(2 S,4 R)-4-hydroxyproline(4-nitrobenzoate): Strong induction of stereoelectronic effects via a readily synthesized proline derivative. Crystallographic observation of a correlation between torsion angle and bond length in a hyperconjugative interaction

(2S,4R)-4-Hydroxyproline(4-nitrobenzoate) was synthesized. The crystal structure revealed an exo ring pucker, with the nitrobenzoate pseudoaxial on the pyrrolidine envelope and antiperiplanar to C^β and C ^δ C-H bonds. The unit cell exhibited variation in C ^δ-H/C^γ-O and C^β-H/C ^γ-O torsion angles, with a 15° increase in torsion angle (148° to 163°) observed to result in a 0.018 Å decrease in C ^δ-H/C^γ-O bond length, consistent with favorable σ_C-H → σ*_C-O hyperconjugative interactions increasing with greater orbital overlap.