Conjugation of selenols with aziridine-2-carboxylic acid-containing peptides

Article abstract of DOI:10.1055/s-2005-871972

The addition of PhSeH to aziridine-2-carboxylic acid containing peptides is described, thus expanding the scope of nucleophiles for the opening of this class of electrophilic peptide substrates. The process offers a new strategy for the generation of usef

Full text of DOI:10.1055/s-2005-871972

LETTER
2011
Conjugation of Selenols with Aziridine-2-Carboxylic Acid-Containing
Peptides
Selenolysis of
Azi
a
r
idine Pepti
t
des han D. Ide, Danica P. Galonić, Wilfred A. van der Donk, David Y. Gin*
Department of Chemistry, University of Illinois, Urbana, IL 61801, USA
Fax +1(217)2448024; E-mail: gin@scs.uiuc.edu
Received 19 May 2005
HAlaOBn promoted by BOP and DIPEA provided the
corresponding TrtSerAlaOBn dipeptide, whose side chain
hydroxyl was then converted to its methanesulfonate es-
ter. Aziridine ring-closure was then effected with Et3N to
Abstract: The addition of PhSeH to aziridine-2-carboxylic acid
containing peptides is described, thus expanding the scope of nu-
cleophiles for the opening of this class of electrophilic peptide sub-
strates. The process offers a new strategy for the generation of
useful phenylselenocysteine derivatives and a-seleno-b-amino incorporate the ‘Azy’ amino acid residue. The N-trityl
acid-containing peptides.
protecting group was then removed using TFA to provide
the corresponding free aziridine HAzyAlaOBn, which
was acylated with BocValOH via a BOP coupling to
form the tripeptide 5.1h,k In like manner, the tripeptide
BocValAzyAlaNHBn (6), incorporating an amide rather
than an ester group at the C-terminus, was prepared from
TrtSerOH (4) and HAlaNHBn as the initial coupling sub-
strates.
Key words: aziridine, peptide, ligation, selenol, solid-phase
The site-selective modification of peptides incorporating
non-natural electrophilic amino acid residues has received
considerable attention due to the potential for attachment
of carbohydrates, lipids, affinity tags, and other biologi-
cally relevant molecules to preformed peptides.1 Among
the most common electrophilic functionalities that have
been synthetically incorporated into peptides are ke-
tones,1a,g dehydroalanines,1c,f cyclic sulfamidates,1d alkyl
halides,1j and aziridines.1h,k
"Azy"
O
O
Me
BocHN
Me
OBn
N
N
a–e
f–j
H
OH
OH
O
Me
5
In the course of our studies on the synthesis and conjuga-
tion of aziridine-2-carboxylic acid derivatives with thiol
nucleophiles,1h,k we have found that selenols (RSeH,
Scheme 1) also function as excellent nucleophiles in this
capacity. The nucleophilic ring-opening of aziridine-2-
carboxylic acid derivatives 1 with selenols can proceed by
addition at either the a- or b-carbon of the aziridine 1,
thereby allowing for site-selective backbone or side-chain
modification of peptides to give the b- or a-amino acid
products 2 and 3, respectively.
TrtHN
O
O
Me
O
BocHN
Me
NHBn
4
N
N
H
O
Me
6
Scheme 2 Reagents and conditions: (a) HAlaOBn·TsOH, BOP, i-
Pr2NEt, CHCl3, 23 °C, 92%; (b) MsCl, Et3N, CH2Cl2, 0 °C; (c) Et3N,
THF, 70 °C, 91% over two steps; (d) TFA, MeOH, CHCl3, 0 °C,
85%; (e) BocValOH, BOP, i-Pr2NEt, CHCl3, 23 °C, 81%; (f) HAla-
NHBn·TFA, BOP, i-Pr2NEt, CHCl3, 23 °C, 51%; (g) MsCl, Et3N,
THF, 0 °C; (h) Et3N, THF, 70 °C, 80% over two steps; (i) TFA,
MeOH, CHCl3, 0 °C, 82%; (j) BocValOH, BOP, i-Pr2NEt, CHCl3,
23 °C, 91%.
O
O
SeR
H
N
RSeH
R'3N
H
α
β
α
β
N
+
N
α
A series of threonine-derived aziridine-peptides were
also accessed in order to study the addition of PhSeH
into these 3-methyl-substituted aziridine-2-carboxylates
(Scheme 3). A simple dipeptide 8 was first accessed via
initial acylation of benzylamine with TrtThrOH (7) using
EDC, HOBt, and Et3N. Aziridine formation was again ac-
complished through threonine hydroxyl mesylation fol-
lowed by Et3N-mediated ring-closure to incorporate the
‘Azy(3-Me)’ residue. Subsequent removal of the trityl
protective group (TFA) and N-acylation with BocAlaOH
under BOP coupling conditions provided the dipeptide 8.
The synthesis of higher homologs of 3-MeAzy-containing
peptides required the synthesis of TrtAzy(3-Me)OH (9).
This proceeded via carboxylate O-alkylation of TrtThrOH
(7) with benzyl bromide, followed by sequential aziridine
ring-closure (via its mesylate precursor) and benzyl ester
Me/H
O
β
RSe
Me/H
Me/H
1
2
3
Scheme 1 Addition of selenols to aziridine-2-carboxylic acid
containing peptides.
Initial efforts focused on the synthesis of several aziri-
dine-carboxylic acid containing peptides to serve as suit-
able electrophiles for selenol ligation. The synthetic
approach used to access the aziridine-2-carboxylates is
based on the earlier synthetic efforts of Moroder2 and
Okawa.3 Coupling of TrtSerOH (4, Scheme 2) with
SYNLETT 2005, No. 13, pp 2011–2014
1
9
.0
8
.2
0
0
5
Advanced online publication: 20.07.2005
DOI: 10.1055/s-2005-871972; Art ID: S04705ST
© Georg Thieme Verlag Stuttgart · New York

2012
N. D. Ide et al.
LETTER
hydrogenolysis (Pd/BaSO4, H2) to provide TrtAzy(3- ment of both the Azy-containing peptides 5 and 6 (entries
Me)OH (9). This amino acid residue was then used to 1 and 2) with PhSeH provided the corresponding PhSe
prepare a number of peptides (10–13). For example, BOP- adducts in 75% and 79% yields, respectively, with regio-
mediated acylation of HAlaNHMe, followed by trityl selectivities favoring the b-PhSe adduct compared to its a-
removal (TFA) and aziridine N-acylation with either PhSe counterpart (i.e., regioselectivity: 0:1 and 1:2 from 5
BocAlaOH or BocLys(Boc)OH with BOP provided the and 6, respectively). From these initial simple examples,
peptides
BocAlaAzy(3-Me)AlaNHMe
(10)
and it is clear that subtle peptide structural modifications, such
BocLys(Boc)Azy(3-Me)AlaNHMe (11), respectively. as the presence of a C-terminal ester versus secondary
Similarly, TrtAzy(3-Me)OH (9) was also used to acylate amide group at the amino acid immediately preceding the
both HPheNHBn4 and HGlyPheNHBn via standard BOP Azy residue, lead to significant changes in the regioselec-
coupling conditions. Each of the resulting peptides was tivity of aziridine opening. Although these results are con-
deprotected at the aziridine N-terminus (TFA) and then sistent with precedent favoring b-nucleophilic addition to
acylated with BocAlaOH in a BOP promoted coupling to Azy-containing peptides using other nucleophiles,1h,k,3b,5
provide the tripeptide BocAlaAzy(3-Me)PheNHBn (12) they do highlight the importance of this particular amide
and the tetrapeptide BocAlaAzy(3-Me)GlyPheNHBn functionality in attenuating the degree of b-Se regioselec-
(13), respectively.
tivity in Azy-selenolysis.6
Investigation of the reactivity of these peptides involved In subsequent studies, selenolysis of Azy(3-Me)-contain-
aziridine selenolysis (Table 1) with PhSeH (2 equiv) at ing peptides was investigated. When the Azy(3-Me)-pep-
23 °C in DMF in the presence of Et3N (1 equiv). Treat- tide 8 was treated with PhSeH (entry 3), a mixture of
adducts was obtained in good yield, favoring the product
'Azy(3-Me)'
derived from the addition of PhSeH to the b-carbon of the
aziridine (a-Se:b-Se, 1:4). However, when the Azy(3-
O
O
O
Me)-containing tripeptide 11 [incorporating a C-terminal
secondary amide group at the amino acid immediately
preceding the Azy(3-Me)-residue] was treated with Ph-
SeH under otherwise identical conditions (entry 4), a re-
versal of regioselectivity favoring the a-PhSe-adduct was
observed (regioselectivity, 3:1). Even higher regioselec-
tivities were observed with the tripeptides 10 and 12 as
well as the tetrapeptide 13 (entries 5–7), providing virtu-
ally exclusive selectivity for the a-PhSe adduct with com-
parable high yields. Indeed, this is a notable contrast to the
BocHN
a–e
TrtHN
HO
N
NHBn
OH
Me
N
8
Me
Me
O
7
O
Me
BocHN
NHMe
N
H
10
Me
O
f-i
Me
O
O
Me
j–l
BocHN
NHMe
N
N
H
Table 1 Addition of PhSeH to Aziridine-2-carboxylic Acid
Containing Peptides
O
BocHN(CH2)4
O
11
12
j, k, m
n–p
Me
TrtN
OH
O
SePh
O
H
N
PhSeH,
Et3N
H
β
α
β
Ph
O
α
β
N
+
Me
O
O
α
N
BocHN
NHBn
Me/H
O
PhSe
Me/H
DMF
9
N
N
Me/H
q–s
H
α-PhSe-adduct
β-PhSe-adduct
Me
Me
O
Entry Aziridine
PhSe adduct PhSe adduct
yield (%)
(a-Se):(b-Se)
O
O
H
1
2
3
4
BocValAzyAlaOBn (5)
BocValAzyAlaNHBn (6)
75
79
0:1a
1:2
1:4
3:1
BocHN
N
N
N
NHBn
H
Me
O
Ph
13
Me
BocAlaAzy(3-Me)NHBn (8) 74
Scheme 3 Reagents and conditions: (a) H2NBn, EDC, HOBt,
CH2Cl2, 0 °C, then 23 °C, 91%; (b) MsCl, Et3N, CH2Cl2, 0 °C; (c)
Et3N, THF, 100 °C, 79% over two steps; (d) TFA, MeOH, CHCl3,
0 °C, 99%; (e) BocAlaOH, BOP, i-Pr2NEt, CHCl3, 23 °C, 84%; (f)
BnBr, THF, 23 °C, 80%; (g) MsCl, Et3N, CH2Cl2, 0 °C; (h) Et3N,
DME, 85 °C, 98% over two steps; (i) Pd/BaSO4 (5% w/w), H2, THF,
23 °C, 99%; (j) HAlaNHMe·HCl, BOP, i-Pr2NEt, CHCl3, 23 °C,
77%; (k) TFA, MeOH, CHCl3, 0 °C, 89%; (l) BocAlaOH, BOP, i-
Pr2NEt, CHCl3, 23 °C, 99%; (m) BocLys(Boc)OH, BOP, i-Pr2NEt,
CHCl3, 23 °C, 35%; (n) HPheNHBn, BOP, i-Pr2NEt, CHCl3, 23 °C,
72%; (o) TFA, MeOH, CHCl3, 0 °C, 73%; (p) BocAlaOH, BOP, i-
Pr2NEt, CHCl3, 23 °C, 63%; (q) HGlyPheNHBn, BOP, i-Pr2NEt,
CHCl3, 23 °C, 81%; (r) TFA, MeOH, CHCl3, 0 °C, 98%; (s) BocAla-
OH, BOP, i-Pr2NEt, CHCl3, 23 °C, 82%.
BocLys(Boc)Azy(3-
Me)AlaNHMe (11)
62
5
6
7
BocAlaAzy(3-Me)AlaNHMe 88
(10)
1:0a
1:0a
1:0a
BocAlaAzy(3-Me)PheNHBn 93
(12)
BocAlaAzy(3-Me)GlyPheN- 90
HBn (13)
a The minor isomers were not detectable by 1H NMR.
Synlett 2005, No. 13, 2011–2014 © Thieme Stuttgart · New York

LETTER
Selenolysis of Aziridine Peptides
2013
a–d
established precedent for the synthesis of a-amino acid
derivatives via regioselective ring-opening of Azy(3-Me)-
peptides at the b-position with other nucleophiles.5a,b,7
AcGlyGlyPheAzyAla
HAla
16
17
e, f
In general, Azy(3-Me)-peptides favor a-PhSe adduct for-
mation (entries 4–7), but the selectivity of these reactions
can be strongly influenced or even reversed by changes in
the peptide backbone or amino acid side-chains (entries 3
vs. 4). Although the more subtle regioselectivity effects
derived from differing periphery peptide structures (i.e.,
entries 1 vs. 2; 3 vs. 4) warrant further detailed study, this
general reactivity profile nonetheless provides efficient
access to a-seleno-b-amino acid containing peptides (en-
tries 4–7), which have been shown to be valuable sub-
strates for stereoselective radical reactions.8
SePh
O
O
O
H
H
AcHN
N
N
N
N
OH
H
H
O
O
Me
Ph
18 (87%, 9 steps)
a, g–i, d
AcAlaPheGlyValAzyLeu
HLeu
19
20
e, j
SePh
O
Me
O
O
H
H
H
While extensive NMR analysis clearly revealed the con-
stitutional make-up of each of the PhSe adducts, the ste-
reochemical consequences of the aziridine selenolysis
reactions remained elusive with this data alone. However,
unambiguous structure determination was achieved
through single crystal X-ray analysis (Figure 1) of the b-
PhSe adduct 14 derived from 8 (Table 1, entry 3) and the
a-PhSe adduct 15 derived from 10 (Table 1, entry 5). The
a-R,b-R and a-R,b-S configurations of 14 and 15, respec-
tively, reveal that nucleophilic substitution proceeds with
inversion, without affecting the integrity of the adjacent
C-stereocenter.
N
N
N
AcHN
N
H
N
OH
Me
H
O
O
O
Ph
Me
Me
Me
21 (68%, 11 steps)
= 2-chlorotrityl-linked polystyrene
Scheme 4 Reagents and conditions: (a) FmocAzyOH, HBTU,
NMM, DMF, 23 °C; DBU, DMF, 23 °C; (b) FmocPheOH, HBTU,
NMM, DMF, 23 °C; DBU, DMF, 23 °C; (c) FmocGlyGlyOH,
HBTU, NMM, DMF, 23 °C; DBU, DMF, 23 °C; (d) Ac2O, pyridine,
23 °C; (e) PhSeH, TEA, DMF, 23 °C; (f) (CF3)2CHOH–CH2Cl2 (20%
v/v), 23 °C, 87%; (g) FmocValOH, HBTU, NMM, DMF, 23 °C;
DBU, DMF, 23 °C; (h) FmocPheGlyOH, HBTU, NMM, DMF,
23 °C; DBU, DMF, 23 °C; (i) FmocAlaOH, HBTU, NMM, DMF,
23 °C; DBU, DMF, 23 °C; (j) (CF3)2CHOH:CH2Cl2 (20% v/v),
23 °C, 68%.
Finally, expansion of the versatility of the generation of
seleno-modified peptides from Azy-containing peptide
precursors is also possible through solid-phase peptide
synthesis protocols. We have previously shown that Azy-
containing peptides can be conveniently prepared on 2- step) and 21 (68%, 11 steps, 97% per step).9 In analogy to
chlorotrityl-linked polystyrene resin and efficiently ligat- the solution-phase selenolysis of Azy-containing tripep-
ed with thiol nucleophiles on solid support.1h,k Similar tide 5 (Table 1, entry 1), these polymer-supported pep-
highly efficient on-bead selenolysis of Azy-peptides is tides exhibit high selectivity for the formation of
also possible (Scheme 4), exemplified by the solid-phase phenylselenocysteine-containing peptides, useful inter-
mediates in the synthesis of dehydroalanines.10,11
peptide synthesis and aziridine opening by PhSeH to af-
ford the Se-derived peptides 18 (87%, 9 steps, 98% per
Figure 1 X-ray data of PhSe adducts 14 and 15.
Synlett 2005, No. 13, 2011–2014 © Thieme Stuttgart · New York

2014
N. D. Ide et al.
LETTER
This report describes the addition of PhSeH to aziridine-
2-carboxylic acid containing peptides, thus expanding the
scope of nucleophiles used for the opening of these elec-
trophilic peptide substrates. The process is capable of pro-
ducing useful phenylselenocysteine derivatives and a-
seleno-b-amino acid-containing peptides. Further studies
aimed at expanding the scope and probing the factors that
influence the regioselectivity of aziridine opening are cur-
rently underway.
References
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Typical PhSeH Conjugation Procedure12
BocAla-b2,3-HAla(a-SePh)GlyPheNHBn
Triethylamine (0.3 mL, 0.002 mmol, 0.1 equiv) was added to a solu-
tion of BocAla-3-MeAzyGlyPheNHBn (13, 10.0 mg, 0.0176 mmol,
1.0 equiv) and PhSeH (3.8 mL, 0.0356 mmol, 2.0 equiv) in DMF
(590 mL) at 23 °C. The resulting yellow solution was stirred for 4 h
at this temperature and then concentrated in vacuo. The resulting
yellow residue was washed with hexanes (3 ꢀ 1.0 mL) and dried in
vacuo to provide BocAla-b2,3-HAla(a-SePh)GlyPheNHBn (11.6
(2) Korn, A.; Rudolph-Böhner, S.; Moroder, L. Tetrahedron
1994, 50, 1717.
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(6) A similar regioselectivity effect was also observed in the
thiolysis of Azy-containing peptides. See ref. 1k.
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1
mg, 90%) as a white solid; mp 235–237 °C. H NMR (500 MHz,
DMF-d7): d = 8.63 (t, J = 5.9 Hz, 1 H), 8.57 (t, J = 5.7 Hz, 1 H),
8.34 (d, J = 8.2 Hz, 1 H), 8.20 (m, 1 H), 7.80 (m, 2 H), 7.50–7.38
(m, 13 H), 6.82 (d, J = 7.6 Hz, 1 H), 4.86 (m, 1 H), 4.53 (m, 3 H),
4.28 (m, 2 H), 4.12 (dd, J = 16.6, 6.1 Hz, 1 H), 3.80 (dd, J = 16.4,
5.3 Hz, 1 H), 3.36 (dd, J = 13.8, 5.5 Hz, 1 H), 3.12 (m, 1 H), 1.56
(s, 9 H), 1.45 (d, J = 7.1 Hz, 3 H), 1.41 (d, J = 6.6 Hz, 3 H) ppm. 13
C
NMR (125 MHz, DMF-d7): d = 172.8, 171.6, 171.5, 169.6, 156.0,
140.1, 138.7, 134.6, 130.0, 129.9, 129.6, 128.9, 128.8, 128.2, 128.0,
127.4, 127.0, 78.7, 55.4, 52.1, 50.8, 47.1, 43.5, 43.1, 38.6, 28.4,
19.0, 18.9 ppm. FT-IR (neat film): 3289, 1654, 1524, 1374, 1245,
1165, 1025, 871 cm–1. HRMS (ESI)+: m/z calcd for C36H46N5O6Se
[M + H]+: 724.2613; found: 724.2609.
Note: Products of PhSeH selenolysis of compounds 10, 12, and 13
were purified in this fashion (hexane wash). Products of PhSeH se-
lenolysis of compounds 5, 6, 8, and 11 were purified by column
chromatography.
Acknowledgment
(9) The seleno-peptide conjugates 18 and 21 were directly
isolated following cleavage from the resin, and were
assessed to be >90% pure by 1H NMR.
This research was supported by the NIH (GM58833). Procter and
Gamble fellowships to D.P.G. and N.D.I. are acknowledged.
(10) (a) Okeley, N. M.; Zhu, Y.; van der Donk, W. A. Org. Lett.
2000, 2, 3603. (b) Gieselman, M. D.; Zhu, Y.; Zhou, H.;
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(11) Although solid-phase synthesis of Azy(3-Me)-containing
peptides is a highly efficient process, attempts at on-bead
selenolysis were not efficient due to competitive
unproductive processes such as, inter alia, aziridine N-
deacylation.
(12) For detailed representative procedures on the solution-phase
and solid-phase synthesis of aziridine-containing peptides,
see ref. 1k.
Synlett 2005, No. 13, 2011–2014 © Thieme Stuttgart · New York