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Paper | Regular issue | Vol 26, No. 8, 1987, pp.2139-2151
Published online, 1st January, 1970
DOI: 10.3987/R-1987-08-2139

Miguel F. Braña, Mercedes Garrido, María L. López Rodríguez, and M. José Morcillo

*Departamento de Química Orgánica I., Facultad de Química, Universidad Complutense de Madrid, Ciudad Universitaria, E-28040 Madrid, Spain


A study of three direct methods for the synthesis of α-alkyl α-amino acids has been applied to the preparation of α-alkyltryptophan esters. Reaction of Nb-benzylidene-L -tryptophan methyl esters 3 with tetrabutyl-ammonium hydrogen sulfate and methyl or ethyl iodide (phase-transfer method) gives α-methyl and α-ethyltryptophan methyl esters 6a,b in poor yield with complete racemization. Reaction of L-tryptophan 1 with dimethylformamide dimethyl or diethyl acetal followed by treatment of 8a,b with LDA and methyl iodide (Fitt and Gschwend method) gives rise to the formation of α,Na -dimethyl-Nb - dimethylsminamethylenetryptophan methyl or ethyl esters hydriodide 9 a,b . Treatment of 9a or 9b with aqueous acid, under mild conditions, fails to hydrolyze the imine functionality. Reaction of 3 with LDA and methyl or ethyl iodide (Bey and Vevert method) followed by hydrolysis provides α-methyl and α-ethyltryptophan methyl esters 6a,b. By carrying out this procedure with isopropyl, benzyl and allyl iodide or bromide the corresponding α-alkyl esters are not obtained.