Pictet-Spengler synthesis of pyrazole-fused β-carbolines

The synthesis of new pyrazolo[4,3-c]beta-carbolines (8a,b) is achieved by condensation of the appropriate aldehyde with 3-(4-amino-1,3-dimethylpyrazol-5-yl)indole (4) under Pictet- Spengler reaction conditions. Regioselective cyclization occurred at the usual indole C-2 position as evidenced from the H-1- and C-13 nmr spectra of 8a,b which lack the pyrrolic H-2 signal, present in 4 (delta 7.26, 1H, d, J(CH-NH) = 2.5 Hz).