A process for preparing a 3-oxo-7-hydroxy-bicyclo[3,3,0]octan-2-ylcarboxylic acid ester of the formula ##STR1## wherein R.sub.1 is alkyl of 1-6 carbon atoms or phenalkyl of 7-10 carbon atoms andR.sub.2 is hydrogen, alkyl or 1-6 carbon atoms, phenalkyl of 7-10 carbon atoms, tetrahydropyranyl, tetrahydrofuranyl, .alpha.-ethoxyethyl, tri-C.sub.1-4 -alkylsilyl, wherein the alkyl moieties can optionally be substituted by phenyl, or ##STR2## wherein R.sub.3 is alkyl of 1-6 carbon atoms or aryl of 6-12 carbon atoms, comprises oxidizing the corresponding 7-hydroxy-2-oxabicyclo [3,3,0]octan-3-ylideneacetic acid ester of the formula ##STR3## wherein R.sub.1 and R.sub.2 are as defined above, to form the corresponding ketone, reacting the latter with a base which opens the ether oxygen containing ring, and then reducing the resultant product to prepare the 3-oxo-7-hydroxy-bicyclo[3,3,0]octan-2-ylcarboxylic acid ester.