2-Aminopyridinium

The neutral complex is more stable than the 2-aminopyridinium formate complex. 

The authors claim that these associations, which are destroyed in fixed compounds, play an important role in the calculation of Ty.The cases of 1,2,4-triazole-5-thiones 74 [97SA(A)699] and of pyridone dimers 15a-15a and 15a-15b were also studied [96MI(13)65]. (3) The recording of IR spectra in solution at different temperatures to determine the effect of the temperature on Kj-, for instance, in pyrazolinones [83JPR(325)238] and in cytosine-guanine base pairs [92MI(9)881]. (4) The determination of the equilibrium 2-aminopyridine/acetic acid 2-aminopyridinium acetate (see Section III.E) in the acid-base complex was carried out by IR (97NKK100). 

Heating the crystalline salt 2-aminopyridinium propiolate (346) at 100 °C in the solid state led to a 10 9 mixture of 2/f-pyrido[l,2-n]pyrimidin-2-one and ( )-3-(2-imino-l,2-dihydro-l-pyridyl)acrylic acid (347). Analysis of differental scanning calorimetry data shows unambiguously that the reaction takes place in the solid state. An endothermic peak at 81.1 °C corresponds to a solid state reaction, and a peak at 122-123 °C is attributed to melting. The product ratio of 2//-pyrido[l, 2-n]pyrimidin-2-one and 347 is 1 2.5 at 60°C, and 1 1.4 at 80°C (94MI12). 

The literature of mechanistic aromatic photochemistry has produced a number of examples of [4 + 4]-photocycloadditions. The photodimerization of anthracene and its derivatives is one of the earliest known photochemical reactions of any type97. More recently, naphthalenes98, 2-pyridones" and 2-aminopyridinium salts100 have all been shown to undergo analogous head-to-tail [4 + 4]-photodimerization. Moreover, crossed [4+4]-photocycloaddition products can be obtained in some cases101. Acyclic 1,3-dienes, cyclohexadienes and furan can form [4 + 4]-cycloadducts 211-214 with a variety of aromatic partners (Scheme 48). 

This may be rationalized from a consideration of resonance in the conjngate acids. The conjngate acids from ring protonation benefit from charge delocalization, which is greater in 4-aminopyridininm that in 2-aminopyridinium. This type of delocalization is not possible in 3-aminopyridinium 3-aminopyridine (p Ta 6.0) is the weakest base of the three aminopy-ridines, and has basicity more comparable to that of pyridine (pATa 5.2). 

An unusual photochemical reaction of 2-pyridones, 2-aminopyridinium salts and pyran-2-ones is photodimerization to give the so-called butterfly dimers. These transformations are outlined in equations (13) and (14). Photodimerization by [2+2] cyclization is also a common and important reaction with these compounds. It has been the subject of particular study in pyrimidines, especially thymine, as irradiation of nucleic acids at ca. 260 nm effects photodimerization (e.g. equation 15) this in turn changes the regular hydrogen bonding pattern between bases on two chains and hence part of the double helix structure is disrupted. The dimerization is reversed if the DNA binds to an enzyme and this enzyme-DNA complex is irradiated at 300-500 nm. Many other examples of [2+2] photodimerization are known and it has recently been shown that 1,4-dithiin behaves similarly (equation 16) (82TL2651). 

According to Chuiguk and Oksanich13 the reaction of 2-aminopyridinium perchlorates and /i-chlorovinylaldehydes led exclusively to the 4-substituted products (9) whereas from 2-amino-6-methylpyridine no cyclic product was obtained. With //-chlorovinyl ketones both the 2- and 4-substituted pyrido-[l,2-u]pyrimidinium salts were detected by H NMR.3 2-Amino-6-methyl-pyridine gave rise only to the 2-substituted isomer (10). 

Fischer14 15 prepared the unsubstituted pyrido[l,2- /]pyrimidinium salt (16) and its 4-phenyl derivative (9 R1 = Ph) by reacting 2-aminopyridinium perchlorate with propargylaldehyde and with ethynyl phenyl ketone, respectively. 

Methyl-4-oxo-4//-pyrido[l,2-fl]pyrimidine (37) was prepared in 90"n yield from 2-aminopyridinium iodide and ethyl acetoacetate in pyridine at reflux temperature.2 The yield dropped to 20", when 1,2-diaminopyridinium iodide was used instead of 2-aminopyridine.2... 

Okamoto et a/.156 cyclized the dinitriles (86 R = CN, R2 = H) by heating in 15° hydrochloric acid to obtain pyrido[l,2-a]pyrimidine-3-carboxylic acids (89 R2 = H). 2-Aminopyridinium chloride and ethoxymethylene-malononitrile at 110CC yielded 3-cyano-4-imino-4H-pyrido[l,2-a]pyrimi-dine (87 R1 = R2 = H, X = NH) and compound 91. Under similar conditions, 2-amino-3-methylpyridine gave a noncyclized product of type 91. 

The cyclocondensation of 2-aminopyridinium perchlorate and 3-ha-loacetylacetones in ethanol at 120-130°C gave 3-halo-2,4-dimethylpyr-ido[l,2-a]pyrimidinium perchlorate 51 (R = Cl, Br) (90ZAK502 91KGS1381). Cyclocondensation of 2-aminopyridinium perchlorate and... 

Gashev et al. observed an unusual reaction when 1.5-5-fold excesses of ethyl acetoacetate were reacted with 2-aminopyridinium perchlorates at 120-170°C. Instead of 2-methyl-4//-pyrido[l,2- ]pyrimidin-4-one, 2,4-dimethylpyrido[l,2-fl]pyrimidinium perchlorates 51 (R = H 9-OH X = H) were obtained (88KGS1288 91KGS1381). 

Carboxylic and sulfonic acid chlorides and anhydrides give acylamino- and sulfonamidopyridines 748 and 749. Evidence exists that initial products of the reaction, the corresponding A -acyl-2-aminopyridinium salts, are unstable and rapidly rearrange to the thermodynamically more stable 2-acylaminoderivatives. 

Figure 14. Molecular structures of 2-aminopyridinium, acridizin-ium, and their photodimers.

Chuiguk and Oksanich described the reaction of 2-aminopyridinium perchlorates and 1-formyl-2-chlorocycloalkenes in refluxing ethanol, leading to the tricyclic products 157. 

Potts et alP treated 1,2-diaminopyridinium iodide with 2-acetylcyclo-hexanone in refluxing pyridine and obtained the pyrido[l,2-fl]quinazolin-ium iodide 158 in 10% yield. When 2-aminopyridinium iodide was used instead of 1,2-diaminopyridinium iodide, the yield of 158 was higher. 

The tricyclic compounds 244 were likewise obtained when 2-aminopyri-dinium halides were treated with fi-oxo esters in boiling pyridine. st yields were attained when 2-aminopyridinium iodides were applied. 

Reaction of 2-aminopyridinium A -imine with 2,3-diphenylcyclopropenone in refluxing benzene gave 3-amino-2,3-diphenyl-Af-(2-pyridyl)acrylainide in 21% yield. 

MAP = 4-methyI-2-aminopyridinium terpy = 2,2 6, 2"-terpyridine. Not accurately evaluated. 

Diaminopyridinium iodide undergoes reaction with ethyl aceto-acetate and acetylacetone to form somewhat unexpected products, the pyrido[l,2-a]pyrimidine derivatives 33 and 34, respectively.154 The initial step has been shown to be decomposition of 1,2-diaminopyridinium iodide to 2-aminopyridinium iodide which undergoes condensation with [i-dicarbonyl compounds. The reaction of 1,2-diaminopyridinium salt with 1,2-dicarbonyl compounds gives pyrido[ 1,2-A] [1,2,4]-triazinium salts 35.161... 

The reactions of 2-aminopyridinium hydrochloride <74CPB243) and its 3,4-benzologue <85CPB3034, 86JOC2988) with ethoxymethylenemalononitrile yielded 3-cyano-4-oxo-477derivatives of (12) and its 8,9-benzo derivative, respectively. The reaction of 1-aminoisoquinoline and methyl 3-pyrrolidino-... 

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