Furanose fonn

As viewed in the drawing, a 120° counterclockwise rotation of C-4 places its hydroxyl group in the proper position. At the same time, this rotation moves the CH2OH group to a position such that it will become a substituent that is up on the five-membered ring. The hydrogen at C-4 then will be down in the furanose fonn. 

Most carbohydrates exist as cyclic hemiacetals. Those with five-membered rings are called furanose fonns those with six-membered rings are called pyranose fonns. 

In nebularine, D-ribose in its furanose fonn is attached to position 9 of purine. The stereochemistry at the anomeric position is j8. 

Treatment of l-0-(a-D-glucopyranosyl)-D-fructose with hydrogen fluoride in pyridine has afforded a l,T 2,2 dianhydride mixture with the fructose moiety in either the pyranose or furanose fonn. Some novel 1,6-anhydro-lactose derivatives for the synthesis of oligosaccharides containing N-acetyl-lactosamine residues are mentioned in Chapter 4, and a 3,6-anhydroglucofuranose derivative resulting from an unusual molecular rearrangement of a xanthate is described in Chapter 7. 

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