Type-Two Intramolecular Diels−Alder Reactions of Pyrazolo-<i>o</i>-quinodimethanes

Two isomeric series of homologous <i>N</i>-(acryloyloxy)alkylated pyrazolo-3-sulfolenes <b>10a</b>−<b>c</b> and <b>18a</b>−<b>c</b> have been efficiently synthesized from a common starting material, 4,6-dihydro-1<i>H</i>-thieno[3,4-<i>c</i>]pyrazole (<b>5</b>). Thermolysis of these fused 3-sulfolenes provides the corresponding <i>o</i>-quinodimethanes which simultaneously undergo “type-two” intramolecular Diels−Alder reactions to form two- and three-atom-bridged tricyclic pyrazoles which are otherwise difficult to prepare. It was also demostrated that, depending on the <i>N</i>-substitution position of the pyrazolo-fused 3-sulfolenes, the temperature required for the thermal extrusion of SO<sub>2</sub> and the regioselectivity of the T2-IMDA reactions were influenced substantially.