Thiazolium ring opening

The kinetics of proton transfer from the C-2a position of 2-(l-methoxybenzyl)thiazolium salts was studied for the p-H and /)-NMeJ derivatives by H NMR spectroscopy. The study was carried out by mixing the salts rapidly with sodium hydroxide in a stopped-flow instmment and monitoring the progress of enamine formation and decomposition in the visible region of the spectmm. Under these conditions the thiazolium ring opened, the H NMR spectrum of the product being consistent with both as- and /ra r-stereochemistry about the newly formed enamine bond (Scheme 23) <1997JA2356>. 

The intermediate is thus a potent nucleophile and adds to carbonyl compounds to form carbon-carbon bonds. The medium has to be basic enough to ensure significant amount of the ylid, but not too basic or significant thiazolium ring opening will occur. 

Certain bifunctional nucleophiles allow cyclization after ring opening. The formation of 2-thiazolium salts (71JHC40S) and the analogous production of 2-amino-2-thiazolines (191) from aziridines and thiocyanic acid fall into this category (72JOC4401). 

Equilibrium studies. Acidities of thiazolium cations, such as 213, cannot be measured in basic aqueous solution because the hydroxyl adduct ( pseudobase ) 216 of the thiazolium cation is formed rapidly and is subject to base-catalyzed ring opening (Scheme 36).151 In DMSO, formation of the carbene dimer 217 from 213 and 214 is a complicating factor.8 If indicator anion (In") was added to a solution of 213, a very rapid drop in absorbance was followed by a somewhat... 

Robert and co-workers (239,240) discovered novel conversions of 2-amino-1,3-dithiolium-4-olates (348) into other mesoionic heterocycles. For example, reaction of 348 with carbon disulfide, phenyl isocyanate, or phenyl isothiocyanate affords l,3-dithiolium-4-thiolates (349), l,3-thiazolium-4-olates (350), and 1,3-thiazolium-4-thiolates (351), respectively. Some of these reactions proceed via the ring-opened ketene tautomer of 348 (240). 

Certain AMmines formed by deprotonation of N-aminoazolium salts also failed to undergo cycloaddition to give azapentalenes. TV-Amino-thiazolium mesylate on treatment with potassium carbonate and DMAD added 2 moles of dipolarophile to give the fused diazepine 219 2°5 4-Amino-l-methyl-s-triazolium iodide and DMAD in the presence of base gave the pyrazole 221, probably by ring opening of the first-formed adduct 220. Dehydrogenation to 222 did not occur.297... 

Oxidation to an azolone is an expected reaction for a pseudo base, but little appears to be known of such reactions. Most commonly, pseudo bases suffer ring fission. Estimated rates of ring opening of 246 are in the ratio 109 104 5 1 for X = 0, S, and NMe, respectively. Thiazolium salts 247 consume two equivalents of OH on titration because the pseudo bases 248 lose a proton to give 249, which then form anions 250. Quaternized oxazoles 251 are readily attacked by hydroxide to give open-chain products such as 252, and quaternized 1,3,4-oxadiazoles behave similarly. Quaternary isothiazoles (e.g., 253) are cleaved by hydroxide, as are 1,2,4-thiadiazolium salts (254 255). 

Basic aqueous solutions of a thiazolium cation contain the ring-opened thiolate anion 74 derived from the pseudobase 72. The observation that the uptake of two equivalents of base for the formation of the thiolate anion... 

Fig. 60. Aminomethylation with ring opening of thiazolium salts.

ROMPgel-supported thiazolium salts <2004OL3377> (ROMPgel = ring opening metathesis polymerization gel) and the effect of pressure on Stetter reactions in the synthesis of hindered aliphatic acyloins and 7-ketonitriles <2004HPR233> were also investigated. 

The reduction of thiamine hydrochloride in neutral buffer is reported to involve a cleavage between the pyrimidine ring and the thiazolium ring and a ring opening of the latter further chemical steps lead to a substituted thiourea [285]. 

The photochemical desulfurization of 4-amino-l,3 tWazolium salts 224 or mesoionic 2,3,5-triaryl-l,3-thiazolium-4-olates 228 in the presence of trialkylphosphite or trialkylphosphine, affording ring-opened cinnamic acid derivatives 226 [88JCS(P1)189] and 230 (83JHC245) or quinolinones 231 [88JCS(PI)189] is assumed to proceed via intermediate azetine derivatives 225 and 229, respectively (Scheme 62). 

The reaction between JV-methylthiazolium iodide or tetrafluoroborate with l,l-bis(diphenyl-phosphino)ferrocene-> -ethene-platinum (57) leads to the formation of a ring expanded adduct (58) by formal C—S insertion <94JA5180>. A mechanism for the ring-opening process based on an initial nucleophilic attack on C-2 of the thiazolium ring is shown in Scheme 9. 

The thiazolium-mediated three-component reaction of thiazolium salts 201, aryl aldehydes and dimethyl acetylenedicarboxylate provides a facile synthesis of 2-amino-2-arylfurans 202 <05OL1343>. The reaction pathway may involve the sequential nucleophilic addition of thiazol-2-ylidene 203 with the aldehyde and DMAD to form the spirocyclic intermediate 204 through the simultaneous formation of two C-C bonds and a C-O bond Selective ring opening of the spirocyclic intermediate 204 followed by hydrolysis leads to 3-aminofuran 202 via 205. 

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