(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

[Image: see text] (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.