O=C(C)CC[S:5]([CH2:8][CH2:9][C:10]([O:12][CH3:13])=[O:11])(=[O:7])=[O:6].C[O-:16].[Na+:17]>C1COCC1.CO>[CH3:13][O:12][C:10](=[O:11])[CH2:9][CH2:8][S:5]([O-:16])(=[O:7])=[O:6].[Na+:17]To a suspension of 12 (141 g, 636 mmol) in THF (1.2 L) and MeOH (700 ml) in an ice bath was added slowly sodium methoxide (144 ml of a 25% solution in MeOH, 1.05 equiv). At the end of the addition, the bath was removed and the mixture stirred at r.t. for 1 h. The solvents were than evaporated and the remaining solid was triturated in ether containing a small amount of EtOAc. The slightly hygroscopic white solid was filtered and dried under high vacuum o.n. to yield 108.7 g (98%) of 13.
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