Abstract: The invention relates to a novel process for the preparation of pregnane derivatives of formula I, ##STR1## wherein R.sup.1 stands for a methyl or an ethyl group,R.sup.2 represents a hydrogen atom or a methyl group, andX is a hydrogen atom or a formyl or acetyl group, and the bond indicated by a dotted and a continuous line stands for a single or a double bond between the two neighboring carbon atoms.According to the invention a trifluoroacetate ester of formula II, ##STR2## wherein R.sup.1 and R.sup.2 are as defined above, is reacted with formic acid or acetic acid in the presence of a catalytic amount of a mercury salt in a dipolar proton-free or basic solvent. The formyl or acetyl group being in the place of X can be split off in a way known per se.The process provides a novel advantageous method for building up the pregnane side chain characteristic of corticoids.