[natalaaa207]

Good morning to all. I have problems with some exercies from stereochemistry, Could somebody help me?

[Borek]

Please read the forum rules, you have to show your attempts at solving the problem to receive help.

[natalaaa207]

I know...but if I know how to solve it I wouldn't ask for help.

[Babcock_Hall]

If you look at the top problem, what is sole difference between the reagents in the two routes?  What sort of chemistry is this reagent capable of doing?  If you solve the top problem, the bottom problem will become easier.

[natalaaa207]

I know that its stereoselective addiction to carbonyl group and that second product is Felking product, but i don't know how to explain the difference, that we obtain different products in different temperatures.

[Babcock_Hall]

The temperature is a difference that I did not notice the first time around.  What do you suppose that Ce³⁺ might do?

[pgk]

Try to write the mechanisms of these reactions, by having in mind that:
1). Hydride anions (as most anions and nucleophiles) prefer to attack from the less hindered side.
2). On the other hand, CeCl3 is a Lewis acid that forms a complex with the two oxygen atoms, which brings both carbonyl groups from the same side.
3). Low temperature usually means kinetic (vs thermodynamic) control. 
4). Carbon anions (carbanions) also prefer to attack from the less hindered side.
5). TiCl4 is also a Lewis acid.