Dihydropyridines, photochemical reaction with

Heteroatom Wittig chemistry also includes reactions of N-sulfonyl imines. It was demostrated that these compounds underwent olefination reactions with nonstabilized phosphonium ylides under mild conditions to afford an array of both Z- and E-isomers of 1,2-disubstituted alkenes, allylic alcohols, and allylic amines.Additionally, studies of the reactions of 5-bromo-4,6-dimethyl-2-thioxo-l,2-dihydropyridine-3-carboni-trile and thiazolidinone with phosphorus ylides have proved the formation of new phosphonium ylides. Annulations via P-ylides are a common occurrence in the literature. For example, on photochemical irradiation, phosphonium-iodonium ylides were shown to undergo 1,3-dipolar cycloaddition reactions with triple bonds, via a carbene intermediate, to yield furans. " Even more common are the reactions of Morita-Baylis-Hillman (MBH) acetates and carbonates. Zhou et al. demostrated that these substrates were able to generate very reactive 1,3-dipoles in the presence of tertiary phosphines the dipoles then underwent cycloaddition reactions to yield annulation products (Scheme 16). ... 

The simple 1,2-dihydropyridine (63) cyclizes to the 2-azabicyclo[2.2.0]hexene (64) derivative upon irradiation (79JA6677). This reaction is the basis of a general synthetic route to dihydropyridines that are difficult to prepare by other means (Scheme 3). Dihydropyridines with more complicated substitution patterns, i.e. (66), undergo a photochemical rearrangement to valence isomers (68) and (69) (71T2957). The 1-azahexatriene (67) is believed to be an intermediate in this reaction (equation 21). 

---