Acylating of Pyrrole and N-Methylpyrrole with 1,3-Benzoxathiolium Tetrafluoroborates. A High-Yield Method for the Synthesis of Diacylpyrroles

2-Substituted 1,3-benzoxathiolium tetrafluoroborates (I) were used as masked acylating agents for pyrrole and N-methylpyrrole. The reactions on pyrrole (11) were regiospecific, and according to the molar ratio of the reagents (1:II = 1:3 or 2.5-3:1), 2-acylpyrroles were obtained in moderate to good yields (3842%) and 2,5-diacylpyrroles were obtained in excellent yields (in most cases, quantitative). The reactions on N-methylpyrrole (111) were not regioselective, and both a- and 6-positions were attacked. So, depending on the molar ratio of the reagents (HI1 = 1:3 or 2.5-3:1), 2- and 3-acyl-N-methylpyrroles (9-51 % and 27-68% yields, respectively) and 2,4- and 2,5-diacyl-N-methylpyrroles(6 0-93% and 17-40%, respectively) were obtained. A very interesting feature of the new method is the possibility of introducing two identical or different acyl groups in the pyrrole ring under mild conditions. 'H and I3C NMR spectra of all the new compounds and IR spectra, recorded in the gas phase, of 2- and 3-acylpyrroles and of 2,4- and 2,5-diacylpyrroles are reported.