Molecular and crystal structures of 2,3,4,6,1′,3′,4′,6′-octa-O-acetyl-β-sophorose, methyl 2,3,4,6,3′,4′,6′-hepta-O-acetyl-β-sophoroside, and methyl 2,3,4,6,3′,4′-hexa-O-acetyl-6′-deoxy-β-sophoroside