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