Electronic Control of Stereoselectivity. 6. Directionality of Singlet Oxygen Addition to 1,4-Dimethoxynaphthalenes Laterally Fused to Bridged Bicyclic Systems

Article abstract of DOI:10.1021/jo01312a019

The photooxygenation in methanol solution of 1,4-dimethoxynaphthalene derivatives having bridged bicyclic systems fused at C2, C3 with rose bengal as sensitizer leads to formation of stereoisomeric epoxynaphthoquinone monoketals.These products, which are readily hydrolyzed in acid solution to the epoxynaphthoquinones, possess an epoxide oxygen which serves as a stereochemical marker for the directionality of singlet oxygen addition.For the norbornyl and norbornenyl cases, endo bonding by ¹O2 is preferred; the reverse is true for the pair of bicyclo<2.2.2>octenyl derivatives studied.This contrasting stereoselection was compared to the stereochemical consequences of alkaline hydroperoxidation of the corresponding naphthoquinones.In each instance, the reaction course was opposite to that observed with singlet oxygenation.Both processes are analyzed from the theoretical viewpoint, use being made of the photoelectron spectra of the substrate molecules, detailed evaluation of frontier orbital effects, and prevailing ?/? interactions.While the alkaline hydroperoxidation results can be interpreted in terms of standard kinetic and steric control, an understanding of the directionality of ¹O2 capture appears dependent on our appreciation of the contributions made by the ? electrons of the bicyclic moieties upon the aromatic ? orbitals.The tilting caused by such interactions is thought to be source of the experimentally observed exo/endo ratios.