Preparation of α-acetonylpiperidines from α-allylated heterocycles by a bromocyclocarbamation reaction

2,6-Disubstituted (alkyl)(allyl)- and diallylpiperidines containing C=C bonds in different environments can be selectively transformed into alpha-acetonylpiperidines, including the alkaloid (+/-)-6-epipinidinone. The key step is the bromocyclocarbamation reaction of the N-Boc-protected alpha-allylheterocycle with N-bromosuccinimide. The bulky tert-butyl group on the piperidine ring can affect the diastereoselectivity of the cyclization by 1,5-asymmetric induction. The struclure of one of the isolated diastereomers was established by X-ray single-crystal diffraction. The prepared bicyclic bromides were readily dehydrobrominated with tBuOK and the corresponding enols were hydrolyzed to alpha-acetonylpiperidine hydrochlorides. The method presented is convenient and simple to perform. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).