Enamides photoreactions

The most widely utilized photoreaction of enamine derivatives is undoubtedly the photocyclization of enamides. Since this subject has already been extensively reviewed3 only a brief discussion is presented here. 

This chapter describes photoreactions of nitrogen containing chromophores which are different from the principal reactions of electronically excited carbonyl compounds. Five groups of substrates were included cyclic and acyclic imines, nitriles, amine oxides, azo and diazo compounds, and enamides. 

Enamide and enamine photocyclizations (9, 13, 14, 113) are central to yet another strategy which has been successfully employed by Ninomiya (9, 13, 14, 61-71), Kametani (74), and Atta-ur-Rahman (72, 73) in the synthesis of yohimbine alkaloids. These approaches typically begin with harmalane (336) which contains the ABC unit of the pentacyclic yohimbine targets (Scheme 3.56). Condensation of harmalane with an appropriately substituted cyclohexene carboxylic acid or benzoyl chloride 337 affords seco D-ring intermediate 338. To effect D-ring closure, the enamide chromophore in 338 is photocyclized to form pentacyclic enamide 339. Alternatively, photoreaction... 

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