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The properties of singlet and triplet excited states are strongly medium-dependent. Hence, these species constitute valuable tools as reporters to probe compartmentalised microenvironments, including drug@protein supramolecular systems. In the present review, the attention is focused on the photophysical properties of the probe drugs (rather than those of the protein chromophores) using transport proteins (serum albumins and α1-acid glycoproteins) as hosts. Specifically, fluorescence measurements allow investigation of the structural and dynamic properties of biomolecules or their complexes. Thus, the emission quantum yields and the decay kinetics of the drug singlet excited states provide key information to determine important parameters such as the stoichiometry of the complex, the binding constant, the relative degrees of occupancy of the different compartments, etc. Application of the FRET concept allows determination of donor–acceptor interchromophoric distances. In addition, anisotropy measurements can be related to the orientation of the drug within the binding sites, where the degrees of freedom for conformational relaxation are restricted. Transient absorption spectroscopy is also a potentially powerful tool to investigate the binding of drugs to proteins, where formation of encapsulated triplet excited states is favoured over other possible processes leading to ionic species (i.e. radical ions), and their photophysical properties are markedly sensitive to the microenvironment experienced within the protein binding sites. Even under aerobic conditions, the triplet lifetimes of protein-complexed drugs are remarkably long, which provides a broad dynamic range for identification of distinct triplet populations or for chiral discrimination. Specific applications of the laser flash photolysis technique include the determination of drug distribution among the bulk solution and the protein binding sites, competition of two types of proteins to bind a drug, occurrence of drug–drug interactions within protein binding sites, enzymatic-like activity of the protein or determination of enantiomeric compositions. The use of proteins as supramolecular hosts modifies the photoreactivity of encapsulated substrates by providing protection against oxygen or other external reagents, by imposing conformational restrictions in the binding pockets, or by influencing the stereochemical outcome. In this review, a selected group of examples is presented including decarboxylation, dehalogenation, nucleophilic addition, dimerisation, oxidation, Norrish type II reaction, photo-Fries rearrangement and 6π electrocyclisation.

ConspectusDiscovered more than eight decades ago, the Diels–Alder (DA) cycloaddition (CA) remains one of the most versatile tools in synthetic organic chemistry. Hetero-DA processes are powerful methods for the synthesis of densely functionalized six-membered heterocycles, ubiquitous substructures found in natural products and bioactive compounds. These reactions frequently employ azadienes and oxadienes, but only a few groups have reported DA processes with thiadienes. The electron transfer (ET) version of the DA reaction, though less investigated, has emerged as a subject of increasing interest.In the last two decades, researchers have paid closer attention to radical ionic hetero-cycloreversions, mainly in connection with their possible involvement in the repair of pyrimidine(6–4)pyrimidone photolesions in DNA by photolyases. In biological systems, these reactions likely occur through a reductive photosensitization mechanism. In addition, photooxidation can lead to cycloreversion (CR) reactions, and re...

Financial support from the Spanish Government (CTQ2009- 13699, CTQ2009-13459-C05-03/PPQ, RIRRAF RETICS), and the Generalitat Valenciana (Prometeo Program) is gratefully acknowledged. Dedicated to Prof. Avelino Corma on the occasion of his 60th birthday.

In the presence of the chiral amide AMX the quinolones (Ia)—(Ic) and (IVa) undergo photocyclization with good to high regio- and enantioselectivity.

The photochemistry of enol esters 1 is dominated by electrocyclic ring opening ( 1b → 3 + 4 ), 1,3-acyl migration ( 1c , d → 6c , d ) and [2 + 2] dimerization ( 1d → 11). Photosensitization with triphenylpyrylium tetrafluoroborate (TPT) enhances formation of oxidation products ( 1b → 5 , 1c , d → 9c , d + 10c , d ), presumably through the intermediacy of radical cations 1b–d + .

Two diaryl acetylenedicarboxylates 1 a,b have been prepared in moderate yields by direct reaction of acetylenedicarboxylic acid with the corresponding phenols catalyzed by sulfuric or p -toluenesulfonic acids and also through acetylenedicarbonyl chloride. The accompanying by-products have been characterized. Attempted esterification using N,N′-dicyclohexylcarbodiimide as condensing agent gives an asymmetric ester amide 11a and a substituted uracil 12a . Photolysis of the aryl esters 1 a,b in benzene solution affords the benzocoumaran-3-ones 14a,b . In the case of 1b , minor amounts of a polycyclic condensed 4-chromanone 17b have also been observed. These results shows that the nature of the substituent attached at the β-position of the C  C triple bond plays an important role in the cyclization of o -hydroxyaryl ethynyl ketones.

Direct photolysis of enol acetates of 3-phenylpropiophenones1a–c gives rise to the parent propiophenones2a–c and the 1,3-acyl shift products3a–c. By contrast, 2,4,6-triphenylpyrylium tetrafluoroborate sensitized photolysis of substrates1a–c affords the α-acetyloxypropiophenones7a–c as the most general products. These results have been rationalized according to the generation of radical pairs in the direct photolysis and radical cations in the photoinduced electron transfer processes.

Single electron transfer (SET) oxidation of [2.2]paracyclophane (1) with cerium ammonium nitrate (CAN) produces the corresponding radical cation (1+.), which undergoes cleavage of the carbon-carbon bond of the ethano bridge to generate the double-benzylic radical cation 2+. Trapping of this intermediate by oxygen and nucleophiles gives rise to aldehydes 3, while a second oxidation and subsequent nucleophilic trapping affords nitrates 4. This facile cleavage of the carbon-carbon bond is explained by alignment of the latter with the π systems in the rigid structure of [2.2]paracyclophane, which enables charge delocalization across both benzene rings. When the reaction is carried out in methanol, aromatic substitution takes also place as a minor reaction pathway, which is responsible for the formation of the polyfunctionalized cyclophane 5d.

Condensation of isatoic anhydride with phenols, followed by treatment of the resulting aryl anthranylates (4) with chloroacetyl chloride gives aryl N-chloroacetylanthranylates (5). Their uv-irradiation results in photo-Fries rearrangement, to afford benzophenones (6). Direct treatment of the latter with ammonia gives rise to 1,3-quinazolines (7); by contrast, treatment of 6 with potassium iodide and subsequently with ammonia leads to 1,3-dihydro-2H-1,4-benzodiazepin-2-ones (1)

The photooxygenation of trans - and cis -stilbene (2a,b) by means of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) led to benzaldehyde (5) and diphenylacetaldeyde (6). Under the same conditions trans - and cis -1,2-diphenyloxirane (4a,b) afforded 6 as major product, together with minor amounts of 5, benzophenone (8) and, in the case of 4b, 1,2-diphenylethanone (7). These data support the intermediacy of the oxiranes in the photooxygenation of the corresponding stilbenes and reveal that rearrangement of the radical cations 4a,b+. occurs by two alternative pathways: phenyl migration and H migration. The TPT-photosensitized oxygenation of 5-methylene-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (9) produced mainly dibenzosuberone (10) and dibenzosuberane (11), together with trace amounts of products arising from dehydrogenation of the dimethylene bridge (14,15,16) and solvent-derived products (12,13,18). The formation of 10 and 11 is justified via the radical cation of the 1,1-diphenylethylene moiety, generated by electron transfer from the olefin to the excited sensitizer. The epoxide 20, aldehyde 17 and dioxetane 19 appear to be key intermediates. Formation of the minor compounds 14, 15 and 16 can be explained through the radical cation of the 1,2-diphenylethane substructure.

A new natural product with in vivo anti-juvenile hormone (JH) activity, N-(2-methyl-3-oxodecanoyl)-2-pyrroline (2), has been isolated from Penicillium brevicompactum Dierckx. Its structure has been...

1,n-Dihaloalkanes can be photochemical precursors of n-haloalkyl and -allyl radicals and cations, which are generated via one-photon processes. Time-resolved techniques have provided considerable information on the structures and reactivity of these intermediates. Low-temperature matrix isolation, two-laser two-color, laser-drop, and laser-jet photolysis techniques are powerful tools to photolyze haloalkyl radicals and to generate carbenes or biradicals via two-photon processes.

An unprecedented stereoselective photoreduction of a chiral BZP is observed in steady state as well as in time-resolved studies.

Combination of the pyrene and benzoylthiophene units constitutes an interesting approach to design bichromophoric photosensitizers with increased intersystem crossing quantum yield and enhanced selectivity. The potential of this strategy has been illustrated in the present work by using a model photoisomerization reaction.

Dyads containing phenol and olefin subunits are versatile models for the investigation of proton, electron and energy transfer processes. As they are readily accessible, a number of analogues (allylphenols, cinnamylphenols and derivatives) have been prepared with a wide range of photophysical and photochemical properties. By means of appropriate structural modification of a very simple initial structure, it is possible to reproduce, at will, different types of behaviour. In addition to providing valuable fluorescence emission data, these systems are chemically productive, giving rise to irreversible photoreactions that constitute a fingerprint for the mechanism involved. They include photocyclisation to 5- and 6-membered ring cyclic ethers, Z/E isomerisation, di-pi-methane rearrangement, formation of ortho-quinone methides, photohydration and photodehalogenation. This rich photochemistry is highly sensitive to the microenvironment experienced, as indicated by the dramatic modifications observed within cyclodextrin cavities. Intramolecular OH...pi interactions, both in their ground and excited states, play a key role in the interesting properties of 2-allylphenol derivatives. This is supported by experimental data and also by theoretical calculations.

A dramatic enhancement of the di-π-methane rearrangement is observed upon excitation of trans-1,3-diphenylpropene (1) to its higher singlet states in acetonitrile, which leads to trans-1,2-diphenylcyclopropane (3) as a major photoproduct.

Par reaction photochimique des dihydro-2,3- et -2,5 furannones-2, on obtient une decabonylation, une decarboxylation, une cyclisation, une dimerisation ou des transpositions de substituants

Photolysis of 3-phenylpropiophenones1 a–d in the presence of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) yields the corresponding α,β-unsaturated ketones2 a–c and1 d (from1 c), together with acetophenone (3), benzophenone (4), benzoic acid (5) and benzaldehyde (6), presumably by fragmentations of the radical cation1+ ·, generated via a single electron transfer process from1 to the excitedTPT.

β-Substitution shows a pronounced influence on the photochemistry of α,2-diacetoxystyrenes. As in the case of the parent compound, intramolecular cyclization with participation of the neighbouring 2-acetoxy group takes place upon irradiation of the enol esters4a–c; however other processes are also observed, depending on the substrate. The phenyl derivative4a gives theE isomer7a and the phenanthrene9. The vinyl derivative4b also undergoescis-trans isomerization and/or photooxidation, to afford7b and10. Finally, a 1,4-acyl migration occurs in the benzyl derivative4c, whereby the 1,4-diketone12 is formed.

The photolysis of the 7-acetoxybenzopyran derivatives 1a-5a has been carried out. Chromene 1a was found to undergo extensive photopolymerization. Chromanone 2a underwent a rather inefficient photo-Fries rearrangement to the 6 - and 8 - acetyl derivatives 2b and 2c . Diacetoxychromene 3a gave the unsaturated ketone 8 as the main product, besides its deacetylation analogue 9 and chromanone 11 . Diacetoxychroman 4a afforded a mixture of four C-acetyl products: 1b, 1c, 2b and 2c , together with Chromene 1a and chromanone 2a. Finally, irradiation of chroman 5a gave rise to a mixture of the two possible photo-Fries products 5b (43%) and 5c (52%). The mechanistic implications of the above results are discussed, with special emphasis on the photoreactivity of the phenyl ester as compared with that of the pyran ring, the enol ester and the benzyl ester moieties. The synthetic applications of these transformations are illustrated with the preparation of eupatoriochromene 1b and encecalin 1e .

Irradiation of the p -methoxyphenyl and p -methylphenyl esters of 2-butynoic, propynoic, 3-(ethylenedioxy)butanoic, 3,3-dimethoxypropanoic and 3-oxobutanoic acids (1–3) affords the corresponding photo-Fries products 4–6. Compound 5a is converted in part into the acetophenone 7a, by way of a Norrish type II photo-reaction, while compound 6a is reluctant to undergo this process, in spite of the fact that it also possess γ-carbonyl hydrogen atoms. From the preparative point of view, the photorearrangement of the esters 1a-d and 2a,c-d is exploitable, while that of 3a proceeds with a lower yield. The differences found in the photochemical behaviour of 2a and 3a show the sharp influence of the acetal group on the course of the reaction. Compounds 4–6 are representative model compounds valuable as direct chromone precursors; in fact, they can be readily cyclized to the chromones 10 under basic or acidic conditions.