Preparation of Phosphonic Acid Analogues of Proline and Proline Analogues and Their Biological Evaluation as δ1-Pyrroline-5-carboxylate Reductase Inhibitors.

Racemic 1-hydroxy-3-butenyl-, 3-chloro-1-hydroxypropyl-, and 3-bromo-1-hydroxypropylphosphonate and the corresponding (S)-enantiomers obtained by lipase-catalyzed resolution of the respective racemic chloroacetates were subjected to functional group manipulations. These comprised ozonolysis, Mitsunobu reactions with hydrazoic acid and N-hydroxyphthalimide, alkylation of hydrazine derivative, removal of phthaloyl group followed by intramolecular substitution, and global deprotection to deliver the racemates and (R)-enantiomers (ee 92-99% by chiral high-performance liquid chromatography) of pyrrolidin-2-yl-, oxazolidin-3-yl-, oxazolidin-5-yl-, pyrazolidin-3-yl-, and 1,2-oxazinan-3-ylphosphonic acids. These phosphonic acids were evaluated as analogues of proline and proline analogues for the ability to inhibit γ-glutamyl kinase, δ1-pyrroline-5-carboxylate synthetase, and δ1-pyrroline-5-carboxylate reductase. Only the latter enzyme was inhibited by two of them at concentrations exceeding 1 mM.

Introduction

Herbicides with favorable properties such as high activity, crop tolerance, and low toxicity to insects and mammals are essential for weed control in modern agriculture to secure food supply for the growing world population.[1] Furthermore, persistence of these agrochemicals should be minimal and biodegradation by the soil microflora should be complete in a short time span. Rapid development of resistant weeds in combination with diffusion of the acquired resistance in the biosphere is a public concern and has forced companies to search for new herbicide targets and active ingredients. Amino acid metabolism is an attractive target for herbicide development.[2] However, little attention has so far been paid to interference with proline biosynthesis. It is accessed by two routes, the ornithine and glutamate pathways, the latter being the main one in plants. Both the pathways share the last reaction step catalyzed by the NAD(P)H-dependent δ1-pyrroline-5-carboxylate (P5C) reductase.[3] Luckily, this fact allows to block both ways with only one inhibitor.

Forlani and Kafarski et al. found that N-phenyl-substituted aminomethylenebisphosphonic acids are inhibitors of P5C reductase with activity in the micromolar to millimolar range.[4−6] These compounds also demonstrated phytotoxicity in vivo, which could be reversed by exogenously supplied amino acids.[7] We reasoned that phosphonic acid analogues of l-proline and proline analogues could be inhibitors of the reductase and other enzymes in the proline metabolism. To test this idea, a series of racemic, chiral nonracemic, and achiral phosphonic acids were synthesized for evaluation first with P5C reductase of plants (Figure ). The selection of compounds comprised five- (1–4) and six-membered (5 and 6) ring systems with one or two heteroatoms, all with an attached phosphonic acid group. The (R)-configured enantiomers of 1–6 correspond to the (S)- or l-configured ones in the carboxyl acid series because of the higher priority of PO3H2 compared to CO2H according to the Cahn–Ingold–Prelog rules. Many azaheterocyclic phosphonates have been synthesized and biologically evaluated in the past.[8]

Figure 1

Phosphonic acid analogues of proline and proline analogues.

Results and Discussion

Synthesis of (±)-, (R)-, and (S)-Phosphaproline

Although racemic, (R)-, and (S)-phosphaproline [(±)-, (R)-, and (S)-1] have been described in the literature,[9−17] we present here a new access from recently prepared racemic and enantiomeric 1-hydroxy-3-butenylphosphonate 7 (Scheme ).[18] It also served as a starting material for phosphonic acids racemic 2 and 6. Hydroxyphosphonate (±)-7 was resolved by lipase-catalyzed kinetic hydrolysis of the chloroacetic ester and delivered the enantiomers with ee >97%.[18] The enantiomer (S)-7 was transformed into (R)-1, (R)-2, and (R)-6 and the enantiomer (R)-7 into phosphaproline (S)-1 by functional group manipulation. Furthermore, (±)- and (S)-7 were already converted to phosphaaspartic acids via azides (±)- and (S)-8 prepared by Mitsunobu reaction.[18] Evans and Weber[19] and later others[20,21] described the formation of pyrrolidines from homoallyl azides upon hydroboration with dicyclohexylborane. As we hoped that azides 8 prepared by the Mitsunobu reaction[18,22] would undergo the same cylization, we reacted them with dicyclohexylborane.[23] Trialkylboranes 9 were unstable and underwent migration of an alkyl group from the boron to nitrogen atom, with loss of nitrogen after attack of the azido group on the boron atom.[19] The phosphaproline derivatives 10 were globally deprotected by refluxing 6 M HCl. Phosphaprolines 1 were isolated by cation-exchange chromatography (Dowex 50W × 8, H+) and crystallized. The enantiomers of 8 delivered the enantiomers of 1 of known configuration, correctly assigned[17] recently.

Scheme 1

Preparation of (±)-, (R)-, and (S)-Proline [(±)-, (R)-, and (S)-1]

The ee of (R)- and (S)-1 was the same (>99%) as that of the starting α-hydroxyphosphonates 7 (>99%), which was proven by chiral high-performance liquid chromatography (HPLC) (Figure S1).

Synthesis of (±)- and (R)-Isoxazolidin-3-ylphosphonic Acid

These phosphonic acid analogues of structural analogues of proline were obtained by functional group manipulation of (±)- and (S)-11[18] (Scheme ). Ozonolysis gave both hydroxyphosphonates 12 in 91% yield. The following Mitsunobu reaction[22] with N-hydroxyphthalimide delivered protected O-alkylhydroxylamines (±)- and (S)-13 in yields of 83 and 84%, respectively. Hydrazinolysis in iPrOH effected removal of the phthaloyl group. The open-chain O-alkylhydroxylamines (±)- and (S)-14 with a leaving group at C-1 immediately cyclized to isoxazolidin-3-ylphosphonates 15. As the 4-nitrobenzenesulfonyloxy (nosyloxy) group was replaced with inversion of configuration, the (S)-enantiomer of 13 was transformed into (R)-15. The crude isoxazolidin-3-ylphosphonates 15 were deprotected with HBr in AcOH at room temperature in 16 h.[24] The phosphonic acids were purified by cation-exchange chromatography (Dowex 50W × 8, H+) and crystallization. The overall yields of (±)- and (R)-2[25] starting from 13 were 79 and 74% (ee 99%, Figure S2), respectively. Surprisingly, deprotection of 15 with bromotrimethylsilane (TMSBr)/allyltrimethylsilane (allylTMS)[26] had a detrimental effect on the yield of (R)-2 (15%).

Scheme 2

Preparation of (±)- and (R)-Isoxazolidin-3-ylphosphonic Acid [(±)- and (R)-2]

Nos = 4-nitrobenzenesulfonyl; DIAD = diisopropyl azodicarboxylate, PhthNOH = N-hydroxyphthalimide.

Synthesis of Phosphapipecolic Acid (±)-5 and (±)- and (R)-1,2-Oxazinan-3-ylphosphonic Acid [(±)- and (R)-6]

Phosphonic acids 5 and 6 differ from phosphaproline by replacing the five-membered ring by a six-membered ring containing an additional CH2 group or an oxygen atom. The preparation of racemic phosphapipecolic acid was recently published.[14] The synthesis of analogues 6 was accomplished starting from 4-hydroxybutylphosphonates 16, derived from nosylates (±)- and (S)-11 (ee 85%) by hydroboration with H3B × tetrahydrofuran (THF), followed by oxidative cleavage of the B–C bond with H2O2/NaHCO3 (Scheme ).[18] The Mitsunobu reaction and the ensuing cyclization induced by hydrazine hydrate were similarly performed for the transformation of 12 into 15 in Scheme and delivered 1,2-oxazinanes (±)- and (R)-18, cyclic oxime ethers.[27] However, this time the hydroxylamine derivatives, the two 1,2-oxazinan-3-ylphosphonates (±)- and (R)-18, were first isolated as homogeneous compounds by flash column chromatography, fully characterized and finally deprotected. Although the five-membered analogue 15 suffered partial decomposition on attempted deprotection with refluxing 6 M HCl, 1,2-oxazinan-3-ylphosphonates (±)- and (R)-18 were smoothly deprotected, as evidenced by their high yields of 94 and 91%, respectively. The ee of 83% for phosphonic acid (R)-6 was increased to 92% upon crystallization from H2O/EtOH (Figure S5).

Scheme 3

Preparation of (±)- and (R)-(−)-1,2-Oxazinan-3-ylphosphonic Acid [(±)- and (R)-6]

Nos = 4-nitrobenzenesulfonyl; DIAD = diisopropyl azodicarboxylate, PhthNOH = N-hydroxyphthalimide.

Synthesis of (±)- and (R)-(+)-Isoxazolidin-5-ylphosphonic Acid [(±)- and (R)-3]

For the preparation of these heterocyclic phosphonic acids, isomeric to (±)- and (R)-2, a separate entry had to be developed (Scheme ). We reasoned that racemic 3-chloro-1-hydroxypropylphosphonate (±)-20 could be the key intermediate for both isoxazolidin-5-ylphosphonic acids, as it could be easily prepared as racemate and resolved enzymatically. Ethyl β-chloropropionate (19) was reduced to the aldehyde with diisobutylaluminium hydride (DIBALH) in dry toluene at −78 °C, to which diisopropyl trimethylsilyl phosphite was added to give α-hydroxyphosphonate (±)-20 in 84% yield in a one-pot reaction.[28] Chloroacetylation with (ClH2CC(O))2O/pyridine furnished α-chloroacetoxyphosphonate (±)-21. This ester was subjected[14] to lipase-catalyzed kinetic hydrolysis in a biphasic system on a preparative scale with 16.3 mmol of substrate. When the enzymatic hydrolysis was stopped at 40% conversion, 5.14 mmol of α-hydroxyphosphonate (+)-20 with an ee of 97% and (S)-configuration were obtained, determined[28,29] by using (R)-(+)-t-Bu(Ph)P(O)SH as the chiral solvating agent in combination with 31P nuclear magnetic resonance (NMR) spectroscopy. Satisfyingly, α-hydroxyphosphonates (±)- and (S)-20 were converted to N-protected O-alkylhydroxylamines (±)- and (R)-22 in yields of 88 and 72% (ee 95% by HPLC, Figure S6), respectively. Luckily, the chloride on the primary C-3 position was a much weaker leaving group than Ph3PO and did not compete with the displacement reaction at C-1. The cleavage of the phthaloyl group was again induced with hydrazine hydrate. Precursors 22 were cyclized to isoxazolidin-5-ylphosphonic acids (±)- and (R)-23, which were directly used for the next step. Global deprotection was accomplished as outlined above. The overall yields of (±)- and (R)-3 starting from 23 were 81 and 63% (ee 97%, Figure S3), respectively. The crystalline isoxazolidinylphosphonic acids 2 and 3 had to be stored at −18 °C to prevent gradual decomposition at room temperature.

Scheme 4

Preparation of (±)- and (R)-(+)-Isoxazolidin-5-ylphosphonic Acid [(±)- and (R)-3]

DIAD = diisopropyl azodicarboxylate, PhthNOH = N-hydroxyphthalimide.

Synthesis of (±)- and (R)-(−)-Pyrazolidin-3-ylphosphonic Acids [(±)- and (R)-4]

Initially, we reasoned that these phosphonic acids could easily be prepared by the reaction of a modified propylphosphonate, with leaving groups at C-1 and C-3, with a protected hydrazine derivative, but were convinced of the contrary. At first, 3-chloro-1-hydroxypropylphosphonate (±)-20 was converted to triflate (±)-24 in 71% yield (Scheme ). It was added to a mixture of N,N′-bis(Boc)-hydrazine[31] and NaH in dimethylformamide (DMF), which had been stirred for 30 min at room temperature and was allowed to react for 18 h at 20 °C and 2 h at 50 °C. The crude product, which did not contain the starting material, was a complex mixture and was therefore discarded. The failure of this experiment was attributed to a combination of the low reactivity from Cl– as the leaving group at C-3 and the high reactivity[32] of TfO– at C-1. The first step, that is, the intermolecular reaction of the deprotonated hydrazine derivative will prefer attack at C-3. The cyclization of the N-substituted hydrazine intermediate should easily proceed to pyrazolidin-3-ylphosphonate (±)-25, as it is an intramolecular process, and TfO– is an excellent leaving group. Substitution at C-1 of phosphonates was first considered unlikely for steric reasons and low reactivity in general. The size of the attacking nucleophile and the shielding of C-1 by the isopropyl-protecting groups at the phosphorus atom disfavor a SN2 reaction. Base-induced elimination of TfOH was more likely than substitution. Consequently, a better leaving group had to be placed at C-3 or/and a less reactive one at C-1.

Scheme 5

Attempted Preparation of Protected (±)-Pyrazolidin-3-ylphosphonic Acid [(±)-27]

To replace chloride by bromide, β-bromopropionate 26 was transformed into 3-bromo-1-hydroxyphosphonate (±)-29 and then into bromo triflate (±)-30a by the same procedures as used for the chloro derivative (Scheme ). It was reacted with N,N′-bis(Boc)-hydrazine in a biphasic system[33] (20% NaOH/toluene) under phase-transfer conditions at room temperature. The extractively obtained crude product was again a complex mixture without a starting material, but contained elimination products as judged by NMR spectroscopy. As found later, when we had compound (±)-25 in hand, this mixture already contained some of it. Importantly, this experiment demonstrated that the trifluoromethanesulfonyloxy group had to be replaced by a less reactive leaving group such as a mesyloxy or 4-nitrobenzenesulfonyloxy (nosyloxy) group to interfere with elimination. The corresponding mesylate (±)-28b and nosylate (±)-28c were obtained in 93 and 86% yield, respectively, by esterification of 3-bromo-1-hydroxyphosphonate (±)-27 with mesyl chloride/Et3N or NosCl/Et3N/dimethylaminopyridine (DMAP) (Scheme ). When the experiment with bis(Boc)-hydrazine/NaOH was repeated with mesylate (±)-28b instead of the triflate at 0 °C and followed by thin-layer chromatography (TLC) monitoring, no new product could be detected besides the starting material after a reaction time of 1 h. Then, the temperature was increased to 19–20 °C. After 1 h, a spot of a new compound less polar than the substrate appeared on the TLC plate, and its intensity increased with the reaction time. After 5 h at 19–20 °C, the reaction mixture was worked up. The 31P NMR spectrum of the crude product displayed resonances for the cyclic phosphonate (±)-25, the open chain product (±)-29b, a compound of unknown structure, and the starting material (±)-28b in molar ratios of 4:100:16:1. Flash column chromatography gave the open chain hydrazine derivative (±)-29b in 75% yield as a colorless gum. As expected, the first substitution occurred at C-3, and the second one at C-1 virtually did not take place at 20 °C. When hydrazine derivative (±)-29b was heated at 80 °C under the same reaction conditions as before except for the higher reaction temperature, cyclization smoothly proceeded to pyrazolidine (±)-25 and was finished after 2.5 h. Workup and purification delivered pyrazolidin-3-ylphosphonate (±)-25 in 75% yield. The reaction with nosylate (±)-29c was performed analogously (56% yield) except that the reaction temperature for both steps was 50 °C instead of 20 °C at the beginning and 80 °C later, owing to the nosyloxy group being a better leaving group than the mesyloxy one. When cyclization was carried out in DMF with KOtBu as the base for 1.5 h at 0 °C and 1.5 h at 20 °C, the yield was somewhat higher (65%). The yield could not be improved any further despite many experiments. It seemed likely that the partial removal of an isopropyl group would give a water-soluble salt and thus decrease the yield. Global deprotection of (±)-25 was more sensitive to forcing conditions such as refluxing 6 M HCl than to the milder conditions with TMSBr/allylTMS at 50 °C. Purification of the crude product by cation-exchange chromatography (Dowex 50W × 8, H+) and crystallization furnished racemic pyrazolidin-3-ylphosphonic acids [(±)-4] in 55% yield.

Scheme 6

Preparation of (±)-Pyrazolidin-3-ylphosphonic Acid [(±)-4]

Nos = 4-nitrobenzenesulfonyl, Tf = trifluoromethanesulfonyl, Ms = methanesulfonyl.

To prepare its enantiomer (R)-4, bromohydroxyphosphonate (±)-27 was chloroacetylated and kinetically resolved in the same way as the chloro analogue (±)-21 (Scheme ). Stopping hydrolysis at a conversion of 40% delivered (+)-α-hydroxyphosphonate (S)-27 of 95% ee, as determined via (R)-Mosher ester in combination with 31P NMR spectroscopy. This enantiomer was nosylated (ee 95% by HPLC, Figure S7) and then cyclized with inversion of configuration at C-1 in analogy to the racemate in a basic biphasic system mediated by a phase-transfer catalyst at 50 °C. Global deprotection of (R)-25 delivered pyrazolidin-3-ylphosphonic acid [(R)-4, ee 92%, Figure S4] by isolation from the crude product as for the racemate.

Scheme 7

Preparation of (R)-Pyrazolidin-3-ylphosphonic Acid [(R)-4]

Nos = 4-nitrobenzenesulfonyl.

Inhibition of Plant P5C Reductase by Phosphonic Acid Analogues of Proline and Proline Analogues

The ability of all obtained compounds to interfere with the catalytic activity of P5C reductase, purified from Arabidopsis thaliana cultured cells,[34] was then evaluated. In the range from 10–4 to 10–3 M, their addition to the assay mixture was found ineffective. On the contrary, when millimolar concentrations of compounds (R)-, (S)-1, and(R)-6 were used, a significant inhibition was evident that was proportional to the dose (Figure ). Interestingly, their effectiveness was higher than that of proline, which exerts product inhibition in the range from 10 to 200 mM.[34,35] The calculated IC50 values were 84 ± 3, 24 ± 4, 21 ± 2, and 45 ± 19 mM for proline, (R)-, (S)-1, and (R)-6, respectively. However, these concentrations were 3–4 orders of magnitude higher than those found for bisphosphonate inhibitors of P5C reductases from higher plants.[4−7] Therefore, although potentially useful for molecular-docking studies aimed at a better comprehension of the product inhibition mechanism, the use of these analogues for weed control seems unfeasible. The ability of phosphonic acid analogues of proline and proline analogues to interfere with the activity of the enzymes that catalyze the first step in proline biosynthesis in bacteria and plants, namely γ-glutamyl kinase and P5C synthetase from Escherichia coli and rice, respectively, was also assessed. However, in no case was their catalytic rate significantly inhibited (data not shown).

Figure 2

Effect of millimolar concentrations of compounds (R)-, (S)-1, and (R)-6 on the activity of A. thaliana P5C reductase. The inhibition brought about by the physiological product of the enzyme, proline, is also shown as a term of comparison.

Conclusions

In summary, we have prepared the racemic 1-hydroxy-3-butenyl-, 3-chloro-1-hydroxypropyl-, and 3-bromo-1-hydroxypropylphosphonate and converted them to chloroacetates for lipase-catalyzed enantioselective hydrolysis. The racemates and (S)-enantiomers of these α-hydroxyphosphonates were transformed into racemic and (R)-configured pyrrolidin-2-yl-, oxazolidin-3-yl-, oxazolidin-5-yl-, pyrazolidin-3-yl-, and 1,2-oxazinan-3-ylphosphonic phosphonic acids by a variety of functional group manipulations. The ee (92–99%) of the (R)-enantiomers were determined after derivatization using chiral HPLC on quinine and quinidine-derived anion-exchange columns. These phosphonic acids, structural analogues of proline and proline analogues, were tested as P5C reductase inhibitors, but displayed negligible activity. They also did not interfere with the enzymes of bacteria and plants catalyzing the first step in proline biosynthesis. These phosphonic acids may also be evaluated as inhibitors of proline-metabolizing enzymes.

Experimental Section

General Information

1H, 13C (J-modulated) and 31P NMR spectra were recorded in CDCl3 or D2O on a Bruker AV 400 (1H: 400.13 MHz, 13C: 100.61 MHz, 31P: 161.98 MHz), AV III 400 (1H: 400.27 MHz, 13C: 100.65 MHz, 31P: 162.03 MHz), AV II+ 500 (1H: 500.32 MHz, 31P: 202.53 MHz), and AV III 600 (1H: 600.25 MHz, 13C: 150.93 MHz, 31P: 242.99 MHz) at 25 °C unless otherwise indicated. Chemical shifts (δ) are reported in parts per million (ppm) relative to CHCl3/CDCl3 (δH 7.24; δC 77.00), toluene-d8 (δH 2.09 for HD2C of toluene-d7), HOD (δH 4.80) and external H3PO4 (85%; δP 0.00) and coupling constants (J) in Hz. Infrared (IR) spectra were recorded on a Bruker VERTEX 70 IR spectrometer in the attenuated total reflection (ATR) mode or of films on a silicon disc.[36] High-resolution mass spectra (HRMS) were obtained using a Brucker Maxis Q-TOF mass spectrometer [electrospray ionization (ESI)]. Optical rotations were measured on a PerkinElmer 341 polarimeter in a 1 dm quartz cell. Analytical HPLC: Shimadzu system comprising components LC-20AT, SIL-20A HT, CTO-20AC, SPD-20A, CMB-20A, column: CHIRALPAK IA (250 mm × 4.6 mm, particle size 5 μm, 1 mL/min, 25 °C). Melting points were measured on a Leica Galen III Thermovar instrument and are uncorrected.

Anhydrous THF was refluxed over potassium and distilled prior to use. Pyridine was dried by refluxing over powdered CaH2, then distilled, and stored over molecular sieves (4 Å). All other solvents, also dry ones, and chemicals were used as purchased. Flash column chromatography was performed using silica gel (particle size 0.040–0.063 mm). Reactions were monitored by analytical TLC using precoated silica gel plates (60 F254, 250 μm thickness). Spots were visualized by ultraviolet (UV) and/or dipping into a solution of (NH4)6Mo7O24·4H2O (25.0 g) and Ce(SO4)2·4H2O (1.0 g) in 10% aqueous H2SO4 (500 mL), followed by heating with a heat gun. The solvent for TLC of phosphonic acids was iPrOH/H2O/NH3 (25%) in the ratio of 6:3:1. Spots of phosphonic acids were visualized by dipping the silica gel plate into a solution of 2% ninhydrin in EtOH and heating with a heat gun.

Determination of Enantiomeric Excesses of Aminophosphonic Acids

The analytical determination of enantiomeric excesses was performed with a commercial CHIRALPAK tert-butyl-QD-AX (150 mm × 4 mm, 5 μm) column and an in-house prepared underivatized OH-QN-AX column[37] (150 mm × 4 mm, 5 μm; CSP1). The mobile phase comprised an aqueous H3PO4 solution in MeOH in the ratio of 1:9 (v/v). Note that the molarity of the aqueous phase as well as the apparent pH (adjusted with triethylamine) of the polar organic mobile phase and the flow rates were optimized depending on the amino phosphonic acid, the column type, and the derivatization type. These chromatographic conditions are provided in the respective figure legends of the corresponding chromatograms summarized in the Supporting Information.

The instrumentation used were a thermoshaker PHMT with PSC24N from Grant-bio (Cambridgeshire, UK) and an Agilent 1100 HPLC–UV–fluorescence detector (FLD) system from Agilent (Waldbronn, Germany), which comprised a binary pump, a temperature controlled column oven, an autosampler, a multiwavelength detector (MWD) and a FLD. For the FLD, a gain of 10, an excitation wavelength of 254 nm, and an emission wavelength of 395 nm were chosen, whereas the MWD was set to 254 nm.

For compounds 1 and 2, a derivatization with the Sanger’s reagent (1-fluoro-2,4-dinitrobenzene, DNFB from Sigma Aldrich) and chiral separation on the CHIRALPAK tert-butyl-QD-AX column were chosen. For compounds 3, 4, and 6, a derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC from Synchem) and chiral separation on an underivatized OH-QN-AX column provided the best results.

AQC Derivatization

Phospha amino acid (10 μL, 20 mM in water) was added to 0.2 M borate buffer (pH 8.8, 70 μL), followed by the addition of AQC (3 mg/mL in dry acetonitrile, 20 μL). The reaction mixture was immediately heated at 55 °C for 10 min.

DNFB Derivatization

To a solution (150 μL, 10 mM) of phospha amino acid in sodium carbonate buffer (0.1 M, pH 9.5), Sanger reagent [2.5% (v/v) in acetonitrile, 50 μL] was added and heated for 15 min at 50 °C. After reaching room temperature and centrifugation, the respective mobile phase (350 μL) was added.

Chromatographic results are summarized in the Supporting Information in Figure S1 [compounds (R)-, (S)-, and (R,S)-1], Figure S2 [compounds (R)- and (R,S)-2], Figure S3 [compounds (R)- and (R,S)-3], Figure S4 [compound (R)-4], and Figure S5 [compounds (R)- and (R,S)-5].

Note that for DNFB-derivatized compounds 1 and 2, an elution order of the (S)-enantiomer before the (R)-enantiomer was observed on the CHIRALPAK tert-butyl-QD-AX column with separation factors of 1.32 and 1.23, respectively. Concerning enantiomeric excess, an ee of 99.3% was determined for compound (R)-1, 99.99% for (S)-1, and 99.2% for (R)-2. The same elution order, (S)- before (R)-enantiomer, was also observed for AQC-derivatized compounds 3 and 4 using the unmodified QN-AX column, providing separation factors of 1.14 and 1.38, respectively. The determined ee values were 97.2% for compound (R)-3 and 98.8% for (R)-4 after crystallization (values for mother liquor and before crystallization: 76.9 and 92.2%). Although compound 6 was an analogue of compound 2, an elution order of (R) before (S) was observed with a separation factor of 1.23 and an ee value of 91.6% after crystallization (values for mother liquor and before crystallization: 56.3 and 83.1%) using AQC derivatization and separation on an unmodified QN-AX column. A control experiment for compound 2 derivatized with AQC and separation on the same unmodified QN-AX column showed the same elution order, (R) before (S), as observed for compound 6; however, no baseline separation could be achieved under standard test conditions (data not shown). The deviation in the elution order was the result of using different derivatization reagents combined with different chiral selector types (QN-AX vs QD-AX) and selector modifications (tert-butyl type vs unmodified CSP) for enantiomer separation.

(±)-, (R)-(+)-, and (S)-(−)-Pyrrolidin-2-ylphosphonic Acid (Phosphaproline) [(±)-, (R)-, and (S)-1]

Cyclohexene (598 mg, 0.74 mL, 7.28 mmol, 4 equiv) was added to a solution of H3B × Me2S (277 mg, 0.35 mL, 3.64 mmol, 2 equiv) in freshly distilled dry 1,2-dimethoxyethane (DME, 5.5 mL) under an argon atmosphere at 0 °C. The reaction mixture was stirred for 15 min at 0 °C and 1 h at room temperature. The resulting suspension of dicyclohexylborane[23] was again cooled at 0 °C. Racemic 1-azido-3-butenylphosphonate (±)-8[18] (475 mg, 1.82 mmol) dissolved in dry DME (1 mL) was added. After stirring for 1 h at 0 °C and 2 h at room temperature, the reaction was quenched with concd HCl (2 mL) and water (5 mL). The organic phase was removed, and the aq one was concentrated under reduced pressure. The residue was refluxed with a mixture of concd HCl (8 mL) and water (5 mL) for 18 h. The solution was cooled and concentrated under reduced pressure. The residue was dried over KOH in a vacuum desiccator and purified by cation-exchange chromatography (Dowex 50W × 8, H+, elution with water). Fractions containing the product (TLC: iPrOH/H2O/NH3 (25%), 6:3:1, Rf = 0.25) were pooled and concentrated under reduced pressure to furnish racemic phosphaproline [(±)-1] (179 mg, 65%) as colorless crystals; mp 270–273 °C (H2O/EtOH). Similarly, (R)-(−)-1-azido-3-butenylphosphonate (R)-8[18] {533 mg, 2.04 mmol; [α]D20 −31.3 (c 1.2, acetone)} was converted to (R)-phosphaproline [(R)-1] (191 mg, 62%); [α]D22 −46.6 (c 0.5, 1 M NaOH) {lit.:[14] [α]D21 −49.1 (c 1.1, 1 M NaOH); lit.:[17] [α]D21 −49.8 (c 1.1, 1 M NaOH); lit.:[9] [α]57820 +64 (c 1.0, 1 M NaOH), +64 (c 1.0, 1 M NaOH)}. Similarly, (S)-(+)-azide [(S)-8] [1.09 g, 4.17 mmol; [α]D20 +32.0 (c 1.08, acetone)] was converted to (S)-(+)-phosphaproline [(S)-1] (451 mg, 71%): [α]57823 +49.5 (c 0.55, 1 M NaOH); [α]D20 +47.3 (c 0.55, 1 M NaOH); mp 280 °C (decomp). Azide (S)-8 (1.670 g, 80%) was prepared from 1-hydroxy-3-butenylphosphonate (R)-7 (1.89 g, 8.0 mmol, 99% ee) by a literature procedure;[18] [α]D20 +32.0 (c 1.08, acetone). The NMR spectra of (±)-, (R)-, and (S)-phosphaproline were identical to those reported in the literature.[14]

(±)- and (S)-(+)-3-Hydroxy-1-(4-nitrobenzenesulfonyloxy)propylphosphonate [(±)- and (S)-12]

Racemic nosylate (±)-11[18] (993 mg, 2.36 mmol) was dissolved in a mixture of methanol (5 mL) and CH2Cl2 (5 mL). Ozone was bubbled through the stirred solution at −78 °C until the blue color persisted. Excess ozone was removed by passing air through the solution. NaBH4 (107 mg, 2.83 mmol, 1.2 equiv, dissolved in 1 mL of ethanol) was added and stirring was continued for 2.5 h at room temperature. The reaction mixture was concentrated under reduced pressure. The residue was dissolved in water (10 mL) and EtOAc (10 mL). The organic layer was separated, and the aq one was extracted with EtOAc (2 × 10 mL). The combined organic layers were dried (Na2SO4) and concentrated under reduced pressure. The residue was purified by flash chromatography (hexanes/EtOAc, 1:1, Rf 0.17) to yield 3-hydroxypropylphosphonate (±)-12 (912 mg, 91%) as colorless crystals; mp 103 °C (CH2Cl2/hexanes). Similarly, nosylate (S)-11[18] (1.334 g, 3.166 mmol) was converted to hydroxypropylphosphonate (S)-12 (1.227 g, 91%) as colorless crystals; mp 93 °C (CH2Cl2/hexanes); [α]D20 +25.5 (c 1.15, acetone). The NMR spectra of (±)- and (S)-12 were identical.

IR (ATR) of (S)-enantiomer ν: 3358, 2985, 1608, 1534, 1375, 1350, 1242, 1186, 988 cm–1. 1H NMR (400.13 MHz, CDCl3): δ 8.40–8.34 (m, 2H), 8.20–8.13 (m, 2H), 5.08 (td, J = 9.4, 4.5 Hz, 1H), 4.68 (2 oct overlapping to a dec, J = 6.4 Hz, 2H), 3.79 (td, J = 11.8, 4.4 Hz, 1H), 3.70 (td, J = 11.8, 3.5 Hz, 1H), 2.55 (br s, 1H, OH), 2.22–2.07 (m, 1H), 2.01–1.93 (m, 1H), 1.298 (d, J = 6.4 Hz, 3H), 1.294 (d, J = 6.4 Hz, 3H), 1.264 (d, J = 6.4 Hz, 3H), 1.248 (d, J = 6.4 Hz, 3H); 13C NMR (100.61 MHz, CDCl3): δ 150.8, 142.3, 129.5 (2C), 124.2 (2C), 75.1 (d, J = 172.6 Hz), 72.6 (d, J = 6.3 Hz), 72.6 (d, J = 6.5 Hz), 57.3 (d, J = 10.3 Hz), 33.5, 24.0 (d, J = 5.2 Hz), 24.0 (d, J = 5.2 Hz), 23.9 (d, J = 5.1 Hz), 23.7 (d, J = 4.9 Hz); 31P NMR (162.03 MHz, CDCl3): δ 16.7. Anal. Calcd for C15H24NO9PS: C, 42.35; H, 5.69; N, 3.29. Found: C, 42.40; H, 5.72; N, 3.29.

(±)- and (S)-(+)-Diisopropyl 1-(4-nitrobenzenesulfonyloxy)-3-(phthalimidooxy)propylphosphonate [(±)- and (S)-13]

Racemic 3-hydroxypropylphosphonate (±)-12 (1.260 g, 2.96 mmol), N-hydroxyphthalimide (507 mg, 3.11 mmol, 1.05 equiv) and Ph3P (1.010 g, 3.85 mmol, 1.3 equiv) were dissolved with stirring in dry THF (12 mL) and dry CH2Cl2 (1.5 mL) under argon. DIAD (779 mg, 0.76 mL, 3.85 mmol, 1.3 equiv) was added dropwise at 0 °C, and the solution was slowly warmed to room temperature in a cooling bath and stirred overnight. Water (a few drops) was added, and after 15 min, the reaction mixture was concentrated under reduced pressure. The residue was flash-chromatographed (hexanes/EtOAc, 3:2, Rf 0.33 for hexanes/EtOAc, 1:1) to yield racemic phthalimidooxyphosphonate (±)-13 (1.399 g, 83%) as a colorless foam. Similarly, (S)-hydroxypropylphosphonate (S)-12 (1.572 g, 2.96 mmol; 97% ee) was converted to (S)-(+)-phthalimidooxyphosphonate (S)-13 (1.399 g, 84%) as a colorless foam; [α]D20 +15.1 (c 0.35, acetone). The NMR spectra of (±)- and (S)-13 were identical.

IR (ATR) of (±)-15 ν: 2981, 1732, 1531, 1373, 1349, 1255, 1185, 984 cm–1. 1H NMR (600.25 MHz, CDCl3): δ 8.45–8.39 (m, 2H), 8.30–8.24 (m, 2H), 7.89–7.84 (m, 2H), 7.81–7.76 (m, 2H), 5.35 (td, J = 9.1, 4.2 Hz, 1H), 4.74 (oct, J = 6.2 Hz, 1H), 4.70 (oct, J = 6.2 Hz, 1H), 4.45–4.38 (m, 1H), 4.32 (ddd, J = 10.2, 8.2, 5.5 Hz, 1H), 2.48–2.39 (m, 1H), 2.27–2.18 (m, 1H), 1.34 (d, J = 6.2 Hz, 3H), 1.32 (d, J = 6.2 Hz, 3H), 1.31 (d, J = 6.2 Hz, 3H), 1.30 (d, J = 6.2 Hz, 3H); 13C NMR (150.93 MHz, CDCl3): δ 163.4 (2C), 150.7, 142.1, 134.6 (2C), 129.7 (2C), 128.7 (2C), 124.2 (2C), 123.6 (2C), 74.1 (d, J = 172.4 Hz), 73.5 (d, J = 10.9 Hz), 72.7 (d, J = 6.1 Hz), 72.6 (d, J = 6.4 Hz), 29.7, 24.0 (d, J = 3.6 Hz), 24.0 (d, J = 3.7 Hz), 23.8 (d, J = 5.0 Hz), 23.7 (d, J = 5.0 Hz); 31P NMR (242.99 MHz, CDCl3): δ 14.2. HRMS (ESI-TOF) m/z: [M + Na]+ calcd C23H27N2O11PSNa, 593.0966; found, 593.0961.

(±)- and (R)-(+)-Isoxazolidin-3-ylphosphonic Acid [(±)- and (R)-2]

Racemic phthalimidooxyphosphonate (±)-13 (762 mg, 1.34 mmol) was dissolved in iPrOH (4 mL) and heated and stirred at 60 °C after addition of N2H4 × H2O (334 mg, 6.68 mmol, 0.33 mL). A crystalline solid was formed rapidly. After 1 h, the solvent was removed at reduced pressure (1 mbar). Diisopropyl ether was added, and the residue was converted to a paste by stirring and breaking up the lumps with a spatula. The mixture was filtered through a Celite pad with suction and carefully washed with iPr2O. The filtrate was concentrated under reduced pressure. The oily, faint yellow residue (355 mg) was mixed with AcOH/HBr[24] (7 mL, 30%) under argon at room temperature and stirred for 16 h. Volatile compounds were removed under vacuum (1 mbar). The residue was purified by cation-exchange chromatography (Dowex 50W × 8, H+; column: o.d. 2 cm × 38 cm, elution with water, fractions of 10 mL; Rf 0.33). Ninhydrin-positive fractions were pooled and concentrated under reduced pressure to give racemic isoxazolidin-3-ylphosphonic acid (±)-2 (162 mg, 79% combined yield for two steps) as colorless crystals; mp 148 °C (decomp) (H2O/iPrOH). Similarly, (S)-(+)-phthalimidooxyphosphonate (S)-13 (928 mg, 1.63 mmol) was converted to (R)-(+)-isoxazolidin-3-ylphosphonic acid (R)-2 (148 mg, 74%) as colorless crystals; mp 180 °C (decomp) (water/ethanol); ee 99% (Figure S2); [α]D20 +6.4 (c 0.63, water) {lit.:[25] [α]D20 +30.8 (c 1.0, CF3CO2H)}. The NMR spectra of (±)- and (R)-2 were identical.

IR (ATR) of (±)-2 ν: 3100–1500 (very br), 1447, 1232, 1169, 1076, 963, 927, 858 cm–1. 1H NMR (600.25 MHz, D2O): δ 4.34 (qd, J = 8.0, 4.0 Hz, 1H), 4.18 (qd, J = 8.0, 0.5 Hz, 1H), 3.84 (td, J = 9.9, 8.4 Hz, 1H), 2.76–2.68 (m, 1H), 2.53–2.42 (m, 1H); 13C NMR (150.93 MHz, D2O): δ 71.1 (d, J = 8.2 Hz), 56.8 (d, J = 139.9 Hz, 30.7); 31P NMR (242.99 MHz, D2O): δ 7.8. Anal. Calcd for C3H8NO4P: C, 23.54; H, 5.27; N, 9.15. Found: C, 23.56; H, 5.05; N, 25.

(±)- and (S)-(+)-Diisopropyl 1-(4-Nitrobenzenesulfonyloxy)-4-(phthalimidooxy)butylphosphonate [(±)- and (S)-17]

The mixture of racemic ω-hydroxynosylate (±)-16[18] (2.334 g, 5.31 mmol), N-hydroxyphthalimide (1.039 g, 6.37 mmol, 1.2 equiv) and dry toluene (30 mL) was concentrated under reduced pressure and dried in vacuo. Triphenylphosphane (1.95 g, 7.43 mmol, 1.4 equiv) was added to the residue, followed by dry THF (20 mL) under argon atmosphere. After cooling at 0 °C, DIAD (1.502 g, 7.43 mmol, 1.4 equiv, 1.46 mL) was added dropwise. Stirring was continued for 3 min at 0 °C and 4 h at room temperature before H2O (0.5 mL) was added. After 5 min, the reaction mixture was concentrated under reduced pressure. The residue was flash-chromatographed (heptanes/EtOAc, 1:1, Rf 0.38) to yield racemic phthalimidooxyphosphonate (±)-17 (2.275 g, 73%) as colorless crystals; mp 130–132 °C [1,2-dichloroethane (DCE)/heptanes]. Similarly, (S)-(+)-ω-hydroxynosylate (S)-18 {2.580 g, 5.87 mmol; [α]D27 +7.7 (c 1.25, acetone), derived from 1-hydroxy-3-butenylphosphonate[18] of 85% ee} was converted to (S)-phthalimidooxyphosphonate (S)-17 (2.574 g, 75%) as colorless crystals; mp 128 °C (CH2Cl2/hexanes); [α]D20 +5.0 (c 1.29, acetone). The NMR spectra of (±)- and (S)-19 were identical.

IR (ATR) of (±)-19 ν: 2978, 2960, 1732, 1367, 1345, 1268, 1182, 1007, 981 cm–1. 1H NMR (600.25 MHz, CDCl3): δ 8.38–8.34 (m, 2H), 8.21–8.16 (m, 2H), 7.83–7.79 (m, 2H), 7.76–7.71 (m, 2H), 4.99 (td, J = 9.1, 4.4 Hz, 1H), 4.72 (oct, J = 6.2 Hz, 1H), 4.63 (oct, J = 6.2 Hz, 1H), 4.22–4.14 (m, 2H), 2.28–2.19 (m, 1H), 2.10–2.02 (m, 1H), 2.02–1.94 (m, 1H), 1.91–1.82 (m, 1H), 1.30 (d, J = 6.2 Hz, 3H), 1.298 (d, J = 6.2 Hz, 3H), 1.27 (d, J = 6.2 Hz, 3H), 1.25 (d, J = 6.2 Hz, 3H); 13C NMR (150.93 MHz, CDCl3): δ 163.5 (2C), 150.7, 142.5, 134.5 (2C), 129.4 (2C), 128.8 (2C), 124.2 (2C), 123.3 (2C), 77.7 (d, J = 171.5 Hz), 77.0, 72.4 (d, J = 7.1 Hz), 72.4 (d, J = 7.1 Hz), 26.7, 24.2 (d, J = 10.0 Hz), 24.1 (d, J = 3.6 Hz), 24.0 (d, J = 3.7 Hz), 23.9 (d, J = 5.0 Hz), 23.7 (d, J = 5.0 Hz); 31P NMR (242.99 MHz, CDCl3): δ 14.4. Anal. Calcd for C24H29N2O11PS: C, 49.31; H, 5.00; N, 4.79. Found: C, 49.40; H, 5.13; N, 4.98.

(±)- and (R)-Diisopropyl 1,2-Oxazinan-3-ylphosphonate [(±)- and (R)-18]

A mixture of racemic phthalimidooxyphosphonate (±)-17 (873 mg, 1.49 mmol), N2H4 × H2O (374 mg, 7.47 mmol, 5 equiv, 0.36 mL) and iPrOH (5 mL) were heated at 80 °C for 1 h. The mixture was cooled and diluted with EtOAc (5 mL). The crystals were collected by suction and washed with EtOAc. The combined filtrates were concentrated under reduced pressure. The residue was flash-chromatographed (EtOAc, Rf 0.36) to give racemic 1,2-oxazinan-3-ylphosphonate [(±)-18] (273 mg, 73%) as a colorless liquid. Similarly, (S)-(+)-phthalimidooxyphosphonate (S)-17 (1.40 g, 2.4 mmol) was converted to (R)-(−)-1,2-oxazinan-3-ylphosphonate (R)-18 (500 mg, 83%); [α]D23 −38.9 (c 0.9, MeOH). The NMR spectra of (±)- and (R)-(−)-18 were identical.

IR (ATR) ν: 3242, 2978, 2939, 1375, 1232, 1058, 974 cm–1. 1H NMR (600.25 MHz, CDCl3): δ 5.42 (br s, 1H), 4.76–4.65 (m, 2H), 3.99–3.93 (m, 1H), 3.73 (td, J = 11.3, 3.1 Hz, 1H), 3.43 (ddd, J = 15.4, 11.2, 3.0 Hz, 1H), 2.00–1.93 (m, 1H), 1.82–1.65 (m, 3H), 1.32 (d, J = 5.8 Hz, 3H), 1.31 (d, J = 6.6 Hz, 3H), 1.30 (d, J = 6.4 Hz, 6H); 13C NMR (150.93 MHz, CDCl3): δ 71.1 (d, J = 6.7 Hz), 71.0 (d, J = 7.0 Hz), 70.5, 56.5 (d, J = 150.7 Hz), 24.5 (d, J = 11.7 Hz), 24.1 (d, J = 3.6 Hz), 24.1 (d, J = 3.7 Hz), 24.0 (d, J = 4.5 Hz), 23.95 (d, J = 4.5 Hz), 23.64 (d, J = 3.7 Hz); 31P NMR (242.99 MHz, CDCl3): δ 20.1. Anal. Calcd for C10H22NO4P: C, 47.80; H, 8.83; N, 5.57; O, 25.47. Found: C, 47.78; H, 9.19; N, 5.74; O, 25.53.

(±)- and (R)-(−)-1,2-Oxazinan-3-ylphosphonic Acid [(±)- and (R)-6]

Racemic 1,2-oxazinan-3-ylphosphonate (±)-18 (479 mg, 1.906 mmol) was dissolved in 6 M HCl (10 mL) and refluxed for 4 h. The solution was concentrated under reduced pressure. The residue was purified by cation-exchange chromatography (Dowex 50W × 8, H+, water as eluent, Rf 0.61) to furnish racemic 1,2-oxazinan-3-ylphosphonic acid (±)-6 (301 mg, 94%) as crystals; mp 183–86 °C (decomp.) (H2O/EtOH). Similarly, (R)-(−)-1,2-oxazinan-3-ylphosphonate (R)-18 (480 mg, 1.91 mmol) was converted to (R)-(−)-1,2-oxazinan-3-ylphosphonic acid [(R)-6] (289 mg, 91%) as crystals; mp 183–85 °C (decomp) (H2O/EtOH); [α]D25 −22.3 (c 0.95, H2O) before crystallization (ee 83.1%; Figure S5); [α]D27 −23.2 (c 0.95, H2O) after crystallization (ee 91.6%). The NMR spectra of (±)- and (R)-6 were identical.

IR (ATR) of (±)-6 ν: 3500–1700 (very br), 1234, 1186, 1168, 1069, 1040, 978, 960, 913 cm–1. 1H NMR (600.25 MHz, D2O): δ 4.31–4.23 (m, 1H), 4.15 (td, J = 12.1, 2.5 Hz, 1H), 3.59 (ddd, J = 13.7, 12.3, 2.5 Hz, 1H), 2.22–2.12 (m, 1H), 1.99–1.78 (m, 3H); 13C NMR (150.93 MHz, D2O): δ 71.4, 56.1 (d, J = 134.5 Hz), 21.8 (d, J = 9.8 Hz), 20.8 (d, J = 2.5 Hz); 31P NMR (242.99 MHz, D2O): δ 9.1. Anal. Calcd for C4H10NO4P: C, 28.75; H, 6.03; N, 8.38; O, 38.30. Found: C, 28.79; H, 5.94; N, 8.21; O, 38.63.

(±)-Diisopropyl 3-Chloro-1-hydroxypropylphosphonate [(±)-20]

Ethyl 3-chloropropionate (19) (4.097 g, 30 mmol) was dissolved in dry toluene (40 mL) and cooled to −78 °C under argon atmosphere. A solution of DIBALH (33 mL, 1 M, toluene) was added dropwise over 10 min.[28] After 2 h of additional stirring at −78 °C, diisopropyl trimethylsilyl phosphite (7.150 g, 30 mmol) was added. The cooling bath was removed and the reaction mixture was stirred at room temperature for 18 h. HCl (2 M, 10 mL, exothermic!) was added dropwise, and after 10 min, more HCl (90 mL) was added and stirring was continued for 30 min (TLC: the silylated hydroxyphosphonate should be absent). The organic phase was separated and the aq one was extracted with EtOAc (3 × 50 mL). The combined organic layers were washed with brine, dried (MgSO4), and concentrated under reduced pressure. The residue was purified by flash chromatography (hexanes/EtOAc, 1:3; Rf 0.24) to give racemic α-hydroxyphosphonate (±)-20 (6.558 g, 84%) as a colorless oil.

IR (ATR) ν: 3272, 2979, 1386, 1376, 1178, 1077, 978 cm–1. 1H NMR (400.27 MHz, CDCl3): δ 4.90–4.50 (m and br s, 3H), 3.99 (td, J = 9.7, 4.4 Hz, 1H), 3.78–3.65 (m, 2H), 2.18–1.99 (m, 2H), 1.30 (d, J = 6.0 Hz, 3H), 1.29 (d, J = 6.2 Hz, 9H); 13C NMR (100.61 MHz, CDCl3): δ 71.5 (d, J = 7.4 Hz), 71.3 (d, J = 7.4 Hz), 64.6 (d, J = 165.7 Hz), 41.2 (d, J = 16.7 Hz), 34.4 (d, J = 2.5 Hz), 24.1 (d, J = 3.7 Hz), 24.0 (d, J = 3.8 Hz), 23.9 (d, J = 4.7 Hz, 2C); 31P NMR (162.04 MHz, CDCl3): δ 22.7. Anal. Calcd for C9H20ClO4P: C, 41.79; H, 7.79; O, 24.74. Found: C, 41.84; H, 7.76; O, 24.84.

(±)-Diisopropyl 3-Chloro-1-(chloroacetoxy)propylphosphonate [(±)-21]

Racemic 3-chloro-1-hydroxypropylphosphonate (±)-20 (4.967 g, 19.2 mmol) was dissolved in dry CH2Cl2 (30 mL) under Ar atmosphere. After cooling to 0 °C, dry pyridine (4.556 g, 57.6 mmol, 4.649 mL, 3 equiv) was added, and the solution was stirred for 10 min. Chloroacetic anhydride (4.924 g, 28.8 mmol, 1.5 equiv) dissolved in dry CH2Cl2 (16 mL) was added and stirring was continued at 0 °C.[38] After 2.5 h (TLC monitoring), when the starting material was consumed, the reaction was quenched with water (4 mL), and stirring was continued for 5 min before more water (20 mL) was added. The organic phase was separated, and the aqueous one was extracted with EtOAc (30 mL). The combined organic phases were washed with HCl (30 mL, 2 M), water (10 mL), saturated aqueous solution of NaHCO3 (20 mL), dried (MgSO4), and concentrated under reduced pressure. The residue was purified by flash chromatography (hexanes/EtOAc, 1:1, Rf 0.33) to give chloroacetate (±)-21 (5.463 g, 85%) as a colorless oil.

IR (ATR) ν: 2981, 2937, 1768, 1387, 1386, 1376, 1246, 1157, 1103, 979 cm–1. 1H NMR (600.25 MHz, CDCl3): δ 5.42 (td, J = 9.2, 4.5 Hz, 1H), 4.74 (sept, J = 6.2 Hz, 1H), 4.73 (sept, J = 6.2 Hz, 1H), 4.10 (AB system, J = 14.8 Hz, 2H), 3.64–3.58 (m, 1H), 3.50 (ddd, J = 11.2, 8.1, 6.6 Hz, 1H), 2.36–2.23 (m, 2H), 1.32, 1.316, 1.309, 1.299 (4 × d, each J = 6.2 Hz, 3H); 13C NMR (150.93 MHz, CDCl3): δ 166.2 (d, J = 4.9 Hz), 72.2 (d, J = 6.7 Hz), 72.0 (d, J = 7.2 Hz), 67.5 (d, J = 171.4 Hz), 40.5, 40.1 (d, J = 14.4 Hz), 32.6, 24.1 (d, J = 3.5 Hz), 24.0 (d, J = 3.9 Hz), 23.95 (d, J = 5.0 Hz), 23.8 (d, J = 5.1 Hz); 31P NMR (242.99 MHz, CDCl3): δ 16.3. Anal. Calcd for C11H21Cl2O5P: C, 39.42; H, 6.32; O, 23.87. Found: C, 39.39; H, 6.23; O, 24.06.

Resolution of Racemic Diisopropyl 3-Chloro-1-(chloroacetoxy)propylphosphonate [(±)-21]

(±)-3-Chloro-1-(chloroacetoxy)propylphosphonate (±)-21 (5.463 g, 16.3 mmol) was dissolved in a mixture of t-BuOMe and hexanes (36 mL, 1:1) and phosphate buffer (25 mM, 120 mL).[14] After the pH had been adjusted to 7.0 using the autotitrator, lipase from Thermomyces lanuginosus (0.4 mL, ≥100 000 U/g, [3.1.1.3], Sigma) was added. The mixture was vigorously stirred at room temperature, and pH of 7.0 was maintained by the addition of NaOH (0.5 M). At a conversion of 40% by consumption of the base (13.04 mL), the pH was brought to 4 by the addition of HCl (2 M). The reaction mixture was extracted with EtOAc (3 × 100 mL). The combined organic phases were washed with a saturated aq solution of NaHCO3 (50 mL), dried (MgSO4), and concentrated under reduced pressure. The residue was purified by flash chromatography (hexanes/EtOAc, 1:2, chloroacetate: Rf 0.44; hydroxyphosphonate: Rf 0.15) to give chloroacetate (R)-21 {2.918 g, 53%; [α]D21 −21.4 (c 1.01, acetone)} and hydroxyphosphonate (S)-20 {1.332 g, 5.14 mmol, 63%; [α]D21 +31.7 (c 1.67, acetone)} as colorless liquids. The ee of 97% for (+)-22 was determined by using(R)-(+)-(t-Bu)(Ph)P(O)(SH) as CSA and 31P NMR spectroscopy:[29,30] major singlet (1.00) at 22.9 ppm and minor one (0.015) at 22.7 ppm.

(±)- and (R)-(−)-Diisopropyl 3-Chloro-1-(phthalimidooxy)propylphosphonate [(±)- and (R)-22]

Dry toluene (20 mL) was added to a mixture of (±)-3-chloro-1-hydroxypropylphosphonate (±)-20 (1.484 g, 5.74 mmol), Ph3P (2.107 g, 8.03 mmol, 1.4 equiv), and N-hydroxyphthalimide (1.310 g, 8.03 mmol, 1.4 equiv) under argon atmosphere. A solution of di-tert-butyl azodicarboxylate (1.850 g, 8.03 mmol, 1.4 equiv) in dry THF (6 mL) was dropwise added under cooling with water at room temperature. After stirring for 2 h, while the color of the reaction mixture had changed from an intense red brown to a faint orange brown, water was added (10 drops). Ten minutes later, the reaction mixture was concentrated under reduced pressure. The residue was flash-chromatographed (heptanes/EtOAc, 1:1; Rf 0.33) to yield phthalimidooxyphosphonate (±)-22 (2.05 g, 88%) as a colorless heavy oil, which eventually crystallized; mp 94–95 °C (CH2Cl2/heptanes). Similarly, (S)-3-chloro-1-hydroxypropylphosphonate (S)-20 (575 mg, 2.5 mmol; ee 96%) was converted to (R)-(−)-3-chloro-1-(phthalimidooxy)propylphosphonate (R)-22 (727 mg, 72%); [α]D15 −28.8 (c 2.4, acetone); mp 92–93 °C (hexanes/EtOAc); ee 95%, determined by chiral HPLC (Figure S6). The NMR spectra of (±)- and (R)-22 were identical.

IR (ATR) of crystalline (±)-24 ν: 2977, 1788, 1729, 1374, 1358, 1273, 1251, 1188, 1173, 1135, 1123, 1107, 982 cm–1. 1H NMR (400.27 MHz, CDCl3): δ 7.83–7.77 (m, 2H), 7.74–7.68 (m, 2H), 4.98 (septd, J = 7.3, 6.2 Hz, 1H), 4.80–4.67 (m, 2H), 3.95 (td, J = 11.0, 7.3 Hz, 1H), 3.84 (dddd, J = 11.0, 6.7, 5.8, 0.9 Hz, 1H), 2.46–2.29 (m, 2H), 1.38 (d, J = 6.2 Hz, 3H), 1.33 (d, J = 6.2 Hz, 3H), 1.32 (d, J = 6.2 Hz, 3H), 1.31 (d, J = 6.9 Hz, 3H); 13C NMR (100.65 MHz, CDCl3): δ 163.0 (2C), 134.5 (2C), 128.9 (2C), 123.6 (2C), 79.3 (d, J = 163.8 Hz), 72.5 (d, J = 6.7 Hz), 71.9 (d, J = 7.1 Hz), 40.7 (d, J = 12.4 Hz), 33.0, 24.249 (d, J = 2.9 Hz), 24.01 (d, J = 3.8 Hz, CH3), 23.98 (d, J = 4.7 Hz), 23.8 (d, J = 5.7 Hz); 31P NMR (162.04 MHz, CDCl3): δ 15.0. Anal. Calcd for C17H23ClNO6P: C, 50.57; H, 5.74; N, 3.47; O, 23.77. Found: C, 50.69; H, 5.52; N, 3.45; O, 23.79.

(±)- and (R)-(+)-Isoxazolidin-5-ylphosphonic Acid [(±)- and (R)-(+)-3]

(±)-3-Chloro-1-(phthalimidooxy)propylphosphonate (±)-22 (848 mg, 2.1 mmol) was converted to (±)-isoxazolidin-5-ylphosphonic acid (±)-3 (259 mg, 81%) by the same procedure as used for the preparation of (±)-isoxazolidin-3-ylphosphonic acid (±)-2 from the respective nosylate (±)-13; mp 173 °C (decomp) (H2O/iPrOH). Similarly, protected (R)-(−)-α-aminooxyphosphonate (R)-22 (662 mg, 1.64 mmol) was converted to (R)-(+)-isoxazolidin-5-ylphosphonic acid [(R)-3] (159 mg, 63%); [α]D15 +20.3 (c 1.67, H2O); mp 192 °C (decomp) (H2O/EtOH); ee 97.2% before crystallization (Figure S3). The NMR spectra of (±)- and (R)-(+)-3 were identical.

IR (ATR) of (±)-3 ν: 3250–1500 (very br), 1453, 1283, 1240, 1136, 1077, 1024, 949, 928, 907, 890 cm–1. 1H NMR (400.27 MHz, D2O): δ 4.47 (dd, J = 9.5, 7.0 Hz, 1H), 3.84 (ddd, J = 10.9, 8.5, 4.2 Hz, 1H), 3.71 (td, J = 10.9, 8.1 Hz, 1H), 2.89–2.77 (m, 1H), 2.66–2.51 (m, 1H); 13C NMR (150.93 MHz, D2O): δ 78.4 (d, J = 155.9 Hz), 47.0 (d, J = 7.8 Hz), 30.0; 31P NMR (162.04 MHz, D2O): δ 10.8. Anal. Calcd for C3H8NO6P: C, 23.54; H, 5.27; N, 9.15; O, 41.81. Found: C, 23.57; H, 5.23; N, 9.01; O, 41.38.

(±)-Diisopropyl 3-Chloro-1-(trifluoromethanesulfonyloxy)propylphosphonate [(±)-24]

(±)-3-Chloro-1-hydroxypropylphosphonate (±)-20 (573 mg, 2.22 mmol, dried by azeotropic distillation with toluene) was dissolved in dry Et2O (9 mL) and cooled at −78 °C under an Ar atmosphere. n-BuLi (0.98 mL, 2.5 M in hexanes, 2.44 mmol, 1.1 equiv) was added, and the solution was stirred for 5 min. Then, (CF3SO2)2O (691 mg, 0.41 mL, 2.44 mmol, 1.1 equiv) was added, and the reaction mixture was stirred for 1 h at −78 °C. The reaction was quenched by adding water (5 mL) and a saturated aq solution of NaHCO3 (5 mL), and stirring was continued for 5 min. EtOAc (20 mL) was added, and the phases were separated. The aq layer was extracted with EtOAc (2 × 15 mL), and the combined organic phases were dried (MgSO4) and concentrated under reduced pressure. The residue was purified by flash chromatography (heptanes/EtOAc, 2:1, Rf 0.40) to yield racemic 3-chloro-1-(trifluoromethanesulfonyloxy)propylphosphonate (±)-24 (618 mg, 71%) as a colorless oil.

IR (ATR) ν: 2984, 1414, 1242, 1205, 1140, 1103, 986, 923 cm–1. 1H NMR (400.27 MHz, CDCl3): δ 5.15 (td, J = 8.2, 5.0 Hz, 1H), 4.88–4.73 (m, 2H), 3.78–3.70 (m, 1H, 1H), 3.60 (ddd, J = 11.5, 7.5, 6.4 Hz, 1H), 2.53–2.33 (m, 2H), 1.369 (d, J = 6.0 Hz, 3H), 1.362 (d, J = 6.2 Hz, 6H), 1.356 (d, J = 5.9 Hz, 3H); 13C NMR (100.65 MHz, CDCl3): δ 118.3 (q, J = 319.6 Hz), 78.7 (d, J = 170.8 Hz), 73.5 (d, J = 6.9 Hz), 73.3 (d, J = 7.3 Hz), 39.1 (d, J = 10.7 Hz), 33.5, 24.0 (d, J = 3.7 Hz), 23.9 (d, J = 3.7 Hz), 23.8 (d, J = 4.6 Hz), 23.6 (d, J = 5.0 Hz); 31P NMR (162.03 MHz, CDCl3): δ 11.9. Anal. Calcd for C10H19ClF3O6PS: C, 30.74; H, 4.90; O, 24.57; S, 8.21. Found: C, 30.60; H, 4.98; O, 23.45; S, 8.24.

(±)-Diisopropyl 3-Bromo-1-hydroxypropylphosphonate [(±)-27]

Ethyl 3-bromopropionate (5.431 g, 30 mmol) was converted to (±)-3-bromo-1-hydroxypropylphosphonate (±)-27 (6.729 g, 74%) by the procedure used for the preparation of the chloro analogue (±)-20; flash chromatography (hexanes/EtOAc, 1:3; Rf 0.24).

IR (ATR) ν: 3267, 2979, 1386, 1376, 1253, 1222, 1106, 987 cm–1. 1H NMR (400.27 MHz, CDCl3): δ 4.80–4.65 (m, 2H), 4.04–3.92 (m, 1H), 3.59 (dd, J = 7.9, 5.7 Hz, 2H), 3.29 (br s, 1H), 2.28–2.10 (m, 2H), 1.32 (d, J = 6.2 Hz, 6H), 1.31 (d, J = 6.2 Hz, 6H); 13C NMR (100.61 MHz, CDCl3): δ 71.6 (d, J = 7.4 Hz), 71.4 (d, J = 7.3 Hz), 65.7 (d, J = 165.0 Hz), 34.5 (d, J = 2.5 Hz), 29.9 (d, J = 17.1 Hz), 24.1 (d, J = 3.2 Hz), 24.1 (d J = 3.3 Hz), 24.0 (d, J = 4.4 Hz), 23.9 (d, J = 4.3 Hz); 31P NMR (162.03 MHz, CDCl3): δ 22.4. Anal. Calcd for C9H20BrO4P: C, 35.66; H, 6.65. Found: C, 35.91; H, 6.50.

(±)-Diisopropyl 3-Bromo-1-(chloroacetoxy)propylphosphonate [(±)-30]

Racemic 3-bromo-1-hydroxypropylphosphonate (±)-27 (7.578 g, 25.0 mmol) was converted to (±)-3-bromo-1-(chloroacetoxy)propylphosphonate (±)-30 (7.022 g, 74%; hexanes/EtOAc, 1:1; Rf 0.33) by the procedure used for the preparation of (±)-21.

IR (ATR) ν: 2981, 1725, 1387, 1377, 1181, 995 cm–1. 1H NMR (400.13 MHz, CDCl3): δ 5.39 (ddd, J = 9.1, 8.1, 5.2 Hz, 1H), 4.79 (sepd, J = 7.1, 6.2 Hz, 2H), 4.09 (AB-system, J = 14.8 Hz, 2H), 3.45 (dddd, J = 10.3, 6.5, 5.8, 1.1 Hz, 1H), 3.34 (td, J = 10.3, 7.5 Hz, 1H), 2.43–2.33 (m, 2H), 1.32 (d, J = 6.2 Hz, 3H), 1.312 (d, J = 6.2 Hz, 3H), 1.31 (d, J = 6.2 Hz, 3H), 1.30 (d, J = 6.2 Hz, 3H); 13C NMR (100.61 MHz, CDCl3): δ 166.2 (d, J = 4.8 Hz), 72.3 (d, J = 6.8 Hz), 72.0 (d, J = 7.3 Hz), 68.6 (d, J = 171.0 Hz), 40.5, 32.8, 27.8 (d, J = 14.6 Hz), 24.0 (d, J = 3.6 Hz), 24. (d, J = 3.0 Hz), 23.95 (d, J = 4.6 Hz), 23.8 (d, J = 5.0 Hz); 31P NMR (162.03 MHz, CDCl3): δ 16.0. Anal. Calcd for C11H21BrClO5P: C, 34.80; H, 5.58. Found: C, 34.52; H, 5.36.

Lipase-Catalyzed Resolution of Racemic Diisopropyl 3-Bromo-1-(chloroacetoxy)propylphosphonate [(±)-30]

(±)-3-Bromo-1-(chloroacetoxy)propylphosphonate (±)-30 (4.144 g, 10.92 mmol) was enzymatically resolved (15 mL of t-BuOMe, 15 mL of hexanes, 50 mL of 25 mM phosphate buffer pH 7.0, room temperature; stopped after addition of 8.37 mL of 0.5 M NaOH, corresponding to 40% conversion in 7 h 40 min) using lipase from T. lanuginosus (0.60 mL) by the method used for racemic 1-chloroacetoxy-3-butenylphosphonate (±)-21. The crude mixture of hydroxyphosphonate and chloroacetate (molar ratio by 31P NMR: 37:63) was flash-chromatographed (hexanes/EtOAc, 1:2, chloroacetate: Rf 0.44, hydroxyphosphonate: Rf 0.15) to give (R)-(−)-chloroacetate (R)-30 {2.40 g, 58%; [α]D26 −24.9 (c 2.04, acetone)} and (S)-(+)-hydroxyphosphonate (S)-27 {1.171 g, 3.86 mmol, 35%; [α]D26 +29.5 (c 2.37, acetone), 95% ee and (S)-configuration by 31P NMR spectroscopy of (R)-(+)-Mosher ester} as colorless oils.

(±)-Diisopropyl 3-Bromo-1-(trifluoromethanesulfonyloxy)propylphosphonate [(±)-28a]

(±)-3-Bromo-1-hydroxypropylphosphonate (±)-27 (1.515 g, 5.0 mmol) was converted to racemic 3-bromo-1-(trifluoromethanesulfonyloxy)propylphosphonate (±)-28a (1.8.71 g, 86%) as a colorless oil by the procedure used for the preparation of the chloro analogue (±)-24.

IR (ATR) ν: 2984, 1414, 1242, 1204, 1140, 1102, 988, 921 cm–1. 1H NMR (400.27 MHz, CDCl3): δ 5.11 (td, J = 8.2, 5.2 Hz, 1H), 4.86–4.71 (m, 2H), 3.61–3.53 (m, 1H), 3.41 (td, J = 10.6, 7.3 Hz, 1H), 2.60–2.39 (m, 2H), 1.36 (d, J = 6.2 Hz, 3H), 1.36 (d, J = 6.2 Hz, 6H), 1.34 (d, J = 5.2 Hz, 3H); 13C NMR (150.93 MHz, CDCl3): δ 118.2 (q, J = 319.5 Hz), 79.7 (d, J = 170.4 Hz), 73.5 (d, J = 6.8 Hz), 73.3 (d, J = 7.3 Hz), 33.6, 26.7 (d, J = 11.1 Hz), 24.0 (d, J = 3.8 Hz), 23.9 (d, J = 3.7 Hz), 23.85 (d, J = 4.5 Hz), 23.6 (d, J = 5.0 Hz); 31P NMR (CDCl3, 162.03 MHz): δ 11.7. Anal. Calcd for C10H19BrF3O6PS: C, 27.60; H, 4.40; O, 22.06; S, 7.37. Found: C, 27.69; H, 4.44; O, 22.22; S, 7.42.

(±)-Diisopropyl 3-Bromo-1-(methanesulfonyloxy)propylphosphonate [(±)-28b]

Et3N (1.518 g, 2.08 mL, 15.0 mmol, 1.5 equiv) and CH3SO2Cl (1.489 g, 1.01 mL, 13.0 mmol, 1.3 equiv, dissolved in 5 mL of dry CH2Cl2) were added to a stirred solution of racemic 3-bromo-1-hydroxypropylphosphonate (±)-27 (3.031 g, 10 mmol) in dry CH2Cl2 (35 mL) at 0 °C under Ar. After 1 h, the cooling bath was removed, and water (10 mL) was added. Stirring was continued for 10 min, before the organic phase was separated, and the aq one was extracted with CH2Cl2 (2 × 15 mL). The combined organic layers were washed with HCl (2 M, 10 mL), washed with water (5 mL), then dried (MgSO4), and concentrated under reduced pressure. The residue was purified by flash chromatography (heptanes/EtOAc, 1:1, Rf 0.37) to yield mesyloxyphosphonate (±)-28b (3.535 g, 93%) as a colorless oil.

IR (ATR) ν: 2981, 2936, 1359, 1260, 1240, 1174, 979 cm–1. 1H NMR (400.27 MHz, CDCl3): δ 4.90 (td, J = 8.5, 4.7 Hz, 1H), 4.83–4.68 (m, 2H), 3.59–3.51 (m, 1H), 3.49–3.40 (m, 1H), 3.20 (s, 3H), 2.44–2.28 (m, 2H), 1.34 (d, J = 6.2 Hz, 9H), 1.35 (d, J = 6.2 Hz, 3H); 13C NMR (150.93 MHz, CDCl3): δ 75.1 (d, J = 171.8 Hz), 72.7 (d, J = 6.8 Hz), 72.5 (d, J = 6.9 Hz), 39.1, 34.0, 27.8 (d, J = 14.8 Hz), 24.1 (d, J = 3.5 Hz), 24.0 (d, J = 4.0 Hz), 23.95 (d, J = 5.0 Hz), 23.8 (d, J = 5.0 Hz); 31P NMR (242.99 MHz, CDCl3): δ 14.9. Anal. Calcd for C10H22BrO6PS: C, 31.51; H, 5.82; O, 25.18; S, 8.41. Found: C, 31.53; H, 5.76; O, 25.43; S, 8.61.

(±)- and (S)-(+)-Diisopropyl 3-Bromo-1-(4-nitrobenzenesulfonyloxy)propylphosphonate [(±)- and (S)-(+)-28c]

A solution of 4-nitrobenzenesulfonyl chloride (1.441 g, 6.5 mmol, 1.3 equiv, = NosCl) and DMAP (40 mg, 0.33 mmol) in dry CH2Cl2 (5 mL) and Et3N (1.214 g, 1.66 mL, 12 mmol, 2.4 equiv) was added to a stirred solution of racemic 3-bromo-1-hydroxypropylphosphonate (±)-27 (1.516 g, 5 mmol, dried by evaporation of a solution in dry toluene) in dry CH2Cl2 (20 mL) at 0 °C under Ar. The reaction mixture was stirred for 4.5 h at 0 °C and then quenched with H2O (5 mL) and concd HCl. (1 mL) 10 min later. The phases were separated, and the aqueous one was extracted with CH2Cl2 (2 × 20 mL). The combined organic phases were washed with a saturated aqueous solution of NaHCO3 (30 mL), dried (MgSO4), and concentrated under reduced pressure. The residue was purified by flash chromatography (hexanes/EtOAc, 1:1, Rf 0.22) to give nosylate (±)-28c (2.148 g, 86%) as a yellowish oil. Similarly, (S)-(+)-3-bromo-1-hydroxypropylphosphonate (S)-27 {1.649 g, 5.44 mmol; ee 95% by Mosher; [α]D18 +30.5 (c 0.95, acetone)} was converted to (S)-(+)-nosyloxyphosphonate (S)-28c {2.313 g, 87, 95.4% ee by chiral HPLC (Figure S7); [α]D23 +19.7 (c 1.86, acetone)}. The spectroscopic data of (±)- and (S)-28c were identical.

IR (ATR) ν: 2982, 1536, 1404, 1377, 1261, 1188, 992 cm–1. 1H NMR (400.13 MHz, CDCl3): δ 8.40–8.35 (m, 2H), 8.22–8.17 (m, 2H), 5.06 (ddd, J = 9.4, 8.3, 4.7 Hz, 1H), 4.70 (sepd, J = 7.0, 6.2 Hz, 1H), 4.64 (sepd, J = 7.2, 6.2 Hz, 1H), 3.54 (ddd, J = 10.4, 6.4, 5.8 Hz, 1H), 3.35 (ddd, J = 10.4, 8.2, 7.0 Hz, 1H), 2.46–2.29 (m, 2H), 1.30 (d, J = 6.2 Hz, 3H), 1.19 (d, J = 6.2 Hz, 3H), 1.27 (d, J = 6.2 Hz, 3H), 1.24 (d, J = 6.2 Hz, 3H); 13C NMR (100.61 MHz, CDCl3): δ 150.8, 142.2, 129.6 (2C), 124.2 (2C), 75.6 (d, J = 172.2 Hz), 72.7 (d, J = 6.8 Hz, 2C), 33.8, 27.9 (d, J = 13.0 Hz), 24.0 (d, J = 4.0 Hz), 23.97 (d, J = 4.9 Hz), 23.9 (d, J = 4.9 Hz), 23.7 (d, J = 4.8 Hz); 31P NMR (161.98 MHz, CDCl3): δ 15.1. Anal. Calcd for C15H23BrNO8PS: C, 36.90; H, 4.75; S, 6.57. Found: C, 37.09; H, 4.75; S, 6.59.

(±)-Diisopropyl 3-[(N,N′-Di-tert-butoxycarbonyl)hydrazino]-1-(methanesulfonyloxy)propylphosphonate [(±)-29b]

Bis(Boc)-hydrazine[31] (799 mg, 3.42 mmol, 1.5 equiv), Bu4NHSO4 (116 mg, 0.34 mmol, 0.15 equiv), toluene (10 mL), and NaOH (4 mL, 20 w/v %) were added to mesyloxyphosphonate (±)-28b (870 mg, 2.28 mmol) at 0 °C. The mixture was stirred vigorously. After 1 h, a sample for the 31P NMR spectrum was withdrawn and diluted with CDCl3. As only the starting product was detected, the cooling bath was replaced by a water bath of 19 °C (the temperature was not allowed to surpass 20 °C). After 2 h at 19–20 °C, another sample was withdrawn for 31P NMR spectroscopy. This time, beside the resonance for the starting material (δP 15.3), a new signal (δP 16.1) appeared which was assigned to the hydrazine derivative (±)-29b (their ratio was 1.0:0.54). Within 5 h at 19–20 °C, the starting material decreased to 1%. Now, the reaction mixture was worked up. EtOAc (20 mL) and a saturated aq solution of NaHCO3 (20 mL) were added. The organic phase was removed, and the aq one was extracted with EtOAc (2 × 10 mL). The combined organic layers were washed with HCl (10 mL, 1 M), dried (Na2SO4), and concentrated under reduced pressure. The residue [(±)-25/(±)-29b/unknown compound/(±)-28b = 4:100:16:1, by 31P NMR] was purified by flash chromatography (heptanes/EtOAc, 1:1, Rf 0.15, starting material 0.22) to give N,N′-bis(Boc)-hydrazinophosphonate (±)-29b (911 mg, 75%) as a colorless gum.

IR (ATR) ν: 3254, 2979, 2935, 1707, 1363, 1248, 1173, 1143, 985, 933 cm–1. 1H NMR (500.32 MHz, toluene-d8, 80 °C): δ 6.26 (br s, 1H), 4.94 (br s, 1H), 4.73–4.51 (m, 2H), 3.83–3.71 (m, 1H), 3.67–3.57 (m, 1H), 2.77–2.70 (m, 3H), 2.43–2.30 (m, 1H), 2.20–2.08 (m, 1H), 1.45–1.40 (m, 9H), 1.40–1.36 (m, 9H), 1.21–1.07 (m, 12H); 31P NMR (202.53 MHz, toluene-d8, 80 °C): δ 16.1, 15.2; ratio: 91:9. Anal. Calcd for C20H41N2O10PS: C, 45.10; H, 7.76; N, 5.26; O, 30.04; S, 6.02. Found: C, 44.71; H, 8.10; N, 4.94; O, 29.59; S, 5.76.

(±)-Diisopropyl 1,2-(Bis-tert-butoxycarbonyl)pyrazolidin-3-ylphosphonate [(±)-25] from (±)-29b

Bu4NHSO4 (64 mg, 0.19 mmol, 0.15 equiv), toluene (5 mL), and NaOH (2 mL, 20 w/v %) were added to 3-hydrazinophosphonate (±)-29b (666 mg, 1.25 mmol). The mixture was heated at 80 °C and stirred vigorously. After 3 h, the mixture was cooled and diluted with EtOAc (20 mL) and water (8 mL). The organic phase was separated, and the aq one was extracted with EtOAc (2 × 15 mL). The combined organic layers were washed with HCl (10 mL, 1 M), dried (Na2SO4), and concentrated under reduced pressure. The residue was flash-chromatographed (heptanes/EtOAc, 1:2, Rf 0.29) to furnish protected pyrazolidinylphosphonate (±)-25 (411 mg, 75%) as a colorless, very viscous oil.

IR (ATR) of (±)-25 ν: 2977, 1702, 1366, 1252, 1166, 1141, 986 cm–1. 1H NMR (400.13 MHz, CDCl3, 50 °C): δ 4.84 (oct, J = 6.2 Hz, 1H), 4.71 (sepd, J = 7.3, 6.2 Hz, 1H), 4.42 (td, J = 9.6, 3.3 Hz, 1H), 3.97 (td, J = 9.5, 7.1 Hz, 1H), 3.21 (td, J = 9.8, 5.3 Hz, 1H), 2.44–2.30 (m, 1H), 2.30–2.11 (m, 1H), 1.46 (s, 9H), 1.45 (s, 9H), 1.31 (d, J = 6.2 Hz, 3H), 1.30 (d, J = 6.2 Hz, 6H), 1.29 (d, J = 6.2 Hz, 3H); 13C NMR (100.61 MHz, CDCl3, 50 °C): δ 156.6, 156.5, 81.8, 80.7, 72.0 (d, J = 7.0 Hz), 70.8 (d, J = 7.2 Hz), 54.1 (d, J = 173.3 Hz), 45.3 (br s), 28.3 (3C), 28.1 (3C), 27.2 (d, J = 1.1 Hz), 24.4 (d, J = 2.4 Hz), 24.1 (d, J = 4.8 Hz), 24.0 (d, J = 3.6 Hz), 23.7 (d, J = 6.1 Hz); 31P NMR (162.04 MHz, CDCl3, 50 °C): δ 21.8. Anal. Calcd for C19H37N2O7P: C, 52.28; H, 8.54; N, 6.42. Found: C, 51.99; H, 8.40; N, 6.34.

(±)- and (R)-(−)-Diisopropyl 1,2-(Bis-tert-butoxycarbonyl)pyrazolidin-3-ylphosphonate [(±)- and (R)-25] from 28c

Method A

(±)-3-Bromo-1-(4-nitrobenzenesulfonyloxy)propylphosphonate (±)-28c (1.465 g, 3 mmol) was dissolved in toluene (10 mL) under Ar. An aq solution of NaOH (6 mL, 20 w/v %), BocNH–NHBoc (1.394 g, 6 mmol) and Bu4NHSO4 (290 mg, 0.9 mmol, dissolved in 5 mL of toluene) was added at room temperature. The reaction mixture was vigorously stirred and heated at 50 °C for 7 h. After cooling to room temperature, the reaction mixture was diluted with EtOAc (10 mL) and neutralized with HCl (2 M). The organic phase was separated, and the aq one was extracted with EtOAc (2 × 20 mL). The combined organic layers were washed with brine, dried (MgSO4), and concentrated under reduced pressure. The residue was purified by flash chromatography (hexanes/EtOAc, 1:1, Rf 0.15) to yield pyrazolidin-3-ylphosphonate (±)-25 (728 mg, 56%) as a yellowish gum.

Method B

BocNH–NHBoc (1.237 g, 5 mmol, 2.5 equiv) and KOtBu (561 mg, 5 mmol, 2.5 equiv) were dissolved in dry DMF (5 mL) under Ar atmosphere. A solution of (±)-3-bromo-1-(4-nitrobenzenesulfonyloxy)propylphosphonate (±)-28c (977 mg, 2 mmol) in dry DMF (5 mL) was added at 0 °C. The reaction mixture was stirred for 1.5 h at 0 °C and at room temperature until the starting material was virtually consumed (1.5 h). Acetic acid (6 drops) was added, and the volatile components were removed under reduced pressure (0.5 mbar). The residue was taken up in H2O/EtOAc (30 mL, 1:1). The organic phase was separated, and the aq one was extracted with EtOAc (2 × 10 mL). The combined organic phases were washed with brine, dried (MgSO4), and concentrated under reduced pressure. Flash chromatography (hexanes/EtOAc, 3:2, Rf 0.23) of the residue gave the desired racemic pyrazolidin-3-ylphosphonate (±)-25 (570 mg, 65%) as a yellowish oil.

Cyclization by Method B with Alternative Work Up

(±)-Bromonosylate (±)-28c (2.300 g, 4.71 mmol) was converted to protected pyrazolidin-3-ylphosphonate (±)-25. When cyclization was finished, the reaction mixture was cooled to room temperature, and HCl (2 M, 25 mL) and water (25 mL) were added (color changed from dark red/brown to yellow). The mixture was extracted with EtOAc (4 × 25 mL). The combined organic phases were washed with HCl (1 M, 2 × 20 mL), NaHCO3 (saturated aq solution, 10 mL), dried (MgSO4), and concentrated under reduced pressure. Flash chromatography of the residue gave the desired (±)-pyrazolidin-3-ylphosphonate (±)-25 (1.250 g, 61%) as a yellowish oil.

(S)-(+)-Bromonosylate (S)-28c {1.464 g, 3 mmol; [α]D23 +19.7 (c 1.86, acetone); ee 96% by chiral HPLC} was converted to protected (R)-(−)-pyrazolidin-3-ylphosphonate (R)-25 {635 mg, 49%; [α]D26 −25.5 (c 1.40, acetone)} by method A. The spectroscopic data were identical to those of the racemate.

(±)- and (R)-(+)-Pyrazolidin-3-ylphosphonic Acid [(±)- and (R)-4]

AllylTMS (1.333 g, 1.85 mL, 11.67 mmol, 3 equiv) and TMSBr (5.955 g, 5.13 mL, 38.9 mmol, 10 equiv) were added to a solution of protected (±)-pyrazolidin-3-ylphosphonate (±)-25 (1.700 g, 3.89 mmol) in dry DCE (20 mL) under Ar atmosphere.[26] After stirring at 50 °C for 14 h and cooling to room temperature, volatile components were removed under reduced pressure (0.5 mbar). The residue was dissolved in DCE (10 mL), and the solvent was again removed under reduced pressure (0.5 mbar). Water (20 mL) was added to the residue, and the mixture was stirred for 10 min before it was extracted with EtOAc (2 × 15 mL). The aq phase was concentrated (5 mL) and applied to a column filled with Dowex 50W × 8, H+ (o. d. 1.5 × 14 cm, water as eluent, fractions of 25 mL). Fractions containing the product (TLC: Rf 0.42) were pooled and concentrated under reduced pressure to give (±)-pyrazolidinylphosphonic acid [(±)-4] (323 mg, 55%); mp 129–131 °C (H2O/EtOH). Similarly, protected (R)-(−)-pyrazolidin-3-ylphosphonate (R)-25 {739 mg, 1.69 mmol; [α]D26 −25.5 (c 1.40, acetone)} was converted to (R)-(+)-pyrazolidin-3-ylphosphonic acid [(R)-4] {136 mg, 53%; [α]D18 +5.09 (c 0.51, H2O)} by the procedure used for the racemic compound; mp 118–119 °C (H2O/EtOH); ee 92.2% before crystallization, after crystallization 98.8% ee (Figure S4). The NMR data of (±)-and (R)-4 were identical.

IR (ATR) of (±)-4 ν: 3277, 1626, 1446, 1234, 1190, 1145, 1028, 957, 924, 894 cm–1. 1H NMR (400.13 MHz, D2O): δ 3.56 (ddd, J = 11.3, 9.2, 4.6 Hz, 1H), 3.53 (td, J = 8.8, 8.0 Hz, 1H), 3.45 (td, J = 11.3, 8.2 Hz, 1H), 2.56–2.44 (m, 1H), 2.34–2.19 (m, 1H); 13C NMR (100.61 MHz, D2O): δ 56.6 (d, J = 151.2 Hz), 47.3 (d, J = 8.1 Hz), 28.327; 31P NMR (161.98 MHz, D2O): δ 16.13. Anal. Calcd for C3H9N2O3P: C, 23.69; H, 5.96; N, 18.42. Found for (±)-4: C, 23.60; H, 5.70; N, 18.34.

P5C Reductase Purification and Assay

A. thaliana P5C reductase was purified from a suspension of cultured cells, as previously described.[34] Enzyme activity was measured at 35 °C as the P5C-dependent oxidation of NADH. The assay mixture contained 50 mM Tris-HCl buffer, pH 7.5, 0.4 mM NADH, 2 mM dl–P5C and 1 mM MgCl2 in a final volume of 1 mL. A limiting amount of enzyme (about 0.2 nkat) was added to the prewarmed mixture, and the decrease in absorbance at 340 nm was determined for up to 10 min by continuous monitoring of the sample. Activity was calculated on the assumption of an extinction coefficient for NADH of 6220 M–1 cm–1. The concentrations causing 50% inhibition (IC50) of P5C reductase activity and their confidence intervals were estimated by nonlinear regression analysis using Prism 6 for Windows (version 6.07; GraphPad Software Inc., San Diego, CA, USA).