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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: Dan on August 01, 2012, 08:11:52 PM

Title: Problem for Aug 2012
Post by: Dan on August 01, 2012, 08:11:52 PM

Rationalise the following transformation:

Title: Re: Problem for Aug 2012
Post by: Dan on August 30, 2012, 07:10:44 AM

No takers?

Hint: Amadori

Title: Re: Problem for Aug 2012 [Still unsolved, hint added]
Post by: Dan on September 15, 2012, 07:34:04 AM

Hint: Carbanionic migration

Title: Re: Problem for Aug 2012 [Still unsolved, hint added] [second hint added]
Post by: yesway on September 15, 2012, 05:18:04 PM

Hi,

with your hints (and a few beers), I put together a suggestion towards the solution.

Best


edit: resize of png

Title: Re: Problem for Aug 2012 [Still unsolved, hint added] [second hint added]
Post by: Dan on September 16, 2012, 06:53:05 AM

Yes, well done!

The benzilic acid rearrangement proceeds with very high diastereoselectivity. Can you rationalise the stereochemistry?

Title: Re: Problem for Aug 2012 [Still unsolved, hint added] [second hint added]
Post by: Rory on October 25, 2012, 07:30:40 AM

Long time no see Dan. How are you doing? :)
My explanation for the diastereoselectivity is that the methyl group migration of C to give D is stereoelectronically forbidden, for a carbonyl group that bears highly distorted bond angles would be formed. So rearragement of A to B is favored.

Title: Re: Problem for Aug 2012 [Still unsolved, hint added] [second hint added]
Post by: Dan on October 30, 2012, 05:01:12 AM

Hi Rory, I'm fine thanks.

I am not convinced by your distorted bond argument. As drawn, both of those migration products have distorted sp² bond angles - this is just an artistic issue, not a real life problem. It is more accurate to draw the carbonyl in a position inbetween the axial and equatorial positions.

I think it is more helpful to consider a 5-membered calcium chelate. Hint: What models are commonly used to rationalise and predict the diastereoselectivity of addition to α-stereogenic aldehydes/ketones?

Title: Re: Problem for Aug 2012 [Still unsolved, hint added] [second hint added]
Post by: Rory on October 30, 2012, 06:19:54 AM

I'm sorry for the inconvincable argument. Does Cram rule account for the stereoselectivity?

Title: Re: Problem for Aug 2012 [Still unsolved, hint added] [second hint added]
Post by: Dan on November 09, 2012, 12:16:00 PM

Yes - the stereoselectivity of the benzilic acid rearrangement can be rationalised by the chelated Felkin-Ahn model. Well done.