Stereospecific, Flexible and Redox-Economic Asymmetric Synthesis of cis- and trans-3-Hydroxypipecolic Acids and Analogs

Both cis- and trans-3-hydroxy-L-pipecolic acids are synthesized from a common chiral intermediate 7 by a short and flexible route. The stereospecific inversion of C-3 was achieved by the formation of an oxazoline followed by acidic ring cleavage. The overall yields are 27% and 30%, respectively, in 12 and 10 linear steps. Several versatile chiral building blocks are also accessible by this diastereodivergent synthesis. Unlike the chiral pool approach, our synthetic strategy is not limited by the availability of starting materials. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)