I know when using fischer projection I have to look if it's a D or L configuration. if it's D: up = right and down = left and if it's L: up = left and down = right. I think the concept is clear to me, however I dont't know if the OH group on the C5 carbon atom is pointing up or down. For example:
enter image description here
When I have to draw those cyclic molecules to fischer projection I never know if the OH group which will be shown in the fischer projection (not in the clyic structure because it's a hemi-acetal then) is pointing up or down. What is the rule for this?
I want to get this:
enter image description here
But I don't know how to know the side of the circled OH group

  • $\begingroup$ I'm not sure what you mean. Generally, a Fisher projection is drawn vertically with the OHs on the left and right. The carbonyl appears at the top. Up and down only make sense if there is a ring. But the C5 carbon doesn't have an OH. The C1 carbon has an OH which is clearly up, but in the Fisher projection, that's the carbonyl carbon so there is no stereochemistry in the projection. Usually, this OH is in the axial position because of the anomeric effect from the adjacent ether's lone pair. See: en.wikipedia.org/wiki/Fischer_projection $\endgroup$ – Zhe Oct 8 '16 at 17:59
  • $\begingroup$ I updated my question hopefully it clear know @Zhe $\endgroup$ – KingBoomie Oct 8 '16 at 18:08
  • $\begingroup$ I think it's as simple as L for left and D for right... I'm not sure what you mean by up or down still. The OH you've circled in red is the O that's in the ring. The other OH of the hemiacetal is from the carbonyl (that's the one pointing up in the ring on the right). It could be down as well. That is still the same Fisher projection. $\endgroup$ – Zhe Oct 8 '16 at 19:36
  • $\begingroup$ What I mean when you have a D Sugar the rule is in the Fischer projection of the linear form OH pointing to the left ==> up in cyclic and OH pointing to the right ==> down in cyclic. However how can you determine to which side to OH is pointing on the 5th C if you don't know if it is pointing up or down (and if you don't know if the cyclic form is a D or L) @Zhe $\endgroup$ – KingBoomie Nov 7 '16 at 18:17
  • $\begingroup$ Your question is complicated by the fact that, in your example, the placement of C-5 hydroxyl in the Fisher projection can't change because it is D. I think the main issue is that you need to learn how to convert a Fisher projection into the ring form and vice versa. I can't tell you explicitly up or down because it matters how you draw the ring in the L- form. To my knowledge, there is no canonical form, but I might be wrong there... $\endgroup$ – Zhe Nov 7 '16 at 19:09

The OH group you've circled is always on the right hand side for D-sugars and on the left for L-sugars as its position is in the very definition of D/L sugars.


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