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Dipivaloylketene ( 2 ) is obtained by flash vacuum pyrolysis of furan-2,3-dione 6 and dimerizes to 1,3-dioxin-4-one 3 , 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 3 generates axially chiral 2,6,9-trioxabicyclo[3.3.1]nonadienes (bisdioxines) 4 . When arylamines are used as the nucleophiles under neutral conditions, decarboxylation occurs during the formation of bisdioxines 8 . However, when water or alcohols are added to 3 under acidic conditions, bisdioxine-carboxylic acids and esters 10 and 11 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 5 . This reaction is decarboxylative when free carboxylic acid functions are present in the bisdioxines, thus forming 21 and 22 , but carboxylic acid derivatives are preserved to yield compounds 20 , 23 , 25 , 28 , and 29 . A hydrogenolysis of the dibenzyl ester 23 yields the free dicarboxylic acid 24 . 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 32 requires microwave heating at 170 °C to form tetraoxaadamantane 33 , and the catenated compound 36 and calix[6]arene derivative 37 did not form tetraoxaadamantanes. The reaction mechanisms of bisdioxine and tetraoxaadamantane formation are discussed.

From dipivaloylketene to tetraoxaadamantanes