The condensation of 3-O-(tert-butyldimethylsilyl)-5-deoxy-1,2-O-isopropylidene-alpha-D-xylo- hexodialdo-1,4-furanose (obtained in six steps (35%) from D-glucurono-6,3-lactone) with the lithium enolate of (+/-)6-endo-chloro-5-endo-(methoxymethoxy)-7-oxabicyclo[2.2.1]heptan-2-o ne (derived in six steps (25%) from the Diels-Alder adduct of furan to 1-cyanovinyl acetate) was highly exo face selective giving two major aldols that were separated readily. One of them was converted to 6,10-anhydro-5-deoxy-1,2-O-isopropylidene-9-O-(methoymethyl)-alpha-D-ara bino-L-ido-7-undeculofuranose-(1,4)-pyranose-(7,3), a semiprotected form of the long-chain carbohydrate moiety of the herbicidins. The synthesis implies the acid-promoted isomerization of 10,11-anhydro-5,7-dideoxy-1,2-O-isopropylidene-9-O-(methoxymethyl)-7-C-[ (2-nitrophenyl)selenomethyl]-beta-L-ido-L-ido-undecofuranose.