in the art can be used e g zinc chloride titanium tetrachloride aluminum chloride boron trifluoride or boron trifluoride etherate good results are obtained with boron trifluoride etherate the lewis acids are advantageously used in an amount of 0 1 mol percent to 5 mol percent relative to the 6 methyl 5 hepten 2 one used it is possible to carry out conversion in the absence of a solvent but an inert solvent is advantageously used for example halogenated hydrocarbons such as methylene chloride or carbon tetrachloride have proved successful usually the ketene is added and the reaction takes place at a temperature of u2212 30 u00b0 c to 40 u00b0 c preferably u2212 30 u00b0 c to 0 u00b0 c depending on the temperature gradient of the ketene addition the 4 methyl 4 4 methylpent 3 en 1 yl 2 oxetanone can form an oligomer of the formula where n is usually a number u2267 1 at low temperatures of u2212 30 u00b0 c to 0 u00b0 c 4 methyl 4 4 methylpent 3 en 1 yl 2 oxetanone of the formula i is generally formed whereas temperatures of 0 u00b0 c to 40 u00b0 c generally give rise to the oligomer of the formula v the desired lactone can be isolated in pure form by methods conventional to the skilled worker e g by removing the ketene and the excess solvent and finally by distillation e g in a thin film evaporator where r 2 is hydrogen or.