Olefination of dialkyl squarates by wittig and horner-emmons reactions. A facile synthesis of 3,4-dioxo-1-cyclobutene-1-acetic acid esters

Olefination of dialkyl squarates 1–5 with several ylides 6 and phosphonates 7 gave the corresponding alkylidene products 8–12 . In the case of Horner-Emmons reactions, stereoselective formation of Z -alkylidene products was observed. An acid-catalyzed hydrolysis of 8–12 gave squarylacetic acid esters 13 .