Syntheses of chiral 1,3-disubstituted tetrahydro-β-carbolines via CIAT process: highly stereoselective Pictet–Spengler reaction of d-tryptophan ester hydrochlorides with various aldehydes

A highly stereoselective Pictet–Spengler reaction of d-tryptophan methyl ester hydrochloride 1-HCl with various aldehydes via a CIAT (crystallization-induced asymmetric transformation) process is described. It was revealed that the CIAT process should be performed in a mixed solvent of nitromethane and toluene, and a fine tuning of the ratio of nitromethane and toluene for each epimer mixture of 2-HCl was necessary in order to get as high yields and stereoselectivities as possible. Enantiomerically pure cis (or trans) 1,3-disubstituted tetrahydro-β-carbolines 2a–2v were obtained by recrystallization or flash chromatography after neutralization of the corresponding hydrochloride salts cis-2-HCl or trans-2-HCl.