mcpba

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.

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.