Pyridines thiazolo

There are six different ways of constructing fused oxazolo- or thiazolo-pyridines. These are the main groups into which such compounds have been classified (Chart 3). 

Among other condensed thiazole ring systems, imidazo[2,l-6]thiazoles (tetramisole is a member of this class), thiazolo-pyrimidines, and thiazolo-pyridines continue to attract the most interest. Attention is concentrated on synthesis and biological activity. Unexpectedly, dihydrothiazolo[3,2-a]pyridinium bromide is readily brominated when the pyridine ring has a 3-nitro-substituent but not when it has a 3-amino-substituent. 

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). 

Anhydro-3-hydroxy-2-phenylthiazolo[2,3-6]thiazolylium hydroxide (407) underwent ready thermal reaction with alkynic and alkenic dipolarophiles in refluxing toluene. With the former dipolarophile sulfur was lost from the intermediate 1 1 cycloadduct (408) to give the substituted 5H-thiazolo[3,2- i]pyridin-5-ones (409). With the latter, the intermediate (410) lost H2S, also forming (409). 

Thiazolo[5,4-6]pyridine, 2-methylthio- H NMR, 6, 674 (76JHC491) Thiazolo[3,2-a]pyridinium... 

Thiazolo[3,2-a]pyridine, dihydro-nucleophilic reactions, 6, 701-702 synthesis, 6, 704... 

Work on the thiazolopyridines has been centered on the 2-methyl derivatives and their conversion into cyanine dyes. Takahashi et al. prepared a number of substituted thiazolo[5,4-6]pyridines (109 Y =.MeO,i6 EtO, Cl,i6 and EtgNi ), all of which form methiodides (110) as evidenced by the high reactivity of the 2-methyl group. A parallel will be seen between these compounds and the l,4-diaza-3fi -indenes. Kiprianov reports the interesting observation that the etho-toluene- -sulfonate of 109 (Y = H) does not contain a reactive methyl group presumably when Y is H the steric and electronic... 

Methyl-thiazolo[4,5-/]quinoline 19 was methylated by methyl iodide on the nitrogen atom of pyridine giving the appropriate methodide. A subsequent oxidation with potassium hexacyanoferrate in alkaline media gave the 2,6-dimethyl-7-0x0-6,7-dihydrothiazolo[4,5-/]quinoline 21 (37LA60). 

The reaction of 6-amino[l,2,4]triazin-5(2//)-ones or its thione derivatives 680 with acetic anhydride gave the 6-acetamido derivatives 681 and 682, respectively. Treatment with phosphorus pentasulfide in pyridine gave thiazolo[5,4-e][l,2,4]triazines 683 (84LA283 87AJC491) (Scheme 142). 

Carboxymethylthioquinoxaline (55) gave thiazolo[3,2-a]quinoxalin-10-ium-l-olate (55a) (AC2O, pyridine, 0°C, 2 h 63% analogs likewise). " ... 

A microwave-assisted variant of the Rosenmund von Braun reaction has also been developed (Scheme 73) [83]. DMF at 200 °C proved not very useful for the cyanodehalogenation of methyl (3R)-6-bromo-5-oxo-2,3-dihydro-5ff-[l,3]thiazolo[3,2-a]pyridine-3-carboxylates since only low yields of the corresponding nitrile were obtained, and there were substantial amoimts of unconverted starting material. Extending the reaction time from 10 to 20 min gave more desired reaction product but also significant amoimts of... 

A novel series of 5//-thiazolo[3,2-a]pyridine-5-ones 64 is prepared by addition of malonic esters 59 to 2-alkynylthiazoles 60 <06H(67)523>. A plausible reaction mechanism involves an intramolecular cyclization of the initial adduct 61 to give the cyclobutenoxide intermediate 62. Ring-opening of this intermediate and subsequent cyclization lead to 5//-thiazolo[3,2-a]pyridine-5-one 64. 

A useful method for the preparation of functionalised thiazoles has been described. Palladium catalysed cross coupling reactions between 4-thiazolyl-5-acetyl triflates 36 and alkynes afforded 4-alkynyl-5-acetylthiazoles 37 in good yields (56-82%). If 37 is then treated with ammonia in methanol, thiazolo[5,4-c]pyridines 39 are formed, probably via the intermediate imine 38 which then undergos a regioselective 6-endo dig cyclisation <99EJOC3117>. 

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