There is interest in structural, conformational and vibrational properties of ciclohexen-2-aryl-pyrimidin-4-ones, mainly due to their pharmacological uses and because its knowledge is relevant to study molecular processes in organic chemistry.1,2
Cis- (1a,c,d,f) and trans- (1b,e) ciclohexen-2-aryl-pyrimidin-4-ones have been selected as a case study aimed at finding out some correspondence between structural properties (ring
fusion, tautomeric forms and conformers) and reactivity in the gas-phase
(analyzed through NMR and UV spectra, and mass spectral fragmentation).
The flash vacuum pyrolysis (fvp) of 1a–f is described in scheme 1. All compounds undergo ring fragmentation to the corresponding products (2,3) between 600 and 640 ºC.
As conclusions we can say: a) the mass spectra of cis compounds are simpler than the trans and its fragmentation is very selective giving only the RDA as confirmed in the fvp reaction. b) The stronger the conjugation shown by the UV spectra, the easier the fragmentation, irrespective of the compound being cis or trans. c) The more rigid trans compounds (like trans decalins) are more stable thermally because they have less strain ring energy.2
References: 1- Fülöp, F.; Bernáth, G.; Pihlaja, K.; Adv. Heterocycl. Chem. 1998, 69, 349-477. 2- Peláez, W. J.; Szakonyi, Z.; Fülöp, F. and Yranzo, G. I.; Tetrahedron, 2008, 64, 1049-1057.