Furostane-Type Steroidal Saponin from Digitalis ciliata

Abstract

A new furostane-type steroidal glycoside and derivative of tigogenin (1) was isolated from aqueous wastes from production of the cardiac drug acetyldigitoxin from leaves of Digitalis ciliata Trautv. (Scrophulariaceae) and characterized. The structure of the glycoside was established using physical constants, chemical transformations, and spectral data Ciliate foxglove (Digitalis ciliata Trautv.) (Scrophulariaceae) is widely distributed in alpine meadows of Georgia and is endemic to both the Republic and the whole Caucases region.We reported previously on three new glycosides isolated from aqueous wastes from production of the drug acetyldigitoxin [1, 2] from leaves of ciliate foxglove. Herein we present results from a study of the chemical structure of a new furostane-type tigogenin glycoside, pentaoside 1.The water-soluble total steroidal glycosides were chromatographed repeatedly over a column of silica gel to isolate a mixture of two compounds with similar R f values (1a and 1b). Spots of both glycosides were colored yellowish-green on spraying chromatograms with Sannie reagent [3]; rosy red, with Erlich reagent [4]. The IR spectrum of the mixture exhibited a weak broad band at 895 cm -1 [5] and lacked bands for the spiro-ketal group characteristic of spirostane-type glycosides [6].The PMR spectrum of glycoside 1b showed a strong 3H singlet for methoxyl at G 3.14 whereas it was missing in the spectrum of 1a. The data were typical of furostane-type compounds. The 22-OH group is readily alkylated by lower alcohols (MeOH, EtOH) in the presence of silica gel due to its high reactivity. Therefore, the native 22-OH-furostane is practically always accompanied by its slightly less polar 22-O-methyl/ethyl ether.Heating the mixture of glycosides in water for 2 h produced 1, total acid hydrolysis of which isolated the aglycon tigogenin and a mixture of monosaccharides. TLC and GC detected D-glucose, D-galactose, and D-fucose in a 3:1:1 ratio.