Enantiomerically Pure 7-Oxabicyclo[2.2.1]Hept-5-En-2-Yl Derivatives (Naked Sugar) as Synthetic Intermediates .15. a New Total Synthesis of D-Threo-L-Talo-Octose

A new approach to the total, asymmetric synthesis of D-threo-L-talo-octose ((-)-1) and its derivatives is presented. It is based on the chemoselective Wittig-Horner monoolefination of a 5-deoxy-D-ribo-hexodialdose derivative 4 obtained by selective reduction of (-)-5-deoxy-2,3-O-isopropylidene-beta-D-ribo-hexofuranurono-6,1-lactone ((-)-3). Allylic bromination of the resulting methyl (E)-oct-6-enofuranuronate (+)-5 followed by intramolecular nucleophilic displacement of the so-obtained bromides gave a 13.3:1 mixture of (-)-methyl (E)-1,4-anhydro-6,7-dideoxy-2,3-O-isopropylidene-beta-L-talo-oct-6-enopy ranuronate ((-)-8) and methyl (E)-1,4-anhydro-6,7-dideoxy-2,3-O-isopropylidene-alpha-D-allo-oct-6-enop yranuronate (9). The double hydroxylation of the enoate (-)-8 followed Kishi's rule and gave the corresponding D-threo-beta-L-talo-octopyranuronate derivative (-)-11 with a good diastereoselectivity. Reduction of ester (-)-11 and deprotection led to pure (-)-1.