CARBON-CARBON BOND-CLEAVAGE OF ALPHA-SUBSTITUTED BENZOINS BY RETRO-BENZOIN CONDENSATION - A NEW METHOD OF SYNTHESIZING KETONES

When alpha-benzylbenzoin (3a, alpha-benzyl-alpha-hydroxybenzyl phenyl ketone) was treated with potassium cyanide (1) in N,N-dimethylformamid e at 80 degrees C for 1h, the carbon-carbon bond was cleaved, resultin g in the formation of deoxybenzoin (4a, benzyl phenyl ketone) and benz aldehyde (2a). This carbon-carbon bond cleavage proceeds through a ret ro-benzoin condensation mechanism. This method of synthesizing ketones was applied to several alpha-substituted benzoins (3), and the corres ponding ketones (4) were formed in good yields. Further, we found that the cyanide ion-donating ability of tetrabutylammomium cyanide (6, Bu 4NCN) is more effective than that of potassium cyanide (1, KCN). As ex pected from the chemical analogy between cyanide ion and azolium ylide , several azolium salts (7) can also be employed in the retro-benzoin condensation as catalysts. The benzoin derivatives 3 were synthesized in the following three ways; reaction of alkyl halide (9) with benzoin (5), Michael addition of benzoin (5) with accepters (10), and Grignar d reaction of benzils (8). Alkylation of the benzoins without isolatio n, followed by carbon-carbon bond cleavage, readily afforded the corre sponding ketones (4).