Anthraquinones by photo-oxidation of 2-methylbenzophenones

Effect of Solvents on the Photoenolization ofo-Methylanthrone at Low Temperatures. Evidence for H-Atom Tunneling from Nonequilibrating Triplets

“…The photoenolization of o -methylaryl ketones has been studied in detail by several authors. − The reaction occurs adiabatically on the triplet surface ,,− by transfer of a hydrogen atom from the o -methyl group to the carbonyl oxygen ( k H ), and it is completed by intersystem crossing to the ground-state enol ( k ISC , Scheme ). − ,− ,− The photoenol is a thermodynamically unstable o -quinodimethane that rearomatizes to the starting ketone in a proton-transfer reaction ( k - H ) to complete an overall cyclic process . Although much of the initial interest in the reaction came from the complex kinetic behavior of conformationally flexible compounds such as o -methylacetophenone 3,5-8,12,14,15 and o -methylbenzophenone, ,,,− , recent studies by us − and others , with conformationally locked compounds have shown that both H-transfer reactions may occur by quantum-mechanical tunneling mechanisms.…”

Photochemical transformations of small-ring carbonyl compounds. 79. Involvement of enol tautomers in the photoisomerization of 3-substituted isochromanones

“…The NMR spectrum contained singlets at 7.61 (3 ), 6.18 (3 H), and 4.79 (2 H) in addition to the aromatic protons. Ozonolysis of 16 followed by an aqueous workup gave a 65% yield of methyl 2-carboxybenzyl oxalate (17).…”

“…Photoenolization of carbonyl compounds is a reaction of considerable importance and current interest. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] The concept of internal photoactivated hydrogen transfer (photoenolization) was originally postulated to explain the apparent lack of bimolecular photoreduction of 2-alkyl-, 2-amino-, and 2hydroxybenzophenones. 25 Yang2 "4 and other workers5"8 have provided chemical and physical evidence which demonstrate that the photoenolization process is an intramolecular photochemical reaction analogous to the Norrish type II process.20 Enols generated in this fashion have relatively long lifetimes4•8 and can be trapped with dienophiles.2•15 The remarkable stability of these enols is probably due principally to the absence of catalytic ionic contaminants which are generally present in nonphotochemical reactions.…”

Metabolites of bird's nest fungi. Part 5. The isolation of 1-hydroxy-6-methyl-8-hydroxymethylxanthone, a new xanthone, from Cyathus intermedius. Synthesis via photoenolisation

“…This process has also been observed in o-tolualdehyde (9, lo), 1-methylanthraquinone (1 l), 3-aroyl-2-benzylchromones (12,13), and other systems (12,13), but examples where the expected photoenolisation does not occur are also known (12,14). These photoenols can be trapped in a Diels-Alder reaction (8,14,15) oxidation at the ?-carbon by reaction with oxygen (10,11,13,16) to give peroxides, aldehydes or ketones, and acids. Photooxidation of the dimethylxanthone 4 should therefore yield a peroxide which can be reduced to 5, thus allowing regioselective hydroxylation of the C-8 methyl of 4.…”

“…The initial oxidation product of a photoenol has been shown, in the few cases in which it has been isolated (10,13,16), to be a cyclic peroxide. Evaporation of the solvent after photooxidation of 4 gave a mixture whose nmr spectrum showed that the major constituent was the peroxide 10.…”