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The critical sections of the potential-energy surface of the excited first triplet state (T1) of the electrocyclic cyclization of compounds that contain a 2H-pyran ring were studied by means of the semiempirical MINDO/3 method. A theoretical explanation is given for the experimentally observed difference in the mechanisms of the photocolorization and photodecolorization of spiropyrans. The activation barriers of the reaction in the first triplet state range from 20 to 110 kJ/mole.

The lack of electrocyclic ring opening in 1,2-dihydroquinolines not containing a substituent at the nitrogen atom is related to the possibility of efficient homolytic dissociation in the excited state.

6(5)H-Benzimidazo[2,1-b]quinazolin-12-one was obtained in high yield by condensation of benzimidazole-2-sulfonic acid with anthranilic acid. Methods for the introduction of substituents selectively into the 5 and 6 positions of this heterocycle were developed.

The mechanisms of the electrocyclic reactions of oxetene (I), 2H-pyran (II), 2-amino-2H-pyran (III), 2H-thiopyran (IV), and chromene (V) in the ground, first singlet, and tripletexcited stateswere studiedby means of the MINDO/3 method. The enthalpies of excitation are 125 (I, So), 32 (I, S1), 113 (I, T1), 188 (II, So), 109 (III, So), 150 (IV, So), and 239 kJ/mole (V, So). The number of isomers of the open form in So is determined by the nature of the substituent attached to the final carbon atom. It was observed that π donors stabilize both the transoid isomer and the cisoid isomer, which is twisted 20 ° with respect to the formally single β-γ carbon-carbon bond. Bulky substituents cause ∼105 ° rotation about the same bond.

Calculations of a large number of compounds that contain a 2H-pyran fragment were made by means of the MINDO/3 method. A correlation between the magnitude of the barrier to opening of the pyran ring and the length of the Cspiro-O bond was noted. Structural factors that determine the length of the Cspiro-O bond were discovered. Spiropyran structures that have extremal lengths of the Cspiro-O bonds were predicted on the basis of the correlations obtained.

Die aus Hydrazin und entsprechenden Chlorameisensaureestern in Wasser bei 50-60°C dargestellten Hydrazindicarbonsaureester (I) (Ausbeute 79-100%) lassen sich mit konz. Salpetersaure bei 25°C zu den Azodicarbonsaureestern (II) (78-92%) oxidieren.