Cross photoreactions

In parallel to this example, Lewis and coworkers have also observed inverse reactivity for electron transfer and cycloaddition efficiency in the interaction of trans-stilbene with unsaturated esters (52). This result is understandable if the exciplex, rather than the radical ion pair, arranges the olefins in the correct geometry for cycloaddition. Analogous results have also been reported in the (2+2) cross photoreaction of cyano-substituted stilbenes with dienes, where all possible regioisomers were formed, eq. 16 (53a) ... 

Osmundsen (1970 b) showed a varying cross-photoreaction pattern between halogenated salicylanilides and related compounds (salicylanilide, hexachloro-phene, trichlorocarbanilide, fenticlor, Jadit, 2,4-dichlorbenzylalcohol, and zinc omadine). 

Later, Wheeler and coworkers demonstrated that chiral sulfonamidecinnamic acids self-assemble into hydrogen-bonded dimers wherein the olefins lie parallel, with the C=C bonds separated by 3.68 The approach was expanded to cross-photoreactions that generate heterodimers. On UV irradiation, the quasiracemate cocrystals underwent asymmetric photodimerizations to afford an enantiomerically pure a-truxillic acid photoproduct (Figure 21). 

The study of quantum yields. The quantum yield is the fraction of absorbed light that goes to produce a particular result. There are several types. A primary quantum yield for a particular process is the fraction of molecules absorbing light that undergo that particular process. Thus, if 10% of all the molecules that are excited to the state cross over to the T state, the primary quantum yield for that process is 0.10. However, primary quantum yields are often difficult to measure. A product quantum yield (usually designated ) for a product P that is formed from a photoreaction of an initially excited molecule A can be expressed as... 

While a complete study of the photoreactions of 4-bromostilbene (6) has not been published yet, preliminary azulene quenching results(1) suggest that upon direct photoreaction about 3570 of the tram singlets intersystem cross to the triplet state ... 

Figure 28.10 Trifunctional label transfer agents contain two arms with terminal reactive groups and a third arm with a label or affinity tag, such as a biotin group. One of the reactive groups typically is thermoreactive to couple with bait proteins, while the second reactive group usually is photoreactive. The thermoreactive arm has a cleavable cross-bridge to facilitate release of the captured protein and transfer of the label of affinity tag to it.

Harrison, J.K., Lawton, R.G., and Cnegy, M.E. (1989) Development of a novel photoreactive calmodulin derivative Cross-linking of purified adenylate cyclase from bovine brain. Biochemistry 28, 6023. 

Jaffe, C.L., Lis, H., and Sharon, N. (1980) New cleavable photoreactive heterobifunctional cross-linking reagents for studying membrane organization. Biochemistry 19, 4423. 

This article will only discuss two particular kinds of photocycloaddition reactions, the photodimerization or cross-cycloaddition of two olefins to yield a cyclobutane derivative, and the photoreaction of an olefin with a carbonyl compound to give an oxetane, Eq. 1 and Eq. 2. The inportance of substituent effects in reactions of these types is pointed... 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

Photoreactive Amino Acids and Heterobifunctional Cross-Linkers 181... 

A photoreactive nucleotide, 8-bromo-2 -deoxy-adenosine, was incorporated post-synthetically into a DNA sequence (35, Fig. 13) within the previously determined DNA contact site for the transcription factor. Upon irradiation the resulting nitrene cross-linked primarily to the 50-kDa subunit and covalently modified Lys-244. On the other hand, that amino acid residue appeared not... 

Calcineurin is a heterodimer consisting of a large calmodulin-binding catalytic subunit A (61 kDa) anda Ca2+-dependent subunit B (19 kDa).A diazo-group containing photoreactive CsA (47, Fig. 16) was prepared after binding to cyclophylin and after incubation with calcineurin. Irradiation resulted in specific cross-linking mainly to the subunit B , with some minor interaction to subunit A . 

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