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Oligosaccharide Analogues of Polysaccharides. Part 4. Synthesis of a monosaccharide‐derived octamer

1995; Wiley; Volume: 78; Issue: 5 Linguagem: Inglês

10.1002/hlca.19950780515

1522-2675

Jawad Alzeer, Andrea Vasella,

biodegradable polymer synthesis and properties

Abstract 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 (AgNO 2 /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 H 2 O; the 1 H‐NMR spectra of 32 , 34 , and 36 in (D 6 )DMSO (relatively low concentration) show no signs of association.