Base-induced conversion of 5-aminothiazolium salts into substituted pyrroles and pyrrolines via 1,3-dipolar cycloadditions with electron-deficient alkynes and alkenes

Abstract

A series of mesoionic thiazoles 2 were generated and converted in situ by treating the 5-aminothiazolium chlorides 1 with several electrophilic dipolarophiles in the presence of DBN or NEt 3. The reactions involved an 1,3-dipolar cycloaddition of the “masked” cyclic azomethine ylides across the olefinic or acetylenic π-bond, yielding unstable N-bridged adducts as the first step. Thus, DMAD and methyl propiolate gave functionalized pyrroles via a subsequent extrusion of isothiocyanate. Dimethyl fumarate, maleate and fumaronitrile afforded a variety of polysubstituted pyrrolines, pyrroles and aminobutadienes through base-catalyzed rearrangements of the primary 1 : 1 cycloadducts, with retention of the elements of isothiocyanate. A similar initial cycloadduct 17 was isolated from N-phenylmaleimide which generally led to the condensed imino thiopyrans 20. Some of the 1,3-dipolar reactional stereoselectivities were deduced from the stereochemistry or distribution of the final products and found to be markedly dependent on the substituent groups present.