Preparation of Methyl 2,3-Anhydro-4,6-<i>O</i>-benzylidene-3-deoxy-3-nitro-β-<scp>d</scp>-allopyranoside and Some <i>C</i>2-Branched-Chain Derivatives of Methyl 4,6-<i>O</i>-Benzylidene-2,3-dideoxy-3-nitro-β-<scp>d</scp>-glucopyranoside