SYNTHESIS AND REACTIVITY OF COMPOUNDS WITH CYCLOBUTANE RINGS(S) .10. SYNTHESES OF SQUARIC ACID, ITS MONOORTHOESTERS, AND RELATED DERIVATIVES VIA [2 + 2] CYCLOADDITIONS OF TETRAALKOXYETHYLENES WITH HETEROSUBSTITUTED KETENES

Efficient syntheses of squaric acid are reported. The synthetic approach begins with the [2 + 2] cycloaddition reaction of tetraalkoxyethylenes with oxyketenes, which are produced in situ by triethylamine-promoted dehydrohalogenation of the corresponding acyl chlorides. The intermediately formed cyclobutanones undergo rapid enolization followed by esterification with the second equivalent of acid chloride to give the key compounds, the cyclobutenol esters 4a-c. Displacement-induced fragmentation of these esters, promoted by Si02/triethylamine or basic A1203, yields smoothly the hitherto unknown monoorthoesters of squaric acid (5a-c). The acid hydrolyses of the cyclobutenol esters 4a, c and the orthoesters 5b, c, as well as the cyclobutenol esters 4d-f formed by similar [2 + 2] cycloadditions involving acetoxyketene and chloroketene, afford squaric acid (6) in high yields. When tetraalkoxyethylenes are reacted with in situ generated methylthioketene and phthalimidoketene, the corresponding cyclobutenol esters 4g and 4h are formed. However, 4g and 4h behave differently in subsequent reactions: 4g gives on acid hydrolysis the monomethyl thioester of squaric acid as the final product; 4h is stable toward Si02/triethylamine, whereas attempted ethanolysis leads to ring-opened products 11 and 12. © 1979, American Chemical Society. All rights reserved.