Rosa Tormos

Photochemistry of naproxen in the presence of β-cyclodextrin

Abstract Irradiation of naproxen ( 1 ) in the presence of β -cyclodextrin (CD) leads to photodecarboxylation products with ethyl ( 2 ), 1-hydroxyethyl ( 3 ) and acetyl ( 4 ) side chains. The presence of CD does not protect against photodegradation, but rather results in a more rapid disappearance of the drug. The most important change associated with CD is the marked predominance of alcohol 3 over ketone 4 . Since 3 is more cytotoxic than 4 , the reduced photohaemolytic activity of 1 in the presence of CD must be attributed to the sequestering and stabilization of the radical intermediates by complexation, rather than to the nature of the stable photoproducts.

Characterisation of the lowest singlet and triplet excited states of S-flurbiprofen.

The photophysical properties of S-flurbiprofen [S-2-fluoro-alpha-methyl-4-biphenylacetic acid], a nonsteroidal anti-inflammatory drug, have been examined using steady-state and time-resolved spectroscopic techniques. The energy of its first singlet excited state is 99 kcal mol(-1). The fluorescence quantum yields and lifetimes (at 300 nm) have been determined in acetonitrile, methanol, hexane and PBS; they are in the range 0.15<phi(F)< 0.33 and 0.7<tau(F)<2.0 ns. The intersystem crossing quantum yields are between 0.45 and 0.71; the lambda(max) of the T-T absorption is 360 nm, and the triplets live from 15 to 106 micros. Steady state photolysis in aqueous medium leads to S-2-hydroxy-alpha-methyl-4-biphenylacetic acid via photonucleophilic aromatic substitution, in addition to the photodecarboxylation products observed in organic solvents.

Photodecar☐ylation of 2-phenylpropionic acid in solution and included within β-cyclodextrin

Abstract Photolysis of 2-phenylpropionic acid ( 1 ) in acetonitrile, methanol or benzene leads to ethylbenzene ( 2 ), 2,3-diphenylbutane ( 3 d,l and meso ), 1-(2-ethylphenyl)-1-phenylethane ( 4 ), 1-(4-ethylphenyl)-1-phenylethane ( 5 ) and acetophenone ( 6 ). In cyclohexane or carbon tetrachloride, solvent derived products are formed. These results involve homolytic cleavage of the C C bond α to the car☐y group, which affords 1-phenylethyl radical (PER) as key intermediate. The α,α coupling of PER in solution to give 3 is nonstereoselective; by contrast, formation of the meso isomer is preferred upon inclusion of 1 within β-cyclodextrin. This is attributed to the coupling of two long-lived PER-CD units

Proton, electron and energy transfer processes in excited phenol–olefin dyads

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.

2,4,6-triphenylpyrylium tetrafluoroborate-photosensitized Reactions of o-cinnamylphenols and o-hydroxystilebenes

Abstract Direct irradiation of o-cinnamylphenols 1a–c led to mixtures of the cis-isomers 3a–c, the dihydrobenzofurans 4a–c and the dihdyrobenzopyrans 5–c. Under the same conditions, o-hydroxystilbenes 2a,b gave their cis-isomers 9a,b as single photoproducts. Using 2,4,6-triphenylpyrylium tetrafluoroborate as photosensitizer the reactions of the methyl- or methoxy- substituted derivatives appear to proceed via single electron transfer. Compounds 1b,c underwent oxidative fragmentation to the aldehydes 8b,c, while 2b cyclized to benzofuran 10b.

Formation of dichloromethyl phenyl ethers as major products in the photo-Reimer-Tiemann reaction without base

Abstract Photolysis of phenols 1a-d in chloroform or carbon tetrachloride afforded the corresponding di- or trichloromethyl phenyl ethers (2a-d or 6a-d, respectively). In chloroform, hydroxybenzaldehydes 4a-d or 5 and phenyl formates 3a-d were obtained as minor photoproducts. In carbon tetrachloride, traces of salicyloyl chloride (7), phenyl salicylate (8) and phenyl p-hydroxybenzoate (9) were detected starting from the parent phenol (1a). The obtained results agree with the involvement of an electron transfer mechanism.

A new synthesis of precocene II and precocene III based on the photo-Fries rearrangement of a sesamol ester

The photo-Fries rearrangement of the sesamol ester of 3-methylcrotonic acid (2) in the presence of potassium carbonate affords a mixture of the o-hydroxyketones (3c) and (3d). Basic cyclization of the latter leads to the chromanones (3c) or (3d), respectively. Treatment of 4c with alkoxide/alcohol results in the regioselective cleavage of the methylenedioxy ring, to give the corresponding 7-alkoxy derivatives (4e) or (4f). Subsequent methylation with methyl iodide gives rise to 4a or 4b and final reduction/dehydration by conventional methods affords the target compounds Precone II (1a) or Precocene III (1b)

PHOTOCYCLIZATION OF 2-CINNAMYLPHENOLS VIA EXCITED STATE PROTON TRANSFER (ESPT) INVOLVING THE LOWEST-LYING STYRENIC SINGLET

Abstract The attachment of substituents to the styrenic ring of trans -2-cinnamylphenol produces a decrease in the singlet energy of the styrenic moiety. As a consequence, the resulting chromophore becomes responsible for the photophysical and photochemical properties of this type of bichromophoric compounds. Both fluorescence emission and photocyclization occur predominantly from the lowest-lying styrenic singlet. The result is a marked regioselectivity towards 6-membered ring products, via an excited state proton transfer.

Two-photon processes in the photo-Claisen and photo-Friesrearrangements. Direct observation of dienic ketenes generated byphotolysis of transient cyclohexa-2,4-dienones

Using the two-laser two-colour technique it has been possible to promote two photon chemistry in the cases of benzyl phenyl ether 1a and phenyl acetate 1b; this is based on the photolysis of the transient cyclohexa-2,4-dienone intermediates 2a,b of the photo-Claisen and photo-Fries rearrangement to the dienic ketenes 4a and 4b.

Stereoselectivity in the triplet decay of chiral benzophenone–naphthalene bichromophoric systems

Chiral recognition in the intramolecular induced quenching of the methoxynaphthalene triplet by benzophenone has been observed by using diastereomeric bichromophores