Structure and Synthesis of a Quercetin Glucoxyloside from Kalanchoe prolifera (Raym.-Hamet)

Abstract

A quercetin glycoside has been isolated from Kalanchoe prolifera.Its structure has been established as quercetin-3-0-P-Dglucopyranosyl-(1-12)-P-D-xylopyranoside on the basis of its synthesis starting either from 1,2-0-isopropylidene-a-D-xylofuranose or from 1.3.4-tri-0-acetyl-a-D-xylopyranose.Structure and natural occurrence of flavonol-3-0-glycosides is well documented.'Nevertheless, no glucopyranosylxylopyranoside belonging to this series has been previously unambiguously described or fully characterized.2We have recently isolated a compound of this type (1) 3, in course of our study of Kalanchoe prolifera R a y m .-H a m e t 4 ( C r a s s u l a c e a e ) , a species widely used in malagasian traditional medicine5.6for the treatment of rheumatism and arthrosis.Its uv spectrum exhibited absorptions typical for a quercetin substituted at OH-3.7 The empirical formula C26H28016.was deduced from high resolution analysis of the quasi-molecular ion (M+Na)+=619 in fab-ms.It accounted for a quercetin, a pentose and a hexose unit, in good agreement with the acid hydrolysis which led to the isolation of quercetin and to the characterization of xylose and glucose.The 1H nmr spectrum exhibited the signals ' OR 13 R=COCH3 Condensation of 4',7-di-0-benzylquercetin25 with 3 under phase transfer-catalyzed conditions led to 4',7-di-0-benzyl quercetin-3-0-(2.3.4.6-tetra-0-acety1)-P-Dglucopyranosyl-(l+2)-3,4-di-0-acetyl-~-~-xylopyranoside ( 1 6 ) in 40% yield.This latter was submitted to hydrogenolysis to give 1 7 .Finally, deacetylation of 1 7 afforded 1 identical in all respects with the natural glycoside whose structure was