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Photoadduct, yields

Our journey begins with the photo-induced union of 3,4-dimethylfuran (19) and / -(benzyloxy)-propanal (18) (see Scheme 5). Irradiation of a solution of these two simple, achiral compounds in benzene with a 450 W Hanovia lamp equipped with a vycor filter results in the exclusive formation of head-to-head, exo photoadduct 17 in 63% yield. As a cw-fused dioxabicyclo[3.2.0]heptene system, intermediate 17 possesses a folded molecular framework to which access is obstructed on the concave face. In the presence of mCPBA, the less hindered convex face of the enol ether double bond is oxidized in a completely diastereoselective fashion, affording intermediate 16 in 80% yield after regioselective opening of... [Pg.325]

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... [Pg.230]

A recent paper 26) also reported that the photo [2 + 2] reaction between the olefin (63) and the ester (56) would yield the desired photoadduct (64), which was then reduced by LiAlH4 to afford the diol (65). The diol (65), upon oxidation, gave the a-methylene-S-lactone (67) via the intermediate lactol (66) (Table 3)26). [Pg.96]

A new formal synthesis of ( )-zizaene (136) via the fragmentation reaction is a typical example 45,46). When the enol acetate (132) was irradiated, the isolable photoadduct (133) was formed together with other isomers. Reduction and mesylation of which afforded the mesylate (134). Then, fragmentation reaction would yield an epimeric mixture of the alkenone (755) in the presence of NaOH in aqueous dioxane at 60 °C, thus formulated a total synthesis of (+)-zizaene (136) 45-46). [Pg.104]

When the two uracil units are joined by a short N -linker group, 7 //-addition is prevented, and thus the 5-bromouracil derivative 466 gave the cis—syn cyclobutane photoadduct 467 as the major product in greater than 90% yield <2002TL5127>. [Pg.176]

Benzo[l,2-A4,5- ]dithiophene 22 undergoes photochemical [2n+2n] cycloaddition with electron-deficient buta-diynes on irradiation at 300 nm to produce photoadducts in low yields. The reaction with dimethyl acetylenedicar-boxylate proceeds directly to produce the secondary photoadduct 56 (Equation 6) <2003JOC8258>. [Pg.1145]

Styrene, leads to [4 + 2]-adducts exclusively (251-258) 242 is also isolated in varying amounts, With methyl sorbate both [4 + 2]- and [4 + 4]-adducts (259-262) are obtained. 1,4-Dicarbomethoxybutadiene does not add to 138 242 is obtained in 90% yield. All photoadducts (251-262) are photolabile on irradiation (z — 253.7 nm) they are reconverted to 138 and the alkene. Significantly for preparative purposes, the product distributions (stereo-, regio-, and peri-selectivity) differ considerably from those obtained in the thermal Diels-Alder reactions (Section IV,B) mechanistic details are given in the original papers. [Pg.207]

A new synthetic route to alkyl-substituted quinones has relied on the photochemical reaction of 2,3-dichloro-l,4-naphthoquinone with a thiophene derivative (77CL851). Irradiation of a benzene solution of the quinone and thiophene by a high pressure mercury lamp gave photoadduct (295) in 56% yield. Desulfurization of this compound over Raney nickel (W-7) gave the 2-butyl-1,4-naphthoquinone derivative (296 Scheme 62). Alkyl-substituted quinones such as coenzyme Q and vitamin K, compounds of important biological activity, could possibly be prepared through such methodology. [Pg.439]

Vinyl sulfones.1 The photoadducts of 1 to alkenes on oxidation (H202) undergo selenoxide elimination to provide vinyl sulfones in 60-95% yield. Example ... [Pg.247]

A crystalline salt of 1 and 2 (20 mg) was crushed and spread between two Pyrex plates and this was irradiated with a 400-W high-pressure mercury lamp for 20 h under an argon atmosphere. The irradiation vessel was cooled from the outside by tap water. After the irradiation, the reaction mixture was analyzed by NMR. The only peaks observable were those for the starting materials (1 and 2) and a cross photoadduct (3) (47% conversion, 39% yield). In a separate experiment, 105 mg of the salt was irradiated under similar conditions. The product 3 could be isolated simply by fractional recrystallization from ethanol (15 mL). Polymeric products (6 mg) first separated and then 18 mg (17% yield) of nearly pure 3 separated as a pale brown powder mp 215-225 °C. [Pg.152]

Jaquet L, Kelly JM, Kirsch-De Mesmaeker A (1995) Photoadduct between tris(1,4,5,8-tetraazaphena nthrene)ruthenium(ll) and guanosine monophosphate - a model for a new covalent binding of metal complexes to DNA. J Chem Soc Chem Commun 913-914 Jay-Gerin J-P, Ferradini C (2000) A new estimate of the OFI radical yield at early times in the radiolysis of liquid water. Chem Phys Lett 317 388-391... [Pg.41]

Rafikov et al. [35] describe a correlation between the electron-donating capacity of substituted benzenes and the efficiency of adduct formation with maleic anhydride. This is only valid if similar compounds are compared. The ionization potentials of benzene and toluene are 9.246 and 8.820 eV, respectively the yields of adduct formation are 70% and 30%, respectively. In the series of halogenobenzenes, the ionization potentials are as follows fluorobenzene, 9.195 eV chlorobenzene, 9.080 eV bromobenzene, 9.030 eV the yields of adducts are 7%, 2%, and <1%, respectively. Anisole and diphenyl ether, with ionization potentials of 8.220 and 8.090 eV, respectively, do not give adducts with maleic anhydride. It thus seems that an increase in the electron-donating capacity of the benzene derivative leads to a decrease in the yield of photoadducts. [Pg.6]

In one of the earliest reports on ortho photocycloaddition, in which the reaction of benzonitrile with 2-methylbut-2-ene is described, a diradical (triplet) intermediate was proposed [73], The structure of the product corresponds to the most stable of the four possible diradical intermediates. When benzophenone was added as a sensitizer in an attempt to increase the yield of the photoadduct, only 0.05% of ortho adduct was isolated along with 54% of an oxetane formed by the addition of benzophenone to 2-methylbut-2-ene. In the absence of benzophenone, the ortho adduct was isolated in 63% yield. It is, however, thermally as well as photochemically unstable and reverts to starting materials, supposedly also via a biradical. The authors propose that benzophenone catalyzes bond cleavage of the adduct more efficiently than ortho addition and this would account for the low yield of photoadduct in the presence of benzophenone. From these experiments, no conclusion about the identity of the reactive excited state can be drawn. [Pg.82]

Job and Littlehailes [127] have irradiated a 1 1 molar mixture of benzene and acrylonitrile under nitrogen at 0°C and obtained 7-cyanobicyclo[4.2.0] octa-2,4-diene. The reaction did not proceed in the absence of ultraviolet irradiation or in a Pyrex apparatus. The yield of photoadduct is little affected by the presence of air, which, according to the authors, strongly supports the intermediacy of a singlet excited complex. They were, however, unable to detect a UV absorption band of a ground-state complex. However, the yields of adduct were low and the possibility is considered that a low-intensity absorption band is hidden by the benzene spectrum. [Pg.83]

Intramolecular photocycloaddition of naphthalene and anthracene has been studied by Chandross and Schiebel [327], At concentrations above 10 3 M, bi-molecular photodimerization of the anthracene occurs in deaerated methylcyclo-hexane solution. In contrast, irradiation of much more dilute ( 2 X 105 M) solutions resulted in the formation of intramolecular adduct 342 (Scheme 94). Bouas-Laurent et al. showed that the CH2—O—CH2 link is more efficient than the (CH2)3 chain in bridging the two chromophores [328], Irradiation of diethyl ether or methylcyclohexane solution of 343 (5 X 10 5 M) with a high-pressure mercury lamp and liquid filter (X > 335 nm) gave a single photoadduct 344, which was isolated quantitatively. The quantum yield of 344 is 10 times higher than that of 342. [Pg.192]

Prior to the work of Cheng et al. Rubin et al. reported a similar [4 + 4] pho-tocyclization reaction [299]. Rubin has shown that a cyclohexadiene derivative underwent a very facile photochemically promoted rearrangement to the stable bridged bisfulleroid 198 (Scheme 76). This process occurs via the initial [4 + 4] photoadduct (not observed), which undergoes a thermally allowed [2 + 2 + 2] cycloreversion to afford a bis-methano[12]annulene structure 198. Alternatively, compound 198 can be obtained by photolysis of the allylic alcohol 197 under reflux and acidic conditions in good yields. The allylic alcohol is obtained by acidic... [Pg.736]

With high indene concentration, high yields of 1 1 photoadducts of indene and 1,3-cyclohexadiene are formed in benzene solution. This concentration dependence was established with 1,4-dicyanonaphthalene as sensitizer, whereas with 9,10-di-cyanoanthracene no such observation was made. [Pg.251]

Since alcohols are less effective as hydrogen donors than amines, a PET photoaddition can occur only when the oxidized component of the reaction is the alkene. Furthermore, if the photosensitizer is chiral, the polar addition would occur in an enantiodifferentiating manner to some degree. Thus, the photoaddition of 2-propanol to 1,1-diphenylpropene, when sensitized by chiral naphthalene(di)carbox-ylates, formed the anti-Markovnikov photoadduct with enantiomeric excesses of up to 58% [53]. Unfortunately, the reaction is far from attracting synthetic interest as the yields are still too low. [Pg.81]

Photocycloaddition of 3-fz-butylcyclopentanone in the presence of 1-octene in a potassium decanoate micelle yields the the head-to-head photoadduct as the major product, consistent with the notion that a micellar interface helps to orient the reactant molecules (Fig. 14). When the orientation of the reactive olefin is further controlled by adding a hydrophilic group,... [Pg.572]

As shown in Equation (4), the sensitized irradiation of a mixture of 4,6-dimethyl-2-pyrone and maleic anhydride, or of 4,6-dimethyl-5-ethoxycarbonyl-2-pyrone and maleimide, gives the mixed photoadducts (23) and (24) in ca. 20% yields (90BCJ3456, 92BCJ354). As with many other [2 + 2] cyclocondensation reactions involving heterocycles, these are thought to proceed via biradical intermediates. [Pg.884]

Photolytic cyclization of the chloroacetamide 129 afforded the azaazu-lenone 130 in 25% yield (72JOC3691). Irradiation of a benzene solution of the pyrrolizine 131 and dimethyl acetylenedicarboxylate (DMAD) gave a 26% yield of the 1 1 photoadduct 132, which on heating at 180-200°C gave 61% of the azaazulene 133 (72JCS(P1)2517). [Pg.59]


See other pages where Photoadduct, yields is mentioned: [Pg.9]    [Pg.9]    [Pg.337]    [Pg.885]    [Pg.885]    [Pg.885]    [Pg.885]    [Pg.315]    [Pg.151]    [Pg.69]    [Pg.384]    [Pg.541]    [Pg.794]    [Pg.127]    [Pg.541]    [Pg.147]    [Pg.50]    [Pg.74]    [Pg.30]    [Pg.84]    [Pg.118]    [Pg.399]    [Pg.115]    [Pg.144]    [Pg.794]    [Pg.885]   
See also in sourсe #XX -- [ Pg.65 ]

See also in sourсe #XX -- [ Pg.65 ]




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Photoadduct

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