Enantiomerically pure Indoloquinolizines from tryptophane

Abstract
Carboline derivatives 8 and 11, obtained from L‐tryptophane methyl ester in a Pictet‐Spengler reaction with the corresponding acetals 6 and 10, add methyl vinyl ketone to form the diastereomeric mixtures 9 and 12. Under phase‐transfer conditions only 12 can be converted into a mixture of the title compounds 2a and 2b. With potassium carbonate as catalyst, however, 2b can be converted into 2c, the enantiomer of 2a.