Sulfoxide-sulfenate -sigmatropic rearrangement

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. 

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,... 

Following studies on the rearrangement of allylic arenesulfinates, Braverman and CO workers have investigated a number of natural extensions of this unique transformation, including the predictable [2,3]sigmatropic rearrangements of allylic sulfenates to sulfoxides and of propargylic sulfenates and sulfinates to allenic sulfoxides and sulfones respectively. The last reaction is described below, while the other two are described in Chapter 14. 

As a continuation to the studies by Darwish and Braverman on the [2,3]-sigmatropic rearrangement of allylic sulfinates to sulfones, and in view of its remarkable facility and stereospecificity (see Chapter 13), Braverman and Stabinsky investigated the predictable analogous rearrangement of allylic sulfenates to sulfoxides, namely the reverse rearrangement of that attempted by Cope and coworkers . These authors initiated their studies by the preparation of the claimed allyl trichloromethanesulfenate using the method of Sosnovsky . This method involves the reaction between trichloro-methanesulfenyl chloride and allyl alcohol in ether at 0 °C, in the presence of pyridine (equation 6). 

Subsequently, Kametani and coworkers observed a similar allylic sulfoxide-sulfenate-sulfoxide rearrangement. These authors reported the exceptionally facile ringopening reaction of condensed cyclobutenes facilitated by arylsulfinyl carbanion substituents. For example, treatment of sulfoxide 68 with butyllithium in tetrahydrofuran at — 30°C for 10 min, followed by normal workup, results in the formation of product 71, which can be explained by the intervention of a double [2,3]-sigmatropic rearrangement of the initial product 69 via 70 (equation 32). A similar double [2,3]-sigmatropic rearrangement of 1,4-pentadienylic sulfoxides has also been reported by Sammes and coworkers. ... 

The synthetic utility of the remarkably facile and efficient [2,3]-sigmatropic rearrangement of propargylic sulfenates has been further demonstrated in a variety of preparations and interesting reactions of allenyl sulfoxides , including the preparation of vinylallenes " which are useful intermediates in organic synthesis in general and natural polyenes, such as Vitamins A and D, in particular Two typical examples, taken... 

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

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