Discussion
The correct absolute configuration for the following compound is :
![Bicyclo[2.2.1]heptane-2,5,7-trione drawn in perspective with the atoms numbered 1 to 7](/uploads/question-images/sppu-mhset-2019-06-23-chemical-sciences-paper-2/q53.png)
Answer: A
The configuration is 1R, 4R.
At C-1 the substituents are C-7 (C=O), C-2 (C=O), C-6 (CH2) and H. C-7 and C-2 tie at the first sphere (O, O, C), but at the next sphere C-7 leads to C-4, a CH with two carbon neighbours, while C-2 leads to C-3, a CH2. So the priorities are C-7 \(\gt\) C-2 \(\gt\) C-6 \(\gt\) H.
In the drawing C-1 is the front bridgehead, its H points towards the viewer, C-7 is up, C-2 lower left and C-6 lower right. Going 7 → 2 → 6 is anticlockwise as seen, and since the lowest-priority H points towards the viewer the descriptor is reversed: R. The molecule has a C2 axis exchanging C-1 with C-4, so C-4 is also R. Options with mixed descriptors are impossible for this symmetric trione, and 1S, 4S is its enantiomer.
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