Theoretical study on the thermal isomerization reaction between meso and dl cyclomers of 4a,6a,10a,10b-tetrahydropyrido[2,1-c]pyrido[1,2-a]piperazine and its lower homologue

Abstract The reaction mechanism of thermal isomerization from a meso to a dl cyclomer of the titled compounds has been studied theoretically using ab initio RHF, GVB, and MCSCF methods with 6-31G(d) and TZV(d,p) basis sets, followed by the second-order Moeller–Plesset perturbation (MP2) calculations. In 4a,6a,10a,10b-tetrahydropyrido[2,1-c]pyrido[1,2-a]piperazine ( 1 ), it is shown that the energy barrier from a singlet diradical intermediate to the dl cyclomer is smaller than that of the meso cyclomer by 3 kcal/mol. Concomitantly, the transition-state structure for the latter case is spatially three times far apart from the intermediate in comparison with that for the former case. This suggests the preference for a formation of the dl cyclomer over the meso cyclomer. On the other hand, it is shown in 4a,4b,8a,9a-tetrahydropyridino[1′,2′-4,3]imidazolidino[1,5-a]pyridine ( 2 ) that no thermal isomerization reaction should take place from the meso to the dl cyclomer. This is because there is essentially no energy barrier from the intermediate to the meso cyclomer but an energy barrier of about 10 kcal/mol from the intermediate to the dl cyclomer. Besides, the transition-state structure for the latter case is spatially far apart in comparison with that for the former case. The present findings are compatible with the available experimental information.

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