Imidazo[1,2-a]pyrazine-3,6-diones derived from a-amino acids: A theoretical mechanistic study of their formation via pyrolysis and silica-catalyzed process
Por:
Contreras-Torres F.F., Basiuk V.A.
Publicada:
1 ene 2006
Resumen:
Imidazo[1,2-?]pyrazine-3,6-diones are unusual compounds composed of three ?-amino acid fragments. These bicyclic amidines (BCAs) form under high temperatures or with the use of strong dehydrating reagents. We gave insight into the mechanisms of BCA formation via gas-phase pyrolytic and silica-catalyzed reactions of glycine (Gly) and ?-aminoisobutyric acid (AIB) with related diketopiperazines (DKPs), using quantum chemical calculations. The entire process requires four steps: (1) O-acylation of DKP with free or silica-bonded amino acid, (2) acyl transfer from the oxygen to the nitrogen atom, (3) intramolecular condensation of the N-acyl DKP into a cyclol, and (4) elimination of water. To study step (1) at silica surface (modeled by H7Si8O12-OH cluster), we employed two-level ONIOM calculations (AM1:UFF, B3LYP/3-21G:UFF and B3LYP/6-31G(d):UFF); all gas-phase reactions were studied at the AMI, B3LYP/3-21G and B3LYP/6-31G(d) levels. The catalytic effect of silica was observed for both Gly and AIB: the activation energy in the O-acylation at the surface was lower by more than 9 kcal mol-1 as compared to the gas-phase process. Contrary to the exothermic O-acylation, the gas-phase transfer reaction (step 2) was exothermic in both cases, but more favorable for Gly. The cyclocondensation of N-acylated DKPs into BCAs (steps 3 and 4) is endothermic for Gly and exothermic for AIB. © 2006 American Chemical Society.
Filiaciones:
Contreras-Torres F.F.:
Instituto de Ciencias Nucleares, Universidad National Autónoma de México, Circuito Exterior C. U., 04510 México D.F, Mexico
Basiuk V.A.:
Instituto de Ciencias Nucleares, Universidad National Autónoma de México, Circuito Exterior C. U., 04510 México D.F, Mexico
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