Reactions of the relatively persistent carboxylic acid enol-2,2-ditipylethene-1,1-diol. The reversibility of ketene hydration

The reactions of solutions of 2,2-ditipylethene-1,1-diol (tipyl = 2,4,6-triisopropylphenyl) (1), which is the enol of ditipylacetic acid, were studied. Oxidation with (p-BrC₆H₄)₃N^•+SbCl ₆^- gave a benzofuranone, i.e., a five-membered lactone (5) by cyclization via the oxygen on the ring, and ketonization with loss of aromaticity of the ring. Bromination also gave 5, presumably via an initial oxidation of the oxygen, as well as a bromine-containing six-membered lactone, a benzopyranone (9), formed by cyclization on an o-i-Pr group and ring bromination. No product resulting from reaction of bromine with the double bond was formed. This is ascribed to shielding of the double bond by steric crowding. Attempted etherification of the enolic OH groups did not give the ketene trimethylsilyl or methyl acetals. With diazomethane a bicyclic trienone 13 was formed, presumably by cyclopropanation of the ditipylketene (2) which is in equilibrium with 1, followed by a vinyl cyclopropanone rearrangement to 13. The formation of 1 by addition of water to 2 was found to be reversible as shown by isolation of a ¹⁸O-labeled-2 from hydration of 2 in H₂¹⁸O. Several diarylketenes react with H₂¹⁸O to give several isotopomeric diarylacetic acids, and the formation of Ar₂CH¹⁸O₂H indicates the generality of the reversibility of the ketene hydration for these species.