Reactions of 1-bromo-7-(2-hydroxyethoxy) cycloheptene 2 and its chloro analogue 3 with potassium t-butoxide in dimethyl sulphoxide or tetrahydrofuran gave cycloheptatriene and cis-8, 11-dioxabicyclo [5.4. 0] undec-2-ene 18 as the major products together with small amounts of the trans-isomer 17, 8, 11-dioxabicyclo [5.4. 0] undec-1 (7)-ene 14, 8, 11- dioxabicyclo [5.4. 0]-undec-1-ene 19, cyclohept-2-enone ethylene ketal 15, cyclohept-3- ...