Thiazolo pyridinium salts

In the case of a pyridinium salt, an indirect proof of the correctness of this mechanism can be found in the presence of a heptamethine thiazolo-cyanine, together with monomethine. which results unambiguously from 19 by the known opening of the pyridine ring (Scheme 25) (52). 

Thiazolo[3,2-a]pyridinium salts, 3-carboxydihydro-optically active synthesis, 6, 705... 

Thiazolo[3,2-a]pyridinium salts, 3-carboxy-2,3-dihydro-8-hydroxy-8-methyl-desulfbrization, 6, 687-688 Thiazolo[3,2-a]pyridinium salts, 6(8)-nitro-2,3-dihydro-... 

A number of syntheses of substituted 2,3 -bipyridines are worthy of note. Tetracyclone heated at 215°C with nicotinonitrile affords 3,4,5,6-tetraphenyl-2,3 -bipyridine, whereas 3,4-di(2-pyridyl)pyridine is obtained by an oxidative degradation of the corresponding 6,7-disubstituted thiazolo[3,2-a]-pyridinium salt. Nicotinic acid on UV irradiation in aqueous solution at pH 4-6 gives 2,3 -bipyridine-5-carboxylic acid, whereas irradiation of picolinic acid in the same pH range in the absence of metal ions gives some 2,3 -bipyridine. 6,6 -Diphenyl-2,3 -bipyridine is thought to be formed from... 

Methylthiazolium salts (119) with a-dicarbonyl compounds give thiazolo[2,3-6]pyridinium salts... 

Heterocyclic cations will not easily react with electrophiles unless substituted by strongly electron-releasing groups. Alternatively, the reaction conditions are chosen in such a way that a reactive anhydro base or pseudobase intermediate is formed. Thus the thiazolo cation (336) can be nitrated at C-3 (78ZOR216) possibly via the ylide (337). The 6(8)-nitro-2,3-dihydrothiazolo[3,2-a]pyridinium salts (338) and (340) are readily brominated in hydroxylic solvents the regioselectivity and the ease of reaction are consistent with pseudobase intermediates (339) and (341) <81H(15)1349). 

Compounds with the general pyridazinium formula (525) and S-alkylated derivatives are claimed to have useful bactericidal and fungicidal properties (73JAP(K)7391092), whereas the thiazolo[3,2-tf]pyridinium salt (526) is reported to have hypoglycemic activity (78JMC489). 

Methylthiazolium salts 116 with 1,2-dicarbonyl compounds give thiazolo[2,3-7]pyridinium salts 117 (Scheme 75) <1986JHC1889>. 

Thiazolo[3,2-a]pyridines (CaNS-CjN).— The established synthesis of thia-zolo[3,2-a]pyridines from pyridine-2-thiones and a-bromo-a -unsaturated carbonyl compounds (these Reports, Vol. 1, p. 432 Vol. 2, p. 698) has been further studied. The use of the isomeric a-bromocrotonic acids yields the expected bicyclic products (202), exclusively of the 2,3-trans-configuration, owing to the concurrent isomerization of the cis- to the trans-acid. Activated acetylenes undergo a comparable Michael-addition-cyclization process. Thus, acetylenedicarboxylic acid affords trans-2,3-dicarboxydi-hydrothiazolo[3,2-a]pyridinium salts (204). The intermediate vinyl... 

Mesoionic systems may be readily substituted by electrophiles. Thus the thiazolo mesoion (342) will couple with diazonium salts despite their relatively weak electrophilicity (80KGS621). Substitution in a fused heteroaromatic betaine azine ring, e.g. (343), also takes place with ease. The resonance form (344) of the mesoion (343) shows that the electrophile will attack at C-6. The substitution in this position is also predicted by MO calculations (73JHC487). Similarly the pyridine ring in pyridinium olates is active towards electrophiles and is substituted in the positions ortho and para to the olate function. Bromination of the 5-methyl derivative (321 R = Me) occurs exclusively in the 7-position which is rationalized via the intermediate (345). In the absence of a 5-substituent, attack in either the 5- or 7-position occurs the dibromide is readily formed. No bromination in the thiazole ring is observed. The 2-bromo derivative (346) has been made, however, by condensation between the appropriate mercaptopyridine and 1,1,2,2-tetrabromoethane. 

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