Enantiospecific synthesis of (-)-5-epi-shikimic acid and (-)-shikimic acid

Diastereoselective reaction of 2,3-O-isopropylidene-D-ribose with allylmagnesium chloride gave a 5 : 1 mixture of triols 4 and 5, which were then converted to nitrones 8 and 9. Intramolecular nitrone cycloaddition gave the isoxazolidines 10 and 11, which on acetylation gave the corresponding acetates 12 and 13 which were separated by repeated crystallisation. The major adduct 12 was converted to (-)-5-epi-shikimic acid 2. Reaction of the ribonolactone derivative 20 with allylmagnesium chloride gave the hemiacetal 21. Reduction of compound 21 with DIBAL afforded exclusively the diol 22, which was desilylated to give the triol 5. Similar chemistry to that employed for the synthesis of (-)-5-epi-shikimic acid 2 with the diol 5 resulted in the synthesis of (-)-shikimic acid 1.