Carbanions with epoxides

Cyclization of 4-thiophenylbutan-2-ols 473, available by the reaction of sulfur-stabilized carbanions with epoxides, affords substituted thiochromans (Scheme 165) <2000JHC1527>. 

Scheme 4 Reactions of dimesitylboryl-stabilized carbanions with epoxides...

Carbanions generated from 1,3-dithianes also react with epoxides to give the expected products. 

With Functionalized Carbanions. Other nucleophilic carbanions, such as lithio-dithiane or sodiomalonate or acetylene anion react easily with epoxides. 

As with epoxides, carbanions can add in a 1,4 fashion to enones or nitrosugars. Nitromethane anion has been used [64], Dithiane anion has been successfully used in the addition to nitroolefins [65] and to enones [66], Accordingly, C-5 branched-chain glucose derivatives 47 and 48 have been prepared from nitroolefin 46 (Scheme 20) [67,68], Sugar-derived enones have been also used as acceptors in free radical reactions to trap alkyl radicals as well as anomeric radicals (see Schemes 29 and 30). 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Internal cyclization to the oxirane (7) rather than to the substitution product (8) is found in the alkaline epoxidation of tetraoyanoethylene (Linn et al., 1965). This may be due to a longer life-time of the intermediate (6) with the cyano leaving group, compared to carbanions with halide leaving groups. 

Silyl-substituted carbanions stabilized by heteroatom substituents reacted with epoxides providing the corresponding 3-siloxy carbanions via the anionic 1,4-silyl migrations, which reacted with various electrophiles (equation 149)361-370. 

Perfluoroalkoxyanions are also generated by reaction of fluoride ion with acid fluorides and with epoxides (see Section niB, below). Reaction of the (CF3)2CO—CsF complex with tetrafluoroethene [135] gives alkoxide 8.34A, not a carbanion 8.34B (Figure 8.34). In the presence of iodine, however, ethers are formed [126], indicating the formation of intermediate hypoiodites, RpOI (Figure 8.35). 

Sulfonium salts react in several ways. They may behave as a leaving group, undergoing substitution by a nucleophile or fragmenting with the formation of an alkene. However, the most important reaction of sulfonium salts involves the formation of an ylide in the presence of a base. The carbanion of this sulfur ylide is stabilized by the adjacent positively charged sulfonium ion. The reaction of the carbanion with a carbonyl group parallels that of a phosphonium ylide in the Wittig reaction. However, the decomposition of the intermediate dipolar species is different and leads to the formation of an epoxide (oxirane) rather than an alkene. 

The reactions between a-metalloalkyl sulfoxides and electrophiles have been extensively studied. Although alkylations of the sodium or potassium salts of dialkyl sulfoxides are not always very efficient since a,a -dialkylated sulfoxides are often produced (or stilbene in the case of methyl-sulfinyl carbanion and benzyl bromide ), those employing the lithioalkyl aryl sulfoxides work more efficiently with alkyl or allyl halides " and with epoxides. " " Typical examples of these alkylations, allylations and hydroxyalkylations (from epoxides) are illustrated in Scheme 86. 

The related (Z)-lithium dialkenylcuprates (147) derived from acetylene itself also react well with epoxides to provide a useful route to (2)-homoallylic alcohols the lack of reactivity with esters allows an easy access to lactones (148) by condensations between epoxy esters and this type of cuprate (Scheme 29). Likewise, the lower homologs (149) and (151), both of which are relatively easy to prepare in optically active forms, can be readily converted into homoallylic and bishomoallylic alcohols (150) and (152) respectively. An ester unit can also be incorporated into the cuprate functions thus, addition of a mixed lithium cuprate, RCuYLi , to ethyl propiolate gives the cuprates (153), which add to epoxides to give unexpectedly the (Z)-crotonates (154). Such isomerization is not uncommon with vinyl carbanions in general, and is obviously a limitation when isomeric mixtures are produced. 

A final group of fp -carbanions which have been found to react well with at least monosubstituted epoxides are the vinylalanes formed either by direct hydroalumination or by zirconium-catalyzed car-boalumination of a terminal alkyne. Both species are insufficiently reactive to couple with epoxides and must first be converted into their corresponding ate complexes, usually by treatment with n-butylli-thium (Scheme 30). 

By treatment of the carbanion of (48) with an aldehyde, the procedure may be modified to yield hydroxy carbonyl compounds (51) (Scheme 25). Such dithian carbanions also react with epoxides e.g. (52), to give hydroxy ketones, e.g. (53). In this reaction, the less hindered end of the epoxide is preferentially attacked (Scheme 26). 

Another way to deactivate carbanion is by reaction of the carbanion with carbon dioxide and dilute acid to give carboxylic terminal group or by reaction with alkene oxide (epoxide) to give a terminal alcohol. 

Bis(methylthio)methane, (MeS)2CH2, in the presence of BuLi, provides the stabilized carbanion which reacts with epoxide (82) to give the corresponding cyclopropane (83), from which (84) and (85) can be... 

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