[O:1]=[C:2](Cl)OC(Cl)(Cl)Cl.Cl.[CH3:10][O:11][C:12](=[O:21])[CH:13]([N:18]=[N+]=[N-])[CH2:14][CH2:15][CH2:16][CH3:17]>C(OCC)(=O)C.[Re]([O-])(=O)(=O)=O.[Na+]>[CH3:10][O:11][C:12](=[O:21])[C:13]([N:18]=[C:2]=[O:1])=[CH:14][CH2:15][CH2:16][CH3:17]27.3 mg (0.1 mmole) of sodium perrhenate and 1.04 grams (5.25 mmoles) of diphosgene were present at 80° C. in 1 ml of ethyl acetate saturated with hydrogen chloride. Then there were added within 20 minutes with vigorous stirring 1.71 grams (10 mmoles) of 2-azidohexanoic acid methyl ester dissolved in 5 ml of ethyl acetate. The reaction was ended after 1 hour uniform development of gas. The reaction mixture was then filtered with suction over a vacuum frit and the ethyl acetate distilled off under reduced pressure. The crude product thus obtained was distilled in a high vacuum (0.0013 mbar) at 50° to 55° C. apparatus temperature in a bulbed tube. There resulted 1.35 grams (79.8% of theory) of analytically pure 2-isocyanato-2-hexenoic acid methyl ester as a colorless, readily mobile liquid:
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