Vinylogous imides photocycloaddition

The utility of /f-enaminones (vinylogous amides) and / -enamidones (vinylogous imides) has been successfully demonstrated in both inter- and intramolecular [2 + 2] photocycloaddition reactions. The vinylogous amide, which reacts as a /Mieteroatom substituted enone, serves as the nitrogen analogue of the enol form of a / -diketone in the de Mayo reaction24. 

Some of the earliest work on the [2 + 2] photocycloaddition chemistry of vinylogous imides was reported by Wiesner and coworkers, who described the isolation of the [2 + 2] photoaddition products resulting from the irradiation of cyclic vinylogous imides with ethyl acrylate and allene, respectively (Scheme 12)29. 

Similarly, Cantrell observed intermolecular photocycloaddition of vinylogous imide 44 with cyclopentene on irradiation through Pyrex to give cyclobutane 45 (equation 13)30. 

A key step in the synthesis of 12-epi-lycopodine reported by Wiesner and coworkers is the intramolecular [2 + 2] photocycloaddition of a vinylogous imide and an allene. Irradiation of photosubstrate 46 produced a single cyclobutane 47, in which the allene added to the vinylogous imide anti to the methyl group (Scheme 13)31. Photoadduct 47 was converted to ketal-alcohol 48 via a three-step sequence of ketalization, epoxida-tion and reduction. Hydrolysis of the ketal unmasked the /Miydroxy ketone functionality. Retro-aldol fragmentation followed by aldol closure gave hydroxyketone 49, which was readily converted to the polycyclic alkaloid 12-epi-lycopodine. 

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