Cation ammonium tertiaire et liaison hydrogène intramoléculaire

Structural Comparison of Homologous Reduced Peptide, Reduced Azapeptide, Iminoazapeptide, and Methyleneoxypeptide Analogues

“…The Piv group was substituted for Boc in 1 ‘, and the Z group was eliminated by catalytic hydrogenolysis to yield the neutral reduced dipeptide 1 , which was then protonated quantitatively to 1 + by PF 6 - ·Et 2 O + H (Aldrich 17,638-9). The PF 6 - anion was selected to obtain an ammonium salt soluble in nonpolar media and for its weak interaction with ammonium anions, thus allowing the ammonium group to participate in intramolecular interactions . The methyleneoxypeptide 2 was prepared from the commercially available Boc-prolinol 10 (Merck 818348), which was treated by NaH and reacted with N -isopropyl-bromoacetamide ( 9 ) (Figure ) …”

“…The PF6anion was selected to obtain an ammonium salt soluble in nonpolar media and for its weak interaction with ammonium anions, 14 thus allowing the ammonium group to participate in intramolecular interactions. 15 The methyleneoxypeptide 2 was prepared from the commercially available Boc-prolinol 10 (Merck 818348), which was treated by NaH and reacted with N-isopropyl-bromoacetamide (9) (Figure 2). 16 The synthesis of the iminoazadipeptides 3 and 3′ and the reduced azadipeptide 4 is summarized in Figure 3.…”

“…Previous studies on ammonia and protonated reduced peptides have shown that the N + H2 stretching frequencies greatly depend on the solvent and the associated anion. 10a, 15 For example, the n Bu2N + H2 group exhibits a sharp peak at 3246 cm -1 when associated with the PF6anion in a low polar solvent (CH2Cl2) but gives rise to a multicomponent absorption shifted down to 2700 cm -1 when solvated in a strong aprotic medium. The n Bu2N + H2 cation has also a weak absorption at 1596 cm -1 in CH2-Cl2, while the PF6anion has no visible absorption in the 2200-2800 and 3100-3600 cm -1 domains.…”