Barbituric Acid as a Substituent at Aryl Methylium Ions

Activation of different benzophenone derivatives with triflic
anhydride for electrophilic aromatic substitution of 5-phenylbarbituric acids leads to regioselective formation of the
ortho-substituted product. The resulting triphenylmethylium
salt can be isolated when the Michlers ketone is used. More
electrophilic cations form cyclic enol ethers such as 1-n-butyl-9,9-diaryl-1,9-dihydro-10-oxa-1,3-diazaphenanthrene-2,4-diones. Alternatively, supramolecular complex formation
with 2,6-diacetamido pyridine as well as carbenium ion
generation have been studied. Although in dilute acid only
protonation of one of the carbonyl oxygens occurs, ring
opening of the cyclic enol ether toward the carbenium ion
is observed in 96% sulfuric acid.