Stereomutations of Conformational Atropisomers of Hindered 1,2-Diaryltetrahydropyrimidines

Abstract

The barriers required to interconvert the conformational enantiomers (atropisomers) of three 2-(o-halophenyl)-1-mesityl-1,4,5,6-tetrahydropyrimidines (the ortho-halogen substituents being I, Br, Cl) have been measured by low-temperature 1 H NMR spectroscopy. In addition, the barrier for the inversion of the heterocyclic six-membered ring has been determined by monitoring the 13 C NMR spectra at even lower

Conformation and stereodynamics of 1,2-diaryltetrahydropyrimidine and of its five- and seven-membered ring analogs

“…The barrier for this third process was estimated to be about 7 kcal/mol. The analogous process, observed in similar six-membered heterocyclic derivatives containing a double bond, had been assigned 7,8,19 to the restriction of the ring inversion process that have barriers of about 7 kcal/mol. DFT computations indicate that the two conformers (invertomers) arising from this motion (Scheme 2) actually correspond to theoretical minima: they were labeled anti or syn, depending as to whether the CH 2 in position 5 is anti or syn to the NO 2 group.…”

“…Tetrahydropyrimidines bearing a variety of substituents display biological and pharmacological properties 1e3 in that they can be used, for instance, as antidepressants, 4 fungicides, 5 and anthelmintics. 6 In the past we had investigated, by NMR spectroscopy, the conformational and stereodynamic properties of a number of diaryl derivatives of these compounds 7,8 that showed, at low temperature, the existence of conformational diastereoisomers due to the restricted rotation about the aryl-pyrimidinyl rings. In the present work we have investigated the behavior of analogous compounds having, in addition to the six-membered ring, also the smaller and the larger fiveand seven-membered rings, respectively.…”