Oligosaccharide Analogues of Polysaccharides Part 25

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 NaN 3 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 β ‐ 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) HOC(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 C 1 ‐symmetric cyclotrimer 45 in moderate yields.