Reactivity of hexahydrocarbazol-4-ones in Michael reactions: stereocontrolled formation of decahydropyrido[2,3-d]carbazoles

Abstract

The reactivity of hexahydrocarbazolones 3 and 7 in Michael reactions has been studied with several reagents: treatment with acrylonitrile, methyl acrylate and the Mannich base of methyl vinyl ketone leads to 4a-substituted compounds 13–16 in good yields (65–93%), as previously observed with nitroethylene. In contrast, unexpected tetracyclic carbazoles 17 and 18 are obtained with methyl vinyl ketone itself. Reductive cyclisation (H₂, PtO₂) of 4a-cyanoethylhexahydrocarbazolones 13 and 15 affords decahydropyrido[2,3-d]carbazoles 20 and 21, potential intermediates in the synthesis of E-homo Aspidosperma alkaloids. Compounds 20 and 21 are isolated as single diastereoisomers with a C/E trans ring junction.†