C5 Glucopyranose cyclic to linear stereochemistry

Question

How do we know whether the hydroxyl on carbon 5 is on the right or left in a linear structure? I understand that for carbons 2, 3, and 4, groups that are down appear on the right and groups that are up appear on the left. What logic do you use for C5? https://i.redd.it/bkp1ruw746j21.png

Answer 1

The answer to this comes down to which isomer of glucose (or any other sugar) we're working with. In biochemistry, it is always safe to assume that we're working with the D- isomer, as the L- isomer does not occur in nature. And we know that the C5 hydroxyl group of a hexose (or the C n-1 for any sugar with n carbons) determines the optical activity (and, subsequently, which enantiomer we have). So, when we draw the Fisher projection of D-glucose, we will always put the last hydroxyl group on the right.

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Answer 2

This can come from your knowledge that the D-glucose is the only isomer that appears in nature. Remembering your sugar nomenclature, D- indicates that the penultimate carbon has the OH on the right. I'm not sure if the etymology here is actually correct, but I remember that L is left and D is dextral (right).

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