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
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.