(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.

Abstract

(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 Cβ and Cδ C–H bonds. The unit cell exhibited variation in Cδ–H/Cγ–O and Cβ–H/Cγ–O torsion angles, with a 15° increase in torsion angle (148° to 163°) observed to result in a 0.018 Å decrease in Cδ–H/Cγ–O bond length, consistent with favorable σC–H → σ*C–O hyperconjugative interactions increasing with greater orbital overlap.