I know how to give R and S configuration for a cyclic compound like the compound above if the lowest priority group is on a wedge or dash bond but on the above compound the lowest priority group is on the plane of the paper so how do I determine the R and S configuration for the chiral carbon on this compound?
The only two ways to do this are either:
- Move yourself (or imagine your eye) to the right place so that the lowest priority substituent is away from you, or
- Rotate the molecule so that the lowest priority substituent is away from you as you look at the page from above.
Either approach takes a bit of mental gymnastics and gets practice to perfect. For (1), I fine it helps to hold the paper up or move my head so I am close to the page, which makes it easier for me to imagine looking along that bond. For (2), it's easier if you abstract away all the detail by replacing the four groups with just the priority numbers. Even if you can't use a model in the exam, I still think models help to get to grips with the mental manipulations.