Pyridinium salts, rearrangement

Compounds containing reactive halogens (ArCH= CHCHjX or ArCOCHjX) readily form pyridinium salts. Rearrangement of these prod-... 

Other heterocycles which rearrange to isoxazoles are pyridazine 1,2-dioxides (77CC856) and pyridinium salts (80CPB2083), although these transformations are of little synthetic importance. 

Pyridinium salts, l-aryl-4-methoxy-2,6-dimethyl-synthesis, 3, 762 Pyridinium salts, N-aryloxy-rearrangements, 2, 354 Pyridinium salts, 1-benzyl-covalent amination, 2, 239 Pyridinium salts, N-benzyl-reactions... 

By analogy with the conversion of 1,2-dialkylpyridinium salts into 2-alkylaminopyridines known as the Kost-Sagitullin rearrangement, the pyridinium salt 252 was treated with two different hydrazides (Equation 33). The [l,2,4]triazolo[4,3-tf]-pyridines 253 with Y = 4-pyridyl and NH2 were obtained with 35% and 38% yields, respectively <2003CHE275>. 

A special rearrangement in the 1,2,4-thiadiazole series concerns the 2-phenylamino-l,2,4-thiadiazolo [2,3,-a]pyridinium salts (275 R = H, Me), which are obtained by oxidation of N-(2-pyridyl)thioureas (Scheme 45). In the case of the unsubstituted derivative (275 R = H X = Br), neutralization with sodium acetate in ethanol produces directly benzothiazole 277... 

Azide ion will catalyze rearrangements of N- (aryloxy)pyridinium salts to give 3-(o-hydroxyphenyl)pyridines (Scheme 114). It is suggested that azide addition to the a-position... 

Diazinium salts resemble pyridinium salts in their behavior. They form pseudo-bases with hydroxide ions which can disproportionate (e.g. 2-methylphthalazinium ion (199) — 2-methylphthalaz-l-one + 2-methyl-l,2-dihydrophthalazine) or undergo ring fission (e.g. 3-methylquinazolinium ion — (200). Aqueous acid converts (201) into (202), presumably by attack of a water molecule on a protonated species with subsequent intramolecular oxidative-reductive rearrangement of an intermediate carbinol base (201a) as shown. 

Base-catalyzed rearrangement of Af-(aryloxy)pyridinium salts (11) leads to 2-arylpyridines (12) via intramolecular attack on the ortho position of the aryloxy ring by the 2-pyridyl carbanion. When X was 3-CChMe, pyrido[3,2-J]coumarins (13 R = NO2, CN) were directly obtained in useful yields from this reaction. When in (11) X was 3-COMe and R was 4-NO2, 10-hydroxy- 10-methyl-6-nitropyrido[2,3-J]benzopyran (14) was obtained in 10.8% yield.89... 

Highly substituted pyridinium salts of type 1 are easily accessible by Hantzsch-type synthesis. They are valuable products in that they can often be transformed easily into otherwise difficultly accessible, highly substituted benzenes or pyridines by base-catalysed rearrangement. For example, treatment of the salt 2 with ethanolic sodium hydroxide at room temperature for one hour gives 2,4-diacetyl-lV,5-dimethylaniline 3 in 85% yield, while 3-acetyl-5-cyano-6-methyl-2-methylamino-4-phenylpyridine 5 is obtained in 88% yield from the salt 4 under the same conditions. 

An enamine rearrangement has been proposed for the formation of indole rings from pyridinium salt (51) (Scheme 35) <95IJC(B)(34)285>. 

CSj, with subsequent alkylation with methyl iodide, led to (i )-l-ferrocenylethyl-5-methyltrithiocarbonate, which involves a rearrangement with retention (Fig. 4-17, top) [100]. Sulfides [101], tertiary phosphines [101], and tertiary amines [102] are also reasonably good nucleophiles and form reactive ionic products, e.g., pyridinium salts [103], but this has only been verified for achiral or racemic substrates. Pyridinium salts may be considered as a storage form of a-ferrocenylalkyl carbocations, and show almost the same behaviour towards nucleophiles [103, 104]. Primary and... 

The reduction of l-methyl-4-cyanopyridinium iodide (72) in aqueous methanol gave solely the tetrahydropyridine 73. However, in methanol/ sodium hydroxide two different temperature-dependent products could be isolated. The [4 + 2] product 74 predominated above — 20°C, whereas the [2 -I- 2] adduct75 was the sole product at or below —45° C. Similar behavior is observed with the 2-cyano derivative 76 (R = H) again, the initially formed [2 + 2] adduct 79 (R = H) thermally rearranges to the [4 -I- 2] product 80 (R = H). In this case pH and temperature control are not as important because enamine reactivity is diminished by the presence of the cyano group. Other pyridinium salts behave similarly in strong base. Reduction... 

The recyclizations of the pyridinium salts 236 to form the aniline derivatives 238, called the Kost—Sagitullin rearrangement, occur in the presence of bases via ring-opened intermediate 237338,339 (equation 95). 

---