Acetals of three new cycloartane-type saponins from Egyptian collections of Astragalus tomentosus.

Three new cycloartane-type saponin ethyl acetals, deacetyltomentoside I (2), tomentoside III (3), and tomentoside IV (4), were isolated along with the known acetal tomentoside I (1) from the aerial parts of Astragalus tomentosus of Egyptian origin. The saponins from which the acetals are most probably derived are also new compounds. The structures of the acetals were established as 6alpha-hydroxy-23alpha-ethoxy-16beta,23(R)-epoxy-24,25,26,27-tetranor-9,19-cyclolanosta-3-O-beta-d-xyloside (2), 6alpha-acetoxy-23alpha-ethoxy-16beta,23(R)-epoxy-24,25,26,27-tetranor-9,19-cyclolanosta-3-O-[beta-d-(4'-trans-2-butenoyl)]xyloside (3), and 6alpha-acetoxy-23alpha-ethoxy-16beta,23(R)-epoxy-24,25,26,27-tetranor-9,19-cyclolanosta-3-O-[beta-d-glucopyranosyl(1 --> 2)]-beta-d-xyloside (4), by detailed spectroscopic and chemical studies.