Photochemical Triggering of the Bergman and Myers-Saito Cyclizations.
A review. Two strategies for the photochem. generation of reactive enediyne compds. and their subsequent cycloaromatization to p-benzyne or α,3-didehydrotoluene derivs. are discussed in this account. The first method employs a photo-Wolff reaction of stable 11- or 12-membered ring precursor enediynes contg. the 2-diazo-1,3-diketone moiety. Irradn. of these compds. results in ring contraction and the formation of two isomeric enediynes possessing an enolized β-ketoester fragment. One of the isomers undergoes the conventional Bergman cyclization, whereas the other isomerizes into the enyne-allene tautomer, which rapidly cyclizes via a Myers-Saito mechanism. The second strategy consists of replacing the triple bond in a cyclic enediyne or enyne-allene structure with a cyclopropenone group, rendering them thermally stable. Photolysis of cyclopropenones results in efficient decarbonylation and the regeneration of a triple bond, restoring the enediyne π-system. The generation of reactive enediynes by nonresonant two-photon excitation using wavelengths within a phototherapeutic window' was also demonstrated. Photogenerated enediynes show significant nuclease activity, efficiently inducing single-strand dDNA cleavage. [on SciFinder(R)]