Stereoselective synthesis of gem-dihalopiperidines via the halo-aza-Prins cyclization reaction: access to piperidin-4-ones and pyridines

An efficient methodology for the synthesis of 4,4-dihalopiperidine derivatives in excellent yields has been developed using N-(3-halobut-3-en-1-yl)-4-methylbenzenesulfonamide and an aldehyde catalyzed by In(OTf)₃. The reaction involves an initial formation of a six-membered carbocation via the aza-Prins cyclization reaction followed by a nucleophilic attack by a halide ion to give 4,4-dihalopiperidine. The dihalopiperidine is converted to tetrahydropiperidinone using Ac₂O/Et₃N in DCM/H₂O (1 : 1). It is also utilized for the synthesis of pyridine scaffolds by treatment with DBU. Furthermore, the dihalopiperidine is transformed to its enol ether derivatives using KOH in alcohol.