SYNTHESIS OF N-ACETYLALLOSAMINE-DERIVED DISACCHARIDES

The 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 38-4O gave 42, the axial. intramolecularly H-bonded OH-C(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 alpha-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 beta-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 alpha-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).