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I have a few questions on this problem I solved:

1) Why does carbon "b" on methyl vinyl ketone attack carbon "5" on the lactone? Why not carbon "2"?

2) The answer key says that the end product should have "trans" stereochemistry. The internal ester group should be opposite the carbonyl group as shown in my picture. Why is this the case? Does this have to do with the endo rule?

  • $\begingroup$ Your diagram shows "b" connected to "2", yet you're saying the correct result is "b" connects to "5". Is that correct? $\endgroup$ – ron Oct 13 '15 at 14:05
  • $\begingroup$ Note that the product is a cyclohexene, which can't adopt a true chair conformation as shown in the last drawing. $\endgroup$ – jerepierre Oct 13 '15 at 15:51

1) The regiochemistry of the product can be predicted by drawing resonance structures of the diene and dienophile, allowing us to assign partial charges to the reactive centers. Doing that shows us that 2 and a are partially positive while 5 and b are partially negative. We'd then predict that 5 attacks a and b attacks 2. This answer has a figure showing prediction of regiochemistry this way.

2) We would predict that the product is the endo- product, which would have the bridging ester on the opposite side of the cyclohexene relative to the methyl ketone. This has to do with the endo-rule, which is classically explained by secondary orbital overlap between the the lobe of the HOMO on carbon 4 interacting with the lobe of the LUMO on carbon c. Keep in mind that this trend does exist, but mixtures of endo and exo are common.

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  • $\begingroup$ Figures will be added when I have time later. $\endgroup$ – jerepierre Oct 13 '15 at 16:04

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