.2, 26.5, 26.6, 26.7, 28.4, 29.5, 36.0, 40.9, 56.8, 60.2, 64.1, 126.89, 126.91, 127.4, 127.6, 128.33, 128.34, 144.2, 144.4, 172.9; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C24H32NO2, 366.2433; found, 366.2431. Spectral data for 34: 1H NMR (600 MHz, CDCl3) δ 1.84 (bs, 2H), 5.20 (s, 1H), 7.18–7.23 (m, 2H), 7.26–7.32 (m, 4H), 7.33–7.37 (m, 4H); 13C NMR (150 MHz, CDCl3) δ 59.7, 126.88, 126.92, 128.4, 145.6. The 1H and 13C NMR spectral data for 34 match those provided by the source for this compound.

Reductive Ring Opening of 35

The general procedure for the reductive ring opening described above was followed, starting with trisubstituted aziridine 35(20) (76 mg, 0.25 mmol, 99% ee) and 5.0 equiv of n class="Chemical">SmI2. The 1H NMR spectrum of the crude reaction mixture indicated that a mixture of anti-36, syn-36, and 37 was present in a ratio of 1.28:0.28:1, respectively. Purification of the products by silica-gel chromatography (18 mm × 200 mm, 1:10 to 1:6 EtOAc/hexanes as eluent) gave anti-36 as a white solid (mp 53–55 °C) in 43% isolated yield (32.7 mg, 0.106 mmol), syn-36 as a white solid (mp 89–91 °C) in 9% isolated yield (6.6 mg, 0.021 mmol), and 37 as a colorless oil in 30% isolated yield (23.2 mg, 0.0755 mmol). Spectral data for anti-36: 1H NMR (300 MHz, CDCl3) δ 1.09 (t, 3H, J = 7.1 Hz), 1.20 (d, 3H, J = 7.0 Hz), 1.39 (s, 9H), 2.87 (brt, 1H, J = 6.5 Hz), 4.01 (q, 2H, J = 7.1 Hz), 4.81 (brs, 1H), 5.81 (brs, 1H), 7.16–7.33 (m, 5H); 13C NMR (125 MHz, CDCl3) δ 14.0, 15.4, 28.3, 45.3, 56.7, 60.6, 79.4, 126.3, 127.3, 128.4, 141.0, 155.4, 174.9; IR (thin film) 3355 (br, w), 2978 (m), 1721 (s), 1171 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C17H26NO4+, 308.1862; found, 308.1860; [α]D20 = −40.1 (c 0.5, CH2Cl2) on 99% ee material. Spectral data for syn-36: 1H NMR (600 MHz, CDCl3) δ 1.11 (t, 3H, J = 7.2 Hz), 1.13 (d, 3H, J = 7.2 Hz), 1.39 (s, 9H), 2.87 (brs, 1H), 3.96–4.08 (m, 2H), 4.97 (brs, 1H), 5.27 (brs, 1H), 7.19–7.25 (m, 3H), 7.26–7.31 (m, 2H); 13C NMR (150 MHz, CDCl3) δ 13.1, 14.0, 28.3, 45.4, 56.5, 60.6, 79.6, 126.8, 127.4, 128.4, 140.2, 155.1, 173.7; IR (thin film) 3380 (s), 2980 (m), 1728 (s), 1686 (s), 1520 (s), 1173 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C17H26NO4+, 308.1862; found, 308.1855; [α]D20 = −26.0 (c 0.5, CH2Cl2) on 99% ee material. Spectral data for 37 (compound 37 appeared to be a colorless-oil mixture of two rotamers at room temperature in a ratio of 1.2:1): 1H NMR (600 MHz, CDCl3) δ 1.21 (t, 3H, J = 7.2 Hz), 1.27–1.50 (m, 12H), 3.82–4.64 (m, 5H), 7.16–7.37 (m, 5H); 13C NMR (125 MHz, CDCl3) δ 14.1, 15.4, 15.8, 28.3, 49.7, 50.8, 54.8, 55.4, 60.9, 80.4, 80.6, 127.0, 127.2, 127.3, 128.0, 128.3, 128.6, 138.2, 139.2, 155.4, 155.5, 172.1, 172.3. The 1H NMR data match those previously reported for this compound.[37]

Determination of the Relative Stereochemistry of anti-36

Trifluoroacetic acid (0.240 mL, 357 mg, 3.13 mmol, 35.6 equiv) was added to a solution of anti-36 (27 mg, 0.088 mmol, 1.0 equiv) in CH2Cl2 (0.24 mL). After the reaction mixture was stirred at room temperature under n class="Chemical">nitrogen overnight, it was concentrated and diluted with 0.3 mL of H2O. The pH of the mixture was adjusted to ∼10 with sat aq NaHCO3 (ca. 10 mL), and then the mixture was extracted with CH2Cl2 (10 mL × 3). The combined organic layer was dried over Na2SO4, filtered, and concentrated. Purification of the product by silica-gel chromatography (18 mm × 150 mm, 1:1 EtOAc/hexanes as eluent) gave ethyl 3-amino-2-methyl-3-phenylpropanoate 60 as a colorless oil in 74% isolated yield (13.5 mg, 0.065 mmol). Spectral data for ethyl 3-amino-2-methyl-3-phenylpropanoate: 1H NMR (500 MHz, CDCl3) δ 0.93 (d, 3H, J = 7.2 Hz), 1.26 (t, 3H, J = 7.2 Hz), 1.64 (brs, 2H), 2.60–2.72 (m, 1H), 4.00 (d, 1H, J = 9.4 Hz), 4.17 (q, 2H, J = 7.2 Hz), 7.21–7.34 (m, 5H); 13C NMR (125 MHz, CDCl3) δ 14.2, 15.4, 48.1, 59.1, 60.4, 127.0, 127.5, 128.5, 143.6, 175.9. The 1H NMR data match those previously reported for the anti isomer but not those of the syn isomer.[38]

Procedures for the Syntheses of Protected Forms of l-DOPA and (R)-β3-DOPA

4-((2S,3S)-1-(Bis(4-methoxy-3,5-dimethylphenyl)methyl)-3-(ethoxycarbonyl)aziridin-2-yl)-1,2-phenylene Diacetate (54)

To a flame-dried 25 mL Schlenk flask equipped with a stir bar and filled with nitrogen were added n class="Chemical">(R)-VAPOL (54 mg, 0.10 mmol), B(OPh)3 (87 mg, 0.30 mmol), amine 52(7d) (599 mg, 2.00 mmol); dry toluene (4 mL) was added to dissolve the reagents. The flask was then sealed, and the reaction mixture was stirred at room temperature for 1 h. Thereafter, 4 Å of powdered molecular sieves (600 mg, freshly flame-dried) were added to the reaction flask, followed by the addition of aldehyde 50(39) (467 mg, 2.10 mmoL, 1.05 equiv). to this solution was rapidly added ethyl diazoacetate (EDA) 53 (0.30 mL, 2.4 mmoL, 1.2 equiv). After the resulting mixture was stirred for 20 h at room temperature, it was diluted by the addition of hexane (12 mL). The reaction mixture was then filtered through a Celite pad into a 100 mL round-bottomed flask. The reaction flask was rinsed with EtOAc (6 mL × 3), and the rinse was filtered through the same Celite pad. The combined filtrate was concentrated in vacuo, followed by exposure to high vacuum (0.05 mmHg) to afford the crude aziridine as a yellow oil. Purification of the crude aziridine by silica-gel chromatography (40 mm × 210 mm column, 2:1 hexanes/EtOAc as eluent) afforded pure cis-aziridine 54 as a white solid (mp 65–67 °C on >98.5% ee material) in 98% isolated yield (1.15 g, 1.95 mmol). The optical purity of 54 was determined to be >98.5% ee by HPLC analysis (CHIRALCEL OD-H column, 85:15 hexane/2-propanol at 222 nm, flow rate: 1 mL/min). Retention times: R = 7.18 min (minor enantiomer, ent-54) and R = 8.46 min (major enantiomer, 54). Spectral data for 54: R = 0.19 (1:2 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 1.02 (t, 3H, J = 7.1 Hz), 2.21 (s, 6H), 2.25 (s, 3H), 2.26 (s, 6H), 2.27 (s, 3H), 2.59 (d, 1H, J = 6.8 Hz), 3.09 (d, 1H, J = 6.8 Hz), 3.65 (s, 3H), 3.68 (s, 1H), 3.69 (s, 3H), 3.91–4.03 (m, 2H), 7.04–7.10 (m, 3H), 7.17 (s, 2H), 7.24–7.29 (m, 2H); 13C NMR (CDCl3, 125 MHz) δ 14.0, 16.15, 16.22, 20.59, 20.60, 46.4, 47.2, 59.5, 59.6, 60.8, 76.8, 122.6, 122.9, 126.0, 127.3, 127.7, 130.6, 130.8, 134.2, 137.5, 137.6, 141.2, 141.4, 155.9, 156.2, 167.7, 168.0, 168.2; IR (thin film) 2932 (m), 1773 (vs), 1746 (s), 1213 (vs) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C34H40NO8+, 590.2754; found, 590.2769; [α]D20 = −28.2° (c 1.0, CH2Cl2) on >98.5% ee (by HPLC) material.

4-((2S,3S)-1-(Bis(4-methoxy-3,5-dimethylphenyl)methyl)-3-(ethoxycarbonyl)aziridin-2-yl)-1,2-phenylene bis(2,2-dimethylpropanoate) (55)

The procedure for the synthesis of aziridine 54 was followed, starting with n class="Chemical">aldehyde 51(40) (643 mg, 2.10 mmoL, 1.05 equiv). Purification of the crude product by silica-gel chromatography (40 mm × 210 mm column, 5:1 hexanes/EtOAc as eluent) afforded pure cis-aziridine 55 as a white solid (mp 64–66 °C on 98% ee material) in 97% isolated yield (1.31 g, 1.94 mmol). The optical purity of 55 was determined to be 98% ee by HPLC analysis (Chiralpak AD column, 95:5 hexane/2-propanol at 222 nm, flow- rate: 0.7 mL/min). Retention times: R = 8.68 min (minor enantiomer, ent-55) and R = 10.23 min (major enantiomer, 55). Spectral data for 55: R = 0.21 (1:5 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 1.04 (t, 3H, J = 7.0 Hz), 1.30 (s, 9H), 1.32 (s, 9H), 2.20 (s, 6H), 2.24 (s, 6H), 2.56 (d, 1H, J = 6.8 Hz), 3.08 (d, 1H, J = 6.8 Hz), 3.64 (s, 3H), 3.66 (s, 1H), 3.68 (s, 3H), 3.90–4.03 (m, 2H), 6.98 (d, 1H, J = 8.0 Hz), 7.07 (s, 2H), 7.11 (d, 1H, J = 1.5 Hz), 7.16 (s, 2H), 7.25 (dd, 1H, J = 8.5, 1.5 Hz); 13C NMR (CDCl3, 125 MHz) δ 14.0, 16.18, 16.22, 27.18, 27.23, 39.0, 39.1, 46.2, 47.3, 59.5, 59.6, 60.7, 76.9, 122.6, 122.8, 125.6, 127.3, 127.7, 130.6, 130.7, 133.7, 137.55, 137.61, 141.7, 141.9, 155.9, 156.1, 167.8, 175.6, 175.8; IR (thin film) 2977 (s), 1761 (vs), 1482 (s), 1119 (vs) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C40H52NO8+, 674.3693; found, 674.3694; [α]D20 = −34.2° (c 1.0, CH2Cl2) on 98% ee (by HPLC) material.

(S)-4-(2-((tert-Butoxycarbonyl)amino)-3-ethoxy-3-oxopropyl)-1,2-phenylene Bis(2,2-dimethylpropanoate) (56)

To a oven-dried 25 mL round-bottomed flask equipped with a stir bar and filled with nitrogen were added n class="Chemical">aziridine 55 (67.4 mg, 0.100 mmol, 98% ee), Pd(OH)2 (28.0 mg, 0.020 mmol, Pd(OH)2 on carbon 20%, moisture ≤50%), ditert-butyl dicarbonate (33 mg, 0.15 mmol), and methanol (10 mL). The flask was sealed with a rubber septum, and a needle connected to a vacuum line was used to apply vacuum in the flask through the septum. The vacuum was applied for a few seconds with vigorous stirring of the reaction mixture. Then the vacuum was stopped, and a hydrogen balloon was connected to the flask by a needle through the septum. This process was repeated four times. The suspension was stirred at room temperature under hydrogen for 17 h and then filtered through a pad of Celite. The filter cake was washed with EtOAc (5 mL) and DCM (3 mL × 3). The combined filtrate was concentrated to give a light-yellow oil. Purification of the crude product by column chromatography on silica gel (20 mm × 160 mm, hexanes/EtOAc 5:1) gave α-amino ester 56 as a colorless oil (42.2 mg, 0.0855 mmol, 86%). Spectral data for 56: R = 0.20 (1:5 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 1.20 (t, 3H, J = 7.0 Hz), 1.30 (s, 18H), 1.41 (s, 9H), 3.06 (d, 2H, J = 6.0 Hz), 4.07–4.18 (m, 2H), 4.50 (dt, 1H, J = 7.5, 6.0 Hz), 5.01 (d, 1H, J = 7.5 Hz), 6.87 (s, 1H), 6.97 (d, 1H, J = 8.2 Hz), 7.02 (d, 1H, J = 8.2 Hz); 13C NMR (CDCl3, 125 MHz) δ 14.1, 27.2, 28.3, 37.5, 39.06, 39.09, 54.3, 61.5, 79.9, 123.2, 124.2, 127.0, 134.5, 141.5, 142.3, 155.0, 171.5, 175.6, 175.8 (one sp3 carbon not located); IR (thin film) 2977 (s), 1761 (vs), 1482 (s), 1119 (vs) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C26H40NO8+, 494.2754; found, 494.2751; [α]D20 = +27.8° (c 1.0, CH2Cl2) on 98% ee material. (The optical purity was assumed to be unchanged from that of 55.)

4-((2S,3S)-3-(Ethoxycarbonyl)-1-tosylaziridin-2-yl)-1,2-phenylene Diacetate (57)

To a flame-dried 100 mL round-bottomed flask equipped with a stir bar and filled with nitrogen were added room-temperature n class="Chemical">aziridine 54 (467 mg, 0.792 mmol) and anisole (4.1 mL). The resulting solution was cooled to 0 °C in an ice bath, and trifluoroacetic acid (4.1 mL) was rapidly added. The ice bath was then removed, and the reaction mixture was stirred for 40 min at room temperature. The reaction mixture was quenched by careful addition of sat aq Na2CO3 (30 mL) and H2O (10 mL), followed by the addition of Et2O (30 mL). The organic layer was separated, and the aqueous layer was extracted with Et2O (30 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated in vacuo, followed by exposure to high vacuum (0.05 mmHg) for 4 h to give a yellow oil, to which was added 6.5 mL of CH2CI2/CHCl3 (1:1) and Et3N (0.33 mL, 2.4 mmol). The resulting solution was cooled to 0 °C, and a portion of tosyl chloride (228 mg, 1.20 mmol) was added. The mixture was then stirred at 0 °C for 15 h. Thereafter, additional portions of both tosyl chloride (228 mg, 1.20 mmol) and Et3N (0.33 mL, 2.4 mmol) were added to the reaction mixture at room temperature. After the mixture was stirred for 26 h at room temperature, the reaction was quenched with 12 mL of sat aq NH4Cl and 2.5 mL of H2O. The aqueous layer was extracted with CH2Cl2 (15 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated to give a dark-brown oil. Purification by silica-gel chromatography (25 mm × 160 mm, 2:1 hexanes/EtOAc as eluent) afforded cis-57 as a light-yellow oil in 70% yield (258 mg, 0.559 mmol). Spectral data for 57: R = 0.14 (1:2 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 0.96 (t, 3H, J = 7.1 Hz), 2.236 (s, 3H), 2.240 (s, 3H), 2.43 (s, 3H), 3.66 (d, 1H, J = 7.5 Hz), 3.89–4.02 (m, 2H), 4.03 (d, 1H, J = 7.5 Hz), 7.07 (d, 1H, J = 8.3 Hz), 7.13 (d, 1H, J = 2.0 Hz), 7.17 (dd, 2H, J = 8.3, 2.0 Hz), 7.34 (d, 1H, J = 8.0 Hz), 7.88 (d, 1H, J = 8.0 Hz); 13C NMR (CDCl3, 125 MHz) δ 13.7, 20.55, 20.60, 21.7, 43.4, 44.3, 61.9, 122.7, 123.3, 125.8, 128.1, 129.9, 130.0, 133.7, 141.8, 142.3, 145.4, 164.1, 167.8, 168.0; IR (thin film) 2986 (w), 1773 (vs), 1734 (m), 1210 (vs), 1165 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C22H24NO8S+, 462.1223; found, 462.1234; [α]D20 = +15.6° (c 1.0, CH2Cl2) on >98.5% ee material. (The optical purity was assumed to be unchanged from that of 54.)

4-((2S,3S)-3-(Ethoxycarbonyl)-1-((2-(trimethylsilyl)ethyl)sulfonyl)aziridin-2-yl)-1,2-phenylene Diacetate (58)

to a flame-dried 100 mL round-bottomed flask equipped with a stir bar and filled with nitrogen were added room-temperature n class="Chemical">aziridine 54 (366 mg, 0.621 mmol) and anisole (3.1 mL). The resulting solution was cooled to 0 °C in an ice bath, and trifluoroacetic acid (3.1 mL) was rapidly added. The ice bath was then removed, and the reaction mixture was stirred for 40 min at room temperature. The reaction mixture was quenched by careful addition of saturated aq Na2CO3 (25 mL) and H2O (10 mL), followed by addition of Et2O (30 mL). The organic layer was separated, and the aqueous layer was extracted with Et2O (30 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated in vacuo followed by exposure to high vacuum (0.05 mmHg) for 4 h to give a yellow oil, which was then dissolved in a mixture of CH2Cl2 (2 mL) and Et3N (0.9 mL). After the solution was cooled to 0 °C in an ice bath, 2-(trimethylsilyl)ethanesulfonyl chloride (0.12 mL, 0.93 mmol) was added dropwise to the reaction mixture at 0 °C. The ice bath was then removed, and the reaction mixture was stirred at room temperature for 17 h. Thereafter, additional portions of both 2-(trimethylsilyl)ethanesulfonyl chloride (0.12 mL, 0.93 mmol) and Et3N (0.9 mL) were added to the reaction mixture at room temperature. After the mixture was stirred for 23 h at room temperature, the reaction was quenched with 2.5 mL of sat aq NH4Cl and 1 mL of H2O. The organic layer was separated, and the aqueous layer was extracted with CH2Cl2 (4 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated to give a dark-brown oil. Purification by silica-gel chromatography (25 mm × 160 mm, 3:1 hexanes/EtOAc as eluent) afforded cis-58 as a light-yellow oil in 78% yield (0.228 g, 0.483 mmol). Spectral data for 58: R = 0.20 (1:3 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 0.05 (s, 9H), 1.03 (t, 3H, J = 7.0 Hz), 1.12–1.20 (m, 2H), 2.26 (s, 6H), 3.16–3.24 (m, 2H), 3.66 (d, 1H, J = 7.5 Hz), 3.97–4.09 (m, 2H), 4.02 (d, 1H, J = 7.5 Hz), 7.15 (d, 1H, J = 8.2 Hz), 7.25 (d, 1H, J = 1.9 Hz), 7.30 (dd, 1H, J = 8.2 Hz, J = 1.9 Hz); 13C NMR (125 MHz, CDCl3) δ −2.1, 9.5, 13.8, 20.57, 20.62, 43.5, 43.6, 49.5, 62.0, 122.8, 123.4, 125.8, 130.1, 142.0, 142.4, 164.2, 167.9, 168.0; IR (thin film) 2955 (m), 1777 (s), 1208 (s), 1177 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C20H30NO8SiS+, 472.1461; found, 472.1449; [α]D20 = +27.5° (c 1.0, CH2Cl2) on >98.5% ee material. (The optical purity was assumed to be unchanged from that of 54.)

4-((2S,3S)-3-(Ethoxycarbonyl)-1-((2-(trimethylsilyl)ethyl)sulfonyl)aziridin-2-yl)-1,2-phenylene Bis(2,2-dimethylpropanoate) (59)

To a flame-dried 100 mL round-bottomed flask equipped with a stir bar and filled with nitrogen were added room-temperature n class="Chemical">aziridine 55 (674 mg, 1.00 mmol) and anisole (8.9 mL). The resulting solution was cooled to 0 °C in a ice bath, and trifluoroacetic acid (8.9 mL) was rapidly added. The ice bath was removed, and the reaction mixture was stirred for 30 min at room temperature. The reaction mixture was quenched by the careful addition of saturated aq Na2CO3 (68 mL) and H2O (35 mL), followed by the addition of Et2O (50 mL). The organic layer was separated, and the aqueous layer was extracted with Et2O (100 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated in vacuo, followed by exposure to high vacuum (0.05 mmHg) for 4 h to give a yellow oil, which was then dissolved in a mixture of CH2Cl2 (3.5 mL) and Et3N (1.5 mL). After the solution was cooled to 0 °C, 2-(trimethylsilyl)ethanesulfonyl chloride (0.20 mL, 1.5 mmol) was added dropwise to the reaction mixture at 0 °C. The ice bath was removed, and the reaction mixture was stirred at room temperature for 14 h. Thereafter, additional portions of both 2-(trimethylsilyl)ethanesulfonyl chloride (0.20 mL, 1.5 mmol) and Et3N (1.5 mL) were added to the reaction mixture at room temperature. After the mixture was stirred for 22 h at room temperature, the reaction was quenched with 4 mL of sat aq NH4Cl and 1.5 mL of H2O. The organic layer was separated, and the aqueous layer was extracted with CH2Cl2 (6 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated to give a dark-brown oil. Purification by silica-gel chromatography (25 mm × 160 mm, 5:1 hexanes/EtOAc as eluent) afforded cis-59 as a light-yellow oil in 86% yield (0.475 g, 0.855 mmol). Spectral data for 59: R = 0.20 (1:5 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 0.05 (s, 9H), 1.06 (t, 3H, J = 7.1 Hz), 1.11–1.21 (m, 2H), 1.31 (s, 9H), 1.32 (s, 9H), 3.17–3.26 (m, 2H), 3.66 (d, 1H, J = 7.5 Hz), 3.98–4.07 (m, 2H), 4.02 (d, 1H, J = 7.5 Hz), 7.09 (d, 1H, J = 8.3 Hz), 7.18 (d, 1H, J = 1.8 Hz), 7.27 (dd, 1H, J = 8.3 Hz, J = 1.8 Hz); 13C NMR (125 MHz, CDCl3) δ −2.1, 9.5, 13.8, 27.10, 27.14, 39.0, 39.1, 43.2, 43.7, 49.4, 61.9, 122.7, 123.3, 125.4, 129.5, 142.4, 142.9, 164.2, 175.4, 175.6; IR (thin film) 2977 (m), 1761 (vs), 1117 (s) cm–1; HRMS (ESI-TOF) m/z: [M+NH4]+ calcd for C26H45N2O8SiS+, 573.2666; found, 573.2675; [α]D20 = +26.9° (c 1.0, CH2Cl2) on 98% ee material. (The optical purity was assumed to be unchanged from that of 55).

Reductive Ring Opening of 57

The general procedure for the reductive ring opening described above was followed, starting with aziridine 57 (228 mg, 0.494 mmol), 2.5 equiv of n class="Chemical">SmI2, and 5 equiv of DMEA. The 1H NMR spectrum of the crude reaction mixture indicated a complex mixture of several products due to partial cleavage of the acetate group on the benzene ring. To this mixture were added Ac2O (0.2 mL) and Et3N (0.16 mL). After the reaction was stirred at room temperature for 30 min, it was quenched by the addition of EtOH (0.1 mL) and H2O (2.5 mL). The resulting mixture was extracted with EtOAc (3 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated to give a yellow oil. Purification by silica-gel chromatography (20 mm × 160 mm, 1:1.5 EtOAc/hexanes as eluent) gave 61A as a colorless oil in 76% isolated yield (173 mg, 0.373 mmol) and 61B as a light-yellow oil in 8% isolated yield (17.5 mg, 0.0377 mmol). Spectral data for 61A: R = 0.23 (1:1.5 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 1.11 (t, 3H, J = 7.1 Hz), 2.22 (s, 6H), 2.33 (s, 3H), 2.68 (dd, 1H, J = 16.2, 6.2 Hz), 2.75 (dd, 1H, J = 16.2, 6.4 Hz), 3.99 (q, 2H, J = 7.1 Hz), 4.70 (dt, 1H, J = 7.5, 6.2 Hz), 5.97 (d, 1H, J = 7.8 Hz), 6.90–6.98 (m, 3H), 7.14 (d, 2H, J = 8.0 Hz), 7.54 (d, 2H, J = 8.0 Hz); 13C NMR (125 MHz, CDCl3) δ 13.9, 20.5, 20.6, 21.4, 41.0, 53.6, 61.0, 121.8, 123.3, 124.5, 126.9, 129.5, 137.1, 138.0, 141.4, 141.8, 143.4, 167.8, 167.9, 170.4; IR (thin film) 3279 (m), 2984 (w), 2930 (w), 1773 (s), 1734 (s), 1211 (s), 1161 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C22H26NO8S+, 464.1379; found, 464.1381; [α]D20 = +45.2° (c 1.0, CH2Cl2) on >98% ee material. (The optical purity was assumed to be unchanged from that of 54.) Spectral data for 61B: R = 0.38 (1:1.5 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 1.13 (t, 3H, J = 7.1 Hz), 2.26 (s, 6H), 2.42 (s, 3H), 3.93 (s, 2H), 3.99 (q, 2H, J = 7.1 Hz), 4.46 (s, 2H), 7.09 (s, 1H), 7.12 (s, 2H), 7.30 (d, 2H, J = 8.2 Hz), 7.73 (d, 2H, J = 8.2 Hz); 13C NMR (125 MHz, CDCl3) δ 14.0, 20.6, 20.7, 21.6, 46.8, 50.5, 61.2, 123.5, 123.6, 126.5, 127.4, 129.6, 134.0, 136.7, 141.9, 142.2, 143.6, 168.1, 168.2, 168.6; IR (thin film) 2984 (w), 2934 (w), 1773 (vs), 1213 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C22H26NO8S+, 464.1379; found, 464.1381.

Reductive Ring Opening of 58

The general procedure for the reductive ring opening described above was followed, starting with aziridine 58 (236 mg, 0.500 mmol), 4 equiv of n class="Chemical">SmI2, and 8 equiv of DMEA. The 1H NMR spectrum of the crude reaction mixture indicated a complex mixture of several products due to partial cleavage of the acetate group on the benzene ring. To this mixture were added Ac2O (0.63 mL) and Et3N (0.5 mL). After the reaction was stirred at room temperature for 40 min, it was quenched by the addition of EtOH (0.32 mL) and H2O (9 mL). The resulting mixture was extracted with EtOAc (10 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated to give a yellow oil. Purification by silica-gel chromatography (20 mm × 160 mm, 1:2 EtOAc/hexanes as eluent) gave 62A as a colorless oil in 69% isolated yield (164 mg, 0.346 mmol) and 62B as a light-yellow oil in 19% isolated yield (45 mg, 0.095 mmol). Spectral data for 62A: R = 0.21 (1:2 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ −0.09 (s, 9H), 0.78–0.93 (m, 2H), 1.19 (t, 3H, J = 7.1 Hz), 2.26 (s, 6H), 2.59–2.78 (m, 2H), 2.80–2.89 (m, 2H), 4.06–4.13 (m, 2H), 4.87 (dt, 1H, J = 8.1, 6.4 Hz), 5.70 (d, 1H, J = 8.1 Hz), 7.16 (d, 1H, J = 8.7 Hz), 7.19–7.28 (m, 2H); 13C NMR (125 MHz, CDCl3) δ −2.2, 10.2, 14.0, 20.575, 20.584, 41.6, 50.1, 53.6, 61.2, 121.9, 123.7, 124.6, 139.1, 141.7, 142.2, 167.8, 168.0, 170.5; IR (thin film) 3283 (m), 2955 (m), 1773 (vs), 1734 (vs), 1211 (s), 1143 (s) cm–1; HRMS (ESI-TOF) m/z: [M+NH4]+ calcd for C20H35N2O8SiS+, 491.1883; found, 491.1894; [α]D20 = +27.0° (c 1.0, CH2Cl2) on >98.5% ee material (the optical purity was assumed to be unchanged from 54). Spectral data for 62B: R = 0.27 (1:2 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 0.04 (s, 9H), 1.08–1.16 (m, 2H), 1.24 (t, 3H, J = 7.1 Hz), 2.26 (s, 6H), 3.04–3.11 (m, 2H), 3.93 (s, 2H), 4.15 (q, 2H, J = 7.1 Hz), 4.52 (s, 2H), 7.12–7.21 (m, 3H); 13C NMR (125 MHz, CDCl3) δ −2.0, 10.2, 14.1, 20.59, 20.63, 46.8, 50.0, 50.9, 61.4, 123.2, 123.7, 126.3, 134.4, 141.9, 142.3, 168.1, 168.2, 169.5; IR (thin film) 2955 (w), 2930 (w), 1773 (s), 1742 (s), 1211 (s) cm–1; HRMS (ESI-TOF) m/z [M+NH4]+ calcd for C20H35N2O8SiS+, 491.1883; found, 491.1897.

Reductive Ring Opening of 59

The general procedure for the reductive ring opening described above was followed, starting with aziridine 59 (111 mg, 0.200 mmol, 98% ee) and 4.0 equiv of n class="Chemical">SmI2. The 1H NMR spectrum of the crude reaction mixture indicated a mixture of C–N cleavage product 63A and C–C cleavage product 63B was obtained with a ratio of 4.3:1. Purification by silica-gel chromatography (20 mm × 160 mm, 1:3 EtOAc/hexanes as eluent) gave 63A as a yellow oil in 74% isolated yield (81.8 mg, 0.147 mmol) and 63B as a light-yellow oil in 16% isolated yield (18.2 mg, 0.0326 mmol). Spectral data for 63A: R = 0.25 (1:3 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ −0.11 (s, 9H), 0.75–0.90 (m, 2H), 1.18 (t, 3H, J = 7.2 Hz), 1.29 (s, 9H), 1.30 (s, 9H), 2.55–2.75 (m, 2H), 2.82 (d, 2H, J = 6.5 Hz), 4.07 (q, 2H, J = 7.1 Hz), 4.84 (dt, 1H, J = 8.0, 6.5 Hz), 5.70 (d, 1H, J = 8.0 Hz), 7.09 (d, 1H, J = 8.4 Hz), 7.13 (d, 1H, J = 2.0 Hz), 7.19 (dd, 1H, J = 8.4, 2.0 Hz); 13C NMR (125 MHz, CDCl3) δ −2.2, 10.2, 14.0, 27.11, 27.13, 39.05, 39.06, 41.6, 50.0, 53.7, 61.1, 121.8, 123.7, 124.3, 138.6, 142.2, 142.6, 170.4, 175.4, 175.5; IR (thin film) 3283 (m), 2977 (s), 1763 (vs), 1736 (s), 1117 (s) cm–1; HRMS (ESI-TOF) m/z [M+NH4]+ calcd for C26H47N2O8SiS+, 575.2822; found, 575.2842; [α]D20 = +26.0° (c 1.0, CH2Cl2) on 98% ee material. (The optical purity was assumed to be unchanged from that of 55.) Spectral data for 63B: R = 0.39 (1:3 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 0.04 (s, 9H), 1.09–1.16 (m, 2H), 1.24 (t, 3H, J = 7.1 Hz), 1.31 (s, 18H), 3.04–3.13 (m, 2H), 3.92 (s, 2H), 4.15 (q, 2H, J = 7.1 Hz), 4.52 (s, 2H), 7.06–7.11 (m, 2H), 7.17 (dd, 1H, J = 8.4, 1.8 Hz); 13C NMR (125 MHz, CDCl3) δ −2.0, 10.2, 14.1, 27.18, 27.21, 39.10, 39.13, 46.7, 50.0, 50.9, 61.4, 123.4, 123.7, 126.1, 133.9, 142.4, 142.8, 169.5, 175.7, 175.8; IR (thin film) 2977 (s), 1761 (vs), 1507 (m), 1337 (m), 1256 (m), 1117 (vs) cm–1; HRMS (ESI-TOF) m/z [M+NH4]+ calcd for C26H47N2O8SiS+, 575.2822; found, 575.2834.

(R)-4-(1-((tert-Butoxycarbonyl)amino)-3-ethoxy-3-oxopropyl)-1,2-phenylene Bis(2,2-dimethylpropanoate) (64)

To a oven-dried 10 mL Schlenk flask equipped with a stir bar and filled with nitrogen were added 63A (38 mg, 0.068 mmol, 1.0 equiv), n class="Chemical">4-dimethylaminopyridine (0.9 mg, 0.007 mmol, 0.1 equiv), Et3N (19 μL, 14 mg, 0.14 mmol, 2.0 equiv), and CH2Cl2 (0.32 mL). The flask was sealed, and the reaction mixture was stirred for 2 h at room temperature and then stirred at 40 °C for 15 h. After the reaction mixture was cooled to room temperature, 0.5 mL of 1 N HCl was added to the flask, followed by the addition of EtOAc (5 mL). The organic layer was separated, washed with brine (0.3 mL × 2), dried with Na2SO4, and concentrated to give a yellow oil, which was then dissolved in THF (0.65 mL) in a 10 mL Schlenk flask filled with nitrogen. To this solution was dropwise added TBAF (0.31 mL, 1 M in THF, 0.31 mmol) at room temperature. After the reaction mixture was stirred at room temperature under nitrogen for 1.5 h, it was concentrated by rotary evaporation and then high vacuum (0.5 mmHg) to give a bright-yellow oil. This oil was dissolved in a mixture of THF (0.2 mL) and Et3N (0.2 mL). Then trimethylacetyl chloride (0.05 mL, 0.4 mmol) was added to the solution at room temperature. After the resulting reaction mixture was stirred for 15 min at room temperature under nitrogen, it was quenched by the addition of EtOH (0.1 mL) and H2O (1.5 mL). The resulting mixture was extracted with EtOAc (2 mL × 3). The combined organic layer was dried with Na2SO4, filtered, and concentrated to afford a yellow oil. Purification by silica-gel chromatography (20 mm × 120 mm, 1:3 EtOAc/hexanes as eluent) gave 64 as a light-yellow oil in 88% isolated yield (29.5 mg, 0.0598 mmol). Spectral data for 64: R = 0.27 (1:3 EtOAc/hexane); 1H NMR (500 MHz, CDCl3) δ 1.16 (t, 3H, J = 7.2 Hz), 1.30 (s, 9H), 1.31 (s, 9H), 1.39 (brs, 9H), 2.62–2.92 (m, 2H), 4.06 (q, 2H, J = 7.2 Hz), 5.06 (brs, 1H), 5.47 (d, 1H, J = 7.5 Hz), 7.03 (d, 1H, J = 1.6 Hz), 7.06 (d, 1H, J = 8.4 Hz), 7.14 (dd, 1H, J = 8.4 Hz, J = 1.6 Hz); 13C NMR (125 MHz, CDCl3) δ 14.0, 27.20, 27.22, 28.3, 39.10, 39.12, 40.7, 50.6, 60.9, 79.8, 121.4, 123.5, 124.0, 139.7, 141.8, 142.5, 154.9, 170.7, 175.6, 175.8; IR (thin film) 3387 (m), 2978 (s), 2936 (m), 2874 (w), 1761 (vs), 1739 (s), 1723 (s), 1713 (s), 1256 (s), 1119 (vs) cm–1; HRMS (ESI-TOF) m/z [M + H]+ calcd for C26H40NO8+, 494.2754; found, 494.2758; [α]D20 = +23.6° (c 1.0, CH2Cl2) on 98% ee material. (The optical purity was assumed to be unchanged from that of 55.)