Reverse Turn Induced π-Facial Selectivity during Polyaniline-Supported Cobalt(II) Salen Catalyzed Aerobic Epoxidation ofN-Cinnamoyll-Proline Derived Peptides

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Abstract
A novel chemo- and diastereoselective aerobic epoxidation of the N-cinnamoyl peptides catalyzed by polyaniline-supported cobalt(II) salen (PASCOS) is described. The N-cinnamoyl proline derived peptides 1 show a high π-facial selectivity during these epoxidations. The origin of this diastereoselectivity in 1 has been attributed to (i) the propensity of the N-cinnamoyl proline amide to exist predominantly as trans rotamer in CDCl3, DMSO-d6, and CH3CN medium and (ii) existence of these peptides as organized structures (γ- and β-turns) due to the presence of intramolecular hydrogen bonds. An extensive solution NMR and MD simulation study on 1d and 1f indicates that the origin of the high π-facial selectivity is due to the well-defined γ- and β-turns which result in the hindrance of one face of the cinnamoyl double bond in the transition state of the epoxidation reaction.