# Assigning R/S configuration

Question: Assign the R/S configurations to the carbons which are attached to different rings of this molecule (carbons 1 and 3 according to IUPAC numbering):

My Solution

I first started out by numbering the carbons. The one at the top would be 1 and the bottom chiral carbon would be 3. Next, According to CIP rules, I have included the phantom atoms by replacing the double bonds while deciding the priority order. Moving on, at carbon 1, My priorities would be:

1. $$\ce{-OH}$$
2. Cyclopentene ring (on the left)
3. Cyclopropene ring (on the right)
4. Carbon 2

This gives me a counter-clockwise turn and since the least priority group is on the vertical line (away from us) the configuration I assign would be S.

Following a similar procedure for carbon 3, The priorities would be :

1. Bottom 5-member ring (contaning $$\ce{O-O}$$ bond)
2. 3 member ring on the right
3. 5 member ring on the left
4. Carbon 2

Again, this gives me a counter-clockwise turn and since the least priority group is away, I would assign the configuration: S.

The answer key, however, says that the answer is 1R, 3S.

Where am I going wrong?

• Of the oxygen containing rings, doesn't the one on the bottom have the lowest priority because there is only one oxygen atom on the carbon attached to the rest of the molecule, whereas for the rings on the left and the right, there are two? Sep 4, 2020 at 1:12
• I have included the phantom atoms by replacing the double bonds while deciding the priority order. - you did it wrong, as Karsten just mentioned Sep 4, 2020 at 1:14
• The configurations are 1R, 3S. For C1, the order is 1, 3, 2, 4 using your numbers. One switch gets you from S to R. For C3 the order is 2, 3, 1, 4. Two switches here keeps the S assignment. You were correct for the wrong reason. I'm preparing and answer but the drawing takes some time. Sep 4, 2020 at 1:40
• @sai-kartik: You may want to look at this link for help with digraphs: ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/isomers/… Sep 4, 2020 at 11:45