[azmanam]

QUESTION: Provide the reagents for the following transformation.  Also, provide a model to account for the relative stereochemistry of the product.

[sjb]

I want to do a vinyl cuprate 1,4 addition to the enone, which will then kick down and form the anion at C₂ and then kick out the bromide - but I'm not convinced at all....

[azmanam]

yeah, that's it.  But how do you account for the relative stereochem?

[sjb]

Simple sterics? The vinyl group is shielding the top face and the bromide can only approach from the bottom? I assume this is relative stereochem, as I don't see any chiral additives to form the drawn diastereomer rather than its antipode. Carbanions don't invert in the same way as e.g. carbocations as the lone pair is in effect in an antibonding orbital

[nj_bartel]

Oh, kept thinking organocuprate but couldn't figure out the addition in the alpha position, nice.

These are fun, by the way.  Thanks for doing them 

[azmanam]

Yes, sterics account for the relative stereochemistry.  Yes, the reaction is diastereoselective, but not enantioselective. 

Actually, though, I'd argue that you'd have a better case for diasteroselectivity with a carbanion over a carbocation.  Carbocations are always sp2 (and therefore trigonal planar and therefore achiral), but non-resonance stabilized carbanions are inherently sp3.  If the carbanion were not resonance stabilized, it would be sp3, and you could see diastereoselectivity.  Although if that were the case, you might expect the methyl and vinyl groups to be anti in the carbanion... that would put the methyl group and hydrogen syn to each other in the product.  see here:http://www.chemicalforums.com/index.php?topic=31905.msg121887#msg121887

Thanks for the words of encouragement

Part 2: These intermediates are useful in the rapid construction of stereoid skeletons.  Step two involves the heating it.  Provide the mechanism and name the steps involved.

[kiwi]

How about this?



Cheers for the problems - good procrastination material  ;-)

[azmanam]

the first step is more a (retro)electrocyclization, but yes.  well done, all.

This week's didn't take long

we still need a few structures for the last problem (http://www.chemicalforums.com/index.php?topic=34303.0) and part 2 is still open for that pesky LAH reduction (http://www.chemicalforums.com/index.php?topic=33985.0)

or if someone else wants to post a problem for us to try for the rest of the week, that'll work too