Silicon lewis acid catalyzed [3+2] cycloaddition reactions of hydrazones/cyclopentadiene: mild access to pyrazolidine derivatives

A first and fairly mild metal-free catalytic route was developed for the [3+2] cycloadditions of different N-acylhydrazones to cyclopentadiene, providing the synthetically andbiologically important five-membered cyclic compounds pyrazolidines. The reaction was successfully conducted in high yields (up to 99 %) with high diastereoselectivities (up to 98:2 dr) by using catalytic amounts of TMSOTf (trimethylsilyl triflate) as a readily available Lewis acidic catalyst. The experimental findings were supported by DFT calculations of the reaction mechanism. The theoretical and experimental diastereomeric excess values show good agreement, rendering the computations an efficient tool for predicting the selectivities in these and related reactions.