Synthesis, structure, and reactions of secosteroids containing a medium-sized ring. Part 17. Structure-reactivity relationship in the solvolysis of 5,10-secosteroidal 3-tosylates

Abstract

Kinetic measurements of the solvolysis of (Z)-3α- and (Z)-3β-, and (E)-3α- and (E)-3β-tosyloxy-5,10-secocholest-1(10)-en-5-ones in buffered aqueous acetone (90 : 10 v/v) reveal that the (Z)-3α-, (E)-3α-, and (E)-3β-tosylates are solvolysed according to a first-order rate law (the relative rates being ca. 1 : 3 : 8), while the (Z)-3β-ester, under the same conditions, reacts at a much slower rate by a complex mechanism, the kinetics of which are best approximated by a second-order law. These data and product analysis indicate that the former three esters are solvolysed with considerable double bond participation [resulting in the case of the (E)-3-esters in intramolecular cyclopropane ring closure], and that this interaction is unimportant for the (Z)-3β-tosylate. On the basis of conformational analysis of the starting tosylates and stereoelectronic requirements for homoallylic interaction, a possible mechanistic pathway for these solvolyses is proposed.