PI-FACIAL SELECTIVITY IN DIELS-ALDER REACTIONS OF [4.3.2]PROPELLA-2,4,8,10-TETRAEN-7-ONE AND ITS DERIVATIVES - SYNTHESIS OF 2,5-ETHENO[4.3.2]PROPELLA-3,8,10-TRIEN-7-ONES

The Diels-Alder additions of dimethyl acetylenedicarboxylate, methyl propiolate, and maleic anhydride to [4.3.2]propella-2,4,8,10-tetraen-7-one (1), and its 8,9- and 10,11-dihydro derivatives, 2 and 3, and [4.3.2]propella-2,4,7,10-tetraene (4) occur steroselectively at the face of cyclohexadiene ring syn to five-membered ring. The-pi-facial selectivity of the additions is rationalized in terms of difference in dihedral angles between the cyclohexadiene and two flanking rings. Treatment of the adduct of maleic anhydride to 1 with Ni(PPh3)2(CO)2 gives 2,5-etheno[4.3.2]propella-3,8,10-trien-7-one (5a). The electronic absorption spectrum of 5a exhibits characteristic longer wave absorptions compared with the corresponding partly saturated derivatives, suggesting the existence of longicyclic interaction among its four-pi-bonds. Irradiation of 5a results in transannular [2 + 2] cycloaddition rather than cleavage into antiaromatic bicyclo[3.2.0]heptatrienone and benzene.