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stereochemistry of reaction

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    Why can the top happen, but the bottom cant? is it to do with trans-diaxial orbital stuff?
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    This doesn't really make any sense, who told you that the top reaction would happen but the bottom one wouldn't? The only thing different between the two enantiomers is the fact the hydrogen on the carbon alpha to the nitrile carbon is equatorial in the top enantiomer and axial in the bottom. However, this hydrogen does not move during the reaction so is of no significance.

    What year are you? Those arrows on the right are retrosynthesis arrows, so this is stating that you cannot form the bottom regioisomer due to there being a problem with the transition state. This reaction is a 4s+2s cyclo-addition, otherwise known as a Diels Alder reaction. The furan is the diene, the acrylonitrile is the dieneophile. As the nitrile is an electron withdrawing substituent, we have an electron deficient dienophile. The oxygen of the furan ring is electron donating, so we have an electron rich diene. Therefore we have normal electron demand, meaning that for the orbital explanation of this reaction, we use the HOMO of the diene and the LUMO of the dieneophile.In the top reaction, we have the exo product. In the bottom product, we have the endo product. In the endo transition state of this Diels Alder reaction, there is a favourable secondary orbital interaction meaning that the endo transition state should be stabilised over the exo, even though there is a higher degree of steric crowding. Hopefully this drawing shows what I mean by the secondary orbital interaction. In the second step, after the bonds have been formed through the cycloaddition, the dieneophile flips round, meaning that the nitrile will be on the same face as the oxygen.. which is why I'm confused.
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