To a mixture of 1-methylindole (4.26 g) and methyl (2R,3R)-N-benzyloxycarbonyl-3-methyl-2-aziridinecarboxylate (2.70 g) in dry methylene chloride (50 ml) was added boron trifluoride etherate (1.33 ml) under ice-bath cooling. The mixture was stirred for five minutes at ambient temperature and the solvent was evaporated off. The residue was chromatographed on silica gel (chloroform and a 1:1 mixture of hexane and ether as eluent) to give (αR,βR)-N-benzyloxycarbonyl-1,β-dimethyltryptophane methyl ester (1.62 g) as an oil.
