Pyranose ring, detection

The composition of solutions of the 2-heptuloses has been determined, and discussed, by Angyal and Tran.92 These ketoses are different from other reducing sugars inasmuch as there are two hydroxymethyl side chains attached to the pyranose ring. In the a-pyranose form, they are cis to each other and will therefore both be equatorial in the preponderant chair form. In the / -pyranose, however, one or other of the hydroxymethyl groups has to be axial and, in consequence, the / anomers are disfavored in only one solution (that of the altro isomer) was the yS-pyra-nose detected in the 13C-n.m.r. spectrum.92... 

Fig. 15 The six detected confonnations of free Ribose are six-membered pyranose rings, adopting C4- or" Ci-chair skeletons of both p- and a-anomers. The experimental conformational energies in red are estimations derived from the relative intensities of the microwave transitions. The blue horizontal lines represent the theoretical (MP2) AG298 predictions...

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