Geometric enantiomerism in cyclic compounds: chiral dibrominated 1,3-dioxanes.

The first geometric enantiomers in the cyclic compounds series are reported. The investigated compounds are 2,2-disubstituted-5-methyl-1,3-dioxane derivatives in which the substituents at position 2 bear chiral centers with identical substituents but with opposite configurations. The structure of the unlike isomers was determined from the solid state molecular structure of the compounds obtained by single crystal X-ray diffractometry and the enantiomers of these diastereoisomers were observed by chiral column HPLC base-line separation. The investigated compounds were obtained by a diastereoselective bromination reaction of the corresponding 2,2-dialkyl and 2,2-dibenzyl-5-methyl-1,3-dioxanes.

Synthesis, structural analysis, and chiral investigations of some atropisomers with EE-tetrahalogeno-1,3-butadiene core.

The atropenantiomers of stable 1,2,3,4-tetrahalo-1,3-butadiene derivatives (where halogeno stands for bromine or iodine) were separated with use of chiral HPLC. The barriers for the enantiomerization process were determined on-line by dynamic HPLC (DHPLC) or off-line by classical kinetic measurements. In the case of the tetrachloro compound, the barrier was too low for DHPLC and its value was obtained by dynamic NMR experiments. The obtained barriers for chloro, bromo, and iodo derivatives correlate with the van der Waals radii of the halogens. The absolute configuration of the isolated enantiomers of the tetraiodo and tetrabromo compounds was assigned by comparison of the experimental and conformations averaged calculated VCD spectra. The identification of a signature band of the absolute configuration of the butadiene core, the sign and location of which are independent from the different conformations and substituents, allowing the safe assignment of the absolute configuration of the enantiomers of chiral 1,3-butadienes, is also reported.

Comparison of isoelectronic 8-HO-G and 8-NH2-G derivatives in redox processes.

8-Oxo-7,8-dihydroguanine (8-oxo-G) is the major lesion of oxidatively generated DNA damage. Despite two decades of intense study, several fundamental properties remain to be defined. Its isoelectronic 8-aminoguanine (8-NH(2)-G) has also received considerable attention from a biological point of view, although its chemistry involving redox processes remains to be discovered. We investigated the one-electron oxidation and one-electron reduction reactions of 8-oxo-G and 8-NH(2)-G derivatives. The reactions of hydrated electrons (e(aq)(-)) and azide radicals (N(3)(*)) with both derivatives were studied by pulse radiolysis techniques, and the transient absorption spectra were assigned to specific tautomers computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The protonated electron adducts of 8-NH(2)-G and 8-oxo-G showed a substantial difference in their absorption spectra, the unpaired electron being mainly delocalized in the imidazolyl ring and in the six-membered ring, respectively. On the other hand, the deprotonated forms of one-electron oxidation of 8-NH(2)-G and 8-oxo-G showed quite similar spectral characteristics. In a parallel study, the one-electron reduction of 8-azidoguanine (8-N(3)-G) afforded the same transient of one-electron oxidation of 8-NH(2)-G, which represents another example of generation of one-electron oxidized guanine derivatives under reducing conditions. Moreover, the fate of transient species was investigated by radiolytic methods coupled with product studies and allowed self- and cross-termination rate constants associated with these reactions to be estimated.