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

[![enter image description here](https://i.stack.imgur.com/BCvlx.png)](https://i.stack.imgur.com/BCvlx.png)


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?



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> Is there any general rule by which I can identify if any given aldose/ketose is in D/L form?
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Your structure is a ketose because it is a hemiketal of a ketone. It is a furanose because it is a 5-membered (furan-oid) ring. Fructose, which your structure is not, is usually drawn as a furanose as in sucrose but fructose itself exists principally in the pyranose (6-membered ring) 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](https://chemistry.stackexchange.com/questions/99721/twisting-stereoisomers-with-rings-to-determine-r-s/99750#99750). 


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.


[![Structures 1a-1c and 2a-2c](https://i.stack.imgur.com/p3w9C.jpg)](https://i.stack.imgur.com/p3w9C.jpg)


Structures **3a** and **3b** are Haworth renderings of sucrose. Both the glucose (pyranose) and fructose (furanose) moieties are of the *R*-configuration and in the D-series. 


[![Structures 3a and 3b](https://i.stack.imgur.com/owK0D.jpg)](https://i.stack.imgur.com/owK0D.jpg)


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