Use of the Pictet–Spengler reaction for the synthesis of 1,4-disubstituted-1,2,3,4-tetrahydro-β-carbolines and 1,4-disubstituted-β-carbolines: formation of γ-carbolines

Microwave-assisted conjugate addition of indole on nitro-olefins furnished nitro compounds, which were reduced to tryptamines. Further, by using Pictet–Spengler condensation, new 1,4-disubstituted-1,2,3,4-tetrahydro-β-carbolines were synthesized in diastereoselective manner. Dehydrogenation of the tetrahydro-β-carbolines produced new 1,4-disubstituted-β-carbolines. As a new observation, in some of the cases, Pictet–Spengler condensation and dehydrogenation gave two products, namely 1,4-disubstituted-β-carbolines and 1,4-disubstituted-γ-carbolines. A mechanism is proposed for this observation.