Reactions of Partially Acylated Aldohexopyranosides, VIII. A New Synthesis of the Tetradeoxydisaccharide in Avermectins

Methyl 4‐ O ‐benzyl‐3‐ O ‐methyl‐2‐ O ‐pivaloyl‐α‐ L ‐rhamnopyranoside ( 6 ) was prepared from L ‐rhamnose in an improved synthesis in six steps (overall yield: 51%) and was used as the key building block. Acetolysis of 6 gave the glycosyl donor, 1,4‐di‐ O ‐acetyl‐3‐ O ‐methyl‐2‐ O ‐pivaloyl‐α‐ L ‐rhamnopyranose ( 7 ), which was coupled with the acceptor, methyl 3‐ O ‐methyl‐2‐ O ‐pivaloyl‐α‐ L ‐rhamnopyranoside ( 8 ), simply prepared by debenzylation of 6 . The glycosylation reaction was carried out in the presence of trimethylsilyl triflate and led exclusively to the α‐linked disaccharide, methyl 4‐ O ‐(4‐ O ‐acetyl‐3‐ O ‐methyl‐2‐ O ‐pivaloyl‐α‐ L ‐rhamnopyranosyl)‐3‐ O ‐methyl‐2‐ O ‐pivaloyl‐α‐ L ‐rhamnopyranoside ( 9 ). Selective removal of the acetate group and irradiation of the 2,2′‐dipivaloates gave directly the tetradeoxydisaccharide, methyl 4‐ O ‐(α‐ L ‐oleandrosyl)‐α‐ L ‐oleandropyranoside ( 11 ). Irradiation of compound 8 afforded methyl α‐ L ‐oleandropyranoside ( 11 ). Irradiation of compound 8 afforded methyl α‐ L ‐oleandropyranoside ( 12 ) as the only product in 80% yield.