Enantioselective synthesis of (2R,4S)- and (2S,4R)-4-hydroxypipecolic acid from D-glucoheptono-1,4-lactone

Enantiomerically pure (2R,4S)-4-hydroxypipecolic acid [(+)-1] was synthesized from D-glucoheptono-1,4-lactone (2) via the 3,5-dideoxy-D-xylo-heptono-1,4-lactone (7). The latter was readily prepared by benzoylation of 2, followed by p-elimination and diastereoselective hydrogenation of the resulting furanones (4). Compound 7 was converted into the 6,7-O-eyclohexylidene derivative 11, which on treatment with tosyl chloride for long periods afforded the 2-chloro derivative 14, the precursor of the azide 15. Hydrogenolysis of 15 and protection of the amine gave the N-benzyloxycarbonyl derivative 19, having the required configuration for the stereocenters at C-2 and C-4. Removal of the cyclohexylidene group by hydrolysis and subsequent oxidative degradation of the resulting glycol system afforded the hexurono-6,3-lactone 21 as a key intermediate. Chemoselective reduction of the aldehyde function of 21 led to the alcohol 23, which was derivatized as the mesylate 24. Releasing of the amino group by hydrogenation, and dissolution of resulting 25 in aqueous alkali, promoted the intramolecular nucleophilic displacement of the mesylate to give (+)-1. Its enantiomer [(-)-1] was prepared by a similar sequence starting from 2.