Analysis of the products formed in the reaction of NO(3)˙ with the N- and C-protected aromatic amino acids 1-5, which was performed under conditions that simulate exposure of biosurfaces to environmental pollutants, revealed insight how this important atmospheric free-radical oxidant can cause irreversible damage. In general, NO(3)˙ induced electron transfer at the aromatic ring is the exclusive initial pathway in a multi-step sequence, which ultimately leads to nitroaromatic compounds. In the reaction of NO(3)˙ with tryptophan 5 tricyclic products 12 and 13 are formed through an intramolecular, oxidative cyclization involving the amide moiety. In addition to this, strong indication for formation of N-nitrosamides was obtained, which likely result from reaction with N(2)O(4) through an independent non-radical pathway.

A dynamic combinatorial library (DCL) consisting of dialdehydes and diamines has been used to facilitate the transport of calcium ions across a supported liquid membrane (SLM). In a dual selection process, the calcium ions first select matching macrocycles 3 or 5 from the DCL. Then only one of the macrocycle–calcium complexes (5·Ca2+) efficiently transports the calcium ions due to its better balance between lipo‐ and hydrophilicity. The special setup of a DCL combined with an SLM directly finds suitable carriers for ion transport starting from diamine and dialdehyde building blocks 1, 2 and 4. The synthesis of the new, more lipophilic 4‐pentoxypyridine‐2,6‐dicarbaldehyde (4) is also described, and the first transport experiments with liposomes are discussed.

Analysis of the products formed in the reaction of nitrate radicals, NO3 •, with the N- and O-methylated and acetylated thymidines 1a and 1b revealed, for the first time, insight regarding how this important atmospheric free-radical oxidant can cause irreversible damage to DNA building blocks. Mechanistic studies indicated that the initial reaction step likely proceeds via NO3 • induced electron transfer at the pyrimidine ring, followed by deprotonation of the methyl group at C5. The oxidation ultimately leads to formation of nitrates 2, aldehydes 4 and, in the case of high [NO3 •], also to carboxylic acids 5. In addition to this, through a very minor pathway, loss of the methyl group at C5 also occurred to give the respective 2′-deoxyuridines 6. The nitrates 2 are highly labile compounds that undergo rapid hydrolysis during work-up and purification of the reaction mixtures, which could lead to serious misinterpretation of the experimental findings and reaction mechanism. Products resulting from NO3 • addition to the C5=C6 double bond in the pyrimidine ring were not observed. Also, no reaction of NO3 • with the 2′-deoxyribose moiety was detected.

A new method to obtain di‐ and trinitrobenzyl substituted pyridines is presented in this paper. By systematic variation of reaction parameters, the reaction conditions were optimized. The novel synthesis circumvents the commonly used nitration of benzyl pyridines, and thus avoids the nitration of the heterocycle which is a common side reaction. Furthermore, the starting materials for the synthesis of a variety of photochromic nitrobenzyl pyridines are easily accessible. The half‐lives of the phototautomers of several new di‐ and trinitrobenzyl‐substituted pyridines were determined. Copyright © 2010 John Wiley & Sons, Ltd.

The well-known photochromic tautomerism of 2-(2,4-dinitrobenzyl)pyridine (1; CH; Scheme 1) was re-investigated by flash photolysis in aqueous solution in view of its potential application as a lightactivated proton pump. Irradiation of 1 yields the enamine tautomer NH (l max ¼ 520 nm) that rapidly equilibrates with its conjugate base CNO À (l max ¼ 420 nm). The pH -rate profile for the first-order decay of NH and CNO À provides a direct determination of the acidity constant of NH, pK NH a;c ¼ 5.94 AE 0.12 (I ¼ 0.1m) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH þ ), pK CHNH a;c ¼ 4.18 AE 0.02, was determined by spectrophotometric titration.1. Introduction. -In 1925, Tschitschibabin et al. [1] reported that exposure of the pale yellow crystals of 2-(2,4-dinitrobenzyl)pyridine (1; labeled CH in Scheme 1) to sunlight developed a blue color that faded away within a day in the dark. They tentatively identified the blue product as the enamine tautomer NH. Irradiation of 2-nitrobenzyl compounds such as 1 generally yields the corresponding aci-nitro compounds by H-atom transfer from the ortho-alkyl group to the NO 2 group [2]. Thus, the tautomer OH is the expected primary photoproduct of 1 (CH) [3]. But numerous studies have confirmed the original assignment identifying the long-lived blue photoisomer as the enamine tautomer NH. Its absorption spectrum (l max % 520 nm in aqueous solution, 530 -580 nm in other media) is similar to that of the corresponding N-methylated enamine NMe [1] [4] [5]. IR-and NMR-spectral data [6], and timeresolved resonance Raman [7] and polarized optical absorption spectra of oriented molecules [8] all confirm that the blue isomer is NH, and X-ray analysis recently established this structure [9]. Studies of 1 by flash photolysis [10 -14] with sufficient time resolution [12 -14] have identified OH as the primary photoproduct (l max % 410 nm) that is subsequently converted to NH in a thermal reaction. Delayed formation of the enamine NH in MeCN was proposed to occur by proton transfer through the solvent [7b], but calculations and experiments indicate that intramolecular proton transfer OH ! NH is also feasible (see Sect. 2.1). In fact, both pathways may be operating; Kleinschmidt and Graness [5] report a quantum yield of 0.019 for formation of NH via OH, and of 0.024 for a direct formation of NH from 1. According to calculations (see Sect. 2.1), the thermochemical, direct (suprafacial, Woodward -