Identification of hexahydroimidazo[1,2- a]pyrazine-3,6-diones and hexahydroimidazo[1,2- a]imidazo[1,2- d]pyrazine-3,8-diones, unusual products of silica-catalyzed amino acid thermal condensation and products of their thermal decomposition using coupled high-performance liquid chromatography–particle beam mass spectrometry and gas chromatography–Fourier transform infrared spectroscopy–mass spectrometry

Abstract

Sublimation of simple aliphatic amino acids (Ala, Aib, Val and Leu) at 230–250°C under reduced pressure in the presence of silica as a catalyst yields cyclic dipeptides piperazine-2,5-diones as major products. In addition, two types of unusual products have been detected. To determine their structures, we utilized a coupled HPLC–particle beam-MS and GC–Fourier transform IR–MS technique with auxiliary computer simulation of IR spectra. Based on the spectral data obtained, we identified the condensation products as substituted bicyclic and tricyclic amidines, hexahydroimidazo[1,2- a]pyrazine-3,6-diones and hexahydroimidazo[1,2- a]imidazo-[1,2- d]pyrazine-3,8-diones. Other peaks in the chromatograms has been identified as products of the amidines thermal decomposition. The general decomposition pattern includes dehydrogenation as well as cleavage of the carbon skeleton. The last process primarily affects the 6-membered pyrazine ring causing elimination of CO or HNCO, or/and the loss of α-substituents without or with α-carbon atom.