Vinylogous urethanes, cyclization with

Reaction of aniline derivatives with 4-chlorobutyroyl chloride followed by cyclization with sodium ethoxide and subsequent thionation promoted by sonication gave the corresponding A -arylpyrrolidine-2-thiones 126. Zinc-mediated condensation of diethyl bromomalonate with 126 using iodine as activator gave the vinylogous urethanes 127 whose cyclization with PPA gave the tricyclic compound 128 which upon hydrolysis afforded the acid 129 (96TL9403). 

An example of an intriguing ring-size dependency of the reaction pathway is that of the cyclization of vinylogous urethanes 328a-c with cinnamaldehyde (08T883). Pyrrolidine- and azepane-based esters 328a and c, respectively, give via aza-[3+3] annulation the expected... 

Vinylogous urethane (248) was employed in a radical-mediated synthesis of epilupinine (Scheme 48), where its cyclization was induced by treatment with tributyltin hydride-AIBN <89T5269>. In another radical process, a very efficient synthesis of berbine was achieved by photolysis of compound (249), which induced a diradical cyclization process promoted by an excited-state electron-transfer desilylation (Scheme 49) <85TL5867>. 

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