C ₂-Symmetry-Chiral Ketene Dithioacetals Derived from 1,1′ -Binaphthalene-2,2′ -dithiol: Diastereoselective Diels-Alder Reaction of the S-Oxides to Cyclopentadiene

The C ₂-symmetry-chiral ketene dithioacetal 1a (4-methylenedinaphtho[2,1-d:1′ ,2′ -f][1,3]dithiepin) derived from 1,1′binapththalene-2,2′dithiol (2) and its mono-, di-, tri- and tetroxide 1b-e were prepared. The sulfoxides 1b,c,d were obtained totally stereoselectively as the pseudoequatorial isomers. All the S-oxides 1b-e gave diastereoselective cycloaddition to cyclopentadiene. The relative stereochemistry of the adducts was assigned on the basis of nuclear Overhauser effect and lanthanide induced shift experiments. The dibenzo derivative 6 (6-methylenedibenzo[d,f][1,3]dithiepin 5,5,7,7-tetroxide) afforded with cyclopentadiene the expected adducts in the same ratio as the binaphthyl homolog, but the major component reverted to the minor one on heating.