The synthesis of functionalised peptides using α-lithio quinuclidine N-oxide (Li-QNO)

Article abstract of DOI:10.1016/j.tetlet.2009.03.105

Deprotonation of protected peptides using lithiated quinuclidine N-oxide (Li-QNO) as a base at 0 °C, followed by addition of an alkyl halide gives C-alkylated peptide derivatives in good yield.

Full text of DOI:10.1016/j.tetlet.2009.03.105

Tetrahedron Letters 50 (2009) 3635–3638
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Tetrahedron Letters
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The synthesis of functionalised peptides using
(Li-QNO)
a-lithio quinuclidine N-oxide
*
Ian A. O’Neil , Inder Bhamra
Robert Robinson Laboratories, Department of Chemistry, University of Liverpool, Crown St, Liverpool L69 7ZD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Deprotonation of protected peptides using lithiated quinuclidine N-oxide (Li-QNO) as a base at 0 °C, fol-
lowed by addition of an alkyl halide gives C-alkylated peptide derivatives in good yield.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 21 January 2009
Revised 6 March 2009
Accepted 13 March 2009
Available online 20 March 2009
The C-alkylation of
a
-amino acid derivatives represents an
N-protected compound 1 was added to a suspension of Li-QNO 7
in THF at À78 °C, and 30 minutes were allowed for lithiation to oc-
cur. Formation of an immobile yellow suspension was considered
indicative of formation of the poorly soluble dianion 2, subsequent
addition of a 1 M solution of BnBr in THF led to increased solubility.
After 2 h at À78 °C, the mixture was allowed to reach room tem-
perature and quenched with aqueous NH4Cl. Flash column chro-
matography of the organic extracts resulted in isolation of C-
alkylated product 3 in 94% yield (Scheme 2).
attractive approach to the synthesis of higher amino acids. The
use of N-acyl or urethane-protecting groups at the amino terminus
leads to the requirement for double deprotonation in order for C-
alkylation to take place (Scheme 1). Regioselective C-alkylation
has been achieved through the use of an additive such as HMPA
or TMEDA.1 The acid terminus is often protected as the methyl es-
ter derivative, although careful consideration as to the choice of
base for the reaction is required, as in many cases alkyllithium
bases afford substituted ketones such as 5 as the products.2
The reaction of methyl N-benzoylglycinate 1 with LDA, followed
by the addition of BnBr or MeI to afford C-alkylated products 3 and
4 is described as requiring 2 equiv of LDA and HMPA at À78 °C, fol-
lowed by addition of the alkyl halide at the same temperature.2
Contrary to an earlier report3 the use of 1 equiv of LDA results in
predominantly N-alkylation. The use of HMPA leads to greater
yields of products than the use of TMEDA. The role ascribed to
the additives is one of deaggregating the intermediate lithium eno-
late 2, (Scheme 1). The monodentate versus bidentate nature of the
additives (HMPA and TMEDA, respectively) is thought to be impor-
tant in controlling the coordination sphere of the metal atom and
hence the reactivity of the enolate.
We then turned to the alkylation of protected dipeptides. The
necessary trianion formation often makes such alkylation reactions
difficult, the trianion being poorly soluble in THF. Seebach and co-
workers5 have described the solubilisation of such intermediates
through the addition of excessive amounts of inorganic salts (usu-
ally LiCl). Given the stability of Li-QNO at higher temperatures we
were intrigued that if the trianion was warmed to 0 °C, then en-
hanced solubility may lead to higher yields of products. We were
also interested in the possibility that the alkylations may show
some stereoselectivity. As a starting point the alkylation of the gly-
cine residue of N-benzoyl-Gly-Ala-OMe 8 was selected. The chiral
substituent on the neighbouring amino acid (the alanine methyl
group in this case) was expected to exert a steric effect making
alkylation on one face of the carbanion more favourable than the
other. The dipeptide N-benzoyl-Gly-Ala-OMe 8 was prepared by
the DCC-mediated coupling of N-benzoyl glycine with alanine
methyl ester hydrochloride. Treatment of dipeptide 8 with 3 equiv
of LiQNO, at À78 °C followed by warming of the solution to 0 °C
gave a solution of the trianion 11. The solution was re-cooled to
À78 °C, followed by addition of BnBr. Disappointingly, the reaction
gave a mixture of three products, the two diasteroisomeric glycine
C-alkylated dipeptides 9a/b in a 1:1 ratio in 35% yield, and the ala-
nine C-alkylated product 10 in 29% yield (Scheme 3).
We have recently reported the use of
a-lithio quinuclidine N-
oxide (Li-QNO) 7 as a powerful non-nucleophilic base/HMPA mi-
metic.4 It is readily prepared by oxidation of quinuclidine 6 using
ozone, followed by deprotonation with t-BuLi at À78 °C. The low
nucleophilicity of Li-QNO towards carbonyl esters suggested that
it may well be a suitable base for the C-alkylation reaction.4 Ulti-
mately, Li-QNO was envisaged as a replacement for LDA and HMPA
in the C-alkylation of amino acid derivatives.
The alkylation of methyl N-benzoylglycinate 1 with BnBr was
used as a starting point, providing a direct comparison between
the use of Li-QNO and the combination of LDA and HMPA. The
Following this result we decided to examine the alkylation of N-
Boc-protected amino acids using Li-QNO as a base. In order to test
the viability of this approach, the benzylation of methyl N-Boc-gly-
cinate 12 was first investigated. Treatment of 12 with 2 equiv of Li-
* Corresponding author.
E-mail address: ion@liv.ac.uk (I.A. O’Neil).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.105

3636
I. A. O’Neil, I. Bhamra / Tetrahedron Letters 50 (2009) 3635–3638
O
O
O
R
RX (1 equiv)
HMPA or TMEDA
2Li
LDA (2 equiv)
OMe
OMe
OMe
Ph
N
H
30-40%
Ph
N
Ph
N
H
1
2
O
O
O
3 R = Bn
4 R = Me
tBuLi
O
1. O3, Et2O, -78 oC
Li-QNO, 7
tBu
2. t-BuLi, THF, -78 oC
Ph
N
H
N
O
Li
N
5
O
6
Scheme 1.
Ph
O
O
2Li
Li-QNO (2 equiv)
THF, -78 ºC
O
BnBr (1 equiv)
OMe
OMe
OMe
Ph
N
H
Ph
N
THF, -78 ºC to r.t.
94%
Ph
N
H
1
2
O
3
O
O
Scheme 2.
Ph
O
H
N
CO2Me
O
Ph
N
H
i) Li-QNO (3 equiv)
THF -78 ºC to 0 ºC
9a/b
H
N
CO2Me
O
+
Ph
N
H
O
ii) BnBr (1 equiv)
THF, -78 ºC
H
8
O
N
CO2Me
Ph
N
H
10
O
Ph
3 Li
O
N
CO2Me
Ph
N
11
O
Scheme 3.
QNO at 0 °C followed by addition of BnBr at the same temperature
gave an excellent 96% yield of the C-alkylated product 13. We ob-
served no addition of the Li-QNO into the ester. When N-Boc ala-
nine methyl ester 14 was used as the substrate under identical
conditions, quenching of the dianion with BnBr gave a 46% yield
of the C-alkylated product 15 (Scheme 4). Unreacted starting mate-
rial 14 was also recovered in 50%. Significantly, these results indi-
cate that it is possible to C-alkylate N-Boc derivatives of simple
amino acids without using additives such as HMPA or resorting
to dianion formation at low temperatures.
With methodology in place for the C-alkylation of dianionic
Boc-protected amino acid derivatives our attention once again
turned to the alkylation of dipeptide derivatives. In order to gauge
whether selective C-alkylation on the amino acid residue at the C-
terminus of the dipeptide could be achieved, N-Boc-Gly-Gly-OMe
16 was prepared by the DCC-mediated coupling of N-Boc glycine
and glycine methyl ester hydrochloride.6 Treatment of the pro-
tected dipeptide 16 with 3 equiv of Li-QNO at 0 °C, followed by
addition of BnBr, again at 0 °C, gave the C-alkylated dipeptide 17
as a single regioisomer in 76% (Scheme 5).
Benzylation on the glycine residue at the C-terminus of the
dipeptide was confirmed by 1H NMR. The
a-proton(s) of this gly-
cine residue exhibited a significantly higher chemical shift than
their counterparts on the residue at the N-terminus in both the
starting material 16 and in the product 17. Several more polar
by-products observed by TLC remained unidentified.
In order to study the stereoselectivity of the alkylation reaction,
N-Boc-Val-Gly-OMe 18 was prepared. It was hoped that the isopro-
pyl group of the valine residue would exert a large enough steric
effect to influence which face of the enolate 19 would undergo
alkylation (Scheme 6). The requisite dipeptide 18 was prepared
by the DCC-mediated coupling of N-Boc valine with glycine methyl
ester hydrochloride. Again, treatment of the protected dipeptide 18
with 3 equiv of Li-QNO at 0 °C, followed by addition of BnBr at 0 °C,

I. A. O’Neil, I. Bhamra / Tetrahedron Letters 50 (2009) 3635–3638
3637
Ph
i) Li-QNO (2 equiv)
THF, 0 ºC.
i) Li-QNO (2 equiv)
THF, 0 ºC.
Ph
ii) BnBr (1 equiv)
THF, 0 ºC to r.t.
BocHN
CO2Me
BocHN
CO2Me
BocHN
CO2Me
BocHN
CO2Me
ii) BnBr (1 equiv)
THF, 0 ºC to r.t.
15,
46%
12
13, 96%
14
Scheme 4.
H
N
i) Li-QNO (3 equiv)
THF, 0 ºC.
H
N
CO2Me
Ph
CO2Me
BocHN
BocHN
ii) BnBr (1 equiv.)
THF, 0 ºC to r.t.
O
17, 76%
16
O
Scheme 5.
3Li
Li-QNO, (3 equiv)
THF, 0 oC
O
O
O
O
H
N
N
ButO
N
H
OMe
ButO
N
OMe
19
18
O
O
BnBr (1 equiv)
THF, 0 °C to r.t.
O
O
O
O
H
N
H
N
ButO
N
H
OMe
ButO
N
H
OMe
O
O
20b
20a
Ph
Ph
Scheme 6.
gave two diasteroisomeric C-alkylated dipeptides 20a and 20b,
for the two diastereoisomers prepared via the coupling of N-Boc-
valine with
-phenylalanine-OMe7 and -phenylalanine-OMe,8
respectively (Fig. 1).
The observed data compared favourably with that reported in
the literature. The diastereoisomeric ratio of 20a:20b was 7:3,
and had occurred in favour of the product incorporating unnatural
L-
arising from benzylation at the
in a combined yield of 81%.
a
-carbon of the glycine residue,
L
D
1H NMR analysis suggested an uneven distribution of the two
diastereoisomers. The methyl groups of the isopropyl side chains,
the
a-hydrogens and the carbamate protons (all associated with
the valine residue), appeared at different chemical shifts corre-
sponding to the two diastereoisomers. Determination of the diaste-
reomeric ratio was possible through comparison of the relative
integrations of the doublets due to the diastereotopic methyl
groups in the isopropyl side chain of the valine residue. The stereo-
chemical assignment of each diastereoisomer was achieved by
comparing the observed data with those reported in the literature
D
-phenylalanine.
Diastereoselectivity in such alkylations is not unfounded. See-
bach9 has reported C-alkylation of sarcosine residues affording
products with R-configuration at the newly formed stereogenic
centre. Ager and coworkers10 have also reported products in which
the newly formed stereogenic centre is of the R-configuration
through the use of HMPA. The systems investigated by Ager appear
0.92
0.96
0.89
0.94
O
O
O
O
2.18
2.12
H
H
N
N
ButO
N
H
OMe
ButO
N
H
OMe
6.48
6.37
O
O
Ph
Ph
N-Boc-L-Val-L-Phe-OMe (20a)
20b
N-Boc-L-Val-D-Phe-OMe (
)
Figure 1. Chemical shifts (d ppm) measured in CDCl3 (400 MHz).

3638
I. A. O’Neil, I. Bhamra / Tetrahedron Letters 50 (2009) 3635–3638
Ph
Ph
O
O
O
i) LDA (3 equiv)
H
N
HMPA (3 equiv)
H
N
R
TsHN
OMe
S
ii) BnBr, 70%
OMe
TsHN
O
22
21
Ph
Scheme 7.
similar to those we have investigated, Scheme 7 illustrates one
such case. Benzylation of the trianion of N-Ts- -Phe-Gly-OMe 21
Supplementary data
L
afforded the C-benzylated product 22 in which the R-configuration
was favoured at the new stereogenic centre, in 57% de.
Interestingly, the largely different approach of Seebach9 (solu-
bilisation of the anion through addition of Li salts) in comparison
to the approach adopted by Ager10 and ourselves (the use of a Li-
coordinating additive) appears to afford products with the same
stereochemistry at the new chiral centre. In all cases an adjacent
(S)-amino acid generates a stereocentre with (R)-configuration.
The difference is not as great as first appears, both approaches
can be described as deaggregating the carbanionic species prior
to alkylation.
In summary, Li-QNO can be used to deprotonate simple N-pro-
tected amino acid esters and protected dipeptides at 0 °C resulting
in the formation of reactive di- and trianions which undergo regio-
selective reaction with BnBr to give the C-alkylated products in
good yield.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.03.105.
References and notes
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Acknowledgement
We would like to thank the EPSRC for the support of a DTA to I.
B.