Synthesis of Monocyclic and Bicyclic Imino Sugars

A diastereoselective aldol reaction between the chelated alanine ester enolate 5 and the protected threose derivative 6 , followed by cyclization under Mitsunobu conditions, gave the pipecolinic acid derivative 9 . This compound could easily be converted into the versatile protected derivative 11 , which could be transformed in excellent yields either into the corresponding piperidine imino sugar 14 or into the unnatural amino acid 17 . The imino alcohol 13, an intermediate in the synthesis of imino sugar 14 , was also used in a straightforward approach to indolizidinone 19 , involving an intramolecular Horner−Emmons reaction for ring‐closure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)