Thermal cycloaddition of carbonyl compounds to a stable cyclobutadiene

In contrast to acid chlorides which undergo addition to 1 to form cyclobutene derivatives 4, aldehydes react with the same antiaromatic starting compound 1 to give the 4+2]-cycloaddition product 5. Activated ketones such as 1,1,1-trifluoroacetone, biacetyl, and acetyl cyanide undergo a clean cycloaddition reaction to tricyclic compounds (1 + 6a?c → 9a?c), whereas trioxoindane yields a bicyclic product (1 + 6d → 8d).