Preparation of unsaturated ketones

A process for the preparation of unsaturated ketones of the formulae Ia and Ib

• where R1, R2, R3, R4 and R5 are each hydrogen or alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl or aralkyl radicals which are substituted or unsubstituted by oxygen-containing groups, it also being possible for R1 and R2 together to form a 5- or 6-membered carboxylic ring, which comprises the combination of the following reactions:
• a) reacting an allyl alcohol of the formula IIa or a propargyl alcohol of the formula IIb
• with an isopropenyl ether of the formula III
• where R6 is alkyl having 1 to 4 carbon atoms, with formation of a ketal of the formula IV
• where R6 has the abovementioned meaning, as a byproduct,
• b) preparation of the isopropenyl ether of the formula III by reacting a ketal of the formula IV with propyne or allene, and a mixture thereof in the gas phase at elevated temperatures in the presence of a heterogeneous catalyst containing zinc or cadmium together with silicon or oxygen, and
• c) feeding the ketal of the formula IV formed in the reaction (a) into stage (b) for the preparation of the isopropenyl ether of the formula III again.