Abstract
Procedures for the synthesis of branched Xylβ(1–3)[Galβ(1–2)]Glc and Xylβ(1–3)[Galβ(1–2)]GlcA trisaccharides β‐linked to the 3‐O moieties of allobetulin and glycyrrhetic acid, respectively, were developed. To this end, β‐selective glucosylation of the two triterpenes with a glucosyl donor permitting selective access to 2a‐O, 3a‐O, and 6a‐O, was studied; this led to glucoside intermediates. Xylosylation of the 2a,3a‐O‐unprotected glucoside was straightforward because, under inverse procedure conditions, exclusively 3a‐O‐reaction was observed. Subsequent 2a‐O‐galactosylation followed by 4a,6a‐O‐debenzylidenation and chemoselective oxidation of the glucose hydroxymethyl group gave the target molecule 1 in high yield after deprotection. The high nucleophilicity of the glycyrrhetinate keto group required a variation in the sequential attachment of the galactosyl and xylosyl residues, so the 2a‐O‐unprotected glucoside was selected. Initial 2a‐O‐galactosylation, affording mainly a disaccharide, and subsequent protecting group manipulation and 3a‐O‐xylosylation gave the target molecule 2b after deprotection. Transformation of the glucose residue in the trisaccharide intermediate into a glucuronate residue furnished target molecule 2a, with the Xylβ(1–3)[Galβ(1–2)]GlcA β‐linked to 3‐O of the glycyrrhetic acid. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)