IDENTIFICATION OF HEXAHYDROIMIDAZO[1,2-A]PYRAZINE-3,6-DIONES AND ROIMIDAZO[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

Sublimation of simple aliphatic amino acids (Ala, Aib, Val and Leu) at 230-250 degrees C under reduced pressure in the presence of silica as a catalyst yields cyclic dipeptides piperazine-2,5-diones as major pr oducts. In addition, two types of unusual products have been detected. To determine their structures, we utilized a coupled HPLC-particle be am-MS and GC-Fourier transform IR-MS technique with auxiliary computer simulation of IR spectra. Based on the spectral data obtained, we ide ntified the condensation products as substituted bicyclic and tricycli c amidines, hexahydroimidazo[1,2-a]pyrazine-3,6-diones and oimidazo[1, 2-a]imidazo-[1,2-d]pyrazine-3,8-diones. Other peaks in the chromatogra ms ha been identified as products of the amidines thermal decompositio n. The general decomposition pattern includes dehydrogenation as well as cleavage of the: carbon skeleton. The last process primarily affect s the 6-membered pyrazine ring causing elimination of CO or HNCO, or/a nd the loss of alpha-substituents without or with alpha-carbon atom. ( C) 1997 Elsevier Science B.V.