Hiroko Niitsuma

Bio: Hiroko Niitsuma is an academic researcher from Tohoku University. The author has contributed to research in topics: Enantioselective synthesis & Alkylation. The author has an hindex of 2, co-authored 4 publications receiving 70 citations.

Enantioselective construction of a quaternary asymmetric carbon center: a versatile synthesis of .alpha.-alkyl .alpha.-amino acids

Abstract: Alkylation enantioselective des dianions derives des monoesters chiraux d'acides maloniques monosubstitues avec des halogenures d'alkyles

Stereoselective alkylation of dianions derived from chiral half-esters of monosubstituted malonic acids: asymmetric synthesis of α-alkyl α-amino acids and key synthetic intermediates for Hunteria and Aspidosperma indole alkaloids

Abstract: Substitution of the chiral half-esters of monosubstituted malonic acids with halides leads to the formations of mixtures of the diastereoisomeric alkyl- or benzyl-malonic half-esters. The (R)-isomers (13A and 15A–22A) were obtained from the phenylmenthyl half-ester 14 of methylmalonic acid in high diastereoisomeric excess. The same stereoisomers were also produced by reaction of the half-esters 9, 23 and 24 with methyl iodide. Their absolute configurations were determined by transforming the major products into the known α-alkyl α-amino acid derivatives 30, 33 and 35. The major product 43, prepared by allylation of the half-ester 9, was converted into two lactones 41 and 42, key intermediates for synthesis of indole alkaloids of the Hunteria and Aspidosperma types. The mechanism of the above alkylation is discussed.

Stereoselective Alkylation of Dianions Derived from Chiral Half‐Esters of Monosubstituted Malonic Acids: Asymmetric Synthesis of α‐ Alkyl α‐Amino Acids and Key Synthetic Intermediates for Hunteria and Aspidosperma Indole Alkaloids.

Abstract: Substitution of the chiral half-esters of monosubstituted malonic acids with halides leads to the formations of mixtures of the diastereoisomeric alkyl- or benzyl-malonic half-esters. The (R)-isomers (13A and 15A–22A) were obtained from the phenylmenthyl half-ester 14 of methylmalonic acid in high diastereoisomeric excess. The same stereoisomers were also produced by reaction of the half-esters 9, 23 and 24 with methyl iodide. Their absolute configurations were determined by transforming the major products into the known α-alkyl α-amino acid derivatives 30, 33 and 35. The major product 43, prepared by allylation of the half-ester 9, was converted into two lactones 41 and 42, key intermediates for synthesis of indole alkaloids of the Hunteria and Aspidosperma types. The mechanism of the above alkylation is discussed.

Catalytic enantioselective synthesis of α-amino acid derivatives by phase-transfer catalysis

Abstract: Described are improved processes for the enantioselective synthesis of α-amino acids which involve combinations of solvents, highly-mixed and low-temperature reaction conditions, and novel catalysts. Also described are novel catalysts and precursors to α-amino acid derivatives.

A New General Approach to Enantiomerically Pure Cyclic and Open-Chain (R)- and (S)-α,α-Disubstituted α-Amino Acids

Abstract: A wide range of cyclic and open-chain alpa,alpha-disubstituted alpha-amino acids 1a-p were prepared. The racemic N-acylated alpha,alpha-disubstituted amino acids were resolved by coupling to chiral amines 15-18 derived from (S)- phenylalanine to form diastereoisomers 19/20 or 21/22 that could be separated by crystallization and/or flash chromatography on silica gel (Scheme 3). Selective cleavage via the 1,3-oxazol-5(4H)-ones 10a-p gave the corresponding optically pure alpha,alpha-disubstituted amino-acid derivatives 11 or 12 in high yield (Scheme 3). The absolute configurations of the alpha,alpha-disubstituted amino acids were determined from X-ray structures of the diastereoisomers 20, 21g', 22d.