Formation of both acetylvinylcyclopropane and 4-acetylcyclopentene during the thermolysis of CHtripleCCH2C(OH)MeCH:CH2 (1) was confirmed in addn. to predicted AcCH2CH2CH:C:CH2. The mechanistic difference between thermolysis of 1 and CHtripleCCH2CH(OH)CH:CH2 was explained in terms of torsional effect in the transition state due to the presence of the Me group. DSJN Chem.Abstr. Vol 72, 131816e.