An improved protocol for the SmI2-promoted C-alkylation of peptides: Degradation and functionalization of serine residues in linear and cyclic peptides

Article abstract of DOI:10.1016/j.tetlet.2004.10.024

The utility of the samarium diiodide promoted C-alkylation of peptides for the introduction of new side chains on peptide strands is dramatically enhanced by the initial oxidative degradation of serine residues in small peptides. In this way, cyclic pepti

Full text of DOI:10.1016/j.tetlet.2004.10.024

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 9091–9094
An improved protocol for the SmI2-promoted C-alkylation of
peptides: degradation and functionalization of serine residues
in linear and cyclic peptides
Peter Blakskjær, Adina Gavrila, Lisbeth Andersen and Troels Skrydstrup*
Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark
Received 4 September 2004; revised 30 September 2004; accepted 5 October 2004
Abstract—The utility of the samarium diiodide promoted C-alkylation of peptides for the introduction of new side chains on peptide
strands is dramatically enhanced by the initial oxidative degradation of serine residues in small peptides. In this way, cyclic peptides
may also be included in this repertoire as a method for the preparation of peptide libraries.
Ó 2004 Elsevier Ltd. All rights reserved.
In 2000, we reported a novel approach for the selective
incorporation of non-natural side chains directly onto
glycine residues of a peptide backbone as a potential
route to peptide libraries.1 As the incorporation of the
side chain only requires a two-step procedure, this
method allows for the preparation of a series of analogs
froma single and intact peptide, without having to
repeat the peptide synthesis for each modification. Our
approach relies on the reductive samariation of a-pyrid-
ylsulfenyl glycyl units generating selectively a glycine
enolate,2 which is subsequently captured by the presence
of a carbonyl electrophile. As the existing chiral centers
of the peptide exert little effect on the diastereoselective
outcome of the alkylation step, the method becomes
suitable for the preparation of non-natural peptide
libraries.
restricting the utility of this alkylation sequence for
peptide modification.4 In this paper, we provide a viable
solution to this problemturning our attention instead
towards the oxidative degradation of serine residues as
a means of initial functionalization of the peptide back-
bone. The method considerably enhances the utility of
the samarium diiodide promoted C-alkylation protocol
of small peptides and is shown to be adaptable to the
functionalization of a cyclic tetrapeptide.
Steglich and co-workers have previously published a
remarkably simple and efficient procedure for the con-
version of serine- or threonine-containing peptides into
their a-acetyloxy glycine counterparts by treatment with
a stoichiometric amount of lead tetraacetate in refluxing
ethyl acetate.5 Taking into consideration the known
ability of these glycyl acetates to participate in nucleo-
philic displacement reactions, makes this oxidative
degradation of serine units a promising route to incor-
porate the 2-pyridyl sulfide group into a peptide.
The method does, however, suffer from one serious
drawback, which is related to the preparation of the a-
pyridylsulfenyl glycyl moieties. The sulfenyl group is
introduced onto the peptide via the selective bromina-
tion of glycine with N-bromosuccinimide3 followed by
nucleophilic displacement of the halide with the pyrid-
ylthiolate. Whereas this reaction works well with
dipeptides, the yields of the bromination step are
reduced significantly with larger peptides,1b thereby
In order to evaluate this procedure, the Boc-protected
dipeptide 1 was treated with 1.5molequiv of Pb(OAc)4
in refluxing dry ethyl acetate (Scheme 1). After complete
conversion to the acetate after just 30min, simple filtra-
tion and extraction of the lead salts with aqueous citric
acid afforded the desired a-acetyloxy derivative 2 in
essentially quantitative yield. This crude acetate was
smoothly converted into the 2-pyridyl sulfide derivative
3 as a 1:1 diastereomeric mixture by treatment with 2-
mercaptopyridine and diisopropylethylamine (DIPEA)
in dichloromethane at À78°C to 20°C in a rewarding
Keywords: Peptides; Serine; Oxidative degradation; Samarium diio-
dide; C-Alkylation.
*
Corresponding author. Tel.: +45 8942 3932; fax: +45 8619 6199;
e-mail: ts@chem.au.dk
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.10.024

9092
P. Blakskjær et al. / Tetrahedron Letters 45 (2004) 9091–9094
Table 1. (continued)
OH
OMe
O
O
OAc
O
Pb(OAc)4
Entry
Yield of
pyridyl
sulfide (%)
C-Alkylation product
(yield and diast. ratio)
BocHN
Ph
BocHN
Ph
OMe
N
H
N
H
EtOAc, reflux
O
1
2
H
89%
2-PyrSH
DIPEA
O
O
(2 steps)
6
83
BocHN
N
OMe
(70%, 1:1)
O
H
H
O
O
(2-Pyr)
S
BnO
O
O
(3 equiv.)
BocHN
Ph
OMe
BocHN
OMe
N
H
N
H
H
SmI2/NiI2 (1%)
THF, -78 oC to 20 o
O
O
O
O
Ph
O
C
H
4
3
7
82
BocHN
N
N
OMe
75% (d.s. = 4.8:1)
H
O
BnO
Scheme 1.
(90%, 1:1)
Me
OBn
Table 1. Oxidative degradation and reductive coupling of serine
residuesa
O
O
H
N
BocHN
N
OMe
8
74
1) Pb(OAc)4, then
OH
H
R1
R2
O
O
OH
OH
(2-Pyr)SH, DIPEA
O
O
Ph
(82%, 1:1)
H
H
N
N
O
2)
R1
N
N
H
SmI2/NiI2(cat.)
THF, -78 oC to 20 oC
H
R2
O
OBn
O
O
H
N
Entry
Yield of
pyridyl
sulfide (%)
C-Alkylation product
(yield and diast. ratio)
BocHN
BnO
N
OMe
9
73
H
O
OH
H
(49%, 1:1)
O
O
a Isolated yields after column chromatography.
1
78
56
89
CbzHN
N
OMe
(75%, 1:1)
H
O
89% yield. It should be noted again, that this peptide
could not have been converted to the 2-pyridyl sulfide
derivative by means of bromination with NBS because
of the labile Boc-protecting group.1b SmI2-promoted
alkylation of 3 with cyclohexanone under NiI2 catalysis
provided the coupling product 4 in a good yield of
75%.6,7
Ph
O
BzHN
OMe
(76%, 1.6:1)
2
N
H
O
OH
With these encouraging results in hand, a small series of
seryl peptides were successfully subjected to the same
conditions. The results listed in Table 1 reveal the viabi-
lity of this method for the introduction of the 2-pyridyl
sulfide and that the reductive coupling with cyclohexa-
none catalyzed by nickel(II) performs well. The three
tripeptides in entries 7–9 contain either an O-benzyl pro-
tected serine and threonine residue, thereby demonstrat-
ing that selective modification at a specific serine residue
is permitted. This is of course an advantage compared to
the bromination approach where it is not possible to
selectively distinguish between two glycine residues in
the same peptide.
Ph
OMe
O
CbzHN
CbzHN
N
3
H
O
(68%, 1:1)
OH
O
OMe
(71%, 1.8:1)
N
4
5
62
70
H
O
OH
A few other ketones were also tested as illustrated in
Table 2, where modest to good yields were obtained
for the coupling step. In entry 3, all four diastereomers
of the C-glycopeptide prepared were identified by
NMR, providing a small library of non-natural C-glyco-
syl tripeptides.
O
BocHN
OMe
(58%, 1:1)
N
H
O
OH

P. Blakskjær et al. / Tetrahedron Letters 45 (2004) 9091–9094
9093
O
Ph
Ph
NH
n
1) CF3CO2H
O
O
OR
N
BocHN
N
O
O
N
N
2) BOP, DMAP
n
H
H
NH
O
CO2R
DMF with 7
THF with 8
OBn
HN
Ph
5 (R = Me, n = 0)
6 (R = Me, n = 1)
LiOH/H2O
MeOH
(quant. yield)
O
Ph
7 (R = H, n = 0)
8 (R = H, n = 1)
9 (R = Bn, n = 0) traces
H2, Pd/C
MeOH/HCl (cat.)
(quant. yield)
10 (R = Bn, n = 1) 85% yield
(2 steps)
11 (R = H, n = 1)
Pb(OAc)4
61% yield
EtOAc
O
O
cyclohexanone
(5 equiv.)
NH
X
NH
N
N
OH
O
O
O
O
SmI2/NiI2 (cat.)
NH
NH
THF, -78 oC to 20 oC
HN
HN
Ph
Ph
55% yield
O
O
Ph
Ph
14
(2:1 diast. mixture)
(2-Pyr)SH
DIPEA, CH2Cl2
80% yield
12 (X = OAc)
13 (X = S(2-Pyr)
Scheme 2.
Finally, the high interest in small cyclic peptides as thera-
peutic agents against a variety of diseases,8 prompted us
to test the C-alkylation approach on a cyclic tetrapep-
tide, related to the histone deacetylase inhibitor, trap-
oxin A.9 Cyclic tetrapeptides are generally difficult to
prepare due to the high ring strain resulting fromthe
transoid nature of the amide bonds.8d Hence, the ability
to prepare a single cyclic peptide and thereafter intro-
duce side chains at selected positions would represent
a more direct method for the preparation of a large body
of peptide analogs.
11 followed by oxidation with Pb(OAc)4 provided the
acetate 12 in a good yield (61%, two steps). Transforma-
tion to the pyridyl sulfide 13 then gave a 1:1 epimeric
Table 2. Reductive coupling with other ketonesa
Entry Coupling yield
(%) (diast. ratio)
Product
Ph
O
O
O
CbzHN
HO
OMe
N
H
1
67 (1:1)
To examine the feasibility of this approach, the linear
tetrapeptide 5 possessing a protected serine residue
was prepared using standard peptide coupling techni-
ques (Scheme 2). However, after hydrolysis of the termi-
nal ester and Boc removal, the use of standard
cyclization conditions in DMF as earlier reported for
the ring closure of similar tetrapeptides failed to provide
the requisite cyclic peptide 9.9 Recently, Fairlie and co-
workers have demonstrated how the replacement of one
of the phenylalanine moieties in a similar tetrapeptide
with its corresponding b-amino acid substantially in-
creases the cyclization yield due to increased flexibility
of the 13-membered cyclic peptide.10
CbzHN
OMe
N
H
OH
2
35 (1.2:1)
O
BnO
BnO
OBn
O
BnO
OH
O
O
3
78 (4 diast.)
H
BocHN
BnO
N
N
H
OMe
Hence, we synthesized the more flexible homolog 6 con-
taining a b-L-Phe unit. Subsequent cyclization in THF
provided the cyclic tetrapeptide 10 in a satisfactory yield
of 85% (two steps). Liberation of the hydroxyl group to
O
a Isolated yields after column chromatography.

9094
P. Blakskjær et al. / Tetrahedron Letters 45 (2004) 9091–9094
1997, 53, 7267–7274; (d) Steglich, W.; Ja¨ger, M.; Jaroch,
S.; Zistler, P. Pure Appl. Chem. 1994, 66, 2167–2170; (e)
Ja¨ger, M.; Polborn, K.; Steglich, W. Tetrahedron Lett.
1995, 36, 861–864; (f) Trojandt, G.; Polborn, K.; Steglich,
W.; Schmidt, M.; No¨th, H. Tetrahedron Lett. 1995, 36,
857–860; (g) Apitz, G.; Ja¨ger, M.; Jaroch, S.; Kratzel, M.;
Scha¨ffeler, L.; Steglich, W. Tetrahedron 1993, 49, 8223–
8232; (h) Apitz, G.; Steglich, W. Tetrahedron Lett. 1991,
32, 3163–3166.
mixture in an 80% yield. Finally, coupling of this com-
pound to cyclohexanone with SmI2 led to the C-alkyl-
ated cyclic tetrapeptide 14 as a 2:1 mixture of
diastereomers in a good 55% yield.11 This result clearly
illustrates the potential of this concept for the introduc-
tion of side chains onto cyclic peptides.
In conclusion, we have developed an alternative ap-
proach for the functionalization of serine residues in
small peptides which has been extended to a cyclic pep-
tide. Further work is in progress to test other cyclic pep-
tides and for the introduction of more relevant side
chains.
6. For some recent reviews on SmI2, see (a) Kagan, H.
Tetrahedron 2003, 59, 10351–10372; (b) Gansa¨uer, A.;
Bluhm, H. Chem. Rev. 2000, 100, 2771–2788; (c) Steel, P.
G. J. Chem. Soc., Perkin. Trans. 1 2001, 2727–2751; (d)
Molander, G. A.; Harris, C. R. Tetrahedron 1998, 54,
3321–3354.
7. The addition of 1% NiI2 to the reaction was found to
increase the yields of the couplings by 20–30%. In the
reaction of cyclohexanone with a dipeptide similar in
structure to 3 a 45% yield of the coupling product was
obtained in the absence of NiI2 (Ref. 1b).
8. (a) Ried, R. C.; Abbenate, G.; Taylor, S. M.; Fairlie, D. P.
J. Org. Chem. 2003, 68, 4464–4471; (b) Tamilarasu, N.;
Huq, I.; Rana, T. M. Bioorg. Med. Chem. Lett. 2000, 10,
971–974; (c) Dechantsreiter, M. A.; Planker, E.; Matha¨,
B.; Lohof, E.; Ho¨lzemann, G.; Jonczyk, A.; Goodman, S.
L.; Kessler, H. J. Med. Chem. 1999, 42, 3033–3040; (d)
Meutermans, W. D. F.; Bourne, G. T.; Golding, S. W.;
Horton, D. A.; Campitelli, M. R.; Craik, D.; Scanlon, M.;
Smythe, M. L. Org. Lett. 2003, 5, 2711–2714, and
references therein.
Acknowledgements
Generous financial support fromthe Danish Natural
Science Research Council, The HolmFoundation, the
Marie Curie Training Site programand the University
of Aarhus are gratefully acknowledged.
References and notes
1. (a) Ricci, M.; Madariaga, L.; Skrydstrup, T. Angew.
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Modern Synthetic Methods; Ernst, B., Leumann, C., Eds.;
VCH: Weinheim, 1995; Vol. 7, pp 1–178; (b) Matt, T.;
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K.; Easton, C. J.; Radom, L. J. Am. Chem. Soc. 2003, 125,
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4. In addition, N-terminal protecting groups including the
Boc, Cbz and Ts groups proved to be incompatible with
the bromination step (Ref. 1b).
10. Glenn, M. P.; Kelso, M. J.; Tyndall, J. D. A.; Fairlie, D. P.
J. Am. Chem. Soc. 2003, 125, 640–641.
11. Data for compound 14(major diastereomer ). 1H NMR
(CDCl3, 400MHz) (major diastereomer) d (ppm) 8.01 (d,
1H, J = 10.4Hz), 7.30–7.12 (m, 10H), 6.40 (br s, 1H), 6.16
(br s, 1H), 4.97 (dt, 1H, J = 5.6, 10.0Hz), 4.67 (d, 1H,
J = 6.8Hz), 4.19 (d, 1H, J = 9.6Hz), 4.11 (br s, 1H), 3.95
(br s, 1H), 3.89 (dt, 1H, J = 3.6, 9.6Hz), 3.18 (t, 1H,
J = 9.6Hz), 3.15 (t, 1H, J = 10.8Hz), 2.99 (t, 1H,
J = 12.8Hz), 2.89 (dd, 1H, J=5.2, 12.8Hz), 2.80 (dd, 1H,
J = 6.0, 13.6Hz), 2.78 (quint, 1H, J = 9.2Hz), 2.41 (dd,
1H, J = 4.4, 14.4Hz), 2.36–2.32 (m, 1H), 2.06 (sextet, 1H,
J = 9.6Hz), 1.80–1.25 (m, 11H) (the OH signal is missing).
13C NMR (CDCl3, 125MHz) d (ppm) 172.6, 172.4, 171.2,
170.2, 138.2, 136.4, 129.4 (2C), 129.3 (2C), 128.9 (2C),
128.8 (2C), 127.3, 127.0, 72.5, 58.2, 57.3, 53.1, 52.8, 47.1,
40.1, 39.7, 36.8, 35.8, 34.3, 25.7, 25.5, 24.8, 22.3, 21.8. ES-
HRMS C32H40N4O5 [M + Na+]; calculated: 583.2896
found: 583.2893.
5. (a) Schuemann, S.; Zeitler, K.; Ja¨ger, M.; Polborn, K.;
Steglich, W. Tetrahedron 2000, 56, 4187–4195; (b) Paulitz,
C.; Steglich, W. J. Org. Chem. 1997, 62, 8474–
8478; (c) Bogensta¨tter, M.; Steglich, W. Tetrahedron