Mislow-Evans sigmatropic rearrangement

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). 

MISLOW BRAVERMAN - EVANS Rearrangement Reversible 2,3- sigmatropic rearrangement of alyfc sulfoxides to alyl sulenates which are cleaved by phosphites to allyNc alcohols. 

In this case, the ability to use a sulfide in place of a sulfoxide, with the oxidation-state adjustment and subsequent Mislow-Evans to occur later, provided a solution to the recalcitrant Claisen rearrangement. This strategy f Scheme 18.51) was particularly appealing in this case, because terminal thioethers are known to be strongly activating for [3,3]-sigmatropic anionic oxy-Cope rearrangements. ... 

Zard has studied the isomerization/Mislow-Evans rearrangement of vinyl sulfoxides such as 237, arising from enolate addition to alkynyl sulfoxides [Scheme 18.60). Isomerization of 237 to the allylic sulfoxide 238 enabled the [2,3]-sigmatropic rearrangement to a-hydroxy-a-vinyl ketone 239. In this case, diastereoselectivity was low in formation of the carbinol center within a steroid framework. Additions to allenyl sulfoxides provide a similar sequence, leading to 2-propenyl substitution at the tertiary alcohol center (not shown). 

There are a number of [2,31-sigmatropic rearrangements related to the Mislow-Evans rearrangement but differing in the heteroatom moiety. While these are not the main focus of the current chapter, they present unique opportunities for synthetic applications, and several recent examples are oudined in the following. Another closely related process, the [2,3]-Meisenheimer rearrangement of allylic IV-oxides, is discussed separately in Chapter IS on ammonium ylide rearrangements. 

An allylic halogen oxide [2,3]-sigmatropic rearrangement is another intriguing analogue of the Mislow-Evans process. In this scenario tScheme 1R.7QT oxidation of an allylic iodide provides iodoso intermediate 282, which rearranges to the corresponding allylic hypoiodite 283. Yamamoto et al. found that the reaction worked best with excess peracid oxidant, as the initially formed hypoiodite was further oxidized to the iodate ester (284), an intermediate... 

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 ... 

What do we mean by transfer of chirality Reactions in which the stereoselective formation of one stereogenic center is connected to the destruction of another stereogenic center involve transfer of chirality. Sigmatropic rearrangements are a large family of intramolecular reactions that often can be used to transfer chirality. We will look at two examples (1) the Claisen rearrangement (of allyl vinyl ethers to j8,7-unsaturated carbonyl compounds) and (2) the Mislow-Evans rearrangement (of allylic sulfoxides to allylic alcohols via sulfenate esters). 

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... 

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