Synthesis of bicyclic 3‐pyrazolidinones via π‐cyclization reactions of exocyclic N‐acylhydrazonium ions

The preparation of several a‐fused 3‐pyrazolidinones is described starting from 5,5‐di‐methyl‐3‐pyrazolidinone and using an acid‐induced cyclization of a π‐nucleophile tethered to N‐1 onto a cationic carbon bonded to N‐2 (an exocyclic N‐acylhydrazonium intermediate). The cyclization reactions proceed best with activated π‐nucleophiles such as allyl‐ and propargylsilanes. In the case of an ester substituent at the cationic carbon, structural analogues of antibacterial bicyclic pyrazolidinones are obtained. The stereochemistry of one product is established by X‐ray crystallographic analysis. Two cyclization products showed NMR spectra with very broad signals due to relatively high nitrogen‐inversion barriers (ca. 13 kcal/mole).