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1.4- Dienes photoisomerizations

Both 7-oxa- and 7-azabicyclo[2.2.l]hepta-2,5-dienes photoisomerize to the corresponding 3-heteraquadricyclanes While the oxygen heterocycles are usually stable enough to be... [Pg.970]

P -Bonding is obviously also the initiating step in the complex photoisomerization sequence of the stereoisomeric 1,5-dien-3-ones (162) and (163) in ethanol. After low conversions of the starting dienones, an isomer containing an analogous chromophoric system [(164) and (165), respectively] was found to build up temporarily in each case. On longer photolysis times, both compound pairs (162)/(164) and (163)/(165), are consumed, and the mixtures of the four diastereomers (166)-(169) were isolated from both runs. According to separate irradiation experiments with each of these products, (166) and (167) on one hand, and (168) and (169) on the other, are... [Pg.328]

A few examples are known of the thermally reversible, photoisomerization of 4//-azepines, e.g. 22, to l-azabicycIo[3.2.0]hepta-2,6-dienes, e.g. 23.83 The yields are high (65 95%) and the alternative isomers, the 6-azabicycIo[3.2.0]hepta-3,6-dienes, resulting from C2-C6 ring closure, do not appear to be formed. [Pg.178]

The non-preservation of cis stereochemistry of dienophiles 24 and 26 in the adducts 25 and 27 is due to a cis-trans photoisomerization of the double bond and to the concerted suprafacial Diels-Alder cycloaddition of diene to the ground state of trans dienophiles. [Pg.25]

Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. [Pg.3]

The conjugated diene dienoestrol (65) was irradiated at 254 nm in 90% aqueous methanol. Rotation and cis-trans photoisomerization gave (66) which underwent a photochemical [1, 5]sigmatropic rearrangement to give (67). Photocyclization followed by enol-keto tautomerism then gave the isolated dihydrophenanthrene dione (68) [56]. [Pg.68]

An X-ray crystal structure determination of calciferol (vitamin D-2,71) showed that steric crowding in the s-cis diene system resulted in a twisted conformation with a dihedral angle of 53° [59], On irradiation with a mercury lamp, it was partially converted into ergosterol (72) and tachysterol (73) [60, 61]. When a solution of calciferol in light petroleum containing a trace of iodine was exposed to diffuse daylight, the vitamin was photoisomerized to (74) [62],... [Pg.69]

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). [Pg.133]

SCHEME E The biradical model for the direct ,Z-photoisomerization of (a) alkenes and (b) conjugated dienes... [Pg.208]

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. [Pg.211]

A photochemical reaction in which the reactant is converted into an isomer. The most common example of such a reaction is photochemical cis-trans isomerization. Another example of photoisomerization is the effect of light on cholesta-3,5-diene. [Pg.559]

Scheme 12 Photoisomerization of 1,3-diene compounds in the crystalline state and solution, a One-way isomerization to EE isomer in the crystalline state, b Usual isomerization in equilibrium observed in solution... Scheme 12 Photoisomerization of 1,3-diene compounds in the crystalline state and solution, a One-way isomerization to EE isomer in the crystalline state, b Usual isomerization in equilibrium observed in solution...
The addition of O2 to acyclic dienes proved to be strongly dependent on terminal substitution and the substituents at other positions of the conjugated system, and, furthermore, it must be accompanied by photoisomerization of ( ,Z)-dienes to ( , )-dienes because singlet oxygen adds exclusively to ( , )-dienes to yield endoperoxides (cf Tables 6(a), 6(b), and 7, and references therein). [Pg.710]

Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review. Unlike tS in which the lowest vertical excited singlet state is the 1 B state and S2 is the 2 Ag state in solution, these two excited states lie very close to each other in all-trans-1,4-diphenyl-1,3-butadiene (DPB). The additional carbon-carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rota-mers. Most spectroscopic studies in solution have concluded that the l B state is S. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically. [Pg.888]

In the triplet counting technique the substance whose intersystem crossing yield is to be determined is used as a sensitizer for the photoisomerization of piperylene (1,3-pentadiene). Under conditions where essentially all of the sensitizer triplets are intercepted by diene, the quantum yield of isomerization is directly proportional to the intersystem crossing yield of the sensitizer. If the diene interferes with sensitizer singlets however, the yield of triplets will fall, resulting in an anomalously low value for 0,. [Pg.277]

A few examples of the photoisomerization of AH- azepines to l-azabicyclo[3.2.0]hepta-2,6-dienes have been noted, e.g. (49) -> (50) (80TL595). [Pg.505]

The products obtained upon direct excitation of butadiene,291 norbornadiene,292 cyclohexadiene,293 and other dienes294 are also completely different from those obtained by photosensitization, again demonstrating quite different chemical reactivity of excited singlet and triplet dienes. However, 1,l -bicyclohexenyl295 and 1,3-cycloocta-diene298 represent exceptions, undergoing both direct and sensitized photoisomerizations to cyclobutene derivatives. [Pg.85]


See other pages where 1.4- Dienes photoisomerizations is mentioned: [Pg.165]    [Pg.143]    [Pg.203]    [Pg.203]    [Pg.211]    [Pg.213]    [Pg.220]    [Pg.231]    [Pg.247]    [Pg.263]    [Pg.297]    [Pg.301]    [Pg.129]    [Pg.46]    [Pg.235]    [Pg.504]    [Pg.560]    [Pg.504]    [Pg.560]   
See also in sourсe #XX -- [ Pg.196 ]

See also in sourсe #XX -- [ Pg.5 , Pg.196 ]

See also in sourсe #XX -- [ Pg.196 ]

See also in sourсe #XX -- [ Pg.5 , Pg.196 ]




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