(Z)-Methyl 2-methoxyimino-3-oxobutanoate

The title compound, C6H9NO4, was prepared stereoselectively as a precursor for 1-azadienes in a study of hetero-Diels–Alder reactions. The configuration of the C=N double bond was found to be Z, corroborating earlier assignments of similar compounds based only on NMR and IR spectroscopic analysis.

The title compound, C 6 H 9 NO 4 , was prepared stereoselectively as a precursor for 1-azadienes in a study of hetero-Diels-Alder reactions. The configuration of the C N double bond was found to be Z, corroborating earlier assignments of similar compounds based only on NMR and IR spectroscopic analysis.
The crystal structure of (I) (Fig. 1) verifies earlier studies by NMR and IR (Buehler, 1967;Karabatsos & Taller, 1968;Levy & Nelson, 1972;Jirman et al., 1990;Corrêa & Moran, 1999) of Z-configured oximes and forms a basis for further studies in the field. Interestingly, the C1/O2/O4 carboxyl group in (I) adopts a dihedral angle of 93° with respect to the coplanar N?C-C?O π-system, which indicates complete absence of electronic conjugation.

S2. Experimental
A stirred solution of 7.25 g (50.0 mmol) of Z-Methyl 2-(hydroxyimino)-3-oxobutanoate (Lu & Arndt, 2007;Fletcher et al., 2006) in anhydrous acetone (50 ml) was cooled to 273 K and potassium carbonate (3.8 g, 27.5 mmol) was added, followed by dimethyl sulfate (5.70 ml, 60.0 mmol). The stirred reaction mixture was warmed to room temperature over 2 h and kept stirring for 10 h (TLC control). The reaction mixture was filtered and the solid residue was rinsed with acetone

S3. Refinement
Anomalous dispersion was negligible and Friedel pairs were merged before refinement.
The H atoms were placed in calculated positions, with C-H = 0.96 Å and were refined as riding, with U iso (H) = 1.5U eq (C); the methyl groups were allowed to rotate but not to tip.  The molecular structure of (I) with displacement ellipsoids shown at the 30% probability level (arbitrary spheres for the H atoms).

(Z)-Methyl 2-methoxyimino-3-oxobutanoate
Crystal data C 6 H 9 NO 4 M r = 159.14 Orthorhombic, Pna2 1 Hall symbol: P 2c -2n a = 8.3410 (17)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.08 e Å −3 Δρ min = −0.11 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.087 (6) Special details 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 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.  (3)