New approach toward the synthesis of deuterated pyrazolo[1,5-a]pyridines and 1,2,4-triazolo[1,5-a]pyridines

An efficient and operationally simple synthesis of 7-deuteropyrazolo[1,5-a]pyridine and 7-deutero-1,2,4-triazolo[1,5-a]pyridine derivatives using α-H/D exchange of 1-aminopyridinium cations in basic D2O followed by a 1,3-cycloaddition of acetylenes and nitriles is presented. A high regioselectivity and a high degree of deuterium incorporation were achieved. The procedure was applied for several 4-R-1-aminopyridinium cations (R = H, Me, OMe).


Introduction
Isotopically labeled compounds find broad applications in studies of chemical and biochemical reaction mechanisms and metabolism pathways. Deuterium is the most common used isotopic label in mechanistic studies. Deuterated organic compounds are widely used in biological [1] and pharmacological [2][3][4][5] investigations. In the last years deuteration became also an efficient tool in drug design [6].

Results and Discussion
N-Aminopyridinium salts are easily available via direct N-amination of parent pyridines. Salt 1a was prepared by N-amination of pyridine with hydroxylamine-O-sulfonic acid followed by the reaction with HBF 4 according to a previously described method [30]. Salts 1b,c were prepared by direct N-amination of the corresponding pyridines with O-mesitylsulfonylhydroxylamine. In view of difficulties in obtaining experimental pK a values of different positions of pyridinium cations we carried out DFT calculations [31] at the M06-2X 6-31+G(d,p) [32] level of theory with SMD [33] solvation ( Figure 1, see also Supporting Information File 1).
As expected, in all cases the NH 2 group is the most acidic. The NH 2 group is usually ≈12-13 pK a units more acidic than α-C-H hydrogens. However, the difference in pK a of NH 2 and CH 3 groups of the 4-methyl-1-aminopyridinium cation is not so high and the NH 2 group is only 2.7 units more acidic. 1-Aminopyridinium and 4-methyl-1-aminopyridinium cations have similar pK a values for NH 2 and α-C-H groups. The 4-methoxy-Naminopyridinium cation possesses significantly lower acidity  Table 1). The deuteration at room temperature even for 24 h led to a significantly lower degree of deuteration (DD, 20%).
Salt 1b gave the corresponding 5-CD 3 -7-D-pyrazolopyridine 5 along with a 93% DD for the methyl group after two runs in D 2 O. The 4-methoxy derivative 1c slowly underwent an H/D exchange at the 2-position of the pyridinium ring probably by reason of a lower acidity due to the electron-donating effect of the methoxy group. Thus, 5-methoxypyrazolopyridine 6 was obtained in 25% yield with a DD of only 58%. A higher DD of 6 could be achieved by increasing the reaction time, however, the yield of pyrazolopyridine decreased possibly due to the    6-31G+(d,p) method ( Figure 2) and 3a -> 2 hydrogen atom migration is a highly exothermic process. However, the formation of 11 is probably kinetically unfavorable due to the prohibited 1,3-hydrogen shift. Thus, no NMR signals corresponding to 11 were found in the reaction mixture before oxidation. Another possible way of deuterium-atom incorporation includes the H/D exchange between the ND-group and the C sp -H hydrogen of ethyl propiolate before the formation of cycloadduct 12 (path b). Rearrangement of 12 into 13 and further oxidation leads to pyrazolopyridine 8.
In order to explore this approach for the synthesis of deutero-1,2,4-triazolo [1,5-a]pyridines the reaction of 1a and MeCN in basic D 2 O solution was studied (Scheme 3, Table 2). The previously reported conditions for such a reaction were applied [18]

Conclusion
We have developed an efficient protocol for the synthesis of deuterium labeled pyrazolo[1,5-a]pyridines and triazolo [1,5a]pyridines. Readily available and cheap D 2 O was employed as the deuterium source. The established system displays notable efficacy under mild reaction conditions in a short reaction time.
A comparative assessment of the pK a values of different positions of N-aminopyridinium cations by DFT calculations allows predicting the direction of deuterium exchange. We assume that this method could also be extended to tritium labelling of pharmaceutically interesting compounds for medicinal applications.

Supporting Information
Supporting Information File 1 Experimental part, NMR spectra, and quantum calculation details.