Cytotoxicity and genotoxicity induced by the photochemical alkoxyl radical source N-tert-butoxypyridine-2-thione in L5178Y mouse lymphoma cells under UVA irradiation

Direct Experimental Evidence for Alkoxyl Radicals Reacting as Hydrogen Atom Donors toward Pyridines

Nanosecond transient absorption spectroscopy was used to generate ethoxyl radicals and demonstrate that they react with 2,6-lutidine and 4-phenylpyridine to give the corresponding N-hydropyridinyl radicals-products of a novel hydrogen atom transfer from the alkoxyl radical to the nitrogen atom of the substituted pyridines. Nanosecond kinetics show that both reactions are rapid (k ∼ 10⁷ M^-1 s^-1) in acetonitrile at room temperature. Rate constants measured for reaction of the ethoxyl vs. d₅-ethoxyl radical with 2,6-lutidine and 4-phenylpyridine show that both reactions exhibit primary H/D kinetic isotope effects for the hydrogen (deuterium) atom transfer reactions.

Hydrogen atom abstraction reactions from tertiary amines by benzyloxyl and cumyloxyl radicals: influence of structure on the rate-determining formation of a hydrogen-bonded prereaction complex

A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k(H)) were measured for the two radicals (k(H)(BnO(•))/k(H)(CumO(•)) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k(H)(BnO(•))/k(H)(CumO(•)) ratios of 13 to 2027 times were observed. k(H) approaches the diffusion limit in the reactions between BnO(•) and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD(2)O(•), BnO(•)-d(2)) and the 3,5-di-tert-butylbenzyloxyl radical.

Synthesis and anti-tumor activities of some new pyridines and pyrazolo[1,5-a]pyrimidines

Cyclocondensation of cyanoacetamide, cyanothioacetamide and 3-aminopyrazols with sodium salt of 3-hydroxy-1-(2-naphthyl)prop-2-en-1-one gives pyridin-2-one, pyridin-2(1H)-thione, and pyrazolo[1,5-a]pyrimidine derivatives. These derivatives showed potent anti-tumor cytotoxic activity in vitro using different human cancer cell lines.

Rational Design of Substituted N-Alkoxypyridine-2(1H)thiones with Increased Stability against Daylight

N-alkoxypyridine-2(1H)thiones serve as valuable photochemical alkoxyl radical precursors in photobiological studies, but due to a broad absorption band at about 360 nm (pi --> pi* excitation), these molecules decompose readily when exposed to daylight. The goal of the present work is to propose N-alkoxypyridine-2(1H)thiones which due to a blue shift of this band become more stable with respect to daylight and consequently are easier to handle. The shift of the pi --> pi* excitation toward shorter wave length shall be achieved by substituents introduced at the pyridine heterocycle. To study the substituent effects, excitations to the first to singlet states were calculated applying the CASPT2 approach and time dependent density functional theory (TD-DFT). The study indeed showed that electron rich substituents (like the methoxylgroup) at the positions 3, 4, and 6 of the pyridinethione heterocycle yield the desired hypsochromic shift. A free rotation of the substituent, however, is expected to quench these effects. Fluorine atoms, employed to model the influence of electron withdrawing substituents, induce also a blue shift for a substitution at the 3, 4, and 6 positions. For the multiply fluorinated molecule N-methoxy-3,4,6-trifluorinepyridine-2(1H)thione a blue shift of even 24 nm is predicted. Substituents that can conjugate with the pi electrons of the heterocycle (NO2 served as a model) only induce strong bathochromic shifts on the pi --> pi* excitation energy and therefore are not able to eliminate the daylight sensitivity of the precursor molecules.

Computer-aided design of promising photochemical alkoxy radical precursors

A computer-aided design of alkoxyl radical precursors is performed. The new precursors should combine the advantages of N-alkoxypyridine-2(1H)thiones (less reactive radicals) and N-alkoxythiazole-2(3H)thiones (stable with respect to daylight). Additionally, the radical liberation process should be initiated by light with a wavelength of around 350 nm. To find promising compounds, 18 test candidates were obtained by a systematic variation of the parent compound N-alkoxythiazole-2(3H)thione. The properties of the test molecules were computed by a protocol that was already successfully used to rationalize the photochemical behavior of N-alkoxypyridine-2(1H)thiones and N-alkoxythiazole-2(3H)thiones. The computations identify two promising new compounds. For N-methoxy-(1,3)dihydro-[1,3]azaphosphole-2-thione (6a), they predict that the fragmentation process will be initiated by an absorption at 348 nm. An analysis of its fragmentation process indicates that the free excess energy of the resulting radicals should more resemble the situation found for N-alkoxypyridine-2(1H)thiones. For N-methoxy-(1,3)dihydro-pyrrole-2-thione (3a), the excitation energy is somewhat higher (330 nm), but the computed fragmentation paths again indicate that the remaining excess energy of the released radicals is quite favorable. The test molecules also contained the experimentally well-known N-methoxypyridine-2(1H)one (1b). For this molecule, our computed data rationalizes nicely the experimental findings.