Question: Sugar Ring Cyclizations

[churroes]

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Hi all,
I know from biochem that sugars like to cyclize as either furanose or pyranose.
Furanose can be formed by a C-4's OH onto C-1 attack on an 5 carbon aldose, or a C-5's OH to C-2 attack on a 6 carbon ketose.

Pyranose can be formed by a C-5's OH onto C-1 attack for a 6C aldose, or a C-6's OH to C-2 attack on a 7 carbon ketose.


My question is:
Why can't the terminal carbon's hydroxyl group attack to form a ring?
For an example using a 5-C aldose, why couldn't the C5 hydroxyl group attack the C1, forming a pyranose?


Thank you!

 

[quicheduty]

It can! Check out the various cyclizations of ribose for example.

 

[churroes]

Doh!
Thanks for that, I also thought it should be possible, but the Kaplan book made it seem like it was: "Have a methoxy group off the ring, or bust" 😛