Oligosaccharide Analogues of Polysaccharides Part 25

Abstract

F¸r Oskar Jeger mit herzlichen Gl¸ckw¸nschen zum 85. GeburtstagThe ethynylated gluco-azide 11 was prepared from the dianhydrogalactose 7 by ethynylation, transformation into the dianhydromannose 10, and opening of the oxirane ring by azide (Scheme 1). The retentive alkynylating ring opening of 11 and of the corresponding amine 12 failed. (2-Acetamidoglucopyranosyl)acetylenes were, therefore, prepared from the corresponding mannopyranosylacetylenes. Retentive alkynylating ring opening of the partially protected b-d-mannopyranose 15, possessing a C(3)ÀOH group, gave a 85 : 15 mixture of 16 and the (E)-enyne 17. The alkyne 16 was deprotected to the tetrol 18 that was selectively protected and transformed into the C(2)ÀO triflate 20. Treatment with NaN 3 in DMF afforded a 85 : 15 mixture of the b-dgluco configured azide 21 and the elimination product 22. Similarly, the a-d-mannopyranosylacetylene 23 was transformed into the azide 26. Retentive alkynylating ring opening of the ethynylated anhydromannose 28 gave the expected b-d-mannopyranosyl 1,4-dialkyne 29 as the main product besides the diol 28, the triol 31, and the (E)-enyne 30 (Scheme 2). This enyne was also obtained from 31 by a stereoselective carboalumination promoted by the cis (axial) HOÀC(2) group. Deprotection of the dialkynylated mannoside 31 led to 32, whereas selective silylation, triflation, and azidation gave a 3 : 7 mixture of the 1-ethynylglucal 35 and the b-d-gluco azide 36, which was transformed into the diethynylated b-d-GlcNAc analogue 38. Similarly, the diethynylated a-dmannopyranoside 39 was transformed into the disilylated a-d-GlcNAc analogue 41, and further into the diol 42 and the monosilyl ether 43 (Scheme 5). Eglinton coupling of 41 gave the symmetric buta-1,3-diyne 44, which did not undergo any further Eglinton coupling, even under forcing conditions. However, Eglinton coupling of the monosilyl ether 43 and subsequent desilylation gave the C 1 -symmetric cyclotrimer 45 in moderate yields.

Nucleophilic Ring‐Opening of 1,6‐Anhydrosugars: Recent Advances and Applications in Organic Synthesis

“…The stereochemistry of the C-glycosylation process is likely to be controlled by the formation of a covalent bond between the axial OH group at C-3 and the alkynylaluminium chloride resulting in an intramolecular alkynyl shift from the metal center to the oxocarbenium cation intermediate (Scheme 8d). [32] Historically, AlCl 3 -mediated acetylide ring-opening was developed in the 1990s by the group of Vasella for the synthesis of mono-and diethynylated analogues of hexopyranoses. These compounds were used as building blocks in the preparation of original neooligosaccharides and carbohydrate-derived macrocyclic compounds.…”

“…These compounds were used as building blocks in the preparation of original neooligosaccharides and carbohydrate-derived macrocyclic compounds. [31][32][33] Different strategies were used to access αor β-C-alkynyl glycosides in a stereocontrolled manner (Scheme 9 and Scheme 10). The use of fully protected 1,6-anhydro-D-hexoses led to an intermolecular alkynyl transfer from the less hindered, axial direction on the most favored half-chair of the glycosyl cation intermediate leading to α-C-mannoside products (Scheme 9a).…”

The synthesis of d-C-mannopyranosides

“…Alkynylation of a protected 1,6-anhydro-b-D-mannopyranose derivative was studied, and gave opposite selectivities depending on the protecting group pattern (Scheme 14) [112,113].…”

Acetylenosaccharides

“…Mannose and from GlcNAc 1,4-Diethynylated GlcNAc derivatives could not be prepared from levoglucosan, as 1,6-anhydro-2-azido-4-deoxy-4-ethynyl-b-d-glucopyranose and the corresponding amine did not react to afford the expected pyranosylacetylenes under a variety of conditions [201]. These GlcNAc derivatives were therefore prepared from 4-ethynyl-d-mannopyranosylacetylenes.…”