Trisaccharides model

Our first synthesis of a polymer-bound anomeric amine involved a trisaccharide model system (Scheme 2.20).45 Disaccharide 66 was extended in standard fashion to trisaccharide 67 and fully protected to give 68. This latter compound was treated with anthracenesulfonamide and Jt.vy/u-coll )2C104 to form the intermediate 69. Reaction of the iodosulfonamide 69 with tetra-n-butylammonium azide followed by acetylation provided the anomeric azide 70. 

On this basis, a trisaccharide model was selected having two rhamnose residues linked to quinovosamine uniformly by 1+3 linkages, with a methoxycarbonyloctyl group in glycosidic attachment to the quinovosamine residue. 

Hen egg-white lysozyme catalyzes the hydrolysis of various oligosaccharides, especially those of bacterial cell walls. The elucidation of the X-ray structure of this enzyme by David Phillips and co-workers (Ref. 1) provided the first glimpse of the structure of an enzyme-active site. The determination of the structure of this enzyme with trisaccharide competitive inhibitors and biochemical studies led to a detailed model for lysozyme and its hexa N-acetyl glucoseamine (hexa-NAG) substrate (Fig. 6.1). These studies identified the C-O bond between the D and E residues of the substrate as the bond which is being specifically cleaved by the enzyme and located the residues Glu 37 and Asp 52 as the major catalytic residues. The initial structural studies led to various proposals of how catalysis might take place. Here we consider these proposals and show how to examine their validity by computer modeling approaches. 

The most important result, however, is the binding of the trisaccharide tri-N-acetylchitotriose, as the model proposed for substrate binding and enzymic hydrolysis is based on this compound. The three substituted /3-D-glucopyranose residues are labeled A, B, and C in Fig. 2. Two observations are the same as for 2-acetamido-2-deoxy-D-glucose residue C has the same binding as the /3-d anomer, and the resulting shift of amino acid residue 62 (L-tryptophan) is 0.75 A. The hydrogen bonds between lysozyme and carbohydrates A and B are shown in Fig. 2 and listed in Table IV. Residue A, which is located... 

A half-chair conformation of a crystalline monosaccharide has not been observed. A half-chair conformation for the fourth 2-acetamido-2-deoxy-/3-D-glucopyranosyl residue (residue D) in the lysozyme substrate has not been detected, although, on the basis of model fitting, its presence has been suggested (see p. 96). 2-Acetamido-2-deoxy-/3-D-glucosyl groups were added to a molecular model constructed by use of data obtained from the nature of the enzyme-trisaccharide complex it was implicit that the lifetime of the half-chair conformation would be quite short. 

Following the identification of KDO as a constituent of LPS, studies by Osborn and her group ( 5) have revealed that, in LPS, KDO (or the KDO region ) is located at the reducing end of the polysaccharide chain, linking the core segment to lipid A. Later, the application of differential color reactions based on thiobarbituric acid (TBA tests) (6, 7) has led to the view that KDO is present, in LPS from Salmonella or E. coli, in the form of a branched trisaccharide (Fig. 1). In this model, a branchpoint KDO residue (KDO I), ketosidically-linked to the second glucosaminyl residue of lipid A, is substituted in position 4 or 5 by a... 

The nature of the protecting groups employed in the preparation of disaccharide discussed above does not permit the extension of the synthesis to the branched type of KDO trisaccharide shown in Fig. 1. To obtain another model compound for the structure elucidation of the natural KDO trisaccharide by spectroscopic and immunochemical methods, a route was devised which would permit the synthesis of the model trisaccharide 12. 

Equatorially positioned methyl-branched derivatives may be obtained by reductive cleavage of spiro epoxides [94], Thus the Peterson olefination of 188gives the exocyclic 3 -methylene function in 189. By means of a Sharpless epoxidation the allylic 4"-hydroxy group should determine the chirality of the resulting epoxide. However, the Sharpless method does not show any reaction neither in a monosaccharide model system nor in this trisaccharide precursor [95]. Amazingly, the classical epoxidation with m-chloroperbenzoic acid is employed to give exclusively the desired (3"R) epoxide 190 in excellent yield. These results may be associated with a sufficient chiral induction of the stereochemical information at C-l", C-4", and C-5". A subsequent reduction furnishes the original E-D-C trisaccharide sequence 191 of mithramycin [95, 96]. 

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