One-pot enantioselective synthesis of tryptophan derivatives via phase-transfer catalytic alkylation of glycine using a cinchona-derived catalyst

摘要

Tryptophans are building blocks for many natural products. This paper describes the enantiospecific synthesis of ring-A substituted tryptophan derivatives from commercially available gramines using chiral phase-transfer conditions. This one-pot reaction avoids protecting/deprotecting the indolylic nitrogen of gramine by choosing a chemoselective quaternization reagent, 4-(trifluoro-methoxy)benzyl bromide, to produce an electrophilic salt intermediate, which is subsequently alkylated in good yield with high enantiomeric excess.