Synthetic applications of photocyclizations reactions : N-allyliminium salt photocyclizations and intramolecular photoarylation of N-haloarylethyl-[beta]-enaminones

Abstract

The development and application of synthetic methods utilizing N-allyliminium salt photocyclization reactions for the construction of spiro-heterobicyclic ring systems and intramolecular photoarylation of N-haloarylethyl-(beta)-enaminones for the synthesis of tricyclic benzazepines is described. These studies were specifically aimed at construction of the ring skeleton of the Cephalotaxus alkaloids which contains these two key features. Irradiation of methanol solutions of simple monocyclic N-allyliminium salts available from O-methylation or O-pivaloylation of the N-prenyl-(beta)-enaminones derived from cyclohexane- or cyclopentane-1,3-dione leads to production of azaspiro{5.4}decanes and azaspiro{4.4}nonanes in moderate yield (30-44%). On the other hand, similar irradiation of tricyclic iminium salts generated by O-protonation, O-methylation, and O-pivaloylation of N-prenyl tricyclic dibenzazepinones results in photocleavage of the prenyl side chain, exclusively. Investigations concerning the photochemistry of iodinated and brominated N-arylethyl-(beta)-enaminones revealed that, while Pyrex-filtered irradiation results in the formation of a variety of photoproducts, Vycor-filtered irradiation leads to the isolation of C-cyclized tricyclic benzazepines, primarily, in good yield (60-85%). These photocyclization reactions, their mechanistic features, and their synthetic utility are discussed in this dissertation.