# Identifying the D/L form of any aldose or ketose in cyclic form

I understand that the compound above is a ketose because it has a furanose ring. But, how do we know that it is a "D" form?

Is there any general rule by which I can identify if any given aldose/ketose is in D/L form?

How do you recognize the D-configuration? Your furanose 1a is in the D-series. Reverse your furanose to the keto form 1b by following the red bonds. Rotate about the $$\ce{C4-C5}$$ bond by $$120^\circ$$ as illustrated by the blue arrow in 1b. This rotation produces the Fischer projection 1c having the $$\ce{C5}$$-hydroxyl on the right. Thus, furanose 1a is of the D-series.
There is another method that can be used with qualification. At any carbon that determines D vs. L in a carbohydrate, as long as the substituents are $$\ce{O, CH2OH, CH(OH)C}$$ and $$\ce{H}$$, the D-series will be of the R-configuration. Conversely, the L-series will be of the S-configuration. In furanose 1a, the priorities are $$\ce{O>C4>C6>H}$$. This determination is easy using this method.
Structures 2a, 2b, and 2c are different representations of $$\alpha$$-D-glucose. All of the $$\ce{C5}$$-carbons are, necessarily, of the R-configuration and therefore in the D-series.