Construction of pyrazole fused spiroketals by a (3 + 2) annulation reaction

Abstract

A stereoselective (3 + 2) annulation process between 4-bromo pyrazolones and benzofuran-derived azadienes was developed. A series of novel pyrazole fused spiroketals were synthesized in the presence of a base. The process has the salient features of simple operation, mild conditions, and fast reaction rate. The asymmetric (3 + 2) annulation reaction was initially explored, and an optimal catalytic system was obtained. The applicability of a few substrates was investigated, highlighting the possibility of asymmetric catalytic synthesis of chiral pyrazole fused spiroketals.