[Cl:1][C:2]1[CH:7]=[CH:6][C:5]([S:8]([N:11]2[CH:16]3[CH2:17][C:18](=[O:20])[CH2:19][CH:12]2[CH2:13]OC3)(=[O:10])=[O:9])=[CH:4][CH:3]=1.[CH:21](OCC)=[O:22].[O-][CH2:27][CH3:28].[Na+]>C1COCC1.C(O)C>[Cl:1][C:2]1[CH:7]=[CH:6][C:5]([S:8]([N:11]2[CH:16]3[CH2:27][CH2:28][CH2:13][CH:12]2[C:19](=[CH:21][OH:22])[C:18](=[O:20])[CH2:17]3)(=[O:10])=[O:9])=[CH:4][CH:3]=1To a stirring mixture of 9-(4-chloro-benzenesulfonyl)-3-oxa-9-aza-bicyclo[3.3.1]nonan-7-one (170 mg, 0.54 mmol) in THF/ethanol (3 mL, 1/1, v/v) was added ethyl formate (400 mg, 5.4 mmol) followed by sodium ethoxide (0.61 mL of 21% solution in ethanol). The resulting mixture was heated to 60° C. for 30 minutes after which the solution was cooled to room temperature and quenched by the addition of a saturated aqueous NH4Cl solution (10 mL). The resulting mixture was extracted with EtOAc (2×20 mL) and the combined organic phases were dried (Na2SO4), filtered and concentrated under vacuum to yield 9-(4-chloro-benzenesulfonyl)-2-hydroxymethylene-9-aza-bicyclo[3.3.1]nonan-3-one as a yellow solid. This crude product was directly taken to the next reaction without any further purification. Retention time (min)=1.651 min, Method [1], MS (ESI) 344.0 (M+H).
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