Ring Opening Reactions of Pyridinium- and Phosphonium Betaines of Squaric Acid in alkaline media

Pyridiniumtrioxocyclobutylides 1 bearing electrondonating substituents undergo ring opening reactions at the four‐membered ring in alkaline media. The structure of one of the resulting bisacyl ylides 3 is confirmed by X‐ray analysis. In acidic media 3 are easily hydrolysed forming quaternary pyridinium salts of pyruvic acid 6; the existence of a stable gem‐diol form 5 in the crystalline state is also proved by X‐ray analysis. Triphenylphosphoniumtrioxocyclobutylide (8) gives the bisacyl ylide 9, which is hydrolysed to (triphenylphosphor‐anylidene)pyruvic acid (10).