We report on the implementation of the concept of a photochemically elicited two‐carbon homologation of a π‐donor–π‐acceptor substituted chromophore by triple‐bond insertion. Implementing a phenyl connector between the slide‐in module and the chromophore enabled the synthesis of cylohepta[b]indole‐type building blocks by a metal‐free annulative one‐pot two‐carbon ring expansion of the five‐membered chromophore. Post‐irradiative structural elaboration provided founding members of the indolo[2,3‐d]tropone family of compounds. Control experiments in combination with computational chemistry on this multibond reorganization process founded the basis for a mechanistic hypothesis.

Embedded medium-sized carbacycles and cyclohepta [b]indoles occur frequently as scaffold elements in natural products and bioactive compounds.Described herein is ac onceptionally novel photochemically triggered cascade process to these scaffolds.K ey to the cascading ring-expansion process is an unprecedented intramolecular alkyne analogue of the de Mayor eaction.