Rhamnoside, methyl

Carbohydrate orthoesters, first reviewed by Pacsu more than 60 years ago [8], were reported by Fischer et al. [9] as by-products of the Koenigs-Knorr reaction [3] of acetobromo-L-rhamnose (1) with methanol. Orthoacetate 3 was isolated along with the expected a- and P-methyl rhamnosides 2 (Scheme 5.2). However, its true structure was assigned only 10 years later by several research groups [10-12]. 

The reaction of nitroethane/sodium ethoxide with the dialdehyde (31), obtained by periodate oxidation of methyl a-L-rhamnopyranoside (30), leads after deionization to a sirupy mixture (32), which was shown by thin-layer chromatography to contain five main components together with traces of other substances. After acetylation one di-O-acetate (33) crystallized from the mixture and was isolated in 12.5% 5deld, based on the rhamnoside (30). Reduction ot (33) with lithium aluminiumhydride gave methyl 3-C-methyl-3-amino-3,6-dideoxy-a-L-glucopyranoside (34), which was additionally characterized as the triacetate (35)... 

Starting with methyl a-D-mannopyranoside there is ready access to the 2,3-0-isopropylidene rhamnoside Treatment of in dichloromethane with DBE and zinc bromide for 5 5 hours at room temperature gives the 2,3-... 

OH-Luteolin 4 -methyl ether-7-(2"-a-rhamnoside-6" -acetyl-(3-glucoside)... 

Glucosyl(l 3)rhamnoside Baicalein 7-methyl ether (negletein)... 

Gossypetin 8-methyl ether 3-Xylosyl(l — 2)rhamnoside Butea superba stems Leguminosae 423... 

Hydroxymyricetin 8-methyl ether 3-Rhamnoside Erica verticillata aerial parts Ericaceae 446... 

Prenylquercetin 4 -methyl ether 3-Rhamnoside (caohuoside C) Epimedium koreanum aerial parts Berberidaceae 475... 

C-Methylated flavonol glycosides 5,7-Dihydroxy-6,8-di-C-methyl-3-methoxyflavone 7-Galactosyl(l —> 2)rhamnoside Cotula anthemoides seeds 478... 

Gossypetin 7-methyl ether 1145. 3-Arabinoside 3-Rhamnoside 3-Galactoside 8-Glucoside 3-Rutinoside... 

Rhamnosyl(l —> 2)rhamnoside (wanepimedoside A) 8-(7-Methoxy-77-dimethyl)propylkaempferol 4 -methyl ether... 

An anomaly appeared in the rhamnose series. When acetobromorhamnose was treated with methanol, and the product was deacetylated with alkali, a methyl rhamnoside acetate was obtained which had retained an acetate group not saponifiable by alkali Fischer named this compound 7-methyl rhamnoside monoacetate. It was not until 1930 that it was shown to be the l,2-(methyl orthoacetate) (25). Today, more orthoacetates of the same kind are known in the sugar series. 

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