1: 2,2-Dimethyl-[1,3]dioxane-4,6-dione; 3-carboxyazetidine-1-carboxylic acid tert-butyl ester With 4-(N,N-dimethlyamino)pyridine; 1,1'-carbonyldiimidazole in dichloromethane T=0 - 20°C; 2: ethanol 6 h; Heating / reflux;