α versus β D-ribopyranose

Question

I understand that sugars are named α/β according to the arrangement of the OH on the anomeric C with respect to the hydroxymethylene group (Ex. glucose), but I'm a little confused how you determine α/β when you don't have that group (Ex. ribopyranose, below).

α-D-ribopyranose

β-D-ribopyranose

Thanks!

Answer 1

It's actually not the hydroxymethelene group that determines the α/β status of a sugar, instead it's the relative orientation of the anomeric hydroxyl to the stereocenter which determines the absolute stereochemistry (D/L) of the sugar. This just so happens to match up with the same side/opposite side heuristic for the common six carbon pyranoses, but that's a quick rule of thumb, not the official definition:

α (alpha), β (beta): Relative stereodescriptors used in carbohydrate nomenclature to describe the configuration at the anomeric carbon by relating it to the anomeric reference atom. For simple cases the anomeric reference atom is the same as the configurational reference atom. Thus in α-d-glucopyranose the reference atom is C-5 and the OH at C-1 is on the same side as the OH at C-5 in the Fischer projection.

IUPAC Gold Book

The configuration at the anomeric centre (that derived from the carbonyl carbon) is denoted alpha- (α-) or beta- (β-) by reference to the stereocentre that determines the absolute configuration. In a Fischer projection, if the substituent off the anomeric centre is on the same side as the oxygen of the configurational (D- or L-) carbon, then it is the α--anomer. If it is directed in the opposite direction it is the β-anomer.

CAZypedia - includes diagrams and examples, including for five carbon pyranoses.