Remote Substituent Effects on the Cheletropic and Homocheletropic Additions of Sulfur-Dioxide to Exocyclic Polyenes Grafted onto Bicyclo[2.2.2]Octane Skeletons
Under conditions of kinetic control (-20-degrees-C), the competition between the cheletropic and homocheletropic addition of SO2 to 5,6,7,8-tetramethylidenebicyclo[2.2.2]oct-2-ene 8 was unaffected by a methyl substituent at C(2). However, the latter induced a regioselectivity rendering the 1,4-diene moiety at C(5), C(8) slightly preferred over that at C(6), C(7) in 2-methyl-5,6,7,8-tetramethylidenebicyclo[2.2.2]oct-2-ene 11. No sulfolane was observed resulting from a homocheletropic addition of SO2 to 5,6,7,8-tetramethylidenebicyclo[2.2.2]octan-2-ol 12 and 7-methoxy-2,3,5,6-tetramethylidenebicyclo[2.2.2]octane 13. The 1,3-diene unit syn with respect to the alcohol function of tetraenol 12 was preferred for the cheletropic addition of the first equivalent of SO2. In contrast, SO2 preferred the 1,3-diene moiety anti with respect to the methoxy group of 13. Whereas pentaenes 8 and 11 refused to generate bis-sulfolenes resulting from two successive cheletropic additions of SO2, exocyclic tetraenes 12 and 13 gave the corresponding bis-sulfolenes 20 and 28, respectively, in good yields.
Keywords: Barrelene effect ; cheletropic addition ; homocheletropic addition ; 5 ; 6 ; 7 ; 8-tetramethylidenebicyclo[2.2.2]octan-2-ol ; 2-methyl-5 ; 6 ; 7 ; 8-tetramethylidenebicyclo[2.2.2]oct-2-ene ; sulfolane ; sulfolene ; hydrogen bridging ; rate constants ; Non-conjugated chromophores ; olefins
Univ lausanne,chim sect,bch dorigny,ch-1015 lausanne,switzerland.
Record created on 2005-11-09, modified on 2016-08-08