Asymmetric synthesis of chaetogline A: expedient preparation of the indolizino[8,7-b]indole template and optical properties in a push–pull configuration

Abstract

The Bischler–Napieralski cyclization of the succinimide of tryptophan was combined with an unusual Mannich coupling of the resulting enamide with carbonyl electrophiles. This one-pot stepwise synthesis formed the indolizino[8,7-b]indole scaffold obtained from enantiopure succinimide 3b (98.7 : 1.3 er) and thus enabled the first asymmetric synthesis of chaetogline A in two steps from N-methyl tryptophan. The optical properties of chaetogline A, as a push–pull chromophore, show a large Stokes shift (>170 nm), while a water soluble pyridinium analogue shows interesting fluorescence properties.