[H-].[Na+].[C:3](OC(C)C)(=[O:7])[C:4]([CH3:6])=O.[S:12]([CH:21](O)[C:22]([O:24][CH:25]([CH3:27])[CH3:26])=[O:23])[CH:13](O)[C:14]([O:16][CH:17]([CH3:19])[CH3:18])=[O:15].Cl>C(O)(C)C>[CH3:6][C:4]1[C:3]([OH:7])=[C:21]([C:22]([O:24][CH:25]([CH3:27])[CH3:26])=[O:23])[S:12][C:13]=1[C:14]([O:16][CH:17]([CH3:19])[CH3:18])=[O:15]Sodium hydride (132 g, 5.52 mol) was added gradually into isopropyl alcohol (2-3 l) with cooling. To the resulting solution was then added a mixture of isopropyl pyruvate (360 g, 2.76 mol) and di-isopropyl 2,2'-thiodiglycolate (648 g, 2.76 mol) over 20-30 minutes with ice-cooling so that the solution was kept at 60°-80° C. The solution was stirred further for 2 hours at a temperature of 60°-40° C. The solution was poured into an ice-cooled dilute hydrochloric acid solution to form a white crystalline mass, which was recovered by filtration and then washed with water. It was recrystallized from methanol (3 l) to give pure di-isopropyl 3-methyl-4-hydroxy-2,5-thiophene-dicarboxylate (270 g, 34%), m.p. 87°-89° C.
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