Aldo-hexoses

Loss of [CH3+(CH3)3SiOH] from TMS derivatives of aldo-hexoses, etc. 

CH,OH Keto-hexose CH,OH Ene-diol CHjOH Aldo-hexose... 

The Composition (%) of Aldo-hexoses and -pentoses, and Some of Their Deoxy Derivatives, in DtO°... 

Figure 10. Hypothetical equilibria between the oxyanionic pyranose form and the oxyanionic aldehydo form of an aldo -hexose and of a 2-deoxy-aldo-hexose (the hydroxyl groups have been omitted from C-3, -4, and -6 for clarity)...

Isbell and co-workers (51) have tried to minimize the oxygen reaction and to maximize the peroxide reaction. When a large excess of peroxide and a low temperature were maintained, they found that the monosaccharides are converted almost quantitatively to formic and two-carbon acids. Table II presents results for the peroxide oxidation of 14 sugars. The total acid produced from aldo-hexoses under favorable conditions is about six moles, consisting almost entirely of formic acid. Aldopentoses react more rapidly than aldo-hexoses and yield about five moles of formic acid per mole of pentose. Fructose and sorbose yield approximately five moles of total acid of which four moles are formic acid. Glycolic acid was identified qualitatively but not determined quantitatively. L-Rham-nose and L-fucose yield about five moles of acid, including four moles of formic acid. Acetic acid was identified only qualitatively. 

Working at low temperatures (0-4°), h.p.l.c. on a cation-exchange resin in the calcium form will separate the pyranose anomers of most of the aldo-hexoses and -pentoses18 under these conditions, mutarotation is slower than separation. The furanoses are not separated, because they interconvert too rapidly, but, at—2 5 to—45 °, the two furanose forms of D-galac-tose and L-fucose have been separated.19 Attempts to separate the various forms of sugars on a preparative scale [p. 24] have not succeeded so far.20... 

The effect of adding the group —CH2OH to C5 or Cl of aldo-hexoses, aldopentoses, and ketohexoses having the same disposition of... 

Pyrazines are formed from transamination reactions, in addition to carbon dioxide and formaldehyde. A requirement is that the carbonyl compound contains a dione and the amino group is alpha to the carboxyl group (16). If the hydrogen on the ct-carbon oI the amino acid is substituted, a ketone is produced. Newell (17) initially proposed a pyrazine formation mechanism between sugar and amino acid precursors. (See Figure 3). The Schiff base cation is formed by addition of the amino acid to the anomeric portion of the aldo-hexose, with subsequent losses of vater and a hydroxyl ion. Decarboxylation forms an imine which can hydrolyze to an aldehyde and a dienamine. Enolization yields a ketoamine, vhich dissociates to amino acetone and glyceraldehyde. 2,5-Dimethylpyrazine is formed by the condensation of the tvo molecules of amino acetone. 

GC was performed on a polyester stationary phase (10% of neopentyl glycol sebacate or Carbowax 20M) at 170°C. The method was applied to a mixture of aldopentoses, aldo-hexoses and aldoheptoses in biological material. 

Procedure of Willstdtter and Scfcudet " To 10 cc. of a solution containing 100 mg. of aldose is added 20 cc. of 0.1 N iodine-potassium iodide solution (11,11 cc. is required for a hexose) and then 30 co. of 0,1 N sodium hydroxide solution is added gradually. The mixture is allowed to stand for ten to twenty minutes. It is then acidified with dilute hydrochloric or sulfuric acid and titrated with 0.1 N thiosulfate solution. Each cc. of 0.1 N iodine consumed is equivalent to 9.005 mg. of an aldo-hexose, 7.004 mg. of an aldopentose and 17.11 mg. of a reducing C12 disaccharide. 

This may be best illustrated by taking glucose the aldo-hexose, the exact constitution of which we shall develop later,... 

In this normal acid the carboxyl group must be joined to the sixth carbon and this sixth carbon must be the one which, in the aldose, was in the condition of carbonyl. Thus the aldehyde group in the original aldo-hexose must have been the end group. The constitution of the aldose sugars is, therefore, as has been written. 

We may then write the constitutional formulas for the aldo-hexose and the keto-hexose as follows ... 

The keto-hexose, corresponding to the aldo-hexose given above, is oxidized as follows ... 

Phenyl-hydrazine Reaction.—Hydrazones.— The reaction with phenyl hydrazine has been given as one of the aldehyde or ketone reactions which prove the presence, in sugars, of the carbonyl group. The compounds resulting from this reaction are known as hydrazonesy or more specifically as phenyl hydrazones. With an aldo-hexose the reaction is as follows ... 

CH2OH Glucose, Aldo-hexose CH20H Glucose phenyl hydrazone... 

Aldo-hexose Acid nitrile (Cyan-hydrine)... 

In practice, these reactions do not take place quite as simply as represented above. After the formation of the oxime the compound is acetylated and the conversion into the acid nitrile and into the aldehyde occurs with these acetyl derivatives. The resulting acetyl derivative of the aldo-hexose is first converted into an acetamide compound which on hydrolysis yields the hydroxyl compound, i.e., the aldo-hexose. 

Aldo-hexose Mono-basic acid Aldo-pentose... 

Naturally occurring substances are known in the two structural forms of aldo-hexoses and keto-hexoses. 

Glucose, CH2OH—CHOH—CHOH—CHOH—CHOH—CHO, Aldo-hexose. 

If, instead of condensing glycerose, we condense the aldo-triose, glyceric aldehyde, by itself, we then obtain the aldo-hexose, or glucose. CH2OH—CHOH—CHO + HCHOH—CHOH—CHO ---->... 

CH2OH—CHOH—CHOH—CHOH—CHOH—CHO, Aldo-hexose. 

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