Résumé
Kinetically controlled reaction of 4.5 equiv. of 2-methoxypropene with some alkyl α- and β-
d-galactopyranosides gave the 2,3:4,6-di-
O-isopropylidene derivatives in high yields (80–85%). With 2 equiv. of 2-methoxypropene, benzyl β-
d-galactopyranoside gave the 4,6- and the 3,4-monoacetals in the ratio 30:1 together with ∼20% of the 2,3:4,6-diacetal. The structures of methyl 2,3:4,6-di-
O-isopropylidene-α- and -β-
d-galactopyranosides were determined by X-ray analysis. The former crystallised in the orthorhombic system,
P2
12
12
1, with
a = 5.503,
b = 16.105,
c = 16.822 Å, and
Z = 4, the latter in the monoclinic system,
P2
1, with
a = 10.400,
b = 13.344,
c = 11.647 Å, β = 111.50, and
Z = 4. The α anomer and the two molecules of the β anomer had the
d-galactopyranoside and the 1,3-dioxane rings in twist-chair conformations and the dioxolane ring in a half-chair conformation. N.m.r. spectroscopy suggested the occurrence of similar conformations in solution.