[faust]

Hello,

I have some troobles with this exercice

I have to explain the selectivity of this reaction by using the Felkin Ahn model and the Cram Chelate model. But When I used it, I've got 4 differents diastereoisomers for the felkin ahn model... Do you see what I did wrong? Thank you for your help again.

[alphahydroxy]

just had a quick glance, but it seems to me that this must be a question regarding relative stereochemistry rather than absolute.

This is because the stereochemistry at the carbon alpha to the the carbonyl is not affected during the reaction but the its stereochemistry is defined in the product.

Thus, the reactions proceed to either syn or anti products


So, while you can draw 4 different newton projections, 2 of them lead to syn products and 2 lead to anti products.


like i say, i've just had a glance and may be wrong here, but i think it's just a slightly misleading question.

[Dan]

I've had a look at this, and I agree, it must be about relative stereochem rather than absolute. Don't forget co-ordination of the oxygens to the metals.

How does Me₂Mg act as a hydride source? I can't find it anywhere.

[faust]

Ho, ok ...

Hum, no Me2Mg is a methyl group, I've made a mistake when copying the question. It is H or Me in the final product

[movies]

Yeah, it's definitely relative stereochem in this question.  Since you start with a racemic starting material, and use no chiral reagents, you must get a racemic product.

NOTE: the four compounds you drew are not all diastereomers!  There are two pairs of enantiomers, the members of each pair are diastereomeric to the other pair.  Remember: enantiomers are not diastereomers.  They are, however, all stereoisomers.  Check out the isomer heirarchy thread for more discussion.