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PART 3. PREPARATION AND PROPERTIES OF THE

SUGARS AND THEIR DERIVATIVES

XXVIII. OPTICAL ACTIVITY, CONFIGURATION, AND STRUCTURE IN THE SUGAR GROUP

1. CONSTITUTION OF THE SUGARS AND THEIR
INTERRELATIONSHIPS

At the dawn of history our aboriginal ancestors were familiar with honey and the sweet exudations of various trees and plants, but the chemistry of the sugars did not begin until the introduction of cane sugar into Europe [1]. With the expansion and development of Europe, the need was felt for a sugar-bearing plant which could be grown in a temperate climate. In the course of the search for a source of sugar, attention was directed to many closely related natural products. Thus, in 1792 Lowitz [2] found that starch on hydrolysis gives a sweet substance, now known as d-glucose or dextrose. Many other natural products were found which, on hydrolysis, gave closely related products. These substances, with few exceptions, contain carbon, hydrogen, and oxygen, in the proportions corresponding to a hydrate of carbon, and hence they were named "carbohydrates.'

The carbohydrates comprise the sugars and the polysaccharides. The polysaccharides are compounds which, by hydrolysis, yield sugars. The sugars are classified as simple sugars or monosaccharides, and compound sugars comprising the disaccharides, trisaccharides, and tetrasaccharides. The simple sugars are classified further as trioses, tetroses, pentoses, methylpentoses, hexoses, heptoses, etc., according to the number of carbon atoms in the sugar molecule. The hexoses, pentoses, and methylpentoses occur in many plant products and play important roles in many biological processes. The two most important simple sugars are dextrose and levulose.

Few organic compounds have been studied as intensively as the hexoses, and in particular, dextrose. This substance was known in ancient times as grape sugar, but was not isolated from starch until 1792. It was found in diabetic urine in 1815 by Chevreul [3], and in cellulose in 1819 by Braconnot [4]. Crystalline dextrose, however, did not find wide use until a successful method for the production of the hard refined crystalline sugar was devised at the National Bureau of Standards [5]. This method in its essential principles was applied commercially, and many millions of pounds of white crystalline dextrose are now produced annually.

During the latter half of the nineteenth century many organic. chemists devoted their attention to the study of dextrose. In 1843 Dumas [6] ascertained that dextrose has the empirical formula CH2O, and in 1870 Baeyer [7] advanced the structural formula

CH2(OH).CH(OH).CH(OH).CH(OH).CH(OH).CHO.

[blocks in formation]

This formula was supported by subsequent molecular-weight determinations [8] and by the following chemical reactions: (1) The sugar yields, on oxidation with bromine water or nitric acid, a monocar boxylic acid (gluconic) which contains the same number of carbon atoms as the parent sugar [9],

CH2OH (CHOH),CHO+ Br2+H2O→CH2OH (CHOH),COOH+2H Br

(gluconic acid)

This proves that the aldehyde or potential aldehyde group lies at one end of the carbon chain. (2) On reduction with sodium amalgam, the sugar yields a hexahydric alcohol [10] (sorbitol),

CH2OH(CHOH),CHO+ H→→CH2OH (CHOH),CH2OH.

(sorbitol)

(3) Reduction of the sugar with hydriodic acid gives normal hexyl iodide, which proves that the carbon atoms are combined in a straight chain. (4) The sugar combines with hydrogen cyanide to give a nitrile, which, after saponification and reduction with hydriodic acid, yields a normal heptanoic acid,

H

1

CH2OH (CHOH),CHO+HCN→CH2OH (CHOH),C-CN→CH3(CH1),COOH

он

(normal heptanoic acid)

This confirms the straight-chain formula and shows the presence of an aldehyde or potential aldehyde group [11]. (5) The sugar yields pentaacetyl and other derivatives, indicating the presence of five hydroxyl groups. (6) Treatment with phenylhydrazine gives glucose phenylhydrazone,

CH2(OH).CH(OH). CH(OH).CH(OH).CH(OH). CH=N―NHCH5, and prolonged action gives glucosazone,

H

CH2OH.CH(OH).CH(OH).CH(OH).C—C—NNHCH5.

N–NHCH

The formation of hydrazones is typical of carbonyl compounds, and the formation of osazones is peculiar to alpha hydroxy aldehydes and ketones. Although these properties support the polyhydroxy aldehydic formula, other properties which will be considered later show that the aldehyde modification is only one of the several tautomeric forms which are characteristic of the sugars. Indeed, the crystalline sugars contain lactol ring structures, and even in solution. the quantity of the free aldehyde modification is extremely small.

While the chemistry of dextrose was being developed, a number of other sugars were being investigated. Some of these resemble dextrose in that on oxidation they give acids containing the same number of carbon atoms, whereas others give acids containing fewer carbon atoms. The most important of the latter group is d-fructose

or levulose, which has the same molecular weight as dextrose but differs fundamentally in that the reducing group lies on the second carbon. One modification of levulose can be represented by the formula:

CH2(OH).CH(OH).CH(OH).CH(OH).C-CH2OH.

[blocks in formation]

This structure is supported by the following evidence: (1) The sugar exhibits the reducing properties characteristic of a carbonyl group, but on oxidation with nitric acid the carbon chain is broken to give tartaric and oxalic acids,

CH2OH.CHOH.CHOH.CHOH.C.CH2OH + HNO3→

COOH.CHOH.CHOH.COOH+COOH.COOH.

This is evidence that the reducing group lies on either carbon 2 or carbon 3. (2) Reduction with sodium amalgam yields two hexahydric alcohols, "sorbitol" and "mannitol,"

[blocks in formation]

The formation of these two products shows that the carbonyl is not on the end of the carbon chain, and the formation of sorbitol from both dextrose and levulose shows that the two sugars are alike except for the groups attached to carbons 1 and 2. (3) Reduction of the sugar with hydriodic acid yields normal hexyl iodide, which proves that the carbon chain is not branched. (4) Treatment with hydrogen cyanide and saponification of the nitrile yields a heptanoic acid which, on reduction with hydriodic acid, gives a-methyl hexanoic acid.

CN

CH2OH(CHOH)3-C--CH2OH + HCN→CH2OH (CHOH)3-C-CH2OH→→

[blocks in formation]

CH2OH (CHOH)3—C— CH2OH→→→CH3(CH2)3— C-CH3.

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OH

H

This shows that hydrogen cyanide adds to the second carbon, and therefore the carbonyl is on this carbon [12]. (5) The sugar yields pentaacetates and other products which require the presence of five hydroxyl groups. (6) Treatment with phenylhydrazine gives glucosazone [13], which is further evidence that, except for carbons 1 and 2, dextrose and levulose are alike. In dextrose the reducing group is located on the first carbon, and therefore the sugar is related to the aldehydes, while in levulose the reducing group is located on the second carbon and therefore levulose is related to the ketones. This distinction is the basis for the further classification of the sugars as aldoses and ketoses according to whether the reducing group lies on the end carbon or on an intermediate carbon.

Even though the sugars give many of the reactions of aldehydes and ketones, at least in neutral solutions they do not give certain specific aldehydic and ketonic reactions, such as the red color with Schiff's reagent, or the intense absorption bands in the ultraviolet region, which are characteristic of free carbonyl groups [14]. The absence of these distinctive aldehydic and ketonic properties is supposedly explained by the interaction of the carbonyl group with the neighboring hydroxyl groups to form cyclic hemiacetals.

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Although the cyclic hemiacetals can be considered as potential aldehydes, they differ fundamentally from the open-chain compounds and exhibit many interesting properties. As early as 1846 Dubrunfaut [15] noted that the optical rotation of freshly dissolved glucose changes

on standing, and he suggested that this change (mutarotation) was due to a change in molecular composition of the sugar [16]. In subsequent years, this explanation received a striking confirmation in the discovery of two forms of this sugar, which, in aqueous solution, spontaneously revert to an equilibrium mixture of the two. Prior to the discovery of the second form of glucose, Colley [17] had suggested that one oxygen atom is combined with two carbon atoms to form a cyclic structure. This idea was taken up by Tollens [18], who recognized that various ring isomers are possible.

Lacking an experimental method for proving the size of the ring, the early workers postulated a 1,4, or furanose ring, for the normal sugars and glycosides, but subsequent work has proved that the normal methyl glycosides and crystalline sugars contain the 1,5, or pyranose ring [19]. Several direct methods are available for determining the structures of the methyl glycosides, but the structures of the free sugars rest on less satisfactory proof. Perhaps the best method for ascertaining the structures of the aldoses is the bromine oxidation method of Isbell and Hudson [20, 21], in which the pyranose and furanose modifications are oxidized to give the delta or gamma lactones, respectively, without rupturing the lactol ring. Presumably the reaction takes place in the following manner:

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The validity of the method rests on the correct identification of the delta and gamma lactones and on the hypothesis that the ring structure does not change either before or during the reaction. The results obtained by this method and by other methods, such as the comparison of the optical rotations of the sugars with the optical rotations of glycosides of known structure, indicate that the crystalline sugars so far investigated, with the exception of mannose-CaCl2. 4H2O

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