Some stereocontrolled ring-opening reactions of 6-substituted 2,3-epoxybicyclo[3.2.0]heptanes

Abstract

The bicyclo[3.2.0]heptenes (7), (9), (10), (12), and (13) were prepared and treated with a brominating agent in an aqueous medium. The alkenes 99) and (10) gave only the bromohydrins (15) and (16), respectively. The bicycloalkenes (7), (12), and (13) gave mixtures of bromohydrins. The mixture obtained from compound (12) gave the epoxides (32) and (35) on treatment with base: similarly the bromohydrins obtained from the alkenes (7) and (13) were converted into the isomeric epoxide pairs (29)/(34), and (33)/(36), respectively. The epoxides (30) and (31) were furnished by base-treatment of the bromohydrins (15) and (16), respectively. The epoxides (29)–(33) were treated with lithium dibutyl cuprate. The epoxides (30) and (31) reacted with high selectivity to give the corresponding bicycloalkan-3-ols (19) and (20) almost exclusively. The epoxides (32) and (33) reacted non-selectively with this cuprate reagent. The parent epoxide (29) displayed an intermediate behaviour, giving a mixture of the alcohols (18) and (23) with the former compound predominating. These results are explained by assuming that a substituent at the 6-position in this ring-system exerts a perannular conformational control which affects the selectively of reaction of an alkene and epoxide unit at C-2 and also influences the conformation of 2,3-disubstituted derivatives.