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amino acid pka and the wittig reaction


mcpba
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Does anyone know the approximate pKa of the alpha CH (the chiral proton) on a protected amino acid such as tyrosine or where i can get this information?

 

My interest in this is because i want to couple together two amino acids using the wittig reaction and i need to choose a base which avoids epimerising the amino acid chiral centres.

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Well, the carbonyl of an amino acid will not undergo a Wittig reaction...you need an aldehyde or a ketone not an ester, amide or carboxylic acid.

 

And for the record, the alpha proton will definatly racemise under Wittig conditions....people in my group have used similar things and managed to convert an enantiomerically pure compound into a racemate.

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I do have both the amine and acid of my amino acid protected as a boc and methyl ester respectively.

 

I have in fact made an unnatural amino acid containing a benzaldehyde. It is this aldehyde i am looking to carry out my Wittig reaction on, coupling it with a very similar amino acid which will be my ylide. Essentially i wish to form a cis-stilbene between two amino acids derived from phenylalanine.

 

If i can understand a little more precisely the pKa of the alpha proton it may help me in choosing the correct base, currently i suspect the pKa is in the region of 18-25 but it would be great if i have some actual literature where this had been calculated.

 

I appreciate the previous comments I hope my problem is a little clearer.

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