Ochem: TPR HL Sci Workbook Passage 1

[Entadus]

In the first passage of the Hyperlearning Science Workbook for ochem, question #5 asks "How many possible stereoisomers does the molecule have?"

There are 4 chiral centers in the molecule . Thus the answer in the back says there should be 16 stereoisomers (2^4).

HOWEVER, both are involved in rings. Therefore I said there should only be 4 isomers. Each of the two rings can either be on one face of the molecule or the other (2^2 = 4)

2 of the stereoisomers are creating an epoxide.
2 of the stereoisomers are forming an "enediyne" 10 membered ring. See the following pic for an example of this:
http://upload.wikimedia.org/wikipedia/commons/1/13/Calicheamicin.png
(It's the ring formed where there is a triple bond, double bond, and triple bond in succession)

In both cases, the molecule in question has the stereocenters going the same way (that is, cis) since the epoxide can either be on one face of the molecule or the other (How could an epoxide be "TRANS" ???). Likewise the larger ring created by the other 2 stereocenters would be horribly strained if the stereocenters were pointing in opposite directions.

Has anyone else seen this? Thoughts?

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[Entadus]

Okay I found a thread to help: http://forums.studentdoctor.net/showthread.php?t=831469
(See post #4 and below)

I was right, the epoxide and similarly strained rings cannot be "TRANS" at neighboring chiral carbons.

 

[sonnybobiche]

You're right, of course. Their answer is ludicrous. Question is, when it comes time for the real exam, should you go with the answer you know to be right, or the answer someone who knew only what's required for the test would give?