[42879-93-6] · C3H3AgN2 · Silver(I) Imidazolate · (MW 174.95)
(preparation of glycosides)
2,3,4,6-Tetra-O-benzyl-D-glucopyranosyl chloride reacts with aglycones such as 1,2:3,4-di-O-isopropylidene-D-a-galactopyranose, in the presence of silver imidazolate and Mercury(II) Chloride, to give predominantly the 1,2-cis-glycoside with lesser amounts of the 1,2-trans-glycoside (eq 1).2 Without either reagent, only degradation of the starting materials occurs. With the more hindered glycone 1,2:5,6-di-O-isopropylidene-a-D-glucofuranose, only the 1,2-cis-glycoside is formed (eq 2).
Tetrabutylammonium chloride greatly enhances glucosidation rates without affecting product ratios. Better yields of the 1,2-trans-glycosides are obtained when the pyranosyl bromide, glycone, silver imidazolate, and Zinc Chloride are reacted in a ratio of 1.5:1:0.75:~6, respectively.3 In this way the unhindered alcohol 1,2:3,4-di-O-isopropylidene-a-D-galactopyranose gives the 1,2-trans-glycoside in 90% yield (eq 3), and the hindered alcohol 1,2:5,6-di-O-isopropylidene-a-D-glucofuranose gives the corresponding 1,2-trans-glycoside in 30% yield (eq 4).
Treatment of 3,4,6-tri-O-acetyl-D-glucal with silver imidazolate and Iodine in the presence of HgCl2 gives an almost quantitative yield of a mixture of 2-iodo-a-D-glucopyranosyl chloride and 2-iodo-b-D-mannopyranosyl chloride in a ratio of 5:1 (eq 5), which is an improvement over the existing method using Iodine Monochloride.4
Duncan R. Rae
Organon Laboratories, Motherwell, UK