Oxazolium salts reaction with

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 course that is taken in reactions of 1,3-oxazolium salts (179) with stabilized carbanions is strongly dependent on the nature of the substituents on the carbanion in some cases, attack at the 2-position leads to 1,4-oxazepines, (180). ... 

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>. 

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>. 

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. 

The oxazolium salt (160), formed from the oxazole (159) by reaction with methyl triflate in acetonitrile, may be reductively ring opened by treatment with phenylsilane in the presence of cesium fluoride to give the azomethine ylide (161) (presumably this species is in tautomeric equilibrium with the corresponding oxazoline). The azomethine ylide can be trapped as an adduct with a suitable dipolarophile, such as dimethyl acetylenedicarboxylate (DMAD). In the case of this reagent the adduct (162) can be ox-... 

Reaction of A, A -dialkyl dichloromethaniminium chlorides 1501 with 2-aminoacetophenones 1502 provides a general route to the formation of 5-aryl substituted 2-(dialkylamino)-l,3-oxazolium salts 1503 these can also be prepared from the corresponding 5-aryl substituted 2-(dialkylamino)-l,3-oxazoles by alkylation with (Me0)2S02. Treatment of 1503 with NH4OAC gives Tsubstituted 4-aryl-2-(dialkylamino)-177-imidazoles 1504 (Scheme 388) <1999HCA1981>. 

Tris(trimethylsilyl)phosphine (Me3Si)3P readily reacts with pyrylium salts, giving substituted phosphabenzenes12, and the reaction can be conveniently extended to include the preparation from 3-azapyrylium tetrafluoroborate of 1,3-azaphosphorines which, with acetylene carboxylates, gives the phosphabenzene with nitrile elimination (equation 6)13. Both 1,3,4-oxadiazolium and oxazolium salts react similarly to form the five-membered azaphospholes (equation 7)14. 

Cycloaddition of 1050 with DMAD gave the tetrasubstituted bispyrrole 1058. In this case, loss of 2 mol HCIO4 from the bis-oxazolium salt was presumed to generate a mesoionic species (not shown) that underwent cycloaddition with DMAD, followed by a retro Diels-Alder reaction and loss of 2 mol of HNCO to yield 1058. 

The authors found that phenylsilane/anhydrous cesium fluoride was the preferred reagent combination for this reaction sequence. Other silanes were effective, whereas tributyltin hydride afforded low yields (< 20%) and only in the presence of a fluoride source. Tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) and tetrabutylammonium fluoride (TBAF) destroyed the silane. Acetonitrile was the best solvent among the ethers and halogenated solvents evaluated." In a subsequent study " the authors converted a 2,5-disubstimted oxazole 15 (Rl = R3 = CH3, CeHs, or OC2H5, R2 = H) to the oxazolium salt followed by treatment with trimethylsilylcyanide (Scheme 1.290). The resulting intermediate 2-cyano-4-oxazoline 1107 spontaneously ring opened to the stabilized azomethine... 

Wenkert and co-workers prepared a series of 5-alkenyl-substituted oxazolium salts and investigated their intramolecular Diels-Alder reactions as a means to construct alkaloids (Scheme 1.427). The starting oxazoles 1662a-c, 1663, and 1664 were prepared from an acyclic methyl ester and lithiomethyl isocyanide. Methylation of 1662a-c, 1663, and 1664 with methyl trillate then afforded the corresponding iV-methyloxazolium salt quantitatively. The oxazolium salts 1665, 1666, and 1667 were stable for up to 3 h at 90°C. 

Oxazolium salts (11) are known to react with stabilized carbanions, nitrogen, oxygen, sulfur, and halogen at both the C-2 and C-5 positions. Reaction at the C-2 site results in the formation of... 

Mechanistically, the authors proposed that this reaction most probably proceeds by cyclodehydration of 499 with TFAA to afford a mesoionic 1,3-oxazolium-5-olate 505. Trilluoroacetylation of 505 yields 506, which is ring opened with trifluoroacetate to produce 507 from which the enol trifluoroacetate 508 is produced by decarboxylation (Scheme 1.137). Cyclization of508 gives the IV-alkyloxazolium salt 509 that is dealkylated by trifluoroactate to furnish 500. This dealkylation is consistent with the observation that 499 (Rj = Bn) are better substrates than 499... 

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