2-Decalone (octahydronaphthalen-2(1H)-one) has stereocentres at the ring-fusion carbons C4a and C8a. How many configurational stereoisomers does it have, and how many enantiomeric pairs are among them?

A4 stereoisomers, 2 enantiomeric pairs (one cis-fused pair, one trans-fused pair)
B2 stereoisomers, 1 enantiomeric pair
C3 stereoisomers, 1 enantiomeric pair (the cis form is meso)
D4 stereoisomers, 4 enantiomeric pairs

Answer: A

The two ring-fusion carbons are both stereocentres, and since the C=O at C2 breaks the symmetry of the skeleton there is no meso form:

2^{2} = 4

configurations. Ring fusion can be cis (4aR,8aS)/(4aS,8aR) or trans (4aR,8aR)/(4aS,8aS), and each is chiral, giving two enantiomeric pairs. (Unsubstituted decalin, by contrast, has only two achiral isomers.)

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2-Decalone (octahydronaphthalen-2(1H)-one) has stereocentres at the ring-fusion carbons C4a and C8a. How many configurational stereoisomers does it have, and how many enantiomeric pairs are among them?

Answer: A

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