Ancillary role of sulfur functionalities in cycloaddition reactions

2 Ancillary role of sulfur functionalities in cycloaddition reactions 

The use of phenyl vinyl sulfoxide as an acetylenic synthon in Diels-Alder reactions was reported in 1978 [525]. By heating phenyl vinyl sulfoxide with anthracene in chlorobenzene at 130°C for 120h, dibenzobarrelene was obtained in an 83% yield through elimination of phcnylsulfinic acid (see Section 3.1) trom the pnmary cycloadduct. 

This early example is one of the numerous synthetic utilizations of the cycloadditions of olefins activated by sulfur atoms at various oxidation levels (vinyl sulfides, sulfoxides and sulfones = -S(0) R, n = 0, 1, 2). Most of the work carried out in this field has been pertinently reviewed and discussed in a 1988 Tetrahedron Report with 204 references [485], Some specific aspects are underlined here and recent examples given. 

Vinyl sulfides are not good dienophiles towards electron-rich dienes. Their reactivity is much enhanced by the presence of a geminal electron-withdrawing group. The corresponding caplo-dative olefins have been widely used, such as the following  

a-methylthio methyl acrylate (1) was shown to react with cyclopentadiene at room temperature. The adduct (2) was isolated in a 94% yield (exo-endo isomeric mixture). Further transformation of (2), according to Trost s method [250], (see also Section 3.6), afforded norbomenone (3) (60% yield), establishing (1) as a useful ketene synthetic equivalent [526]. 

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