The carcinogens 4-aminobiphenyl, 2-aminofluorene and their N-acetyl derivatives form DNA adducts in vivo with the aryl nitrogen attached at C-8 of guanine. These adducts are proposed to arise through the reaction with the DNA base of a nitrenium ion obtained by N-0 heterolysis of a hydroxylamine ester obtained metabolically by oxidation and esterification. Routes have now been discovered for the photochemical generation of these nitrenium ions, the N-acetyl derivatives via a photoheterolysis reaction and the N-H derivatives by protonation of the singlet nitrene photochemically generated from the azide precursor. The nitrenium ions are observed as transient intermediates with laser flash photolysis, and the kinetics of their reaction with various nucleophiles directly monitored. These results, coupled with competition kinetics experiments, show that the nitrenium ions derived from the above amines and amides are relatively long-lived in water, with a remarkably high selectivity in water for reaction with 2'-deoxyguanosine and its 5'-phosphate. The C-8 adduct is the product of this reaction. These behaviours differ considerably from those of analogous carbenium ions. Reactions with vinyl ethers have also been investigated, and also show significant differences between the two types of electrophile.
E,E-2,3-Dibenzylidenesuccinic acid (2) and its anhydride (1) were synthesized, their crystal structures and 1H and 13C nuclear magnetic resonance spectra were determined, and electron charge densities of carbon atoms were calculated. These results were related to the corresponding data available for E-cinnamic acid (3) in order to evaluate the effect of structural changes in a series 3 → 2 → 1 on the molecular parameters and spectroscopic properties of the cinnamic system. In the molecule of 2 most of the steric strain is released by the rotation about the C(α)—C(α′) bond giving rise to a structure consisting of two, approximately independent, cinnamic acid moieties. In 1, severe steric strain is introduced, as demonstrated by the unusually large values of the CCC bond angles exocyclic with respect to the anhydride ring, as well as by significant deviations from the plane of the cinnamic skeleton. The geometry of 1 results in an intramolecular shielding of the aromatic hydrogen atoms due to the proximity of the two benzene rings; this shielding effect for the ortho, meta, and para hydrogen atoms correlates well with the intramolecular distances between the corresponding positions of both rings. The common linear relationship between 13C chemical shifts and the electron charge densities on the given carbon atom has been obtained for compounds 1, 2, and 3.Crystal data. Anhydride (1): space group P21/c with a = 13.536(3), b = 14.391(3), c = 7.159(1) Å; β = 98.69(1)°; Rw = 0.029 and R = 0.055. Succinic acid (2): space group [Formula: see text] with a = 8.881(3), b = 9.786(1), c = 11.355(3) Å; α = 85.56(1), β = 88.76(2), γ = 69.46(2)°; Rw = 0.048 and R = 0.081. This compound cocrystallizes with one molecule of the solvent (acetic acid). Keywords: photochromic, cinnamic, succinic.
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