Sequential Cycloaddition-Cycloreversion-Cycloaddition-Cope Rearrangement with an Annelated Norbornadiene and Electrophlic Dienes. Unusual 2s + ?2a + ?2a>Transformation of a Pentacyclo-7,10.04,13.06,11>octadeca-3,8,14,17-tetraene.

Article abstract of DOI:10.1016/S0040-4020(01)81298-9

Hexachloronorbornadiene efficiently forms a 1:1 adduct 3 with 5,6-bismethylenenorbornene.Adduct 3 on reaction with 1,2,3,4-tetrachlorothiophene dioxide (TCTD) under mild conditions gives an SO2 bridged adduct,which loses SO2, the product cycloreverting to give 1,2,3,4-tetrachlorobenzene and an annelated cyclopentadiene 8; 8 reacts further with TCTD to give, in 1:1 ratio, two compounds (14 and 15) shown by mass, 13C and 1H NMR spectroscopy and single crystal X-ray structure determination to be the stereoisomeric products of a rare type of specific diene capture by TCTD.E vidence is presented that this result (and analogous examples) is best understood in terms of ''normal'' <4+2>? cycloadditions 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 expected towards electron-deficient diene tetracyclone; the <4+2>? cycloadduct decarbonylates on mild thermolysis, concomitant cycloreversion also giving diene 8.At much higher temperatures in the presence of excess tetracyclone, diene 8 likewise behaves as dienophile; the carbonyl-bridged major cycloadduct decarbonylates to give an intermediate which undergoes an unusual thermally-allowed 2_s + ?²_a + ?²_a> 1,3-shift/cycloaddition, delivering a symmetrical cage-like structure 24, as indicated by mass and 13C and 1H NMR spectroscopy.