A process for the preparation of a compound (I) ##STR1## wherein R is a hydroxyl protecting group which comprises isomerising a compound of formula (I) wherein R is a hydroxyl protecting group by reacting the compound of formula (I) with a sterically hindered organic base in the presence of a Lewis acid and a compound of forumula (III) ##STR2## wherein R.sub.1 and R.sub.2 each independently represent cyano, COR.sub.5 or COR.sub.2 R.sub.6 or R.sub.1 andR.sub.2 together with the carbon atom to which they are attached from a C.dbd.O group;R.sub.5 represents alkyl, cycloalkyl, amino, alkylamino, dialkyl amino or optionally substituted phenyl or phenylalkyl group;R.sub.6 represents alkyl, cycloalkyl or optionally substituted phenyl or phenylalkyl group;R.sub.3 and R.sub.4 represent each independently hydrogen, alkyl, alkoxy or optionally substituted phenyl group.