New Method for Derivatization of Squaric Acid to Highly Substituted Cyclobutenones: Lewis Acid-Catalyzed Reaction of Cyclobutene-1,2-dione Monoacetal and Its Vinylog with Unsaturated Organosilanes, and Subsequent Ring Transformation of the Adducts

Article abstract of DOI:10.1246/bcsj.69.1353

Described herein is a novel method for regio-controlled synthesis of highly substituted cyclobutenones having an unsaturated substituent at 4-position, starting from commercially available squaric acid. Both cyclobutene-1,2-dione monoacetal (4,4-diethoxycyclobutenone) and its vinylog (2,4-diethoxycyclobutenone), which were easily obtained from diethyl squarate, reacted with allylsilanes in the presence of Et₂O-BF₃ to afford 4-allyl-4-ethoxycyclobutenones having various substituents at 2-position regioselectively. These products were efficiently transformed to highly substituted bicyclo[3.2.0]heptenones by refluxing in xylene. The synthetic utility of this process was demonstrated in the construction of tricyclic ring systems. Further extension of the Lewis acid-catalyzed reaction of the monoacetal using an allenylsilane, a silyl enol ether, and a silyl ketene acetal also afforded the corresponding 4-substituted products. In contrast to the above 4-allylated products, 4-propargylated and 4-acylmethylated products did not undergo an analogous ring transformation under the same conditions.