Hemisynthesis and spectroscopic characterization of three glycosylated 4-hydroxylonchocarpins from Dorstenia barteri Bureau

The hemisynthesis of β-galactopyranosides 2, 3 and β-D-glucosaminide 4 from readily available and natural starting material, 4-hydroxylonchocarpin (1), has been achieved. 4Hydroxylonchocarpin (1) is the major constituent of the herbaceous plant D. barteri. The synthetic pathway employed in this work involved the catalytic glycosylation of 1 under phase transfer catalysis (PTC) with different glycosyl donors under various reaction conditions. All these compounds were obtained using chromatographic methods. The structural elucidation of synthetic compounds was done by mass spectrometry and NMR analysis. The hydrogen-bonded phenolic hydroxyl group of 1 was not glycosylated under the employed conditions. A key feature of this reaction is the formation of pyran ring C in the product, β-galactopyranoside (3), and the site specific glycosylation at position C-4 of ring B. The results also confirm the isomerism equilibrium between chalcone 2 and its flavanone analog 3 which also existed in the natural sources via enzymatic reaction. This is the first report of the glycosylation of natural 4hydroxylonchocarpin (1) using phase transfer catalysis.