Regioisomeric iminium ions

Cyclic bridged enamines or fused iminium ions were similarly prepared and successively reduced to the corresponding amines, e.g. 32-34139. The formation of regioisomeric products 33 and 34, not separable by chromatography, is due to the rearrangement of the intermediate carbonium... 

Unfortunately, in 3-alkyl-substituted piperidine IV-oxides there is generally little discrimination between the ring a-carbon centers, and mixtures of regioisomeric products are obtained. When a double bond is present in the starting TZ-oxide, however, this functionality will direct the Polonovski reaction towards elimination of one of the allylic hydrogens. Thus, the (Z)-ethylidene product (18) is obtained from (16) after reduction of the conjugated iminium ion intermediate (17 equation S). ... 

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