Studies in the chemistry of pyrrolo (2, 1-b) thiazole

Abstract

A number of synthetic approaches to the pyrrolo [2.1-b] this sole system have been investigated. Two useful preparative methods have been developed. The first is based on the cyclisation of the quaternary salts from 2-methyl- or 2-methylene- thiatoles and o-haloketones using sodium acetate in acetic snhydride and leads to a variety of 6-alkyl and 6-aryl pyrrolo [2,1-b] thiszoles. The second method involves the thermal cyclisation of the product from 2'-thiaxolyl lithium and epichlorohydrin and affords the parent base, although in low yield. An investigation of the properties of the pyrrolo [2,1-b] thiazole system has been carried out. The ready formation of trint trobenzene complexes of the alkyl pyrrolo [2,1-b] thiazoles together with the evidence afforded by ultra-violet and nuclear magnetic resonance spectroscopy, is indicative of the aromatic nature of the system. Pyrrolo [2,1-b] thiezoles readily from salts with picric and perchloric acid. Nuclear magnetic resonance studies have shown that this process, the simplest form of electrophilic attack, involves the 5-position of the nucleus. A brief examination of the electrophilic substitution reactions of 6-methylpyrrolo [2,1-b] thiazole has shown that substitution takes place extremely readily and that the first substituent group enters at position 5 the second at position 7. An examination of the properties of groups attached to the nucleus has been limited to methyl groups and it has been found in contrast to the analogous indolizine system a methyl group in the 3-position of pyrrolo [2,1-b] thiazole is not sufficiently acidic to allow proton abstraction by n butyl lithium. However methyl groups at the 5- and 7-positlon readily suffer abstraction of a hydride ion in analogy to the methyl indolizines. The results obtained, taken as a whole, indicate a lower degree of polarization of the ground state of pyrrolo [2,1-b] thiazole than exists in indolizine. Furthermore in the transition state of electrophilic substitution at positions 5 and 7 there appears to be a lesser degree of accommodation of the positive charge on the thiazole ring as compared with similar inter-mediates in the indolizine series.