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

Abstract

The Me$-C( 1) bond of the bis-(trimethylsily1)ethynylated anhydroalditol2 is selectively cleaved with BuLi to yield 3/4, while AgN02/KCN in MeOH cleaves the Me3Si-C(2') bond, leading to 5 (Scheme I ) . Both Me,Si groups are removed with NaOH in MeOH (+ 7), the (i-Pr)$i group is selectively cleaved with HC1 in aq. MeOH (+ 6 ) ; all silyl substituents are removed with Bu,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 viu 10. Oxidative dimerisation of either 3 or 5, or of a mixture 3/57 yields only the homodimers 15 and 16 (Scheme 2); treatment of 16 with AgN02/KCN yielded 17, deprotection proceeding much more slowly than the cleavage of the Me,Si-C(Z') group of 2. The iodoalkyne 20, required for the cross-coupling with 5 according to Cadiof-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 A. The conditions for deprotecting 2 were applied to 21, and led to 25 (AgN02/KCN), 26 (as. NdOH), 27 (Bu,NF), and 29 (HCI/MeOH; Scheme 3). Attempted deprotection of the propargylic-ether moiety with BuLi, however, failed. The dimer 27 was further deprotected to 28. Acetolytic (Ac,O/Me,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,S) yielded the octaacetate 41, but proceeded in substantially lower yields (50%). J = 11.3, PhCH); 4.84 (d, J = 11.3, PhCH); 4.80 (d, J = 11.3, PhCH); 4.77 (d, J = 11.0, PhCH); 4.60 (d, J = 11.3, PhCH); 4.47 (d, J = 12.1, PhCH); 4.36 (d, J = 12.1, PhCH); 3.99 (dd, J = 9.6, 0.7, H-C(5')); 3.73 (d, J = 9.3, H-C(3)); 3.71 (t, J = 9.6, 9.2, H-C(8)); 3.66 ( t . J =9.3, H-C(4)); 3.65 (dd, J = 11.3, 3.9, H-C(10')); 3.64 (m, H-C(8)); 3.61 (dd, J = 11.1, 1.8, H'-C(lO')); 3.60 (t, J = 9.6, 9.2, H-C(6)); 3.48 (t, J = 9.0, H-C(7')); 3.45 (td, J = 12.4, 5.0, H'-C(8)); 3.22 (ddd, J = 10.4, 8.2, 3.9, H-C(5)); 3.17 (ddd, J = 9.8, 3.7, 1.9, H-C(9')); 2.96 (ddd, J = 10.2, 5.0, 2.3, H-C(7)); 2.53 (br. t, J = 10.3, H-C(6)); 1.88 (d, J =3.9, OH-C(5)); 1.37 (t. J = 6.3, OH-C(8)); 1.34-1.21 (m, (i-Pr),Si); 0.15 (s. Me,Si). 13C-NMR (50 MHz,

Oligosaccharide Analogues of Polysaccharides, Part 19, Synthesis of 2-(Naphthalen-1-yl)ethyl Cellooligoglycosides and [(Naphthalene-1,8-diyl)di(ethane-2,1-diyl)] Bis[cellooligoglycosides]

“…Workup and FC (hexane/AcOEt 8 : 2) gave 26 3501w, 3090w, 3058m, 2868m, 1954w, 1875w, 1817w, 1605w, 1490w, 1448m, 1401w, 1359m, 1311w, 1206w, 1116s, 1090s, 1064s, 1027m, 990w, 916w, 826w. (27). A suspension of 26 (2.967 g, 1.66 mmol), AgOTf (2.5 g, 3.32 mmol) and 3-molecular sieves in 1,2-dichloroethane (25 ml) was stirred for 1 h, cooled to À 408, treated dropwise with a soln.…”

“…Cross-coupling of the diiodide 6 [26] with the octynitol 5 [27] gave 39% of the dialkyne 7, besides 5% of the corresponding monoalkynylated monoiodide (Scheme 1) 4 ). However, in view of the low stability of 7 (it decomposed at 238 within a few days), we quickly abandoned this type of compounds.…”

Oligosaccharide Analogues of Polysaccharides, Part 16, Cross-Coupling of Partially Protected Dialkynyl Monosaccharides

“…There are relevant differences for the HOÀC (5) and to some extent also for the HOÀC (8) signals. For the spectra of 5, 7, 12, and 13 in CDCl 3 , the J(HOÀC(5),HÀC(5)) value of ca.…”

“…Since bromoalkynes have led to a higher ratio of hetero-to homocoupling products than iodoalkynes [12], we brominated 7 with N-bromosuccinimide (NBS) in the presence of AgOCOCF 3 [2] [13] to give 97% of the bromoalkyne 10. It is remarkable that bromodesilylation [13] of the propargylic alkynyl group (i.e., the alkynyl group at C(3)) of 5 with excess NBS was not complete after 65 h, while the analogous bromodesilylation of a homopropargylic alkynyl group (i.e., the alkynyl group at C(6)) has been successful [2], reflecting the higher nucleophilicity of homopropargylic alkynyl ethers [8] 4 ). To remove the DOPS group of 5, we had to selectively cleave the primary TIPS ether.…”

Oligosaccharide Analogues of Polysaccharides, Part 17, Synthesis and Characterization of a Hexadecamer: Evidence for the Essential Structural Role of the Intramolecular, Interresidue C(3)−OH⋅⋅⋅O−C(5) Hydrogen Bond in Celluloses

“…Iodination with NIS and AgOCOCF 3 was no longer selective, partially removing the THP group of 55, but I 2 in the presence of morpholine [14] [19] transformed 55 in 86% to the iodoalkyne 56. Similarly, the removal of the THP group with Amberlyst 15 was no longer appropriate, yielding only 5 ± 7% of the H-DOPS alkyne 57.…”