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

The title compound, C15H13N3O8, comprises two crystallographically independent molecules in the asymmetric unit. In the crystal, intermolecular C—H⋯O hydrogen bonds link the molecules and short C=O⋯π contacts are seen.


Comment
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-4carboxylate 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-Cg2 iv = 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).
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.

Refinement
All H atoms were found in a difference Fourier map and placed in geometrically idealized and constrained to ride on their parent atoms [C-H = 0.93-0.98 Å and U iso (H) = 1.2 (1.5 for methyl groups) × U eq (C)].