question about stereocontrol?

[gradstudent]

Hi everyone I have two questions.

1. How can I predict the stereochemistry of the reaction of (S)2-methoxy-®4-methoxy-3-hexanone with tetramethyl ammonium triacetoxy borohydride. What face will it attack from? 2. The reaction of 3(S)-hydroxy-4(S)-isopropylcyclohexanone with sodium triacetoxy borohydride. What will be the stereocenter of the aclohol? What face will it attack from in this case? I heard my TA joined this forum and I would like to get an answer from her or someone in her group that took the class from the same prof before. He is on my comps committee and I need an answer to these questions to finish my molecule.

 

Cheers

[chemist]

Hi everyone I have two questions.

1. How can I predict the stereochemistry of the reaction of (S)2-methoxy-®4-methoxy-3-hexanone with tetramethyl ammonium triacetoxy borohydride. What face will it attack from? 2. The reaction of 3(S)-hydroxy-4(S)-isopropylcyclohexanone with sodium triacetoxy borohydride. What will be the stereocenter of the aclohol? What face will it attack from in this case? I heard my TA joined this forum and I would like to get an answer from her or someone in her group that took the class from the same prof before. He is on my comps committee and I need an answer to these questions to finish my molecule.

 

Cheers

 

Hey - sorry for the delayed response I wasn't really aware you'd asked me a question until it was brought to my attention by a lab mate - I'm not really active on this forum anymore - and also the others would prefer if you didn't give out *their* info as it pertains to this forum to other students - so that they can remain active on this forum... they don't want to be "expected" to answer student questions here but I will however take a stab at this question since this will not be a problem for me in the future and I'm quite certain that this is due for you by Monday....

 

Now.. since it's for comps and I'm your former TA I can't give you an exact answer but I can point you in the right direction and perhaps if you still don't understand someone else will come along and SOS...

 

That being said...

First of all if you must do the first reaction as described... then you will likely get attack from the face that has the least steric hindrance... use the Cram Chelation model in this case because you have an alpha alkoxy group.... but I would suspect that you'd get chelation to the alpha position bearing the methyl and therefore the reagent would attack accordingly....

 

however... there is not much of a differentiable difference between the methyl and the ethyl substituents so if you proceed as planned you will likely get a mixture no matter what you do... It's just that it should favor the methyl... but how much I couldn't tell you without actually running the reaction...

 

Might I suggest TBS protecting the undesired location in order to facilitate attack from the intended face and with the intended regiochemistry....

 

For the second question...

First determine what group will prefer to be axial and which will prefer to be equitorial... I would suggest using the charts given in the handouts early in the semester.... paying close attention to section on ring conformation and the sterics invovled.... Also remember that you have a bulky reducing agent... so therefore it will attack from an equitorial face - i.e. the hydride will be delivered to the equitorial position - to give the axial alcohol....

 

hope this helps

good luck.

 

p.s. if you get desperate and the others won't come along to help you can email me at my uni account which is listed on the first page of the old discussion syllabus...

[gradstudent]

Thank you! Don't worry I won't give out any info on anybody to other students.