Pd-Catalyzed Three-Component Chemospecific Allylic Substitution Cascade for the Synthesis of N-Carbonylmethylene-2-Pyridones

Abstract

Functionalized N-carbonylmethylene-2-pyridones are some of the most important structural motifs and exist in many natural products and bioactive compounds. Thus, the efficient construction of such skeletons has attracted much attention. Generally, the synthesis of N-carbonylmethylene-2-pyridones is realized via an intermolecular nucleophilic substitution of 2-hydroxypyridines and appropriate electrophiles. However, the above reactions often suffer from low yields caused by poor O/N chemoselectivities due to the dual nucleophilicity of the 2-hydroxypyridines. As far as the structure is concerned, N-carbonylmethylene-2-pyridones can be divided into three sections:a pyridone, a carbonylmethyl group and a side chain. When the side chain is a H atom, the N-substituted pyridones can be constructed conveniently via a reaction of 2-hydroxypyridines and primary α-bromocarbonyl compounds in high yields with excellent chemoselectivities. However, when the side chain is not a H atom, for example an alkyl group, only limited examples have been reported and only moderate yields of the desired N-substituted pyridine products are obtained by a combination of 2-hydroxypyridines and bulky secondary α-bromocarbonyl compounds, mainly due to the poor O/N chemoselectivities. To achieve a general synthetic pathway for the latter, the following practical strategy was designed. 2-Hydroxypyridines were first treated with primary α-bromocarbonyl compounds to generate the unique N-substituted intermediates in situ, which then reacted with the side chain electrophiles to give only the N-alkylated final products. Thus, a Pd-catalyzed three-component chemospecific allylic substitution cascade has been developed for the synthesis of N-carbonylmethylene-2-pyridone derivatives, with the desired products being obtained in up to 98% yield. No O-alkylated by-product was observed. The results suggested that the N-carbonylmethylene-2-pyridones are constructed via a cascade reaction consisting of a nucleophilic substitution followed by an allylic alkylation. The reaction was performed on a gram scale and the corresponding alkylated product was conveniently converted to a pyridone-containing unnatural amino acid. This methodology allows for the highly chemoselective synthesis of biologically important N-carbonylmethylene-2-pyridone derivatives.