Preparation and structural characterization of N-glycated amino acid and linear or cyclic dipeptides containing the 6-amino-6-deoxy-1,2:3,4-di-O-isopropylidene-α-d-galactopyranose moiety

N-Glycated derivatives of glycine, glycylglycine, and 2,5-piperazinedione, containing the 6-deoxy-1,2:3,4-di-O-isopropylidene-α-d-galactos-6-yl moiety, were synthesized and studied by X-ray crystallography and NMR spectroscopy. The crystal structures of N-(6-deoxy-1,2:3,4-di-O-isopropylidene-α-d-galactos-6-yl)glycine (2), its glycylglycine analogue (3), and 2,5-piperazinedione congener (4) were determined. The crystals of 2 are monoclinic; space group P21 with a = 10.5098(5), b = 5.7632(5), c = 13.0938(7) Å, β = 90.245(5)°, Z = 2. The compounds 3 and 4 crystallized in the orthorhombic space group P212121 with a = 5.3429(9), b = 15.1484(4), c = 22.853(2) Å, Z = 4 (3) and a = 28.69(6), b = 15.478(3), c = 15.504(2) Å, Z = 4 (4). In the solid state the α-d-galactopyranose moiety of 2 and 3 existed in the twisted oT2 conformation, whereas in 4 a transition state between oT2 and oS2 was recorded. 1H NMR spectroscopy revealed that the conformation in solution for the galactopyranose moiety of compounds 2–4 closely resembled that found in the crystals.