Synthesis of 5-Alkyl- and 5-Aryl-L-Tryptophan Analogues via Palladium-Catalyzed Cross-Coupling of an Iodinated Cyclic Tryptophan Tautomer

David E. Zembower; Matthew M. Ames

doi:10.1055/s-1994-25708

Synthesis 1994; 1994(12): 1433-1436
DOI: 10.1055/s-1994-25708

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Synthesis of 5-Alkyl- and 5-Aryl-L-Tryptophan Analogues via Palladium-Catalyzed Cross-Coupling of an Iodinated Cyclic Tryptophan Tautomer

David E. Zembower^* , Matthew M. Ames

^*Division of Developmental Oncology Research, Mayo Comprehensive Cancer Center, Mayo Clinic and Foundation, Rochester, Minnesota 55905, USA

Abstract

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Cyclic tryptophan tautomers have been shown to be useful intermediates in the preparation of optically pure tryptophans substituted in the benzenoid ring. We have utilized cyclic tautomer 2a for the preparation of L-tryptophan derivatives bearing 5-alkyl and 5-aryl functionalities. Treatment of 2a with 4 equivalents of iodine monochloride provided the 5-iodo species 3a in 81% yield. Cross-coupling of 3a with arylboronic acids or B-alkyl-9-borabicyclo[3.3.1]nonane derivatives, catalyzed by 3 mol% [bis(1,1′-diphenylphosphino)ferrocene]palladium(II) chloride, afforded the 5-aryl and 5-alkyl cyclic tryptophan tautomers 3b-g in 59-79% yields. The cyclic tautomers were easily decyclized and deprotected to give the corresponding 5-aryl- and 5-alkyl-L-tryptophans.

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