(1RS,4RS,5RS)-Methyl 2-(3,5-di­nitro­benzo­yl)-2-oxa-3-aza­bi­cyclo­[3.3.0]oct-7-ene-4-carboxyl­ate

Abstract

In organic synthesis, the usual techniques as NMR, mass or infra-red spectrometry and elemental analysis are often not enough for the unequivocally determination of a structure of a compound. When it is possible to crystallize desired compound, the X-ray crystallography is the ultimate analysis. 3,5-dinitrobenzoylation of 2-oxa-3-azabicyclo[3.3.0]oct-7- ene-4- carboxylate leaded to title compound (I) that was unambigously analysed by X-ray analysis. The two independent molecules of the title compound (I) are coupled by π···π interactions of the 3,5-dinitrobenzoyl rings (Fig. 1) [Cg1-Cg2iv = 4.2295 Å, symmetry code: (iv) 1 + x, y, z]. It is also possible to verify the existence of the three stereogenic centres of the same chirality in both molecules of the asymmetric unit. As the space group is centrosymmetric, a racemate is present in a crystal. No other stereoisomers of methyl 2-oxa-3-azabicyclo[3.3.0]oct-7- ene-4-carboxylate are obtained from the reported synthetic methodology (Sousa et al. 2008). In the crystal structure, each pair of the molecules are linked by π··· π contacts between the 3,5-dinitrobenzoyl rings along [100] direction (Fig. 2) (Cg1-Cg2iii = 4.4862 Å, Cg2-Cg1iii = 4.4862 Å; symmetry code: (iii) x, y, z). Intermolecular interactions between carbonyl and nitro groups (distance C···O ≈ 3.0 Å), between nitro groups (distance N···O ≈ 3.0 Å) and C—H···O intermolecular hydrogen bonds (Table 1) generate an assembly by packing these chains along [010] direction (Fig. 3). Table 2 lists the interactions between aromatic rings (resulting in a π···π stacking assembly). The carbonyl and nitro groups are very electronegative; as a result, the electronic density of the 3,5-dinitrobenzoyl rings is delocalized from the centre of π-system towards the electronegative O atoms. This delocalization origins from electrostatic intermolecular interactions between the oxygen atoms and the centre of the π-system (Table 3). This analysis suggest that the most important intermolecular interactions in compound (I) are due to the 3,5-dinitrobenzoyl ring (including the nitro and carbonyl groups), which seems to be the main reason why compound (I) is a solid