organic compounds
Dimethyl 5-acetyl-1-hydroxy-4-methyl-1H-pyrrole-2,3-dicarboxylate, an oxidation-resistant N-hydroxypyrrole
aDepartment of Pharmaceutical Sciences, School of Pharmacy, University of Maryland, Baltimore, Maryland 21201, USA, bCenter for Biomedical Engineering and Technology, School of Medicine, University of Maryland, Baltimore, MD 21201, USA, cDepartment of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA, and dDepartment of Physiology, School of Medicine, University of Maryland, Baltimore, MD 21201, USA
*Correspondence e-mail: jkao@umaryland.edu
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.
CCDC reference: 978380
Related literature
The title compound was obtained as part of an effort to synthesize aromatic 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 appears in the review by Guin et al. (2011). Examples of hydrogen bonding affecting the redox properties of are discussed by Gupta & Linschitz (1997) and Feldman et al. (2007).
and was prepared by a published procedure (HekmatshoarExperimental
Crystal data
|
|
Data collection: APEX2 (Bruker, 2010); cell 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).
Supporting information
CCDC reference: 978380
10.1107/S1600536813034466/ld2115sup1.cif
contains datablock I. DOI:Structure factors: contains datablock I. DOI: 10.1107/S1600536813034466/ld2115Isup2.hkl
Supporting information file. DOI: 10.1107/S1600536813034466/ld2115Isup3.cml
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 CH2Cl2, 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.
Positions of all H atoms were calculated from geometric considerations. H atoms were refined as riding on the attached C atoms. Orientation of CH3 groups was optimized. For all H atoms, Uiso was refined but constrained to be equal within CH3 groups.
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
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 (see Guin et al. (2011) for review, Gupta and Linschitz (1997) and Feldman 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.The title compound was part of an effort to synthesize aromatic
and was prepared by a published procedure (Hekmatshoar et al., 2008). The compound could 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 appears in the review by Guin et al. (2011); examples of hydrogen bonding affecting the redox properties of are offered in the reports by Gupta & Linschitz (1997) and Feldman et al. (2007).Data collection: APEX2 (Bruker, 2010); cell
SAINT (Bruker, 2010); data reduction: SAINT (Bruker, 2010); 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).Fig. 1. Molecular structure of the title compound with non-hydrogen atoms labeled. Displacement ellipsoids are shown at the 60% probability level. |
C11H13NO6 | F(000) = 536 |
Mr = 255.22 | Dx = 1.439 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 13127 reflections |
a = 10.3893 (8) Å | θ = 2.4–31.0° |
b = 15.1803 (12) Å | µ = 0.12 mm−1 |
c = 7.5789 (6) Å | T = 150 K |
β = 99.630 (1)° | Prism, colourless |
V = 1178.45 (16) Å3 | 0.52 × 0.43 × 0.31 mm |
Z = 4 |
Bruker SMART APEXII diffractometer | 3437 independent reflections |
Radiation source: fine-focus sealed tube | 2836 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.014 |
Detector resolution: 8.333 pixels mm-1 | θmax = 30.0°, θmin = 2.0° |
φ and ω scans | h = −14→14 |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | k = −21→21 |
Tmin = 0.888, Tmax = 0.964 | l = −10→10 |
19259 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.031 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.064 | All H-atom parameters refined |
S = 1.00 | w = 1/[σ2(Fo2) + (0.01P)2 + 0.4962P], P = (max(Fo2,0) + 2Fc2)/3 |
3437 reflections | (Δ/σ)max < 0.001 |
215 parameters | Δρmax = 0.35 e Å−3 |
0 restraints | Δρmin = −0.19 e Å−3 |
C11H13NO6 | V = 1178.45 (16) Å3 |
Mr = 255.22 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 10.3893 (8) Å | µ = 0.12 mm−1 |
b = 15.1803 (12) Å | T = 150 K |
c = 7.5789 (6) Å | 0.52 × 0.43 × 0.31 mm |
β = 99.630 (1)° |
Bruker SMART APEXII diffractometer | 3437 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | 2836 reflections with I > 2σ(I) |
Tmin = 0.888, Tmax = 0.964 | Rint = 0.014 |
19259 measured reflections |
R[F2 > 2σ(F2)] = 0.031 | 0 restraints |
wR(F2) = 0.064 | All H-atom parameters refined |
S = 1.00 | Δρmax = 0.35 e Å−3 |
3437 reflections | Δρmin = −0.19 e Å−3 |
215 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 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. |
x | y | z | Uiso*/Ueq | ||
N1 | 0.58967 (7) | 0.15991 (5) | 0.01532 (11) | 0.02245 (15) | |
C1 | 0.57608 (9) | 0.25121 (6) | 0.00825 (12) | 0.02170 (17) | |
C2 | 0.69238 (9) | 0.28546 (6) | 0.10473 (12) | 0.02101 (16) | |
C3 | 0.77256 (8) | 0.21282 (6) | 0.16540 (12) | 0.02102 (16) | |
C4 | 0.70478 (8) | 0.13542 (6) | 0.10655 (12) | 0.02093 (17) | |
O1 | 0.49892 (7) | 0.09969 (5) | −0.06513 (11) | 0.03059 (16) | |
H1 | 0.4367 (15) | 0.1346 (10) | −0.121 (2) | 0.055 (4)* | |
C5 | 0.45803 (9) | 0.28874 (6) | −0.09308 (13) | 0.02522 (18) | |
O5 | 0.37225 (7) | 0.23953 (5) | −0.17286 (11) | 0.03488 (18) | |
C6 | 0.44010 (11) | 0.38675 (7) | −0.10178 (16) | 0.0318 (2) | |
H61 | 0.3632 (15) | 0.3989 (10) | −0.190 (2) | 0.056 (4)* | |
H62 | 0.4261 (14) | 0.4098 (10) | 0.012 (2) | 0.048 (4)* | |
H63 | 0.5176 (13) | 0.4162 (9) | −0.1332 (17) | 0.039 (4)* | |
C7 | 0.72552 (11) | 0.38090 (6) | 0.13774 (14) | 0.02725 (19) | |
H71 | 0.8031 (14) | 0.3861 (10) | 0.227 (2) | 0.050 (4)* | |
H72 | 0.6559 (14) | 0.4127 (9) | 0.1794 (18) | 0.044 (4)* | |
H73 | 0.7376 (13) | 0.4103 (9) | 0.0300 (19) | 0.044 (4)* | |
C8 | 0.90325 (9) | 0.21540 (6) | 0.27407 (13) | 0.02425 (18) | |
O8 | 0.96516 (8) | 0.28077 (5) | 0.32002 (13) | 0.0425 (2) | |
O9 | 0.94825 (7) | 0.13382 (5) | 0.31885 (10) | 0.02999 (16) | |
C9 | 1.07658 (11) | 0.13084 (9) | 0.42886 (18) | 0.0394 (3) | |
H91 | 1.0973 (16) | 0.0679 (11) | 0.439 (2) | 0.062 (5)* | |
H92 | 1.1394 (14) | 0.1618 (10) | 0.3656 (19) | 0.046 (4)* | |
H93 | 1.0726 (13) | 0.1588 (10) | 0.543 (2) | 0.046 (4)* | |
C10 | 0.74086 (8) | 0.04013 (6) | 0.13628 (12) | 0.02129 (17) | |
O10 | 0.70851 (7) | −0.00387 (4) | 0.25304 (10) | 0.02722 (15) | |
O11 | 0.81287 (7) | 0.01306 (5) | 0.01819 (10) | 0.02943 (16) | |
C11 | 0.86684 (14) | −0.07523 (8) | 0.04835 (18) | 0.0383 (3) | |
H111 | 0.9238 (14) | −0.0816 (9) | −0.0377 (19) | 0.046 (4)* | |
H112 | 0.9172 (15) | −0.0775 (11) | 0.170 (2) | 0.060 (5)* | |
H113 | 0.7977 (15) | −0.1177 (11) | 0.037 (2) | 0.059 (5)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
N1 | 0.0195 (3) | 0.0176 (3) | 0.0295 (4) | −0.0019 (3) | 0.0019 (3) | −0.0014 (3) |
C1 | 0.0216 (4) | 0.0181 (4) | 0.0260 (4) | 0.0008 (3) | 0.0056 (3) | 0.0002 (3) |
C2 | 0.0233 (4) | 0.0185 (4) | 0.0220 (4) | −0.0006 (3) | 0.0061 (3) | −0.0007 (3) |
C3 | 0.0203 (4) | 0.0196 (4) | 0.0232 (4) | −0.0016 (3) | 0.0038 (3) | −0.0009 (3) |
C4 | 0.0192 (4) | 0.0186 (4) | 0.0250 (4) | −0.0002 (3) | 0.0040 (3) | −0.0004 (3) |
O1 | 0.0225 (3) | 0.0219 (3) | 0.0438 (4) | −0.0053 (3) | −0.0046 (3) | −0.0031 (3) |
C5 | 0.0224 (4) | 0.0257 (4) | 0.0284 (4) | 0.0036 (3) | 0.0068 (3) | 0.0028 (4) |
O5 | 0.0246 (3) | 0.0322 (4) | 0.0446 (4) | 0.0018 (3) | −0.0036 (3) | 0.0005 (3) |
C6 | 0.0282 (5) | 0.0256 (5) | 0.0414 (6) | 0.0066 (4) | 0.0055 (4) | 0.0056 (4) |
C7 | 0.0338 (5) | 0.0184 (4) | 0.0291 (5) | −0.0026 (4) | 0.0039 (4) | −0.0019 (4) |
C8 | 0.0223 (4) | 0.0251 (4) | 0.0253 (4) | −0.0020 (3) | 0.0037 (3) | 0.0002 (3) |
O8 | 0.0323 (4) | 0.0298 (4) | 0.0591 (5) | −0.0082 (3) | −0.0103 (4) | −0.0025 (4) |
O9 | 0.0226 (3) | 0.0286 (4) | 0.0362 (4) | 0.0011 (3) | −0.0027 (3) | 0.0030 (3) |
C9 | 0.0268 (5) | 0.0473 (7) | 0.0399 (6) | 0.0063 (5) | −0.0063 (4) | 0.0017 (5) |
C10 | 0.0193 (4) | 0.0186 (4) | 0.0247 (4) | −0.0003 (3) | 0.0001 (3) | −0.0014 (3) |
O10 | 0.0305 (3) | 0.0218 (3) | 0.0303 (3) | −0.0020 (3) | 0.0079 (3) | 0.0013 (3) |
O11 | 0.0368 (4) | 0.0229 (3) | 0.0308 (4) | 0.0106 (3) | 0.0120 (3) | 0.0057 (3) |
C11 | 0.0488 (7) | 0.0259 (5) | 0.0442 (6) | 0.0169 (5) | 0.0190 (6) | 0.0075 (5) |
N1—C4 | 1.3304 (11) | C7—H71 | 0.963 (15) |
N1—O1 | 1.3799 (10) | C7—H72 | 0.965 (14) |
N1—C1 | 1.3932 (11) | C7—H73 | 0.957 (14) |
C1—C2 | 1.4033 (12) | C8—O8 | 1.2020 (12) |
C1—C5 | 1.4503 (13) | C8—O9 | 1.3470 (12) |
C2—C3 | 1.4118 (12) | O9—C9 | 1.4501 (13) |
C2—C7 | 1.5005 (13) | C9—H91 | 0.980 (17) |
C3—C4 | 1.4037 (12) | C9—H92 | 0.991 (15) |
C3—C8 | 1.4661 (12) | C9—H93 | 0.971 (15) |
C4—C10 | 1.5018 (12) | C10—O10 | 1.2007 (11) |
O1—H1 | 0.888 (16) | C10—O11 | 1.3242 (11) |
C5—O5 | 1.2399 (12) | O11—C11 | 1.4561 (12) |
C5—C6 | 1.4995 (14) | C11—H111 | 0.956 (14) |
C6—H61 | 0.968 (15) | C11—H112 | 0.981 (16) |
C6—H62 | 0.963 (15) | C11—H113 | 0.958 (16) |
C6—H63 | 0.985 (14) | ||
C4—N1—O1 | 122.20 (8) | C2—C7—H72 | 112.0 (8) |
C4—N1—C1 | 111.99 (7) | H71—C7—H72 | 108.4 (11) |
O1—N1—C1 | 125.78 (8) | C2—C7—H73 | 111.6 (8) |
N1—C1—C2 | 106.00 (8) | H71—C7—H73 | 110.1 (12) |
N1—C1—C5 | 118.84 (8) | H72—C7—H73 | 104.9 (11) |
C2—C1—C5 | 135.11 (8) | O8—C8—O9 | 122.66 (9) |
C1—C2—C3 | 106.86 (8) | O8—C8—C3 | 125.82 (9) |
C1—C2—C7 | 126.74 (8) | O9—C8—C3 | 111.52 (8) |
C3—C2—C7 | 126.40 (8) | C8—O9—C9 | 114.87 (8) |
C4—C3—C2 | 108.24 (8) | O9—C9—H91 | 104.2 (10) |
C4—C3—C8 | 124.69 (8) | O9—C9—H92 | 108.9 (8) |
C2—C3—C8 | 127.06 (8) | H91—C9—H92 | 110.4 (12) |
N1—C4—C3 | 106.90 (8) | O9—C9—H93 | 109.0 (8) |
N1—C4—C10 | 121.82 (8) | H91—C9—H93 | 113.3 (13) |
C3—C4—C10 | 131.24 (8) | H92—C9—H93 | 110.7 (12) |
N1—O1—H1 | 101.8 (10) | O10—C10—O11 | 125.81 (8) |
O5—C5—C1 | 119.78 (9) | O10—C10—C4 | 123.55 (8) |
O5—C5—C6 | 120.16 (9) | O11—C10—C4 | 110.64 (8) |
C1—C5—C6 | 120.06 (9) | C10—O11—C11 | 115.20 (8) |
C5—C6—H61 | 107.5 (9) | O11—C11—H111 | 104.6 (9) |
C5—C6—H62 | 111.0 (9) | O11—C11—H112 | 108.1 (10) |
H61—C6—H62 | 108.4 (12) | H111—C11—H112 | 110.1 (12) |
C5—C6—H63 | 111.2 (8) | O11—C11—H113 | 110.0 (9) |
H61—C6—H63 | 111.1 (11) | H111—C11—H113 | 114.3 (13) |
H62—C6—H63 | 107.6 (11) | H112—C11—H113 | 109.5 (13) |
C2—C7—H71 | 109.7 (9) | ||
C4—N1—C1—C2 | −0.55 (11) | C2—C3—C4—C10 | −177.93 (9) |
O1—N1—C1—C2 | −178.69 (8) | C8—C3—C4—C10 | 1.01 (15) |
C4—N1—C1—C5 | 177.39 (8) | N1—C1—C5—O5 | −0.77 (14) |
O1—N1—C1—C5 | −0.75 (14) | C2—C1—C5—O5 | 176.43 (10) |
N1—C1—C2—C3 | 0.48 (10) | N1—C1—C5—C6 | 179.45 (9) |
C5—C1—C2—C3 | −176.97 (10) | C2—C1—C5—C6 | −3.36 (16) |
N1—C1—C2—C7 | −179.43 (9) | C4—C3—C8—O8 | 176.89 (10) |
C5—C1—C2—C7 | 3.13 (17) | C2—C3—C8—O8 | −4.37 (16) |
C1—C2—C3—C4 | −0.26 (10) | C4—C3—C8—O9 | −3.19 (13) |
C7—C2—C3—C4 | 179.64 (9) | C2—C3—C8—O9 | 175.56 (9) |
C1—C2—C3—C8 | −179.17 (9) | O8—C8—O9—C9 | 0.77 (15) |
C7—C2—C3—C8 | 0.73 (15) | C3—C8—O9—C9 | −179.15 (9) |
O1—N1—C4—C3 | 178.61 (8) | N1—C4—C10—O10 | −82.60 (12) |
C1—N1—C4—C3 | 0.39 (11) | C3—C4—C10—O10 | 94.99 (12) |
O1—N1—C4—C10 | −3.29 (14) | N1—C4—C10—O11 | 97.95 (10) |
C1—N1—C4—C10 | 178.50 (8) | C3—C4—C10—O11 | −84.45 (12) |
C2—C3—C4—N1 | −0.07 (10) | O10—C10—O11—C11 | −6.69 (15) |
C8—C3—C4—N1 | 178.87 (8) | C4—C10—O11—C11 | 172.74 (9) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O5 | 0.888 (16) | 1.746 (16) | 2.5583 (11) | 150.8 (14) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O5 | 0.888 (16) | 1.746 (16) | 2.5583 (11) | 150.8 (14) |
Acknowledgements
This work was supported in part by grants GM056481 (JPYK) and ER2034 (GMR) from the US National Institutes of Health.
References
Bilton, C., Allen, F. H., Shields, G. P. & Howard, J. A. K. (2000). Acta Cryst. B56, 849–856. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Bruker (2010). APEX2, SAINT and XSHELL. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Feldman, K. S., Hester, D. K. II & Golbeck, J. H. (2007). Bioorg. Med. Chem. Lett. 17, 4891–4894. Web of Science CSD CrossRef PubMed CAS Google Scholar
Galek, P. T. A., Fábián, L. & Allen, F. H. (2010). Acta Cryst. B66, 237–252. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Guin, P. S., Das, S. & Mandal, P. C. (2011). Int. J. Electrochem. vol. 2011, Article ID 816202, 22 pages, doi:10.4061/2011/816202. Google Scholar
Gupta, N. & Linschitz, H. (1997). J. Am. Chem. Soc. 119, 6384–6391. CrossRef CAS Web of Science Google Scholar
Hekmatshoar, R., Nouri, R. & Beheshtiha, S. Y. Sh. (2008). Heteroat. Chem. 19, 100–103. Web of Science CrossRef CAS Google Scholar
Keana, J. F. W., Heo, G. S., Mann, J. S., Van Nice, F. L., Lex, L., Prabhu, V. S. & Ferguson, G. (1988). J. Org. Chem. 53, 2268–2274. CSD CrossRef CAS Web of Science Google Scholar
Macrae, C. F., Bruno, I. J., Chisholm, J. A., Edgington, P. R., McCabe, P., Pidcock, E., Rodriguez-Monge, L., Taylor, R., van de Streek, J. & Wood, P. A. (2008). J. Appl. Cryst. 41, 466–470. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.
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 and Linschitz (1997) and Feldman 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.