Structure in the solid state and conformational analysis in solution of 1,4-diacetyl-1,4-dihydro-2,3-diphenylpyrazine, a potential antiaromatic heterocyclic compound

Abstract

It is shown by X-ray crystallography that the 1,4-dihydropyrazine skeleton of 1,4-diacetyl-1,4-dihydro-2,3-diphenylpyrazine 2 has a boat shape. The C(2)–C(3) and C(5)–C(6) double bonds are localized; therefore 2 does not exist as an azahomoaromatic entity with 6 electrons delocalized on the ring and two electrons localized on one nitrogen atom. In the solid state 2 is in a Z, Z conformation. The 1H and 13C nmr spectra indicate that 2 exists as an equilibrium mixture of Z,E, E,E, and Z,Z conformers in CD2Cl2 at −80 °C. An unambiguous assignment of the different sets of signals has been obtained by nOe experiments performed at −80 °C. The conformer distribution is the following: 65% (Z,E), 22% (E,E), and 13% (Z,Z). The relatively low barrier to rotation about the carbonyl nitrogen bonds of the two amide groups [Formula: see text] is in line with a rather large C—N bond length (~1.375 Å) in the crystal.

Preparation, structure, and conformation in solution of diacetyl derivatives of 6,7-dimethyl-1,4-dihydroquinoxaline and of dihydropyridotriazines

“…The signals of the methyl protons of the acetyl groups lie too close to be selectively irradiated in order to perform nOe experiments. However, the structural analogy between l and 2 and the similarity of their nmr data permit an unambiguous assignment of the signals due to H2 and H3, as ethylenic protons are less shielded when they are syn to the carbonyl oxygen (E) than when they are anti (2). An identical sequence is observed for the peri protons H5 and H8.…”

“…'H nuclear magnetic resonance and especially the nuclear Overhauser effect (nOe) have been shown to be the methods of choice in conformational studies of tertiary amides. This type of experiments, performed at low temperature, has been successfully used to carry out a conformational study of monacetyl derivatives of nitrogen heterocycles (1) and extended by our group (2) to the N,N1-diacetyl derivative 1. ;::: 'JS-6 = 5.5…”