Pentopyranoside anomers

D-en/fftro-pentopyranosides, the a anomer consumes lead tetraacetate almost twice as rapidly as does the /S.124... 

It was observed by Jackson and Hudson that the products from an anomeric pair of methyl pentopyranosides have equal, but opposite rotations. Since they believed that the products were dialdehydes, this was reasonable, as the anomeric carbon atom was the only asymmetric center. However, it is now known that these products exist as hemialdals, and, since the optical rotations are high, it is certain that they are in the same form in solution. The hemialdal form has three asymmetric centers and, therefore, the two anomeric forms would be expected to have different rotations. It is the author s opinion that a conformational change occurs in the a-D anomer (assuming a conformation), such that the final hemi-... 

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