Indole-fused cyclic anhydrides earlier deemed unreactive in the Castagnoli-Cushman reaction with imines have been rendered valid participant in this process. The new reaction format involves the use of respective indole-based dicarboxylic acids and in situ cyclodehydration of the latter by acetic anhydride. This finding validates a fundamentally new approach to synthesizing compounds based on the privileged 1,2,3,4-tetrahydropyrazino[1,2-a]indole core characterized by hitherto undescribed substitution pattern.