Discovery of Some New Heterocyclic Systems from Carbene‐Based Reactions of Indigo and N‐Methyl Derivatives

Reactions of N,N′‐dimethylindigo with dichlorocarbene or rhodium carbenoids have been investigated leading to the one‐pot production of new, complex heterocyclic derivatives. One of these arises from an overall ring interconversion process with ring expansion to afford a novel eight‐membered ring‐fused benz[6,7][1,5]oxazocino[3,2‐b]indol‐13‐one derivative. Alternatively, an overall ring construction process gives rise to an unprecedented spirocyclic 2H,4H‐spiro[furo[3,2‐b]indole‐3,2′‐indoline] system. A number of products with an embedded 6H‐pyrido[1,2‐a:3,4‐b′]diindole core from reaction with a rhodium malonyl ester carbenoid intermediate are also reported, plus some other carbenoid‐derived products. A solvent dependence is apparent with these reactions. Reaction path differences are also seen using N‐methylindigo or the parent indigo as the substrate and possible mechanisms are proposed.