Synthesis and conformational analysis of plant hormone (auxin) related compound 2-(indol-3-yl) ethyl β-d-galactopyranoside and its 2,3,4,6-tetra-O-acetyl derivative

The synthesis, structure analysis by X-ray diffraction and NMR spectroscopy (including NOEs) of 2-(indol-3-yl)ethyl 2,3,4,6-tetra-O-acetyl-β-d-galactopyranoside (4) are described. The per-O-acetylated derivative of 2-(indol-3-yl)ethyl β-d-galactopyranoside (4∗) was prepared along with the isomeric α-d-galactopyranose 1,2-orthoacetates (3 exo- and endo-stereomers) by condensation of 2,3,4,6-tetra-O-acetyl-α-d-galactopyranosyl bromide (1) with the aglycone alcohol (2). The analogous condensation of 1 and 2-phenylethanol yielded a mixture of 2-phenylethyl 2,3,4,6-tetra-O-acetyl-β-d-galactopyranoside (6) and isomeric α-d-galactopyranose 1,2-orthoacetates (5). Compound 4 crystallized in the monoclinic system, space group C2, a = 22.385(2), b = 7.865(2), c = 28.761(3) Å, β = 102.1(1)°, Z = 8, with two symmetrically independent molecules in the asymmetric unit. In both molecules β-d-galactopyranose rings are in the 4C1 chair conformation. The conformational analysis of 4 and the analogous unprotected conjugate (4∗), based on molecular mechanics calculations and molecular dynamics simulations, is presented. The conformational stability about the bond of conjugation and relative orientation of the indole ring towards the β-d-galactopyranose moiety are discussed.