Construction of Unique Eight- or Nine-Membered Polyheterocyclic Systems via Multicomponent Reaction of l‑Proline, Alkyl Propiolate, and Isatin

The multicomponent reaction of l-proline, isatins, and
excess of methyl propiolates in methanol unexpectedly afforded 2-(oxoindolin-3-ylidene)­propylidene)­pyrrolidin-1-yl)­acrylates
in good yields, and the products subsequently converted to unique
nine-membered pyrrolo­[1′,2′:1,9]­azonino­[6,5,4-cd]­indoles in the presence of p-TsOH. Furthermore,
the multicomponent reaction of l-proline, N-benzylisatins, and excess alkyl propiolates in refluxing chloroform
unprecedentedly resulted in eight-membered azocino­[1,2-a]­benzo­[c]­[1,5]­diazocines as the main product. The
molecular structures of the polyheterocyclic compounds were confirmed
by determination of 19 single crystal structures. The reaction mechanism
was believed to contain the sequential 1,3-dipolar cycloaddition,
addition of cyclic tertiary amine to excess of propiolate, and fascinating
ring opening as well as rearrangement process. The N-substituent on
isatins played a critical role in the different reaction patterns.