Photoreduction intramolecular

Facile synthesis of isooxyskytanthine (300), a monoterpene alkaloid, was performed by photoreductive intramolecular cyclization of the 5-oxocyclopentanecarboxamide... 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

Nitroanilines and nitronaphthylamines (group III) show remarkable stability towards both photoreduction and photosubstitution. Photoreduction (including intramolecular hydrogen abstraction) occurs, however, after acylation of the amino group in nitroanilines 46-48). Xhe stability of liitroanilines towards photoreduction is evidently due to the chcirge transfer character of the lowest excited state. Another possibility could be the increased probability of radiationless deactivation due to smaller separation of the ground state and the lowest excited states. 

In aliphatic amines (diethylamine or triethylamine) the intramolecular hydrogen abstraction is quenched almost completely. Instead, smooth photoreduction of the nitro group without participation of the side chain is observed with 1,3,5-tri-fezf-butyl-2-nitrobenzene (5) and 14, R = C(CH3)3 ). Products derived from the respective phenylhydroxylamines were isolated in both cases. Again, an electron transfer, which does not seem to suffer from steric restrictions, is operative (see also Section A. 1.3). 

In the intramolecular photoreduction kinetics scheme, catalytic metallic nuclei are formed in the intramolecular ligand-to-metal electron-transfer process. For example. 

The photochemistry of phthalimide systems was thoroughly investigated by many groups over the last two decades. This chromophore shows a broad spectrum of reactivity leading mainly to cycloaddition and photoreduction products by either intermolecular or intramolecular processes. In the presence of electron donors, the electronically excited phthalimide could also undergo electron transfer and act as an electron acceptor. 

Recent work on the photochemistry of ketimines has shown that they do not undergo reduction unless ketones are present. Thus chemical sensitization is entirely responsible for the photoreduction of benzophenone methylimine (22)105 while intramolecular chemical sensitization has been suggested as the mechanism for reduction of the acylketimine (23).119 In related work,... 

In the intramolecular photoreduction kinetic scheme, catalytic metallic nuclei are formed in the intramolecular ligand-to-metal electron-transfer process. For example, catalytic metallic Cu nuclei can be formed in the photochemical reaction (A < 3500 A) of cupric acetate (CuA) ... 

In this enantiodifferentiating photoreduction, the chiral amine plays two roles, as a chiral inductor and as an electron donor. Irradiation of 25 (Scheme 10) in a hexane slurry of unmodified NaY zeolite gave only the intramolecular hydrogen abstraction product 26. However, photolysis of 25 coimmobilized with ephedrine, pseudoephedrine, or norephedrine in NaY supercages afforded the reduction product 27 along with 26. It is clear that the immobilized amine plays the decisive role in the photoinduced electron-transfer reduction of 25, since 27 was not formed in unmodified or (— )-diethyl tartrate-modified zeolites. Consequently, the ee of obtained 27 was independent of the loading level of the chiral inductor. 

Intramolecular proton transfer in electronically excited transfer in, say, salicylic acid ester and other aromatic compounds leads to deexcitation of the energized electron [43-45]. In photoreduction processes, electron transfer often precedes proton transfer [46] the stability of the protonic bond is at least partially due to an n-a interaction [47]. The strength of the protonic interaction appears to be proportional to the ionization potential of the donor and is sensitive to solvent polarity [48]. These effects have hardly been touched on in biologically important transitions and represent an important new field of research. 

The effects of chain-length, solvent, and temperature on the intramolecular photoreduction of benzophenone linked to diphenylamine (53) has also been explored in detail [165, 166],... 

A study of the photoreduction of the oxoamides (103) in acetonitrile/triethylamine has shown that the oxoamides (103 a,b) afford the alcohols (104) in high yield. However, the amides (103 c,d) and (105) are also reactive and afford the cyclized compounds (106 - 108) respectively. The failure of the oxoamides (103 a,b) to undergo cyclization is presumed to be due to intramolecular hydrogen bonding. The ease with which this reductive cyclization occurs with amides or unsaturated ketones has been exploited in a new synthesis of hirsutene (109). Thus irradiation (254 nm) of the ketone (110) in acetonitrile/triethylamine affords the alcohol (111, 20X) and the desired cyclic compound (112, S8X). This latter material is readily converted to hirsutene. The ketone... 

The photoisomerization of cyclobutanones to transient car-benes has been used as part of an interesting synthesis of muscarine (Pirrung and DeAmicis). Intramolecular oxetan formation has been used as part of novel syntheses of medium-ring ethers (1) and the tricyclo-octane (2) (Carless et al. Gleiter and Kissler). Cossy et al. have employed the photoreductive cycliza-... 

One of the earliest photoreactions to be studied was the photoreduction of benzophenone (Ciamician and Silber, 1900)—that is, the conversion of a carbonyl compound into an alcohol by an intermolecular hydrogen abstraction reaction. Intramolecular hydrogen abstraction by the carbonyl group, usually from the y site, is referred to as a Norrish type II reaction. Hydrogen abstraction by olefins and heterocycles has also been observed. 

A polymer containing pe,ndant benzophenone groups (XV) is found to photocrosslink. The mechanism is inter.molecular coupling of radicals formed via photoreduction in the triplet state. In solution, crosslinking is concentration dependent as intramolecular cyclization competes (21). 

Compared with photooxygenation, i.e. the photochemical activation of oxygen, the definition of photoreduction is more difficult. Many photochemical processes involve redox reactions where one part of the substrate (in intramolecular reactions) or a second molecule (in intermolecular reactions) is reduced and the original substrate is oxidized (or... 

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