An expeditious synthesis of 4-fluoropiperidines via aza-Prins cyclization

The reaction of aldehydes with N-tosyl homoallylamine in the presence of a solution of tetrafluoroboric acid-diethyl ether complex in dichloromethane at ambient temperature gave the 4-fluoropiperidines in good yields and with high cis-selectivity. This aza-Prins-type cyclization has a wide scope and the use of HBF4·OEt2 makes this procedure simple, convenient, and cost-effective for the preparation of 4-fluoropiperidines.