A Glycal Approach to the Synthesis of Pregnane Glycoside P57: A Glycal Approach to the Synthesis of Pregnane Glycoside P57†

Pregnane glycoside P57 is known as an active component of Hoodia gordonii, which has long been used as a diet with appetite suppressant property. Chemical synthesis of P57 demands installation of -cymarosyl linkage onto the steroid aglycone. The stereoselective formation of this special 2-deoxy--glycosidic linkage has previously been proven to be difficult and thus a limiting step in the synthesis of P57 and its congeners. Herein, we report the glycosylation reactions with glycals as donors and TPHB (triphenylphosphine hydrobromide) as promoter and a convergent synthesis of P57 via a -selective glycosylation with a trisaccharide glycal donor.