Sulfoxide-sulfenate -sigmatropic

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. 

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. 

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. ... 

In addition to the above-mentioned 1,3-chirality transfer, another mode of transferring centro chirality from one atom to the other is the S C transfer. This feature of the reaction has already been discussed by Mislow in his 1968 paper [40], where he delineates all important stereochemical aspects of [2,3]-sigmatropic rearrangements using the sulfoxide-sulfenate rearrangement as an example. For the sulfinate-sulfone rearrangement under consideration here, the work of Hiroi in the early 1980s should be mentioned (Scheme 16) [44,... 

A close relative of the previous tandem [3,3]-[3,3] sigmatropic rearrangement invokes its tandem [2,3H2,3] counterpart the allylic sulfoxide-sulfenate rearrangement. As a key step in the synthesis of 5-deoxyleukotriene D, Corey applied the sequence of transformations illustrated in Scheme 12. The anion of allylic sulfoxide (149) undergoes 1,2-addition to methyl 5-formylpentanoate followed by low temperature benzoylation. Upon wanning the reaction mixture to ambient temperature, a facile sulfox-... 

The interconversion of C—O and C—S bonds is realized by the 2,3-sigmatropic shift of p,7-unsatu-rated sulfoxides/sulfenates. In a higher oxidation state a sulfone-sulfinate 2,3-rearrangement is known this is of little importance in synthetic chemistry as it is associated with a stepwise rearrangement mechanism which interferes even in slightly hindered substrates. ... 

The [2,3]-sigmatropic interconversion of sulfoxides and sulfenate esters is easily reversible, and this is why chiral allylic sulfoxides typically racemize at room temperature. The sulfoxide-sulfenate ester equilibrium usually favors the sulfoxide. If a reactive thiophile that cleaves the O-S bond of the sulfenate ester is introduced into the reaction mixture, then the equilibrium is displaced and the allylic alcohol is formed with the possibility of transfer of chirality (Figure 10.10). Starting from... 

Even though the sulfilimine (83) undergoes a [2,3]-sigmatropic rearrangement, unlike the sulfoxide/sulfenate rearrangement the equilibrium here lies toward the sulfenamide (84) (Scheme 14). This difference in reactivity plays a very critical role in the stereochemical outcome in the conversion of some dihydrothiazine imines to vicinal diamines. 

This transformation presumably proceeds with initial formation of allylic sulfilimine 55, which rearranges via an envelope-like transition state to sulfenamide 56 (cf. Scheme 1-XIII). Interestingly, NMR analysis showed that this [2,3]-sigmatropic rearrangement lies totally on the side of this sulfenamide, unlike the allylic sulfoxide-sulfenate ester system, which lies predominantly to the side of the sulfoxide. Similarly, C-3 epimeric dihydrothiazine imine 58 can be converted by an identical pathway to E-erythrp vicinal diamine 59 in an efficient and totally stereoselective manner [Eq. (28)]. 

The acid-catalysed rearrangement of two enantiomerically pure epoxides derived from sesquiterpenic himachalenes with Lewis and Brpnsted acids results in polycyclic compounds and mechanisms to explain the products have been proposed. Treatment of acyclic a-hydroxy and a-tosylamino sulflnyl dienes with amines gives enantiopure 1,4-diol or 1,4-hydroxysulfonamide derivatives.This one-pot proce- (g) dure entails a conjugate addition that triggers a diastereoselective sulfoxide-sulfenate [2,3]-sigmatropic rearrangement (Scheme 61). 

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. 

In summary, the evidence described above demonstrates three main mechanistic features of the rearrangement of allylic sulfenates to sulfoxides (1) spontaneous and wholly concerted [2,3]-sigmatropic shift of allyl or a-substituted allyl esters (7 a, b) at one extreme (2) complete stability of the y-aryl and y,y-dialkyl substituted allyl sulfenates as well as... 

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