Carbazomycin A

The structures of carbazomycins A and B are elucidated to be 3,4-dimethoxy-1,2-dimethylcarbazole and 4-hydroxy-3-methoxy-1,2-dimethylcarbazole, respectively. Further, 13C-chemical shifts of carbazomycins and their derivatives are assigned simultaneously.

Facile syntheses of imperative carbazole alkaloids carbazomycin A, carbazomycin B, hyellazole, chlorohyellazole, and clausenaline D have been demonstrated starting from readily available Boc-protected 3-formylindole and dimethyl maleate. The suitably substituted aromatic rings have been designed comprising three/four significant C-C bond forming reactions. The competent Wittig reaction, selective monoalkylations, one-pot regioselective Weinreb amide formation and Boc-deprotection, well designed Grignard reactions, dehydrative intramolecular cyclizations, and Baeyer-Villiger rearrangement of aromatic aldehydes were the main features.

A new, one-pot domino benzannulation reaction between indole-3-ynones and various nitromethane derivatives has been explored for a general entry to diversely functionalized carbazole frameworks (28 examples). The scope of this new benzannulation has been extended to variants like 2-chloroindole-3-ynones to eventuate in chemo-differentiated 1,2,3,4-tetrasubstituted carbazoles with retention of the nitro group. The efficacy of this strategy has been demonstrated through concise total synthesis of natural products, viz. carbazomycin A, calothrixin B, and staurosporinone (K252c).