CONFIGURATIONAL AND CONFORMATIONAL STEREOSELECTIVITY IN THE ACID-CATALYZED RING-OPENING OF 1-PHENYLCYCLOHEXENE OXIDES

The stereoselectivities of several reactions of the conformationally mobile aryl-substituted oxiranes la, b under acidic conditions were compared with those of its rigid teri-butyl analogues 2 and 3. The amounts of syn adducts formed in the reactions of 2 and 3 are always higher in the case of 3. Furthermore, the tendency toward the syn opening observed for 1 is higher than that obtained for either of the rigid epoxides. This last result can not be rationalized simply on the basis of the Winstein-Holness hypothesis that the (erf-butyl is merely an anchor. How ever, the syn stereoselectivity of the reaction of 3 resembles more closely that of the nonrigid epoxide 1, suggesting that 1 reacts preferentially through conformation lb corresponding to 3. The differences in stereoselectivities observed between the reactions of 2 and 3 and the higher reactivity of 1 through conformation lb have been rationalized in terms of the hypothesis that one of the determining factors in these reactions is the preferential C-0 bond cleavage of the protonated 1, 2-epoxycyclohexane in such a way as to give an axial hydroxy group (—axial cleavage–).This observation can probably be generalized to explain the well-known Furst-Plattner rule of the diaxial ring opening of 1, 2-epoxycyclohexanes. © 1979, American Chemical Society. All rights reserved.