Heteropolysaccharides, /3-eliminative degradation

The endocyclic, enolacetal-forming, 8-eliminative degradation of dermatan sulfate (33) possesses only one other notable stereochemical feature, namely, the hexopyranuronate residue of this heteropolysaccharide is that of L-idopyranuronic acid, in which the C-5 proton and the C-4 leaving-group are both axially attached (as was proved with synthetic, model compounds, discussed later) in the JC(l) conformation. Therefore, the /8-eliminative degradation of dermatan sulfate can be accepted as being a diaxial procedure, as in 38. 

This fragmentation of the heparin heteropolysaccharide chain proved to be a jS-elimination reaction in which diazomethane acts as the proton acceptor. It is interesting that aryldiazoalkanes (for example, phenyl or substituted-phenyl diazomethanes) cannot be used for a similar j8-eliminative degradation of quaternary ammonium salts of sulfated heteropolysaccharides. 

Hence, the stereochemistry of the 8-eliminative degradation of alginic acid seems to be similar to that of the heteropolysaccharide chain of heparin. As already described, in tbe latter 8-eliminative degradation, both the D-glucopyranuronate and the L-idopyranuronate, possessing the same stereochemical disposition of the groups on C-5 and C-4 of the ring, are involved. 

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