Alkynide anions, formation

The displaced group X often is a halide ion (chloride, bromide, or iodide), and if the entering nucleophile Nu e is an alkynide anion, the reaction leads to formation of a carbon-carbon bond ... 

Several examples of alkynic ketone formation have been recorded since Weinreb s first examples. A Diels-Alder strategy for Ae synthesis of mevinolin required the preparation of alkynic ketone (24). Standard methods, calling for the addition of the alkynide anion to an aldehyde followed by oxidation, lead to extensive degradation and by-product formation. The Weinreb methodology was clearly more effective (Scheme 8). ... 

The alkylation of j p-carbon can, in principle, involve the alkyne (acetylene) either as the nucleophile or the electrophile. In practice by far the most important process involves the alkyne as nucleophile since the acidity of the alkyne proton (pK = 25) allows the ready formation of alkynide ions. These are excellent nucleophiles and they readily undergo acylation and alkylation with appropriate electrophiles. The recent introduction of palladium-catalyzed reactions, usually involving copper(I) salts but also other cations, has greatly increased the use made of arylation and vinylation reactions. In this chapter only the alkylation of the alkynide ion will be discussed acylation, vinylation and arylation reactions are discussed elsewhere. The alkylation of alkynide anions is a reaction of considerable synthetic use and has been extensively reviewed. ... 

The following are general and specific examples of carbon—carbon bond formation by alkylation of an alkynide anion with a primary alkyl halide. 

Deprotonation of 2-pentyne generates a resonance-stabihzed anion, which is then protonated by NH3 to give an allene (a compound with C=C=C). The allene is then deprotonated to give a resonance-stabilized anion, which is then protonated by NHj to give 1-pentyne. Deprotonation of this terminal alkyne gives an alkynide ion. Formation of this alkynide ion pushes the equilibrium to favor this isomerization process. 

Recent evidence for the formation of hemiketal intermediates iqmn acylation of alkynides has been obtained from glucopyranolactones. Treatment of tetrabenzyl (55) with the anion from l-benzyloxy-3-bu-tyne gave a quantitative yield of hemiketal (56) which showed IR absorptions for the OH and alkyne portions of the molecule (X = 3350 cm" and 2250 cnr, respectively). This compound was stereospecifi-cally reduced to the C-glycoside (57) with triethylsilane/BFs-etherate (overall yield 72% Scheme 18). None of the other stereoisomer or ring-opened product was obtained. ... 

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