Thiazoliums 3 - -, ring opening with

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>. 

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... 

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

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. 

Various azole ylides react with alkynes or alkenes with subsequent ring opening and recyclization. Thus pyrazolium ylide (348) gives compound (349) (Equation (188)) <93JCS(P1)883>, imidazolium ylide (350) gives (351) (Equation (189)) <91H(32)2217>, and thiazolium ylide (352) gives (353) (Equation (190)) <76JOC187>. 

The thiazolium salts (99 R = Me or PhCH2) undergo ring-opening and -contraction to the methylenethietans (100) on treatment with aqueous base. Electrochemical reduction of (101) gives the radical anion of the isomeric compound (102). ... 

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