N-Protected amino acid bromides: Efficient reagents for the incorporation into peptides of extremely hindered α,α-dialkyl- and α-fluoroalkyl-amino acids

Article abstract of DOI:10.1016/S0040-4039(01)00584-6

N-Protected amino acid bromides were found to be exceptionally well suited for the coupling of extremely hindered amino acids. Bromides were generated in situ under neutral conditions and used for coupling with a number of α,α-dialkyl- and N-Me-amino acids, affording configurationally pure peptides in very high yields. For the first time, peptide bond formation on the amino group of α-fluoroalkyl-amino acids is described with satisfactory yields.

Full text of DOI:10.1016/S0040-4039(01)00584-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3925–3927
N-Protected amino acid bromides: efficient reagents for the
incorporation into peptides of extremely hindered a,a-dialkyl-
and a-fluoroalkyl-amino acids
Alma DalPozzo,* Roberto Bergonzi and Minghong Ni
Istituto di Ricerche Chimiche e Biochimiche G. Ronzoni, via Colombo 81, 20133 Milan, Italy
Received 8 March 2001; accepted 6 April 2001
Abstract—N-Protected amino acid bromides were found to be exceptionally well suited for the coupling of extremely hindered
amino acids. Bromides were generated in situ under neutral conditions and used for coupling with a number of a,a-dialkyl- and
N-Me-amino acids, affording configurationally pure peptides in very high yields. For the first time, peptide bond formation on
the amino group of a-fluoroalkyl-amino acids is described with satisfactory yields. © 2001 Elsevier Science Ltd. All rights
reserved.
The incorporation into peptides of unnatural amino
acids containing a quaternary carbon atom at the 2-
position is of common interest in the synthesis of
peptidomimetics. However, standard peptide chemistry
is mostly unsuccessful for this purpose, due to the
reactivity of these molecules being rather different from
that of proteinogenic amino acids.
amino group remains unresolved, except for the least
bulky a-Tfm-alanine.5 In fact, beside the steric hin-
drance, this amino function becomes unreactive (pKa
around 6), because of the polarizing effect of the
fluoroalkyl group.6 Activation of the nitrogen via the
isocyanate gave only very poor yields in most cases.7
On the other hand, the amino group of a-Tfm-AA was
reported to react with N-protected amino acid chlorides
in the presence of TEA, but this happens with total
racemization of the non fluorinated amino acid. In fact,
as we have demonstrated,† the substrate does not react
with the chloride, but with a ketene formed through
hydrogen halide elimination induced by TEA. This
drastic reaction is limited to substrates where epimer-
ization is not possible.8,9
In recent years, many difficult condensations have been
improved using new potent carboxylic group activators,
such as phosphonium and aminium/uronium salts, or
N-protected amino acid fluorides in the presence of
silylating agents.1 These reagents proved to be very
efficient for the coupling of well known, problematic,
a,a-disubstututed and N-substituted amino acids. Nev-
ertheless, none of the new reagents succeeded in the
coupling of a-fluoroalkyl-amino acids. Peptides con-
taining an a-trifluoromethyl residue (a-Tfm) at the N-
terminal position have been obtained,2–4 but the
problem of chain elongation by derivatization of the
Prompted by a need to synthesize peptides containing
a-Tfm- or a-Dfm-amino acids (a-difluoromethyl) at
different positions and considering that acyl bromides
are known to be much more reactive than chlorides
Scheme 1.
Keywords: amino acid bromides; difficult couplings; a-fluoroalkyl-amino acids.
* Corresponding author. Tel.: +39-02-70600223; fax: +39-02-70633007; e-mail: dalpozzo@ronzoni.it
† Unpublished results.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00584-6

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A. DalPozzo et al. / Tetrahedron Letters 42 (2001) 3925–3927
Table 1. Synthesis of peptides with N-protected amino acid bromides (the numbers shown between brackets indicate the
equivalents of bromide+equivalents of collidine employed)
Reagent
Substrate
Time
Producta
Yield (%)
Pht-Phe-Br
(5+2)
Pht-Phe-Br
(3+1)
Pht-Phe-Br
(5+2)
Pht-Val-Br
(3+4)
Cbz-Pro-Br
(3+4)
Cbz-Pro-Br
(3+4)
Cbz-Pro-Br
(5+2)
Fmoc-Pro-Br
(3+4)
a-Tfm-(R,S)-Phe-OEt
a-Dfm-(R,S)-Phe-OEt
a-Dfm(R or S)b-Phe-Ala-OMe
N-Me-Val-OMe
Aib-OtBu
2 h
Pht-Phe-a-Tfm-(R,S)-Phe-OEt
Pht-Phe-a-Dfm-(R,S)-Phe-OEt
Pht-Phe-a-Dfm(R or S)b-Phe-Ala-OMec
Pht-Val-N-Me-Val-OMec
Cbz-Pro-Aib-OtBu
94
90
83
89
91
96
98
94
2 h
12 h
10 min
10 min
10 min
10 min
10 min
N-Me-Val-OMe
a-Tfm-(R,S)-Phe-OEt
Aib-OtBu
Cbz-Pro-N-Me-Val-OMec
Cbz-Pro-a-Tfm-(R,S)-Phe-OEt
Fmoc-Pro-Aib-OtBu
1
a Each peptide was isolated, purified by flash chomatography and characterized by H NMR, 19F NMR and FAB-mass spectroscopy. In all cases,
the purity was >98%, as detected by RP–HPLC.
b The absolute configuration is unknown.
c For the products derived from the coupling of bromides with
L-amino acids, it was possible to ascertain by HPLC the absence of racemization.
toward amines (up to two orders of magnitude), we
sought a general method that could afford pure bro-
mides from different amino acids.
the Boc, Cbz or Fmoc groups, mostly undergo sponta-
neous decomposition to the corresponding oxazolones
or Leuch’s anhydrides. Whereas, for the other halides,
the intramolecular cyclization cannot compete with the
coupling reaction, the prerequisite for the overactivated
bromides to survive their own preparation is to be
N-protected as diacylamines.§
Amino acid bromides have never been used in chemical
practice, probably because they are unstable and
difficult to obtain in a pure state. We were able to
obtain pure amino acid bromides from N-protected
carboxylic acids with 1-bromo-N,N-2-trimethyl-1-
propenylamine under very mild and neutral conditions,
following a method described in the literature10,11 for
simple acyl bromides (Scheme 1).
In conclusion, suitably N-protected amino acid bro-
mides were shown to be very efficient reagents for the
in situ acylation of building blocks bearing highly hin-
dered amino groups.
Other potential brominating agents have been proposed
in the literature, such as BroP, PyBroP or TBFH, but,
in our hands, none of them succeeded, yielding little if
any peptide formation with Tfm-AA.‡
However, their most impressive application is for the
formation of the peptide bond at the N-terminal posi-
tion of a-Tfm-amino acids, which failed to react in
reasonable yields with all other known reagents.
The generality of our method has been assessed using a
variety of bromides with a variety of hindered sub-
strates. In every case, very pure peptides were obtained
in high yields by flash chromatography, after separation
of the excess acids and reagents. All products were
confirmed by 1H NMR, 19F NMR and FAB mass
spectroscopy. When configurationally pure amino acid
esters were employed as substrates, the optical purity of
the final peptides was ascertained by HPLC.
In a typical procedure, to a solution of a suitably
N-protected amino acid in dry DCM, bromoenamine
was added (from 1 to 2 equiv. according to its purity)
and the solution stirred under argon for 15 min; the
total conversion of the acids to bromides was checked
by TLC after quenching with MeOH. When the conver-
sion was complete, a premixed solution of the amino
acid esters and collidine was slowly added at 0°C. The
ratio between bromides, amino acid esters and collidine
were varied according to the substrates and are given in
Table 1.
It is worth pointing out that, unlike chlorides or
fluorides, amino acid bromides, when protected with
‡ It is interesting to note that, whereas Carpino’s reagent TFFH
(tetramethylfluoroformamidinium hexafluorophosphate)12 can give
pure and isolable fluorides, its bromo analogue led, within a few
minutes, to the formation of the corresponding unreactive anhy-
drides under the same coupling conditions. This was demonstrated
by HPLC and IR.
§ As shown in Table 1, only Cbz- and Fmoc-Pro-Br survived long
enough to acylate the hindered AA esters, while Cbz- and Fmoc-N-
Me-Val promptly gave Leuch’s anhydrides. Evidently, the rigidity
of the proline cyclic side-chain excludes certain transition states that
are an integral part of the cyclization process.

A. DalPozzo et al. / Tetrahedron Letters 42 (2001) 3925–3927
3927
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