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Topic: Problem of the Week - 10/26/09  (Read 18257 times)

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Offline azmanam

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Problem of the Week - 10/26/09
« on: October 26, 2009, 09:30:20 PM »
This time, I'm not going to answer after every guess.  The community can discuss and combine and come back with a consensus answer.  I like that better than the 'is this right?' 'no.' 'how bout this?' 'sorta, think about it this way.' ... approach to problems.  So discuss and argue and debate amongst yourselves and let me know when you think you've got the answer.

QUESTION: Provide a complete, arrow pushing mechanism for the following transformation and predict the product:
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Offline Heory

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Re: Problem of the Week - 10/26/09
« Reply #1 on: October 27, 2009, 02:45:32 AM »
No stereoselectivity in the reaction?

Offline Heory

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Re: Problem of the Week - 10/26/09
« Reply #2 on: October 27, 2009, 06:18:35 AM »
I prefer the old way of discussing, for few people are willing to post their viewpoints.

Offline azmanam

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Re: Problem of the Week - 10/26/09
« Reply #3 on: October 29, 2009, 08:31:03 AM »
It looks like you're right, no one wants to play my way.  Oh well.

There will be stereoselectivity issues to discuss later, but first we'll focus on just the arrow pushing.  You have chosen the wrong nucleophile after L.A. activation, and as a result, the rest of the mechanism is off track.  Reassess with a different nucleophile and see what else looks interesting.
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Offline Dan

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Re: Problem of the Week - 10/26/09
« Reply #4 on: October 29, 2009, 10:44:48 AM »
This one's tough. I got to a tricyclic intermediate but I'm not sure where to go from there, or even whether I'm in the right place anyway...
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Offline azmanam

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Re: Problem of the Week - 10/26/09
« Reply #5 on: October 29, 2009, 10:47:10 AM »
Diels Alder.  Interesting, but no.  (plus, I think the bottom face opposite the isopropyl group would be more accessible if that were to occur :))
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Offline nj_bartel

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Re: Problem of the Week - 10/26/09
« Reply #6 on: October 29, 2009, 11:17:22 AM »
For the first mechanism proposed - are there any mechanisms involving vinylic carbocations that are generally accepted?

Offline azmanam

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Offline Heory

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Re: Problem of the Week - 10/26/09
« Reply #8 on: October 29, 2009, 11:34:08 AM »
 :)

Offline Heory

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Re: Problem of the Week - 10/26/09
« Reply #9 on: October 29, 2009, 11:38:10 AM »
I'm really very sorry. Your drawing always looks rather beautiful and I just want mine to be better. Well, I will use the ACS style.

Offline nj_bartel

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Re: Problem of the Week - 10/26/09
« Reply #10 on: October 29, 2009, 11:38:29 AM »
addition of HX to alkynes

http://www.chemistry.ccsu.edu/glagovich/teaching/311/content/alkyne/reactions/alkynerxn2.html

Ah alright.  When I took organic I, that mechanism was presented to me as still pretty widely disputed due to the instability of that cation.  An alternative possibility presented was an X'onium intermediate.

Edit:  I like your new one Heory.

Offline azmanam

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Re: Problem of the Week - 10/26/09
« Reply #11 on: October 29, 2009, 12:18:18 PM »
I had no idea this question would be this intriguing.  Three completely different guesses with three completely different modes of activity, yet none of them get to the product of the reaction.  Very fun :)

Perhaps it would be useful to think about and list in words what the various functionalities of the two reagents are.  Then maybe try to think about how they might react with each other under lewis acidic conditions.  Look for best nucleophiles, best electrophiles and see if we can talk through it that way?

Heory: not sure what you're talking about.  Your schemes are nice and quite easy to follow.  Keep up the good work. (btw, I don't use ACS, I use TotSyn style, downloaded from here:  http://totallysynthetic.com/blog/?page_id=287)
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Offline Heory

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Re: Problem of the Week - 10/26/09
« Reply #12 on: October 29, 2009, 12:29:58 PM »
I am totally confused...
OH is not a good nuleophilic reagent in the presence of LA
i need more clues
now it is 0:30 here and i must go to sleep
see you tomorrow

Offline azmanam

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Re: Problem of the Week - 10/26/09
« Reply #13 on: October 29, 2009, 12:33:16 PM »
;)  see you tomorrow.  Let visions of terpenes dance in your dreams :)
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Offline Dan

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Re: Problem of the Week - 10/26/09
« Reply #14 on: October 29, 2009, 12:35:50 PM »
Diels Alder.  Interesting, but no.  (plus, I think the bottom face opposite the isopropyl group would be more accessible if that were to occur :))

My structure also breaks Bredt's rule... Schoolboy error...

This is tough, I need more time to think about it. I'm liking Heory's diene nucleophile though.
My research: Google Scholar and Researchgate

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