Figure 2.19. Manipulations of Fischer ProjectionsAgain, determining the (R)/(S) designation of a Fischer projection of a compound follows the same
rules as described previously. But what if our lowest-priority group is pointing to the side and, as
such, pointing out of the page? Just as before, we’ve got a couple of different tricks to help
determine the right stereochemistry.
Option 1: Make 0 Switches
Go ahead and determine the order of substituents as normal, drawing a circle from 1 to 2 to 3.
Remember, number 4 doesn’t count, so just skip right over it when determining the order. Then,
obtain the (R)/(S) designation. The true designation will be the opposite of what you just obtained.
Option 2: Make 1 Switch
Swap the lowest-priority group with one of the groups on the vertical axis. Obtain the (R)/(S)
designation and, once again, the true designation will be the opposite of what you just found.
Option 3: Make 2 Switches
In this method, start with option 2, moving the lowest-priority group into the correct position. Then,
switch the other two groups as well. Because we made two switches, this molecule will have the
same designation as the initial molecule. This is the same as holding one substituent in place and
rotating the other three in order.