AZA-PEPTIDES .2. X-RAY STRUCTURES OF AZA-ALANINE AND AZA-ASPARAGINE-CONTAINING PEPTIDES

In order to determine the structural consequences of the N-alpha/(CH)- H-alpha exchange in aza-peptides, we have solved the crystal molecular structures of some derivatives containing the aza-analogue of asparag ine [Z-AzAsn(Me)-NMe2 (1), Z-AzAsn(Me)-Pro-NHiPr (2) and Piv-Pro-AzAsn (Me)-NHiPr (5)], aspartic acid [Z-AzAsn(OEt)-Pro-NHiPr (3) and alanine (Boc-AzAla-Pro-NHiPr (4)], by using X-ray diffraction. They reveal th at the alpha-nitrogen accommodates a pyramidal (1-4) or planar (5) str ucture: depending on the sequence, When pyramidal, the alpha-nitrogen assumes the R (D-like) chirality, AU of the derivatives but 1 adopt ei ther a beta(I)-folded (2-4) or beta(II)-folded (5) structure in which the (AzAsn)(NH)-H-delta bond is intramolecularly hydrogen-bonded to th e alpha-nitrogen. (C) Munksgaard 1997.