Asymmetric Diels–Alder reactions. Part 2. A model to account for the diastereofacial reactivity of (E)-1-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)-3-trimethylsiloxybuta-1,3-diene and its 2-methyl derivative. X-Ray molecular structure of (E)-1-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)-3-trimethylsiloxybuta-1,3-diene and of (1R,6R,9R,10S)-9-methyl-10-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)bicyclo[4.4.0]dec-3-ene-2,5,8-trione