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.