Facile synthesis of cleistetroside-2, a partially acetylated oligorhamnoside from Cleistopholis glauca and patens.

A tetrasaccharide, dodecanyl 4-O-acetyl-alpha-l-rhamnopyranosyl-(1-->3)-2,4-di-O-acetyl-alpha-l-rhamnopyranosyl-(1-->3)-4-O-acetyl-alpha-l-rhamnopyranosyl-(1-->4)-alpha-l-rhamnopyranoside (cleistetroside-2), was synthesized via '2+2' convergent strategy. Sequential regioselective 3-O-glycosylation of isopropyl 1-thio-alpha-l-rhamnopyranoside (4) with 4-O-acetyl-2,3-O-isopropylidene-alpha-l-rhamnopyranosyl trichloroacetimidate (8), and isopropyl 4-O-acetyl-2,3-O-isopropylidene-alpha-l-rhamnopyranosyl-(1-->3)-2,4-di-O-acetyl-alpha-l-1-thio-rhamnopyranoside (10) with dodecanyl 4-O-acetyl-alpha-l-rhamnopyranosyl-(1-->4)-2,3-O-isopropylidene-alpha-l-rhamnopyranoside (12), greatly facilitate the target availability.