Oligosaccharide Analogues of Polysaccharides, Part 23, Synthesis of a Dimeric Acetyleno Cyclodextrin from a Mannopyranose-Derived Dialkyne

Abstract

The 1,4-cis-diethynylated a-d-mannopyranose analogue 11 has been prepared from 1,6 : 2,3-dianhydro-b-dallopyranose (6) by alkynylating epoxide and acetal opening (Scheme 2). Eglinton coupling of 11 gave the cyclodimer 18 (Scheme 3). Crystal-structure analysis of the corresponding bis(methanesulfonate) 19 revealed substantially bent butadiyne moieties; one mannopyranosyl ring adopts the 4 C 1 and the other one a slightly distorted O S 2 conformation ( Fig. 1). Hydrogenation of 18, followed by deprotection, gave the stable butane-1,4-diyl-bridged cyclodimer 21 (Scheme 3). Crystal-structure analysis shows the 4 C 1 conformation of the mannopyranosyl units (Fig. 2). The two butane fragments are characterised by a combination of gauche and antiperiplanar arrangements.Introduction. ± In the context of the synthesis of alkynylated saccharides, we have so far prepared up to hexadecameric cellulose analogues where the glycosidic O-atom is replaced by a butadiyne group, interrupting intramolecular, inter-residue H-bonds so as to evaluate their contribution to the supramolecular structure of cellulose (see [2] and earlier publications cited there). For this, we have synthesised glucopyranose derivatives possessing 1,4-trans-oriented ethynyl substituents [3] [4]. We have also synthesised the isomeric a-d-glucopyranose analogues, possessing 1,4-cis-oriented ethynyl substituents [5]. These have been incorporated in cyclotrimeric, cyclotetrameric, and cyclohexameric analogues of cyclodextrins [6]. Remarkably, oxidative oligomerisation of the dialkyne 1 has led in a single step to the C 1 -symmetric cyclotrimer 2 (Scheme 1), while the larger cyclooligomers were prepared by selective cross-coupling. We have also used 4-, or 1-monoethynylated glucopyranose derivatives to transform maltohexaose derivatives to maltooctaoses possessing terminal ethynyl groups, and further to cyclomaltooctaose analogues, in which a butadiyne-1,4-diyl group is replaced by one glycosidic O-atom [1].In the course of the synthesis of these acetyleno sugars, we have devised methods for the invertive [5] or retentive [3] alkynylating opening of 1,6-anhydro-b-dglucopyranoses, providing a rapid access to a-or b-d-glucopyranosyl-acetylenes. It was tempting to also prepare the analogous branched chain a-d-mannopyranosylacetylene and to examine its oxidative cyclooligomerisation. A few mannopyranosylacetylenes have been prepared [7 ± 10]. Reductive dehydroxylation of an ulose

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].…”

Oligosaccharide Analogues of Polysaccharides Part 25

“…± In the context of the synthesis of dialkynylated saccharides, we have prepared C(4)-ethynylated a-dand b-d-glucopyranosylacetylenes [2 ± 5] and C(4)ethynylated a-d-mannopyranosylacetylenes [6]. These monomers were transformed into oligomeric buta-1,3-diynylated cellulose analogues (up to a hexadecamer [7] [8]) and into gluco-and manno-configured analogues of cyclodextrins that are devoid of intramolecular, inter-residue H-bonds [6] [9 ± 11].…”

“…Protection, in the 1,6anhydro-b-d-mannopyranose 5, of both HOÀC (2) and HOÀC(3) by the TIPS group prevents an intramolecular alkynyl shift. Opening of the anhydro ring, ring flip, and intermolecular, inverting alkynyl transfer from the less hindered, axial direction has led in good yields to the a-d-mannopyranosylacetylene 6 [6].…”

“…a-d-Mannopyranosylacetylenes have been prepared selectively by intermolecular alkynylating ring opening of 1,6-anhydromannoses [6] and by treatment of a mannopyranosylacetate with 1-(tributylstannyl) 2-(trimethylsilyl)acetylene and trimethylsilyl triflate [18]. Cross-coupling of a mannopyranosyl bromide with an alkyne under conditions of the Sonogashira reaction led, in modest yields, to anomeric mixtures of mostly the a-d-mannopyranosylacetylenes [19].…”

“…Even the reduction of the hemiketal prepared by addition of an acetylide to 2,3,4,6-tetra-Obenzyl-d-mannonolactone led to a mixture of anomers, and provided no more than 55% of the desired b-d-anomer [20]. Considering that the intramolecular, alkynylating ring opening of 1,6-anhydro-b-d-mannopyranoses might well be the method of choice to prepare either anomer, we selectively protected commercial 1,6-anhydro-b-d- (3), HOÀC(4), and HOÀC (6), and activation of HOÀC(2) (Scheme 2). This was achieved by acetalisation of 18 with butane-2,3-dione in the presence of HC(OMe) 3 and BF 3 ¥ OEt 2 [21], yielding the diol 19.…”