Rearrangements Mislow

Mislow-Evans Rearrangement (Evans (-Mislow) Rearrangement)... 

Another sigmatropic reaction is the Mislow rearrangement of allyl sulfoxides, which is invisible because it is thermodynamically unfavourable, but the ease with... 

What is the mechanism of the Evans-Mislow rearrangement in step c ... 

Chuard et al. used allylsulfoxides as precursors of oxygen-centered radicals (Scheme 83) [229]. The Evans-Mislow rearrangement of allylsulfoxides leads to allyl sulfonyl ethers. The authors decided to use those as source of oxygen radicals. In addition, when the allyl sulfoxide was branched on a cyclobutene, the tandem reaction delivered a cyclobutyloxy radical that collapsed and underwent ring expansion in good overall yield. Thus, starting from sulfoxide 283,... 

One of the most widely used allyl alcohol syntheses uses the Evans-Mislow rearrangement (39, arrows) of allyl sulphoxides17) (Scheme 1). Since the allyl sulphoxide is usually made from some allylic electrophile by substitution and oxidation, this strategy requires a specific allyl cation equivalent. 

The ester group is transformed into an ethyl group in simple fashion 116 and the position and stereochemistry of all the chiral centres corrected by an Evans-Mislow rearrangement. Oxidation to the sulfoxide 118 allows a [2,3] sigmatropic shift across the top face of the molecule giving an unstable sulfenate ester 119 from which sulfur is removed by a phosphite thiophile to give the alcohol 117 with the correct anti-stereochemistry. The alkene returns to its original position and stereochemistry. The rest of the synthesis involves the construction of the polyene chain.16... 

Another quite different looking sigmatropic reaction is the Mislow rearrangement 6.11 > 6.12, which is invisible because it is thermodynamically unfavourable, but the ease with which it takes place explains why allyl sulfoxides, with a stereogenic centre at sulfur, racemise so much more easily than other sulfoxides.660 Here, one end of the C S bond moves from the sulfur (S- F) to the oxygen atom (0-2 ) and the other end moves from C-1 to C-3. This is therefore called a [2,3]-shift, the bond marked in bold moving two atoms at one end and three at the other. 

MISLOW BRAVERMAN EVANS Rearrangemenl Reversible 2.3- sigmatiopic rearrangement o( aHyUc sulloxides to ailyl suVenates which are cleaved by phosphites to allyUc alcohols. 

The spontaneous rearrangement of allyl p-toluenesulphenates to allyl sulphoxides was independently recorded by Mislow and coworkers and Braverman and Stabinsky. Mislow and colleagues201 have demonstrated that simple allyl alcohols such as 149, on conversion to the corresponding lithium alkoxides followed by treatment with arenesulphenyl chlorides, may be smoothly transformed at room temperature via the sulphenate esters into allylic sulphoxides 150 (equation 83). Braverman and Stabinsky202 have found that when the more reactive trichloromethanesulphenyl chloride is treated with allyl alcohol and pyridine in ether at — 70°, it affords trichloromethyl allyl sulphoxide and not allyl trichloromethanesulphenate as reported by Sosnovski203 (equation 84). 

The allyl sulphenate-allyl sulphoxide rearrangement is a general reaction and is applicable to structurally diverse allyl alcohols204,205 (Table 13). Mechanistically, it represents a typical example of a [2,3]-sigmatropic rearrangement as shown by the detailed investigations of Mislow and Braverman and their coworkers. 

Treatment of (—)-(S)-276 with allyl Grignard reagents gives optically active allylic sulphoxides 288. This reaction, however, involves an allylic rearrangement via transition state 289 as evidenced by Mislow and his collaborators362 (equation 160). 

In conclusion, it may be noted that the pioneering investigations by both the Mislow and Braverman groups have contributed to the elucidation of the mechanism of this concerted [2,3]-sigmatropic rearrangement, and have thus laid the ground for the following stereochemical and synthetic studies. 

In further studies on thiabenzenes, Mislow and co-workers (170) demonstrated that deprotonation of optically active 2-chloro-lO-(2,5-xylyl)-10 thioxanthenium perchlorate 132, resolved via the (+> camphor-10-sulfonate salt, affords optically active 2-chloro-10-(2,5-xylyOthiaanthracene 133, which subsequently rearranges to the corresponding 9-substituted thioxanthene. 

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