Chemical consequences of long-range orbital interactions in cholestane-3,7-diol monosulfonate esters. A seven-center fragmentation

The manifestation of through-five-bond interactions in the reactions of the rigid 1,5-diol monosulfonate esters (1-3), having an ideal all-trans geometry of the sigma-relay, with sodium tert-amylate is investigated. It has been shown that the deprotonation of the alcohol group in 2 and 3, which results in an increased electrofugal ability of this group, finds expression in a seven-center fragmentation. This fragmentation also illustrates that effective through-five-bond interactions exist between the alcoholate group and the carbocationic center which is generated during the heterolysis. The reaction outcome of mesylate 1 does not indicate effective through-bond interactions, and only elimination is observed. This difference can be attributed to the alkyl substituents on the gamma- and alpha-positions to the mesylate group in 2 and 3, respectively, which stimulate the seven-center fragmentation. Though a reasonable amount of fragmentation product is obtained from 3, the through-five-bond interaction is not strong enough to dominate the reaction course completely and typical El-like processes, i.e., elimination and rearrangement, are competitive. As expected, only a 1,2 Me-shift is observed in the reaction of the axial mesylate 4 where a gauche interaction is present in the geometry of the sigma-relay. The presence of through-bond interactions in the reactions of 3 and 4 becomes apparent by comparison of the reactivity of 3 and 4 with their O-silylated analogs 5 and 6.