C-Glycosidations Using Unprotected Sugars and Its Application to Synthesis of Bioactive Natural Products

Practical and highly stereo-selective aryl and allyl C-glycosidations using unprotected sugars as glycosyl donors have been developed. Aryl C-glycosidations of several unprotected 2-deoxy sugars with phenol and naphthol derivatives by the combined use of trimethylsilyl trifluoromethanesulfonate (TMSOTf)-AgC10₄ or TMSOTf exclusively gave the corresponding unprotected o-hydroxyaryl β-C-glycosides which appear in many biologically attractive aryl C-glycoside antibiotics as the key subunit. On the other hand, allyl C-glycosidations of several unprotected glycals with allyltrimethylsilane by TMSOTf afforded the corresponding unprotected and 2,3-unsaturated allyl α-C-glycosides in high yields which are versatile synthetic intermediates for the syntheses of optically active natural products. Furthermore, the total synthesis of urdamycinone B, a prototypical member of the C-glycosyl angucycline antibiotics, was accomplished by the successful application of the present aryl C-glycosidation of unprotected sugar as the key step.