1: (1S,6R)-diethyl 5-oxo-3-oxabicyclo[4.1.0]heptane-7,7-dicarboxylate With sodium tetrahydroborate in tetrahydrofuran T=20 - 60°C; 6 h; Inert atmosphere; 2: methanol in tetrahydrofuran T=60°C; 1.16667 h; Inert atmosphere; 3: With acetic acid in tetrahydrofuran; methanol T=0°C; Inert atmosphere;