Abstract Experiments are presented which show that base treatment of the 7-halobicyclo- [3.2. 0] hept-2-en-6-ols 1 to 4 produces ring contraction to the aldehydes 5 and 6, except in those cases of the isomer series 4 where the substituent on the endo-side of C (7) is larger than a chlorine atom, in which case endo-hydrogen migration occurs to give the ketones 7.
