Selective synthesis of <i>gem</i>-dihalopiperidines and 4-halo-1,2,3,6-tetrahydropyridines from halogen substituted homoallylic benzenesulfonamides and aldehydes

An efficient synthesis of <i>gem</i>-dihalopiperidines and 4-halo-1,2,3,6-tetrahydropyridines <i>via</i> aza-Prins cyclization reaction of homoallylic benzenesulfonamides and aldehydes has been described. The reaction proceeds <i>via</i> aza-Prins followed by base-mediated elimination reaction, giving moderate to good yields. The reaction is highly diastereo- and regio-selective. Furthermore, the <i>gem</i>-dihalopiperidines can be easily converted to 2-substituted-1-tosylpiperidin-4-one and pyridine in good yields. Additionally, 4-halo-1,2,3,6-tetrahydropyridines can be employed to afford their corresponding Sonogashira coupling products in good yield.