Intramolecular cycloaddition of azoalkenes derived from terminal alkenoic and alkynoic acids

Abstract

The azoalkenes (1) derived from terminal alkenoic acids H₂CCH(CH₂)_nCO₂H (n= 1, 3, and 4) have been generated in order to determine whether intramolecular cycloaddition would take place. The azoalkenes derived from hex-5-enoic acid (n= 3) underwent intramolecular cycloaddition and four cycloadducts, the pyridopyridazines (6), have been isolated in moderate yield. The azoalkenes derived from but-3-enoic acid (n= 1) and from hept-6-enoic acid (n= 4) did not undergo intramolecular cycloaddition: the intermolecular cycloadduct (5) was formed when the chiorohydrazone (2a) was dehydrochlorinated in the presence of cyclopentadiene.

The intramolecular cycloadducts (8) and (10) have also been isolated from reactions of the hydrazones (7) and (9), derived from cyclopent-2-enylacetic acid and pent-4-ynoic acid, respectively, with sodium carbonate. The adducts (10) are formed by cycloaddition to an unactivated triple bond, a reaction which has not previously been observed with azoalkenes. Compounds (10) are unstable in air, and a long-lived radical, which has been formulated as compound (11), has been detected as an oxidation product of the dihydropyridazine (10c).