ORGLIST: Re: Peptide bond formation from tfa salt

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From: Jack Kellum (jkellum$##$ucsd.edu)
Date: Tue Mar 21 2000 - 20:36:02 EST

Chris,

There's no need to convert your TFA salt to the HCl salt. The critical
factor is your choice of base. Whereas TEA would neutralize a TFA salt just
as readily as an HCl salt, the problem lies in the fact that you're coupling
with a highly activated carboxyl component. Your title said "peptide bond
formation" so I assume that the two components you're attempting to couple
are amino acids. In this case, using TEA with a amino acid chloride will
likely result in formation of an N-carboxyanhydride or an oxazolone
depending on the nature of your N protecting group. If the use of an amino
acid chloride is your only choice, you'll want to employ a severely hindered
tertiary amine base such as 2,6-di-tert-butylpyridine (Carpino, JOC, 1991,
2635).

If possible, why not utilize some of the more traditional coupling methods
for amino acids. An excellent review by Humphrey and Chamberlin appears in
Chem. Rev., 1997, 2243.

Good luck,

Jack
----- Original Message -----
From: "Christopher Paul Borella" <cborella$##$ic.sunysb.edu>
To: "Multiple recipients of list orglist" <orglist$##$dq.fct.unl.pt>
Sent: Tuesday, March 21, 2000 3:45 PM
Subject: ORGLIST: Peptide bond formation from tfa salt

Hello,
I have a simple question, I have generated a TFA salt of a free amine, and
want to couple it with an acid chloride insitu. I can't seem to find any
reference for this reaction, there are plenty with HCl salts but not
TFA. Would TEA be strong enough to break up this salt or do I need to do
something else? A co-worker sugested exchanging it to the HCl salt by
reacting with excess HCl, however my compound is not stable to those
conditions (silyl protecting groups). I would appreciate it if someone
could give me some advise as to how to solve this or at least point me to
a reference.

Thanks,

Chris

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