Synthesis of the Glycoalkaloids of Selaginella doederleinii and Structure Revision of One of Them

Abstract

AssTucr.-The syntheses of hordenine-a-L-rhamnopyranoside 111 and of the three possible isomers resulting from its glycosylation by (E)-6-O-cinnamoylglucose, as their acetyl derivatives 3 , 4 , and 5, have been achieved. These results have led us to revise the structure of the acylated glycoalkaloid previously isolated from Selaginella hderIeinii from 2 to 6. In addition, the structure of a minor glycoside isolated from the same plant has been established as 7, on the basis of the synthesis of its acetyl derivative 14.We have recently reported the isolation from Selaginella hederleinii Hieron. (Selaginellaceae) of several hordenine-derived glycosides ( 1). The main constituent belonging to this series is hordenine-a-L-rhamnopyranoside 117. It is accompanied in the plant extract by several minor glycosides of the general structure 6-O-cinnamoyl-(or 4hydroxycinnamoyl-)-glucopyranosyl-rhamnopyranosyl-hordenine. The structure of one of them has been concluded to be (E)-hordenine-I6-O-cinnamoyl-f3-~glucopyranosyll-( 1~4)-a-~-rhamnopyranoside 127 on the basis of its 13C-nmr data ( l), compared with those of flavonoid glycosides with a closely related sugar moiety, previously isolated from Ginkgo biloba (2,3). Nevertheless, more recently, an alternative structure with a different position of linkage between glucose and rhamnose units has been postulated for these flavonoids, on the basis of similar 13C-nmr data (4). Finally, the correct structures of Ginkgo flavonoids were unequivocally established, using 'Hnmr multiple-pulse experiments ( 5 ) . It was therefore obvious that '3C nmr did not permit us to assign unambiguously the site of glucosylation, at C-2, C-3, or C-4 on the rhamnose unit, in such derivatives (6).We report here the synthesis of hordenine-a-L-rhamnopyranoside 111 and of the three possible isomers resulting from its glycosylation by (E)-6-O-cinnamoyl glucose, as their acetyl derivatives 3 , 4 , and 5. These results permitted us to determine with certainty the structure of the acylated glycoside previously isolated from S . dwdereinii, which should be revised from 2 to 6. In addition, the same syntheses using 4-hydroxycinnamic acid as acylating unit allowed us to depict as 7 the structure of a minor glycoalkaloid isolated initially in too small an amount to record its 13C-nmr spectrum. RESULTS AND DISCUSSIONTo our knowledge, glycosidation reactions targeted towards the synthesis of 0 -a -Lrhamnosides of phenols have not been systematically explored. Therefore, it was logical to try to synthesize 1 by various reactions previously described either for the rhamnosidation of alcohols or for the glycosidation of phenols by various carbohydrate units. 'This work has been previously presented at the Second Sino-French Symposium on the Chemistry of Journal of Natural Products mol. 53, No. 4 equimolecular mixture of the orthoester 12 and the expected protected glycoside 11 in 20% overall yield. Finally, hordenine-(2,3,4-tri-O-acetyl)-O-a-~-rhamnopyranoside 1117 could be readily obtained in 46% yield by treatment o...

Chemodiversity in Selaginella: a reference system for parallel and convergent metabolic evolution in terrestrial plants

“…This class of specialized compounds often exhibit significant pharmacological and psychoactive effects and are widely used as medicines and mood modulators (Facchini, 2001). Five N -methyltyramine-derived phenolic alkaloids were reported in S. doederleinii ( 103 – 107 ; Chao et al, 1987, 1990; Lin et al, 1991; Figure 7A ). The first committed step for N -methyltyramine biosynthesis from tyrosine to tyramine is catalyzed by tyrosine decarboxylase, gene homologs of which can be found in the S. moellendorffii and P. patens genomes (Kawalleck et al, 1993).…”

Nuclear Magnetic Resonance Spectroscopic Approaches for the Determination of Interglycosidic Linkage and Sequence in Oligosaccharides

“…Various pulse sequences (such as FLOCK, COLOC, XCORFE etc) have been employed in order to observe these long-range heteronuclear couplings under the ''C detection mode for deducing IGL as well as the sequence in flavonoid glycosides (Anderson et al, 1991a, b;Hasler et al, 1992;Hu et al, 1992;Mizuno et al, 1990). in saponins (Bader et al, 1992;Ding et al, 1992;Jiang et al, 1994;Kawashima et al, 1991a,b;Kubo et al, 1992a,b;Mimaki and Sashida, 1990a,b;Nagao et al, 1991a,b,c;Orsini et al, 1994;Reznicek et al, 1992;Seaforth et al, 1992;Xu et al, 1992a,b), in glycoalkaloids (Chao et al, 1990), in a bufanoloide glycoside (Krenn et al, 1991), in mono-and di-terpenoid glycosides (Orihara et al, 1991;Yahara et al, 1991), in phenolic glycosides (Ikeya et al, 1991;Noda et al, 1992a,b) and in oligosaccharides (Enriquez, 199 1 ;McIntyre and Vogel, 1989;Saitoh et al, 1993a,b;. The "Cdetected methods for the determination of long-range heteronuclear connectivity require more sample, by at least an order of magnitude, than the one-bond connectivity experiment and, therefore, these have been largely replaced by inverse heteronuclear techniques.…”

Rapid Screening and Structural Characterization of Antioxidants from the Extract ofSelaginella doederleiniiHieron with DPPH-UPLC-Q-TOF/MS Method

“…It has the function of eliminating wind, scattering frigidity, detumescence, and treating cough [1]. In the reported research, different types of compounds from Selaginella doederleinii , including biflavonoid, alkaloid, xylogen, sterol, and organic acid, have been exhibited [2–6]. …”