Allylic ester, -sigmatropic rearrangement

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... 

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) . ... 

O-Allyl imidate esters undergo [3,3]-sigmatropic rearrangements to /V-allyl amides. Trichloromethyl imidates can be made easily from allylic alcohols by reaction with trichloroacetonitrile. The rearrangement then provides trichloroacetamides of IV-allylamines.260... 

Selenium dioxide is a useful reagent for allylic oxidation of alkenes. The products can include enones, allylic alcohols, or allylic esters, depending on the reaction conditions. The mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a [2,3]-sigmatropic rearrangement that restores the original location of the double bond, and (c) solvolysis of the resulting selenium ester.183... 

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]. 

Heating of an allylic alcohol with an excess of trialkyl orthoacetate in the presence of trace amounts of a weak acid to give a mixed orthoester. The orthoester loses ethanol to generate the ketene acetal, which undergoes [3,3]-sigmatropic rearrangement to give a Y,5-unsaturated ester. 

There are several variations of the Claisen rearrangement that make it a powerfid tool for the synthesis of y,<5-unsaturated carboxylic acids. The ortho ester modification of the Claisen rearrangement allows carboalkoxymethyl groups to be introduced at the /-position of allylic alcohols.157 A mixed ortho ester is formed as an intermediate and undergoes sequential elimination and sigmatropic rearrangement. 

This procedure illustrates a general method for the preparation of rearranged allylic amines from allylic alcohols.2,3 The method is experimentally simple and has been used to prepare a variety of allylic prim-, sec-, and tert-carbonyl amines as illustrated in Table I. The only limitation encountered so far is a competing ionic elimination reaction which becomes important for tri-chloroacetimidic esters of 3-substituted-2-cyclohexen-l-ols.3 4 The rearrangement is formulated as a concerted [3,3]-sigmatropic rearrangement on the basis of its stereo- and regiospecificity3,5 which are... 

Methyl (2E,4E)Senoates. The dianion (2) of 1 can be prepared by treatment with I.DA and then sec-butyllithium in THF at —78°. The dianion is alkylated selectively at the a-allylic position to give 3 as the major product. Treatment of 3 with LDA induces a [2,3]sigmatropic rearrangement to 4. Remaining steps to the diunsaturated ester (6) are methylation, oxidation, and dchydrosulfcnylation.1... 

Compound 87 isolated in a trapping experiment681 undergoes a [2,3] sigmatropic rearrangement to yield a selenite ester (88). Hydrolysis of the latter gives the product allylic alcohol. 

Allylic thionesters, dithiocarbamates and thionocarbamates are starting materials for various synthetic purposes [203, 492,493]. General synthetic routes, from Nakai et al. [492], to a,0-unsaturated esters and ketones via the [3,3] sigmatropic rearrangement of allylic thionocarbamates (1) are featured in the accompanying schemes. 

Ortho ester Claisen rearrangement.2 The [3,3]sigmatropic rearrangement of allyl vinyl alcohols prepared from this ester results in a regiospecific synthesis of diethyl allylmalonates. 

It has been reported that treatment of methyl 1-methylsulfanylvinyl sulfoxides with sodium thiophenolate in methanol affords l-methylsulfanylalk-l-en-3-ols. A sequence has been proposed218 (see Scheme 45) in which the thiophilic base first causes an in situ isomerization of the vinyl sulfoxide moiety into an allylic sulfoxide which then undergoes a [2,3]-sigmatropic rearrangement and subsequent thiophilic cleavage of the intermediate sulfenic ester. 

Some ester- or aldehyde-substituted bicyclo[2.2.2]oct-2-ene and bicyclo[2.2.1]hept-2-ene systems (25) have been shown to undergo nitrosation adjacent to the carbonyl group followed by [3,3]-sigmatropic rearrangement to give oxazine products (26).23 Stereodeflned monoprotected allylic 1,2-diol (27) and its diastereomer have been ( ) converted into stereodeflned propenylimidazolidinone (30) and its diastereomer by... 

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