Thiazolo thiazolium salts

The thiazole-2-thione (51) with an a-halo ketone gives the intermediate (52) which is cyclized by strong acid into the thiazolo[2,3-/ thiazolium salt (53) (77HC(30-i)l). A wide variety of [5,5]-fused systems are prepared in this way. 

Carbene structure, of thiazolium salts in basic medium, 31.34 Carbocation, in thiazolium salts, 32 Carbocyanines. see Trimethine thiazolo-cyanines... 

Many syntheses of imidazo-fused systems involve cyclization of amino-substituted heterocycles (or their tautomers), suitably substituted at an adjacent ring position. This provides a convenient route to imidazo[2,l-A]thiazoles, imidazo [l,2- ]imidazoles (Scheme 22), imidazo[l,2-A]pyrazoles, imidazo[l,5- ]imidazoles, and imidazo[l,5-c]thiazoles. Similarly, the thiazolium salt 33 easily undergoes the cyclization resulting in thiazolo[3,2-Mbenzimidazoles 34 after a subsequent basic treatment (Scheme 23) <2005MOL327, CHEC-III(11.04.9.1.1)152>. 

The usual precursor to the thiazolo-fused systems is a cyclic thioamide or its tautomeric form . A convenient approach to the synthesis of bicyclic [5-5] systems, e.g., the tricyclic thiazolium salt 46 and 2,3-dihydroimidazo[2,l-A]thiazol-5-one 48, involves the N,S-dialkylation of appropriate precursors 45 and 47 with 1,2-dihaloalkanes (Scheme 31) <1999JMC2828, 2002EJC777>. 

The thiazolo[2,3-6]oxazole (104) has also been prepared by a base-catalyzed type A cyclization <85CC454>. In this case, the cyclization of thiazolium salt (102) is believed to occur by intramolecular nucleophilic attack of a hydrate dianion intermediate (103) on the positively charged ring. However, other workers have cast doubt on the structure of the product from this reaction claiming, on the basis of x-ray analysis, that it exists as (105) in the solid phase <85CC1113>. 

Nucleophilic Substitution. Several nucleophilic replacements in benzo-thiazolium salts (82) and (83) have been reported. The reactions (see Scheme 4) were studied chiefly for the purpose of comparison with those of the analogous thiazolo[2,3-Z ]benzothiazoles (see p. 702). The results agree with the general experience that quaternization of benzothiazole enhances its susceptibility towards nucleophilic reagents. ... 

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