Stereodynamics of Nitrogen Chiral Centers in aza-β3-Cyclodipeptides

• Published in: Chirality (New York, N.Y.) N.Y.), 2013-06, Vol.25 (6), p.341-349
• Main Authors: Le Grel, Philippe, Asprogenidi, Aikaterini, Huez, Philippe, Le Grel, Barbara, Salaün, Arnaud, Roisnel, Thierry, Potel, Michel, Rasti, Elham, Hocquet, Alexandre
• Format: Article
• Language: English
• Subjects: Aza Compounds - chemistry; aza-β3-pseudopeptides; Chemical Sciences; chiral nitrogen atom; conformation; diasteromerization; inversion barrier; Magnetic Resonance Spectroscopy; Models, Molecular; Nitrogen - chemistry; Organic chemistry; Peptides, Cyclic - chemistry; pyramidal inversion; quantum chemical calculation; Stereoisomerism; VT-NMR; X-ray structure
• ISSN: 0899-0042; 1520-636X
• DOI: 10.1002/chir.22160

ABSTRACT The present work is devoted to the synthesis, conformational analysis, and stereodynamic study of aza‐β3‐cyclodipeptides. This pseudopeptidic ring shows E/Z hydrazide bond isomerism, eight‐membered ring conformation, and chirotopic nitrogen atoms, all of which are elements that are prone to modulate the ring shape. The (E,E) twist boat conformation observed in the solid state by X‐ray diffraction is also the ground conformation in solution, and emerges as the lowest in energy when using quantum chemical calculations. The relative configuration associated with ring chirality and with the two nitrogen chiral centers is governed by steric crowding and adopts the (P)SNSN/(M)RNRN combination which projects side chains in equatorial position. The nitrogen pyramidal inversion (NPI) at the two chiral centers is correlated with the ring reversal. The process is significantly hindered as was shown by VT‐NMR experiments run in C2D2Cl4, which did not make it possible to determine the barrier to inversion. Finally, these findings make it conceivable to resolve enantiomers of aza‐β3‐cyclodipeptides by modulating the backbone decoration appropriately. Chirality 25:341–349, 2013. © 2013 Wiley Periodicals, Inc.