COC([C:5]1([CH2:16]/[CH:17]=[CH:18]\[CH2:19][CH3:20])[C:9](=[O:10])[CH:8]=[CH:7][CH:6]1[CH2:11][C:12]([O:14][CH3:15])=[O:13])=O.[Cl-].[Na+]>CS(C)=O.O>[CH3:15][O:14][C:12]([CH2:11][CH:6]1[CH:5]([CH2:16]/[CH:17]=[CH:18]\[CH2:19][CH3:20])[C:9](=[O:10])[CH:8]=[CH:7]1)=[O:13]A 360 mg quantity of 5-methoxycarbonyl-4-methoxycarbonylmethyl-5-(cis-2-pentenyl)-2-cyclopentenone is dissolved in 8 ml of dimethyl sulfoxide containing 23 mg of water. The solution is reacted in a sealed tube with addition of 20 mg of sodium chloride. After the solution has been heated at 175° C. for 4 hours, the product is cooled to room temperature and then extracted with ethyl ether. The ethereal layer is washed, dried and distilled for the removal of the solvent. Purification of the residue by a silica gel column affords a compound (1-a) and compound (1-b) (R2 =CH3 in each). Yield of compound (1-a): 62.5%. Yield of compound (1-b): 28.5%. Combined yield: 91%. B.p. 87°-91° C./2.5 mm Hg.
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