Oligosaccharide analogues of polysaccharides: Part 25. Synthesis of mono- and diethynylated analogues of 2-acetamido-2-deoxy-D-glucopyranose

Article abstract of DOI:10.1002/1522-2675(200208)85:8<2235::AID-HLCA2235>3.0.CO;2-I

The 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 β-D-mannopyranose 15, possessing a C(3)-OH 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)-O triflate 20. Treatment with NaN₃ in DMF afforded a 85:15 mixture of the β-D-gluco configured azide 21 and the elimination product 22. Similarly, the α-D-mannopyranosylacetylene 23 was transformed into the azide 26. Retentive alkynylating ring opening of the ethynylated anhydromannose 28 gave the expected α-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) HO-C(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 β-D-gluco azide 36, which was transformed into the diethynylated β-D-GlcNAc analogue 38. Similarly, the diethynylated α-D-mannopyranoside 39 was transformed into the disilylated α-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 C₁-symmetric cyclotrimer 45 in moderate yields.