Diversity-oriented synthesis of privileged benzopyranyl heterocycles from s-cis-enones

A novel strategy for the construction of benzopyranyl heterocyclic series with maximized diversity in the polar surface area on rigid scaffolds has been developed through a divergent synthetic pathway with high efficiency. s-cis-Enones embedded in a benzopyran skeleton were identified as versatile key intermediates for the synthesis of four different heterocycle libraries fused with a benzopyran substructure. These four novel core skeletons were designed by a creative recombination of the privileged skeletons: benzopyran, pyridine, pyrazole, pyrazolopyrimidine, and pyrimidine. The regioselective synthesis of each core skeleton was achieved by the introduction of three s-cis enone intermediates. This paper also explores the regioselective formation of arylpyrazole through the condensation of beta-keto aldehyde with arylhydrazine under three different conditions and presents the mechanistic information that was obtained from the regioisomeric ratio of arylpyrazole based on the substituent's electronic effect and reaction temperature. It appears that the regioselective synthesis of arylpyrazole was achieved through the intriguing interplay of the nucleophilicity on arylhydrazine and the electrophilicity on dielectrophiles.