Amino sugars, catalytic oxidation

An l-hydroxy-l,2-benziodoxol-3(17/)-one 1-oxide (IBX)-mediated process provides another efficient and stereoselective preparative route for the s)uithesis of amino sugars. For example, reaction of the D-glucal derivative 113 with p-methoxybenzene isocyanate in the presence of a catalytic amount of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) followed by treatment with IBX furnishes the cyclic carbonate 115, through urethane 114. Subsequent removal of the p-methoxybenzyl (PMB) protecting group by CAN affords the protected amino sugar 116 (O Scheme 56) [94]. 

D-Glycopyranosiduronic Acids from Amino Sugars, Prepared by Catalytic Oxidation... 

Apart from the oxidation at C-6 to form uronic acids from hexoses, the reactions of sugar nucleotides show a marked tendency towards the formation of deoxy and deoxy-amino sugars, especially in prokaryotes. Moreover, many of the enzymes of sugar nucleotide metabolism have requirements for NAD or, less often, for NADP. Among such enzymes are the 4-epimerases, which catalyse conservative reactions, yet need catalytic amounts of NAD. 

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

The catalytic hydrogenation of nitro alcohol to amino alcohol has been applied to the synthesis of an intermediate leading to sugar alcohols. The condensation of ni-tromethane with pentoses in the presence of sodium methoxide, followed by hydrogenation over platinum oxide and deamination with nitric acid, gave the alditols with one more carbon atom.22 The reaction sequence is as follows pentose — 2-epimeric sodio aci-nitro alcohols —> 2-epimeric hexitylamines —> 2-epimeric hexitols + 2-epi-meric 1,4-anhydrohexitols. The 2-epimeric sodio a -nitro alcohols were hydrogenated to 2-epimeric hexitylamines over platinum oxide in acetic acid. An example is shown in eq. 9.8 with D-ribose. 

Sugar-derived /,5-unsaturated alcohols can he efficiently transformed into C-vinyl furanosides using an oxidative cyclization procedure (eq 75). Thus, treatment of a DMSO solution of the relevant substrate with catalytic quantities of Pd(OAc)2, sodium acetate and oxygen provides the expected cyclization products which serve as precursors to C-linked amino acids and glycosides. " ... 

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