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. (C) Wiley-VCH Verlag GmbH Co.