The formylation of pyrrolo [1,2-a]pyrazines containing alkyl, aryl, or hetaryl substituents in positions 1 and 6 of the heterocycle has been studied. It has been shown that formylation of 1-phenyl-and 1-(2 thienyl)pyrrolo [1,2-a]pyrazine occurs selectively at the α-position of the pyrrole ring. In all of the remaining examples the reaction course depends on substituent, reagent ratio, and reaction time.The broad spectrum of biological activity of heteroaromatic polycyclic systems with a common bridging nitrogen atom and containing a pyrrole ring is the basis for the continuing interest in the synthesis of this class of compounds. One such structure is the aromatic pyrrolo[1,2-a]pyrazine bicyclic system which contains both a π-electron-rich pyrrole ring and a π-electron-deficient pyrazine ring.Within the scope of a systematic investigation of electrophilic reactions in such a series of compounds we have studied the formylation of 1-and 6-alkyl-, aryl-, and hetaryl-substituted pyrrolo[1,2-a]pyrazines 1a-g.It is known that unsubstituted pyrrolo[1,2-a]pyrazine is formylated at the β'-position of the pyrrole ring in 60% yield [1]. However, the 1 H NMR spectrum of this compound reported by the authors casts doubt upon the correctness of this structural identification. The low field shift of the H-4 proton signal (9.38 ppm) points to α-rather than β'-formylation.In our laboratory we have also previously studied the formylation of the 3,4-dihydro analogs of pyrrolo[1,2-a]pyrazines. It was found that the reaction occurs nonspecifically and the result is governed by the structure of the starting substrates [2].We have found that in the case of 1-substituted aromatic pyrrolo[1,2-a]pyrazines only 1-phenylpyrrolo[1,2-a]pyrazine (1a) and 1-(2-thienyl)pyrrolo[1,2-a]pyrazine (1b) are selectively formylated to give the 6-formyl derivatives 2a and 2b respectively.The low field shift of the pyrazine proton signal at position 4 in the formylated products when compared with the starting 1-phenyl-and 1-(2-thienyl)pyrrolo[1,2-a]pyrazines (9.44 and 9.39 ppm compared with 7.75 and 7.64 ppm respectively) indicates that the formyl group occurs at position 6 of the heterocycles. N R N N R N H O POCl 3 1a,b 2a,b DMF 0-20°C 1, 2 a R = Ph, b R = 2-thienyl __________________________________________________________________________________________