93254-84-3Relevant articles and documents
Tetraalkylammonium-templated stereoselective Norrish-Yang cyclization
Yamada, Shinji,Iwaoka, Azusa,Fujita, Yuka,Tsuzuki, Seiji
supporting information, p. 5994 - 5997 (2014/01/06)
Tetrabutylammonium salts serve as templates for the Norrish-Yang cyclization of 2-benzyloxy-acylbenzenes to give trans-dihydrobenzofuranols in high stereoselectivities. The dual cation-π interactions between an ammonium with a benzene ring and a carbonyl group play a key role in changing the conformation of the substrate, which was supported by ab initio calculations.
Photocyclization Reactions. Part 7 [1]. Solvent and Substituent Effects in the Synthesis of Dihydrobenzofuranols Using Photocyclization of α-(2-Acylphenoxy)toluenes and Ethyl 2-Acylphenoxyacetates
Sharshira, Essam Mohamed,Horaguchi, Takaaki
, p. 1837 - 1849 (2007/10/03)
Photocyclization reactions were carried out on α-(2-acylphenoxy)toluenes 1a-e and 2-acylphenoxyacetates 2a-e in three solvents of different polarity (benzene, acetonitrile and methanol) to examine solvent and substituent effects on the cyclization of 1,5-biradical intermediates to dihydrobenzofuranols. Irradiation of 1a-e in benzene gave cis-dihydrobenzofuranols cis-4b-e selectively in 14-84% yields along with rearranged products, 2-acylbenzophenones 5b-d (23-39% yields). Photocyclization of 1a-e in acetonitrile or methanol gave a mixture of cis- and trans-dihydrobenzofuranols 4a-e in 28-81% yields and small amount of 2-acylbenzophenones 5b-c in 6-12% yields. In polar acetonitrile or methanol, the cis and trans stereoselectivity of dihydrobenzofuranols decreased. The decrease in stereoselectivity was attributed to intermolecular hydrogen bonding between the hydroxyl group of 1,5-biradicals and solvents. On the other hand, irradiation of esters 2a-e in benzene afforded cis-dihydrobenzofuranols cis-13a-e selectively in 48-74% yields. Similarly, photocyclization of 2a-e in acetonitrile or methanol produced dihydrobenzofuranols 13a-e in a fair to good yields. In the photocyclization of 2b-d, not only in nonpolar benzene but also in polar acetonitrile or methanol, increasing the size of alkyl group from methyl (R = Me) to ethyl or isopropyl group (R = Et or i-Pr) gave cu-dibydrobenzofuranols cis-13b-d predominantly. Conformational, solvent and substituent effects on the cyclization of 1,5-biradicals and reaction pathways are discussed.
The photocyclization of o-alkoxy phenyl ketones
Wagner, Peter J.,Meador, Michael A.,Park, Bong-Ser
, p. 5199 - 5211 (2007/10/02)
Several o-alkoxybenzophenones and o-(benzyloxy)benzophenones and -acetophenones photocyclize to 3-hydroxy-2,3-dihydrobenzofurans. Quantum yields generally are quite high, except for o-(benzyloxy)acetophenone. The o-ethoxy and o-benzyloxy ketones form two diastereomeric products, the Z isomer being highly preferred in hydrocarbon solvents, the E isomer being formed in comparable yield in methanol or with added pyridine. The reaction involves δ-hydrogen abstraction by the ketone triplets followed by cyclization of the 1,5-biradical intermediates. The biradicals have such short lifetimes that they usually cannot be detected by flash spectroscopy or trapped by thiols. Triplet state lifetimes, determined both by steady-state quenching studies and by flash kinetics, reveal that hydrogen abstraction rate constants are quite low. Arrhenius plots for triplet decay indicate activation energies of 3-5 kcal/mol and A values of 109 for the δ-hydrogen abstraction. MMX calculations and spectroscopic data all indicate that the ketones exist primarily in conformations with the carbon α to the ether oxygen twisted well away from the carbonyl. The low observed rate constants are ascribed to even lower equilibrium populations of conformers in the geometry required for reaction in the triplet state than in the ground state. 2,6-Diacylphenyl ethers show ten times the triplet reactivity of their monoacyl equivalents. In these cases, the ether function is twisted 90° such that the target C-H bond is much closer to a carbonyl. The large solvent effects on the stereochemistry of cyclization despite short biradical lifetimes suggest that bond rotations may induce intersystem crossing of the triplet biradicals. The low cyclization quantum yield from o-(benzyloxy)acetophenone and the formation of o-benzoylacetophenone as a major side product suggest that the 1,5-biradicals partially cyclize into the benzene ring to generate spiroenol intermediates. Rate constants for quenching of the triplet ketones by 2,5-dimethyl-2,4-hexadiene were measured. The kq values are ≥5 × 109 M-1 s-1 for the o-methoxy ketones but only 1-3 × 109 for the o-benzyloxy ketones. This rare steric effect on triplet energy transfer is attributed to twisting of the benzoyl chromophores caused by steric congestion.
