The problem with the reduction of the ester function in a ketoester lies with the fact that the reducing reagent, e.g. LAH, would not only reduce the ester but also would reduce the ketone. To solve this problem one reacts the ketoester with a a 1,3-diol which leads to the conversion of the ketone into a cyclic ketal. Ketals are not attacked by nucleophiles and will therefore remain unaffected by LAH reduction of the ester. The ketone can be regenerated later on by treatment of the ketal with dilute acid.