Regioselectivity in the formation of norbornene-fused pyrazoles: Preparation of 1-substituted derivatives of 4,5,6,7-tetrahydro-1H-4,7- methanoindazole

摘要

The structural characteristics of (±)-(exo,exo)-3-(hydroxymethylene) -5,6-(isopropylidenedioxy)bicyclo[2.2.1]heptan-2-one make the reactions between this β-diketone and hydrazines particularly interesting for elucidating the mechanism of pyrazole formation. The isolation and X-ray structure determination of two 5-hydroxy substituted Δ2-pyrazolines [(±)-(3aR*,4R*,5R*,6S*,7R*,7aR*) -7a-hydroxy-5,6-(isopropylidenedioxy)-3a,4,5,6,7,7a-hexahydro-4, 7-methano-1H-indazole] and [(±)-(3aS*,4R*,5R*, 6S*,7R*,7aR*)-7a-hydroxy-5,6-(isopropylidene-dioxy) -1-phenyl-3a,4,5,6,7,7a-hexa-hydro-4,7-methano-1H-indazole] has been determinant for proposing a mechanism. Besides B3LYP/6-31G* calculations have been carried out on all intermediate dihydroxypyrazolidines and 5-hydroxypyrazolines. Finally, the annular tautomerism of the NH-methanotetrahydroindazoles has been studied both experimentally (13C NMR) and theoretically: the Mills-Nixon effect favours the 2H-tautomer. © 2006 Elsevier Ltd. All rights reserved.