Rearrangements photo-induced

A comparison of the electron impact (El) and chemical ionization (Cl-methane) mass spectra of 1//-azepine-1-carboxylates and l-(arylsulfonyl)-l//-azepines reveals that in the El spectra at low temperature the azepines retain their 8 -electron ring structure prior to fragmentation, whereas the Cl spectra are complicated by high temperature thermal decompositions.90 It has been concluded that Cl mass spectrometry is not an efficient technique for studying azepines, and that there is no apparent correlation between the thermal and photo-induced rearrangements of 1//-azepines and their mass spectral behavior. 

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

Oxogambirtannine (344) has been synthesized by Irie et al. (187) via photo-induced rearrangement of spirotetrahydro-P-carboline 346, obtained by Pictet-Spengler condensation of tryptamine and indandione (345). 

But the method suffers from the difficulty of obtaining starting materials and photo induced rearrangement of silyl ketones... 

Characteristics and energetics of the photo-induced electron-transfer degenerate Cope rearrangement of 2,5-diarylhexa-l,5-dienes have been reported in detail. Oxygentrapping experiments have established the two-step nature of the rearrangement of... 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

P. M. Collins and F. Famia, The photochemistry of ketones derived from carbohydrates. Part 10. A study of stereochemical influences on photo-induced rearrangements and ring expansions of 3-oxacyclopentanones using l,2-0-isopropylidenefuranos-3-ulose derivatives, J. Chem. Soc. Perkin. Trans. 1, (1985) 575-581. 

Using the above definition, the process of a photo-induced isomerization involves an excitation followed by a deexcitation involving a rearrangement. Rearrangement and dissociation were two of the few possibilities for deexcitation known during the 1920-30 s. 

Anderson, J.C. and Reese, C.B. (1962) Photo-induced rearrangement involving aryl participation. Tetrahedron Letters, 3, 1-4. 

Photo induced interconversion of the configuration at an asymmetric C atom, has not been reported. For tyrosine it has been excluded even under high-intensity laser irradiation [66]. This chiral unit seems to be very stable. A bicyclic ketone system with (formal) double epimerization is described by Zandomeneghi et al. [67] (Eq. 33). This reaction proceeds, however, by a complex rearrangement mechanism and will be discussed in Sec. II.B. 

In a series of papers, Zandomeneghi, Cavazza and coworkers [67,83] investigate the photo induced double (oxa)-di-n-methane rearrangement of a bicylcic ketone structure 22. 

The photo-induced rearrangements of a series of 4,5 epoxy-3-ketones (12 lead to contraction of ring A and expansion of ring B to give diketones (13) [60-62]. The reaction seems likely to involve an excited singlet state, for it was unaffected by triplet quenchers. Rearrangement proceeds with retention of stereochemistry at C(io> (lOa- or lOjS-series) and at C(4>... 

The l-aza-S-oxa-di-iT-methane rearrangement of (25) does not occur. Instead, cis-trans isomerization and photoelimination of methanol are observed for this oxime (equation 21) Finally, the l-oxa-4-aza-di-iT-methane rearrangement of compound (26) also did not take place. This substrate gave rise to the photo-induced 1,5-benzoyl migration shown in equation (28). ... 

The related photocyclizations of divinylamines to pyrroles and of diarylamines to dihydrocarbazoles are well established in the literature. A further example of this type of behaviour has been reported in the N-unsubstituted divinylamine (23) which is converted on irradiation in diethyl ether into the hexahydroindolone (24). In contrast, xanthone-sensitized excitation of 1 -(1-phenylvinyl)-3,4-dihydroisoquinoline (25) yields the spirobenzylisoquinoline (26), but in only 6% yield. The presence of vitamin C during the photo-induced rearrangement of... 

The photoisomerisation kinetics and other properties of the 1 1 inclusion complexes formed between aromatic derivatives of norbomadiene and P-cyclo-dextrin have been measured." (S)- or (R)-2-Chloropropiophenone affords partially racemised (S)- or (R)-2-phenylpropionic acid respectively by a photo-induced rearrangement via what is probably an ion or radieal intermediate," and (Z)-N-substituted benzoyl-a-dehydrophenylalanines such as (7) are photo-isomerised to 1-azetidine derivatives (8) by a 1,3-acyl migration. Irradiation of 9,9 -bifluorene-9,9 -diol (9) gives a mixture of fluoren-9-one and spiro[9H-fluorene-9,9 (10 -H)-phenanthren]-10 -one (10) whose composition is solvent dependent with the more polar solvents favouring (10). Laser flash photolysis shows the presence of two transients, one of which can be identified with the 9-fluorenyl cation (11), and which originates from photoheterolysis of the diol (9). There is also evidence to support the view that unimolecular rearrange-... 

Photooxidation rates of propan-2-ol in aqueous Ti02 suspensions are reported to be increased by ultrasound radiation, an observation which has been rationalised in terms of mass transport of the substrate and activation of the solid catalyst. The value of the newly described photochemical rearrangement of 2-phenylthio-l,3-cyclohexanediols such as (69) to deoxysugars (70) which are in equilibrium with the closed form (71) has been illustrated by its application to the synthesis of (+)-m-rose oxide (72), and the same authors have also described the regioselective photorearrangement of 2-phenylthio-3-aminocyclohexanols (73) to deoxyazasugars (74) this has proved to be useful in the synthesis of various piperidines (75), amino-sulfones, -sulfoxides and -acids. Hydroxy(alkoxy)methyl radicals have been generated by photo-induced electron transfer. ... 

Pyrex-filtered irradiation of methanol solutions of the pyridone (94) results in the formation of the (2+2)-cycloadduct (95). The route to (95) is thought to involve (4+4)-photocycloaddition to yield the adduct (96) which is thermally unstable and undergoes a facile Cope rearrangement to yield the cyclobutane isomer (95). A full account has been published of the photo-induced mixed addition between (97) and (98). A 1 1 ratio of these compounds yields the two adducts (99) and (100) in a 6 22 ratio. A 1 4 ratio of the reactants gives the same products but in a respective ratio of 21 6. 

The photochromism of 1 is achieved by atom rearrangement into the intramolecular oxygen-atom insertion product 2, in contrast to the frequently reported organic photochromic systems based on photo-induced cyclization, cis/trans isomerization, or H atom transfer [1]. The quantum yield of the photoreaction from 1 to 2 at 509 nm in acetonitrile without 02 is 0.14 0.01. In solution, photoreaction of 1 causes the oxidation reaction by atmospheric oxygen, resulting in a mixture of 2 and further oxidation products such as [(RhCp )2((i-CH2)2( i-S03)] and [(RhCp )2 ( r-CH2)2( t-S04)]. In contrast, in the crystalline state, the photochromic system between 1 and 2 is stable and repeatable with essentially 100% interconversion ratio. 

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