Stability and removal of selected avobenzone's chlorination products.

Stability studies of two avobenzone transformation products: chloro-avobenzone [2-chloro-1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione] and dichloro-avobenzone [2,2-dichloro-1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione] have been performed at different pHs values as well as under UV A light and compared with the stability of avobenzone, one of the mostly used UV A filter present in sunscreens. We have additionally investigated the possibility of application of TiO2 photocatalysis as a method for the removal of them. Results have shown the differences in pH stability of all three studied compounds with much slower degradation rate under neutral conditions in comparison to the acidic ones for all three studied compounds. In the case of photolytic experiment, performed under UV A light, dichloro-avobenzone exhibited the lowest UVA stability (half-life 22.4 ± 0.7 min), while avobenzone and chloro-avobenzone are much more stable and have shown quite similar degradation pattern (half-lives 126 ± 16 min and 128 ± 25 min). Under the photocatalytic conditions the less stable was dichloro-avobenzone (half-life 14.1 ± 0.6 min), while chloro-avobenzone and avobenzone were much more stable (half-lives 41 ± 3 min and 79 ± 13 min). Dichloroavobenzone is significantly more reactive than avobenzone and its monochloro-derivative. On the basis of the formation of various stable degradation products, including substituted acetophenones, benzoic acids and phenols, identified by GC-MS, the degradation pathway has been proposed.

Structures in solid state and solution of dimethoxy curcuminoids: regioselective bromination and chlorination.

BACKGROUND: Several papers described the structure of curcumin and some other derivatives in solid and in solution. In the crystal structure of curcumin, the enol H atom is located symmetrically between both oxygen atoms of the enolone fragment with an O···O distance of 2.455 Å, which is characteristic for symmetrical H-bonds. In the solution, the geometry of the enolone fragment is attributed to the inherent disorder of the local environment, which solvates one of the basic sites better than the other, stabilizing one tautomer over the other. In this paper, how the position of methoxy groups in dimethoxy curcuminoids influence the conformation of molecules and how the halogen atoms change it when they are bonded at α-position in keto-enol part of molecules is described. RESULTS: Six isomers of dimethoxy curcuminoids were prepared. Conformations in solid state, which were determined by X-ray single crystallography and 1H MAS and 13C CPMAS NMR measurements, depend on the position of methoxy groups in curcuminoid molecules. In solution, a fast equilibrium between both keto-enol forms exists. A theoretical calculation finding shows that the position of methoxy groups changes the energy of HOMO and LUMO. An efficient protocol for the highly regioselective bromination and chlorination leading to α-halogenated product has been developed. All α-halogenated compounds are present mainly in cis keto-enol form. CONCLUSIONS: The structures in solid state of dimethoxy curcuminoids depend on the position of methoxy groups. The NMR data of crystalline solid samples of 3,4-diOCH3 derivative, XRD measurements and X-ray structures lead us to the conclusion that polymorphism exists in solids. The same conclusion can be done for 3,5-diOCH3 derivative. In solution, dimethoxy curcuminoids are present in the forms that can be described as the coexistence of two equivalent tautomers being in fast equilibrium. The position of methoxy groups has a small influence on the enolic hydrogen bond. Theoretical calculations show that the energy gap between HOMO and LUMO depend on the position of methoxy groups and are lower in solution. Chlorination and bromination on α-position of 1,3-diketone moiety do not change the preferential form being cis keto-enol as in parent compounds.

Photocyclization of 2-chloro-substituted 1,3-diarylpropan-1,3-diones to flavones.

The photochemical behavior of 2-halo-substituted 1,3-diarylpropan-1,3-dione strongly depends on the nature of the halogen atom bonded and the presence of electron-donor groups on the phenyl ring. In the case of 2-chloro-1,3-diphenylpropan-1,3-dione and 1-(3,5-dimethoxyphenyl)-3-phenylpropan-1,3-dione, cyclization to flavones was the sole reaction pathway, whereas in the case of 2-chloro-1,3-di(4-methoxyphenyl)propan-1,3-dione, only products derived from alpha-cleavage were observed. 2-Fluoro derivatives of 1,3-diarylpropan-1,3-diones were photostable; on the other hand, 2-chloro-2-fluoro derivates resulted in 3-fluoroflavones.

Primary photoreactions of the 3',5'-dimethoxybenzoin cage and determination of the release rate in polar media.

3',5'-dimethoxybenzoin (DMB) is an important photoremovable protecting group. The primary photoreactions of DMB acetate and fluoride following photoexcitation by a subpicosecond laser flash were investigated by pump-probe spectroscopy. The primary photoproduct is identified as a preoxetane biradical intermediate that decays by different pathways depending on solvent polarity. In polar solvents (acetonitrile, water), the biradical decays by releasing acetate or fluoride with a lifetime of about 2 ns. Thus, DMB is an excellent protecting group for the investigation of fast processes such as protein folding.

Catalytic regioselective sulfonylation of alpha-chelatable alcohols: scope and mechanistic insight.

This paper describes a convenient protocol for the regioselective sulfonylation of alpha-chelatable alcohols. Typically, the reaction of alpha-heterosubstituted alcohols with 1 equiv of p-TsCl and 1 equiv of Et(3)N in the presence of 2 mol % of Bu(2)SnO leads to rapid, regioselective, and exclusive monotosylation. The pK(a) of the amine was correlated to the reaction rate. A plausible mechanism for this reaction has been proposed on the basis of (119)Sn NMR studies.