The carbon-13 magnetic resonance (c.m.1.) spectra of oligosaccharides, derived from yeast mannans by partial acetolysis, contain signals at 6,93.1-105.4 (C-1's) and 79.2-81.2 ('2-2's and/or C-3's substituted by mannopyranosyl units). They were assigned on the basis of ( a ) carbons deshielded by attached mannopyranosyl units have resonances at lower field than those of unsubstituted carbons and (b) comparison under carefully controlled conditions of the relative sizes of signals of comparable carbons, which have the same resonances, in members of two different homologous series of oligosaccharides. The c.m.r. spectra of certain mannans containing (1 -t 6)-linked a-D-mannopyranose main-chains could then be rationalized since they have many signals with resonances in common with those of oligosaccharides, which are derived by partial acetolysis and contain the structures of the side chains. For personal use only.

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A structural elucidation of polysaccharides extracted from the aposymbiotically cultured mycobiont of the lichen Ramalina peruviana was carried out in order to determine whether the polysaccharides found previously in the symbiotic thalli are produced by the mycobiont or photobiont or both. The mycobiont isolate was cultivated on a solid malt-yeast extract-medium and the freeze-dried colonies were defatted and the polysaccharides extracted successively with hot water and aq. 2% KOH, each at 100 degrees C. The alkaline extract was obtained in much higher yield (31.5%) and submitted to a freeze-thawing treatment, giving rise to a precipitate (PK2) of a mixture of (1-->3),(1-->4)-alpha-glucan (1.2:1 ratio, nigeran) and a (1-->3)-beta-glucan (laminaran). The mother liquor was treated with Fehling solution to give a precipitate (galactomannan). This had a (1-->6)-linked alpha-d-mannopyranosyl main chain, substituted at O-4 and in small proportion at O-2,4 by beta-Galp units. All three polysaccharides have previously been found in the symbiotic thalli of R. peruviana, showing that these are produced by the fungus, without the participation of the Trebouxia photobiont. Surprisingly, isolichenan, a cold-water soluble (1-->3),(1-->4)-alpha-linked-glucan (3:1 ratio) was not found in the isolated mycobiont, despite being the main polysaccharide found in the thalli.

The reduction of hexofuranose and hexopyranose acetals with LiAlH,-AICI, (1 :1, probabiy AIH,CI) to the corresponding ethers proceeds in similar fashion as that of sinlple acetais and ketals (Scherne I) (refs. 1-7). Certain 0-methyl and 0-benzyl derivatives, not readily obtainable otherwise, are conveniently synthesized from appropriate acetals in this v,ay. Cyclic orthoesters of glucofuranose may be selectively reduced to acetals; the steric course of such reduction has been shown unequivocally in the case of orthoformates (Scheme 11). Of several cleavable groups, the ease of hydrogenoiysis is ( a ) cyclic orthoester > isopropylidene ketal, cyclohexylidene ketal > benzyiidene acetal > ethylidene acetal > formal and (b) 5,6-0-linked or 3,5-0-linked acetal or ketal > I ,2-0-linked acetal or ketal.Canadian Journal of Chern~stly, 47, 1195Chern~stly, 47, (1969 The reductive cleavage of 1,3-dioxolanes, 1,3-dioxanes, 1,3-oxathiolanes, O-tetrahydropyran-2-yl, and 0-tetrahydrofuran-2-yl derivatives to ethers with LiAIH,-AlCl, has been investigated in detail by several workers, especially in the groups of Eliel (1-3) and Brown (4-7). The effects of various s~lbstituents at C-2 and C-4 (Scheme I) on the position of cleavage ha\ e been established (4). Although polar effects have been found to predominate, steric factors can also play a role. For example, in 1 electron donor substituents at C-4 result mainly in C2-0, bond cleavage due to the formation of an intermediate ion (2) which is partially stabilized, in contrast to the other possible intermediate (3). Steric effects arise (8) in the cleavage of compou~lds that are di-substituted at both C-2 and C-4. ]in this case scission of the C,-0, bond occurs since the intermediate species is destabilized by nonbonded interactions between the C-2 and C-4 substituents. A rarer type of effect has been observed by Brown and co-workers (9) in hydrogenclysis of compounds in which the approach of reducing species is sterically impeded.In the light of the existing knowledge of the reductive cleavage with LiAlH,-AICI,, we were interested ill hydrogenolyzing various carbohydrates with the reagent in order to evaluate ~t s synthetic utility and also to test the validity of the reported mechanistic principles in somewhat more complex systems. Various carbohydrates containing cyclic orthoester, 0-isopropylidene, 0-cyclohexylidene, 0-propylidene, 0-ethylidene, 0-metliylene, and 0-benzylidene groups were prepared by conventional procedures and each treated with an equimolar mixture of lithium alumillium hydride and alurni~lium chloride. The condition of hydrogenolysis was generally similar to that used by Brown et al. (4-7). The crude reaction procl~~cts were examined by thililayer chromatography (t.1.c.) and in some cases by gas-liquid cbromatography (g.1.c.) in order to ascertain the number and proportion of various componeilts. The principal reaction product(s) ( Table 1) obtained from various cotnpounds were isolated and characterized.The routes of cleavage of 0-methylene derivatives t...

Galactomannans were isolated from Cladonia signata, C. furcata, C. imperialis, and C. clathrata via successive alkaline extraction and precipitation with Fehling solution and Cetavlon. They were investigated using "C-NMR spectroscopy, methylation analysis, and Smith degradation, and their specific rotations and monosaccharide compositions determined. As with galactomannans of other Cladonia species, they contained (l->6)-linked main chains of a-mannopyranose, which were non-substituted (structure 4 in Fig. 2), monosubstituted at O-2 with a-mannopyranose (structure 6) or cc-galactopyranose (structure 1), O-4 with (5-galactopyranose (structure 2), and disubstituted at O-2 and O-4 with a-mannopyranosyl and p-galactopyranosyl units, respectively (structure S). Disubstitution was present to a greater extent in the galactomannans of C. clathrata and C. imperialis than in those of C. signata and C. furcata. In the case of the galactomannans of C. furcata, C. clathrata, and C. imperialis, substitution also occurred at O-2 with 0-p-galactofuranosyl-(l->6)-O-a-mannopyranosyl units (structure 7). As observed in previous investigations, the C-l portion of the "C-NMR of mannose-containing polysaccharides is typical of the lichen species. However, those of galactomannans of C. imperialis and C. clathrata are almost identical and, although other chemical data showed many structures in common, some differences were evident.C O 1997 The British Lichen Society