Synthesis of both enantiomers of hydroxypipecolic acid derivatives equivalent to 5-azapyranuronic acids and evaluation of their inhibitory activities against glycosidases

We have synthesized 3-hydroxy- and 3,4,5-trihydroxypipecolic acid derivatives corresponding to 5-aza derivatives of uronic acids and evaluated their inhibitory activities against various glycosidases including β-glucuronidase. Compounds 4 and 5 were chosen as common intermediates for the synthesis of 3,4,5-trihydroxypipecolic acids and 3-hydroxypipecolic acids as well as for 3-hydroxybaikiain, a unique natural product isolated from a toxic mushroom. Cross aldol reaction of N-Boc-allylglycine derivative with acrolein followed by the ring-closing metathesis gave 4 and 5 as a mixture of diastereomers which could be separated by silica gel column chromatography. By employing lipase-catalyzed kinetic resolution, the synthesis of both l- and d-isomers of 3,4,5-trihydroxy- and 3-hydroxypipecolic acids was achieved. None of the compounds tested showed inhibitory activity against α- and β-glucosidases. On the other hand, l-23 and l-29 were found to have potent inhibitory activity against β-glucuronidase. In addition, it is interesting that some uronic-type azasugar derivatives showed moderate inhibitory activities against β-N-acetylglucosaminidase.