2,3,4-Trioxo-1-(1 H -pyrrolo[2,3- b ]pyridin-7-ium-7yl)-cyclobutan-1-ide

: 2,3,4-Trioxo-1-(1 H -pyrrolo[2,3- b ]pyridin-7-ium-7-yl)-cyclobutan-1-ide was obtained by reaction of squaric acid with 7-azaindole in acetic anhydride. Abstract: 2,3,4 ‐ Trioxo ‐ 1 ‐ (1 H ‐ pyrrolo[2,3 ‐ b ]pyridin ‐ 7 ‐ ium ‐ 7 ‐ yl) ‐ cyclobutan ‐ 1 ‐ ide was obtained by reaction of squaric acid with 7 ‐ azaindole in acetic anhydride.


Results and Discussion
The linkage of the four-membered ring with the pyridine-N of the azaindole in 1 was proven by two-dimensional NMR techniques (see Supplementary Materials). The 1 H-NMR spectrum shows a sharp singlet at 13.37 ppm compatible with the indole-NH. In the H,H COSY-spectrum, this NHsignal exhibits a correlation with the signal at 7.98 ppm (H-2), which is coupled with the H-3-signal at 6.97 ppm. Furthermore, H-5 appears at 7.74 ppm and shows a correlation with H-4 at 8.75 ppm and H-6 at 9.19 ppm. Additionally, a H,C-HMBC spectrum was made. It displays a correlation between C-1 of the four-membered ring at 165.4 ppm and H-6 at 9.19 ppm.

Syntheses
To a solution of squaric acid dichloride (3, 0.151 g, 1.00 mmol) in tetrahydrofuran (20 mL) was added a solution of 7-azaindole (2, 0.354 g, 3.00 mmol) in the same solvent (20 mL) with stirring at room temperature. After stirring for 30 min at room temperature, the precipitate was removed by filtration and washed with ethanol. Crystallization from glacial acetic acid yielded a yellow solid (0.051 g, 24%).

Scheme 2.
Independent synthesis of the title compound 1.

Results and Discussion
The linkage of the four-membered ring with the pyridine-N of the azaindole in 1 was proven by two-dimensional NMR techniques (see Supplementary Materials). The 1 H-NMR spectrum shows a sharp singlet at 13.37 ppm compatible with the indole-NH. In the H,H COSY-spectrum, this NH-signal exhibits a correlation with the signal at 7.98 ppm (H-2), which is coupled with the H-3-signal at 6.97 ppm. Furthermore, H-5 appears at 7.74 ppm and shows a correlation with H-4 at 8.75 ppm and H-6 at 9.19 ppm. Additionally, a H,C-HMBC spectrum was made. It displays a correlation between C-1 of the four-membered ring at 165.4 ppm and H-6 at 9.19 ppm.

Instrumentation
Melting points were determined in open-glass capillaries on an electric variable heater (Electrothermal IA 9100, Bibby Scientific, Stone, UK). FT-IR absorption spectra were recorded on a Thermo Nicolet FT-IR 200 spectrometer (Thermo Nicolet, Madison, WI, USA) using KBr pellets.  To a solution of squaric acid dichloride (3, 0.151 g, 1.00 mmol) in tetrahydrofuran (20 mL) was added a solution of 7-azaindole (2, 0.354 g, 3.00 mmol) in the same solvent (20 mL) with stirring at room temperature. After stirring for 30 min at room temperature, the precipitate was removed by filtration and washed with ethanol. Crystallization from glacial acetic acid yielded a yellow solid (0.051 g, 24%).