Pinching group stabilization. The synthesis and thermal isomerization of 10-Oxapentacyclo[6.3.2.13,6.01,8.02,7]tetradeca-4,12-diene

Abstract

Sensitized irradiation (benzophenone, 0�, N2, pyrex filter, medium pressure Hg lamp) of dimethyl tricyclo[4.2.1.02,5]nona-3,7-diene-3,4-dicarboxylate (11) in (E)-1,2-dichloroethene yielded a mixture of 1 : 1 adducts (13) and (14) by site selective [2 π+2 π] cycloaddition at the cyclobutene π-bond. Reduction of the (Z)-dichloro isomer(13) with lithium aluminium hydride formed the related diol (16) which is the immediate precursor to the cyclic ether (18). Dechlorination of (18) with zinc in ethanol forms the title diene (19). Thermolysis of the polycyclic diester (20) affords the fragmentation products cyclopentadiene and dimethyl benzene-1,4-dicarboxylate. In contrast, the title compound (19) containing the cyclic ether ring was more stable and yielded the novel isomer (28) as the major product only upon flash vacuum pyrolysis at 560�(1.5 × 10-2 Torr). This difference in behaviour is attributed to a pinching group effect exerted by the cyclic ether present in (19).