作者:Jean-Luc Maloisel、Andrea Vasella
DOI:10.1002/hlca.19920750505
日期:1992.8.13
AbstractThe protected disaccharide 44, a precursor for the synthesis of allosamidin, was prepared from the glycosyl acceptor 8 and the donors 26–28, best yields being obtained with the trichloroacetimidate 28 (Scheme 6). Glycosidation of 8 or of 32 by the triacetylated, less reactive donors 38–40 gave the disaccharides 46 and 45, respectively, in lower yields (Scheme 7). Regioselective glycosidation of the diol 35 by the donors 38–40 gave 42, the axial, intramolecularly H‐bonded OHC(3) group reacting exclusively (Scheme 5). The glycosyl acceptor 8 was prepared from 9 by reductive opening of the dioxolane ring (Scheme 3). The donors 26–28 were prepared from the same precursor 9 via the hemiacetal 25. To obtain 9, the known 10 was de‐N‐acetylated (→ 18), treated with phthalic anhydride (→ 19), and benzylated, leading to 9 and 23 (Schemes 2 and 3). Saponification of 23, followed by acetylation also gave 9. Depending upon the conditions, acetylation of 19 yielded a mixture of 20 and 21 or exclusively 20. Deacetylation of 20 led to the hydroxyphthalamide 22. De‐N‐acetylation of the 3‐O‐benzylated β‐D‐glycosides 11 and 15, which were both obtained from 10, was very sluggish and accompanied by partial reduction of the O‐allyl to an O‐propyl group (Scheme 2). The β‐D‐glycoside 30 behaved very similarly to 11 and 15. Reductive ring opening of 31, derived from 29, yielded the 3‐O‐acetylated acceptor 32, while the analogous reaction of the β‐D‐anomer 20 was accompanied by a rapid 3‐O→4‐O acyl migration (→34; Scheme 4). Reductive ring opening of 21 gave the diol 35. The triacetylated donors 38–40 were obtained from 20 by debenzylidenation, acetylation (→36), and deallylation (→37), followed by either acetylation (→38), treatment with Me3SiSEt (→39), or Cl3CCN (→ 40).