OLIGOSACCHARIDE ANALOGS OF POLYSACCHARIDES .2. REGIOSELECTIVE DEPROTECTION OF MONOSACCHARIDE-DERIVED MONOMERS AND DIMERS

The Me(3)Si-C(1) bond of the bis-(trimethylsilyl)ethynylated anhydroalditol 2 is selectively cleaved with BuLi to yield 3/4, while AgNO2/KCN in MeOH cleaves the Me(3)Si-C(2') bond, leading to 5 (Scheme 1). Both Me(3)Si groups are removed with NaOH in MeOH (-->7), the (i-Pr)(3)Si group is selectively cleaved with HCl in aq. MeOH (-->6); all silyl substituents are removed with Bu(4)NF (-->8). Acetolysis transformed 9 into 13, which was desilylated to 14, while thiolysis of 9 led to a mixture 11/12. The tetraacetate 14 has also been obtained from 9 via 10. Oxidative dimerisation of either 3 or 5, or of a mixture 3/5 yields only the homodimers 15 and 16 (Scheme 2); treatment of 16 with AgNO2/KCN yielded 17, deprotection proceeding much more slowly than the cleavage of the Me(3)Si-C(2') group of 2. The iodoalkyne 20, required for the cross-coupling with 5 according to Cadiot-Chodkiewicz, was prepared by deprotection of 3/4 to 18, methoxymethylation]ation (-->19), and iodination. Cross-coupling yielded mostly 21, besides the homodimer 22. Similarly, cross-coupling of 20 and 23 (obtained from 5) led to 24 and 22. The structure of 24 was established by X-ray analysis (Fig.), showing a C(6)-C(5') distance of 5.2 A. The conditions for deprotecting 2 were applied to 21, and led to 25 (AgNO2/KCN), 26 (ag. NaOH), 27 (Bu(4)NF), and 29 (HCl/MeOH; Scheme 3). Attempted deprotection of the propargylic-ether moiety with BuLi, however, failed. The dimer 27 was further deprotected to 28. Acetolytic (Ac2O/Me(3)SiOTf) debenzylation of the dimer 30, obtained from 10, gave 31 (83%) which was deacetylated to 32 (Scheme 4). Cross-coupling of 5 and the bromoalkyne 33, obtained from 10, yielded 34; again, acetolysis proceeded well, leading to 35. The cellobiose derivative 38 was prepared from the lactone 36 via 37. The glycosidic linkage of 38 proved resistant to the conditions of acetolysis, leading to 39. Acetolysis of the benzylated thiophene 40 (from 30 with Na2S) yielded the octaacetate 41, but proceeded in substantially lower yields (50%).