Sulfation of O-isopropylidenated lactose derivatives at the 2'-position induces an unusual 3,0B-boat conformation of the D-galactosyl residue

Article abstract of DOI:10.1016/0008-6215(90)84234-L

2,3:5,6:3',4'-Tri-O-isopropylidenelactose dimethyl acetal was converted into the 2'-sulfate derivatives 9 and 10 and the 2',6'-di-sulfate derivative 2.A 2'-sulfate group was shown to induce a change of the ring conformation of the galactosyl residue to a ^3,0B conformation. 2'-Sulfated lactose derivatives were synthesized which differed only in the group linked to O-6', namely the 6'-O-tert-butyldimethylsilyl 9, the 6'-O-triphenylmethyl 10, and the 6'-sulfate 2, all of which adopt a conformation close to a ^3,0B boat.The conformation of the galactosyl ring is slightly influenced by the group at O-6', with 2 showing the most pronounced effect, and 9 showing the smallest changes.It was proved by various n.m.r. spectroscopic parameters, which included coupling constants, n.O.e.s, and T₁ values, that the galactosyl residues of 9, 10, and 2 adopt a ^3.0 B conformation.However, other protective groups at O-2', such as an O-acetyl group in 7, an O-benzyl group in 8, or the hydroxy group itself in 1, 4, 5, and 3, do not cause a change of the ring conformation.Using the GESA program, we found that the observed change of the conformation cannot be explained by unfavorable sterical interactions between the 2'-sulfate group and other parts of the molecule.However, the conformational change observed here could be attributable to electronic effects that are unique to the sulfate group.