Phototransformations of selected pharmaceuticals under low-energy UVA–vis and powerful UVB–UVA irradiations in aqueous solutions—the role of natural dissolved organic chromophoric material

The kinetics of simulated low-energy daylight (UVA–vis) and powerful combined ultraviolet B and A (UVB–UVA) induced direct and indirect phototransformations of four pharmaceuticals, i.e., ibuprofen, metoprolol, carbamazepine, and warfarin, which were investigated in dilute solutions of pure laboratory and natural humic waters. The results strengthen the essential function of natural chromophores in dissolved organic material (CDOM) as principal photosensitizer toward indirect phototransformations of pharmaceuticals in natural conditions under available low-energy UVA–vis and slight UVB radiations. The results confirmed that organic micropollutants are able to undergo a direct photolysis if their absorbance spectra overlap the spectral range of the available radiation but only if the radiation is strong enough, e.g., ibuprofen is able to undergo only indirect photolysis via different pathways in all realistic conditions. The action of nitrate anions as photosensitizers in the applied conditions proved to be of little importance. High-performance size-exclusion chromatographic experiments verified that the rate constants obtained under the low-energy UVA–vis and powerful UVB–UVA irradiations for the decreased amounts of the two largest molecular size fractions of CDOM were quite close to the rate constants detected for the increased amounts of the next five molecular size fractions with smaller molecular sizes. The decreased contents of the two largest molecular size fractions correlated quite well with the decreased contents of the studied pharmaceuticals under the low-energy UVA–vis irradiation process but somewhat less under the powerful UVB–UVA irradiation. The photochemically induced decomposition of the CDOM aggregates appears to increase the amounts of smaller molecular size fractions and simultaneously produce via CDOM-stimulated radical reactions indirect structural transformations of pharmaceuticals. Apparent quantum yields were estimated for the transformation–degradation of the two largest molecular-size CDOM aggregates under low-energy UVA–vis and powerful UVB–UVA irradiations. Figure Structural difference between CDOM and pharmaceuticals studies     

Selective photochemical cleavage of an α-ketoamide in a highly functionalised macrolide ascomycin

A novel photochemical amide cleavage reaction of (6S)-methoxyascomycin opening a pathway for the selective cleavage of the pipecolic acid, is described. The scope of this reaction with several analogues carrying suitable protecting groups is examined.     

Synthesis of 6-vinyl and 5-vinylproline analogues of ascomycin

6-Vinyl (12) and (5R)- and (5S)-vinylproline (18, 19) analogues of ascomycin are synthesised starting from the known suitably protected (6S)-methoxy-9-hydroxy derivative (4) of ascomycin. The strategy involves hydrolytic cleavage of the C_ε-N bond of the pipecolic acid moiety, extension of the amino acid side chain by two or one carbon units, functional group manipulations, Pd-catalysed reinstallation of the C_ε-N or C_δ-N bonds, followed by deprotection and oxidation.     

Online phototransformation–flow injection chemiluminescence determination of triclosan

A highly selective and sensitive chemiluminescence method for the determination of triclosan is proposed. The method is based on the phototransformation of triclosan to a light-emitting precursor in the presence of fluorescein in alkaline medium and the chemiluminescence reaction is then triggered by strong base or oxidants such as N-bromosuccinimide. Based on this reaction an online phototransformation–flow injection manifold was developed, in which the photoreactor comprises a 150-cm-long × 0.8-mm-i.d. piece of PTFE tubing coiled around a 25-W fluorescent lamp, and the phototransformed products were then injected into a carrier stream of borate buffer. After mixing with the oxidant stream the produced light was detected by a photomultiplier. A wide calibration range from 8.0 × 10⁻⁸ to 1.0 × 10⁻⁴ mol L⁻¹ was obtained under the optimized conditions, and the detection limit was as low as 5.0 × 10⁻⁸ mol L⁻¹. The whole process of analysis, including the online phototransformation and subsequent chemiluminescence detection, could be completed in 6 min. Most of the foreign substances tested showed high tolerance levels, and the proposed method was directly applied to the determination of triclosan in toothpaste samples without any pre-separation procedure. Figure Schematic representation of the phototransformation of triclosan and subsequent chemiluminescence reaction     

Phototransformation studies of halo-substituted cinnamates by Raman spectrometry and quantum-chemical computations

The comprehension of the cinnamic derivative phototransformation mechanisms is particularly important when these molecules are used as addressable photosensitive layers. In this work we show that the halo-substituted cinnamate sensitivity to the phototransformation is a function of the excitation wavelength and the substituent nature and position. With this intention, we underline the existence of various isomers and rotatomers by Raman spectroscopy and we assign the observed vibrational modes with the help of quantum-chemical calculations. These various aspects of our work clarify the relative roles of the steric, inductive and mesomeric effects according to the considered substitution.     

Phototransformation products of the alkaloid lappaconitine: Multinuclear NMR study

The NMR technique has been applied to characterize the phototransformation products of the natural alkaloid lappaconitine. It was demonstrated that the photolysis of lappaconitine 1 results in cleavage of the ester bond with elimination of N-acetylanthranilic acid. The final reaction product was found to be an immonium salt 4 of N-acetylanthranilic acid and enamine 3. An equilibrium between the imine cation and the enamine 3 was detected.