[C:1]([CH:4]([CH2:9][C:10]([CH3:12])=[CH2:11])C(OC)=O)(=[O:3])[CH3:2].[OH-].[K+].[N:15]([O-])=[O:16].[Na+].S(=O)(=O)(O)O.C(=O)=O>>[CH3:12][C:10](=[CH2:11])[CH2:9][C:4](=[N:15][OH:16])[C:1](=[O:3])[CH3:2]At 10° C., 200 g of methyl 2-acetyl-4-methylpent-4-enoate (preparation: cf. Tetrahedron (1985) 4633) in 2000 ml of aqueous 10% strength potassium hydroxide solution were admixed with 85 g of sodium nitrite, and the mixture was stirred at 23° C. for 18 hours. With ice-cooling, 1000 ml of 10% strength sulfuric acid were subsequently added dropwise such that the internal temperature remained below 10° C. The mixture was then stirred at 10° C. until evolution of CO2 had ceased. For work-up, the mixture was extracted with methyl tert-butyl ether, the combined organic phases were extracted with 3 N aqueous sodium hydroxide solution and the alkaline phases were adjusted to pH 1 using 20% strength sulfuric acid. They were then extracted with methylene chloride, the extract was dried over sodium sulfate and the solvent was removed under reduced pressure. This gave 142 g of the title compound as a yellow oil which crystallized on standing.
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