Ketose-type pyranoses

In the past, ketose-type pyranose carbasugars were mainly investigated in the search for sucrose mimics to employ as non-nutritive sweeteners [25a,b]. Probably due to this seemingly unappealing potential and the inherent difficulties related to their stereoconstitutional features, up until 1990 only one report was found to have been published dealing with the asymmetric synthesis of D- and L-carba-analogues of fructopyranose [34]. 

It was observed (Verstraeten, 1964 1966) that 2-ketoses show type I bands at 875 cm and type IIA bands at 817 cm , regardless of whether the 2-ketoses are pyranose or furanose. These bands were ascribed to the presence of structural feature XVI and were tentatively assigned to a skeletal vibration. However, six aldoses also... 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

The introduction of an additional asymmetric carbon atom leads to the existence of two isomers, (a and j8) of each type of ring, which are described as anomers, and C-i is now referred to as the anomeric carbon atom (C-2 in ketoses). In the D-series of sugars the a-hydroxyl is written on the right of the sugar chain in the cyclic form of the Fischer formulae and trans to the CHjOH in the Haworth formulae. It is necessary to relate the pento-pyranoses to the hexoses to determine whether the anomeric hydroxyl... 

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