1- Amino-2- pyridinium

Most of the synthetic approaches toward this ring system utilize N-amino pyridinium salts functionalized at the a-position with a carbonyl group. Thus, the amination of 2-(l,3-dioxolan-2-yl)pyridine with tosyl-hydroxylamine gave 78, whose reaction with urea in the presence of boron trifluoride-acetic acid gave 79, which gave the thermally unstable... 

A-(Monosubstituted amino)pyridiniums (972) are in prototropic equilibrium with A-imides (973) and dications (971). For R = H or allyl, the A-imides (973) are very strong bases and cannot usually be isolated if R is an electron-withdrawing group (e.g. acyl, sulfonyl, nitro), then the imide (973) is less basic and more stable. The cations (971) are only obtained in very strongly acid media. 

A 6n electrocyclic closure could be implicated in the photocyclisation reaction reported for the N-amino pyridinium ion (257) and some of its ring substituted derivatives to give (258). This type of reactivity was also seen for compound (233). A summary of the previously published work and of new results for the 67t photocyclisation of photochromic ethylenes substituted by derivatives of cyclopentadiene anion and pyridinium cation has appeared.The basic skeleton involved in the rearrangement is shown in structure (259) which is in photochemical equilibrium with (260). 

A 2-aminophenylacetate spontaneously cyclizes in acetic acid or with PPA [2416] to give an oxindoie. An unexpected cyclization observed when the amino-pyridinium salt (48.1) is heated with benzylamine is a Dimroth rearrangement. 

Pyrazolo[l,5-a]pyridines can be prepared by cycloaddition of pyridinium A -imides (produced by iV-deprotonation of iV-amino-pyridinium salts with base ) with alkynes or A -amination of 2-alkynyl-pyridines. The cycloaddition of 3-benzyloxypyridinium AT-imide involves preferentially the more hindered C-2. ... 

Dehydration of Aldoximes. A range of alkyl, aryl, and heteroaryl aldoximes undergo smooth dehydration with4-(dhnethyl-amino)pyridinium chlorosulfite chloride at —10 to +10 °C to give the corresponding nitriles in 70-100% yield (eq 3). ... 

Single crystals of ionic liquids with chiral cations have mostly been investigated as racemates, including [l- l-(ethoxycarbonyl)ethyl -3-methylimidazolium][NTf2] (mpt. 45°C) [138], [1-(2-hydroxy-2-methyl-2-phenylethyl)-3-tert-butylimidazolium]Cl (mpt. 64-66°C) [217], [(1-phenylethyl)ammonium][CH2ClC02] (mpt. 95°C) [420], [ l-(2-naphthyl)prop-2-en-l-yl]amino pyridinium]l (mpt. 103-105°C) [485] and [6,6 -spirobi(l-methyl-l,5,6,7-tetrahydropyrrolo[l,2-a]imidazol-4-ium][NTf2]2 (mpt. 112°C Fig. 11.9] [547]. Exceptions... 

Methylphenyl)sulfonyl]amino] pyridinium inner salt... 

The formation of imidazopyridines is known to be a two-step process. It was shown earlier [46] that the first step in the reaction - the alkylation of 2-aminopyridine with bromomethyl (adamantan-l-yl) ketone was successfully carried out with a high yield of l-[3-adamantan-l-yl)-2-oxoethyl]-2-amino-pyridinium bromide 29a by boiling the reagents for 1 h in ethanol or ethyl acetate. They have established that the introduction of an electron-acceptor substituent into the pyridine ring, considera-bly hinders the reaction. When 2-aminopyridine, 2-amino-4-methylpyridine, or 2-amino-3-methylpyridine were heated with bromomethyl(adamantan-l-yl) ketone in ethyl acetate for 1 h, the yields of compounds 29a, 52a, 56a varied within the limits of 80-95% and, whereas with 2-amino-6-bromo and 2-amino-5-chloropyridine the corresponding alkylation products - compounds 49a, 57a were obtained in yields of only 50%. In the case of 2-amino-5-bromo-3-trifluoromethylpyridine, the formation of only a negligible amount of compound 58a was observed after 1 h and the yield reached 42% only after 5 h... 

Amino-5-nitrosopyrimidines also condense with benzoylacetonitrile, phenacyl-pyridinium bromide and acetonylpyridinium chloride in the presence of sodium cyanide to produce. 7-amino-6-pteridinyl ketones (63JOC1197). Pteridine syntheses from pyridinium salts are not limited to the preparation of pteridyl ketones since pyridinium acetamide... 

The 3-chloroacrylaldehyde (218) (Type 1, Scheme 6) reacted readily with 2-amino-l,3,4-selenadiazole (219). The final product was the [l,3,4]selenadiazolo[3,2-a]pyridinium salt (220) (76ZC337X... 

Thiazolo[3,2-a]pyridinium, 6-amino-2,3-dihydro- H NMR, 6, 675 (78ACS(B)70) Thiazolo[3,2-a]pyridinium, anhydro-7-bromo-3a-carboxy-2,3-dihydro-8-hydroxy-2ft5-dimethyl-1 or-0X0-... 

---