Ring c functionalised diterpenoids. Part VII. Formolysis of (16S)-ent-12α-p-tolylsulphonyloxykaurane

Abstract

Buffered formolysis of the title tosylate (1b) at room temperature followed by mild hydrolysis gave chiefly the corresponding alcohol (1a), smaller amounts of the epimeric ent-atisan-13-(6a and c) and -16-ols (7a and c), together with traces of the ent-kauranol (1d) and the ent-14(13→12)abeo-kauranol (5a). The product distribution is rationalised in terms of the relative stabilities of intermediate carbocations. Formolysis of the deuteriated tosylate (1g) demonstrated that the ent-atisan-13-ols are formed mainly via a classical ion [(B) in the Scheme]. A mechanism involving formation of ent-trachylobane (9a) and a corner-protonated cyclopropane (H) is proposed to account for the loss of deuterium associated with the conversion of (1g) into the ent-atisan-16-ols.