Acyl a-l-rhamnopyranosides

The acyl a-L-rhamnopyranosides were first isolated in 2000 from different strains of Streptomycetes. Such secondary metabolites are formed by different carboxylic... 

Studies on biosynthesis of acyl a-L-rhamnopyranosides and rhaumosyllactones involved only feeding experiments with labeled precursors (Figure 5.63) [263]. 

Although the 2,3-dimethyl and 3,4-dimethyl ethers of L-rhamnose were known, the 2,4-dimethyl ether had not been synthesized prior to its preparation through a trifluoroacetyl intermediate. - The synthesis started from methyl 2,3-0-isopropylidene-a-L-rhamnopyranoside this was methylated and the acetal group removed, to give methyl 4-0-methyl-a-L-rhamnopyranoside (6). Conversion to the 2,3-bis(trifluoroacetate) (7) was readily achieved with trifluoroacetic anhydride in the presence of sodium trifluoroacetate. As expected, the trifluoroacetate (7) was completely de-acylated by treatment with alcohol, regenerating (6) this process was complete after 25 min. at room temperature. The procedure for selective de-esterification was based on the observation that, if excess carbon tetrachloride (6 vol.) is present, very little methanolysis occurs. By use of a mixed methanol-carbon tetrachloride solvent (65 35 vol./vol.), the meth-... 

---