Homoarcyriaflavin:  Synthesis of Ring-Expanded Arcyriaflavin Analogues

The construction of the ring-expanded carbazole system, forming arcyriaflavin homologues, is efficiently accomplished by the reaction of 2,2‘-bridged bis-indoles with 3,4-dibromo-2,5-dihydro-1H-2,5-pyrroledione derivatives under Grignard conditions. A ring size of up to nine members in the central ring is achievable. Substitutions either at the indole system or at the imide-N are also possible. The conformation of homoarcyriaflavins as a cross-link between the rigid arcyriaflavins and the flexible arcyriarubins was investigated by NMR, X-ray, and semiempiric quantum chemical calculation methods.