Structural Studies of Intermediates in Antibiotic Synthesis. II. The Structures of Methoxyiminomalonic Acid and its Methyl and Benzhydryl Monoesters

Article abstract of DOI:10.1107/S0108768183002700

The crystal and molecular structures of methoxyiminomalonic acid monohydrate (I), methyl methoxyiminomalonate (II) and the sodium salt of benzhydryl methoxyiminomalonate trihydrate (III), were determined by direct methods from diffractometer data and refined to R values of 0.03, 0.07 and 0.04 for 1305, 1229 and 2484 observed reflections, respectively.The crystal data are: C4H5NO5.H2O, M_r=165, P<*>, a=6.735(1), b=8.035(2), c=7.609(5) Angstroem, α=72.20(4), β=69.58(3), γ=70.84(3) deg, Z=2, V=355.8 Angstroem³, D_m=1.54, D_x=1.538 g cm^-3, Mo Kα, μ=0.89 cm^-1, T=293 K for (I); C5H7NO5, M_r=161, P2₁/n, a=7.874(2), b=8.179(2), c=11.722(3) Angstroem, β=96.97(2) deg, Z=4, V=749.3 Angstroem³, D_m=1.42, D_x=1.425 g cm^-3, Cu Kα, μ=8.88cm^-1, T=293 K for (II); C17H14NO5^-.Na⁺.3H2O, M_r=389, C2/c, a=15.940(8), b=5.795(1), c=40.392(13) Angstroem, β=98.11(3) deg, Z=8, V=3693.6 Angstroem³, D_m=1.4, D_x=1.397 g cm^-3, Mo Kα, μ=0.88cm^-1, T=83 K for (III).In all three molecules the methoxime group is antiplanar to a free carboxyl group, while the syn-oriented carboxyl group is perpendicular to that plane.These findings show that basic half-hydrolysis of the diester of methoxyiminomalonic acid takes place exclusively on the anti carboxyl group and is attributed to steric effects, whereas half-esterification of the corresponding diacid with diphenyldiazomethane, in a non-polar medium, yields preferentially the syn ester probably because of the different relative acidities of the two carboxylic acid groups.