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Abstract:
Hydrolytically labile ω-chloro-ω-phenyhhioglycosides. obtained from the
corresponding ω-phenylthioglycosides on treatment with NCS in CCL4 are readily dealkoxyhalogenated with Zn/Ag-graphite in anhydrous solvents affording enantiomerically pure synthons bearing an aldehyde function at the one end and a vinylthioether group at the other end of the ring-opened product. This fragmentation technique turned out to be general and was used for the synthesis of a building block exhibiting the substitution pattern of the C9-C14 segment common to all trienomycin-, mycotrienin- and ansattienin macrolide antibiotics. Zn/Ag-graphite was also used to reductively ring-open the 6-deoxy-6-iodopyranoside 37, easily accessible from D-glucose, to enal 38. which served as central precursor for the synthesis of both the C1-C6 as well as C7-C13 unit of the polyhydroxylated chain of amphoteronolide B. Combining these two segments 45 and 51 by ketophosphonatelaldehyde coupling afforded enone 54 which is synthetically
equivalent to an intermediate of a former total synthesis of this macrolide. Thus, a
convergent synthesis of the C1-C13 fragment of amphotericin B based upon the hidden C2-symmetry of this target molecule was achieved using metal-graphite promoted ring-opening of carbohydrates in the key step in combination with formal inversion of the
configuration of the resulting hex-5-enals.
