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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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.