Tadashi Shiraiwa

OPTICAL RESOLUTION BY PREFERENTIAL CRYSTALLIZATION OF DL-METHIONINE HYDROCHLORIDE

DL-Methionine hydrochloride (DL-Met·HCl) was found to exist as a conglomerate, based on the infrared spectrum, solubility, and melting point. The optical resolution of DL-Met·HCl was successfully achieved by preferential crystallization into D-and L-Met·HCl. Treatment of the purified D- and L-Met·HCl with triethylamine gave D- and L-methionine of 100% optical purities. Chirality, 9;48–51, 1997.

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.

Optical Resolution by Preferential Crystallization of (RS)-2-Amino-3-(2-carboxyethylthio)propanoic Acid.

Electrophilic additions of DL- and L-Cys to propenoic acid afforded (RS)- and (R)-2-amino-3-(2-carboxyethylthio)propanoic acids [(RS)- and (R)-ACE], respectively. (RS)-ACE was found to exist as a conglomerate based on its melting point, solubility, and infrared spectrum. (RS)-ACE was optically resolved by preferential crystallization to yield (R)- and (S)-ACE. The obtained (R)- and (S)-ACE were efficiently recrystallized from water, taking account of the solubility of (RS)-ACE, to give them in optically pure form.

Optical resolution by preferential crystallization of (RS)-2-amino-3-chloropropanoic acid hydrochloride

The racemic structures of (RS)-2-amino-3-chloropropanoic acid [(RS)-ACP] and (RS)-2-amino-3-chloropropanoic acid hydrochloride [(RS-ACP·HCl] were examined to obtain (R)- and (S)-ACP via optical resolution by preferential crystallization. The melting point, infrared spectrum, solubility, and ternary solubility diagram suggested that (RS)-ACP·HCl exists as a conglomerate and that (RS)-ACP forms a racemic compound. Optical resolution by preferential crystallization of (RS)-ACP·HCl was successfully achieved to yield (R)- and (S)-ACP·HCl. Optically pure (R)- and (S)-ACP were obtained from the purified (R)-and (S)-ACP·HCl, respectively.

Preparation of Optically Active Allothreonine by Separating from a Diastereoisomeric Mixture with Threonine

A simple procedure is described to obtain D- and L-allothreonine (D- and L-aThr). A mixture of N-acetyl-D-allothreonine (Ac-D-aThr) and N-acetyl-L-threonine (Ac-L-Thr) was converted to a mixture of their ammonium salts and then treated with ethanol to precipitate ammonium N-acetyl-L-threoninate (Ac-L-Thr·NH3) as the less-soluble diastereoisomeric salt. After separating Ac-L-Thr·NH3 by filtration, Ac-D-aThr obtained from the filtrate was hydrolyzed in hydrochloric acid to give D-aThr of 80% de, recrystallized from water to give D-aThr of >99% de. L-aThr was obtained from a mixture of the ammonium salts of Ac-L-aThr and Ac-D-Thr in a similar manner.

Optical resolution by the replacing crystallization of DL-threonine with L-alanine as an optically active cosolute

DL-Threonine (DL-Thr) was optically resolved by replacing crystallization with L-alanine (L-Ala) as an optically active cosolute. D-Thr was preferentially crystallized from a supersaturated aqueous solution of DL-Thr in the presence of L-Ala. Optical resolution was successfully achieved to afford D-Thr with an optical purity of 96-98% and L-Thr of 91-95%. The partially resolved D- and L-Thr were recrystallized from water, taking account of the solubility of DL-Thr, to efficiently yield both enantiomers in an optically pure form.

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 α-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.

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.

Synthesis of Four Stereoisomers of 1,4-Thiazane-3-carboxylic Acid 1-Oxide via the Asymmetric Transformation (combined isomerization-preferential crystallization) of 1,4-Thiazane-3-carboxylic Acid

In order to synthesize four stereoisomers of 1,4-thiazane-3-carboxylic acid 1-oxide (TCA•SO), (S)-1,4-thiazane-3-carboxylic acid [(S)-TCA], which is one of the precursors, was prepared by the asymmetric transformation (combined isomerization-preferential crystallization) of (RS)-TCA. This asymmetric transformation was used (2R, 3R)-tartaric acid [(R)-TA] as a resolving agent and salicylaldehyde as the epimerization catalyst in propanoic acid at 110°C to afford a salt of (S)-TCA with (R)-TA in 100% de with a yield of over 90%. Optically pure (S)-TCA was obtained by treating the salt with triethylamine in methanol in a yield of over 80%, based on (RS)-TCA as the starting material. In addition, asymmetric transformation of (R)-TCA gave (S)-TCA in a yield of 60-70%. (S)-TCA was oxidized by hydrogen peroxide in dilute hydrochloric acid to selectively crystallize (1S, 3S)-TCA•SO. (1R, 3S)-TCA•SO of 70% de from the filtrate was allowed to form a salt with (R)-TA after a treatment with triethylamine to give (1R, 3S)-TCA•SO as a single diastereoisomer. (1R, 3R)- and (1S, 3R)-TCA•SO were also prepared by starting from (R)-TCA that had been synthesized from L-cysteine.

Structures and properties of a diastereoisomeric molecular compound of (2S,3S)- and (2R,3S)-N-acetyl-2-amino-3-methylpentanoic acids.

An X-ray crystal structural analysis revealed that (2S,3S)-N-acetyl-2-amino-3-methylpentanoic acid (N-acetyl-L-isoleucine; Ac-L-Ile) and (2R,3S)-N-acetyl-2-amino-3-methylpentanoic acid (N-acetyl-D-alloisoleucine; Ac-D-aIle) formed a molecular compound containing one Ac-L-Ile molecule and one Ac-D-aIle molecule as an unsymmetrical unit. This molecular compound is packed with strong hydrogen bonds forming homogeneous chains consisting of Ac-L-Ile molecules or Ac-D-aIle molecules and weak hydrogen bonds connecting these homogeneous chains in a fashion similar to that observed for Ac-L-Ile and Ac-D-aIle. Recrystallization of an approximately 1:1 mixture of Ac-L-Ile and Ac-D-aIle from water gave an equimolar molecular compound due to its lower solubility than that of Ac-D-aIle or especially Ac-L-Ile. The results suggest that the equimolar mixture of Ac-L-Ile and Ac-D-aIle could be obtained from an Ac-L-Ile-excess mixture by recystallization from water.