An efficient stereoselective synthesis of Δ4,5-pipecolic esters

The synthesis of racemic and enantiomerically homogeneous pipecolic esters from 1-amino-3-buten-2-ols is reported. The synthesis of enantiomerically homogeneous N-methylpipecolic esters requires four chemical steps from N-t-BOC-protected amino esters. The key step of the sequence is a conformationally restricted Claisen rearrangement. The method affords complete control of the absolute and relative stereochemistry of all three stereogenic centers in pipecolic ester 22 which is obtained in 33% overall yield from N-t-BOC-L-alanine ethyl ester 16a