Abstract: 9-Xanthenecarboxaldehyde (721) and methyl (7b), mesityl (7c), 2, 4, 6- triisopropylphenyl (Tip)(7d), and tert-butyl (7e) 9-xanthenyl ketones were prepared and found to be ketonic in CDC13. In DMSO-d6, 7a, 7c, 7d, and diphenylacetaldehyde 16 are in rapid equilibrium with the enols 15a, 15c, 15d, and 17, respectively. Keto== enol equilibrium constants, Keno], were measured in DMSO-d, at various temperatures. The values at 294 ...