Enantioselective synthesis of (+)-shikimic acid and (+)-5-epi-shikimic acid by asymmetric Diels-Alder reaction of (S)-alpha-sulfinylacrylates

Enantiopure (S) benzyl and t-butyl 2-p-tolylsulfinylpropenoates (1 and 2) were readily prepared by Mannich condensation of (R) p-tolylsulfinylacetates (CH2O+Me2NH), followed by in situ nitrogen quaternization. These new chiral dienophiles reacted with furan at high pressures (4-13 Kbar) at rt to afford mainly a 2:1 mixture of both endo adducts (endo-B+endo-A). Enantiopure endo-4B and endo-4A were stereoselectively transformed into (+)-shikimic acid and (+)-5-epi-shikimic acid respectively, by dihydroxylation of the double bond, removal of the sulfinyl group, basic opening of the oxabicyclic skeleton and hydrolysis of the ester moiety. (C) 1997 Elsevier Science Ltd.