4-Nitrobenzyl 5-oxo-2-(2-oxopiperidin-1-yl)- tetrahydrofuran-2-carboxylate

Lactivicin [LTV, (1)] is a natural product antibiotic that targets penicillin-binding proteins (PBPs), a class of enzymes involved in the final steps of bacterial cell wall biosynthesis (Nozaki et al., 1987, 1989). As part of a research programme aimed at identifying new antibiotics, we are interested in antibiotics that do not possess a -lactam ring. We have prepared an LTV analogue from the title compound, (2), where a six-membered cyclic hydroxamate unit acts as a surrogate of the naturally occurring isoxazolidine-3-one core (Wolfe, Akuche et al., 2003; Wolfe, Wilson et al., 2003).

The title compound, C 17 H 18 N 2 O 7 , is a synthetic racemic analogue of lactivicin, a natural product antibiotic that targets penicillin-binding proteins. There are two almost identical molecules in the asymmetric unit.

Comment
Lactivicin [LTV,(1)] is a natural product antibiotic that targets penicillin-binding proteins (PBPs), a class of enzymes involved in the final steps of bacterial cell wall biosynthesis (Nozaki et al., 1987(Nozaki et al., , 1989. As part of a research programme aimed at identifying new antibiotics, we are interested in antibiotics that do not possess a -lactam ring. We have prepared an LTV analogue from the title compound, (2), where a six-membered cyclic hydroxamate unit acts as a surrogate of the naturally occurring isoxazolidine-3-one core .
As is common in Z 0 = 2 structures (Collins, 2006), one molecule of (2) is well ordered and the other has resolvable disorder. If the minor component of the disorder is selected, the two molecules have very similar geometries (Fig. 2), with the major discrepancy being in the orientation of the nitro group. If this group is also omitted, the two molecules are essentially identical (r.m.s. positional deviation = 0.10 Å , r.m.s. bond length deviation = 0.016 Å and r.m.s. torsion angle deviation = 3.07 )  and related by a pseudo glide plane at (0.42 À x, 0.50 + y, 0.00 + z) (Fig. 3).
There are no hydrogen bonds in the crystal structure of (2), which consists of bilayers with the nitro groups dominating the exposed faces (Fig. 4).

Figure 2
A least-squares fit of the minor component of the disordered molecule (blue) to the undisordered molecule. The major differences in conformation are in the nitro group. H atoms have been omitted.

Figure 3
A view along the pseudo-glide plane (0.42 À x, 0.50 + y, 0.00 + z) that relates the two independent molecules. C atoms in the independent molecules are coloured green (disordered) and orange.

Figure 4
A cross section through the bilayers in the structure of (2). The interface rich in nitro groups lies parallel to ab at c = 0.5. Compound (2) crystallizes as thin plates which are always twinned. Initial structure determination and refinement were from data collected on an in-house Nonius KappaCCD diffractometer using a sealed-tube source (data set 1 in Table 1). The low completeness even at = 25 generated two level A checkCIF alerts. We were advised to obtain new data from the UK EPSRC National Crystallography Service (NCS) rotating anode diffractometer. A new sample was prepared (also twinned) and a suitable crystal was selected (data set 2 in Table 1). It was observed that the mosaicity deteriorated reversibly on lowering the temperature: the optimal mosaic spread occurred at 200 K. The structure from the NCS data, which generated no level A alerts, is reported in this paper. All three analyses yield the same structural information, although the rotating anode data are undoubtably crystallographically superior (Table 1). The quality of an analysis is undoubtedly limited by the quality of the samples nature provides.

Data collection
H atoms were located in a difference map, but those attached to C atoms were repositioned geometrically and initially refined with soft restraints on bond lengths and angles to regularize their geometry (C-H in the range 0.93-0.98 Å and O-H = 0.82 Å ) and U iso (H) values (in the range 1.2-1.5 times U eq of the parent atom), after which the positions were refined with riding constraints. The disordered atoms were refined with bond length similarity and anisotropic displacement parameter similarity restraints. C160, C170 and attached H atoms are disordered over two sites, with occupancy factors 0.665(9) and 0.335(9).