A Tandem, Bicatalytic Continuous Flow Cyclopropanation-Homo-Nazarov-Type Cyclization

Continuous flow processing represents an emerging technology in the chemical and pharmaceutical industries. Herein, we describe a tandem, bicatalytic continuous flow cyclopropanation-homo-Nazarov-type ring-opening cyclization to form hydropyrido­[1,2-a]­indoles, which represents a naturally occurring chemical scaffold present in many bioactive and therapeutically relevant molecules. The tandem flow reactions provided high conversions (>97%) with product throughputs on the order of 3–5 g h–1. The individual transformations (cyclopropanation and ring-opening cyclization) were separately optimized in the batch then successfully transferred to the flow. Significantly, this represents the first literature example of continuous flow cyclopropane ring-opening cyclizations; hydropyrido­[1,2-a]­indoles are formed on a multigram scale (>4 g h–1 throughput) in near-quantitative yields from N-indolyl-1,1-cyclopropyl β-amidoesters. Overall, the continuous flow technology exhibited superior yields, relative to the batch reactions, for both the ring-opening cyclizations and the tandem, bicatalytic reactions. These results provide the basis for large-scale implementation of bicatalytic cyclopropanation-ring-opening cyclization reactions for complex synthesis and represent initial efforts toward the development of an industrially viable, four-step continuous flow synthesis of hydropyrido­[1,2-a]­indoles.