2-Formyl-6,7-dihydro-4H-pyrazolo[1,5-a]pyrazine-5-carboxylic acid 4-nitrobenzyl ester (2.71 g) was added to the dry acetonitrile (164 mL) solution of anhydrous MgBr2 (cont. 98%) (6.17 g) under a nitrogen atmosphere at room temperature. The dry THF solution (164 mL) of (5R,6S)-6-bromo-7-oxo-4-thia-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid 4-nitrobenzyl ester (cont. 96.5%) (3.27 g) was added to the mixture, cooled to −20° C., and Et3N (cont. 99%) (9.24 mL) was added in one portion. The reaction vessel was covered with foil to exclude light. The reaction mixture was stirred for 1.5 h at −20° C. and treated with acetic anhydride (cont. 97%) (3.19 mL) and DMAP (cont. 99%) (203 mg) in one portion. The reaction mixture was warmed to 0° C. and stirred for 1 h at 0° C. Acetic anhydride (3.19 mL) was added to the solution 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), followed by concentration under reduced pressure. The residue was purified with silica-gel column chromatography three times (n-hexane-AcOEt (1/1 to 2/3), CHCl3-acetone (29/1 to 19/1) and CHCl3-acetone (29/1)). The titled compound was obtained as yellow amorphous (diastereo-mixture (64:36), 3.30 g, 53%).
