Diels‐Alder Reactions of 5,8‐Ethano‐5,8‐dihydro‐1,4‐naphthoquinone with Cyclic Dienes

Diels‐Alder cycloadditions of 1,3‐cyclohexadiene and cyclopentadiene to π ‐facially dissymmetric 5,8‐ethano‐5,8‐dihydro‐1,4‐naphtboquinone (1) were examined. The stereochemical outcome of the reactions, determined by a combination of chemical and spectral methods, indicates that addition of cyclic dienes to 1 occurs preferentially at the face syn to the etheno‐bridge of 1. Cycloadducts 7a/7b and 8a/8b obtained from Diels‐Alder reactions of 1 with cyclopentadiene and 1,3‐cyclohexadiene undergo [2+2]‐photocyclization to give caged compounds 9a/9b and 10a/10b , respectively. Stereoselective reduction of the enedione double bond in 7a/7b and 8a/8b with aqueous TiCl 3 in acetone affords cis‐bis‐bicyclic ring‐fused 1,4‐cyclo‐hexadiones 11a/11b , and 12a/12b , respectively.