Synthesis of Optically Active 1,4-Thiazane-3-carboxylic Acid via Optical Resolution by Preferential Crystallization of (RS)-2-Amino-3-[(2-chloroethyl)sulfanyl]propanoic Acid Hydrochloride

Optically active 1,4-thiazane-3-carboxylic acid [TCA] was synthesized from cysteine via optical resolution by preferential crystallization. The intermediate (RS)-2-amino-3-[(2-chloroethyl)sulfanyl]propanoic acid hydrochlo-ride [(RS)-ACS?HCl] was found to exist as a conglomerate based on its melting point, solubility and IR spectrum. (RS)-ACS?HCl was optically resolved by preferential crystallization to yield (R)- and (S)-ACS?HCl. (R)- and (S)-ACS?HCl thus obtained were recrystallized from a mixture of hydrochloric acid and 2-propanol, taking account of the solubility of (RS)-ACS?HCl, efficiently yielding both enantiomers in optically pure forms. (R)- and (S)-TCA were then respectively synthesized by the cyclization of (R)- and (S)-ACS?HCl in ethanol in the presence of triethylamine.     

Preparation of optically active 2-aminobutanoic acid via optical resolution by replacing crystallization

An attempt was made to use a simple procedure to obtain (R)- and (S)-2-aminobutanoic acids [(R)- and (S)-1] which are non-proteinogenic alpha-amino acids and are useful as chiral reagents in asymmetric syntheses. Compound (RS)-1 p-toluenesulfonate [(RS)-2], which is known to exist as a conglomerate, was optically resolved by replacing crystallization with (R)- and (S)-methionine p-toluenesulfonate [(R)- and (S)-3] as optically active co-solutes. When (S)-3 was employed as the co-solute, (R)-2 was preferentially crystallized from a supersaturated solution of (RS)-2 in 1-propanol, as was (S)-2 in the presence of (R)-3. (R)- and (S)-2 recrystallized from 1-propanol were treated with triethylamine in methanol to give (R)- and (S)-1 in optically pure forms.     

Efficient route to (S)-azetidine-2-carboxylic acid

A new and efficient route to (S)-azetidine-2-carboxylic acid (>99.9% ee) in five steps and total yield of 48% via malonic ester intermediates was established. As the key step, efficient four-membered ring formation (99%) was achieved from dimethyl (S)-(1'-methyl)benzylaminomalonate by treating with 1,2-dibromoethane (1.5 eq) and cesium carbonate (2 eq) in DMF. Krapcho dealkoxycarbonylation of dimethyl (1'S)-1-(1'-methyl)benzylazetidine-2,2-dicarboxylate, the product of this cyclization procedure, proceeded with preferential formation (2.7:1, 78% total yield) of the desired (2S,1'S)-monoester, with the help of a chiral auxiliary which was introduced on the nitrogen atom. The undesired (2R,1'S)-isomer could be converted to that with proper stereochemistry, by a deprotonation and subsequent re-protonation step. Finally, lipase-catalyzed preferential hydrolysis of the (2S,1'S)-monoester and subsequent deprotection provided enantiomerically pure (S)-azetidine-2-carboxylic acid in a 91% yield from the mixture of (2S,1'S)- and (2R,1'S)-isomers.     

Asymmetric synthesis of L-azetidine-2-carboxylic acid and 3-substituted congeners--conformationally constrained analogs of phenylalanine, naphthylalanine, and leucine.

Enantiopure L-azetidine-2-carboxylic acid, the (3R)-phenyl, (3R)-naphthyl and (3S)-isopropyl analogs were prepared based on a zinc-mediated asymmetric addition of allylic halides to the camphor sultam derivative of glyoxylic acid O-benzyl oxime.     

Intramolecular reductive cyclization strategy to the synthesis of (-)-6-methyl-3-hydroxy-piperidine-2-carboxylic acid, (+)-6-methyl-(2-hydroxymethyl)-piperidine-3-ol and their glycosidase inhibitory activity

The first stereoselective synthesis of (2S,3R,6S)-6-methyl-3-hydroxy-piperidine-2-carboxylic acid (-)-6 and (2R,3R,6S)-6-methyl-(2-hydroxymethyl)-piperidine-3-ol (+)-7 was achieved starting from readily available d-glucose in 14 steps with 17% overall yield for both the compounds. The key feature of the present strategy includes the Wittig-olefination for the preparation of required conjugated keto-azide 9 and construction of 2,3,6-trisubstituted piperidine skeleton 11 by applying intramolecular reductive cyclization of conjugated keto-azide intermediate. The glycosidase inhibitory activity of compounds 6 and 7 towards several glycosidases has been evaluated.     

Synthesis and evaluation of [123I] labeled iodovinyl amino acids syn-, anti-1-amino-3-[2-iodoethenyl]-cyclobutane-1-carboxylic acid, and 1-amino-3-iodomethylene-cyclobutane-1-carboxylic acid as potential SPECT brain tumor imaging agents

syn- and anti-1-amino-3-[2-iodoethenyl]-cyclobutane-1-carboxylic acid (syn-, anti-IVACBC 16, 17) and their analogue 1-amino-3-iodomethylene-cyclobutane-1-carboxylic acid (gem-IVACBC 18) were synthesized and radioiodoinated with [(123)I] in 34-43% delay-corrected yield. All these amino acids entered 9L gliosarcoma cells primarily via L-type transport in vitro with high uptake of 8-10% ID/1 x 10(6) cells. Biodistribution studies of [(123)I]16, 17 and 18 in rats with 9L gliosarcoma brain tumors demonstrated high tumor to brain ratios (4.7-7.3:1 at 60 min post-injection). In this model, syn-, anti-, and gem-[(123)I]IVACBC are promising radiotracers for SPECT brain tumor imaging.     

Asymmetric transformation of a racemic α-(phthalimidooxy)arylacetic ester by a combination of preferential crystallization and simultaneous racemization

An asymmetric transformation of racemic t-butyl 2-(3,4-O-carbonyldioxy-phenyl)-2-(phthalimidooxy)acetate [(RS)- 2b ] into one of its optically active forms was carried out by a combination of preferential crystallization of a desired enantiomer and the simultaneous racemization of the antipode. (R)- 2b was easily racemized in diethylketone in the presence of a small amount of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Under the conditions for racemization, the asymmetric transformation was achieved successfully to give (S)- 2b with 84% optical purity in 80% yield. A potent antipseudomonal cephalosporin M-14659 ( 1 ) was prepared from the pure (S)- 2b which was obtained by the recrystallization of the crude (S)- 2b . © 1993 Wiley-Liss, Inc.     

2-Amino-3-(3-hydroxy-1,2,5-thiadiazol-4-yl)propionic acid: resolution, absolute stereochemistry and enantiopharmacology at glutamate receptors

In order to identify new subtype-selective (S)-glutamate (Glu) receptor ligands we have synthesized (RS)-2-amino-3-(3-hydroxy-1,2,5-thiadiazol-4-yl)propionic acid [(RS)-TDPA]. Resolution of (RS)-TDPA by chiral chromatography was performed using a Crownpac CR(+) column affording (R)- and (S)-TDPA of high enantiomeric purity (enantiomeric excess=99.9%). An X-ray crystallographic analysis revealed that the early eluting enantiomer has R-configuration. Both enantiomers showed high affinity as well as high agonist activity at (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptors, determined using a [(3)H]AMPA binding assay and an electrophysiological model, respectively. The affinities and agonist activities obtained for (R)-TDPA (IC(50)=0.265 microM and EC(50)=6.6 microM, respectively) and (S)-TDPA (IC(50)=0.065 microM and EC(50)=20 microM, respectively) revealed a remarkably low AMPA receptor stereoselectivity, (S)-TDPA showing the highest affinity and (R)-TDPA the most potent agonist activity. In addition, (S)-TDPA was shown to interact with synaptosomal Glu uptake sites displacing [(3)H](R)-aspartic acid (IC(50 ) approximately 390 microM). An enantiospecific and subtype-selective agonist activity was observed for (S)-TDPA at group I metabotropic Glu (mGlu) receptors (EC(50)=13 microM at mGlu(5) and EC(50)=95 microM at mGlu(1)).     

Asymmetric synthesis and biological evaluations of (+)- and (-)-6-dimethoxymethyl-1,4-dihydropyridine-3-carboxylic acid derivatives blocking N-type calcium channels

An efficient asymmetric synthesis of 1,4-dihydropyridine derivatives is described. The key step is the stereoselective Michael addition using t-butyl ester of l-valine as a chiral auxiliary to achieve good ee (>95% for all the tested experiments) and moderate yield. With this method, (+)-4-(3-chlorophenyl)-6-dimethoxymethyl-2-methyl-1,4-dihydropyridine-3,5-dicarboxylic acid cinnamyl ester was obtained and was characterized as a promising N-type calcium channel blocker with improved selectivity over L-type compared to its (−)- and racemic isomers.     

3-Aminopyrrolidine-4-carboxylic acid as versatile handle for internal labeling of pyrrolidinyl PNA

Conformationally restricted pyrrolidinyl PNAs with an α/β-dipeptide backbone consisting of a nucleobase-modified proline and a cyclic five-membered amino acid spacer such as (1S,2S)-2-aminocyclopentanecarboxylic acid (ACPC) (acpcPNA) can form very stable hybrids with DNA with high Watson-Crick base pairing specificity. This work aims to explore the effect of incorporating 3-aminopyrrolidine-4-carboxylic acid (APC), which is isosteric to the ACPC spacer, into the acpcPNA. It is expected that the modification should not negatively affect the DNA binding properties, and that the additional nitrogen atom in the APC should provide a handle for internal modification. Orthogonally-protected (N(3)-Fmoc/N(1)-Boc and N(3)-Fmoc/N(1)-Tfa) APC monomers have been successfully synthesized and incorporated into the acpcPNA by Fmoc-solid-phase peptide synthesis. T(m), UV and CD spectroscopy confirmed that the (3R,4S)-APC could substitute the (1S,2S)-ACPC spacer in the acpcPNA with only slightly decreasing the stability of the hybrids formed between the modified acpc/apcPNA and DNA. In contrast, the (3S,4R) enantiomer of APC caused substantial destabilization of the hybrids. Furthermore, a successful on-solid-support internal labeling of the acpc/apcPNA via amide bond formation between pyrene-1-carboxylic acid or 4-(pyrene-1-yl) butyric acid and the pyrrolidine nitrogen atom of the APC spacer has been demonstrated. Fluorescence properties of the pyrene-labeled acpc/apcPNAs are sensitive to their hybridization states and can readily distinguish between complementary and single-mismatched DNA targets.     

Synthesis and biological evaluation of (R)-N-(diarylmethylthio/sulfinyl)ethyl/propyl-piperidine-3-carboxylic acid hydrochlorides as novel GABA uptake inhibitors

A series of new (R)-1-(2-diarylmethylthio/sulfinyl)ethyl-piperidine-3-carboxylic acid hydrochlorides 5a-d/6a-d and (R)-1-(3-diarylmethylthio)propyl-piperidine-3-carboxylic acid hydrochlorides 5'a-d were synthesized and evaluated as gamma-aminobutyric acid uptake inhibitors through cultured cell lines expressing mouse GAT1. Biological screening results demonstrated that the compounds 6a-d with diarylmethylsulfinyl ethyl side chain show more potent GAT1 inhibitory activities than 5a-d/5'a-d with diarylmethylthio ethyl/propyl moieties. Some of them, such as 6a, exhibited excellent inhibitions of [(3)H]-GABA uptake in cultured cells, which is 496-fold higher than (R)-nipecotic acid and 11.5 times less than tiagabine. The synthesis and structure-activity relationships are discussed.     

Preparation of (2S,4R)‐2,4‐thiazolidinedicarboxylic acid by separation of diastereoisomeric salts with organic amines

L ‐Cysteine was condensed with glyoxylic acid monohydrate in acetic acid at 30°C to give (4R)‐2,4‐thiazolidinedicarboxylic acid [(4R)‐TDA] as a mixture of two diastereoisomers, (2R,4R)‐ and (2S,4R)‐TDA. An attempt was made to separate (2S,4R)‐TDA from the diastereoisomeric salts of (4R)‐TDA with 1‐propylamine, 2‐methyl‐2‐propylamine, benzylamine, and (R)‐ and (S)‐1‐phenylethylamines [(R)‐ and (S)‐PEA]. The salts of (2S,4R)‐TDA were preferentially crystallized as less soluble diastereoisomeric salts. When the salt with (R)‐PEA was employed, the separation was successfully achieved to afford optically pure (2S,4R)‐TDA in a yield of 41%, based on the starting amount of the diastereoisomeric mixture of (4R)‐TDA. Chirality 11:326–329, 1999. © 1999 Wiley‐Liss, Inc.     

Optical resolution by preferential crystallization of (2RS,3SR)-2-amino-3-chlorobutanoic acid hydrochloride

(2RS,3SR)-2-Amino-3-chlorobutanoic acid hydrochloride [(2RS,3SR)-ACB · HCl] was found to exist as a conglomerate based on the melting point, infrared spectrum, and solubility. Optical resolution by preferential crystallization of (2RS,3SR)-ACB · HCl was achieved to yield both (2R,3S)- and (2S,3R)-ACB · HCl of 80–100% optical purities. The obtained (2R,3S)- and (2S,3R)-ACB · HCl were recrystallized, taking into account the solubility of (2RS,3SR)-ACB · HCl, to give efficiently optically pure (2R,3S)- and (2S,3R)-ACB · HCl. Treatment of the purified (2R,3S)- and (2S,3R)-ACB · HCl with triethylamine gave optically pure (2R,3S)- and (2S,3R)-2-amino-3-chlorobutanoic acid, respectively. Chirality 9:656–660, 1997. © 1997 Wiley-Liss, Inc.     

Stereoselectivity of the aliphatic hydroxylation of 6-n-propylchromone-2-carboxylic acid in rat and guinea pig

Following administration of 6-n-propylchromone-2-carboxylic acid (6-n-PCCA) (500 μmol/kg) to male rats, three metabolic products were detected and isolated from the 0–24 h urine. All were identified as resulting from oxidation exclusively along the 6-n-propyl moiety. Some 66% of the dose was excreted in the 0–24 h urine, 55% of which was 6-PCCA, with 15% as (6-1′-hydroxypropyl)chromone-2-carboxylic acid (6-1′-HPCCA), 22% as 6-(2′-hydroxypropyl)chromone-2-carboxylic acid (6-2′-HPCCA), and 4% as (6-3′-carboxypropyl)chromone-2-carboxylic acid (6-3′-CPCCA). Derivatization of the methyl esters of the hydroxylated metabolities with S-α-methoxy-α-(trifuloromethyl)-phenylacetyl chloride (Mosher's reagent) allowed the evaluation of urinary enantiomeric composition by HPLC and assignment of their absolute configurations by NMR. This was found to be 90:10 (R/S) for 6-2′-HPCCA, and 7:93 (R/S) for 6-1′-HPCCA. When rats were dosed with the racemic 1′- and 2-hydroxy metabolites; no stereoselective metabolism or excretion was observed. Administration of 6-n-PCCA to male guinea pigs revealed that this species was unable to metabolise this compound. © 1993 Wiley-Liss, Inc.     

Coformational aspects of polypeptide structure XL. The synthesis of poly((S)-thiazolidine-4-carboxylic acid)

The synthesis of poly[(S)-thiazolidine-4-carboxylic acid] is described. The polymer is obtained by the polymerization of the N-carboxyanhydride of (S)-thiazolidine-4-carboxylic acid in pyridine or nitrobenzene using triethylamine as an initiator. The amino acid is prepared by the condensation of cysteine and formaldehyde. N-Acetyl-(S)-thiazolidine-4-carboxylic acid methyl ester is also prepared as a model compound by standard acetylation and esterification reactions.     

Structure-based design, synthesis, and biological evaluation of novel 1,4-diazepines as HDM2 antagonists

Crystallographic analysis of ligands bound to HDM2 suggested that 7-substituted 1,4-diazepine-2,5-diones could mimic the alpha-helix of p53 peptide and may represent a promising scaffold to develop HDM2-p53 antagonists. To verify this hypothesis, we synthesized and biologically evaluated 5-[(3S)-3-(4-chlorophenyl)-4-[(R)-1-(4-chlorophenyl)ethyl]-2,5-dioxo-7-phenyl-1,4-diazepin-1-yl]valeric acid (10) and 5-[(3S)-7-(2-bromophenyl)-3-(4-chlorophenyl)-4-[(R)-1-(4-chlorophenyl)ethyl]-2,5-dioxo-1,4-diazepin-1-yl]valeric acid (11). Preliminary in vitro testing shows that 10 and 11 substantially antagonize the binding between HDM2 and p53 with an IC(50) of 13 and 3.6 microM, respectively, validating the modeling predictions. Taken together with the high cell permeability of diazepine 11 determined in CACO-2 cells, these results suggest that 1,4-diazepine-2,5-diones may be useful in the treatment of certain cancers.     

Stereoselective synthesis of the optically active samin type of lignan from L-glutamic acid

The optically active samin type of lignan, (1R,2S,5R, 6S)-6-(2-methoxy-4,5-methylenedioxyphenyl)-3,7-dioxabicyclo[3.3.0]octan-2-ol, was stereoselectively synthesized from L-glutamic acid via (2R,3R)-2-[(1S and R)1-[(tert - butyldimethylsilyl)oxyl-1-(2-methoxy-4,5methylenedioxyphenyl)methyl]-3-[(tert- butyldiphenylsilyl)oxylmethyl-1,4-butanediol.     

Enantiomers of 2-methyl- and 2,4-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid: Preparation by resolution,determination of absolute configuration,and Curtius rearrangement

Both enantiomers of 2-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid 2 and 2,4-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid 3 were prepared via resolution of the corresponding racemic carboxylic acids with (R)- and (S)-1-phenylethylamine, respectively. Absolute configuration of (−)-(R)-2-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid was determined by X-ray crystallography. Curtius rearrangement of acyl azides prepared from enantiomers of these heterocyclic carboxylic acids carried out in benzyl alcohol afforded enantiomers of the corresponding benzyl carbamates, which upon hydrogenolysis gave racemic 2-amino-2-methyl-3,4-dihydro-2H-1,4-benzoxazin-3-one 4 and 2-amino-2,4-dimethyl-3,4-dihydro-2H-1,4h-benzoxazin-3-one 5. Chirality 10:791–799, 1998. © 1998 Wiley-Liss, Inc.     

Synthesis of [O-methyl-11C]1-(2-chlorophenyl)-5-(4-methoxyphenyl)-4-methyl-1H-pyrazole-3-carboxylic acid piperidin-1-ylamide: a potential PET ligand for CB1 receptors

Synthesis and in vitro evaluation of [O-methyl-(11)C]1-(2-chlorophenyl)-5-(4-methoxyphenyl)-4-methyl-1H-pyrazole-3-carboxylic acid piperidin-1-ylamide ([(11)C]-1), a potential imaging agent for CB(1) receptors using PET is described. 1-(2-Chlorophenyl)-5-(4-hydroxyphenyl)-4-methyl-1H-pyrazole-3-carboxylic acid piperidin-1-ylamide (5), the precursor for radiolabeling, was synthesized from 4-OTBDPS-propiophenone (2) in five steps with 30% overall yield. The reaction of alcohol 5 with [(11)C]MeOTf at 60 degrees C afforded [(11)C]-1 with an average radiochemical yield of 14.5% (EOS) and >2000 Ci/mmol specific activity. The radiotracer was found to selectively label CB(1) receptors in slide-mounted sections of postmortem human brain containing prefrontal cortex as demonstrated by in vitro autoradiography using phosphor imaging.