Efficient Synthesis Of Lablabosid F

The Lablaboside F is a natural saponin compound that was separated and identified from the seed of the Legumnosage plant (Dolichos lablab L). It shows excellent ajuvant activilies corpared to the well-known ajuvant QS-21. Therefore, development of an efficient synthese approach to Lablaboside F is demanding. Based on our previous experience on the chemical syntheses of complex saponins, starting from the oleanolic acid and the glucose glycosyl ortho-alkynylbenzoates, galactose glycosyl ortho-alkynylbenzoates and rhamnose glycosyl ortho-alkynylbenzoates, we accomplished the synthesis of Lablaboside F in a efficient manner, in which the glycosidie linkages were constructed by the gold(I) catalyzed glycosylation reaction.Based on the literature, the glucose donor was synthesized from the acetyl group protected glucose. After bromination of Ac protected glucose at C-1position using HBr/HOAc, followed by ortho esterization with trimrthyl orthoacetate, the ortho ester was prepared. Deacetylation of the ortho ester at3,4and6positions, and then benzylation using NaH/BnBr followed by opening the ortho ester under the acidic conditions. The obtained intermediate was acetylated at1and2position under Ac2O/Py conditions, and was selectively removed acetyl group at1position. The compound with the ortho-alkynylbenzoates was prepared for the donor. Deacetylation of the opened ortho ester was completed under the acidic conditions with NaOMe. Benzolytion of the obtained compound at1and2position under BzCl/DMAP conditions, and then selectively removing of Bz group at C-1position, then the compound with the ortho-alkynylbenzoates was prepared for the donor. The galactose donor was also prepared similar to that of the glucose. The rhamnose donor was preparated similar to that of the synthesis of Lobatoside E. In this way, the total synthesis of the Lablaboside F was acomplished in15steps and in14%yield.