SEQUENTIAL CYCLOADDITION-CYCLOREVERSION-CYCLOADDITION-COPE REARRANGEMENT WITH AN ANNELATED NORBORNADIENE AND ELECTROPHILIC DIENES - UNUSUAL]OCTADECA-3,8,14,17-TETRAENE(PI(2)(A)+SIGMA(2)(A)] TRANSFORMATION OF A PENTACYCLO)

Hexachloronorbornadiene efficiently forms a 1:1 adduct 3 with 5,6-bism ethylenenorbornene. Adduct 3 on reaction with 1,2,3,4-tetrachlorothiop hene dioxide (TCTD) under mild conditions gives an SO2 bridged adduct, which loses SO2, the product cycloreverting to give 1,2,3,4-tetrachlo robenzene and an annelated cyclopentadiene 8; 8 reacts further with TC TD to give, in 1:1 ratio, two compounds (14 and 15) shown by mass, C-1 3 and H-1 NMR spectroscopy and single crystal X-ray structure determin ation to be the stereoisomeric products of a rare type of specific die ne capture by TCTD. Evidence is presented that this result (and analog ous examples) is best understood in terms of ''normal'' [4+2]pi cycloa dditions to 8 as dienophile towards electron-deficient TCTD, followed by rapid Cope rearrangement of the SO2-bridged intermediate adducts to give the observed products. At low temperatures adduct 3 behaves as e xpected towards electron-deficient diene tetracyclone ; the [4+2]pi cy cloadduct decarbonylates on mild thermolysis, concomitant cycloreversi on also giving diene 8. At much higher temperatures in the presence of excess tetracyclone, diene 8 likewise behaves as dienophile; the carb onyl-bridged major cycloadduct decarbonylates to give an intermediate which undergoes an unusual thermally-allowed [pi2s + pi2a + sigma2a] 1 ,3-shift/cycloaddition, delivering a symmetrical cage-like structure 2 4, as indicated by mass and C-13 and H-1 NMR spectroscopy.