A-o-Glucopyranose

Mirror images a-o-glucopyranose-4Ci (upper) and a-L-glucopyranose-1C4 (lower) 2-Carb-8. Aldoses 2-Carb-8.1. Trivial names... 

It is possible to separate the two anomeric forms of glucose by careful recrystallization from water. The two forms have different specific optical rotations (see Section 3.4.1), [aJo + 112° for a-o-glucopyranose, and [ajo + 18.7°... 

Reaction of peracetylated l-thio-a-o-glucopyranose (10a) with peracetylated 6-deoxy-6-iodomaltose (18), 6 -deoxy-6 -iodomaltose (21a) and 6 -deoxy-6 -iodomalto-oligosaccharides (21 b,21 c) gave the corresponding S-a-glucosylthio-maltose derivatives (20,23a-c) in excellent yields (Scheme 7) [24,28,29aj. 

The methylation and periodate data appear to eliminate all structural possibilities except the following (a) o-glucopyranose units linked 1 —> 4 (b) D-glucofuranose units linked 1 —> 5 (c) a combination of (a) and (b) and (d) open-chain n-glucose units linked by acetal oxygen bridges 1 —4, 1 — 5 to the adjacent D-glucose unit. 

The coplanar effect did not create as much excitement with the theoreticians as did the anomeric effect and its cause is not known with certainty. The simplest explanation is that the 1,3-diaxial interaction of a methyl group at position 2 of the oxane with the C-H bond at position 6 is increased because these two substituents are closer than if they were separated by -CH2- instead of -O-. The calculation for the crystalline a-o-glucopyranose, with carbon-oxygen bonds of 1.439 and 1.427 A making an angle of 113.7° between them, gives 2.400 A for the C-l-C-5 distance, whereas the corresponding value for cyclohexane is at least equal to 2.5 A. It is a well-known fact that steric strain increases rapidly as the intemuclear distance decreases. 

Norrild and coworkers showed that this structure is only valid as an initial complex formed under completely non-aqueous conditions. In the presence of water, a rapid rearrangement from the a-o-glucopyranose form to the a-o-glucofuranose occurs. In the latter form, all five free hydroxy groups of glucose are covalently bound to the sensor molecule (Figure B.10.4.2). 

The conformational structures of the two anomers of the pyranose forms of a-o-glucopyranose in Figure 4.3 and P-D glucopyranose in Figure 4.4 are also shown. 

The a and p stereoisomeric forms of carbohydrates are capable of independent existence, and many have been isolated in pure form as stable, crystalline solids. When crystallized from ethanol, for example, o-glucose yields a-o-glucopyranose, mp 146°C, [a]o -f 112.2°. Crystallization from a water-ethanol mixture produces P-D-glucopyranose, mp 148-155°C, [a]o -1-18.7°. In the solid state the two forms do not interconvert and are stable indefinitely. Their stmctures have been unambiguously confirmed by X-ray crystallography. 

Mutarotation occurs slowly in neutral aqueous solution, but can be catalyzed by either acid or base. Mechanism 23.1 shows a four-step, acid-catalyzed mechanism for mutarotation starting with a-o-glucopyranose. Steps 1 and 4 are proton transfers and describe the... 

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