CS(C)=O.C(Cl)(=O)C(Cl)=O.[OH:11][CH2:12][CH2:13][CH:14]([C:24]1[CH:29]=[CH:28][CH:27]=[CH:26][CH:25]=1)[C:15]([N:17]1[CH2:23][CH2:22][CH2:21][CH2:20][CH2:19][CH2:18]1)=[O:16].C(N(CC)CC)C>ClCCl>[CH:12]([CH2:13][CH:14]([C:24]1[CH:29]=[CH:28][CH:27]=[CH:26][CH:25]=1)[C:15]([N:17]1[CH2:23][CH2:22][CH2:21][CH2:20][CH2:19][CH2:18]1)=[O:16])=[O:11]
Dimethylsulphoxide (3.97 g, 50.9 mmol) in dichloromethane (25 ml) was added to a solution of oxalyl chloride (3.09 g, 24.4 mmol) at -50° C. in dichloromethane (25 ml). The reaction mixture was kept at this temperature for 5 min, treated with a solution of the product of step (a) (5.54 g, 21.2 mmol) in dichloromethane (30 ml), and, after 1 h, treated with triethylamine (10.72 g, 0.106 mol). The cooling bath was removed and the reaction mixture allowed to warm to room temperature. Water (100 ml) was added and the separated aqueous phase washed with dichloromethane (50 ml). The combined organic phases were washed with brine (50 ml), water (50 ml), dried (Na2SO4) and concentrated in vacuo to afford the title compound as an oil (5.30 g, 96.4%).
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