Total synthesis of C-glycosylangucycline, urdamycinone B, using an unprotected sugar

The total synthesis of urdamycinone B (1), a prototypical member of the C-glycosylangucycline antibiotics, was achieved by a novel and effective strategy without any protecting group in the sugar moiety. The unprotected C-glycosyljuglone 6 was effectively synthesized by the aryl C-glycosidation of 1,5-naphthalenediol (16) with the totally unprotected D-olivose (8) and the subsequent regioselective photooxygenation of the resultant C-glycosylnaphthalenediol 17. On the other hand, the diene 7 was prepared from 3-methyl-2-cyclohexen-1-one (9) in a short step via the cross-coupling of the vinyl triflate 20 and vinylbutyltin (21) and the Wittig reaction of the aldehyde 24 and the phosphine 25. Finally, the regioselective Diels-Alder reaction of the unprotected C-glycosyljuglone 6 and the diene 7, followed by the regioselecitive introduction of the ketone function at the C1 position, led to the total synthesis of 1.