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 38–40 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 Me 3 SiSEt (→ 39 ), or Cl 3 CCN (→ 40 ).