1,3-dithiane carbanions from

The widespread use of compounds derived from the 1,3-dithiane carbanion or its homologues as acyl anion equivalents (for applications see Section 4.2.1.1) followed the pioneering works of Corey and Seebach on nucleophilic acylation (for reviews, see [43] and [44]). 

Dithioacetals of aldehydes are sources of carbanions and hence may be used to form new C-C bonds in reactions in which the formerly electron-deficient character of the aldehydic carbon has been reversed. The 1,3-dithianes derived from formaldehyde or a higher aldehyde may be metallated and then alkylated (Scheme 2.27). Hydrolysis of the dithioac-etal is usually carried out in the presence of a thiophilic (sulfur seeking) metal salt such as a mercury salt. The insoluble sulfides cause the equilibrium to move in favour of the parent carbonyl compound. 

In dithioacetals the proton geminal to the sulfur atoms can be abstracted at low temperature with bases such as Bu"Li. Lithium ion complexing bases such as DABCO, HMPA and TMEDA enhance the process. The resulting anion is a masked acyl carbanion, which enables an assortment of synthetic sequences to be realized via reaction with electrophiles. Thus, a dithioacetal derived from an aldehyde can be further functionalized at the aldehyde carbon with an alkyl halide, followed by thioacetal cleavage to produce a ketone. Dithiane carbanions allow the assemblage of polyfunctional systems in ways complementary to traditional synthetic routes. For instance, the p-hydtoxy ketone systems, conventionally obtained by an aldol process, can now be constructed from different sets of carbon groups. ... 

Unlike the carbanions from 1,3-dithians (see Chapter 3, p. 31 and Chapter 6, p. 90), the sulfinyl carbanions have the ability to undergo Michael additions (conjugate or 1,4-additions) with a,p-unsaturated carbonyl compounds. For instance, the secondary carbanion (44) from the ethyl ethylthiomethyl sulfoxide (45) may be sucessively reacted with ethyl iodide and 3-butene-2-one to give heptan-2,5-dione (46) via the tertiary carbanion (47), as shown in Scheme 23. The carbanion (47) may also be condensed with propyl bromide, and hydrolysis of the product yields ethyl propyl ketone (48) (Scheme 23). 

Carbanion generation. A 1 1 mixture of CsOH and CsF suspended in dichloromethane or THF is very effective for generation of carbanions from 1-trimethyl-silylalkynes, 2-trimethylsilyl-l,3-dithiane, trifluoromethyltrimethylsilane, and silyl enol ethers, all through desilylation. 

Veratraldehyde 3 and veratric acid methyl ester 4 were isolated from vanillic aldehyde 5 and vanillic acid methyl ester 6 in the original reaction mixture. Silica gel chromatography of this reaction mixture was used to isolate small amounts of these primary oxidation products. These compounds had lost two carbon atoms from the origind propenoidic chain. A further component isolated from the methylated mixture was 2-methoxyhomoveratric aldehyde 7, which suggested the presence of 2-hydroxyhomoveratric aldehyde 8 in the oxidation reaction mixture. Its structure was confirmed by independent synthesis. Treatment of the reaction mixture from the catalytic oxidation of E-ferulic acid 1 with 1,3-dithiane followed by methylation formed the thioketal 9, which was also prepared by reaction of veratraldehyde 3 with 1,3-dithiane carbanion 10 followed by methylation. 

The carbanions derived from thioacetals, however, are typical -synthons. Most frequently used are 1,3-dithianes and C -silylated thioethers (see p. 33f. D. Seebach, 1969, 1973 B.-T. Grobel, 1974,1977). In these derivatives the proton is removed by butyllithium in THF. 

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

Other carbanionic groups, such as acetylide ions, and ions derived from a-methylpyridines have also been used as nucleophiles. A particularly useful nucleophile is the methylsulfinyl carbanion (CH3SOCHJ), the conjugate base of DMSO, since the P-keto sulfoxide produced can easily be reduced to a methyl ketone (p. 549). The methylsulfonyl carbanion (CH3SO2CH2 ), the conjugate base of dimethyl sulfone, behaves similarly, and the product can be similarly reduced. Certain carboxylic esters, acyl halides, and DMF acylate 1,3-dithianes (see 10-10. )2008 Qxj(jatjye hydrolysis with NBS or NCS, a-keto aldehydes or a-... 

Pyridyl-l,3-dithianes (50) result from the reaction of picolyl lithium reagents with 1,2-dithiolanes in the presence of HMPT. An initial ring opening is followed by reaction at the carbanion site with a second mole of dithiolane <96PS(112)101>. 

From the synthetic point of view, the most important a-sulfinyl carbanions appear to be those derived from dithioacetal S-oxides which are a synthon for acyl anions65. However, the yields of the alkylation reaction were found to be low. In spite of the fact that dithiane S-oxides have been intensively studied66 63, their synthetic applications are limited,... 

With Dithiane. Dithiane is one of the most popular functionalized carbanion used for the ring opening of epoxides. This reaction has been studied by different groups [31,56], The anion of bis(phenylthio)methane has been successfully used for the opening of epoxides derived from furanose [56]. This method is an excellent way to prepare formyl derivatives that are suitable for chain extension by olefination. Alternatively formyl derivatives can be reduced to hydroxymethyl derivatives. A recent application of epoxide 33... 

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). 

The stabilizing effects of the 1,3-sulfur atoms on the carbocations, radicals, and carbanions generated from 2-aryl-l,3-dithianes and -dithiolanes has permitted the measurement of a variety of bond-making and bond-breaking energies (in DMSO and sulfolane) and their correlation with electron-transfer energies.148... 

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