Irradiation coumarin

Keywords hydroxybenzaldehyde, ethyl acetate, Knoevenagel condensation, microwave irradiation, coumarin... 

CqHaOj. Colourless crystals m.p. lOS C. Obtained by boiling coumarin with sodium ethoxide. Irradiation of o-coumaric acid produces coumarinic acid. The stable form of the acid is the trans form. 

For the synthesis of coumarins, the Pechmann reaction [145] is one of the most popular synthetic routes. As the reaction is conventionally carried out at high temperature, two microwave-assisted versions have been recently described. Besson and co-workers described the cyclocondensation of different m-amino phenols 226 with /1-ketoesters 227 on graphite/montmorillonite KIO support (Scheme 83). The use of graphite was crucial in the development of the reaction conditions. In fact, microwave irradiation of the reagents using different conditions gave poor results in terms of yields and purity. The optimized conditions, using a monomode microwave system, employed... 

The nOe difference measurements not only help in stereochemical assignments but also provide connectivity information. A large nOe at 8 6.91 (G-8H), resulting from the irradiation of the methyl singlet of the methoxy group (8 3.94), confirms their proximity in space. This nOe result is consistent with structure A for the coumarin. 

The photodimerization of coumarin has been rather extensively investi-gated.<90 92) Irradiation of coumarin in solution yields a mixture of four stereoisomeric dimers (78)—(81) with dimers (78) and (79) strongly predominating ... 

In order to study any chemoselectivity influences of microwave irradiation on the domino Knoevenagel/hetero-Diels-Alder process (the so-called Tietze reaction), Raghunathan and coworkers [31a] investigated the transformation of 4-hydroxy coumarins (10-85) with benzaldehydes 10-86 in EtOH to afford pyrano[2,3-c]cou-marin 10-87 and pyrano[2,3-b]chromone derivatives 10-88. Normal heating of 10-85a and 10-86a at reflux for 4h gave a 68 32 mixture of 10-87a and 10-88 in 57% yield, whereas under microwave irradiation a 97 7 mixture in 82% yield was obtained. Similar results were found using the benzo-annulated substrates 10-85b and 10-86b. 

Some 7-aminocoumarins-4-carboxylates [60 a] were synthesized by the Pechmann reaction, by microwave irradiation of the reactants on solid supports (graphite-K10) (Scheme 8.41). Synthesis of unsubstituted coumarins (C-4 position) has been also reported [60 b]. 

Another illustrative example concerns the photodimerization of coumarin. The direct irradiation in ethanol, Eq. 49,... 

Photodimerization of coumarin and its derivatives have been widely investigated and used.[37-45] It is known that under a UV irradiation with light wavelength longer than 300 nm, coumarin and its derivatives can undergo [2+2] dimerization to form four types of dimmers depending on the reaction conditions. Importantly, such formed photodimers can be... 

Figure 4 shows typical SEC profiles of two PS-3-PI diblock copolymers before and after the photodimerization reaction. [46] The appearance of a peak after the UV irradiation at a short retention time with a doubled molar mass indicates the formation of PS-3-PI-C-PI-3-PS copolymer chains. In contrast, the irradiation of SI 42 in THF without the self-assembly only leads to a slight increase of the molar mass. It confirms that the self-assembly helps to concentrate and expose the reactive ends of precursor diblock copolymers on the periphery of the core-shell micelles, which greatly increases the coupling efficiency of the photodimmerization reaction between two coumarin end groups. 

While this reaction to form coumarin derivatives can be completed in mineral acids, research shows that the reaction was much faster in ILs even at room temperature. The same group used l-butyl-3-methylimidazolium hexafluoro-phosphate IL at high temperatures without employing any acid catalyst. The yields were comparable to chloroaluminate ILs with catalytic amounts of acid at room temperature. They also concluded that Bronsted acidity (produced by HF when [bmimJIPF ] contacts water) was not responsible for the observed activity. Singh et al have used l-butyl-3-methylimidazolium hydrogen sulfate IL in combination with microwave irradiation. They were able to synthesize coumarins in quantitative yields with drastic reduction in reaction times. Soares et al have used [bmim][NbCl6] IL to perform the Pechmann reaction using various phenols with ethyl acetoacetate to produce coumarin in moderate yields (-35%). 

The Pechmann and Knoevenagel reactions have been widely used to synthesise coumarins and developments in both have been reported. Activated phenols react rapidly with ethyl acetoacetate, propenoic acid and propynoic acid under microwave irradiation using cation-exchange resins as catalyst <99SL608>. Similarly, salicylaldehydes are converted into coumarin-3-carboxylic acids when the reaction with malonic acid is catalysed by the montmorillonite KSF <99JOC1033>. In both cases the use of a solid catalyst has environmentally friendly benefits. Methyl 3-(3-coumarinyl)propenoate 44, prepared from dimethyl glutaconate and salicylaldehyde, is a stable electron deficient diene which reacts with enamines to form benzo[c]coumarins. An inverse electron demand Diels-Alder reaction is followed by elimination of a secondary amine and aromatisation (Scheme 26) <99SL477>. 

Phenylbenzoic acid is converted into benzo[c]coumarin in the Su z system, that is irradiation when mixed with trivalent iodine compounds and iodine the 2-iodo derivative may also be formed <99JCS(P1)1713>. 

The photocyclodimerization of coumarin (2//-l-benzo[b]pyran-2-one) in direct and sensitized experiments is known to proceed rather inefficiently in the absence of Lewis acids and in better yields in their presence.42,43 In contrast, 2-quinolone (22, X = NH)44 or 1-thiocoumarin (2//-l-benzo[b]thiopyran-2-one 22, X = S)45 afford anti head-to-head dimers 23 on direct irradiation in very good yields. Psoralens (7//-furo[3,2-g][l]benzopyran-7-ones) usually exhibit a behavior similar to that of coumarins.46... 

Another example of a carbonyl compound whose excited singlet reacts before crossing over to a triplet is provided by coumarin. Direct irradiation in ethanol gives very low yields of a cw-fused dimer.432 However, inclusion of benzophenone results in only trans-fused dimer, even if all the light is initially absorbed by the coumarin.433 Apparently, excited singlet coumarin transfers its singlet excitation to benzophenone, which then crosses over to its triplet and transfers its triplet excitation to coumarin. Triplet coumarin then dimerizes fairly efficiently ( = 0.25) to a tranj-fused system. Morrison has demonstrated sizeable solvent effects on this photodimerization.434 Yields of the cis-dimer are increased by polar solvents and probably arise from excited singlet excimers.119... 

Pyridone (260), unlike quinolin-2-one and coumarin, does not yield a dimer of the cyclobutane type, but on irradiation in concentrated solution is converted into the dimer (261) by a novel 1,4-cycloaddition process.266,267 The same dimerization has been observed in a number of nuclear-substituted pyridones.286,268 An analogous process is reported to occur in various 2-aminopyridines [Eq. (66)], on irradiation in acid solution.267... 

The study of this cycloaddition and its mechanism is complicated by the formation in small yield of additional photoproducts. Irradiation of phenanthraquinone (308) in, for example, 2-methylpropene affords the oxetane (309), arising by 1,2-cycloaddition of the alkene to the carbonyl, in addition to the expected 1,4-dioxene (310).334 Oxetanes are the principal products of photoaddition of phenanthraquinone to benzofuran, furano[3,2-y]coumarin, and isocoumarin.329 A further product has the structure (311), and is undoubtedly the... 

De la Hoz, A., Moreno, A. and Vazquez, E., Use of microwave irradiation and solid acid catalyst in an enhanced and environmentaly friendly synthesis of coumarin derivatives, Synlett, 1999,608-610. 

Bogdal, D., Coumarins fast synthesis by knoevenagel condensation under microwave irradiation, /. Chem. Res. (S), 1998, 468-469. 

Other y-lactone containing systems that have been studied include derivatives of 3-oxabicyclo[3.1.0]hexan-2-one173 and 3-(2-hydroxybenzylidene)-4,5-dihydrofuran-2(3//)-one (213), which on irradiation in methanol undergoes a trans-cis photoisomerization followed by a photochemically induced trans esterification to give the coumarin (214).174... 

Addition to six-membered oxygen heterocycles is also common. The photocycloaddition of 5,7-dimethoxycoumarin to tetramethylethylene has been described,269 and 4-hydroxycoumarin (326) undergoes facile addition to cyclohexene on direct irradiation to give the cyclobutane (327)270 analogous additions to a variety of other alkenes have been reported, and the cycloaddition of 4-methoxycoumarin to 2-methylpropene has been employed in a synthesis of l,2-dihydrocyclobuta[c]coumarin.271 Photoaddition of the 1,2-bisenol lactone (328) to tran.s-stilbene yields propellane (329),272 and [ 2 + 2] cycloaddition is observed along with other competing photoreactions on irradiation of chromone in the presence of alkenes.273... 

A mixture of a hydroxybenzaldehyde 1 (100 mmol), carbonyl compound 2 (110 mmol) and piperidine (0.20 g, 2.4 mmol) was irradiated with microwaves for the time indicated in Table 1. At the end of exposure to microwaves, the reaction mixture was cooled to room temperature, and the crude product recrystallized from an appropriate solvent to obtain the coumarins 3. 

Keywords aromatic hydroxyaldehyde, ethyl malonate, methyl malonate, Knoe-venagel reaction, microwave irradiation, 3-substituted coumarin... 

Powdered single crystals of coumarins kept in a petri dish were irradiated with a Hanovia 450-W medium-pressure mercury arc lamp from a distance of about 2 ft. Samples were turned around periodically to provide uniform exposure. Progress of the irradiation was monitored by variation in melting point and NMR and IR spectra. After complete conversion, the time of which was dependent on the nature of the coumarin, the dimer was separated from the monomer by TLC. Dimers were identified by their spectral properties. 

Photocycloaddition of C-P4-C. The photochemistry of coumarin and its derivatives has been the subject of numerous investigations, mainly as a consequence of its importance in biological systems [119-127], Irradiation of 7,7 -polymethylene-di(oxycoumarin)s at low concentrations yields only syn head-to-head and head-to-tail intramolecular cyclomers. Steric factors introduced in the corresponding molecule by methyl groups at the 4-positions of the coumarins led predominantly the syn head-to-tail cyclomer (Fig. 12). At higher concentrations, the amounts of intermolecular photoproducts increase as expected. 

Enantioselective photodimerization of coumarin (97) was accomplished in the inclusion complex (100) with 12a. For example, irradiation of the 1 1 inclusion compound of 97 with 12a gave the anti-head-to-head dimer (—)-98 of 96% ee in 96% yield [52], This photochemical reaction of 100 to the complex (101) of 98 was also found to proceed by a single-crystal to single-crystal manner. However, 1 2 inclusion compound of 11 and 97 gave the. vw-head-to-head dimer (99) upon irradiation in 75% yield [53],... 

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