SULFATION OF O-ISOPROPYLIDENATED LACTOSE DERIVATIVES AT THE 2'-POSITION INDUCES AN UNUSUAL (3,0)B-BOAT CONFORMATION OF THE D-GALACTOSYL RESIDUE

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 3,0B 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. © 1990.