STEREOSELECTIVE SYNTHESIS AND DIELS-ALDER REACTIONS OF (Z)-1,2-BIS(PHENYLTHIO)-1,3-BUTADIENE AND (E)-1,2-BIS(PHENYLTHIO)-1,3-BUTADIENE

Bromination of 3-phenylthio-2-s ulfolene (2) with N-bromosuccinimide g ave 2-bromo-3-phenylthio-2-sulfolene (3) which was converted mainly to 2,3-bis(phenylthio)-2-sulfolene (4) by treatment with sodium phenylth iolate. Thermal desulfonylation of 4 at different temperatures in the presence of a base (DBU) yielded stereoselectively the (Z)- and (E)-1, 2-bis(phenylthio)-1,3-butadiene (6). These two geometric isomers could be thermally interconverted. The Diels-Alder reactions of 6 were also investigated. Only the (Z)-diene 6a could undergo the Diels-Alder rea ction; the (E)-diene 6b was in situ converted to the Z isomer before u ndergoing the Diels-Alder reaction. The reaction of 6a with N-phenylma leimide gave the cycloaddition product 7 with complete endo selectivit y, but under daylight or during chromatography it readily underwent a thioallylic rearrangement to yield 8 with inversion of configuration. The cycloaddition of 6a with methyl acrylate proceeded regiospecifical ly, but generating a mixture of endo and exo isomers. The endo/exo rat io could be increased by using ZnCl2 as the catalyst.