Enol elimination reactions. Part IV. Some improvements to the synthesis of conjugated acetylenic acids

Abstract

The reaction of the enolates (2) of acylmalonates with arenesulphonyl halides gives enol sulphonates in poor yield, partly because of competing halogenation which gives α-halogeno-ketones (6). The halogenation is reversible, and, because the reverse reaction leads to re-establishment of the original sulphonation conditions, it is possible to make enol sulphonates from the reaction of an α-halogeno-ketone with an arenesulphinate ion. This reaction is the sulphur equivalent of the Perkow reaction, but it is not preparatively useful. The halogenation reaction can be avoided altogether by using arenesulphonic anhydrides as sulphonating agents. The result is that the synthetic sequence: acylmalonate → enol sulphonate → acetylenic acid is now a useful one for the synthesis of conjugated acetylenic acids. For some purposes the use of t-butyl esters in place of ethyl esters has advantages.