Destroyer stereochemistry question...

[Decan]

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Hello. I had a question on #91 in the destroyer (ochem section). I understand why all the choices are S, but can anyone explain why choice B needs to be rotated twice to figure out the stereochemistry around the chiral carbon? I'm horrible with stereochemistry so any help would be greatly appreciated!

 

[levyusupov]

you need to rotate twice only if the lowest substitued is not a dash or a triangle. thus, you rotate to figure out what is the stereochem of the 4th position.

 

[orgoboy]

The reason you would "rotate" the molecule in the first place is so that the lowest ranking species is pointing away. However the rule is when doing one rotation it has to be accompanied by another to conserve stereo chemistry. The next time you confront a similar problem 1) rotate two species pointing the lower ranking molecule away, 2) rotate the two remaining species to conserve stereochem.
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