Tandem Beckmann and Huisgen–White rearrangement of the9-azabicyclo[3.3.1]nonan-3-one system. Part 2.1 The secondmode of the rearrangement leading to 6-(prop-1-enyl)piperidin-2-ylaceticacid, a versatile intermediate for the syntheses of piperidine alkaloids(+)-pinidine and (+)-monomorine I

The second mode of the Huisgen–White rearrangement of the bicyclic lactam, (-)-2-ethyl-4-oxo-3,10-diazabicyclo[4.3.1]decane (-)-13, leading to cis-[6-(prop-1-enyl)piperidin-2-yl]acetic acid (-)-9a under alkaline conditions is described. A reasonable reaction mechanism accounting for the preferable formation of the (E)-propenyl isomer (E)-9a is presented. Conversions of the olefinic acid 9a into two piperidine alkaloids (+)-pinidine (+)-10 and (-)-dihydropinidine (-)-21, and (-)-cis-2-formyl-6-methylpiperidine (-)-22, a key synthetic intermediate for an ants’ trail pheromone (+)-monomorine I (+)-11, are also described.