Oxazolium salt

Thiazolo[2,3-6]oxazolium salts, 2,3-dihydro-synthesis, 6, 992 Thiazolo[2,3-6]pteridines structure, 3, 284 Thiazolo[3,2-a]pteridines structure, 3, 284 Thiazolopurines, 5, 566... 

Benzoxazoles are produced in high yield from a-acylphenol oximes by a Beckmann rearrangement using zeolite catalysts <95SC3315>. The reaction of the o-benzoquinone 40 with aromatic aldehyde oximes produces the benzoxazoles 41 <95ZOR1060>. The fused oxazolium salts 43 (R = Me, Et, Pr , or Ph R2 = Me or Pr ) are formed from tropone and nitrilium hexachloroantimonates 42 <96JPR598>. 

Vedejs developed an enantiocontrolled synthesis of aziridinomitosenes involving internal alkylation of the oxazole 132 to produce an oxazolium salt 133 followed by nucleophilic addition of cyanide providing the adduct 134 <00JA5401>. Electrocyclic ring opening of 134 to the azomethine ylide 135 with internal [2+3] trapping produces the tetracyclic product 137 via the pyrroline 136. 

The photocycloaddition of aliphatic and aromatic aldehydes with 2,4,5-trimethyloxazole (131) gave bicyclic oxetanes 132 in almost quantitative yields hydrolitic cleavage led selectively to erytro a-amino-P-hydroxy methyl ketones 133 <00CC589>. The oxazolium salt 134 was converted to the azomethine ylide 136 via electrocyclic ring opening of the oxazoline 135. Intramolecular cycloaddition afforded 137 in 66% overall yield which was transformed into the aziridinomitosene derivative 138 . 

Azomethine ylides. Reduction of oxazolium salts (1) with phenylsilane and cesium fluoride provides an unstable 4-oxazoline (2), which can react as an azomethine ylide with a dipolarophile such as DM AD to give a dihydropyrrole (3). ... 

Aminodihydrodiazepinium salts have been prepared by the reaction of ethylenediamine with oxazolium salts (76GP25125I0). 

The reactions of nitrilium salts with nitrones usually give tars. However, pyridine-A -oxides afford oxazolium salts, which rearrange in solution to more stable amides (Scheme 105) <71TL1947,75JOC4l>. 

A series of publications by Vedejs et al. (17-19) outlined the use of oxazolines in the generation of azomethine ylides. Reduction of the oxazolium salts 71 (R = Ph or Me) with PhaSiH/CsF led to formation of the unstable 4-oxazolines (72) and their valence bond azomethine ylide tautomers (17), which could be trapped in situ with DM AD to afford the bicyclic adducts 73, where R and R = Ph or Me, in 75% yield via [2 + 2] cycloaddition (Scheme 3.18). 

This French group (145) has also been able to divert the usual Diels-Alder cycloaddition pathway of Reissert salts with oleftnic esters to a 1,3-dipolar cycloaddition pathway by the addition of triethylamine. In addition, mtinchnone imine 243 can be trapped with dipolarophiles to furnish 244 (146). No Diels-Alder cycloadducts derived from the oxazolium salt were detected. In contrast, fumarate... 

The thiazolium salt (399) undergoes base-induced ring expansion via (400) and (401) to give (402) (80TL2429), a direct parallel of the conversion of analogous oxazolium salts into 1,5-benzoxazepines. 

Reaction of the oxazolium salts (591) with methylhydrazine led to the isolation of 6-hydroxytetrahydro-l,2,4-triazines (592) (74ZOR2429), while the oxazolium or thiazolium salts (593 X = 0,S) with hydrazine gave dihydro-1,2,4-triazines (594) (78HC(33)189, p.576). 2-Amino-3-phenacyloxadiazolium salts (595) and hydrazine afforded the 3,4-diamino compounds (596) (71LA(749)125>. 

The reactivity of these methyl groups is normally enhanced by nitrogen atom quaternization. Benzotroponopyridinium salt 505 with two molecules of azulene aldehyde condenses to afford blue-black bis(azulenylvinyl) dye 506 (Amax 536 nm 62ZC369). Furthermore, oxazolium salt 507 condenses with activated benzothiazolium salt 508 (Scheme 134) to form unsymmetri-cal trimethine cyanine 509 (63UP1). (Dyes 512a,b arise from methylthio exchange Section IV,A,5,e.)... 

Oxazolium salts are readily attacked by nucleophilic reagents at position 2 (c/. 102) ring fission usually ensues and may be followed by recyclization, as in the transformation into imidazoles (see Section 4.18.3.1.2(iv)). JV-Alkyl-2-chlorobenzoxazolium salts (103), because of the lability of the chlorine atom, are useful for effecting various condensation and dehydration reactions (see Section 4.18.3.1.2(iv).). 

Oxazolium salts are easily attacked by nucleophiles. The hydrolysis of oxazoles and benzoxazoles by hydrochloric acid has long been known. Triphenyloxazole gives benzoic acid, benzaldehyde and ammonium chloride 2-methylbenzoxazole affords o-acetamido-phenol, and 5-ethoxy-2-pentyloxazole is rapidly converted into the acylamino ester (132) by dilute hydrochloric acid at room temperature (Scheme 4). 

Quaternary oxazolium salts are easily cleaved by alkali iV-methylbenzoxazolium iodide affords iV-methyl-2-formamidophenol (135). iV-Methyloxazolium fluorosulfonates react with ethanolic ammonia to give imidazoles with sodium hydrogen sulfide, thiazolium salts are formed. Hydrolysis or aminolysis of 2-amino-lV-phenacyloxazolium salts (136) is followed by cyclization to yield imidazolones (137) or imidazoimidazoles (138), respectively (Scheme 5). 

The oxazoliumcarboxylic acid (147) is easily decarboxylated via the ylide (148) the neutral compound (149) is much more stable due to the low equilibrium concentration of the zwitterionic tautomer (150 Scheme 7). Oxazolium salts lacking substituents at the 2-position react with dialkyl acylphosphonates in the presence of triethylamine to give mixtures of l,4-oxazin-3-ones and 2-azetidinones the reaction (see Scheme 8) proceeds by electrophilic attack of the phosphonate on an oxazolium ylide, e.g. (151), followed by insertion of oxygen into the carbon-phosphorus bond, ring-opening, and formation of the enolate anion (152) which can cyclize in two alternative ways with expulsion of the phosphonate group. 

Nucleophilic substitution in the azine ring is a normal reaction. The fused oxazole ring, however, is susceptible to nucleophilic ring opening reactions, especially in oxazolium salts and dihydro derivatives. When fused to pyrimidine, substituents in the pyrimidine ring may promote a Dimroth rearrangement of the latter. 

The oxazole ring possesses less aromatic stabilization than a thiazole ring and is readily opened in many of its fused derivatives, especially in oxazolium salts. In the pyridazinium derivative (191) the oxazole ring is opened by oxygen, sulfur or carbon nucleophilic attack at the C-8a ring junction (77YZ422). In the mesoionic pyridine (192) an amine attacks at C-2, which is a pseudocarbonyl carbon atom (70JCS(C)1485). 

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