7-Benzyl-5,6,7,8-tetrahydroimidazo[1,2-a]pyrazine-2-carbaldehyde (139 mg) was added to the dry acetonitrile (8.7 mL) solution of anhydrous MgBr2 (325 mg) under a nitrogen atmosphere at room temperature. The dry THF solution (8.7 mL) of (5R,6S)-6-bromo-7-oxo-4-thia-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid 4-nitro-benzyl ester (223 mg) was added to the mixture, cooled to −20° C., and Et3N (0.24 mL) was added in one portion. The reaction vessel was covered with foil to exclude light. The reaction mixture was stirred for 5 h at −20° C. and treated with acetic anhydride (0.11 mL) and DMAP (7 mg) in one portion. The reaction mixture was warmed to 0° C. and stirred for 15 h at 0° C. The mixture was diluted with ethyl acetate and washed with 5% citric acid aqueous solution, saturated sodium hydrogen carbonate, water and brine. The organic layer was dried (MgSO4) and filtered. The filtrate was concentrated under reduced pressure. The residue was applied to silica gel column chromatography, then the column was eluted with n-hexane—AcOEt (3/1˜1/1). The titled compound was obtained as two diastereo mixture (80/20, purple amorphous solid, 233 mg, 61%).
