of a solvent at temperatures of between 0 u00b0 and 90 u00b0 for a period of between 1 and 10 hours solvents that can be used include tetrahydrofuran ethers such as diethyl ether and hydrocarbons such as benzene and toluene the preferred temperature and reaction time ranges are 30 u00b0 to 80 u00b0 c for 3 to 4 hours respectively this is a key step in the process since it provides for the preparation of iv in high yields the trione iv may be used in step 2 without separating it from the reaction or it may be used after recovery from the mixture conventional methods of recovery such as crystallization evaporation chromatography and combinations thereof may be used it is preferred not to separate trione iv from the mixture for use in step 2 the mixed anhydride v may be prepared in accordance with chart b wherein r is the same as defined above this reaction is conducted in the presence or absence of a solvent and an acid acceptor at a temperature of between 0 u00b0 and 60 u00b0 c for a period of about 0 5 to 3 hours solvents that can be used include tetrahydrofuran diethyl ether dioxane and toluene the preferred solvent is tetrahydrofuran acid acceptors include trialkyl amines such as triethylamine triisopropylamine and heterocyclic amine such as dabco the preferred acid acceptor is triethylamine the preferred.