A,P-Unsaturated sulfoxides and sulfones

This review continues from one entitled The Chemistry of a,P-Unsaturated Sulfoxides which was recently published in a monograph of organosulfur chemistry. 1 In addition, the coverage has been extended to include the analogous a,P-unsaturated sulfones. The focus of the present review are recent advances in the chemistry of a,P-unsaturated sulfoxides and sulfones, especially those which have been published since 1993. The synthesis of a,P-unsaturated sulfoxides and sulfones has already been reviewed extensively by Rayner2 and hence will not be covered here. Only the chemistry of vinyl (alkenyl) sulfoxides and sulfones will be considered. The chemistry of other a,P-unsaturated sulfoxides and sulfones such as dienyl, allenyl, and propargyl (alkynyl) sulfoxides and sulfones is beyond the scope of this review. Considerable emphasis has been placed on stereo- and enantioselective reactions, reflecting the current interest in this area. 

RECENT ADVANCES IN THE CHEMISTRY OF a,P-UNSATURATED SULFOXIDES AND SULFONES Ian Forristal and Christopher M. Rayner 155... 

Davis and coworkers have used an intramolecular asymmetric Michael addition to an optically pure a,(3-unsaturated sulfoxide as part of the synthesis of the dihydropyridine sulfone (132), a potent antihypertensive agent [108], The Hantzsch reaction (treatment with 3-aminocrotonate (130) in MeOH under reflux) of the a-acyl-a,P-unsaturated sulfoxide (129) gave the product (131) as a single diastereoisomer in 48% yield, resulting from an asymmetric Michael addition followed by dehydrative cyclization. The sulfoxide was then oxidized to the corresponding sulfone (132) (Scheme 5.44). 

There is generally little or no competition from 1,2 addition (to the C=0). However, when R is allylic,l,4 addition is observed with some substrates and 1,2 addition with others. The compound R2CuLi also add to a,P-unsaturated sulfones but not to simple aP-unsaturated nitriles. Organocopper reagents RCu (as well as certain R2CuLi) add to ocP-unsaturated and acetylenic sulfoxides. ... 

The addition of electrophilic reagents to chiral a,/3-unsaturated sulfoxides is also accompanied by asymmetric induction. Stirling and Abbott (318,322) found that the addition of bromine to the optically active (.R)-vinyl-p-tolyl sulfoxide 319 yields a mixture of diastereo-meric a,/3-dibromosulfoxides 320. Oxidation of this mixture gives the optically active sulfone 321, with a center of chirality at the a-carbon atom only. The optical purity (32%) of this sulfone was estimated by comparing its specific rotation with that obtained as a result of oxidation of diastereomerically pure sulfoxide (/ )-320. The assignment of configuration at the a-carbon atom was based on the analysis of the polarizabilities of substituents. 

The synthesis of electron-deficient diene 285 was achieved by the Stille coupling of P-trifluoromethanesulfonyl-a,P-unsaturated sulfone (284) with a 3-stannyl-a,P-unsaturated ester (283) (Scheme 74).148 Similarly, the preparation of a diverse range of enantiomerically pure 1- and 2-sulfinyl dienes has been achieved via Stille coupling of halovinyl sulfoxides and vinyl stannanes.149,150 Enantiomerically pure 1- and 2-sulfinyl dienes have been used extensively in asymmetric Diels-Alder reactions.111... 

Sulfoxide, sulfinate and sulfonate are used as activators of acetylenic or vinyl units. Several a, P unsaturated synthons, namely acetylenic sulfoxide (1), vinyl sulfoxide (2), acetylenic sulfinate (3), acetylenic sulfonate (4), and l-propene-l,3-sultone (5) are developed. Their applications in Diels-Alder reactions, heterocycle and alkaloid syntheses are also investigated. For the chiral acetylenic sulfoxide, the sulfoxide moiety not only enables chemical activation of the acetylene unit, it can also induce stereochemical control at the adjacent carbon centers to achieve enantioselective synthesis. 

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