A new example of reversible thermal isomerization of 3H-pyrazole into 4H-pyrazole

Abstract

9-Diazofluorene reacted with phenyl prop-1-yn-1-yl sulfone in diethyl ether at 20°C (3–4 days) to give 5'-(benzenesulfonyl)-4'-methylspiro[fluorene-9,3'-pyrazole] (3H-pyrazole) as the only 1,3-dipolar cycloaddition product formed according to the von Auwers rule. On heating in boiling ethanol for 0.5 h, 5'-(benzenesulfonyl)-4'-methylspiro[fluorene-9,3'-pyrazole] underwent partial reversible isomerization into a 4H-pyrazole derivative, 3-(benzenesulfonyl)-3a-methyl-3aH-dibenzo[e,g]indazole, whereas in toluene at 180°C (40 min) it was completely converted into thermodynamically more stable 2-(benzenesulfonyl)-3-methylpyrazolo[1,5-f]phenanthridine with a 1H-pyrazole fragment. 3a-Methyl-2H-dibenzo[e,g]indazol-3(3aH)-one was formed when the 3H- and 4H-pyrazoles were kept in glacial acetic acid at 20°C in the presence of a catalytic amount of sulfuric acid, followed by treatment with water. Alcoholic potassium hydroxide promoted reversible isomerization of the 3H-pyrazole into 5'-(benzenesulfonyl)-4'-methylidene-2',4'-dihydrospiro[ fluorene-9,3'-pyrazole].