Synthesis of new stable pseudobases

Pyrido[1,2-b ][1,2,4]triazinium salts easily form covalent hydrates in the presence of traces of water and readily react with various nucleophilic anions (e


Introduction
Earlier we published 1,2 that N-amination of 2-aminopyridines and a subsequent condensation reaction of the resulting diaminopyridinium salt with α-dioxo reagents proved to be a simple and straightforward protocol for the synthesis of pyrido [1,2-b]-as-triazinium salts 1 (Scheme 1).We have also shown 1 that this bicyclic heteroaromatic ring system is ready to form covalent hydrates 2. This water-sensibility of the heteroaromatic cation prompted us to reinvestigate the reactivity of 1 towards nucleophiles.

Scheme 2
The solution of 5a in dimethyl sulfoxide at room temperature was found in accordance with our early observations 1 ) to undergo a slow change as monitored by the NMR spectrum and after 24 h formation of the covalent hydrate 6a was detected.NMR experiments revealed that, similar to 7a, the hydroxyl group attached to the C-2 atom.Thus, the transformation 5a → 7a can be rationalized either as a nucleophilic addition of the hydroxide ion on 5a or as a rapid formation of 6a followed by a deprotonation step.
As both pathways are basically facilitated by the electron-defficient nature of the starting aromatic salt, introduction of further electron withdrawing groups seemed to assist such transformations.Upon this consideration we also have prepared derivative 5b containing an 9chloro and 7-trifluormethyl substituent and subjected both 5a and 5b to reactions with nucleophiles.Synthesis of 5b was carried out according to our well etablished procedure: 2amino-3-chloro-5-trifluoromethylpyridine (3) was N-aminated by tosyl hydroxylamine to the diamino salt 4 and reaction with benzil yielded the desired fused triazinium salt.
The new pyrido[1,2-b][1,2,4]triazinium salt (5b) also easily reacted with sodium carbonate solution and gave the expected addition product 7d in good yield.Similar reactions of 5a and 5b were experienced also with sodium methoxide and sodium cyanide to give the addition products 7b,c and 7e,f, respectively.
Proton assignment for both 7b and 7e was fulfilled by decoupling and homonuclear correlation experiments.The carbons were assigned with heterocorrelated methods.HSQC measurements supplied the one-bond proton-carbon connectivities, while quaternary carbons were determined by applying the HMBC method.In the case of 7b the protons and the corresponding carbons at positions 6,7,8, and 9 could be determined staightforwardly.The protons and carbons of the two phenyl groups were distinguished on the basis of their long range heterocorrelations.The carbon at 84.7 ppm gave crosspeaks with both protons H-2 + H6 at 7.36 ppm and with the methoxy protons (3.18 ppm).These correlations were diagnostic from the point of view of the methoxy position at C-2 instead of at C-9a.The situation was rather similar for compound 7e.
Preliminary experiments indicated that similar to the nucleophilic anions, also secondary amines can react with triazinium salt 5.In this case, however, a different course of the reaction was also observed in some cases 6,7 (i.e.ring opening of the pyridine moiety to afford dieneamines).Investigation of this ambident reactivity towards amines is in progress and will be published elsewhere.Also, because of the potential biological use from the viewpoint of pharmaceutical applications, various tests are planned.

Experimental Section
General Procedures.Melting points were determined by a Büchi apparatus and are uncorrected.The IR spectra were recorded with a Nicolet Magna 750 FT-IR, spectrophotometers; the 1 H and 13 C NMR spectra were recorded using a Varain UNITY-INOVA spectrometer, equipped with a 5-mm indirect detection probe, operating at 399.9 MHz for 1 H and at 100.5 MHz for 13 C nuclei.Approximately 20 mg of each sample was dissolved in 0.65ml CDCl 3 or DMSO-d 6 .The temperature was stabilized at 303K.All 1D and 2D experiments were run using the Varian software library.Assignments of the phenyl-protons and carbons were marked by 2′-H, 3′-H, 2′-C, etc. in the case of the 2-phenyl group, and 2″-H, 3″-H, 2″-C, etc. with the 3-phenyl group.

General procedure for the synthesis of pseudobases 7 from pyrido[1,2-b][1,2,4]triazinium salts 5
To a suspension of 5a (0.37 g, 1mmol) or 5b (0.47 g, 1 mmol) in acetonitrile (2 ml) was added the appropriate reagent (i.e.aqueous sodium carbonate solution, methanolic sodium methoxide, or aqueous potassium cyanide, 3 mmol) with stirring at 5 o C. A yellow solution was formed from which the product separated either spontaneously or by addition of ice.The product was filtered off and recrystallized from cyclohexane.