Hydrodehalogenation of alkyl iodides with base-mediated hydrogenation and catalytic transfer hydrogenation: application to the asymmetric synthesis of N-protected α-methylamines.

We report a very mild synthesis of N-protected α-methylamines from the corresponding amino acids. Carboxyl groups of amino acids are reduced to iodomethyl groups via hydroxymethyl intermediates. Reductive deiodination to methyl groups is achieved by hydrogenation or catalytic transfer hydrogenation under alkaline conditions. Basic hydrodehalogenation is selective for the iodomethyl group over hydrogenolysis-labile protecting groups, such as benzyloxycarbonyl, benzyl ester, benzyl ether, and 9-fluorenyloxymethyl, thus allowing the conversion of virtually any protected amino acid into the corresponding N-protected α-methylamine.

In the field of peptidomimetic drug development, the goal is to incorporate nonproteinogenic amino acid surrogates into candidate compounds to introduce new ligand–receptor interactions, to reduce proteolytic susceptibility, and to increase bioavailability. α-Methylamines represent a class of amino acid replacements in which the carboxyl group of the amino acid is converted to a methyl group and which, therefore, can serve as C-terminal amino acid mimics. Indeed, our laboratory reported that replacing the CONHBn group of a C-terminal Gln-NHBn with a methyl group increased cellular potency approximately 50-fold in a set of STAT3 phosphorylation inhibitors[1] (Figure 1). In addition to peptidomimetics, α-methylamines are components of several important drug classes, such as adrenergic receptor agonists,[2−5] α-methylhistamine and analogues,[6,7] inhibitors of phenylethanolamine N-methyl transferase,[8] amphetamine analogues,[9] dopamine analogues,[10] potassium channel modulators,[11] and cathepsin K inhibitors,[12] to name but a few (Figure 1).

Figure 1

α-Methylamines utilized in pharmacological applications.

Complete reduction of the carboxyl group of α-amino acids to a methyl group is a common strategy for the stereospecific synthesis of α-methylamines. This is a multistep process, and typical strategies involve reduction of the carboxyl group to a hydroxymethyl group, followed by derivatization to moieties that are further reduced to the methyl group. The following examples illustrate varying approaches to the several steps in the synthesis. Starting with l-histidinol (in which the carboxyl group of histidine was reduced to the hydroxymethyl group), Friary et al. converted the hydroxyl group to the chloride, then reduced the chloromethyl group with catalytic transfer hydrogenolysis using ammonium formate in refluxing methanol.[7] Donner reduced free carboxyl groups of N-Boc-protected amino acids with borane to hydroxymethyl groups and converted these to the corresponding ethyl thioethers via tosylates.[13] Desulfurization with Raney nickel afforded the N-Boc-protected α-methylamine. This methodology was applied to a variety of amino acids, including bis-Boc histidine, Boc-Tyr(Me), and Boc-Ser(Bn). The side-chain benzyl protecting group of serine was removed by the Raney nickel. Khono et al. reported the synthesis of (R)-2-amino-1-(4-methoxyphenyl)propane starting with tyrosine in which the amino, side-chain hydroxyl, and carboxyl groups were protected with benzyloxycarbonyl, methyl, and ethyl groups, respectively (Cbz-Tyr(Me)-OEt).[4] The carboxyl ester was reduced to the hydroxymethyl group with NaBH4. After activation as a tosyl ester, the tosylate was reduced with Zn powder in the presence of NaI in refluxing aqueous dimethoxyethane. The Cbz group was stable to these conditions, and it was subsequently removed using standard hydrogenation. Quagliato et al. reduced the carboxyl group of free amino acids using LiBH4, followed by Boc protection of the amino group.[11] The hydroxymethyl group was transformed to an iodomethyl group with I2 and polymer supported triphenylphosphine. The iodomethyl group was reduced to the methyl group using L-Selectride. Some of these methods required elevated temperatures, and not all amino and side-chain protecting groups were stable to all of the reduction steps. We, therefore, sought a methodology that could be used to convert α-amino acids bearing standard protecting groups to N-protected and “side-chain”-protected α-methylamines.

Our laboratory reported a catalytic transfer hydrogenation (CTH) method using triethylsilane (TES) and 10% Pd/C to remove benzyl-type protecting groups and to reduce nitro groups, imines, azides, and both conjugated and nonconjugated multiple bonds under neutral conditions.[14] Though a wide variety of functional groups were reduced, TES/Pd–C-mediated reduction of organic halides was not explored. Herein, we report that CTH as well as conventional hydrogenation under alkaline conditions is an efficient and chemoselective method of hydrodehalogenation of alkyl iodides derived from the carboxyl groups of protected α-amino acids to produce chiral α-methylamines.

As illustrated with tert-butyloxycarbonyl-leucine (Scheme 1), protected amino acids were reduced to the corresponding β-amino alcohols by activation of the carboxyl group as a mixed carbonic anhydride (MCA) using iso-butyl chloroformate (IBCF), followed by in situ reduction with sodium borohydride.[15−17] The hydroxyl group was replaced by an iodide by treatment with iodine, triphenylphosphine, and imidazole to give the iodomethyl intermediate, 1c.[18]

Scheme 1

Using standard CTH conditions, addition of TES to a stirred suspension of 1c in MeOH/THF in the presence of 10% Pd–C under argon resulted in the immediate evolution of hydrogen gas, as is normally observed.[14] However, there was no evidence of deiodination, even after stirring for the extended reaction time of 24 h. Conventional hydrogenation in MeOH/THF in the presence of 10% Pd–C also failed to reduce the iodo intermediate. Interestingly, addition of 2 equiv of the tertiary amine, diisopropylethylamine (DIEA), to the TES Pd/C-mediated CTH reaction resulted in complete reduction of the iodide to the corresponding N-Boc-α-methylamine (1d). Conventional hydrogenation in MeOH/THF over 10% Pd–C in the presence of 1.5–2 equiv of DIEA also resulted in reductive deiodination to the α-methyl group. This suggested that the formation of HI poisons the Pd catalyst and that DIEA neutralizes the acid. Dehydrohalogenation of iodomethyl intermediates is complete in 4 h for both reduction methods as judged by reverse phase HPLC.

Reductive deiodination of alkyl iodides derived from a variety of protected amino acids was performed under both conditions, CTH with TES/Pd–C and conventional hydrogenation with H2/Pd–C, in the presence of DIEA (Table 1). These basic conditions are selective for deiodination and leave untouched benzyl-based amino or hydroxyl protecting groups, such as benzyloxycarbonyl (Z), benzyl ester (OBzl) and benzyl ether, and 9-fluorenyloxymethyl (Fmoc), which are cleaved by standard hydrogenation reactions. Thus, our methodology allows the preparation of α-methylamines containing a wide variety of protected functionalities, e.g., the side-chain amino group of lysine (entry 6), the side-chain carboxyl group glutamic acid (entry 7), and by inference aspartic acid, the hydroxyl groups of tyrosine and serine (entries 8 and 9, respectively), the indole of tryptophan (entry 10), and the imidazole of histidine (entry 11). In our studies, we have chosen to keep the amino α-protecting groups intact for stability on storage. At the appropriate time, these protecting groups can be removed using standard deprotection conditions.

Table 1

Yields of the Conversion of α-Iodomethylamines to α-Methylamines by Base-Mediated Dehydrohalogenation

Yields are calculated from weights of final products after chromatographic purification.

The N-protected α-methylamine was not separable from nonpolar, silane-based side products resulting from the TES in the CTH reaction.

Oba and colleagues introduced deuterium in leucine derivatives by free-radical-mediated Bu3SnD/AIBN reduction of methyl iodides.[19,20] In a similar vein, we found that the iodine atom could be readily replaced with a deuterium using TES-d in the catalytic transfer hydrogenation reaction, as illustrated for Fmoc-PheψCH2D (5e) in Scheme 2. Although methanol is a potential hydrogen source, no trace of the trihydrogen species was found, indicating that, during TES-mediated CTH, triethylsilane is the sole source of hydrogen.

Scheme 2

In the case of 4-iodophenylalanine, mass spectroscopic analysis of the hydrogenation reactions, using both H2 and TES, at various time points revealed that the aromatic side chain is deiodinated first. This is in keeping with dehalogenation of aromatic halides with hydrogenation, which is known to be very rapid.[21] Attempts to deiodinate the iodomethyl derivative of 4-nitrophenylalanine, using both procedures, resulted in mixtures of compounds, indicating that the conditions are not selective with respect to the reduction of nitro groups.

To test for epimerization of the α-carbon during these procedures, we converted both l- and d-Boc-Thr(Bn)-OH to the corresponding α-methylamines, 12d and 13d, respectively, using both H2 and TES conditions. As threonine has two chiral centers, it was reasoned that the potential diastereomers resulting from the synthesis would be readily detected. Chiral HPLC indicated the presence of single diastereomers. Neither 1H nor 13C NMR spectra indicated the presence of more than one stereoisomer. As further proof, the Boc group was removed from 12d and the α-methylamine was derivatized with Mosher’s reagent to give a product with three stereocenters (14, Scheme 3). Chiral HPLC of the unpurified product showed one peak, and 1H and 19F NMR showed only one set of resonances. Thus, the chirality of the α-carbon remains intact during all phases of the transformation. This is in keeping with the work of Quagliato et al., who reported that reduction of α-iodomethylamines derived from N-Boc-amino acids either with H2/Pearlman’s catalyst or with L-Selectride to form α-methylamines did not epimerize the α-carbon.[11]

Scheme 3

In summary, dehydrohalogenation of the alkyl amines derived from N-protected amino acids can be accomplished by conventional hydrogenolysis or CTH in the presence of a tertiary amine. Hydrogenation under alkaline conditions leaves intact side-chain protecting groups, thus allowing the conversion of virtually any protected amino acid into the corresponding α-methylamine. Removal of the amino protecting group would permit facile incorporation into peptidomimetic or other drug candidates.

Experimental Section

Preparation of N-Protected β-Amino Alcohols (General Procedure)[16,17]

The preparation of Boc-LeuψCH2OH, 1b, is given as an example. A solution of Boc-Leu-OH (4.34 g, 18.8 mmol) in dry THF (40 mL) was cooled to −10 °C under an argon atmosphere. N-Methylmorpholine (NMM) (2.5 mL, 22.6 mmol, 1.2 equiv) was added, followed by dropwise addition of isobutyl chloroformate (2.9 mL, 22.6 mmol, 1.2 equiv). After stirring at −10 °C for 10 min, the reaction was allowed to increase to room temperature over 60 min. The precipitated NMM hydrochloride was filtered off, and the filtrate cooled to −10 °C. NaBH4 (1.07 g, 28.2 mmol, 1.5 equiv) was added. After 15 min, the reaction was quenched with 2% KHSO4 (25 mL) until the effervescence ceased. Ethyl acetate (200 mL) was added, and the solution was washed with water (200 mL) and 5% NaHCO3 (100 mL), followed by brine (100 mL), before drying over MgSO4. Filtering and removal of the solvent under reduced pressure gave the protected amino alcohol of sufficient purity to proceed to the next reaction. Analysis was done by TLC and/or HPLC and NMR.

Alternate Procedure: Preparation of Boc-Leucinol, 1b

In this case, a procedure different from the above was utilized. Leucine methyl ester hydrochloride (1 equiv; 27.5 mmol) and Boc-anhydride (1.5 equiv; 41.3 mmol) were dissolved in CH2Cl2 (135 mL), and triethylamine (1.5 equiv, 41.3 mmol, 5.5 mL) was added. The mixture was stirred for 3 h at room temperature. The solution was washed with 2% KHSO4 (100 mL) and 5% NaHCO3 (100 mL), followed by brine (50 mL), before drying over MgSO4. Removal of the solvent under reduced pressure gave the Boc-leucine methyl ester as a clear oil. The Boc-leucine methyl ester and NaBH4 (4 equiv, 88.9 mmol) were dissolved in dry THF (110 mL) under an argon atmosphere, and methanol (24 equiv, 22 mL) was added dropwise. The vigorous effervescence was controlled by a cold water bath. After 3 h, the volume of the solution was reduced under reduced pressure to ca. 80 mL and quenched with water (100 mL) before extracting with ethyl acetate (2 × 100 mL). The extract was washed with brine (100 mL) and dried over MgSO4. Filtering and removal of the solvent gave the product as a clear oil.

Preparation of Protected Amino Iodides (General Procedure)[18]

The preparation of Boc-LeuψCH2I, 1c, is provided as an example. Iodine (7.99 g, 27.6 mmol, 2 equiv) was added to triphenyl phosphine (7.23 g, 27.6 mmol, 3 equiv) and imidazole (3.13 g, 46 mmol, 5 equiv) dissolved in CH2Cl2 (60 mL), and the mixture was stirred for 30 min. Boc-leucinol (2.00 g, 9.2 mmol, 1 equiv) dissolved in CH2CL2 (20 mL) was added dropwise. After 2 h, the white precipitate was filtered and the filtrate was washed with brine (2 × 100 mL) before drying over MgSO4. Removal of the solvent under reduced pressure gave a brown solid that was purified by silica gel chromatography using 10–20% ethyl acetate in hexane. Yield 2.0 g, 67%, starting from 9.2 mmol of 1b. Identical to that reported by Caputo et al.[22]1H NMR (300 MHz, CDCl3) δ 0.93–1.00 (m, 6H), 1.36 (t, J = 7.0 Hz, 2H), 1.46 (s, 9H), 1.57–1.72 (m, 1H), 3.26–3.33 (m, 1H), 3.36–3.52 (m, 2H), 4.5 (brs, 1H).

Z-LeuψCH2I, 2c

Yield 2.5 g, 88%, starting from 7.96 mmol of 2b. 1H NMR (300 MHz, CDCl3) δ 0.90–0.94 (m, 6H), 1.36 (m, 2H), 1.62 (m, 1H), 3.26 (m, 1H), 3.4–3.54 (m, 2H), 4.9 (d, J = 9.0 Hz, 1H), 5.1 (s, 2H), 7.3–7.38 (m, 5H). 13C NMR (75 MHz, CDCl3): δ 15.4, 22.5, 22.7, 24.7, 44.2, 48.5, 66.9, 128.1, 128.2, 128.6, 136.4, 155.6. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C14H21INO2 362.0617; Found 362.0662.

Boc-PheψCH2I, 3c

Yield 2.16 g, 65%, starting from 9.2 mmol of 3b. Identical to that reported by Quagliato et al.[11] and Caputo et al.[22]1H NMR (300 MHz, CDCl3) δ 1.45 (s, 9H), 2.8 (dd, J = 8.1 and 13.2 Hz, 1H), 2.93 (dd, J = 5.7 and 13.5 Hz, 1H), 3.18 (dd, J = 4.0 and 10.1 Hz, 1H), 3.42 (dd, J = 4.1 and 10.0 Hz, 1H), 3.62 (m, 1H), 4.7 (brs, 1H), 7.25–7.36 (m, 5H).

Z-PheψCH2I, 4c

Yield 1.2 g, 85%, starting from 4.2 mmol of 4b. Identical to that reported by Caputo et al.[22]1H NMR (300 MHz, CDCl3) δ 2.82 (dd, J = 8.0 and 13.6 Hz, 1H), 2.95 (dd, J = 6.0 and 13.9 Hz, 1H), 3.2 (dd, J = 4.0 and10.3 Hz, 1H), 3.43 (dd, J = 4.4 and 10.1 Hz, 1H), 3.71 (m, 1H), 4.95 (d, J = 7.7 Hz, 1H), 5.1 (s, 2H), 7.22–7.43 (m 10H).

Fmoc-PheψCH2I, 5c

Yield 2.3 g, 85%, starting from 5.2 mmol of 5b. Identical to that reported by Caputo et al.[22]1H NMR (300 MHz, CDCl3) δ 3.0–2.8 (m, 2H), 3.2 (dd, J = 3.6 and 10.0 Hz, 1H), 3.45 (dd, J = 4.2 and 10.2 Hz, 1H), 3.7 (m, 1H), 4.24 (t, J = 6.8 Hz, 1H), 4.42 (d, J = 6.7 Hz, 2H), 4.93 (d, J = 8.1 Hz, 1H), 7.26–7.36 (m, 7H), 7.4–7.46 (m, 2H), 7.56–7.62 (m, 2H), 7.8 (d, J = 7.5 Hz, 2H).

Z-Lys(Boc)ψCH2I, 6c

Yield 2.0 g, 76%, starting from 5.46 mmol of 6b. 1H NMR (300 MHz, CDCl3) δ 1.3–1.6 (m, 15H), 3.1 (m, 2H), 3.31 (m, 1H), 3.41 (m, 2H), 4.6 (brs, 1H), 5.0 (brs, 1H), 5.1 (s, 2H), 7.3–7.4 (m, 5H). 13C NMR (75 MHz, CDCl3): δ 14.1, 22.8, 28.5, 29.7, 34.6, 40.1, 50.4, 66.9, 79.1, 128.1, 128.2, 128.6, 136.3, 155.7, 156.1. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C19H30IN2O4 477.1250; Found 477.1288.

Fmoc-Glu(OBn)ψCH2I, 7c

Yield 1.3 g, 68%, starting from 3.5 mmol of 7b. 1H NMR (300 MHz, CDCl3) δ1.9 (m, 2H), 2.43 (m, 2H), 3.3 (m, 1H), 3.4 (m, 1H), 3.5 (m, 1H), 4.2 (t, J = 7.0 Hz, 1H), 4.41 (m, 2H), 4.93 (d, J = 9.0 Hz, 1H), 5.1 (s, 2H), 7.27–7.42 (m, 9H), 7.6 (d, J = 7.5 Hz, 2H), 7.75 (d, J = 8.0 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 13.5, 30.1, 30.6, 47.3, 50.3, 66.6, 66.8, 120.0, 125.0, 127.1, 127.7, 128.3, 128.4, 128.6, 135.7, 141.3, 143.8, 155.6, 172.8. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C27H27INO4 556.0985; Found 556.1021.

Z-Tyr(Bn)ψCH2I, 8c

Yield 2.3 g, 89%, starting from 5.2 mmol of 8b. 1H NMR (300 MHz, CDCl3) δ 2.68 (dd, J = 8.0 and 13.2 Hz, 1H), 2.8 (dd, J =5.6 and 13.2 Hz, 1H), 3.13 (dd, J = 3.34 and10.5 Hz,1H), 3.35 (dd, J = 3.5 and 9.4 Hz, 1H), 3.6 (m, 1H), 4.86 (d, J = 8.0 Hz, 1H), 5.00 (s, 2H), 5.05 (s, 2H), 6.8 (d, J = 8.1 Hz, 2H), 7.1 (d, J = 8.1 Hz, 2H), 7.25–7.6 (m, 10H). 13C NMR (75 MHz, CDCl3): δ 157.9, 155.4, 137.0, 136.3, 133.3, 133.2, 131.3, 131.2, 130.2, 129.6, 129.0, 128.8, 128.7, 128.6, 128.6, 128.2, 128.1, 128.0, 127.5, 115.1, 70.1, 66.9, 51.7, 39.7, 13.5. HRMS (ESI-TOF) m/z:[M + NH4]+ Calcd for C24H28IN2O3 519.1145; Found 519.1154.

Fmoc-Ser(Bn)ψCH2I, 9c

Yield 1.8 g, 72%, starting from 4.95 mmol of 9b. 1H NMR (300 MHz, CDCl3) δ 3.38 (m, 2H),3.5 (m, 1H), 3.7 (m, 1H), 3.8 (m, 1H), 4.2 (t, J = 7.4 Hz, 1H), 4.42 (m, 2H), 4.52 (s, 2H), 5.2 (d, J = 8.2 Hz, 1H), 7.27–7.42 (m, 9H), 7.6 (d, J = 7.5 Hz, 2H), 7.75 (d, J = 7.5 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 47.3, 51.1, 67.0, 70.4, 73.5, 120.1, 125.1, 127.1, 127.8, 127.8, 128.0, 128.2, 128.6, 137.6, 141.4, 143.9, 155.6. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C25H25INO3 514.0879; Found 514.0890.

Fmoc-Trp(Boc)ψCH2I, 10c

Yield 1.4 g, 57%, starting from 4.0 mmol of 10b. 1H NMR (600 MHz, CDCl3) δ 1.71 (s, 9H), 2.98 (m, 1H), 3.09 (m, 1H), 3.32(m, 1H),3.48 (m, 1H),3.9 (m, 1H), 4.27 (t, J = 7.0 Hz,1H), 4.41–4.49 (m, 2H), 5.1 (d, J = 7.75 Hz, 1H),7.3–7.46 (m, 7H), 7.56–7.63 (m, 3H), 7.7 (d, J = 8.0 Hz, 1H), 7.8 (d, J = 7.6 Hz, 2H), 8.18 (s, 1H). 13C NMR (150 MHz, CDCl3): δ 155.6, 149.6, 143.8, 141.3, 135.5, 130.2, 127.8, 127.1, 125.1, 124.7, 124.2, 122.8, 120.1, 119.1, 115.7, 115.4, 83.8, 67.0, 50.3, 47.2, 30.5, 28.2, 13.4. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C31H32IN2O4 623.1407; Found 623.1409.

Fmoc-His(Boc)ψCH2I, 11c

Yield 1.6 g, 63%, starting from 4.4 mmol of 11b. 1H NMR (500 MHz, CDCl3) δ 1.63 (s, 9H), 2.90 (m, 1H), 3.04 (m, 1H), 3.26 (m, 1H), 3.39 (m. 1H), 3.97 (m, 1H), 4.26 (m, 1H), 4.38 (m, 2H), 6.06 (d, J = 8.1 Hz, 1H), 7.29 (s, 1H), 7.32–7.35 (m, 2H), 7.41–7.43 (m, 2H), 7.61–7.64 (m, 2H), 7.78 (d, J = 7.5.Hz, 2H), 8.08 (s, 1H). 13C NMR (125 MHz, CDCl3) δ: 155.6, 146.8, 143.9, 141.3, 138.6, 137.0, 127.7, 127.1, 125.2, 120.0, 115.1, 85.9, 66.9, 50.9, 47.2, 31.6, 27.9, 18.9, 10.9. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C26H29IN3O4 574.1203; Found 574.1214.

Boc-l-Thr(OBzl)ψCH2I, 12c

Yield 1.0 g, 73%, starting from 3.4 mmol of Boc-l-Thr(OBzl)-CH2OH. 1H NMR (600 MHz, CDCl3) δ 1.22 (d, J = 6.3 Hz, 3H), 1.44 (s, 9H), 3.24–3.3 (m, 2H), 3.72 (m, 1H), 4.02 (m, 1H), 4.41 (d, J = 11.3 Hz, 1H), 4.63 (d, J = 11.3 Hz, 1H), 5.00(d, J = 9.0 Hz, 1H), 7.27–7.38 (m, 5H). 13C NMR (75 MHz, CDCl3): δ 6.9, 16.7, 28.4, 56.3, 71.3, 73.5, 79.7, 127.9, 128.0, 128.5, 138.0, 155.5. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C16H25INO3 406.0879; Found 406.0872.

Boc-d-Thr(OBzl)ψCH2I, 13c

Yield 1.04 g, 76%, starting from 3.4 mmol of Boc-d-Thr(OBzl)-CH2OH. 1H NMR (600 MHz, CDCl3) δ 1.22 (d, J = 6.3 Hz, 3H), 1.44 (s, 9H), 3.24–3.3 (m, 2H), 3.72 (m, 1H), 4.02 (m, 1H), 4.40 (d, J = 11.3 Hz, 1H), 4.63 (d, J = 11.3 Hz, 1H), 5.00(d, J = 9.0 Hz, 1H), 7.27–7.38 (m, 5H). 13C NMR (75 MHz, CDCl3): δ 6.9, 16.7, 28.4, 56.3, 71.3, 73.5, 79.7, 127.9, 128.0, 128.5, 138.0, 155.5. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C16H25INO3 406.0879; Found 406.0882.

Dehalogenation of N-Protected β-Amino Iodides (General Procedure)

The conversion of Boc-LeuψCH2I, 1c, to Boc-LeuψCH3, 1d, is given as an example.

Method A, Conventional Hydrogenation

Boc-LeuψCH2I, 1c (2.00 g, 6.11 mmol, 1 equiv), and DIEA (1.60 mL, 9.17 mmol, 1.5 equiv) were dissolved in a mixture of THF (27 mL) and methanol (55 mL), and 10% palladium on charcoal (0.200 g) was added. The mixture was hydrogenated at 49 psi and room temperature for 24 h in a Parr apparatus. The solvent was removed, and the residue was dissolved in 100 mL of EtOAc and filtered through Celite. The filtrate was concentrated under vacuum, and the crude product was purified by silica gel chromatography eluting with 10–20% ethyl acetate in hexane.

Method B, Catalytic Transfer Hydrogenation with TES

Boc-LeuψCH2I, 1c (0.700 g, 2.14 mmol1 equiv, 2.14 mmol), triethylsilane (3.4 mL, 21.4 mmol; 10 equiv), and DIEA (0.56 mL, 3.21 mmol, 1.5 equiv) were dissolved in a mixture of THF (24 mL) and methanol (47 mL), and 10% palladium on charcoal (70 mg) was added into it. The mixture was stirred at room temperature for 24 h.

The solvent was removed under vacuum, and the residue was dissolved in 50 mL of EtOAc and filtered through Celite. Solvent was removed, and the crude was then purified by silica gel chromatography as above.

Boc-LeuψCH3, 1d

Yield: Method A: 1.1 g, 88%, starting from 6.11 mmol of 1c; Method B: 0.391 g, 91%, starting from 2.14 mmol of 1c. 1H NMR (300 MHz, CDCl3) δ 4.26 (bs, 1H), 3.70 (bs, 1H), 1.68 (m, 1H), 1.43 (s, 9H), 1.36 (m, 2H), 1.10 (d, J = 7.0 Hz, 2H), 0.92 (m, 6H). 13C NMR (125 MHz, CDCl3): δ 155.3, 78.9, 77.3, 77.0, 76.8, 46.7, 44.7, 28.6, 28.4, 25.0, 22.7, 22.6, 21.8. HRMS (ESI-TOF) m/z: [M + Na]+ Calcd for C11H23NNaO2 224.1626; Found 224.1616.

Z-NHLeuψCH3, 2d

Yield: Method A: 1.2 g, 64%, starting from 8.03 mmol of 2c. 1H NMR (300 MHz, CDCl3) δ 7.37–7.29 (m, 5H), 5.13–5.04 (m, 2H), 4.51 (d, J = 8.0 Hz, 1H), 3.82–3.77 m, 1H), 1.69–1.60 (m, 1H), 1.39–1.16 (m, 2H), 1.14 (d, J = 7.0 Hz, 3H), 0.93 (m, 6H). 13C NMR (125 MHz, CDCl3): δ 155.8, 136.7, 128.6, 128.2, 128.2, 128.1, 128.1, 66.5, 46.6, 45.3, 25.0, 22.8, 22.5, 21.8. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C14H22NO2 236.1651; Found 236.1611.

Boc-PheψCH3, 3d(11)

Yield: Method A: 1.23 g, 91%, starting from 5.73 mmol of 3c. 1H NMR (300 MHz, CDCl3) δ 7.24–7.17 (m, 5H), 4.36 (bd, 1H), 3.91 (m, 1H), 2.87 (m, 2H), 1.45 (s, 9H), 1.14 (d, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ 155.2, 138.3, 129.7, 129.5, 129.3, 128.6, 128.5, 128.3, 126.5, 126.3, 49.1, 47.4, 43.0, 28.4, 20.2, 19.1. HRMS (ESI-TOF) m/z: [M + Na]+ Calcd for C14H21NNaO2 258.1470; Found 258.1471.

Z-PheψCH3, 4d

Yield: Method A: 0.28 g, 81%, starting from 1.27 mmol of 4c; Method B: 0.310 g, 76%, starting from 1.52 mmol of 4c. 1H NMR (300 MHz, CDCl3) δ 7.40–7.15 (m, 10H), 5.09 (s, 2H), 4.60 (m, 1H), 4.00 (m, 1H), 2.8 (m, 2H), 1.14(d, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ 155.6, 137.9, 136.6, 129.5, 128.6, −128.4, 128.1, 126.5, 66.51, 48.0, 42.8, 20.2. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C17H20NO2 270.1494; Found 270.1536.

Fmoc-PheψCH3, 5d

Yield: Method A: 0.340 g, 89%, starting from 1.03 mmol of 5c; Method B: 1.105 g, 82%, starting from 3.66 mmol of 5c. 1H NMR (300 MHz, CDCl3) δ 7.80 (d, J = 7.5 Hz, 2H), 7.58 (m, 2H), 7.43–7.15 (m, 9H), 4.59 (bd, 1H), 4.48–4.18 (m, 4 H), 2.83 (m, 2H), 1.14 (d, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ = 155.6, 144.0, 141.4, 137.9, 129.6, 128.4, 127.7, 127.1, 126.5, 125.1, 125.1, 120.0, 66.4, 48.0, 47.3, 42.7, 20.2. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C24H24NO2 358.1807; Found 358.1829.

Z-Lys(Boc)ψCH3, 6d

Yield: Method A: 0.795 g, 83%, starting from 2.85 mmol of 6c; Method B: 0.24 g, 65%, starting from 1.06 mmol of 6c. 1H NMR (300 MHz, CDCl3) δ 7.36–7.30 (m, 5H), 5.08 (s, 2H), 4.57 (m, 2H), 3.73 (m, 1H), 3.11 (m, 2H), 1,27–1.54 (m, 15H), 1.14 (d, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ 156.1, 155.9, 136.6, 128.5, 128.2, 128.1, 66.5, 47.0, 40.3, 36.7, 29.8, 23.1, 21.3. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C19H31N2O4 351.2284; Found 351.2285.

Fmoc-Glu(OBn)ψCH3, 7d

Yield: Method A: 0.28 g, 90%, starting from 0.72 mmol of 7c; Method B: 0.383 g, 81%, starting from 1.1 mmol of 7c. 1H NMR (300 MHz, CDCl3) δ 7.77 (d, J = 7.5 Hz, 2H), 7.59 (d, 2H, J = 7.5 Hz), 7.43–7.30 (m, 9H), 5.10 (s, 2H), 4.62 (d, J = 8.7 Hz, 2H), 4.40 (d, J = 7.0 Hz, 2H), 4.00 (t, J = 7.0 Hz, 1H), 3.77 (m, 1H), 2.43 (t, J = 7.5 Hz, 2H), 1.60–1.93 (m, 2H), 1.18 (d, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ 173.4, 155.9, 144.0, 143.9, 141.3, 135.9, 128.6, 128.3, 127.70, 127.52, 127.46, 127.07, 125.1, 120.22, 120.00, 66.4, 66.4, 60.46, 47.3, 46.9, 31.9, 31.1, 21.4. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C27H28NO4 430.2018; Found 430.2011.

Z-Tyr(Bn)ψCH3, 8d

Yield: Method A: 0.295 g, 78%; Method B: 0.310 g, 83%, starting from 1.0 mmol of 8c. 1H NMR (600 MHz, CDCl3) δ 1.14 (d, J = 7.0 Hz, 3H), 2.7 (m, 1H), 2.8 (m, 1H), 3.98 (m, 1H), 4.62 (m, 1H), 5.07 (s, 2H), 5.11 (s, 2H), 6.93 (d, J = 8.5 Hz, 1H), 7.10 (d, J = 8.0 Hz, 1H), 7.33–7.42 (m, 6H), 7.45–7.48 (m, 3H), 7.52–7.55 (m, 1H), 7.58–7.63 (m, 2H). 13C NMR (150 MHz, CDCl3): δ 157.5, 137.1, 133.3, 133.2, 131.3, 130.5, 130.2, 129.4, 129.1, 128.8, 128.7, 128.6, 128.5, 128.1, 127.9, 127.5, 114.8, 70.0, 66.5, 48.1, 41.9, 20.2. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C24H26NO3 376.1913; Found 376.1942.

Fmoc-Ser(OBn)ψCH3, 9d

Yield: Method A: 0.345 g, 92%; Method B: 0.330 g, 88%, starting from 0.97 mmol of 9c. 1H NMR (300 MHz, CDCl3) δ 1.21 (d, J = 6.0 Hz, 3H), 3.45 (m, 2H), 3.92 (m, 1H), 4.2 (t, J = 7.0 Hz, 1H), 4.38 (d, J = 7.0 Hz, 2H), 4.52 (m, 2H), 4.98 (brs, 1H),7.25–7.43 (m, 9H), 7.6 (d, J = 7.5 Hz, 2H), 7.75 (d, J = 7.5 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 18.1, 47.3, 66.6, 73.1, 73.2, 120.0, 125.1, 127.0, 127.6, 127.7, 127.8, 128.5, 138.1, 141.3, 144.0, 155.9. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C25H26NO3 388.1913; Found 388.1924.

Fmoc-Trp(Boc)ψCH3, 10d

Yield: Method A: 0.124 g, 77%; Method B: 0.135 g, 84%, starting from 0.32 mmol of 10c. 1H NMR (600 MHz, CDCl3) δ 1.22 (m, 2H), 1.69 (s, 9H), 2.82 (m, 1H), 3.03 (m, 1H), 4.15 (m, 1H), 4.24 (m, 1H), 4.4–4.48 (m, 2H), 4.87 (m, 1H), 7.28 (m, 1H), 7.30–7.37 (m, 3H), 7.40–7.48 (m, 3H),7.57–7.61 (m, 2H), 7.66 (m, 1H), 7.79 (m, 2H), 8.16 (s, 1H). 13C NMR (150 MHz, CDCl3): δ 155.8, 149.7, 144.0, 141.3, 130.9, 127.7, 127.1, 125.1, 124.4, 123.9, 122.6, 120.0, 119.3, 116.9, 115.3, 83.6, 66.6, 47.3, 47.0, 32.2, 28.2, 20.5. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C31H33N2O4 497.2440; Found 497.2456.

Fmoc-His(Boc)ψCH3, 11d

Yield: Method A: 0.200 g, 85%; Method B: 0.190 g, 81%, starting from 0.53 mmol of 11c. 1H NMR (600 MHz, CDCl3) δ 1.22 (d, J = 6.0 Hz, 3H), 1.62 (s, 9H), 2.75 (m, 1H), 2.82 (m, 1H), 4.08 (m, 1H), 4.25 (m, 1H), 4.34 (m, 1H), 4.40 (m, 1H), 5.63 (d, J = 6.5 Hz, 1H), 7.20 (s, 1H), 7.31–7.34 (m, 2H), 7.39–7.43 (m, 2H), 7.60–7.63 (m, 2H), 7.78 (d, J = 7.5 Hz, 2H), 8.06 (s, 1H). 13C NMR (150 MHz, CDCl3): δ 155.8, 147.0, 144.1, 141.3, 136.7, 127.6, 127.0, 125.2, 120.0, 114.5, 85.5, 66.6, 47.3, 46.8, 34.2, 27.9, 20.5. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C26H30N3O4 448.2236; Found 448.2257.

Fmoc-PheψCH2D, 5e

To a stirred suspension of Fmoc-phenylalanino iodide (100 mg, 0.21 mmol), DIPEA (0.1 mL, 0.63 mmol), dissolved in a mixture of 10 mL of THF–MeOH (9:1), and 10% Pd–C (10 mg), was added Et3SiD (10 equiv; 0.3 mL). It was stirred at room temperature for 12 h (HPLC analysis showed quantitative conversion), and the mixture was then centrifuged. The supernatant was then concentrated and purified by RPHPLC. The pure fractions were then collected and lyophilized to get a white solid (48 mg, Yield: 65%). 1H NMR (600 MHz, CDCl3) δ 1.14 (s, 2H), 2.76 (m, 1H), 2.86 (m, 1H), 4.03 (m, 1H), 4.24 (t, J = 6.5 Hz, 1H), 4.37 (m, 1H), 4.48 (m, 1H), 4.65 (m,1H), 7.20 (m, 2H), 7.26 (m, 1H), 7.31–7.36 (m, 4H), 7.44 (m, 2H), 7.60 (m, 2H), 7.80 (d, J = 7.5 Hz, 2H). 13C NMR (150 MHz, CDCl3): δ 155.6, 144.0, 141.4, 137.9, 129.5, 128.4, 127.7, 127.1, 126.5, 125.1, 125.0, 120.0, 66.4, 47.9, 47.4, 42.7. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C24H23DNO2 359.1870; Found 359.1886.

Boc-l-Thr(OBzl)ψCH3, 12d

Yield: Method A: 0.30 g, 88%; Method B: 0.28 g, 81%, starting from 1.23 mmol of Boc-l-Thr(OBzl)-CH2I (12c). 1H NMR (600 MHz, CDCl3): δ 1.16–1.18 (m, 6H). 1.44 (s, 9H), 3.5 (m, 1H), 3.72 (m, 1H), 4.42 (d, J = 11.6 Hz, 1H), 4.6 (d, J = 11.6 Hz, 1H), 4.73 (m, 1H), 7.25–7.36 (m, 5H). 13C NMR (75 MHz, CDCl3): δ 16.1, 18.1, 28.4, 50.3, 71.0, 78.9, 127.6, 127.7, 128.3, 138.6, 155.8. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C16H26NO3 280.1913; Found 280.1914. Chiral HPLC R: 22.06 min. (Chiralpak IC Column; mobile phase: water/MeCN, gradient 55–75% MeCN/50 min, 0.5 mL/min, monitored at 254 nm).

Boc-d-Thr(OBzl)ψCH3, 13d

Yield: Method A: 0.29 g, 84%; Method B: 0.276 g, 80%, starting from 1.23 mmol of Boc-d-Thr(OBzl)-CH2I (13c). 1H NMR (600 MHz, CDCl3) δ1.16–1.18 (m, 6H), 1.44 (s, 9H), 3.5 (m, 1H), 3.72 (m, 1H), 4.42 (d, J = 11.6 Hz, 1H), 4.61 (d, J = 11.6 Hz, 1H), 4.74 (m, 1H), 7.26–7.36 (m, 5H). 13C NMR (75 MHz, CDCl3): δ 16.1, 18.1, 28.4, 50.2, 71.0, 78.9, 127.6, 127.7, 128.3, 138.6, 155.8. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C16H26NO3 280.1913; Found 280.1917. Chiral HPLC R: 22.86 min. (Chiralpak IC Column; mobile phase: water/MeCN, gradient 55–75% MeCN/50 min, 0.5 mL/min, monitored at 254 nm).

Preparation of Mosher’s Adduct, 14

Boc deprotection of 12d (100 mg) was done by treating with 0.5 mL of trifluoroacetic acid (TFA) for 1 h. Excess TFA was removed under vacuum and stripped off residual amount with CCl4. The crude TFA salt (50 mg, 0.28 mmol) was then dissolved in 1.0 mL of dry CH2Cl2. To this solution, cooled in ice, 120 μL of triethylamine (0.84 mmol) followed by 52 μL, (0.28 mmol) of (S)-(+)-α-methoxy-α-trifluoromethylphenylacetyl chloride (Mosher’s acid chloride), was added. After completion, as monitored by reverse phase HPLC, 100 μL of water was added and the mixtures were passed through a short silica gel column that was washed with an additional 10 mL of CH2Cl2. The organic eluate was concentrated under vacuum, yielding a colorless oil. (82 mg, 75%). 1H NMR (500 MHz, CDCl3) δ 1.18 (d, J = 6.7 Hz, 3H), 1.2 (d, J = 6.3 Hz, 3H), 3.4 (s, 3H), 3.56 (m, 1H), 4.1 (m, 1H), 4.43 (d, J = 11.8 Hz, 1H), 4.65 (d, J = 11.8 Hz, 1H), 7.0 (d, J = 8.5 Hz, 1H), 7.26–7.41 (m, 8H), 7.50–7.55 (m, 2H). 19F NMR (470 MHz, CDCl3) δ −68.75 (s, 3F). HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C21H25F3NO3 396.1787; Found 396.1790. Chiral HPLC R: 26.64 min. (Chiralpak IC Column; mobile phase: water/MeCN, gradient 50–80% MeCN/30 min, 0.5 mL/min, monitored at 254 nm).