BASE-INDUCED CONVERSION OF 5-AMINOTHIAZOLIUM SALTS INTO SUBSTITUTED PYRROLES AND PYRROLINES VIA 1,3-DIPOLAR CYCLOADDITIONS WITH ELECTON-DEFICIENT ALKYNES AND ALKENES

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 ate olefinic or acetylenic pi-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.