Coumarin photoreaction

Baranowska-Kortylewicz, J., and Kassis, A.I. (1993b) Labeling of immunoglobulins with bifunctional, sulfhydryl-selective, and photoreactive coumarins. Bioconjugate Chem. 4, 300-304. 

Other coumarin-related compounds include furans, found in moldy sweet potatoes, and the furan coumarin complexes (furanocoumarins) found in parsnip leaves (psoralens). They are photoreactive compounds (primary photosensitizers) that exacerbate sunburn to psoralen exposed skin. 

In the above cases, the photoreactivity of indole, coumarin, pyridine, and so forth, does not compete effectively with PFR, which becomes the major reaction. However, in the case of the chromene depicted in Scheme 28, the electrocyclic ring opening prevails over PFR. By contrast, the analogous chromane undergoes clearly acyl migration [86]. 

As in five-membered heterocycles, the introduction of a carbonyl group produces a dramatic effect on the photoreactivity. Pyran-2-ones undergo other electrocyclic processes in addition to those discussed in preceding sections.213 Photorearrangement, however, does not occur in the coumarin nucleus, although a photo-Fries rearrangement of 3-benzoyloxy-6,7-... 

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... 

Since the cross-linker is cleavable, SAED provides a means of fluorescent transfer of the coumarin tag to a second molecule, which interacts with the initially modified protein (Fig. 181). For example, soybean trypsin inhibitor (STI) was labeled with SAED and then allowed to interact with trypsin. After photoreactive cross-linking of the two interacting molecules, the complex was reduced with DTT, breaking the conjugate and transferring the fluorescent tag to trypsin near the STI binding site (Thevenin et al., 1991). 

Since photochemical reactions in inclusion compounds have been described in one chapter of the previous book [1], enantioselective (3-lactam formation reactions are summarized in this present chapter as a typical application of the inclusion technique for enantioselective photosynthesis. In addition, as a representative enantioselective single-crystal-to-single-crystal photoreaction, the photodimerization reactions of coumarin and thiocoumarin in their inclusion compound with a chiral host are also described. Furthermore, a host-catalyzed photodimerization reaction of chalcone and 2-pyridone in the solid state is also added to this chapter as a unique example of the application of inclusion techniques to selective photoreaction. 

CDx and in particular 7-CDx are known to accommodate two aromatic moieties under certain circumstances. Hence if two appropriate prochiral guest molecules are included in the same CDx cavity, regio- and enantioselective bimolecular photoreactions are expected to occur [108]. Indeed, it is known that the presence of CDx not only accelerates the rate but also modifies the product distribution of photocyclodimerizations of anthracene derivatives [109-111], coumarin derivatives [113-115], stilbene derivatives [116,117], stilbazole [118], and tranilast [119]. For instance, Tamaki and coworkers reported significantly enhanced quantum yields of photodimerization of anthracenesulfonates and anthracenecarboxyl-ates in the presence of 3- and 7-CDx [109-111]. These anthracene derivatives form 2 2 and 2 1 guest-host complexes with (3- and 7-CDx, respectively, and... 

Figure 2.19 Schematic representation of the anisotropic crosslinking of a photoreactive coumarin polymer by the action of polarised UV light to produce an anisotropic network as a non-contact alignment layer. E indicates the polarisation direction of the incident beam. ...

If the C=C bonds are more than 4.6 A apart, the reaction does not take place. As molecules need to have double bonds aligned and about 4 A apart for such photoreactions, it is sometimes possible to force the required molecule to crystallize with a unit cell dimension of this length. For example, mercuric chloride crystallizes with one unit cell dimension of 4.33 A, and its complex with coumarin maintains this unit-cell length. The required 4 A packing distance can also be obtained by use of chlo-... 

Recently De Schryver and co-workers supposed the intermediacy of exclmers in photochemical reactions of blchromophorlc systems in which the two chromophores were linked by a chain with much more units. In the case of the blsmalelmldes and the bls-coumarins, because of the very efficient collapse to photoproduct, no emission from an excited state complex is observed. In a series of bisanthracenes they could show as well excimer emission (Fig. 2) as photoreaction (vide infra). 

Photoreactions of coumarin (99) in EtOH both in the absence and presence of benzophenone as a sensitizer give, respectively, a mixture of, vyn-head-to-head dimer 100 and 5vn-head-to-tail dimer 101 [59], and an//-head-to-head dimer 102 together with a small amount of nnh-head-to-tail dimer 103 [60]. On the other hand, photoirradiation of a 1 2 inclusion complex of 10 with 99 and of a 1 2 inclusion complex of (5,5)-(-)-l,6-(2,4-dimethylphenyl)-l,6-diphenylhexa-2,4-diyne-1,6-diol (104) with 99 in the solid state gave 100 (74.5%) and rac-103 (94%), respectively, in the yields indicated [61]. 

They reported even the effect of added impurities coumarin, which is photoinert in the solid state, photodimerized when admixed (by cooling the melt) with various foreign substances such as antipyrine (Scheme 60) [97]. Compounds which are photoreactive in solution may frequently be photoinert in the solid state owing to the rigid crystalline environment. Addition of external substances disrupts the crystal lattice and the compounds recover their photoreactivity. This is very commonly observed. 

In the (keto)coumarin/amine/ferrocenium salt system, the ferrocenium salt plays a crucial role that is rather complex. In a three-component photoinitiator system [238,239] consisting of a coumarin, an iron arene complex such as CpFe +Ar and a phenylglycine derivative as an amine, the first step of the photoreaction occurs between the dye and the complex according to an electron process. The amine reacts with the radical (created on the complex) through hydrogen abstraction. Therefore, no detrimental ketyl radicals are formed. 

Other similar photoreactive groups such as chalcones (48), coumarins (49), and dibenzazepines (SO) have been proposed for similar applications. 

Scheme 8. Enantioselective Photoreactions in TADDOL Inclusion Compounds with a Cou-marin, a Methacryl Anilide, and an Oxocyclohexenyl-carboxamide. In the first case, the packing of the coumarin molecules in the mixed crystal is such that the double bonds are predisposed for the (2+2) cycloaddition. In the second example, a photochemical electrocychc reaction is followed by a sigmatropic H shift. The third reaction is an intramolecular (2+2) cycloaddition with dia- and enantioselective formation of three new stereogenic centers. There are several more reactions of this type, described in the literature [54], and the Toda group has determined the crystal structures of a number of inclusion compounds to show the correlation between the crystal packing and the configuration of the photoproducts. EMastereoselective solid-phase reactions of chiral guests in TADDOL-host lattices have also been described by the...

Additional complications may arise in some cases when nn excitations are involved in photoreactions, and the main mechanism is not so clear. For definiteness, take two rather popular molecular systems, thymine and coumarin. In the former. So Si transition is of nn nature, whereas in the latter is of jtjt nature. With this. So -> Ti is the nn transition for both molecules. The photodimerization is possible for both the systems [54, 85]. There are various viewpoints on the mechanism of the photoreactions (compare [85] and [86]), but, in our opinion. 

Y. TANAKA, S. SASAKI, and A. KOBAYASHI / Solid-State Photoreaction on an Inclusion Compound of Coumarin with S-Cyclodextrin... 

SOLID-STATE PHOTOREACTION ON AN INCLUSION COMPOUND OF COUMARIN WITH e-CYCLODEXTRIN... 

ABSTRACT 3-Cyclodextrin was found to form a crystalline inclusion complex with coumarin. The solid-state photoreaction of the inclusion compound was studied at 25-27 C, and compared with those of coumarin and of a mixture of coumarin with the cyclodextrin. Under irradiation at a wavelength longer than 300 nm, coumarin with or without 3-cyclodextrin and in the inclusion complex converted to a photodimer, cis-head-to-head dimer in the solid state. The conversion rate of coumarin was higher than that in the mixture, and the latter was higher than that in the complex. These indicate that 3-cyclodextrin retards the photodimerization of coumarin, but does not affect the course of the reaction in the solid state. 

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