Esters acyloin coupling reaction

NECESSARY REACTION CONDITIONS FOR THE ACYLOIN COUPLING REACTIONS OF ESTERS... 

Early applications of the acyloin coupling reaction to the esters of aliphatic monocarbocyclic acids, before the introduction of the TMS-Cl trapping technique, gave variable yields and the separation of the... 

Attempts to bring about the acyloin coupling reaction with the diester (31) were unsuccessful. The failure can be attributed to the severe nonbonded interactions between the two ester groups in the cisoid conformation required for coupling to occur. 

Acyloin-type reactions of esters provide the simplest route to 1-siloxy-l-alkoxycyclopropane [21,22] Eq. (6). The reaction of commercial 3-halopropionate with sodium (or lithium) in refluxing ether in the presence of Me3SiCl can easily be carried out on a one mole scale [21]. Cyclization of optically pure methyl 3-bromo-2-methylpropionate [23], available in both R and S form, gives a cyclopropane, which is enantiomerically pure at C-2, yet is a 1 1 diastereo-meric mixture with respect to its relative configuration at C-l Eq. (7). Reductive silylation of allyl 3-iodopropionate with zinc/copper couple provides a milder alternative to the alkali metal reduction [24] Eq. (8). 

A further general route to the 1,2-dicarbonyl system involves the oxidation of a-ketols (acyloins) (cf. the preparation of benzil from benzoin, Expt 6.143). The acyloins may be prepared from carboxylate esters by a radical coupling reaction involving the use of finely divided sodium metal in anhydrous ether, benzene, or toluene.144... 

The bimolecular reductive coupling of carboxylic esters by reaction with metallic sodium in an inert solvent under reflux gives an a-hydroxyketone, which is known as an acyloin. This reaction is favoured when R is an alkyl. With longer alkyl chains, higher boiling solvents can be used. The intramolecular version of this reaction has been used extensively to close rings of different sizes, e.g. paracyclophanes or catenanes. 

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