7. MONOSACCHARIDES AND OLIGOSACCHARIDES
297
very probably occurs in cultures of Shigelh flexneri (18), and its optical
enantiomorph L-glycero-D-mannoheptose (XLII) has been identified in
the lipoglycoprotein complex of the cell wall of Escherichia colt B (19).
The first natural octose, D-glycero-D-mannoctulose (XXXIX) has recently been isolated from an aqueous extract of the Californian avocado
(20) [the well-known source of D-mannoheptulose (XXXVIII)] and, as
will be seen from its formula, is related in structure. It also occurs in
Sedum spp. (20a).
8. Monosaccharides as Heterocylic Compounds
Although we have up to this point considered the monosaccharides
as open-chain compounds, it must be emphasized that they rarely, if
ever, exist free and unsubstituted in this condition. As solids and in
solutions, except at extremes of pH, they behave chemically and physically as substances devoid of carbonyl groups in any appreciable
amount. Neither infrared nor ultraviolet spectroscopies indicate the
presence of the >C=0 system. Moreover, they will react reversibly,
under suitable conditions, with equimolar proportions of alcohols,
phenols, and suitable nitrogen bases, through the elimination of equimolar amounts of water, to form glycosyl derivatives. (The glycosyl
radical is [C*(H 2 0)*] minus OH; cf. CH 3 OH and CH 3 —). These glycosyl derivatives are no longer capable of reducing, as do free sugars,
alkaline solutions of polyvalent cations or anions such as Cu
2+ , Bi
3+ ,
Ce
4+ , or Fe(CN) 6
3- .* Moreover, the glycosides do not react with carbonyl reagents nor do they undergo oxidation (>C=0->—C0 2 H)
nor reduction (>C=0-> >CHOH). Hence the aldehyde group of the
aldoses, and the ketone grouping of the ketoses, must have become
transformed in the generation of the glycosyl derivatives.
When a crystalline reducing sugar (aldose or ketose) is dissolved
in an ionizing solvent, the optical activity of the solution changes progressively from an initial level to a steady value; this phenomenon is
termed mutarotation. In a number of instances, two physically distinct
crystalline modifications of a single sugar can be obtained that differ in
their initial optical rotatory power on being dissolved; on mutarotating,
both modifications ultimately attain the same equilibrium value. For
example, the so-called V and "ß" D-glucoses, when dissolved in water
show, respectively, initial [«] D values of +Π0° and +19° and both
ultimately attain the equilibrium value of +52.5°.
All the aforementioned phenomena are consistent with the possession by the sugar of a ring structure in which an intramolecular rear* Ketoses reduce these reagents because they have the structure in the openchain form of α-hydroxy ketones.
297
very probably occurs in cultures of Shigelh flexneri (18), and its optical
enantiomorph L-glycero-D-mannoheptose (XLII) has been identified in
the lipoglycoprotein complex of the cell wall of Escherichia colt B (19).
The first natural octose, D-glycero-D-mannoctulose (XXXIX) has recently been isolated from an aqueous extract of the Californian avocado
(20) [the well-known source of D-mannoheptulose (XXXVIII)] and, as
will be seen from its formula, is related in structure. It also occurs in
Sedum spp. (20a).
8. Monosaccharides as Heterocylic Compounds
Although we have up to this point considered the monosaccharides
as open-chain compounds, it must be emphasized that they rarely, if
ever, exist free and unsubstituted in this condition. As solids and in
solutions, except at extremes of pH, they behave chemically and physically as substances devoid of carbonyl groups in any appreciable
amount. Neither infrared nor ultraviolet spectroscopies indicate the
presence of the >C=0 system. Moreover, they will react reversibly,
under suitable conditions, with equimolar proportions of alcohols,
phenols, and suitable nitrogen bases, through the elimination of equimolar amounts of water, to form glycosyl derivatives. (The glycosyl
radical is [C*(H 2 0)*] minus OH; cf. CH 3 OH and CH 3 —). These glycosyl derivatives are no longer capable of reducing, as do free sugars,
alkaline solutions of polyvalent cations or anions such as Cu
2+ , Bi
3+ ,
Ce
4+ , or Fe(CN) 6
3- .* Moreover, the glycosides do not react with carbonyl reagents nor do they undergo oxidation (>C=0->—C0 2 H)
nor reduction (>C=0-> >CHOH). Hence the aldehyde group of the
aldoses, and the ketone grouping of the ketoses, must have become
transformed in the generation of the glycosyl derivatives.
When a crystalline reducing sugar (aldose or ketose) is dissolved
in an ionizing solvent, the optical activity of the solution changes progressively from an initial level to a steady value; this phenomenon is
termed mutarotation. In a number of instances, two physically distinct
crystalline modifications of a single sugar can be obtained that differ in
their initial optical rotatory power on being dissolved; on mutarotating,
both modifications ultimately attain the same equilibrium value. For
example, the so-called V and "ß" D-glucoses, when dissolved in water
show, respectively, initial [«] D values of +Π0° and +19° and both
ultimately attain the equilibrium value of +52.5°.
All the aforementioned phenomena are consistent with the possession by the sugar of a ring structure in which an intramolecular rear* Ketoses reduce these reagents because they have the structure in the openchain form of α-hydroxy ketones.