From dipivaloylketene to tetraoxaadamantanes.

Dipivaloylketene () is obtained by flash vacuum pyrolysis of furan-2,3-dione and dimerizes to 1,3-dioxin-4-one , which is a stable but reactive ketene. The transannular addition and rearrangement of enols formed by the addition of nucleophiles to the ketene function in generates axially chiral 2,6,9-trioxabicyclo[3.3.1]nonadienes (bisdioxines) . When arylamines are used as the nucleophiles under neutral conditions, decarboxylation occurs during the formation of bisdioxines . However, when water or alcohols are added to under acidic conditions, bisdioxine-carboxylic acids and esters and are obtained. Acid hydrolysis of the bisdioxines proceeds through the addition of water to a C=C double bond and results in a second transannular oxa-Michael-type reaction and generation of tetraoxaadamantanes . This reaction is decarboxylative when free carboxylic acid functions are present in the bisdioxines, thus forming and , but carboxylic acid derivatives are preserved to yield compounds , , , , and . A hydrogenolysis of the dibenzyl ester yields the free dicarboxylic acid . The tetraoxaadamantanes are formed in high yields (65-95%) in most cases, but the addition of water to the concave inside of the bisdioxines becomes severely hindered in cyclic derivatives, so that the 38-membered ring compound requires microwave heating at 170 °C to form tetraoxaadamantane , and the catenated compound and calix[6]arene derivative did not form tetraoxaadamantanes. The reaction mechanisms of bisdioxine and tetraoxaadamantane formation are discussed.