No abstract

A total of 14 ocotillol-type ginsenosides were conveniently synthesized employing glycosylation of ocotillol sapogenin derivatives with glucosyl ortho-alkynylbenzoate donors under the promotion of a gold(I) catalyst as the key step. Relying on a rational protecting group strategy and the unexpected regioselectivity of the glycosylation of the 3,25-diol sapogenins (2a/2b, 5a/5b) for the tertiary 25-OH, mono 3-O-glucosyl ocotillol-PPD, 6-O-glucosyl ocotillol-PPT, 25-O-glucosyl ocotillol-PPD/PPT and 3,25-di-O-glucosyl ocotillol-PPD/PPT ginsenosides were prepared in which the configuration at the C-24 is either R or S.

Four representative Δ-ginsenosides, namely, ginsenosides Rh (1), (20E)-Rh (2), Rg (3), and Rk (4) from Panax Ginseng, were chemically synthesized for the first time. Dehydration of the naturally occurring 20(S)-protopanaxatriol and 20(S)-protopanaxadiol provided all types of Δ-sapogenins, which were separated due to a judicious choice of protecting groups. The Δ-sapogenins were then directly glycosylated with glycosyl ortho-alkynylbenzoate donors under the catalysis of PhPAuNTf as key steps. The neutral conditions of the glycosylations were crucial to prevent the acid-labile Δ double bond from isomerization.