Chemistry of thienopyridines. IX. Direct nitration of thieno[2,3‐b]pyridine and thieno[3,2‐b] pyridine

Thieno[ 2,3-b]pyridine (I) and thieno[3,2-b]pyridine (II) were nitrated at C-3 in ca. 50% yield by means of nitric-sulfuric acids. Reduction of 3-nitro-I (Ia) with tin and hydrochloric acid gave 3-amino-I (Ib) (39%), which was converted to 3-acetylamino-I (Ic) and to dipyrido[2,3-b:2′,3′-d]thiophene (V). Reduction of 3-nitro-II (IIa) with iron and acetic acid at 100° gave 3-acetyl-amino-II (IIb) (25%), while use of tin and hydrochloric acid at room temperature gave 2-chloro-3-amino-II (IIc). Assignments of position of nitration are based on nmr spectral data (which indicate. that substitution occurs in the thiophene ring) and on non-identities of Ic, IIb, and V with authentic samples of 2-acetylamino-I, 2-acetylamino-II, and dipyrido[2,3-b:3′,2′-d]thiophene (VI), respectively. Results are consistent with reactivity indices for I and II as calculated from simple Huckel Molecular Orbital theory and with observations on nitration and reduction in the isoteric quinoline system.