Lewis-acid-catalysed oxygenation of l, l′-bicyclohexenyl and α-terpinene. reactions in dichloromethane and liquid sulphur dioxide

1, 1′-Bicyclohexenyl and α-terpinene are converted into the corresponding cyclic peroxides in the presence of oxygen and Lewis acids in dichloromethane. In liquid sulphur dioxide under oxygen, the major product from 1, 1′-bicyclohexenyl in the presence of SnCl₄, SbF₅ and SbCl₅ is the corresponding cyclic sulphone, whereas the cyclic peroxide is almost exclusively formed in this medium with MoCl₅, WCl₆ and VOCl₃. Under the same conditions, α-terpinene is quantitatively oxygenated in a dark reaction, in the absence of added Lewis acids. In a 1: 1 dichloromethane-sulphur dioxide medium, ergosteryl acetate requires irradiation for conversion into the corresponding peroxide. Mechanistic interpretations of these observations are presented.