Abstract:
Organic chemists have always confronted the difficult task of synthesizing carbohydrates of with regio- and sterospecific specific glycosydic linkages, some of them frequently found in nature, with exert significant biological activities. To overcome this hurdle, they introduced an ingenuous synthetic method called "Intramolecular Glycosylation".
Intramolecular glycosylation is the reaction between a glycosyl donor and acceptor that are both linked to a tether. The tether can be a temporary tether which is cleaved during glycosylation process, or a stable tether which does not participate in the glycosylation reaction, but prearranges both glycosyl donor nad acceptor in a certain orientation such as to enforce glycosylation regio-and stereoselectively thus creating a specific O-glycosydic bond.
The development of such an efficient method opened the gateways to significant achievements ments such as, the synthesis of beta-D-mannosides, beta-D-fructofuronosides, beta-glycosydic bonds between L-rhamnose and glucose which are very difficult or almost impossible to obtain through conventional glycosydic methods.
We introduce herein a significant achievement with the intramolecular glycosylation with a stable tether toward a regio-and stereoselective synthesis of the beta(1-3) glycosydic linkage between a galactose donor and a glucose acceptor that bears a free 2-OH and 3-OH. This glycosidic bond is found as repeating unit in certain capsular polysaccharides, which are extracted from various bacterium, cerebrosides which exert an important function in constituting the human cell membrane, also in Saponins, which are extracted from various genuses of plants and are used as anti-hepatitis, anti-Leischmanial, and cardiac agents. The importance of such glycosydic bond sparkled us do devise a regio- and stereoselective synthesis of this beta(1-3) linkage. As spacer of choice was a succinylamide alkyl in which the alkyl is a pentyl, butyl, propyl and ethyl spacer in which the monopeptide controls the flexibility of the alkyl part of the spacer, thus orienting both glycosides such as to allows complete stereo-and regioselective glycosylation of one OH group and leaving the other hydroxyl almost fully discriminated. When applying this method we succeeded in constructing the beta(1-3) glycisydic bond with exellent to good stereoselectivity and good yield. Intramolecular glycosylation was successfully effected between a 3,4,6-O-tribenzylated galactose thioglycoside donor and a 4,6-O-benzyli- denated glucosyl acceptor were the 3-OH has been almost completely chosen for stereo-and regioselective glycosylation for the first three spacers and the 2-OH discriminated.
Only with the more constrainted ethyl spacer was the 3-OH fully discriminated, and the 2-OH underwent glycosylation to afford alpha-(1-2) glycosydic bond as major product and alpha-(1-3) glycosydic linkage as minor product. An attempt to glycosylate the embedded 2-OH of the glucose acceptor of the first synthesized cyclic disaccharide containing the pentyl spacer, with a tribenzoylated (L)-rhamnose thioglycoside gave no formation of trisaccharide, thus no reaction.
We delved into further investigation by conducting a molecular modelling analysis which would provide further explanations of the anomeric outcomes and the non-reactivity of the free 2-OH toward other reactions.
With molecular modelling we have been able to obtain surprising results and explanations. First, the course of all intramolecular glycosylations reactions are oriented by aromatic stacking triads between the benzylether protecting groups of the galactose donor and the benzylidene protecting group of the ajacent glucose acceptor. Moreover, the free embeded 2-OH of the glucose acceptor forms a bifurcated hydrogen bond with the monopetide and the ester moiety of the spacer. consequently is the nucleophilicity of the free 2-OH considerably decreased, thus unreactive. The cavity of the macrocylic ring of the cyclic disaccharides is too small to accomodate another sugar, even a benzoylation or methylation revealed not obtainable.
The molecular modelling informs us on the solvent's choice, since the intramolecular reaction was performed in a mixture of dichloromethane/acetonitrile 1:1. The use of toluene would result in a different outcome . This stems from the fact that, toluene being an aromtic solvent can intercalate within the benzylether and benzylidene protecting groups. The dichloromethane and acetonitrile solvents does not intercalate, thus allowing maximum aromatic stacking between benylether of the galactose donor and benzylidene groups of the glucosyl acceptor, and all the anomeric outcomes of the glycosylation reactions.
The case of the succunylamide ethyl spacer distinguished itself from all of the others with the formation of alpha-(1-2) glycosydic linkage as major product. During glycosylation, the thiophenyl group is rapidly expulsed from the glycosydic center generating an acylium ion which because of the more constrainted ethyl spacer attcks preferntially the more embedded 2-OH and leaving the more accessible 3-OH discriminated. For the case of the ethyl spacer, molecular modelling 2 aromatic stacking diads stacking, one between the 3 and 4-O-benzylether group of the galactose donor and another between the 6-O- benzylether group and the 4.6-O- benzylidene group of the ajacent glucosyl accetor.
Another surprising result, is the formation of two intramolecular hydrogen bonding (bifurcated hydrogen bonding) significantly (7 kcal/mole) more stale than in the beta(1-3) products, leading to the formation of an intramolecular hydrogen beta-turn like bond, observable on the surface of proteins and, in polypeptides constituting of aromatic or constrained amino acids. This analysis also provides information on the role of the sugar which act as a rigid template that acts as a bifurcated hydrogen bond donor.
Finally, the molecular modelling analysis, reveals itself completely concordant with our synthesis the, 1-D and 2-D NMR analysis, the chosen solvents and protecting groups.
These succinylamide alkyl cyclic disaccharides can be cleaved under normal basic conditions, leaving free two hydroxyls that can be selectively protected followed by the glycosylation of the other hydroxyl with a another sugar. Moreover, the presence of protecting groups that can be partially cleaved allows access to a more complicated saccharide. The side chain linked glycosydically to the glucose acceptor with its free carboxylic group can be activated and react with a polypetide, glycopetide, or any other amines. Another possibility, is the unactivated carboxylic group of the polyfunctional side chain to bind a protein. This methodology emulates the efficiency and elegance of our strategy.
Despite the significant contributions achieved through various methods of Intramolecular Glycosylation and "Intramolecular Aglycon Delivery", this method remains classifyable as a novel synthetic strategy in carbohydrate chemisty. It opens bright perpectives for the obtention and formation of other important glycosydic linkages that are present in many natural compounds, and also for the construction of more complex saccharides.
glycosylation , cyclic disaccharides