Enamides cyclizability

Synthesis of alkaloids enamide cyclizations for, 22, 189 (1983) lead tetraacetate oxidation in, 36, 70 (1989)... 

While some of the mechanistic details for the examples described in this chapter have not yet been fully elucidated, it is clear from the scope of the examples discussed herein that the photochemistry of enaminones and enamidones is a fascinating research area. The novel sequence for the annelation of imidazole rings onto a preexisting structure, the synthesis of perhydroindoles via vinylogous amide [2 + 2] photocycloaddition chemistry and the enamide cyclization reactions all underscore the enormous utility of these chromophores in the development of new reactions and novel synthetic methods. 

Based on the above result, a newly isolated alkaloid alamarine (33, structure, see p. 240) was synthesized by the route involving enamide cyclization under both thermal and photochemical conditions, giving predominantly the desired lactam (32) (Scheme 28) (73). 

One of the major areas of research in enamide chemistry has been the application of enamide cyclization to the synthesis of natural alkaloids and related heterocyclic compounds. Ever since Ninomiya and co-workers discovered nonoxidative photocyclization of the A-benzoylenamine-type ena-mides in 1969 (9), several types of alkaloids have been synthesized, starting from enamides of rather simple structures, by applying enamide cyclization under both photochemical and thermal conditions. 

By taking advantage of these features, enamide cyclization has shown its great versatility in the synthesis of a wide variety of isoquinoline and indole alkaloids. 

In this chapter, we first summarize more or less basic works on the synthesis of the common and basic skeletons of various types of natural alkaloids, aiming at their application to total synthesis, and also those works that seem applicable to the above syntheses, and then collected works on total syntheses of alkaloids via the route involving enamide cyclization. 

The cyclization of l-methylene-2-nicotinoylisoquinolines 27 under three different conditions, photochemical, thermal, and acylating conditions, are compared (73). Starting from the above enamide (27), Ninomiya et al. (73) synthesized 10-azaberbine as the major product by simple heating at 195-200°C, whereas 12-azaberbine was the major product under acylating condition as shown in Table II. Further, total synthesis of alamarine (33), the only alkaloid having an azaberbine structure, was synthesized via the route involving enamide cyclization under both thermal and photochemical conditions (73) as shown in Scheme 71. 

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