Photo- and thermochromic spirans. 13. Calculation of the pathways of the thermal electrocyclic reactions of chromenes and their structural analogs by the MINDO/3 method

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, S_o), 32 (I, S₁), 113 (I, T₁), 188 (II, S_o), 109 (III, S_o), 150 (IV, S_o), and 239 kJ/mole (V, S_o). The number of isomers of the open form in S_o is determined by the nature of the substituent attached to the final carbon atom. It was observed that donors stabilize both the transoid isomer and the cisoid isomer, which is twisted 20 ° with respect to the formally single - carbon-carbon bond. Bulky substituents cause 105 ° rotation about the same bond.See [1] for Communication 12.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1028–1036, August, 1982.