Type-Two Intramolecular Diels−Alder Reactions of Pyrazolo-o-quinodimethanes
journal contributionposted on 09.08.2000 by Ta-shue Chou, Hong-Chuan Chen, Woei-Chang Yang, Wan-Sheung Li, Ito Chao, Shwu-Jiuan Lee
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Two isomeric series of homologous N-(acryloyloxy)alkylated pyrazolo-3-sulfolenes 10a−c and 18a−c have been efficiently synthesized from a common starting material, 4,6-dihydro-1H-thieno[3,4-c]pyrazole (5). Thermolysis of these fused 3-sulfolenes provides the corresponding o-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 N-substitution position of the pyrazolo-fused 3-sulfolenes, the temperature required for the thermal extrusion of SO2 and the regioselectivity of the T2-IMDA reactions were influenced substantially.