[Cl:1][C@H:2]1[C:23](=[O:24])[N:4]2[C:5]([C:16]([O:18][C:19]([CH3:22])([CH3:21])[CH3:20])=[O:17])=[C:6]([CH2:11][O:12][C:13](=[O:15])[CH3:14])[CH2:7][S:8](=[O:10])(=[O:9])[C@H:3]12.[CH2:25](Cl)Cl.Cl.CNC.C=O>C(O)(C)(C)C>[Cl:1][C@H:2]1[C:23](=[O:24])[N:4]2[C:5]([C:16]([O:18][C:19]([CH3:20])([CH3:22])[CH3:21])=[O:17])=[C:6]([CH2:11][O:12][C:13](=[O:15])[CH3:14])[C:7](=[CH2:25])[S:8](=[O:9])(=[O:10])[C@H:3]12A mixture of t-butyl 7α-chloro-3-acetoxymethyl-3-cephem-4-carboxylate-1,1-dioxide (1.9 g), methylene chloride (3 ml), dimethylamine hydrochloride (0.817 g), t-butanol (80 ml) and formaldehyde solution (I.10 ml, 37% w/v in water) was heated to reflux at 90° C. for 3 hours. Solvent was removed under reduced pressure and the residue was dissolved in methylene chloride, washed with water, brine, dried and concentrated to give 1.6 g (81.6%) of t-butyl 7α-chloro-2-methylene-3-acetoxymethyl-3-cephem-4-carboxylate-1,1-dioxide, m.p. 148°-149° C. This product without further purification was used in the next step.
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