Dimethyl 5-acetyl-1-hydroxy-4-methyl-1H-pyrrole-2,3-dicarboxylate, an oxidation-resistant N-hydroxypyrrole

The title compound, C11H13NO6, exhibits an intramolecular O–H⋯O=C hydrogen bond between the N-hydroxyl H atom and carbonyl O atom of the neighboring acetyl group. This finding contradicts a previously published model in which the hydrogen bond was postulated to occur with the neighboring carbomethoxy group. This relatively strong hydrogen bond [O—H⋯O: D = 2.5583 (11) Å and θ = 152°] may underlie the resistance of the title compound to oxidation into the corresponding nitroxide.

The title compound, C 11 H 13 NO 6 , exhibits an intramolecular O-HÁ Á ÁO C hydrogen bond between the N-hydroxyl H atom and carbonyl O atom of the neighboring acetyl group. This finding contradicts a previously published model in which the hydrogen bond was postulated to occur with the neighboring carbomethoxy group. This relatively strong hydrogen bond [O-HÁ Á ÁO: D = 2.5583 (11) Å and = 152 ] may underlie the resistance of the title compound to oxidation into the corresponding nitroxide.

Related literature
The title compound was obtained as part of an effort to synthesize aromatic nitroxides and was prepared by a published procedure (Hekmatshoar et al., 2008). The compound could not be converted to the corresponding nitroxide under commonly used oxidation conditions (Keana et al., 1988). For analysis of intramolecular hydrogen-bond parameters in organic crystals, see: Bilton et al. (2000); Galek et al. (2010). A survey of the effect of intramolecular hydrogen bonding on the reduction potential of quinones appears in the review by Guin et al. (2011). Examples of hydrogen bonding affecting the redox properties of quinones are discussed by Gupta & Linschitz (1997) Table 1 Hydrogen-bond geometry (Å , ). Data collection: APEX2 (Bruker, 2010); cell refinement: SAINT (Bruker, 2010); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2012 (Sheldrick, 2008); molecular graphics: XSHELL (Bruker, 2010) and Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Comment
The title compound was synthesized by a published procedure (Hekmatshoar et al., 2008) with the goal of preparing the corresponding nitroxide by oxidation. The molecular structure is shown in Fig. 1. The compound proved resistant to several oxidizing conditions commonly used to convert N-hydroxylamines to nitroxides. Resistance of the title compound to oxidation may be attributable to the relatively strong hydrogen bond formed between the N-hydroxyl hydrogen and the carbonyl oxygen of the acetyl group in the 5-position. Modulation of redox properties by hydrogen bonding has been documented for quinones (see Guin et al. (2011) for review, Gupta andLinschitz (1997) andFeldman et al. (2007) for studies on specific series of benzoquinones and naphthoquinones, respectively). Existence of the intramolecular H-bond in the title compound is unsurprising, since in organic crystals where intramolecular hydrogen bonding would result in a planar 6-membered ring structure, the H-bond is almost always observed (Bilton et al., 2000). The observed O-H···O donor-acceptor distance (2.558 Å) is significantly shorter than the mean of 2.692 Å found for 8493 organic crystal structures in the Cambridge Structural Database in which the H-bond closes a 6-membered ring (Galek et al., 2010).
Likewise, the observed O-H···O bond angle (150.8°) is significantly greater than the mean of 137.8° found for the same set of 8493 structures (Galek et al., 2010). These comparisons suggest that the intramolecular H-bond in the title compound is stronger than average. Finally, it may be interesting to note that, in the original paper reporting the synthesis of the title compound (Hekmatshoar et al., 2008), the authors suggested H-bonding between the N-hydroxyl hydrogen and the carbonyl oxygen of the ester group in the 2-position of the pyrrole ring.

Experimental
The title compound was prepared by the procedure of Hekmatshoar et al. (2008) in an effort to synthesize the corresponding nitroxide. The compound was subjected to four oxidation reactions: 1) m-chloroperbenzoic acid in CH 2 Cl 2 , 2) hydrogen peroxide-sodium tungstate in methanol/acetonitrile, 3) nickel peroxide in benzene, and 4) lead dioxide in benzene (Keana et al., 1988). In each case, no nitroxide was isolated, and only the title compound was recovered.

Refinement
Positions of all H atoms were calculated from geometric considerations. H atoms were refined as riding on the attached C atoms. Orientation of CH 3 groups was optimized. For all H atoms, U iso was refined but constrained to be equal within CH 3 groups.  (Sheldrick, 2008); molecular graphics: XSHELL (Bruker, 2010) and Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Figure 1
Molecular structure of the title compound with non-hydrogen atoms labeled. Displacement ellipsoids are shown at the 60% probability level. 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 > σ(F 2 ) is used only for calculating R-factors(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. For all H atoms both coordinate and isotropic displacement parameters were freely refined. All H atoms were located from the difference Fourier maps and refined unconstrained including isotropic displacement parameters.