Direct peptide coupling of novel amino acid derivatives produced by rearrangement of catalytically generated ammonium ylides

Article abstract of DOI:10.1016/S0040-4039(03)01778-7

Protected amino acids can be prepared from substrates in which a diazo ester is aryl-tethered to an allylic amine, by catalytic intramolecular ammonium ylide generation and [2,3] rearrangement. When the aryl tether is sufficiently electron-deficient, direct coupling of the rearrangement product with a hindered amino acid ester to give a dipeptide is possible, and ammonium ylide generation, rearrangement and peptide coupling can be accomplished in a one-pot fashion.

Full text of DOI:10.1016/S0040-4039(03)01778-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 7031–7034
Direct peptide coupling of novel amino acid derivatives produced
by rearrangement of catalytically generated ammonium ylides
J. Stephen Clark* and Mark D. Middleton
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
Received 16 June 2003; revised 3 July 2003; accepted 18 July 2003
Abstract—Protected amino acids can be prepared from substrates in which a diazo ester is aryl-tethered to an allylic amine, by
catalytic intramolecular ammonium ylide generation and [2,3] rearrangement. When the aryl tether is sufficiently electron-
deficient, direct coupling of the rearrangement product with a hindered amino acid ester to give a dipeptide is possible, and
ammonium ylide generation, rearrangement and peptide coupling can be accomplished in a one-pot fashion.
© 2003 Elsevier Ltd. All rights reserved.
The demand for novel non-natural amino acids has
increased dramatically in recent years. Much of this
demand has been driven by the replacement of natu-
ral amino acids with non-natural surrogates in order
to generate peptidomimetics with therapeutic potential
and enhanced metabolic stability.1 The incorporation
of a,a-disubstituted amino acids into peptides to pro-
mote the formation of specific secondary structural
elements such as b-turns and helices, and deliver pre-
determined tertiary structure has also become an
important area of research.2 In addition, unusual a,a-
disubstituted amino acids are encountered in a wide
variety of bioactive targets (e.g. oxazolomycin and
myriocin)3 and some have been shown to function as
enzyme inhibitors4 and CNS-active glutamate ana-
logues.5
Scheme 1.
Keywords: amino acid; ammonium ylide; rearrangement; peptide coupling.
* Corresponding author. Tel.: +44 (0)115 9513542; fax: +44 (0)115 9513564; e-mail: j.s.clark@nottingham.ac.uk
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01778-7

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J. S. Clark, M. D. Middleton / Tetrahedron Letters 44 (2003) 7031–7034
ylide rearrangement display a greater propensity to
undergo peptide coupling. We now present the results
of our studies with these second-generation substrates.
In a recent publication, we reported that it was possible
to prepare unusual protected non-natural amino acids
from the diazo esters 1 (Scheme 1).6 In the reaction,
copper-catalysed carbenoid formation is followed by
ammonium ylide formation and immediate rearrange-
ment gives the azalactones 2 or 3. In the case of the
azalactones 2, lactone cleavage followed by standard
catalytic hydrogenolysis affords the amino acid esters 4.
Alternatively, the azalactones 2 are sufficiently reactive
to undergo direct coupling with glycine ethyl ester to
give the N,N-diprotected dipeptides 5, but extended
reaction times are required. However, the azalactones
are not sufficiently reactive to undergo direct coupling
with esters of other amino acids.6
It is well known that activated N-protected amino acid
aryl esters such as p-nitrophenyl and pentafluorophenyl
esters are very efficiently coupled to other amino acids.7
Thus, we expected that incorporation of one or more
electron-withdrawing substituents into the aryl unit
would deliver rearrangement products that displayed
enhanced reactivity with regard to direct coupling with
protected a-amino acids. However, we were aware that
we might encounter stability problems prior to and
during ylide generation if we were to make the aryl
system too electron-deficient. We therefore explored
two sets of substrates in which the position para to the
phenolic ester group is substituted with a fluoro group
or the more electron-demanding nitro group. The pre-
cursors were prepared from the aldehydes 9 by simple
reaction sequences (Scheme 2).8 Reductive amination of
the aldehydes with benzylamine and then reaction with
allyl bromide gave the allylic amines 10a and 10b. The
4-nitro-substituted phenol 10b was converted into the
diazo ester 11a by deprotonation with sodium hydride
and acylation with succinimidyl diazoacetate; the corre-
sponding deuterated diazo ester 11b was then obtained
by deuterium exchange. The ethyl esters 11c and 11d
were obtained by acylation of the phenols 10a and 10b
with ethyl diazomalonyl chloride.
In our original studies, we also demonstrated that the
azalactones 3 bearing an unsaturated N-substituent
undergo ring-closing metathesis to give the protected
cyclic amino acid derivatives 6.6 Sequential lactone
cleavage and hydrogenation affords the cyclic amino
esters 7, and direct coupling of the protected cyclic
amino acid derivatives 6 with glycine delivers the dipep-
tides 8.
The ability to couple the fully protected amino acid
derivatives 2 and 6 directly to other amino acids and
polypeptides without further activation or deprotection
is one of the most attractive features of our methodol-
ogy. However, the synthetic utility of this route is
limited by the fact that these azalactones are not suffi-
ciently reactive to couple directly with a-substituted
amino acids. In order to expand the scope of our
methodology, we have redesigned our substrates so that
the protected amino acid derivatives resulting from
The copper-catalysed cyclisation reactions of the sub-
strates 11 generally proceeded as expected (Eq. (1),
Table 1). Treatment of the substrates 11 with an appro-
priate copper(II) catalyst delivered reasonable yields of
,
Scheme 2. Reagents and conditions: i PhCH2NH2, 4 A sieves, NaBH4, EtOH, rt (X=F 81%); ii PhCH2NH2, NaBH(OAc)3,
ClCH2CH2Cl, rt (X=NO2 83%); iii BrCH2CHCH2, K2CO3, CH2Cl2 or MeCN (10a X=F 87%, 10b X=NO2 69%); iv NaH,
succinimidyl diazoacetate, THF, rt (11a 75%); v K2CO3, THF, D2O, rt (11b 74%, >95% D-incorporation); vi ClCOC(N2)CO2Et,
2,6-lutidine, CH2Cl2, rt (11c 65%, 11d 73%).
Table 1.
(1)
Entry
Ligand (L)
Substrate
X
Y
Product
Yield (%)
1
2
3
acac
hfacac
hfacac
11a
11c
11d
NO2
NO2
F
H
CO2Et
CO2Et
12a
12c
12d
29
63
75

J. S. Clark, M. D. Middleton / Tetrahedron Letters 44 (2003) 7031–7034
7033
the azalactones 12 arising from ammonium ylide for-
mation and rearrangement. Unfortunately, the azalac-
tone 12a proved to be so reactive to nucleophilic attack
that substantial hydrolysis of this compound occurred
during purification by column chromatography on Flo-
rasil® or silica gel and the isolated yield was signifi-
cantly reduced.
relatively hindered partner valine methyl ester was com-
plete within two hours (entry 5, Table 2), and the
corresponding reaction of the less hindered azalactone
12a was complete in just over one hour. In all cases,
excellent yields were obtained during the peptide cou-
pling reactions of the nitro-substituted azalactones.
The rapid and high-yielding direct coupling of hindered
amino acids to the protected amino acid derivatives
obtained by ylide generation and rearrangement, sug-
gested that it might be possible to perform both opera-
tions in a one-pot fashion. In order to explore the
feasibility of this reaction, we performed copper-medi-
ated cyclisation of the substrates 11a–c followed by
immediate addition of (S)-valine methyl ester (Eq. (3),
Table 3). One-pot cyclisation and coupling reactions
with this relatively hindered amino acid ester did indeed
proceed in excellent yield, and the coupling reactions
were complete in one hour when they were performed
at reflux.
Having successfully converted our cyclisation precur-
sors 11 into the azalactones 12, we then explored the
direct coupling of these intermediates to a variety of
amino acids (Eq. (2), Table 2). In previous studies, we
had discovered that it took five days for complete
coupling of azalactones such as 12 (X=H, Y=H) with
glycine ethyl ester. The fluoro-substituted azalactone
12d was found to be much more reactive than the
compound without this substituent (X=H), and cou-
pling to glycine methyl ester was complete in just over
four hours (entry 1, Table 2). It was also possible to
couple the azalactone 12d to alanine methyl ester (entry
2, Table 2), although this reaction did take 24 hours. A
remarkable increase in reactivity was observed when
the nitro-substituted azalactone 12c was employed as
the reacting partner. Coupling of this compound with
glycine methyl ester was complete in just 20 minutes
(entry 3, Table 2) and the corresponding reaction with
alanine methyl ester was also rapid (entry 4, Table 2).
Remarkably, coupling of the azalactone 12c to the
In summary, we have demonstrated that our nitro-sub-
stituted precursors 11 (X=NO2) undergo efficient car-
benoid formation, cyclisation and ylide rearrangement
to give the azalactones 12 (X=NO2), and that these
compounds display greatly enhanced reactivity with
regard to direct coupling with esters of a-amino acids.
In addition, we have shown that it is possible to
Table 2.
(2)
Entry
Substrate
X
Y
R
Reaction time
Product
Yield (%)
1
2
3
4
5
6
12d
12d
12c
12c
12c
12a
F
F
NO2
NO2
NO2
NO2
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
H
H
Me
H
Me
i-Pr
i-Pr
4 h 15 min
24 h
20 min
45 min
2 h
13a
13b
13c
13d
13e
13f
73
84
87
92
89
88
1 h 5 min
Table 3.
(3)
Substrate
Ligand (L)
Y
Temp. (ii)
Time (ii) (h)
Product
Yield (%)
11a
11a
11b
11c
11c
acac
acac
acac
hfacac
hfacac
H
H
D
CO2Et
CO2Et
rt
Reflux
rt
rt
Reflux
10–20
1
10–20
10–20
1
13f
13f
13g
13e
13e
50
49
52
81
74

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J. S. Clark, M. D. Middleton / Tetrahedron Letters 44 (2003) 7031–7034
perform these operations in a one-pot fashion without
isolation of the intermediate azalactones 12.
Altorfer, M.; Lehmann, C.; Scho¨nholzer, P.; Mu¨ller, K. J.
Org. Chem. 1996, 61, 4080; (d) Yokum, T. S.; Gauthier, T.
J.; Hammer, R. P.; McLaughlin, M. L. J. Am. Chem. Soc.
1997, 119, 1167.
Now that we have addressed reactivity issues, the final
task is to perform ylide formation and rearrangement
in an enantioselective fashion. Studies toward this goal
are currently in progress and results of this work will be
reported in due course.
3. (a) Mori, T.; Takahashi, K.; Kashiwabara, M.; Uemara,
D. Tetrahedron Lett. 1985, 26, 1073; (b) Kluepfel, D.;
Bagli, J.; Baker, H.; Charest, M. P.; Kudelski, A. J.
Antibiot. 1972, 25, 109.
4. Berkowitz, D. B.; Jahng, W.-J.; Pederson, M. L. Bioorg.
Med. Chem. Lett. 1996, 6, 2151.
Acknowledgements
5. (a) Curry, K.; Peet, M. J.; Magnuson, D. S. K.; McLen-
nan, H. J. Med. Chem. 1988, 31, 864; (b) Monn, J. A.;
Valli, M. J.; Massey, S. M.; Hansen, M. M.; Kress, T. J.;
Wepsiec, J. P.; Harkness, A. R.; Grutsch, J. L., Jr.;
Wright, R. A.; Johnson, B. G.; Andis, S. L.; Kingston, A.;
Tomlinson, R.; Lewis, R.; Griffey, K. R.; Tizzano, J. P.;
Schoepp, D. D. J. Med. Chem. 1999, 42, 1027; (c) Bradley,
D. M.; Mapitse, R.; Thomson, N. M.; Hayes, C. J. J. Org.
Chem. 2002, 67, 7613.
We thank the University of Nottingham for the award
of a studentship to M.D.M. and Aventis Pharmaceuti-
cals, Bridgewater, New Jersey for financial support. We
also thank Dr. Paul Cox (Aventis Pharmaceuticals,
Bridgewater, New Jersey) for his interest in this work.
6. Clark, J. S.; Middleton, M. D. Org. Lett. 2002, 4, 765.
7. (a) Kovacs, J.; Mayers, G. L.; Johnson, R. H.; Cover, R.
E.; Ghatak, U. R. Chem. Commun. 1970, 53; (b) Kovacs,
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U. R. J. Org. Chem. 1970, 35, 1810; (c) Kisfaludy, L.;
Roberts, J. E.; Johnson, R. H.; Mayers, G. L.; Kovacs, J.
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