1- Aminopyridinium iodide

The reaction of 1-aminopyridinium iodide (429) with dimethyl chlorofumarate in ethanol/K2C03 to form, ultimately, a pyrazolo[l,5-n]pyridine also occurs via a 1,5-dipolar mechanism. The initially formed 1 1 adduct (430), stabilized by delocalization of the negative charge, underwent disrotatory ring closure as shown to give (431) in which the 3... 

The imine (161), obtained from 1-aminopyridinium iodide and potassium carbonate, combines with dimethyl acetylenedicarboxylate yielding in the first place (162) which then with more ester gives dimethyl fumarate and the pyrazolopyridine (163), isolated in 15% yield. A corresponding reaction with isoquinoline imine gave 75% of the primary adduct [(cf. (162)]. ... 

Fourteen milliliters (22 g., 0.10 mole) of 57% hydriodic acid is added to the filtrate, and the resulting solution is stored at — 20° for 1 hour (Note 3). The solid that separates is collected weight 15.5-17.5 g. Recrystallization of this solid from about 100 ml. of absolute ethanol gives 14-16 g. (63-72%) of 1-aminopyridinium iodide as almost-white crystals, m.p. 160-162° (Note 4). 

The temperature is kept at — 20° or lower by a bath of dry ice and methanol. If the temperature rises above —20°, an appreciable quantity of 1-aminopyridinium iodide may redissolve and be lost. 

Monocyclic 2H-[ 1,2,3 Idiazaphospholes (B) are easily accessible from the condensation of the four-membered chain incorporated in hydrazones or azoalkanes with phosphorus trichloride making available a large number of representatives that have been intensively studied [2, 4, 7], In contrast, their 1//-isomers (A) are less known and are obtained only as second minor product during the synthesis of 2//-[l,2,3]diazaphospholes in some cases. A facile synthesis for pyrido-anellated azaphospholes has been developed in our group by making use of 1,2-disubstituted pyridinium salts for condensation with phosphorus trichloride [8, 13-15], Accordingly, cyclocondensation of 2-alkyl-1-aminopyridinium iodides (1) with phosphorus trichloride in the presence of triethylamine affords pyrido-anellated l//-[l,2,3]diazaphospholes, i.e. l,2,3]diazaphospholo 1,5-a] pyridines (2) (Scheme 1) [16],... 

Alkyl-l-aminopyridinium iodides (88) condense with PCI3 in presence of NEtj to give 1,2,3-diazaphospholo[l,5-a]pyridines (89). 2-Methyl-1-aminopyridinium iodide, on reacting with two equivalents PCI3 under these conditions, forms 1 -dichlorophosphino-1,2,3-diazaphospholo[l, 5-a]py-ridine (89), R = PCI2 (Equation (8)) <95S173>. 

Cyanides can react with 1-aminopyridinium salts to give 2-substituted triazolopyridines, possibly via the pyridinium ylide. With 1-aminopyridinium iodide and cyanide ion the intermediate 4-cyanopyridine reacts with the aminopyridinium salt to give 2-(4-pyridyl)triazolopyridine (45).51 When acetonitrile or benzonitrile are used, 2-methyl- and 2-phenyltriazolo-pyridines are obtained.58 60 The reaction is thought to involve a dipolar cycloaddition of the N- mi nopyridine with the nitrile, as shown in Eq. (4). 

Aminopyridinium iodide with dimethyl chlorofumarate forms a readily aromatized dihydropyr-azolo[l,5-a]pyridine (29) via an initially formed 1 1 adduct (28) for a similar example see Section... 

When a solution of 1-aminopyridinium iodide (143) in DMF was treated with anhydrous potassium carbonate, the mixture developed the deep blue color characteristic of pyridine 1-imine (144) addition of EP caused an immediate exothermic reaction giving the pyrazolopyridine 145 in 48% yield311 hydrolysis and decarboxylation gave a high yield of pyrazolo[l,5-a]pyridine (146).311 Many reactions of this type have now been carried out,312 and the regioselectivity for the cydization of 3-substituted derivatives has been examined,313... 

Amination. Another use is for the amination of an amine to produce an un-symmetrically substituted hydrazine. Thus 1-aminopyridinium iodide is prepared by adding 0.3 mole of pyridine to a freshly prepared solution of 0.1 mole of hydroxyl-amine-O-sulfonic acid in cold water, heating for 20 min. on the steam bath, cooling,... 

Atnino groups are converted into pyrazole rings on heating the compound with a methyl ketone under acidic conditions an intermediate imine may be involved. 1-Aminopyridinium iodide reacts with diketene to give the aceto-acetylimino ylide which cyclizes under the influence of a base. 

Pyridinium A -imide, the ylide produced by removal of a proton from 1-aminopyridinium iodide, serves as a 1,3-dipole and reacts with propiolate (shown above) or fumarate to give bicyclic compounds." " ... 

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