Hydrolysis furanose derivatives

We may return now to the polysaccharides present in the peanut for a brief consideration of the relationship of the other components present in the pectic materials to the araban constituent. All the evidence indicates that the pectic acid portion of the peanut is identical with normal pectic acid and, as was indicated in the previous section, this material, which is very stable to acid hydrolysis and possesses a high positive rotation contains a main chain which is built up of D-galac-turonic acid residues of the pyranose type. If, therefore, the araban associated with the pectic acid had been derived directly from the pectic acid by decarboxylation without intermediate hydrolysis of the poly-galacturonide, the sugar residues in the araban should also be in the pyranose form. The experimental evidence shows clearly, however, that the arabinose residues in araban are furanose in type and it follows that any hypothesis concerning the direct conversion of pectic acid into the araban by decarboxylation is untenable. 

Similar products (but having R = /3-D-galactopyranosyl) were obtained from octa-O-acetyllactobiononitrile56 in 25% aqueous ammonia. The furanose structure of the monoamido derivative 61 (and that of its analog N-acetyl-3-0-/3-D-galactopyranosyl-D-arabinofuranosyl-amine56) was ascertained by methylation, hydrolysis, and identification of 2,5-di-O-methyl-D-arabinose. 

Conclusive proof of the structures of the glycosides B and C was obtained by methylation, hydrolysis, oxidation and comparison of the rates of hydrolysis of the lactones so obtained. From the glycoside B a 1,5-lactone was obtained,141 whereas a 1,4-lactone was derived from C, showing that the glycoside B had a 1,5-pyranose lactol ring and C a 1,4-furanose lactol ring. Since A and B were a- and /3-anomers, it followed that A also had a pyranose structure. 

Since L-arabinose is a constituent unit of many natural products a number of methylated derivatives of this sugar have been isolated by the hydrolysis of methylated polysaccharides. Thus methylated peanut (Arachis hypogea) araban and sugar beet araban give trimethyl-L-arabofuranose, 2,3-dimethyl-L-arabinose and 2-methyl-L-arabinose in equal proportions.84 From methylated gum arabic trimethyl-L-arabo-furanose and 2,5-dimethyl-L-arabinose have been isolated.85 These same compounds have also been isolated from methylated cherry gum86 and methylated damson gum,87 whereas methylated mesquite gum88 affords 3,5-dimethyl- and 2,3,5-trimethyl-L-arabinose on hydrolysis. 

Bell114 has suggested that the very ready hydrolysis of a -diisopro-pylidene-D-fructose might possibly indicate a furanose structure, namely, that a-diisopropylidene-D-fructose is l,2 4,6-diisopropylidene-D-fructo-furanose. Hydrolysis would have to be accompanied by change in the size of the ring since the structure of the mono-isopropylidene derivative is, without doubt, pyranose. In this connexion, however, it should be... 

In the aldohexose series, 5-(benzylamino)-l,2-0-cyclohexylidene-5-deoxy-L-idurononitrile gives, on acid hydrolysis, an almost quantitative yield of N-benzyl-2-cyano-5-hydroxypyridinium chloride. On partial, catalytic hydrogenation of this aminonitrile, 5-(benzylamino)-I,2-0-cyclohexylidene-5-deoxy-L- do-hexodialdo-l,4-furanose is obtained. This compound is reducible with sodium borohydride to crystalline 5-(benzylamino)-l,2-0-cyclohexylidene-5-deoxy-L-idofur-anose which, on removal of the cyclohexylidene group with acid, forms the intermediate 5-(benzylamino)-5-deoxy-L-idopyranose this then loses three molecules of water per molecule, to give N-benzyl-5-hydroxy-2-(hydroxymethyl)pyridinium chloride. It is therefore clear that the transformation of 5-aminoaldoses into pyridine derivatives in acid solution is not prevented by the monoalkylation of the amino group. 

Acetamido-6-deoxy-D-fructose exists as a furanose form, and gives with methanolic hydrogen chloride an anomeric mixture of the furano-sides. 6-Acetamido-6-deoxy-L-xf//o-hexulose was obtained as a syrup which showed a distinct Amide II band. Regardless of whether the free 6-acetamido-6-deoxy-L-xi/Zo-hexulose was prepared by hydrolysis of 6-acetamido-6-deoxy-2,3-0-isopropylidene-a-L-3C[/Io-hexulofuranose or by N-acetylation of 6-amino-6-deoxy-L-xyZo-hexulo-furanose hydrochloride with acetic anhydride—triethylamine in aqueous methanol, there appeared in both cases only the furanose form of the L-xj/Zo-hexulose derivative. ... 

They then prepared the completely methylated derivatives of adeno-gine . and guanosine by simultaneous deacetylation and methylation of the acetylated nucleosides. In this way, trimethyl-iV-methyl adenosine and trimethyl-A -methyl guanosine were formed, and isolated as the hydrochlorides. On hydrolysis of the adenosine derivative by means of dilute hydrochloric acid, 6-iV-methyladenine and trimethyl-D-ribo-furanose were isolated. The same trimethyl sugar was isolated from the methylated guanosine and was identified in each case by oxidation, first to trimethyl-7-D-ribonolactone and then to meso-dimethoxy succinic acid. It follows that the sugar component has the furanose ring-structure. 

There are many instances in which the introduction of amines with specific substitution patterns is desired. With the abundance of structurally diverse commercially available amines from which to choose, the use of these compounds as nucleophiles is extremely valuable. While the example illustrated in Scheme 6.29 focuses on the hydrazine displacement of a secondary tosylate from a furanose sugar derivative [56], this type of reaction is easily achieved with most primary and secondary amines. Where the introduction of direct NH2 groups is desired, the best methods rely on the reduction of azides or nitro groups, or the hydrolysis (hydrazinolysis) of imides [32]. 

---