Synthesis of fused imidazo[1,2-a]pyridines derivatives through cascade C(sp2)–H functionalizations

Abstract

An efficient and convenient synthesis of diversely substituted naphtho[1′,2′:4,5]imidazo[1,2-a]-pyridine derivatives from the cascade reactions of 2-arylimidazo[1,2-a]pyridines with a-diazo carbonyl compounds via Rh(III)-catalyzed regioselective C(sp²)–H alkylation followed by intramolecular annulation is presented. Interestingly, when simple 2-arylimidazo[1,2-a]pyridines were used as the substrates, 5,6-disubstituted naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines were efficiently obtained, whereas using 2-arylimidazo[1,2-a]pyridine-3-carbaldehydes as the substrates afforded naphtho[1′,2′:4,5]imidazo[1,2-a]-pyridine-5-carboxylates as the dominating products. Compared with literature methods for the synthesis of naphtho[1′,2′:4,5]imidazo[1,2-a]pyridine derivatives, the protocol presented herein has advantages such as easily obtainable substrates, simple operational procedure, high efficiency and excellent regio- and chemoselectivity.