1,3-Dipolar Cycloadditions to Oxidopyraziniums

Abstract

The dipolar cycloadditions of a range of dipolarophiles to 1,5dimethyl-a n d I-arylmethyl-5-methyl-3-oxidopyraziniums are described.This paper is dedicated to Professor Rolf Huisgen, in acknowledgement of his major contributions to the recognition and development of dipolar cycloaddition processes.We have previously reported' some dipolar cycloaddition reactions of 1,5-dimethyl-3oxidopyrazinium (la), prepared by the N-4-quaternisation of 6-methylpyrazin-2-one2 with iodomethane followed by N-deprotonation using triethylamine.The oxidopyrazinium was shown to react in the regiochemical sense implied by a resonance contributor (2), for example with methyl acrylate and acrylonitrile -only the regioisomers (3, R=Me02C, NC) were detected.Products from reactions with other electron-poor dipolarophiles, diethyl maleate, methyl propiolate, dimethyl acetylenedicarboxylate (DMAD) and maleimide, were also described.'In all of the examples studied the (presumed) initial adducts (4) were not isolatedtautomers (3) (Scheme 1) having an exocyclic methylene and therefore a complete (cyclic) secondary amide L Scheme 1