[Dan]

Rationalise the following transformation:

[Dan]

No takers?

Hint: Amadori

[Dan]

Hint: Carbanionic migration

[yesway]

Hi,

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

Best


edit: resize of png

[Dan]

Yes, well done!

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

[Rory]

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.

[Dan]

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?

[Rory]

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

[Dan]

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