Photochemistry of dihydromayurone. Novel solvent participation in a photoisomerization

Abstract

When irradiated in 2-propanol, both cis-and trans-dihydromayurone (1 and 6) react to give a mixture of isomerized (2 and 3) and reduced (4 and 5) products. The isomeric photoproducts (2 and 3) are unusual in that a reducing solvent is necessary for their formation although a net reduction has not occurred. An exchange of hydrogen atoms during isomerization was demonstrated by irradiation of dideuterio-trans-dihydromayurone (8) in 2-propanol to give monodeuterated hydrindanone 9 and irradiation of 1 in 2-propanol-d1 to give monodeuterated hydrindanone 7. Hydrindanone 2 is a secondary product derived from the photoisomerization of 3. The difference between the photoreduction of 1 and that of simpler bicyclo[4.1.0] heptan-2-ones is attributed to the gem-dimethyl group, which sterically hinders hydrogen abstraction.The photoreduction of conjugated cyclopropyl ketones in 2-propanol has been shown2 to proceed with hydrogen abstraction by the n-n* excited carbonyl oxygen atom, followed by a ground-state rearrangement of the a-hydroxycyclopropylcarbinyl radical. The products obtained from bicyclo [4.1.0]heptan-2-ones are cyclohexanones derived from the reductive opening of the cyclopropyl ring (eq 1). The alternate 0 ? excited ground state state OH products, cycloheptanones, are normally not observed owing to both the thermodynamic preference for opening to a six-membered ring as well as preference for opening of the outer cyclopropyl bond which is geometrically aligned for better overlap, as compared to the inside bond, with the adjacent carbonyl center. The irradiation of either cis-or trans-dihydromayurone (1 and 6 , respectively) in 2-propanol does not follow this simple reaction course, but rather gives a complex mixture of products (Scheme I).The ketones were irradiated in 2-propanol with a Corex-filtered (>260 nm) 450-W Hanovia mediumpressure mercury lamp and the reaction progress was monitored by gas-liquid chromatography (glc). After removal of the solvent and oxidation of the total mixture to convert any alcohols to the corresponding ketones, the products were isolated by a combination of alumina and silica gel chromatography and recrystallization. The isolated yields are listed in Table I. 3a,7,7-Trimethyl-7a-vinylhexahydro-l-indanone (2) was identified on the basis of its spectral data. The nmr spectrum clearly shows a vinylic group at a quaternary center, three methyl singlets, and two hydrogens a to the carbonyl group (see Experimental Section). This ketone is isomeric with 1 (mol wt 206) and the ir (1) The results of this research were first disclosed a t the X X I I I IUPAC