Collect. Czech. Chem. Commun. 1982, 47, 117-123

Mechanism of hypobromous acid addition to unsaturated steroids. Competition between neighboring group participation and external nucleophile attack

Pavel Kočovskýa, František Turečekb and Václav Černýa

a Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, 166 10 Prague 6
b Jaroslav Heyrovský Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 121 38 Prague 2


Mechnism of hydrobromous acid addition to 2,3- and 5,6-unsaturated steroids bearing a 19-acetoxy group were established by means of isotopic labeling. In the cleavage of 2α,3α-bromonium ion III participation by the 19-acetoxyl predominates over external attack, 5(O)n process being the major (88%) and 7(O)π,n process the minor (10%) reaction. The external attack by water contributes only 2%. The 5α,6α-bromonium ion XI is cleaved by participation of the 19-acetoxyl at C(5) in a 6(O)π,n process (78%) via XII - XIII. Cleveage at C(6) leads to XIV and occurs partially with 7(O)π,n participation by the 19-acetoxyl (4%) and partially by external attack of water (8%). These results bring evidence for the formation of a seven-membered acetoxonium ion ring as intermediate in participation processes involving ester carbonyl groups.