Thiazolium salts, ring opening

Neat exploitations of this process, include the synthesis of medium-sized heterocycles as products of a three stage sequence involving addition of hydroxide to w-iodoalkyl thiazolium salts, ring-opening and then reclosure by intramolecular S-alkylation, illustrated below for the formation of an eight-membered ring, " and the introduction of sulfur to the imidazole 2-position using phenyl chlorothionofor-mate. ... 

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). 

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). 

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

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>. 

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 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 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. 

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). ... 

Dimoization.—Quaternary thiazolium salts are ring-opened in aqueous, but dimerized in anhydrous, alkaline media. Preliminary results (Scheme 7) show that anhydro-bases of iV-phenacyl-4-methylthiazolium salts (113) are dimerized by methanolic sodium methoxide to yellow labile products that... 

Y 77%.—Unexpectedly, not the reactive 2-position but the negatively charged S-atom in the opened nucleus is attacked. Thiazolium salts may undergo ring opening in alkaline soln. F. e. 

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