Chapter One

Acyloin Ester Condensation

[alpha]-Hydroxyketones from carboxylic esters

Upon heating of a carboxylic ester 1 with sodium in an inert solvent, a condensation reaction can take place to yield a [alpha]-hydroxy ketone 2 after hydrolytic workup. This reaction is called Acyloin condensation, named after the products thus obtained. It works well with alkanoic acid esters. For the synthesis of the corresponding products with aryl substituents (R = aryl), the Benzoin condensation of aromatic aldehydes is usually applied.

For the mechanistic course of the reaction the diketone 5 is assumed to be an intermediate, since small amounts of 5 can sometimes be isolated as a minor product. It is likely that the sodium initially reacts with the ester 1 to give the radical anion species 3, which can dimerize to the dianion 4. By release of two alkoxides R'[O.sup.-] the diketone 5 is formed. Further reaction with sodium leads to the dianion 6, which yields the [alpha]-hydroxy ketone 2 upon aqueous workup:

An intramolecular reaction is possible with appropriate substrates containing two ester groups, leading to the formation of a carbocyclic ring. This reaction is especially useful for the formation of rings with ten to twenty carbon atoms, the yield depending on ring size. The presence of carbon-carbon double or triple bonds does not affect the reaction. The strong tendency for ring formation with appropriate diesters is assumed to arise from attachment of the chain ends to the sodium surface and thereby favoring ring closure.

A modified procedure, which uses trimethylsilyl chloride as an additional reagent, gives