Stereochemical Assignments For Xylomollin Stereochemistry



You have selected to use the R and S Method for determining the stereochemical relationships between molecules drawn in different cyclic three-dimensional representations. Using this method requires four steps:

Step 1: Determine that the molecules have the same molecular formula and connectivity.

Step 2: Find the stereocenters on each molecule. Note that a cyclic molecule must have at least one stereocenter and must not have a plane of symmetry in order to be a chiral molecule. If a cyclic molecule has no stereocenters, the molecule is achiral. Two cyclic molecules with the same connectivity and no stereocenters are identical.

Step 3: Determine whether the stereocenters are R or S.



Assign an R or S orientation to the stereocenters above.

Priorities can be assigned to the groups on both molecules, using the priority rules. The priorities for each stereocenter bearing a chlorine are colored in teal, and the priorities for each stereocenter bearing a bromine are colored in pink. This is shown above.

Then consider the molecule on the left. It can be seen that the first three groups on the stereocenter bearing the bromine are oriented in the counterclockwise direction, but since the fourth group is oriented upward, the final assignment is clockwise, or R. Furthermore, it can be seen that the first three groups on the stereocenter bearing the chlorine are oriented in the clockwise direction, and since the fourth group is oriented downward, the final assignment is R. This is shown above.

Next consider the molecule on the right. As this molecule is written in wedge-dash notation, the assignment of the S orientation for both stereocenters is straightforward. This is shown above.

Step 4: Consider the relative orientations of the stereocenters:
  1. If each molecule has a plane of symmetry or a center of symmetry, the molecules are achiral.

  2. If each molecule has at least one stereocenter, and
    • all of the stereocenters match exactly (each individual pair of stereocenters has an R orientation in both or an S orientation in both), the molecules are not stereoisomers. Specifically, the molecules are identical.
    • all of the stereocenters do not match (each individual pair of stereocenters has an R orientation in one molecule and an S orientation in the other), the molecules are stereoisomers. Specifically, the molecules are enantiomers.
    • some of the stereocenters match and some do not match (at least one individual pair of stereocenters has an R orientation in both or an S orientation in both AND at least one individual pair of stereoisomers has an R orientation in one molecule and an S orientation in the other), the molecules are stereoisomers. Specifically, the molecules are diastereomers.
Using the relative orientations of the stereocenters in the molecules above, determine the stereochemical relationship between the molecules.

Click Next to check your answer and continue on.


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