Ring-Closing Alkyne Metathesis. Application to the Total Synthesis of Sophorolipid Lactone

The first total synthesis of a major component of the microbial biosurfactant sophorolipid has been achieved. This approach to the 26-membered macrolide 1 containing a Z-configured alkene group in its lipidic tether spanning the sophorose backbone is based on a ring-closing metathesis reaction of diyne 21 catalyzed by Mo[N(t-Bu)(Ar)]3 (5; Ar = 3,5-dimethylphenyl) activated in situ by CH2Cl2, followed by Lindlar reduction of the resulting cycloalkyne 22. The two β-glycosidic linkages of compound 21 were installed by means of the glucal epoxide method and a modified Koenigs−Knorr reaction promoted by AgOTf/lutidine, respectively