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 (1 R , 2 R , 4 R )-4-(3-Furyl)-1,2-isopropylidenedioxytetrahydrofuran