Esters, sulfonate carbanions from

The cyclopropane cyclizations by elimination of triflinic acid (CF3S02H) are readily effected by basic treatment of triflones (trifluoromethyl alkyl sulfones) with activated /-protons (equations 46 and 47)39. The cyclopropane diesters 45 are formed on treatment of 44 with potassium hydride in DMSO or sodium methoxide in methanol (equation 48). In contrast, the monoester 46 failed to give the desired cyclopropane40. Addition of carbanions derived from /f, y-unsaturated phenyl sulfones to a, /i-unsaturated carboxylic esters and subsequent elimination of benzenesulfinate ion give cyclopropanes possessing the unsaturated side chain and the ester function in trans positions (equation 49)41. 

The catalytic conditions (aqueous concentrated sodium hydroxide and tetraalkylammonium catalyst) are very useful in generating dihalo-carbenes from the corresponding haloforms. Dichlorocarbene thus generated reacts with alkenes to give high yields of dichlorocyclopropane derivatives,16 even in cases where other methods have failed,17 and with some hydrocarbons to yield dicholromethyl derivatives.18 Similar conditions are suited for the formation and reactions of dibromocar-benc,19 bromofluoro- and chlorofluorocarbene,20 and chlorothiophenoxy carbene,21 as well as the Michael addition of trichloromethyl carbanion to unsaturated nitriles, esters, and sulfones.22... 

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-... 

Nitroalkenes react with lithium dianions of carboxylic acids or with hthium enolates at -100 °C, and subsequent treatment of the Michael adducts with aqueous acid gives y-keto acids or esters in a one-pot operation, respectively (Eq. 4.52).66 The sequence of Michael addition to nitroalkenes and Nef reaction (Section 6.1) provides a useful tool for organic synthesis. For example, the addition of carbanions derived from sulfones to nitroalkenes followed by the Nef reaction and elimination of the sulfonyl group gives a,P-unsaturated ketones (Eq. 4.53).67... 

Another important reaction in synthetic chemistry leading to C-C bond formation is the Michael addition. The reaction typically involves a conjugate or nucleophilic 1,4-addition of carbanions to a,/l-unsaturated aldehydes, ketones, esters, nitriles, or sulfones 157) (Scheme 21). A base is used to form the carbanion by abstracting a proton from an activated methylene precursor (donor), which attacks the alkene (acceptor). Strong bases are usually used in this reaction, leading to the formation of byproducts arising from side reactions such as condensations, dimerizations, or rearrangements. 

Carbonyl compounds from sulfones.2 a-Sulfonyl carbanions react with 1 to form a-sulfonyl boronic esters, which are oxidized to aldehydes or ketones by m-chloroperbenzoic acid or sodium m-chloroperbenzoate (C1C6H4C03H -I- NaH in CH2C12, -60°). 

Mechanism The n-BuLi abstracts a proton from sulfone and produces the phenyl sulfonyl carbanion A, which reacts with an aldehyde to form an alkoxide B. The alkoxide B is esterified in situ to yield the corresponding ester C. The sodium amalgam reduction of ester... 

A number of (3-substituted ethyl esters have been proposed for carboxy protection in peptide synthesis, which are cleaved by fragmentation via an ElcB base-catalyzed (3-elimination (Scheme 2). Appropriate electron-withdrawing substituents such as sulfones facilitate proton abstraction from the (3-carbon under mild alkaline conditions. An elimination step to stabilize the carbanion occurs which provides the release of the free carboxy group (Scheme 2). 

The most widely explored method is the formation of tri- and tetrasubstituted cyclopropanes by reaction of vinylsulfonium salts with methylene compounds activated by ester, ketone, nitrile, or sulfone substituents. A series of examples is collected in Table 25. A variant of this method, where cyclopropanedicarboxylates cis-1 were actually obtained by intramolecular cyclization of the sulfonium salt Cj building block is also shown.Alkylthiocyclopropane derivatives 8 were obtained by the reaction of a ketene dithioacetal monosulfonium salt with carbanions derived from doubly activated methylene compounds. ... 

C-Methylation. Methyl iodide is an active alkylating agent employed in the C-methylation of carbanions derived from ketones, esters, carboxylic acids, amides, nitriles, nitroalkanes, sulfones, sulfoxides, imines, and hydrazones. The quantity of methyl iodide utilized in methylations varies from a slight (1.1 equiv) to a large excess (used as solvent). 

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