Methyl 1-{4-[(S)-2-(methoxycarbonyl)pyrrolidin-1-yl]-3,6-dioxocyclohexa-1,4-dien-1-yl}pyrrolidine-2-carboxylate

The complete molecule of the title diproline ester quinone, C18H22N2O6, is generated by a crystallographic twofold axis, which passes through the centre of the benzene ring. Both –CO2Me groups are orientated to the same side of the benzene ring, with the carbonyl groups pointing roughly towards each other. The conformation of the proline residue is an envelope. In the crystal, a three-dimensional network is sustained by C—H⋯O interactions involving both the quinone and carbonyl O atoms.

The complete molecule of the title diproline ester quinone, C 18 H 22 N 2 O 6 , is generated by a crystallographic twofold axis, which passes through the centre of the benzene ring. Both -CO 2 Me groups are orientated to the same side of the benzene ring, with the carbonyl groups pointing roughly towards each other. The conformation of the proline residue is an envelope. In the crystal, a three-dimensional network is sustained by C-HÁ Á ÁO interactions involving both the quinone and carbonyl O atoms.

Comment
Oxidative nucleophilic addition of amines to quinones results in the formation of aminoquinone products (Lyons & Thomson, 1953). As part of a study into concise methodology for the synthesis of heterocyclic systems, we envisaged that oxidative addition of α-amino acid derivatives to benzoquinone could yield a suitably functionalized precursor for cyclization to yield pyrroloindole quinones, a structural motif present in the mitomycin anticancer drugs (Tomasz, 1995). The title diproline ester quinone, (I), was synthesized in this context.
The molecule of (I), Fig. 1, exists about a crystallographic 2-fold axis of symmetry passing through the centre of the benzene ring. This has the result that the two -CO 2 Me groups are orientated to the same side of the benzene ring. The carbonyl groups are tucked in under the benzene ring. The conformation of the proline residue is an envelope with the C5 atom lying above the plane through the remaining atoms. The conformational descriptors (Cremer & Pople, 1975) The crystal packing features C-H···O contacts, Table 1. The quinone-O1 atom accepts two such interactions, one from a methylene-H and the other from a methyl-H, whereas the carbonyl-O2 accepts a quinone-H. The C-H···O interactions combine to give a 3-D network, Fig. 2.
The maximum and minimum residual electron density peaks of 0.64 and 0.63 e Å -3 , respectively, were located 1.59 Å and 0.85 Å from the H5b and C8 atoms, respectively. In the absence of significant anomalous scattering effects, 750 Friedel pairs were averaged in the final refinement. However, the absolute configuration was assigned on the basis of the chirality of the L-proline starting material. Fig. 1. The molecular structure of (I) showing displacement ellipsoids at the 50% probability level. Unlabelled atoms are generated by (-x, y, -z).

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.