Hexoses isolation

III. Tables of Derivatives of Pentoses, Desoxyhexoses and Hexoses Isolated from Plant Gums and Mucilages. 283... 

Problem 22.17 ( + )-Glucose. ( + )-mannose and (+ )-galactose are among the hexoses isolated after three Kiliani-Fischer step-ups. beginning with d-(+ )-glyceraldehyde. Do these hexoses belong in the d or l families ... 

The u-gluco configuration tentatively assigned to a 2,3-diamino-2,3-dideoxy-hexose isolated from the lipid A moiety of lipopolysaccharides from Rhodopseudomonas viridis has now been established by comparison of the c.d. spectrum of the di-A-acetyl derivative with that of synthetic 2,3-diacetamido-2,3-dideoxy-D-glucose. ... 

Because there are four chirality centers and no possibility of meso forms there are 2" or 16 stereoisomeric hexoses All 16 are known having been isolated either as natural products or as the products of chemical synthesis... 

Several features of the rearrangement have been elucidated. Although in the treatment of the ester 26 with acetic acid the products were isolated in only 64% yield, evidence was obtained (22) that finally no 1,2-unsaturated compounds remained, since the noncrystalline portion on hydrogenation and deacetylation afforded only 3-deoxy-D-ribo and -d-arabino-hexose and no 1,5-anhydrohexitols. That the components of the final mixture were in equilibrium was indicated by the observation that the main component 27 underwent reaction in boiling acetic acid to give a solution with the same optical activity as that of the original reaction mixture. Thus the 2,3-unsaturated compounds are more stable than the hydroxyglycal derivatives and the a isomer 27 is more stable than its anomer 28. 

From the rearrangement of tetra-O-acetyl-2-hydroxy-D-galactal in boiling acetic acid it was possible to isolate l,2,4,6-tetra-0-acetyl-2,3-dide-h.ydro-3-deoxy-a-jy-threo-hexose (32) (58%) and a small amount of 1,2,3,4,6-penta-O-acetyl-jS-D-galactopyranose. In the reaction mixture the presence of some a-pentaacetate was demonstrated chromatographically but NMR spectroscopy indicated no resonances corresponding to the / anomer of compound 32. These spectral measurements indicate that compound 32 constituted 80% of the mixture of products. 

In similar manner 3,6-dideoxyhexoses have been prepared from esteri-fied 6-deoxy-2-hydroxyglycals. 2,3,4-Tri-0-acetyl-6-deoxy-2-hydroxy-D-glucal was converted into the a and / forms of l,2,4-tri-0-acetyl-2,3-didehydro-3,6-dideoxy-D-en/t/iro-hexose. The a anomer was the main product (77%, 55% isolated crystalline) and, in addition to the ft anomer (19%), a small amount (4%) of saturated products was obtained. On hydrogenation, the major product also suffered some hydrogenolysis but afforded two tri-0-acetyl-3,6-dideoxyhexoses which were shown by NMR spectroscopy to be present in the ratio 12 13 and to have the a configuration. Deacetylation of the reduction products gave... 

Rimington and Rowlands165 have isolated an extremely active preparation from the serum of pregnant mares. This material was obtained in a high degree of purity and was shown to contain hexose and hexosamine in the molar ratio of 2/1. In the fractionation it appeared that, at first, the hexose content decreases as the potency is raised, but later this trend is reversed. It was shown that hexose content could not be taken as indication of gonadotropic activity. The hexose/hexosamine ratio was 2/1 as in normal serum mucoproteins, so that this gonadotropic hormone may be a modified serum mucoprotein. 

Alternatively, if we suppose that the hexose polysaccharide is produced first and is then transformed into pectic acid and araban, at least two types of galactan, other than the one already isolated from pectic materials, would be required to account for the types of structure present respectively in pectic acid and araban. This would seem to be somewhat unlikely since, on this view, one would expect to encounter other types of pectic acid and araban, whereas on the evidence now available the pectic acid isolated from all samples of pectic materials examined appears to be the same, and a similar conclusion applies in the case of the araban. 

Acetals and Ketals.—When D-glucosone condenses with acetone in the presence of sulfuric acid, crystalline 1,2 2,3 5,6-tri-0-isopropylidene(2-hydroxy-D-orobmo-hexose) (XLI) may be isolated.7 On graded hydrolysis... 

In the same way, condensation experiments have been performed with v-glycero-D-gulo-heptose and ethyl acetoacetate, and a crystalline product has been isolated this also gave the aldehyde V16 when oxidized with periodate. Hence, in this case also, the reaction is analogous to that with the hexoses, although the yield is lower. 

These reversible polymerisations of aldehydes are to be distinguished from the condensations which they can also undergo. Thus formaldehyde is converted by quite weak alkalis (Ca(OH)2, CaC03) into glycollic aldehyde and glyceraldehyde, and further into a mixture of hexoses (Butlerow, 0. Loew) from which E. Fischer isolated the so-called... 

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