An Improved Synthesis of Optically Pure (R)-4-Bromo-N-methyl-1-tosyltryptophan Derivative, a Key Intermediate in the Synthesis of Ergot Alkaloids

Optically pure (R)-N-Boc- or (R)-Cbz-4-bromo-N-methyl-1-tosyltryptophan methyl ester (11a or b) was synthesized in 2 steps from (R)-N-acetyl-4-bromo-l-tosyltryptophan methyl ester (7) by simultaneous N-methylation and deacetylation using methyl trifluoromethanesulfonate (TfOMe) in the presence of 2,6-di-tert-butyl-4-methylpyridine (DTBMP).