The Pictet-Spengler Reaction of Nb-Hydroxytryptamines and Cysteinals. II. Temperature Effects, Stereochemistry and Mechanism

The effect of temperature on the Pictet-Spengler reaction of Nb-hydroxytryptamines (1) and cysteinals (2) has been examined. The optically active nitrones (3), obtained from 1 and 2, gave the corresponding Nb-hydroxy-β-carbolines (4α and 4β) exclusively at -78°C in the presence of an excess of trifluoroacetic acid. The mechanism is discussed in relation to the stereochemistry.