Cyclobutane Ring-Opening of Allene-Enone Photoadducts by Boron Trifluoride Etherate; A Convenient Method for the Introduction of the Isopropenyl Group into α,β-Unsaturated Ketones - Duc, DO Khac Manh; Fetizon, Marcel; Hanna, Issam; Lazare, Sylvain (Synthesis, 1981)