Synthesis of screening substrates for the directed evolution of sialic acid aldolase: towards tailored enzymes for the preparation of influenza A sialidase inhibitor analogues

The stereoselective synthesis of two epimeric screening substrates, (4R, 5R, 6R)- and (4S, 5R, 6R)-6-dipropylcarbamoyl-2-oxo-4,5,6-trihydroxy-hexanoic acid, for the directed evolution of sialic acid aldolase is described. The complementary methods relied on stereoselective indium-mediated additions of ethyl alpha-bromomethyl acrylate to functionalised aldehydes. With an alpha-hydroxy aldehyde, (2R, 3R)-2,3-dihydroxy-4-oxo butanoic acid dipropylamide, the addition was chelation controlled, and the syn product, (6R, 5R, 4S)-6-dipropylcarbamoyl-2-methylidene-4,5,6-trihydroxy-hexanoic acid ethyl ester, was obtained. In contrast, the stereochemical outcome of the addition to (2R, 3R)-N,N-dipropyl-2,3-O-isopropylidene-4-oxobutyramide was consistent with Felkin-Anh control, and the anti adduct, (4R, 5R, 6R)-6-dipropylcarbamoyl-2-methylidene-4-hydroxy-5,6-O-isopropylidene-hexanoic acid ethyl ester, was the major product. Ozonolysis and deprotection gave the screening substrates as mixtures of furanose and pyranose forms, in good yields.     

Does activation of the anti proton, rather than concertedness, determine the stereochemistry of base-catalyzed 1,2-elimination reactions? Anti stereospecificity in E1cB eliminations of β-3-trifluoromethylphenoxy esters, thioesters, and ketones

As part of a comprehensive investigation on the stereochemical aspects of base-catalyzed 1,2-elimination reactions, we have studied a set of acyclic carbonyl substrates that react by an irreversible E1cB mechanism with largely anti stereospecificity. (2)H NMR data show that these reactions using KOH in EtOH/H(2)O under non-ion-pairing conditions produce a minimum of 85-89% anti elimination on stereospecifically labeled tert-butyl (2R*,3R*)- and (2R*,3S*)-3-(3-trifluoromethylphenoxy)-2,3-(2)H(2)-butanoate, S-tert-butyl (2R*,3R*)- and (2R*,3S*)-3-(3-trifluoromethylphenoxy)-2,3-(2)H(2)-butanethioate, and the related ketones, (4R*,5R*)- and (4R*,5S*)-5-(3-trifluoromethylphenoxy)-4,5-(2)H(2)-3-hexanone. With both diastereomers of each substrate available, the KIEs can be calculated and the innate stereoselectivities determined. The elimination reactions of the β-3-trifluoromethylphenoxy substrates occur by E1cB mechanisms with diffusionally equilibrated enolate-anion intermediates. Thus, it is clear that anti elimination does not depend solely upon concerted E2 mechanisms. Negative hyperconjugation provides a satisfactory explanation for the anti stereospecificity exhibited by our carbonyl substrates, where the leaving group activates the anti proton, leading to the enolate intermediate. The activation of the anti proton by negative hyperconjugation may also play a role in the concerted pathways of E2 mechanisms. We have also measured the rates of the hydroxide-catalyzed elimination reactions of butanoate, thiobutanoate, and ketone substrates in EtOH/H(2)O, with β-tosyloxy, acetoxy, and 3-trifluoromethylphenoxy nucleofuges.     

Highly enantioselective conjugate addition of nitroalkanes to alkylidenemalonates using efficient phase-transfer catalysis of N-spiro chiral ammonium bromides

Highly enantioselective conjugate addition of nitroalkanes to alkylidenemalonates has been accomplished for the first time by the utilization of efficient phase-transfer catalysis of N-spiro C2-symmetric chiral quaternary ammonium bromide 1. For instance, simple mixing of nitropropane (2, R1 = Et), diisopropyl benzylidienemalonate (3, R2 = Ph), Cs2CO3 (1 equiv), and (S,S)-1 (1 mol %) in toluene at 0 degrees C for 2.5 h gave rise to the desired conjugate addition product 4 (R1 = Et, R2 = Ph) quantitatively (anti/syn = 86:14) with 97% ee (anti isomer). The applicability of this procedure has been demonstrated with other representative alkylidenemalonates and nitroalkanes. Since 4 can be readily transformed into the corresponding gamma-amino acid hydrochloride 5 without loss of diastereo- and enantioselectivity, the present method provides a new and practical access to various optically active gamma-amino acid derivatives.     

3-Pyrrolidinecarboxylic acid for direct catalytic asymmetric anti-Mannich-type reactions of unmodified ketones

We report the development of direct catalytic, enantioselective, anti-selective Mannich-type reactions between unmodified ketones and alpha-imino esters under mild conditions. The reactions were performed using 5-10 mol % of (R)-3-pyrrolidinecarboxylic or (R)-beta-proline as catalyst in an environmentally benign solvent, 2-PrOH, at room temperature. The anti-Mannich products were obtained in good yields with high diastereo- and enantioselectivities (up to anti/syn >99:1, 99% ee). While (3R,5R)-5-methyl-3-pyrrolidinecarboxylic acid is an excellent catalyst for the anti-Mannich-type reactions of aldehydes, it did not efficiently catalyze the corresponding Mannich-type reactions of ketones; (R)-3-pyrrolidinecarboxylic acid did efficiently catalyze the Mannich-type reactions of ketones. (S)-Proline or (S)-2-pyrrolidinecarboxylic acid has been reported to catalyze the Mannich-type reactions of ketones to afford the syn-products. Thus, the position of the carboxylic acid group on the pyrrolidine ring directs the stereoselection of the catalyzed reaction, providing either syn- or anti-Mannich products.     

Additions of enantioenriched allenylzinc and indium reagents to lactic aldehyde ethers

Allenylzinc and indium reagents were generated in situ through Pd(0)-catalyzed metalation of (R)- and (S)-3-butyn-2-ol methanesulfonate with Et(2)Zn and InI. These reagents add to the benzyl and TBS ethers of (S)-lactic aldehyde to afford diastereomeric stereotriads in moderate to high yield. The (S)/(S) combination afforded the anti,anti adducts with 94:6-100:0 diastereoselectivity. The (R)/(S) combination was mismatched, affording a mixture of anti,syn and syn,anti adducts in diastereomeric ratios of ca. 80:20-85:15. Addition of the racemic allenylmetal reagents to the (S)-lactic aldehyde ethers afforded the products of matched and mismatched pairings in equal amount.     

酶催化拆分制备手性2-氯-1-(2,4-二氟苯基)乙醇及 光学活性抗真菌药物的合成

用Pseudomonas stutzeri脂肪酶催化转酯化反应动力学拆分外消旋体法制备高对映体纯的手性2-氯-1-(2,4-二氟苯基)乙醇, 得到95.8% ee值的(R)-异构体和94.5% ee值的(S)-异构体, 以此手性醇为关键中间体合成了4种具有抗真菌活性的光学活性化合物α-氯代苄氧基-β-(1-咪唑基)-2,4-二氟乙苯硝酸盐. 纸片扩散法测试体外抗真菌活性结果表明, 对各种念珠菌(Candida species) (R)-5a和(R)-5b具有与氟康唑相当的抗菌活性, 特别是对氟康唑耐药的烟曲霉菌(Aspergillus fumigatus), 5a, 5b及其两种光学活性异构体均有优异的抗菌活性, 而且(R)-异构体的活性明显高于(S)-异构体和外消旋体.     

Enhancement of diastereoselectivity in photodimerization of alkyl 2-naphthoates with chiral auxiliaries via inclusion within γ-cyclodextrin cavities

Irradiations of alkyl 2-naphthoates are known to result in four isomeric "cubane-like" photodimers: anti(HH)-2, syn(HH)-2, anti(HT)-2, and syn(HT)-2 where the anti(HH)-2, anti(HT)-2, and syn(HT)-2 consist of pairs of diastereomers. Here, chiral auxiliary and chiral microreactor strategies have been combined to achieve high diastereoselectivity in photodimerizations of an enantiomeric pair of 2-naphthoates with (R)- and (S)-1-methoxycarbonylethyl esters as chiral auxiliaries (1R and 1S). Thus, irradiations of their γ-cyclodextrin (γ-CD) complexes have been conducted. Fluorescence, IR, and NMR spectra of both enantiomers of 1 demonstrate that their γ-CD complexes are mainly 2:2 with the molecules of 1 in head-to-head orientations. Irradiation of the complexes in the solid state mainly resulted in anti(HH)-2. The absolute configuration of each diastereomer of anti(HH)-2 has been established for the first time here. The diastereomeric excesses (de's) of anti(HH)-2 from 1R and 1S were 94% and 86%, respectively. These de's are much higher than those found from irradiations in solution (55% for 1R and 1S), where the opposite diastereomeric form is in excess! Calculations of the energies of various conformations of the head-to-head 2:2 inclusion complexes were performed using the PM3 approach. The predicted major diastereomers based on the calculation are consistent with those found experimentally.     

Influence of carrier ligand NH hydrogen bonding to the O6 and phosphate group of guanine nucleotides in platinum complexes with a single guanine ligand

Coordinated N,N',N"-trimethyldiethylenetriamine (Me3dien) has several possible configurations: two have mirror symmetry (R,S configurations at the terminal nitrogens) and the terminal N-Me's anti or syn with respect to the central N-Me (anti-(R,S) and syn-(R,S) isomers, respectively), and two are nonsymmetrical (R,R and S,S configurations at terminal nitrogens, rac denotes a 1:1 mixture of the two isomers). For each configuration, two Me3dienPtG atropisomers can be formed (anti or syn orientation of central N-Me and G 06, G = guanine derivative), and these can be observed since the terminal N-Me's decrease the rate of G rotation about the Pt-N7 bond. In symmetrical syn-(R,S)-Me3dienPtG derivatives with G = 9-EtG and 3'-GMP, the anti rotamer, which can form O6-NH H-bonds, was slightly favored over the syn rotamer but never more than 2:1. This anti rotamer is also favored by lower steric repulsion between the terminal N-Me's and G O6; thus, the contribution of O6-NH H-bonding to the stability of the anti rotamer could be rather small. With G = 5'-GMP, an O6-NH H-bond in the anti rotamer and a phosphate-NH H-bond in the syn rotamer can form. Only the syn rotamer was detected in solution, indicating that NH H-bonds to 5'-phosphate are far more important than to O6, particularly since steric factors favor the anti rotamer. Interconversion between rotamers was faster for syn-(R,S)- than for rac-Me3dien derivatives. This appears to be determined by a smaller steric impediment to G rotation of two "quasi equatorial" N-Me's, both on one side of the platinum coordination plane (syn-(R,S) isomer), than one "quasi equatorial" and one "quasi axial" N-Me on either side of the coordination plane (rac isomer).     

Designer chiral quaternary ammonium bifluorides as an efficient catalyst for asymmetric nitroaldol reaction of silyl nitronates with aromatic aldehydes

Designer chiral quaternary ammonium bifluoride 1 has been prepared, and both its catalytic and its chiral efficiency have been clearly demonstrated by achieving the first catalytic asymmetric nitroaldol reaction of silyl nitronate with aldehydes. For instance, the reaction of trimethylsilyl nitronate 2 (R(1) = Me) with benzaldehyde (R(2) = Ph) in THF in the presence of (S,S)-1 (2 mol %) proceeded smoothly at -78 degrees C, giving the corresponding nitroaldol adduct 3 (R(1) = Me, R(2) = Ph) in 92% isolated yield (anti/syn = 92:8) with 95% ee (anti isomer). The method was found to be successfully applicable to other aromatic aldehydes and silyl nitronates, and a high level of anti selectivity and enantiomeric excess was constantly observed. This finding should lead to the further development of fluoride ion-catalyzed asymmetric carbon-carbon bond-forming reactions.     

Stereospecificity of an enzymatic monoene 1,4-dehydrogenation reaction: conversion of (Z)-11-tetradecenoic acid into (E,E)-10,12-tetradecadienoic acid

In this article, we report the first stereochemical study of an enzymatic 1,4-dehydrogenation reaction, namely, the transformation of (Z)-11-tetradecenoic acid into (E,E)-10,12-tetradecadienoic acid, involved in the sex pheromone biosynthesis of the moth Spodoptera littoralis. The investigation was carried out using the labeled substrates (R)-[10-(2)H]- and (S)-[10-(2)H]-tridecanoic acids ((R)-2 and (S)-2, respectively) and (R)-[2,2,3,3,13-(2)H(5)]- and (S)-[2,2,3,3,13-(2)H(5)]-tetradecanoic acids ((R)-1 and (S)-1, respectively). Probes (R)-2 and (S)-2 were prepared as described in a previous article.(1) The synthesis of the pentadeuterated chiral substrates (R)-1 and (S)-1 was accomplished by kinetic resolution of the racemic 12-tridecyn-2-ol (6) with immobilized porcine pancreatic lipase. The enantiomerically pure alcohols (R)-6 and (S)-6 were transformed into the final acids (S)-1 and (R)-1, respectively, by a sequence of well-established reactions. The analyses of methanolyzed lipidic extracts from glands incubated separatedly with each individual probe showed that in the transformation of (Z)-11-tetradecenoic acid into (E,E)-10,12-tetradecadienoic acid, both pro-(R) hydrogen atoms at C-10 and C-13 are removed from the substrate. This is the first example reported of a desaturase with pro-(R)/pro-(R) stereospecificities that gives rise to (E)-double bonds. A mechanistic explanation for the stereochemical outcome of this reaction is advanced.     

Imprinting structural information from a GpG ligand into the configuration of a chiral diamine ligand through second-sphere communication in platinum(II) complexes

Cisplatin forms the cis-Pt(NH3)2(d(GpG)) cross-link with DNA. We have recently created novel d(GpG) conformations by using "retro models" (complexes having bulky carrier ligands designed to slow d(GpG) dynamic motion). Our results define four conformer classes: HH1, HH2, delta HT1, and delta HT2, with a head-to-head or head-to-tail base orientation and a phosphodiester backbone with a normal (1) or opposite (2) propagation direction. Moreover, each G residue can be syn or anti, and the base canting can be left-handed (L) or right-handed (R). Thus, 32 variants of cis-Pt(NH3)2(d(GpG)) are conceivable, but the adduct is too dynamic to study. Thus far, by using retro models, we have obtained evidence for five variants with d(GpG) but only four with GpG. We therefore selected Me2DAPPt(GpG) complexes for study by 1H and 31P NMR spectroscopy, CD spectroscopy, and molecular mechanics and dynamics (MMD) calculations. Coordinated Me2DAP (N,N'-dimethyl-2,4-diaminopentane) has N, C, C, N chiral centers designated, for example, as R,R,R,R. This ligand has greater flexibility and more readily inverted N centers than ligands used previously in GpG retro models. One goal was to determine whether the GpG ligand can control the configuration of a carrier ligand. (R,R,R,R)-Me2DAPPt(GpG) forms the anti, anti HH1 R variant almost exclusively. Equal populations of the two possible linkage isomers of (S,R,R,R)-Me2DAPPt(GpG) are formed, both favoring the anti, anti HH1 R, variant; however, the isomer with the 5'-G cis to the S nitrogen has sharper signals, suggesting that interligand interactions are more favorable. Indeed, this linkage isomer was the major product of isomerization when (R,R,R,R)-Me2DAPPt(GpG) was kept at pH approximately 9.5 to allow N center equilibration. Steric clashes between the Me2DAP C-Me groups and the G O6 atoms found by MMD calculations appear to disfavor the HH1 conformer of (S,S,S,S)-Me2DAPPt(GpG) and (S,S,S,R)-Me2DAPPt(GpG) complexes. These two complexes have a significant population of the anti, syn delta HT1 conformer, as indicated by broad 1H NMR signals and by 31P NMR and CD data. Equilibration of (S,S,S,R)-Me2DAPPt(GpG) at pH 9.5 leads to a mixture of (S,S,S,S)-Me2DAPPt(GpG) and at least one isomer of (S,S,S,R)-Me2DAPPt(GpG). Thus, second-sphere communication (hydrogen bonding and steric interligand interactions) influences both GpG conformation and Me2DAP configuration.     

Catalysis of 3-pyrrolidinecarboxylic acid and related pyrrolidine derivatives in enantioselective anti-Mannich-type reactions: importance of the 3-acid group on pyrrolidine for stereocontrol

The development of enantioselective anti-selective Mannich-type reactions of aldehydes and ketones with imines catalyzed by 3-pyrrolidinecarboxylic acid and related pyrrolidine derivatives is reported in detail. Both (3R,5R)-5-methyl-3-pyrrolidinecarboxylic acid and (R)-3-pyrrolidinecarboxylic acid efficiently catalyzed the reactions of aldehydes with alpha-imino esters under mild conditions and afforded anti-Mannich products with high diastereo- and enantioselectivities (anti/syn up to 99:1, up to >99% ee). For the reactions of ketones with alpha-imino esters, (R)-3-pyrrolidinecarboxylic acid was an efficient catalyst (anti/syn up to >99:1, up to 99% ee). Evaluation of a series of pyrrolidine-based catalysts indicated that the acid group at the beta-position of the pyrrolidine ring of the catalyst played an important role in forwarding the carbon-carbon bond formation and in directing anti-selectivity and enantioselectivity.     

Synthesis of new chiral sulfinyldiacetic acid derivatives and attempt at chemoselective asymmetric Pummerer reaction

(R(S))-1 (85% ee) was prepared by utilizing a porcin pancreatic lipase-promoted hydrolysis of sulfinyldiacetic acid dimethyl ester (8) which was derived from thiodiacetic acid (7). (R(S))-1 (99% ee) and (S(S))-1 (99% ee) were readily obtained by methanolysis of (R(S),S)-12 and (S(S),S)-12 with MeONa in MeOH. (R(S),S)-12 and (S(S),S)-12 were furnished by chromatographic separation of the diastereomeric mixture, obtained by oxidation of thiodiacetic mono-carboxylic acid (11) with 30% H2O2 followed by dehydrative condensation of the resultant sulfinyldiacetic mono-carboxylic acid with 4(S)-isopropyl-1,3-thiazolidine-2-thione. (R(S))-1 (99% ee) was successively treated with (TMS)2NLi, Ac2O, and TMSOTf to give a major chiral-3 product in 75% ee and in a highly chemoselective manner (chiral-3:chiral-2=93:7).     

The practical synthesis of (2S)-7-methoxy-1,2,3,4-tetrahydro-2-naphthylamine via optical resolution of 2-(3-methoxybenzyl)succinic acid

We describe the practical synthetic route for (2S)-7-methoxy-1,2,3,4-tetrahydro-2-naphthylamine 1(2S)-2-amino-7-methoxytetraline; (S)-AMT]. (2R)-2-(3-Methoxybenzyl)succinic acid [(R)-1] was obtained by the optical resolution of 2-(3-methoxybenzyl)succinic acid (1) as the salt of (1R,2S)-2-(benzylamino)cyclohexylmethanol (7), and (R)-1 was converted to the optically active (2S)-7-methoxy-1,2,3,4-tetrahydro-2-naphthoic acid [(S)-2] by the intramolecular Friedel-Crafts reaction followed by catalytic hydrogenation. (S)-AMT was obtained from the acid (S)-2 by Hofmann rearrangement without racemization.     

Synthesis and characterization of some new C2 symmetric chiral bisamide ligands derived from chiral Feist's acid

The hemilabile chiral C2 symmetrical bidentate substituted amide ligands (1R,2R)-5(a-d) and (1S,2S)-6(a-d) were synthesized in quantitative yield from (1R,2R)-(+)-3-methylenecyclo-propane-1,2-dicarboxylic acid (1R,2R)-3 and (1S,2S)-(-)-3-methylene-cyclopropane-1,2-dicarboxylic acid (1S,2S)-3, in two steps, respectively. The chiral Feist's acids (1R,2R)-3 and (1S,2S)-3 were obtained in good isomeric purity by resolution of trans-(±)-3-methylene-cyclopropane-1,2-dicarboxylic acid from an 8:2 mixture of tert-butanol and water, using (R)-(+)-α-methylbenzyl amine as a chiral reagent. This process is reproducible on a large scale. All these new synthesized chiral ligands were characterized by 1H-NMR, 13C-NMR, IR, and mass spectrometry, as well as elemental analysis and their specific rotations were measured. These new classes of C2 symmetric chiral bisamide ligands could be of special interest in asymmetric transformations.     

3-Fluoro-N-methyl-D-aspartic acid (3F-NMDA) stereoisomers as conformational probes for exploring agonist binding at NMDA receptors

N-Methyl-D-aspartate (NMDA) is the prototypical agonist of the NMDA receptor subtype of ionotropic glutamate receptors. Stereogenic placement of a C-F bond at the 3-position of (S)-NMDA generates either the (2S,3S)- or (2S,3R)- diastereoisomers of 3F-NMDA. The individual diastereoisomers were prepared by synthesis in enantiomerically pure forms and it was found that (2S,3S)-3F-NMDA is an agonist with a comparable potency to NMDA itself, whereas the (2S,3R)-diastereoisomer has negligible potency. The difference in potency of these stereoisomers is attributed to a preference of the C-F bond (2S,3S)-3F-NMDA to adopt a gauche conformation to the C-N(+) bond in the binding conformation, whereas the (2S,3R)-3F-NMDA forces these bonds anti, losing electrostatic stabilisation, to achieve the required binding conformation. These observations illustrate the utility of stereoselective fluorination in influencing the molecular conformation of β-fluorinated amino acids and thus probing the active conformations of bioactive compounds at receptors.     

5-enolpyruvylshikimate 3-phosphate synthase: chemical synthesis of the tetrahedral intermediate and assignment of the stereochemical course of the enzymatic reaction

A chemical synthesis of both diastereomers of the tetrahedral intermediate involved in 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) catalysis has been accomplished. Combination of methyl dibromopyruvate with a protected shikimic acid derivative, phosphorylation, and lactonization afforded the intermediates (S)-15 and (R)-15, whose configurations were assigned by NMR. After introduction of the 3-phosphate group and deprotection, photoinitiated radical debromination of the dibromo analogues (S)-5 and (R)-5 was accomplished with tributyltin hydride in mixed aqueous solvents in the presence of surfactant to give the pyruvate ketal phosphates (R)-TI and (S)-TI, respectively. These compounds are stable at high pH, but decompose at pH 7 with a half-life of ca. 10 min. (R)-TI proved to be inert to EPSPS, while (S)-TI was converted by the enzyme to a mixture of 5-enolpyruvylshikimate 3-phosphate, shikimate 3-phosphate, and phosphoenolpyruvate. The demonstration that the enzymatic intermediate possesses the S-configuration at the ketal center confirms the mechanism as an anti addition followed by a syn elimination. Furthermore, it appears that the syn stereochemistry of the second step requires the phosphate leaving group to serve as the base in catalyzing its own elimination.     

Synthesis of enantiomerically pure anti-1,2-diaryl and syn-1,2-alkylaryl vic-selenoamines

Phenylselenyl benzylcarbanion stabilized by an (S)-2-p-tolylsulfinyl group evolves in a highly stereoselective way in the reactions with (S)-N-(p-tolylsulfinyl)imines at -98 °C affording diastereomerically pure 1,2-selenoamino derivatives in good yields. The syn or anti relationship of the obtained compounds depends on the alkyl or aryl character of the imine. They are easily transformed into enantiomerically pure (1R,2S)-1-aryl[or (1S,2S)-1-alkyl]-2-(phenylseleno)-2-phenylethylamines by reaction with t-BuLi and subsequent methanolysis of the generated sulfinamide derivatives with TFA.     

Diastereoselective hydroxylation of 6-substituted piperidin-2-ones. An efficient synthesis of (2S,5R)-5-hydroxylysine and related alpha-amino acids

The synthesis of (2S,5R)-5-hydroxy-6-oxo-1,2-piperidinedicarboxylates (5) and related (3S,6R)-3-hydroxy-6-alkyl-2-oxo-1-piperidinecarboxylates has been developed. The approach is based on the asymmetric hydroxylation of enolates generated from the corresponding N-protected-6-substituted piperidin-2-ones. The utility of 5a as a precursor in the synthesis of (2S,5R)-5-hydroxylysine (1), an amino acid unique to collagen and collagen-like proteins, has also been demonstrated. (2S)-6-oxo-1,2-piperidinedicarboxylates (6) required for hydroxylation studies were prepared in 38-74% yield, starting from conveniently protected aspartic acid as inexpensive chiral adduct. Hydroxylation of 6 to 5 proceeds in high yield and excellent diastereoselectivity by treatment of their Li-enolate with (+)-camphorsulfonyloxaziridine at -78 degrees C. Ring opening of di-tert-butyl (2S,5R)-6-oxo-1,2-piperidinedicarboxylate ((5R)-5a) under reductive conditions afforded the corresponding 1,2-diol (17) in 91%, which was further transformed to (2S,5R)-5-hydroxylysine in four steps (84%). 17 is also a versatile intermediate in the preparation of tert-butyl (2S,5R)-2-[(tert-butoxycarbonyl)amino]-5-hydroxy-6-iodohexanoate (3) and tert-butyl (2S)-2-[(tert-butoxycarbonyl)amino]-4-[(2R)-oxiranyl]butanoate (4), two amino acid derivatives used in the total synthesis of the bone collagen cross-link (+)-pyridinoline (2a).     

Preparation of optically active threo-2-amino-3-hydroxy-3-phenylpropanoic acid (threo-beta-phenylserine) via optical resolution

To obtain optically active threo-2-amino-3-hydroxy-3-phenylpropanoic acid (1), (2RS,3SR)-2-benzoylamino-3-hydroxy-3-phenylpropanoic acid [(2RS,3SR)-2] was first optically resolved using (1S,2S)- and (1R,2R)-2-amino-1-(4-nitrophenyl)-1,3-propanediol as the resolving agents to afford (2R,3S)- and (2S,3R)-2 in yields of 73% and 66%, based on half of the starting amount of (2RS,3SR)-2. Next, the racemic structures of ammonium and some organic ammonium salts of (2RS,3SR)-2 were examined based on melting point, solubility, and infrared spectrum, with the aim of optical resolution by preferential crystallization. The benzylammonium salt of (2RS,3SR)-2 was suggested to exist as a conglomerate at room temperature, although it forms a racemic compound at the melting point. The optical resolution by preferential crystallization of the racemic salt afforded the (2R,3S)- and (2S,3R)-salts with optical purities of 90-97%. The (2R,3S)- and (2S,3R)-2 obtained from the purified salts were hydrolyzed by reflux in hydrochloric acid to give (2R,3S)- and (2S,3R)-1.