OLIGOSACCHARIDE ANALOGS OF POLYSACCHARIDES .4. SYNTHESIS OF A MONOSACCHARIDE-DERIVED OCTAMER

NaSMe in toluene leads to regioselective de-C-silylation of the bis[(trimethylsilyl)ethynyl]saccharide 2, but to decomposition of butadiynes such as 1 or 12. We have, therefore, combined the known reagent-controlled, regioselective desilylation of 2 and of 12 (AgNO2/KCN) with a substrate-controlled regioselective de-C-silylation, based on C-silyl groups of different size. This combination was studied with the fully protected 3 which was mono-desilylated to 4 or to 5 (Scheme 1). Triethylsilylation of 5 (-->6) was followed by removal of the Me(3)Si group (-->7), introduction of a (t-Bu)Me(2)Si group (-->8) and removal of the Et(3)Si group yielded 9; these high-yielding transformations proceed with a high degree of selectivity. Iodination of 4 gave 10. The latter was coupled with 5 to the homodimer 11 and the heterodimer 12, which was desilylated to 13. The second building block for the tetramer was obtained by coupling 14 (from 7) with 5, leading to 15 and 16. Removal of the Me(3)Si group (-->17) and iodination led to 18 which was coupled with 13 to the homotetramer 20 and the heterotetramer 19 (Scheme 2). Deprotection of 19 gave 21, which was, on the one hand, iodinated to 22, and, on the other hand, protected by the (t-Bu)Me(2)Si group (-->23). Removal of the Et(3)Si group (-->24) and coupling afforded the homooctamer 26 and the heterooctamer 25. Yields of iodination, silylation, and desilylation were consistently high, while heterocoupling proceeded in only 50-55%. Cleavage of the (i-Pr)(3)SiC and MeOCH(2)O groups of 11 (-->27), 15 (-->28), 20 (-->29) and 26 (-->30) proceeded in high yields (Scheme 3). Complete deprotection in two steps of the heterocoupling products 16 (-->31 -->32), 19 (-->33 -->34), and 25 (-->35 -->36) gave the unprotected dimer 32, tetramer 34, and octamer 36 in high yields (Scheme 4). Only the dimer 32 is soluble in H2O; the H-1-NMR spectra of 32, 34, and 36 in (D-6)DMSO (relatively low concentration) show no signs of association.