(2S,4R)-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-beta and C-delta C-H bonds. The unit cell exhibited variation in C-delta-H/C-gamma-O and C-beta-H/C-gamma-O torsion angles, with a 15 degrees increase in torsion angle (148 degrees to 163 degrees) observed to result in a 0.018 angstrom decrease in C-delta-H/C-gamma-O bond length, consistent with favorable sigma(C-H) -> sigma*(C-O) hyperconjugative interactions increasing with greater orbital overlap.