Intramolecular Hydrogen Bond-Controlled Prolyl Amide Isomerization in Glucosyl 3′(<i>S</i>)-Hydroxy-5′-hydroxymethylproline Hybrids: Influence of a <i>C</i>-5′-Hydroxymethyl Substituent on the Thermodynamics and Kinetics of Prolyl Amide <i>Cis</i>/<i>Trans</i> Isomerization Kaidong Zhang Robel B. Teklebrhan G. Schreckenbach Stacey Wetmore Frank Schweizer 10.1021/jo9003458.s001 https://acs.figshare.com/articles/journal_contribution/Intramolecular_Hydrogen_Bond_Controlled_Prolyl_Amide_Isomerization_in_Glucosyl_3_i_S_i_Hydroxy_5_hydroxymethylproline_Hybrids_Influence_of_a_i_C_i_5_Hydroxymethyl_Substituent_on_the_Thermodynamics_and_Kinetics_of_Prolyl_Amide_i_Cis_i_i_Trans_i_Isomerizati/2856871 Peptide mimics containing spirocyclic glucosyl-(3′-hydroxy-5′-hydroxymethyl)proline hybrids (Glc3′(<i>S</i>)-5′(CH<sub>2</sub>OH)HypHs) with a polar hydroxymethyl substituent at the <i>C</i>-5′ position, such as <i>C</i>-terminal ester Ac-Glc3′(<i>S</i>)-5′(CH<sub>2</sub>OH)Hyp-OMe and <i>C</i>-terminal amide Ac-Glc3′(<i>S</i>)-5′(CH<sub>2</sub>OH)Hyp-<i>N</i>′-CH<sub>3</sub>, were synthesized. <i>C</i>-Terminal esters exhibit increased <i>cis</i> population (23−53%) relative to Ac-3(<i>S</i>)HyPro-OMe (17%) or Ac-Pro-OMe (14%) in D<sub>2</sub>O. The prolyl amide <i>cis</i> population is further increased to 38−74% in the <i>C</i>-terminal amide form in D<sub>2</sub>O. Our study shows that the stereochemistry of the hydroxymethyl substituent at the <i>C</i>-5′ position of proline permits tuning of the prolyl amide <i>cis</i>/<i>trans</i> isomer ratio. Inversion−magnetization transfer NMR experiments indicate that the stereochemistry of the hydroxymethyl substituent has a dramatic effect on the kinetics of prolyl amide <i>cis</i>/<i>trans</i> isomerization. A 200-fold difference in the <i>trans</i>-to-<i>cis</i> (<i>k</i><sub>tc</sub>) isomerization and a 90-fold rate difference in the <i>cis</i>-<i>to</i>-<i>trans</i> (<i>k</i><sub>ct</sub>) isomerization is observed between epimeric <i>C</i>-5′ <b>3</b> and <b>4</b>. When compared to reference peptide mimics Ac-Pro-OMe and Ac-3(<i>S</i>)Hyp-OMe, our study demonstrates that a (13−16)-fold decrease in <i>k</i><sub>tc</sub> and <i>k</i><sub>ct</sub> is observed for the <i>C</i>-5′(<i>S</i>), while a (5−24)-fold acceleration is observed for the <i>C</i>-5′(<i>R</i>) epimer. DFT calculations indicate that the pyrrolidine ring prefers a C<sup>β</sup> exo pucker in both Ac-Glc3′(<i>S</i>)-5′(CH<sub>2</sub>OH)Hyp-OMe diastereoisomers. Computational calculations and chemical shift temperature coefficient (Δδ/Δ<i>T</i>) experiments indicate that the hydroxymethyl group at <i>C</i>-5′ in Ac-Glc3′(<i>S</i>)-5′(CH<sub>2</sub>OH)Hyp-OMe forms a stabilizing intramolecular hydrogen bond to the carbonyl of the <i>N</i>-acetyl group in both epimeric <i>cis</i> isomers. However, a competing intramolecular hydrogen bond between the hydroxymethyl groups in the pyrrolidine ring and pyran ring stabilizes the <i>trans</i> isomer in the <i>C</i>-5′(<i>S</i>) diastereoisomer. The dramatic differences in the kinetic properties of the diastereoisomeric peptide mimics are rationalized by the presence or absence of an intramolecular hydrogen bond between the hydroxymethyl substituent located at <i>C</i>-5′ and the developing lone pair on the nitrogen atom of the <i>N</i>-acetyl group in the transition state. 2009-05-15 00:00:00 diastereoisomeric peptide mimics D 2O pyrrolidine ring hydroxymethyl substituent DFT NMR epimeric cis isomers intramolecular hydrogen bond C β exo pucker prolyl amide cis population isomerization