... Tt is assumed that the 16a-acetoxy group is in the usrial coplanar conformation of an ester group (GW Wheland, Resonance in Orqanic Chemistry, J. Wiley and ... is possihle only if the distance of these atoms is less than 3.3 A. (19) J. W. Corcoran and H. Hirschmann, J . Am ... Chem. ...