Branched-Chain Sugars: Improved Synthetic Approach to 3-O-Substituted Derivatives of 5,6-Dideoxy-5-C-hydroxymethyl-1,2-O-isopropylidene-α-D-xylo-hept-5-enofuranurono-7,5'-lactone and (1R, 2R, 4R)-4-(3-Furyl)-1,2-isopropylidenedioxytetrahydrofuran