Sulfoxides 2,3-sigmatropic rearrangement

As already mentioned, the ry -elimination of selenoxides was discovered around 197010 and had a major impact on the development of organoselenium chemistry. This reaction is about three orders of magnitude more rapid than the elimination of the corresponding less polar and less basic sulfoxides. Sigmatropic rearrangements proceed at markedly lower temperatures. These reactions are discussed in detail in Section 9.11.2.5. 

Scheme 18 Alkatrienyl sulfoxides via sulfenate-to-sulfoxide sigmatropic rearrangement and their electrophile-induced reactions...

B. The Reversible [2,3]-Sigmatropic Rearrangement of Allylic Sulfenates to Sulfoxides... 

Since its discovery two decades ago, the reversible interconversion of allylic sulfenates to sulfoxides has become one of the best known [2,3]-sigmatropic rearrangements. Certainly this is not only because of the considerable mechanistic and stereochemical interest involved, but also because of its remarkable synthetic utility as a key reaction in the stereospecific total synthesis of a variety of natural products such as steroids, prostaglandins, leukotrienes, etc. 

The data presented demonstrate that allylic sulfoxides can provide an easy and highly stereoselective route to allylic alcohols taking advantage of the facility of the allylic sulfoxide-sulfenate [2,3]-sigmatropic rearrangement. This is of considerable synthetic utility, since a number of stereoselective and useful transformations of allylic alcohols and their derivatives have become available in recent years107-109. 

In addition to the synthetic applications related to the stereoselective or stereospecific syntheses of various systems, especially natural products, described in the previous subsection, a number of general synthetic uses of the reversible [2,3]-sigmatropic rearrangement of allylic sulfoxides are presented below. Several investigators110-113 have employed the allylic sulfenate-to-sulfoxide equilibrium in combination with the syn elimination of the latter as a method for the synthesis of conjugated dienes. For example, Reich and coworkers110,111 have reported a detailed study on the conversion of allylic alcohols to 1,3-dienes by sequential sulfenate sulfoxide rearrangement and syn elimination of the sulfoxide. This method of mild and efficient 1,4-dehydration of allylic alcohols has also been shown to proceed with overall cis stereochemistry in cyclic systems, as illustrated by equation 25. The reaction of trans-46 proceeds almost instantaneously at room temperature, while that of the cis-alcohol is much slower. This method has been subsequently applied for the synthesis of several natural products, such as the stereoselective transformation of the allylic alcohol 48 into the sex pheromone of the Red Bollworm Moth (49)112 and the conversion of isocodeine (50) into 6-demethoxythebaine (51)113. 

Recently, a simple and general synthetic method for the preparation of N-alkylisothioazolidines involving [2,3]-sigmatropic rearrangement of appropriately substituted allylic sulfoxides to corresponding sulfenates, followed by intramolecular substitution of the latter, has been described (equation 28)126. 

Apparently, the first report of a double [2,3]-sigmatropic rearrangement of an allylic sulfoxide was published by Gaoni132. This author observed that the 1,4-pentadienyl... 

Another version of the double [2,3]-sigmatropic rearrangement, involving the sequence sulfenate - sulfoxide - sulfenate, has also been observed. For example, an effective one-pot epimerization procedure of 17a-vinyl-l 7/i-hydroxysteroids to the rather inaccessible 17-epimers has been achieved by the use of such a rearrangement (equation 35)137. Thus treatment of alcohol 76a with benzenesulfenyl chloride afforded the sulfoxide 77 as a single isomer and E-geometry of the olefinic double bond. Exposure of 77 to trimethyl phosphite in refluxing methanol produced a mixture of 76b and 76a in a 73 27 ratio. 

In another synthetic application, first reported by Smith and Stirling142, the bis-2,3-(phenylsulfinyl)-l, 3-butadiene 94 has been prepared in low yield by two spontaneous sequential [2,3]-sigmatropic rearrangements of the Ws-sulfenate ester (93). More recently, the yield of this reaction (equation 42) has been improved159, and a related dienyl sulfoxide 95 has been reported (equation 43)160. This type of sulfoxide is of considerable interest in view of recent studies on Diels-Alder reactions of polysubstituted butadienes161-164. 

Analogous with the rearrangement of allylic sulfoxides is the [2,3]-sigmatropic rearrangement of propargylic sulfoxides to allenic sulfenates. This process, which has been relatively little studied so far, appears to be the first step in the facile and quantitative rearrangement of sulfoxide 98 to the hemithioacetal 101 (equation 45)167. This reaction,... 

Recently, Block and coworkers220 reported a striking difference between exo and endo-sulfoxide 160. While the former remained unchanged even after refluxing in toluene for 20 h, the endo-sulfoxide 160, rearranged at room temperature, presumably via a [2,3]-sigmatropic shift (Section II.B), to 4-ethyl-2-oxa-3-thiabicyclo[3,3,0]oct-7-ene (161), a rare example of an isolable sultene185 (equation 66). 

Another [2,3] sigmatropic rearrangement converts allylic sulfoxides to allylically... 

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