Synthesis and conformational analysis of the plant hormone (auxin) related 2-(indol-3-yl)ethyl and 2-phenylethyl β-d-xylopyranosides and their 2,3,4-tri-O-acetyl derivatives

The synthesis, structure analysis by X-ray diffraction and NMR spectroscopy (including also 1H 1HNOE measurements), as well as molecular mechanics and dynamics of d-xylopyranose conjugates of 2-(indol-3-yl)ethanol (1) and 2-phenylethanol (2) are described. The per-O-acetylated derivatives of 2-(indol-3-yl)ethyl β-d-xylopyranoside (4β) and 2-phenylethyl β-d-xylopyranoside (7β) were prepared [along with corresponding α-d-isomers (4α and 7α) and 1,2-O-orthoacetates (5,8)] by Koenigs-Knorr condensation of the aglycone alcohols with 2,3,4-tri-O-acetyl-α-d-xylopyranosyl bromide. Glycoside 4β was deprotected to yield 2-(indol-3-yl) ethyl β-d-xylopyranoside (6). The crystal structures of 4β and 7β were determined. The crystals of both compounds are monoclinic, space group P 21 with a = 11.985(1), b = 7.317(1), c = 12.428(1) Å. β = 93.2(1)°, Z = 2 (4β); a = 5.809(1), b = 19.833(1), c = 8.912(1) Å, β = 106.0(1)°, Z = 2 (7β). The β-d-xylopyranose rings are in the 4C1 chair conformation. The results of the theoretical conformational analysis are compared with the values obtained from the experimental measurements in solid state and solution.