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

Abstract

Abstract 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 Hückel Molecular Orbital theory and with observations on nitration and reduction in the isoteric quinoline system.