B.R. Organic Ch. 4, #60

[Labminion]

For those of you with the BR organic chemistry books (Ch.4, #60): I was wondering why there were 8 instead of 4 total products from this asymmetric Diels Alder reaction. The answer key claims there are two possible structural isomers, each with two chiral centers. Therefore 4 stereoisomers for each structural isomer, for a total of 8 products. However, the Diels Alder reaction is stereospecific with syn addition, so it seems to me there would only be two stereoisomers for each structural isomer, for a total of 4 unique products. Shouldn't each structural isomer just consist of a pair of enantiomers?

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[justadream]

I don't understand this question at all either.

Also, are endo/exo products stereoisomers?

 

[The Brown Knight]

OP was on the right track but didn't account for the endo/exo stereoisomers to get the other 4 stereoisomers.

Think of it like this:
1) Suppose the diene is ABOVE the dienophile
For this, you can orient the dienophile in 4 different ways: the acetyl group in top right, top left, bottom right, and bottom left. Each will give a different stereoisomer.
2) Suppose the diene is BELOW the dienophile
For this as well, you can orient the dienophile in 4 different ways: the acetyl group in top right, top left, bottom right, and bottom left. Each will give a different stereoisomer.

In total, you should have 8 stereoisomers.

 

[justadream]

@The Brown Knight

All the configurations in which the diene is above the dienophile are "endo", correct?