acetic anhydride in dichloromethane stirring with anhydrides (both in slight stoich. excess) for 24 h (roomtemp.); collection (filtration), washing (ether), stirring with CH2Cl2 for 30 min, filtration, solvent removal (vac.), chromy. (SiO2, AcOEt/CH2Cl2=1:1;second fraction); elem. anal.; product ratio depending on ratio of anhydrides;
