Synthesis of Δ 20 -Ginsenosides Rh 4 , (20E)-Rh 3 , Rg 6 , and Rk 1 : A General Approach To Access Dehydrated Ginsenosides.

Four representative Δ20 -ginsenosides, namely, ginsenosides Rh4 (1), (20E)-Rh3 (2), Rg6 (3), and Rk1 (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 Δ20 -sapogenins, which were separated due to a judicious choice of protecting groups. The Δ20 -sapogenins were then directly glycosylated with glycosyl ortho-alkynylbenzoate donors under the catalysis of Ph3 PAuNTf2 as key steps. The neutral conditions of the glycosylations were crucial to prevent the acid-labile Δ20,21 double bond from isomerization.