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.