LEWIS ACID-MEDIATED S(N)2 TYPE DISPLACEMENT BY GRIGNARD-REAGENTS ON CHIRAL RHYDROPYRIDO[2,1-B]PYRROLO[1,2-D][1,3,4]OXADIAZINE - CHIRALITY INDUCTION IN ASYMMETRIC-SYNTHESIS OF 2-SUBSTITUTED PIPERIDINES

Et2AlCl-mediated nucleophilic alkylation with Grignard reagents on chi ral rhydropyrido[2,1-b]pyrrolo[1,2-d][1,3,4]oxadiazine has proved to p roceed via an S(N)2 mechanism at low temperatures (below -80 degrees C ) with high to excellent inversive stereoselection, while, at an eleva ted temperature, hydrazonium ions are formed preferentially leading to retentive stereoselection. This methodology provides useful access to enantioselective preparation of 2-substituted piperidines and is used for asymmetric synthesis of (+)-coniine. (C) 1997 Elsevier Science Lt d.