The 1,4-cycloaddition reactions of the singlet (1Δg) oxygen with s-cis-1,3-butadiene and benzene, with the formation of 3,6-dihydro[1,2]dioxin and 2,3-dioxabicyclo[2.2.2]octa-5,7-diene, respectively, were studied by means of the CAS MCSCF/CAS MCQDPT2 ab initio method with the 6-31G* basis set. In the case of butadiene the reaction was found to be exoenergetic and the product was found to have C 2 symmetry, with the peroxide moiety in the gauche configuration. In the case of benzene the reaction was found to be endoenergetic and the bicyclic product formed was found to have C 2 v symmetry, with the peroxide moiety in the syn configuration. Three possible reaction routes were studied:  (i) concerted cycloaddition, (ii) stepwise cheletropic cycloaddition with the formation of zwitterionic 2,5-dihydrofuran l-oxide as an intermediate, and (iii) stepwise cycloaddition with the formation of a linear intermediate. In the case of butadiene routes (i) and (ii) were excluded, because only second-order saddle points were found on the corresponding reaction pathways. The linear intermediate (I1) found in route (iii) has a biradical character, and its energy relative to that of the separate reactants is 4.1 kcal/mol. The dominant activation barrier corresponds to the transition structure T1 leading to I1 and amounts to 9.9 kcal/mol. The rearrangement of I1 to the product (P) involves only a minor activation barrier of 7.5 kcal/mol (relative to I1). In the case of benzene the reaction occurs in a concerted manner with a single transition structure having C 2 v symmetry; the activation barrier is 25.3 kcal/mol. This difference in binding mechanism can be explained in terms of the configuration of the peroxide moiety in the adduct.

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The geometrical and electrical properties of four tautomers of 2-OH-adenine (2-OH-A) and 8-oxoadenine (8-oxo-A) and three tautomers of fapy-adenine (fapy-A) have been estimated on the basis of a 6-31G ab initio RHF method followed by single-point 6-3 1G** and MP2 calculations. The resulting relative stability of different tautomeric forms suggests that the most favorable in vacuum are the enol-amino forms of 2-OH-A, the keto-amino tautomer of 8-oxo-A, and the diamino form of fapy-A. After taking into account for the polarization functions and electron correlation contribution the energetical succession of the studied tautomers was not changed. The smaller contribution of the electron correlation was observed for the most stable tautomers and the highest for enol-imino isomers. The dipole moment strongly depends on the tautomeric form and has the lowest value for all enol-amino forms of adenine derivatives. The nonplanarity of all studied tautomers was noticed. The amino groups are nonsymmetrical pyramids with out of plane hydrogen atoms.For 2-OH-A and 8-oxo-A atom H62 is tumed above and atom &I below the ring plane. The opposite orientation is observed in the case of fapy-A. However, the 2-OH-A tautomers are characterized by a much smaller deviation from the strict planar structure then 8-oxo-A. The use of the prefix oxo for Cs-substituted adenine and hydroxyl for the derivative modified at the C2 position is justified by the most stable tautomeric forms.