Protonation of the 1:1 intermediate produced in the reaction between dialkyl acetylenedicarboxylates and triphenylphosphine by ethyl 4,4,4-trifluoroacetoacetate leads to vinyltriphenylphosphonium salts: the positively charged ion reacts with the anion of ethyl 4,4,4-trifluoroacetoacetate to form ylides 5a–c, which undergo intramolecular Wittig reaction to produce 1-ethyl 2,3-dialkyl 4-(trifluoromethyl)-3-cyclobutene-1,2,3-tricarboxylate, which undergo electrocyclic ring-opening reactions in boiling toluene to produce highly electron-deficient 1, 3-dienes in fairly good yields.
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