Sulfur-based rearrangements extrusion

Diallylsulfonium salts undergo intramolecular allylic rearrangement with strong bases to yield 1,5-dienes after reductive desulfurization. The straight-chain 1,5-dienes may be obtained by double sulfur extrusion with concomitant allylic rearrangements from diallyl disulfides. The first step is achieved with phosphines or phosphites, the second with benzyne. This procedure is especially suitable for the synthesis of acid sensitive olefins and has been used in oligoisoprene synthesis (G.M. Blackburn, 1969). 

Another major group of methods is based on employment of various other heterocycles. Insertion of oxygen, sulfur, and other atoms and groups into Si—Si, Si—C and Si—Fe bonds of small heterocycles have been developed in continuation of efforts prior to 1980. Elimination or extrusion of structural units from larger heterocycles and rearrangement of various heterocycles are novel lines of development in the 1980s and 1990s. 

Internal alkylation takes place on reaction of a-haloalkyl alkyl sulfones with bases and produces epi-sulfones as intermediaries. These intermediates are usually unstable and provide alkenes by thermal extrusion of sulfur dioxide. This reaction, known as the Ramberg-Backlund rearrangement, has been widely used in synthesis and has been thoroughly reviewed (Scheme 107, entry e).36-3. 43<.473... 

Photolysis of naphtho[8- /, ]-l,3-diseleninylidene 1-oxides 58 provides an efficient method for generating ketenes which can be trapped by various nucleophiles (Scheme 10) <1999TL5211>. The proximity of the selenium atoms in 58 may allow for a ready rearrangement of the excited selenoxide to the corresponding selenenic ester. Based on the previously mentioned short Se-Se distance in 57 and on structural studies of the analogous sulfur species, the corresponding Se-Se distance in the selenoxide would be predicted to be even shorter, favoring the observed, ready ketene extrusion. 

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