Reduction of 1-(2-hydroxy-3,5-di-t-butylphenyl)pyridinium halides and their zwitterions. Formation of di- and tetra-hydropyrido[2,1-b]benzoxazoles

Abstract

Reduction of 1-(2-hydroxy-3,5-di-t-butylphenyl)pyridinium halides (1a–d) and/or their zwitterions (2a–d) with Raney Ni–Al alloy, Raney Ni (W₂) and NaBH₄ were investigated. When (1) was treated with Ni-Al alloy in a mixture of methanol and 30% aqueous KOH, novel cyclization products, 1,2,3,4-tetrahydro-(5a–d) and 3,4-dihydro-pyrido[2,1-b]benzoxazoles (6a) and (6b) were obtained. Raney Ni catalyzed hydrogenation of (1a), (1b), (2a), (2b), and (2d) gave (5) and/or 2-piperidinophenols (4). Reduction of (1b) and (1d) with NaBH₄ was re-examined and 1,2-dihydro-(7) and 1,4-dihydro-pyrido[2,1 -b]benzoxazole (8) were isolated, respectively, as byproducts.