The improved practicalsynthesis of 6,8,10-tri-O-acetyl-3-C-(acetoxymethyl)-3,5,7,9,11-pentadeoxy-1,2-O-isopropylidene-5,7,9,11-tetra-C-methyl-L-erythro-D-altro-β-L-talo-dodecodialdofuranose-(1,4) 12-(diethyl acetal) (3), a useful synthetic segment for the rifamycin W ansa-chain, is described. The key intermediate, 3-O-benzyl-2,4,7-trideoxy-5,6-O-isopropylidene-2,4-di-C-methyl-aldehydo-D-allo-heptose