Electrophilic bromination of bicyclo[3.2.0]hept-2-en-6-ones
Bromination of bicyclo[3.2.0]hept-2-en-6-one (3) and 7,7-dimethylbicyclo[3.2.0]hept-2-en-6-one (2) in a variety of solvents gave high yields of the corresponding 2-exo-bromo-3-endo-substituted bicycloheptanones. The configurations of the products were deduced from n.m.r. spectra. The observed stereospecificity of the reaction is due to the preferential formation of the intermediate bromonium ion on the exo-face of the cyclopentene ring. 7-Chlorobicyclo[3.2.0]hept-2-en-6-ones (1) and (4) gave mixtures of the corresponding 2,3-disubstituted bicycloheptenones and 3,8-disubstituted 2-oxatricyclo[3.2.1.03,6]octanones when brominated in solvents capable of addition across the activated carbonyl group. Spectral evidence suggests that the 3-endo- and 7-endo-substituents determine the preferred conformation of the bicycloheptanones.