Dimethyl photoaddition

Paal-Knorr synthesis, 4, 118, 329 Pariser-Parr-Pople approach, 4, 157 PE spectroscopy, 4, 24, 188-189 photoaddition reactions with aliphatic aldehydes and ketones, 4, 232 photochemical reactions, 4, 67, 201-205 with aliphatic carbonyl compounds, 4, 268 with dimethyl acetylenedicarboxylate, 4, 268 Piloty synthesis, 4, 345 Piloty-Robinson synthesis, 4, 110-111 polymers, 273-274, 295, 301, 302 applications, 4, 376 polymethylation, 4, 224 N-protected, 4, 238 palladation, 4, 83 protonation, 4, 46, 47, 206 pyridazine synthesis from, 3, 52 pyridine complexes NMR, 4, 165... 

A [2 + 2] photoaddition-cycloreversion was applied to the enantioselective synthesis of the natural product byssocMamic add (Figure 6.11). Desymmetrization of a meso-cyclopentene dimethyl ester with PLE in pH 7 buffer-acetone (5 1) provided a monoacid, one of the photopartners. It is noteworthy that both enantiomers of this natural product were synthesized from the same monoacid [58]. 

As mentioned in chapter 4.2.3, aliphatic imines are photochemically rather unreactive. When the C—N double bond is conjugated to an electron withdrawing group (e.g. carbonyl group), as in O-alkyl derivatives of succinimide and phthalimide, the reactivity increases and azetidines are obtained in cycloadditions to olefins486). A somehow similar example is the photoaddition of a 6-azauracil derivative to 2,3-dimethyl-2-butene... 

A novel 1,8-photoaddition of dimethyl 1,4-naphthalenedicarboxylate and 1,4-dicyanonaphthalene to alkenes, a formal [3-1-2] cycloaddition, was reported by Kubo and co-workers. Although yields of the desired products were low, Cu(OAc)2 showed a profound impact on the reaction by increasing the yields considerably [85]. 

Dimethyl-5-methoxyangelicin 11 and 4,4 -dimethyl-5-methoxypsoralen 12 were added separately to 12 synthetic oligonucleotides, and their interaction and subsequent photoaddition reaction on UVA irradiation was studied <2000PCB4992>. The kinetics constant of the overall photoaddition process was determined accurately, the different intercalation sites were identified, and the value of the kinetics constant relative to each type of site was determined. 

Additions to Aromatic Hydrocarbons. A variety of photochemical additions to aromatic hydrocarbons have been reported. Benzene and its derivatives add to maleic anhydride74-76 as well as to simple olefins,77-80 isoprene,81 acetylene derivatives,79,82 and alcohols.83 The mechanism of the maleic anhydride-benzene reaction is discussed in Section IV. A.4. Naphthalene forms a photoadduct with dimethyl acetylenedicarboxylate62 and with acrylonitrile8211 while anthracene behaves similarly with maleic anhydride84 and with 1,2-benzanthracene.85 The photoaddition of several aromatic amines to anthracene has been reported to proceed via a charge transfer complex86,87 in fact, the majority of these addition reactions may proceed in this manner. 

The double bonds in certain heterocyclic compounds, such as furans, Af-acylpyrroles and A-acylindoles are also susceptible to photoaddition of carbonyl compounds to form oxetanes (equation 106) (77JHC1777). A wide range of carbonyl compounds can be used, including quinones, a-diketones, acyl cyanides, perfluorinated aldehydes and ketones and esters. A remarkable case of asymmetric induction in oxetane formation has been reported from optically active menthyl phenylglyoxylate and 2,3-dimethyl-2-butene the oxetane product obtained after hydrolysis of the ester group had an optical purity of 53% (79AG(E)868). 

Unlike coumarin, chromone (206) undergoes efficient unsensitized photoaddition to tetramethylethylene, cyclopentene, ketene dimethyl acetal, and but-2-yne.180 The major product of such an addition to tetramethylene is the cis-fused cyclobutane derivative (207) the formation of the two minor products (208 and 209) is easily rationalized. Added benzophenone has no visible effect on this cycloaddition, which is therefore believed to involve the attack of triplet chromone on the ground-state alkene. Photoaddition to furo-chromones has also been studied,179 and the photosensitized cyclo-... 

The photodecomposition of oxiran and alkyloxirans both in the gas phase and in solution has been extensively investigated. Processes arising by carbon-oxygen bond homolysis and hydrogen abstraction have been reported, and the subject has been reviewed in detail elsewhere.48 The most recent studies include the photoaddition of methanol to alicyclic epoxides, a process that appears to be promoted by acid,49 and the interesting if unusual photochemically induced conversion of the epoxyalcohol (56) to sugiresinol dimethyl ether (57).50... 

The photoaddition of the pyran-2-one (330) to ethylene to give the adduct (331) has been employed in a synthesis of grandisol,274 and the cyclobutane derivatives (332 and 333) were obtained on irradiation of the pyran-4-one (334) in 2-methylpropene.275 Photocycloadditions of dehydroacetic acid276 and 2,4-dioxo-3,3-dimethyl-2,3-dihydropyran277 have also been described, and the synthetically useful addition of 2,2,6-trimethyl-1,3-dioxolenone (335) to 2,3-dimethylbut-2-ene has been reported.278... 

A number of examples of photoaddition to alkynes has been described. Dimethyl acetylenedicarboxylate has been found to add to methyl-substituted 3-benzoylthiophens301 and to thiophen and 2,5-dimethylthiophen302 on irradiation. Benzo[f>]thiophen also undergoes cycloaddition reactions with alkynes,303 and in the case of dimethylacetylene dicarboxylate, product formation has been shown to be wavelength dependent.304 Intramolecular [ 2 -(- 2] cycloaddition has been observed on both direct and triplet-sensitized irradiation of the alkyne (353) and gives the cyclobutene (354)305 the isomer (355) is formed on prolonged irradiation. 

In certain cases benzene will undergo photocycloaddition to oxygen and sulfur heterocycles. The two major photoproducts of irradiation of a mixture of furan and benzene are adducts 356 and 357, arising, respectively, by [ 2 + 2] and [ 4 + 4] cycloaddition processes.306 Irradiation of benzene and 2,2-dimethyl-l,3-dioxol (358) similarly affords adducts 359 and 360, together with dimer 361.307 Thiochromone 1,1-dioxide also undergoes photoaddition to benzene.308... 

Thiets have been proposed as intermediates in the corresponding photoaddition of thiones to alkynes.369 The stable spirothiet (421) was obtained in this way along with the 1,2-dithiole (422) on irradiation of l,3-dimethyl-2-thioparabanic acid (423) with diphenylacetylene370 the proposed pathway is outlined in Scheme 28. Stable thiets have also been detected in the photoaddition of xanthenethione to bis(tert-butylthio)ethynes.371... 

Additions of tetrahydrofuran to dimethylacetylenedicarboxylate395 and to tetracyanoethylene396 and ofxanthene to 3,3-dimethyl-l,2-indanedione397 have also been described. Few systematic investigations of addition to sulfur heterocycles have been made and the topic merits further study. Primary and secondary amines, however, do undergo photoaddition to benzo[h]thiophen (453), as shown in Scheme 30.398... 

In the presence of trifluoroacetic acid, irradiation of dimethyl acetylenedi-carboxylate and benzene leads to the formation of dimethyl fumarate and dimethyl maleate in appreciable amounts in the absence of acid, only traces of these compounds were detected [36,61], Irradiation of dimethyl acetylenedicarboxylate and benzene in the presence of tetracyanoethylene (TCNE) produces a 1 1 1 adduct at the expense of cyclooctatetraene. The photoaddition still proceeds readily behind a fdter transmitting at wavelengths >290 nm, where only the acetylene absorbs to a significant degree [60],... 

Scheme 10 Mechanism of the photoaddition of dimethyl acetylenedicarboxylate to benzene.

A kinetic scheme was proposed [122] with the fluorescent exciplex as precursor of the photoproducts (ortho as well as meta adducts). Quantum yields of adduct formation, exciplex emission, and benzene fluorescence were measured as a function of alkene concentration. The kinetic data fit the proposed reaction scheme. The authors have also attempted to prove the intermediacy of the exciplex in the photoaddition by adding a quencher to the system benzene + 2,2-dimethyl-... 

Organ, M.G., Froese, R.D.J., Goddard, J.D., Taylor, N.J., and Lange, G.L. (1994) Photoadditions and dialkylcuprate additions to 2-tert-butyl-2,6-dimethyl-l,3-dioxin-4-one and related heterocycles. Experimental, ab initio theoretical, and X-ray structural studies of facial seledivity and enone pyramidalization. Journal of the American Chemical Society, 116, 3312-3323. 

Guerry and Neier have reported both the intra- and intermolecular photoaddition of a dihydropyridone, in which the vinylogous amide nitrogen is part of a six-membered ring. Irradiation of 83 in the presence of dimethyl maleate led to the formation of cyclobutane 84 in good yield (Scheme 20)41. The intramolecular version of this reaction was observed on irradiation of 85, which led to the formation of photoadduct 86 in good yield (n — 2 or 3). 

As has been mentioned earlier, it is often very difficult to distinguish between and identify the roles of exciplexes (and excimers) and biradicals in cycloaddition reactions. Caldwell and Creed (1978b) have studied the cycloaddition of dimethyl fumarate to phenanthrene and found that the quantum yield of the cyclobutane photoaddition product is increased in the presence of oxygen. It was suggested that oxygen enhances intersystem crossing in the triplet biradical formed between the two reactants. Nitroxide radicals have also been found to increase intersystem crossing (Sj -> Tj) in carbocyanines when nonpolar solvents are used (Kuzmin et al., 1978). When polar solvents are employed full electron transfer takes place. 

The same photoaddition takes place with N-nitrosodimethylamine. The stereochemistry of both the Ar,Af-dimethyl-2-nitroso-1-cyclohexanamine and 2-[hydroxy(nitroso)amino]-AT V-di-methyl-l-cyclohexaneamine was established by reduction with lithium aluminum hydride and A -acetylation to give both rrans -/V-acctyl-/Vr, Ar-dimethyl-l,2-cyclohexanediamine ( H NMR). 

Miscellaneous examples of the photoaddition of solvent and other simple molecules to nitrogen-containing systems have been described. Methoxylation, methylation, and hydroxymethylation arising via ionic and radical pathways have been observed on irradiation of dimethyl pyridine-2,4-dicarboxylate in methanol. The photomethoxylation of methyl pyridine-2-carboxylate in acidified methanol is facilitated by added 4-substituted pyridines such as 4-cyanopyridine an excited complex is thought to be involved. 

The germanium analogue (299 M = Ge) behaves in a similar fashion to the l,2-disilacyclohexa-3,5-diene (299 M = Si) on irradiation both give the corresponding fluorene (300) by extrusion of dimethylgermylene or dimethyl-silylene, respectivelyThe [ 2-l- 2] photoaddition of 2,3-dimethylmaleic... 

Olefinic dienophiles such as maleic anhydride and maleimide add to 4,6-dimethyl-2-pyrone to afford mono Diels-Alder adducts. These on irradiation afford, by a photo-decarboxylation, a diene which can be trapped by a dienophile to afford the products (311). - - Acetone-sensitized photoaddition of the oxazolones (312) to maleic and dimethyl maleic anhydride yields the adducts (313). - ... 

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