A facile two-step synthesis of novel ring-A double substituted tryptophan building blocks for combinatorial chemistry

A fast synthesis of ring-A disubstituted Fmoc and Boc protected L-tryptophan analogs was achieved starting from the appropriate 2,4- or 2,3-disubstituted phenylhydrazines and optically active N,N-diprotected L-glutamic acid γ-aldehydes, utilizing a Fischer-indole synthesis as a key step. Unlike most of the previously reported methods, that required the multistep stereoselective generation of a chiral carbon, this fast methodology is useful for generating optically active ring-A disubstituted protected tryptophans starting from a simple and common chiral precursor. These building blocks have a wide application scope in peptide and combinatorial chemistry fields.