Oligosaccharide Analogues of Polysaccharides. Part 2. Regioselective deprotection of monosaccharide‐derived monomers and dimers

The Me 3 SiC(1) bond of the bis‐(trimethylsilyl)ethynylated anhydroalditol 2 is selectively cleaved with BuLi to yield 3 / 4 , while AgNO 2 /KCN in MeOH cleaves the Me 3 SiC(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 AgNO 2 /KCN yielded 17 , deprotection proceeding much more slowly than the cleavage of the Me 3 SiC(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 (→ 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 Å. The conditions for deprotecting 2 were applied to 21 , and led to 25 (AgNO 2 /KCN), 26 (aq. 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 (Ac 2 O/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 Na 2 S) yielded the octaacetate 41 , but proceeded in substantially lower yields (50%).