THIELE ACETYLATION OF p-TROPOQUINONE, AN ABNORMAL RING CONTRACTION
Thiele type acylation reaction of p-tropoquinone 1, a novel nonbenzenoid quinone, in the presence of sulfuric acid, resulted in the unexpected ring contraction to a benzaldehyde derivative 2, in addition to a tropone 3. It was proved in this reaction that acetoxyl group attacked C3 exclusively and that the carbon became aldehydic. In the reaction catalyzed by BF3, regioselectivity of the nucleophilic attack was reduced to 4:1 (C3:C4) yielding 9, 10, and 11, all seven-membered ring compounds. Plausible mechanisms for the all reaction pathways are proposed.