tert-Butyl N-((1S)-2-hydroxy-1-{N′-[(E)-2-hydroxy-4-methoxybenzylidene]hydrazinecarbonyl}ethyl)carbamate

The molecule of the title compound, C16H23N3O6, is twisted about the chiral C atom with the dihedral angle formed between the amide residues being 76.9 (3)°. Overall, the molecule is curved with the terminal organic groups lying to the same side. The conformation about the imine bond [1.291 (5) Å] is E and an intramolecular O—H⋯N hydrogen bond generates an S(6) ring. In the crystal, O—H⋯O and N—H⋯O hydrogen bonds involving the hydroxy, amine and carbonyl groups lead to the formation of supramolecular layers, which stack along the c-axis direction.

The molecule of the title compound, C 16 H 23 N 3 O 6 , is twisted about the chiral C atom with the dihedral angle formed between the amide residues being 76.9 (3) . Overall, the molecule is curved with the terminal organic groups lying to the same side. The conformation about the imine bond [1.291 (5) Å ] is E and an intramolecular O-HÁ Á ÁN hydrogen bond generates an S(6) ring. In the crystal, O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds involving the hydroxy, amine and carbonyl groups lead to the formation of supramolecular layers, which stack along the c-axis direction.

Comment
The analysis of the title compound, (I), was conducted in the context of developing N-acylhydrazone derivatives from L-serine for use in anti-tumour testing Pinheiro et al., 2011a;Pinheiro et al., 2011b;Howie et al., 2011;Tiekink et al., 2011) owing to the known anti-tumour activity of L-serine derivatives (Jiao et al., 2009;Yakura et al., 2007).
The absolute structure of (I) could not be determined experimentally but, the assignment of the S-configuration at the C10 atom is based on a starting reagent, L-serine. The structure of (I), Fig. 1, is isomorphous with the analogue not featuring the hydroxyl group in the ring (Pinheiro et al. 2011b). The molecule adopts a curved conformation with both the benzene ring and tert-butyl group lying to the same side of the molecule. Nevertheless, there is a twist in the molecule, at the chiral centre, as seen in the dihedral angle formed between the two amide residues, i.e. N2,C9,O3 and N3,C12,O5, of 76.9 (3) °.
As with related structures in this series, hydrogen bonds dominate the crystal packing, Table 1

Experimental
To a stirred solution of (S)-t-BuOCONHCH(CH 2 OH)CONHNH 2 (Howie et al., 2011) (1.0 mmol) in ethanol (10 ml) at room temperature was added 2-hydroxy-4-methoxybenzaldehyde (1.05 mmol). The reaction mixture was refluxed for 4 h, rotary evaporated and the residue purified by washing with cold ethanol (3 x 10 ml), affording the title compound, M.pt. 455 K, yield 84%. The sample for the structure determination was recrystallized from EtOH to afford colourless laths of (I The C-bound H atoms were geometrically placed (C-H = 0.95-1.00 Å) and refined as riding with U iso (H) = 1.2-1.5U eq (C).
The O-and N-bound H atoms were located from a difference map and refined with the distance restraints O-H = 0.84 ± 0.01 and N-H = 0.88±0.01 Å, and with U iso (H) = zU eq (carrier atom); z = 1.5 for O and z = 1.2 for N. In the absence of significant anomalous scattering effects, 1703 Friedel pairs were averaged in the final refinement. However, the absolute configuration was assigned on the basis of the chirality of the L-serine starting material. Fig. 1. The molecular structure of (I) showing displacement ellipsoids at the 50% probability level.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(F 2 ) is used only for calculating Rfactors(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.