Preparative Use of Uronates Having an Endocyclic, Enolacetal Linkage

Preparative Use of Uronates Having an Endocyclic, Enolacetal Linkage 

Pyranuronates and furanuronates having an enolacetal linkage are readily prepared, for example, by the alkali-catalyzed, sulfonate or phosphate j3-elimination reaction. Such unsaturated uronates have proved to be valuable starting-materials in the preparation of divers rare sugars and sugar conjugates. Nevertheless, only a few publications in this interesting field have thus far appeared. 

5-Unsaturated 4-deoxyhexopyranuronates, especially their 0-benzyl-ated derivatives, such as 195, proved to be readily converted into the corresponding 5-(hydroxymethyl) derivatives (196), by complex metal hydrides, such as sodium borohydride or lithium aluminum hy-dride.  

The anomeric effect of the a and p anomers of alkyl 4,5-unsaturated 4-deoxyhexopyranuronates in the hydrogenation was proved exactly by Schmidt and Neukom by using catalytic hydrogenation. It was found that the two anomers of 4,5-unsaturated 4-deoxyhexopyranuronates exist in two different H and H ) conformations and that this influences the stereochemistry of addition of hydrogen to the enolacetal double bond. 

By catalytic hydrogenation with hydrogen in the presence of 10% palladium-on-charcoal in dry methanol at 22-24°, methyl (methyl 4-deoxy-j8-L-threo-hex-4-enopyranosid)uronate (200) gives a mixture of methyl (methyl 4-deoxy-a-D-xy/o-hexopyranosid) uronate (201) and methyl (methyl 4-deoxy-j8-L-arabino-hexopyranosid)uronate (202) in the ratio 7 43. 

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