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

Abstract

Triplet products have also been predicted by Richardson on the basis of thermochemical calculations; W.

Laser flash photolysis study of the photochemistry of o-methylbenzils

“…In the case of the Z-enol, this intramolecular hydrogen bond should stabilize this species and lower the rate of reketonization leading to an unusually long lifetime for this enol; in the case of the E-enol, it should provide the appropriate geometrical arrangement favoring cyclization to the hydroxyindanone 2 by an internal aldol-type condensation. 21 Several authors have suggested intramolecular hydrogen bonding 17,28 to explain the fact that no deuterium incorporation is observed when the irradiation is performed in deuterated methanol. 14 The decays recorded for the enols showed multiexponential behavior and the half-lives were too long for our experimental set-up; it is thus rather difficult to attribute precise lifetimes to specific enols.…”

“…While these were not identified, they are likely to result from the photoreaction of 2-phenyl-2-hydroxyindan-1-one (2) due to its high photochemical reactivity, leading probably to the corresponding ketoaldehyde by α-cleavage. 21 Lamp irradiation of 3 in benzene also leads to the corresponding hydroxyindanone with a quantum yield of Φ ≈ 0.25, i.e., significantly higher than that for 1 (Burkoth and Ullman reported that 3 is less reactive than 1-(o-tolyl)propane-1,2-dione 28 ). Lamp irradiation in methanol shows few other photoproducts beside the photocyclization product (Φ ≈ 0.16).…”

Photochemical transformations of small ring heterocyclic compounds. 88. Photochemical rearrangements of 4,7-dimethyl-3-chromanone and related compounds

“…The ethereal extracts were washed with a 5% sodium hydroxide solution and water, and dried over magnesium sulfate. Concentration of the ether solution left a yellow oil which was distilled under reduced pressure to give 14 g (46%) of 4,7-dimethylchroman-3-ol: bp 128-132 °C (0.5 mm); mp 58-60 °C; IR (neat) 2.96 µ; NMR (CDCI3,60 MHz) 8.80 (d, 3 H, J = 7.0 Hz), 7.79 (s, 3 H), 7.26 (m, 1 ), 6.90 (s, 1 H, exchangeable with D2O), 6.33 (m, 1 ), 6.03 (m, 2 H), and 3 -2.9 (m, 3 H).…”

“…The resulting oil was chromatographed on a silica gel column using a 20% ether-hexane mixture as the eluent. The oil obtained was distilled to give 6.4 g (81%) of 4,7-dimethylchroman-3-one (8): bp 65-67 °C (0.06 mm); IR (neat) 5.75,6.12,6.30,6.65,7.89,9.48, and 12.26 µ; NMR (CDCI3, 60 MHz) 8.56 (d, 3 H, J = 7.0 Hz), 7.72 (s, 3 H), 6.52 (q, 1 H, J = 7.0 Hz), 5.67 (AB q, 2 H, JAB = 17 Hz), and 3.3-2.9 (m, 3 H); UV (methanol) 302 (shoulder) and 276 nm (e 490 and 2100); m/e 176 (M+), 161,145, 133 (base), 105, 91, and 77.…”

“…A solution containing 200 mg of 4,7-dimethyl-3-methoxychromene (25) in 175 mL of methanol was irradiated with a 550-W Hanovia lamp equipped with a Pyrex filter for 1 h. Removal of the solvent under reduced pressure followed by purification of the residual oil by thick layer chromatography using a 10% acetone-hexane mixture as the eluent gave 185 mg (79%) of 2,3-dimethoxy-3-( ,=Z = 3.0 Hz), 5.68 (d, 1 ,=/ = 3.0 Hz), 3.5-3.1 (m, 3 H), and 1.77 (s, 1 H, exchangeable with D20); UV (methanol) 281 (shoulder) and 276 nm (c 2620 and 2690); m/e 222 (M+), 207,190,175,159 (base), 147,133,115,105,91, and 77. In addition to the signals described above, the crude photolysate contained peaks at 7.75 (s), 6.41 (s), 5.6-6.0 (m), 4.89 (m), 4.18 (m), 3.1-3.5 (m, 3 H), and 1.68 (s, 1 H, exchanged with D20) which were assigned to 2-methoxy-3-(2-hydroxy-4-methylphenyl)-l,3-butadiene (27).…”