Dear Sandie:
Your observations indicate that Arg(Pbf) doubling (presumably via
premature Fmoc cleavage) and Arg(Pbf) coupling proceed at comparable
rates, c.a. (0.3-0.7):1, respectively.
Why this is happening to Arg specifically? That is the question...
Initially, I was looking for possible explanations for abnormally high
rate of Fmoc cleavage and found no satisfactory chemical/structural
reasons for this.
On the other hand, the same ratio may be attributed to abnormally low
coupling rate and this can be easily rationalized.
It is well-documented that sulfonyl-protected guanidino function of Arg
is prone to cyclization into delta-lactam during activation/coupling.
Delta-lactams are still activated species, although they are by the
factor of 1000 less reactive than symmetrical anhydrides. In
Boc-chemistry lactam formation results in longer couplings only.
Clearly, this is not the case with Fmoc which is intrinsically unstable
towards free amino/imino groups participating in coupling. While
favorable coupling rates do not result in detectable amounts of doubling
by-product, contribution of Fmoc cleavage pathway may become
significant in case of difficult couplings with slow kinetics. Positive
ninhydrine tests you observed at Args are in line with this
interpretation.
Possible cure: Extra equivalent of HOBt (e.g.
Boc-Arg(Tos)-OH:HOBt:DCC=1:2:1) suppresses delta-lactam formation with
DCC, but this may not work in case of uronium salts. Try to carry out
preactivation with cooling, avoid any excess of tertiary amine and add
it last with efficient stirring (NMM, not DIEA).
Hope this helps.
Regards,
Igor Rodionov
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