Sulfoxides 2,3 sigmatropic rearrangement of allylic

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. 

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 investigators 3 jj... 

One of the most interesting reactions in sulfur chemistry is the reversible [2,3]sigmatropic rearrangement of allyl sulfoxides to the corresponding sulfenate esters, which are achiral at sulfur. However, in the case of suitably substituted allyl sulfoxides a new chiral center may be generated at the a-carbon in this process, as shown in eq. [137]. 

Sigmatropic rearrangement of allylic sulfoxide to allylic alcohol. 

E. Guittet and S. JuliS, Synth. Commun., 1981, 11, 723. Recently the high transmission of stereochemistry in the 2,3-sigmatropic rearrangement of allyl sulfoxide systems has been used in an allylic hydroxylation process via an asymmetric ene reaction of N-sulfinylcarbamates J. K. Whitesell, J. F. Carpenter, H. K. Yaser and T. Machajewski, J. Am. Chem. Soc., 1990, 112, 7653. 

MISLOW-BRAVERMAN-EVANS Rearrangement Reversible 2,3-sigmatropic rearrangement of allylic sulfoxides to allyl sulfenates which are cleaved by phosphites or thiols to allylic alcohols (see 1st edition). 

Sigmatropic rearrangements of allyl sulfoxides to aUyl sulfenates are known as the Mislow-Evans rearrangements. AUyUc sulfoxide with a bulky substituent at the allylic position was converted to the corresponding E-allyUc alcohols with high stereoselectivity. Some illustrative examples are [124-127] as follows ... 

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

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). 

It has been demonstrated that cycloadducts (129), which are enolphosphates obtained by regio- and stereospecific [4 + 2] cycloaddition reactions of dienes (130) to a variety of dienophiles, are functionalized versatile synthons having fixed stereochemistry. Their [2,3] sigmatropic rearrangement via allylic sulfoxides and selenoxides (131) provides a direct sterospecific entry to new functionalized bi- and tricyclic allylic alcohol systems (133). The latter has been transformed into the corresponding a-hydroxy ketones (132), key structural subunits of natural products and valuable synthetic intermediates (examples are given in Scheme 32). ... 

E)-Trisubstituted allylic bromides. The stereoselectivity in the [2,3]sigmatropic rearrangement of the sulfoxide of 1 by the Evans technique (6, 30-31) is low (E/Z alcohols = 64 36). A new stereoselective method involves treatment of 1 with CH3Br (excess) and LiBr in DMF (20°, 10 hours). The desired (E)-allylic bromide 2 is obtained as the major product in 60% yield. A similar reaction of 1 with CH31 and Nal leads to the iodide corresponding to 2 (80-85% yield). A possible pathway proceeds via a. 

Concerted [2,3]-sigmatropic rearrangements of allyl and benzyl ammonium ylides, allyl sulfonium ylides, allyl sulfoxides, selenoxides, amine oxides and anions of allyl ethers have been reported. These are illustrated by die following general examples ... 

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... 

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... 

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