Benzoins, ring-substituted

Benzoin condensation.1 The benzoates of cyanohydrins of aromatic aldehydes undergo benzoin condensation with an aromatic aldehyde in 50% NaOH/C6H6 in the presence of a phase-transfer catalyst, benzyltriethylammonium chloride. Theoretically two symmetrical and two unsymmetrical benzoins are possible, but in practice only one unsymmetrical benzoin is formed, that in which the carbonyl group is adjacent to the benzene ring substituted by the more electron-donating group. 

The base-catalysed rearrangement of ring-substituted benzoins in aqueous methanol is reported to be initiated by rate-determining a-hydrogen abstraction rather than a mechanism with initial hydroxyl hydrogen abstraction as in the general a-ketol rearrangement (Scheme 93). ... 

In an attempt to use an acyl anion equivalent to open an aziridine, Wu and co-workers isolated an unexpected ring opened product 316 (Eq. 31) [158], The authors found that the presence of oxygen was the determining factor between benzoin formation and ester formation. No desired ketones were ever formed. Various aromatic substituted aldehydes were treated under standard reaction conditions to afford esters in good yields. 4-Methoxybenzaldehyde provided product in only 40% yield, presumably due to the ease of aldehyde oxidation. 

The conjugate addition of carbonyl anions catalysed by thiazolium salts (via umpol-ung) that is fully operative under neutral aqueous conditions has been accomplished. The combination of a-keto carboxylates (157) and thiazolium-derived zwitterions (e.g. 160) in a buffered protic environment (pH 7.2) generates reactive carbonyl anions that readily undergo conjugate additions to substituted o /3-unsaturated 2-acylimidazoles (158) to produce (159). The scope of the reaction has been examined and found to accommodate various a-keto carboxylates and /3-aryl-substituted unsaturated 2-acylimidazoles. The optimum precatalyst for this process is the commercially available thiazolium salt (160), a simple analogue of thiamine diphosphate. In this process, no benzoin products from carbonyl anion dimerization were observed. The resulting 1,4-dicarbonyl compounds (159) can be efficiently converted into esters and amides by way of activation of the A-methylimidazole ring via alkylation.181... 

A pyrrole ring is the core structure found in the skeletons of many naturally occurring biomolecules such as globins, porphyrins, vitamins, and so on and constitutes a unit of pharmaceuticals. Most commonly, pyrroles are synthesized by multistep synthetic methods. A one-pot three-component synthesis of 2,3,4,5-tetrasubstituted pyrroles from benzoin, 1,3-dicarbonyls, and ammonium acetate was carried out using SSA under solvent-free conditions (Tamaddon and Farahi, 2012) (Scheme 5.21). Veisi (2010) reported a room temperature synthesis of N-substituted... 

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