Competition of 5(O)n and 6(O)n participation by 19a-substituent in hypobromous acid addition to 2,3- and 5,6-unsaturated 19-homocholestane derivatives

19-Substituted 2,3-unsaturated cholestane derivatives IIIa-IIIc react with hypobromous acid to give the cyclic bromo ether VI as sole or major product arising by cleavage of the 2α,3α-bromoion V with 6(O)n participation by the 19a-group. The 5,6-unsaturated alcohol IVa reacts with hypobromous acid to yield the cyclic bromo ether X as product of 5(O)n participation. Under the same conditions, the methoxy derivative IVb yields two products, X and XI, with violation of Fürst-Plattner rule. In similar manner as in the preceding case, the products arise by cleavage of the bromonium ion IXb with 5(O)n participation by the 19a-methoxyl. In contrast, the 5,6-unsaturated acetoxy derivative IVc reacts without participation via two diastereoisomeric bromonium ions VIIIc and XV to give the corresponding diaxial bromohydrins XVI and XVII which undergo spontaneous cyclization to the epoxides XVIII and XIX. The course of these reactions, comparison with the lower homologs of the type I and II, the role of Markovnikov and Fürst-Plattner rules and capability of the particular functional groups to participate in 5(O)n and 6(O)n processes are discussed.