Prolonged stability by cyclization: Macrocyclic phosphino dipeptide isostere inhibitors of β-secretase (BACE1)

Article abstract of DOI:10.1016/j.bmcl.2009.05.053

Cyclization of recently reported linear phosphino dipeptide isostere inhibitors of BACE1 via side chain olefin metathesis yielded macrocyclic BACE1 inhibitors. The most potent compound II-P1 (IC₅₀ of 47 nM) and the corresponding linear analog I were tested for serum stability. The approach led to three times prolonged half life serum stability of 44 min for the macrocyclic inhibitor II-P1 compared to the linear compound I.

Full text of DOI:10.1016/j.bmcl.2009.05.053

Bioorganic & Medicinal Chemistry Letters 19 (2009) 4427–4431
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Prolonged stability by cyclization: Macrocyclic phosphino dipeptide isostere
inhibitors of b-secretase (BACE1)
Timo Huber a, , Florian Manzenrieder a, , Christian A. Kuttruff a, Cornelia Dorner-Ciossek b, Horst Kessler a,
*
a Center of Integrated Protein Science at the Department Chemie and Institute for Advanced Study, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany
b A ZNS Forschung, Boehringer-Ingelheim Pharma GmbH & Co. KG, Birkendorferstr. 65, 88397 Biberach, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Cyclization of recently reported linear phosphino dipeptide isostere inhibitors of BACE1 via side chain
olefin metathesis yielded macrocyclic BACE1 inhibitors. The most potent compound II-P1 (IC50 of
47 nM) and the corresponding linear analog I were tested for serum stability. The approach led to three
times prolonged half life serum stability of 44 min for the macrocyclic inhibitor II-P1 compared to the
linear compound I.
Received 8 April 2009
Revised 13 May 2009
Accepted 14 May 2009
Available online 18 May 2009
Ó 2009 Published by Elsevier Ltd.
Keywords:
BACE1
Alzheimer’s disease
b-Secretase
Macrocyclic
Serum stability
Phosphino dipeptide isostere
Almost 10% of today’s population over the age of 65 and 40%
over the age of 80 suffer from Alzheimer’s disease (AD), a major
neurodegenerative disorder.1 A major factor in the pathogenesis
of AD is the cerebral deposition of amyloid fibrils as senile pla-
ques.2 These plaques consist mainly of an insoluble form of Ab
amyloid, a 40–42 amino acid (AA) long peptide produced by prote-
olytic processing of the b-amyloid precursor protein (APP).3,4 APP
is cleaved by at least three proteases. b-secretase (BACE1) initiates
the pathogenic processing of APP by cleaving at the N-terminus.
The resulting C99 membrane bound C-terminal peptide can then
inhibitors in complex with BACE1 were available, showing the
main features of the enzyme–inhibitor interactions. One of the best
inhibitors of BACE1 is OM00-3 (Fig. 1), which has an IC50 of 6 nM
(Table 1) in our test system.
Our group has recently shown that the phosphino dipeptide
(PDP) isostere is a suitable replacement of the hydroxy ethylene
isostere in BACE1 inhibitors (Fig. 1). This exchange resulted in
pseudo peptidic inhibitors (compound I) of same potency (Table
1) as OM00-3.15,16 Consequently we were interested to develop a
macrocyclic inhibitor containing a PDP isostere, as cyclization is
known to enhance serum stability. In addition those molecules
could be used as reporter molecules for 31P NMR based screening.17
In our search for conformationally restrained PDP isostere BACE1
inhibitors we speculated that the P1 and P3 cyclization would lock
the active conformation (due to their close proximity in Tang’s
crystal structure) of the linear pseudo peptidic inhibitor I (Fig. 1)
in the N-terminal region (Figs. 1 and 2A). Analysis of the crystal
structure of OM00-3 bound to BACE1,14 using the programs Sybyl
be hydrolyzed by
c-secretase to form the 40 AA long peptide
Ab40. Thus, BACE1 is a molecular target for therapeutic interven-
tion in AD.5–10 The peptide Ab42, also resulting from the cleavage
of APP by BACE1, has an even higher propensity to aggregate and
is the principal Ab species found in amyloid plaques. Studies of
BACE1 KO mice demonstrated the viability and the absence of
gross phenotypic changes that are observed, for example, with
presenilin knock-outs. Moreover, these mice showed the complete
absence of Ab in their brains.11 BACE1 is a unique member of the
pepsin family of aspartic proteases and has recently been identified
as the principle b-secretase in neurons.12
and AUTODOCK,
18 revealed that the ideal macrocycle should consist of
a 13-membered cycle. The macrocycle consists of a peptidic back-
bone chain and of a hydrocarbon chain, arising from the side chains
of unnatural amino acids (Fig. 1). During the course of our investi-
gations this linkage has also been described for BACE1 inhibitors
with structures derived from a hydroxyethylene19 as well as an
ethanolamine core.20 Many other side chain cyclizations have also
been reported.20–23 A low-energy conformation of our macrocyclic
inhibitor was able to perfectly emulate the bioactive conformation
In 2000 and 2002, Tang and co-workers reported nanomolar
inhibitors of BACE1.13,14 Subsequently, X-ray structures of these
* Corresponding author. Tel.: +49 89 289 13300; fax: +49 89 289 13210.
E-mail address: kessler@ch.tum.de (H. Kessler).
These author contributed equally.
0960-894X/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2009.05.053

4428
T. Huber et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4427–4431
P3
mely attractive option for peptide cyclization.25–28 We used this
P1
concept for the synthesis of macrocycle II (Fig. 1), starting with
the Fmoc protected tripeptide sequence Val-Glu-Phe on TCP-resin
(Scheme 1). In this approach key fragments such as the PDP isoste-
re 12, Fmoc-Asp(OtertBu)-OH, Fmoc-homoallylglycin-OH (5) and
Fmoc-Glu(OtertBu)-OH should be subsequently coupled to form the
linear precursor for ring closing metathesis (RCM) (13, Scheme 1).
Despite the commercially available Fmoc protected amino acids
the two Fmoc building blocks carrying the alkene moieties for
RCM had to be synthesized in solution. Synthesis of 5 was accom-
plished according to a route described by Rojo et al.19 First, Boc-
serine-OH (1) was transformed into the methyl ester 2 using
methyl iodide. In order to substitute the hydroxy functionality
for the corresponding iodide derivative triphenylphosphine, imida-
zol and iodide were used in line with a procedure published by
Pavé et al.29 However, the elimination side product 6 was predom-
inantly formed instead of desired compound 3 (Scheme 2) contrary
to an alternative procedure using the same reagents in inverse or-
der reported by Trost et al.30 Iodide 3 was transformed into a Kno-
chel cuprate, which was subsequently reacted with allyl chloride to
give allylic derivative 4 (Scheme 2).31 The methyl ester was sapon-
ified by LiOH in a mixture of dioxane/water. After complete con-
version acidification yielded H-homoallylglycine-OH. The
aqueous solution was adjusted to pH 8–9 by addition of NaHCO3
and Fmoc-OSu was added to give the final compound 5 ready for
SPPS. Initial attempts to access the key fragment 12 using the
chemistry reported by Baylis et al.32 and Boyd et al.33 failed. An
alternative approach to synthesize 12 relies on a method worked
out by Matziari et al. (Scheme 3).34,35 The alkyl phosphinic acid 9
was synthesized according to literature procedures.33 In this work
the Matziari protocol was slightly modified as we used the car-
boxymethyl ester 9 to protect the phosphinic acid right away
(Scheme 3). Compound 10 and 1-adamantyl bromide were re-
fluxed in chloroform under successive addition of Ag2O to intro-
duce the adamantly protection group (Scheme 3).36,37
O
O
O
O
H
N
H
N
H2N
OH
OH
N
H
N
H
N
H
N
H
O
O
OH
O
O
O
O
COOH
COOH
P3
COOH
OM00-3
P1
O
O
O
O
H
N
H
N
O
P
OH
H2N
N
H
N
H
N
H
N
H
COOH
COOH
COOH
I
P3
P1
10
11
9
12
8
13
O
P
OH
O
O
O
O
H
N
5
H
N
2
4
6
H2N
OH
3
N
N
N
H
N
H
1
7
H
H
O
O
O
COOH
COOH
II
COOH
Figure 1. OM00-3, linear PDP inhibitor
inhibitor II.
I and the corresponding macrocyclic
Table 1
IC50 values against BACE1
Retention timeb (min)
a
Compound
Inhibition IC50 (nM)
OM-003
I
II-P1
II-P2
II–P3
II–P4
6 ( 0.7)c
—
—
12 ( 2)c
47 ( 10)
320 ( 30)
522 ( 50)
808 ( 70)
15.0
15.5
17.4
17.6
The obtained diastereomeric mixture was parted in three frac-
tions by RP-HPLC in a ratio of 1:1:3 (11a/11b/11c). These fractions
were handled separately in further reaction steps to facilitate the
later purification of the four diastereomers of II. Next, the methyl
ester was removed under standard conditions; saponification using
one equivalent of LiOH under ice cooling did not result in a com-
plete turnover. Hence, three equivalents were added in portions.
Unfortunately, under these conditions also the Fmoc group was
partially removed. Therefore, the free amine was in situ reprotect-
ed to yield 12.
The resin loaded with the tripeptide sequence was split into
three fractions with a ratio of 1:1:3 according to the three fractions
of the PDP isostere 12 and the following synthetic steps were car-
ried out in parallel for each fraction. The three fractions of PDP iso-
stere 12 were attached using HATU/HOAt (1.25 equiv each) and
a
Values are means of two experiments, standard deviation is given in
parentheses.
b
c
Detectable by HPLC–ESI-MS (gradient 15–40%, 30 min).
Data taken from previous measurement with the full-domain of BACE1.15
of OM00-3 (Fig. 2A). Superposition and docking of this low-energy
conformation in BACE1 indicated that the macrocyclic PDP isostere
ring system would fit and bind similar than OM00-3 to the BACE1
binding cleft (Fig. 2B). Therefore, we decided to synthesize side
chain cyclized PDP isostere inhibitors of BACE1.24
An essential tool for different macrocyclizations is the olefin
metathesis. The functional group tolerance and the—during
SPPS—unreactive alkene moieties, as well as the big arsenal of
modern generation catalysts make the use of metathesis an extre-
Figure 2. (A) Overlay of the X-ray conformation of OM00-3 (grey) co-crystallized with BACE1 and the macrocyclic PDP isostere inhibitor II (green). (B) hypothetical binding
mode of inhibitor II in the BACE1 binding site as predicted by AUTODOCK 3.0 (N-terminal region with macrocycle).

T. Huber et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4427–4431
4429
O
O
O
a
H
N
H
N
O
P
O
Fmoc peptide
peptide
Fmoc
N
H
N
H
O
O
Ad
O
O
O
peptide : = VE(tBu)F
13
Scheme 1. SPPS of the linear precursor 13. Reagents and conditions: (a) (i) 20% piperidine/NMP (2 Â 10 min); (ii) 12 and HATU, HOAt (1.25 equiv each), DIPEA (5 equiv), NMP,
12 h; (iii) wash NMP (5 Â 5 min); (iv) 20% piperidine/NMP (2 Â 10 min) (v) amino acid, HOBt and TBTU (2.5 equiv each), DIPEA (5 equiv), NMP, 2.5 h; (vi) wash NMP
(5 Â 5 min)–repeat; (iv)–(vi) until sequence is complete, amino acids used in this order: Fmoc-Asp(OtBu)-OH, Fmoc-homoallylglycine-OH [HATU/HOAt (1.5 equiv) and 12 h
instead] and Fmoc-Glu(OtBu)-OH.
O
O
O
X
H
N
H
N
H
N
c
d, e
Boc
O
Fmoc
OH
Boc
OR
O
O
O
O
H
N
H
N
H
N
O
P
O
OH
Fmoc
N
H
N
H
O
N
H
N
H
O
O
O
4
5
Ad
a
O
O
O
O
O
O
H
N
1
2
3
X:= OH R:= H
a
Boc
O
X:= OH R:= Me
X:= I R:= Me
14
b
6
Scheme 2. Synthesis of homoallylglycine 5 via Knochel cuprate. Reagents and
conditions: (a) DMF, K2CO3, MeI (2 equiv), 12 h (76%); (b) (i) DCM, P(Ph)3
(1.25 equiv), imidazole (1.3 equiv), I2 (1.3 equiv), rt 10 min; (ii) then addition of 2,
0 °C?rt 1 h (68%); (c) (i) Zn (6 equiv), THF, 1,2-dibromethane (0.3 equiv), 60 °C
3 min; (ii) TMS-Cl, 25 °C 30 min; (iii) 3 in THF, 45 °C 50 min; (iv) À10 °C, CuCN
(1 equiv) and LiCl (2 equiv) in THF?0 °C 10 min; (v) À25 °C, allyl chloride
(1.3 equiv)?rt 12 h (57%); (d) (i) dioxane/water (4:1), LiOH (2.2 equiv), 0 °C?rt
12 h; (ii) concd HCl (pH 1–1.5), 8 h; (e) satd NaHCO3 (pH 8–9), Fmoc-OSu
(1.3 equiv), 0 °C?rt 12 h (62% over three steps).
O
O
O
O
H
N
H
N
H
N
O
P
O
OH
Fmoc
N
H
N
H
O
N
H
N
H
O
O
O
Ad
b
O
O
O
O
O
15
collidine (4 equiv) for 12 h in order to allow for complete coupling.
The following couplings were again accomplished under standard
SPPS conditions with just 5 to be coupled with HATU/HOAt and
only 1.5 equiv each for a reaction time of 12 h (Scheme 1).
O
O
O
O
H
N
H
N
O
P
O
H2N
OH
N
H
N
H
O
N
H
N
H
Encouraged by previously reported RCM reactions performed
on solid support in our group by Schmiedeberg et al.25 we first ap-
plied the solid phase methodology to this molecule. As indicated
by HPLC–MS all of the starting material was consumed, but the
product was cleaved from the resin. Therefore, hexafluoroisoprop-
anole in DCM was used to cleave each of the peptidic sequences of
the three fractions 13 from the resin whereby all protecting groups
remained on the molecule to yield 14. The RCM was accomplished
in solution with each of the three fractions in parallel (Scheme 4)
by use of Grubbs 2nd generation catalyst.38,39 HPLC–MS analysis
of conversion of 14 fraction c revealed clean conversion to the de-
sired macrocycle 15. Also the RCM conversion of the other fractions
of 14 gave excellent results. The reduction of the olefin to give the
saturated analog was accomplished by hydrogenation which
O
O
O
Y
R2O
O
OR2
O
OR2
16 Y:= Ad R2:= C(CH3)3
II (P1-P4) Y:= R2:= H
c
Scheme 4. Synthesis of compound II from the linear precursor 14. Reagents and
conditions: (a) (i) 14, abs DCM (degassed), 2.5% Grubbs 2nd, reflux 4 h; (ii) 2.5%
Grubbs 2nd, reflux 12 h (complete conversion as indicated by HPLC–MS); (b) DCM
and tert-butanol (1:1), 20% Pd/C, H2 atm, >36 h (quantitative conversion as
indicated by HPLC–MS); (c) (i) TFA/TIPS/water (95:2.5:2.5), rt 12 h; (ii) semi
preparative RP-HPLC.
results in simultaneous removal of the N-terminal Fmoc group.
Several difficulties in first hydrogenation attempts led to the deci-
O
O
O
O
P
O
P
H
HO
Fmoc
a
O
NH2
N
H
OR1
O
OY
O
10 Y:= H R1:= Me
11 Y:= Ad R1:= Me
12 Y:= Ad R1:= H
b
c
7
8
9
Scheme 3. Three component condensation to yield PDP isostere 12. Reagents and conditions: (a) (i) AcCl/HOAc (3:1), 9; (ii) 7, 0 °C; (iii) 8, 0 °C?rt 12 h (64%); (b) (i) CHCl3,
1-Ad-Br, reflux; (ii) Ag2O (45%; 40% 10 recovery); (c) (i) THF, water, LiOH (3 equiv), 0 °C?rt; (ii) 1 N HCl (pH 6–7); (iii) 2 N NaHCO3 (pH 9), Fmoc-Cl (1 equiv) (60%, three
fractions in ratio 1:1:3).

4430
T. Huber et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4427–4431
sion to use DCM together with tert-butanol as protic solvent
(Scheme 4). The reaction time was quite long as it typically took
more than 36 h for the conversion to compound 16. Finally deriv-
ative 16 was stirred in a mixture of TFA/TIPS/water (95:2.5:2.5) for
12 h in parallel while monitoring deprotection by HPLC–MS. Final-
ly, each mixture was purified and the different isomers separated
by standard semi-preparative RP-HPLC. It was possible to separate
all of the four diastereomers which were named after their chrono-
logical appearance from the HPLC-column as follows: II-P1, II-P2,
II-P3 and II-P4. Overall II P1-4 was isolated in equimolar ratio with
a yield of 60% over four steps and HPLC purification relative to the
loading of the resin.
The inhibitory effects of these macrocyclic phosphino peptides
against BACE1 were examined by an assay in which the ectodo-
main of BACE1 secreted from HEK393 cells was incubated with la-
beled substrate peptide and test compound as described in the
Supplementary data. The results and analytical data are shown in
Table 1. Two of the compounds have activities of 47 nM for II-P1
and 320 nM for II-P2. The fraction containing II-P3 has an activity
of 522 nM whereas II-P4 exhibited 808 nM. With this result it
seems most likely that the most active macrocycle II-P1 consti-
tutes the chirality that belongs to the side chain orientation in nat-
ural amino acids. The most active compound II-P1 was chosen
together with the linear analog I to study enhancement of stability
in serum achieved by cyclization.
It is well known that some major problems limit the use of
peptides as effective drugs, among others are their often bad cel-
lular activity, serum stability or brain penetration (across blood
brain barrier). Particularly their fast degradation in vivo by pro-
teases leads to their inherent instability. In order to protect bio-
logically active peptides from in vivo decomposition there are
different approaches, for example, alteration of the peptide bond,
conjugation to carrier molecules, cyclization, N-methylation and
the incorporation of non-proteinogenic amino acids.40–42 For
in vitro enzymatic stability studies mainly isolated enzymes such
as carboxypeptidase A, aminopeptidase M, proteinase A, carboxy-
lic inhibitor II-P1 was more stable and degraded with a half-life
of about 43.9 min. While inhibitor I is totally degraded and not
detectable after approximately 120 min, cyclic inhibitor II-P1 is
still detectable after approximately 160 min. After this time
about 20% of the inhibitor is still measurable by UV detection.
In total the stability (half life) increased by a factor of approxi-
mately three.
This study reports the synthesis of potent macrocyclic PDP iso-
stere inhibitors with enhanced stability in human serum compared
to their linear analogs. The synthesis was accomplished by SPPS of
key fragments. A new useful protected PDP isostere building block
for macrocyclization via metathesis bearing a terminal alkene
functionality for SPPS was efficiently synthesized. After assembly
of the precursor on solid support the RCM, hydrogenation and
deprotection reactions were performed in high yield to obtain
the desired macrocycle.
The data obtained from human serum incubation showed
enhancement of stability against degradation in human serum by
a factor of three compared to the linear analog. The remaining
instability is possible due to the still very peptidic C-terminal part
of the inhibitor and further development by replacing this part
showed led to increased stabilities. Future investigations and opti-
mization of pharmacokinetics together with known prodrug con-
cepts, such as phosphinic acid esters (Monopril),43 could led to
macrocyclic PDP isostere inhibitors with improved pharmaceutical
properties and thereby making them promising candidates for fur-
ther drug development.
Acknowledgments
We thank B. Cordes for ESI-MS analysis and M. Wolff for tech-
nical support.
Supplementary data
Supplementary data (detailed protocols of the peptide digestion
in human serum, molecular docking, SPPS, biological evaluation,
experimental data of synthesized compounds) associated with this
article can be found, in the online version, at doi:10.1016/
j.bmcl.2009.05.053.
peptidase Y,
a-chymotrypsin or complex biological fluids, such
as human serum and urine, human plasma or rat liver lysosomes
are in use. The present investigation used fresh human serum
preparations. Therefore compounds I and II-P1 were treated
with fresh human serum of a single donor and the amount of in-
tact ligand was determined by quantitative HPLC and HPLC–MS
analysis over the time (Fig. 3). The linear inhibitor I is rapidly
degraded with a half-life of about 14.8 min. As assumed, the cyc-
References and notes
1. Evans, D. A.; Funkenstein, H. H.; Albert, M. S.; Scherr, P. A.; Cook, N. R.; Chown,
M. J.; Hebert, L. E.; Hennekens, C. H.; Taylor, J. O. JAMA 1989, 262, 2551.
2. Selkoe, D. J. Neuron 1991, 6, 487.
3. Selkoe, D. J. Nature 1999, 399, A23.
4. Sinha, S.; Lieberburg, I. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 11049.
5. Hussain, I.; Powell, D.; Howlett, D. R.; Tew, D. G.; Meek, T. D.; Chapman, C.;
Gloger, I. S.; Murphy, K. E.; Southan, C. D.; Ryan, D. M.; Smith, T. S.; Simmons, D.
L.; Walsh, F. S.; Dingwall, C.; Christie, G. Mol. Cell. Neurosci. 1999, 14, 419.
6. Ghosh, A. K.; Bilcer, G.; Harwood, C.; Kawahama, R.; Shin, D.; Hussain, K. A.;
Hong, L.; Loy, J. A.; Nguyen, C.; Koelsch, G.; Ermolieff, J.; Tang, J. J. Med. Chem.
2001, 44, 2865.
100
80
60
40
20
0
7. Ghosh, A. K.; Shin, D. W.; Downs, D.; Koelsch, G.; Lin, X. L.; Ermolieff, J.; Tang, J.
J. Am. Chem. Soc. 2000, 122, 3522.
8. Tung, J. S.; Davis, D. L.; Anderson, J. P.; Walker, D. E.; Mamo, S.; Jewett, N.; Hom,
R. K.; Sinha, S.; Thorsett, E. D.; John, V. J. Med. Chem. 2002, 45, 259.
9. Tamamura, H.; Kato, T.; Otaka, A.; Fujii, N. Org. Biomol. Chem. 2003, 1, 2468.
10. Shuto, D.; Kasai, S.; Kimura, T.; Liu, P.; Hidaka, K.; Hamada, T.; Shibakawa, S.;
Hayashi, Y.; Hattori, C.; Szabo, B.; Ishiura, S.; Kiso, Y. Bioorg. Med. Chem. Lett.
2003, 13, 4273.
11. Luo, Y.; Bolon, B.; Kahn, S.; Bennett, B. D.; Babu-Khan, S.; Denis, P.; Fan, W.;
Kha, H.; Zhang, J.; Gong, Y.; Martin, M.; Louis, L.; Yan, J. C.; Richards, Q.; Citron,
W. G.; Vassar, R. Nat. Neurosci. 2001, 4, 231.
12. Vassar, R.; Bennett, B. D.; Babu-Khan, S.; Kahn, S.; Mendiaz, E. A.; Denis, P.;
Teplow, D. B.; Ross, S.; Amarante, P.; Loeloff, R.; Luo, Y.; Fisher, S.; Fuller, J.;
Edenson, S.; Lile, J.; Jarosinski, M. A.; Biere, A. L.; Curran, E.; Burgess, T.; Louis, J.
C.; Collins, F.; Treanor, J.; Rogers, G.; Citron, M. Science 1999, 286, 735.
13. Hong, L.; Koelsch, G.; Lin, X.; Wu, S.; Terzyan, S.; Ghosh, A. K.; Zhang, X. C.;
Tang, J. Science 2000, 290, 150.
0
20
40
60
80
100
120
140
160
Time [min]
14. Hong, L.; Turner, R. T., 3rd; Koelsch, G.; Shin, D.; Ghosh, A. K.; Tang, J.
Biochemistry 2002, 41, 10963.
Figure 3. Stability in human serum of I (blue) and II-P1 (red).

T. Huber et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4427–4431
4431
15. Manzenrieder, F.; Frank, A. O.; Huber, T.; Dorner-Ciossek, C.; Kessler, H. Bioorg.
Med. Chem. 2007, 15, 4136.
28. Kazmaier, U.; Hebach, C.; Watzke, A.; Maier, S.; Mues, H.; Huch, V. Org. Biomol.
Chem. 2005, 3, 136.
16. Manzenrieder, F.; Huber, T.; Frank, A.; Dorner-Ciossek, C.; Kessler, H. J. Pept. Sci.
2006, 12, 166.
29. Pavé, G.; Usse-Versluys, S.; Viaud-Massuard, M. C.; Guillaumet, G. Org. Lett.
2003, 5, 4253.
17. Manzenrieder, F.; Frank, A. O.; Kessler, H. Angew. Chem., Int. Ed. 2008, 47,
2608.
18. Morris, G. M.; Goodsell, D. S.; Halliday, R. S.; Huey, R.; Hart, W. E.; Belew, R. K.;
Olson, A. J. J. Comput. Chem. 1998, 19, 1639.
30. Trost, B. M.; Rudd, M. T. Org. Lett. 2003, 5, 4599.
31. Knochel, P.; Yeh, M. C. P.; Berk, S. C.; Talbert, J. J. Org. Chem. 1988, 53, 2390.
32. Baylis, K. E.; Campbell, C. D.; Dingwall, J. G. J. Chem. Soc., Perkin Trans. 1 1984,
12, 2845.
19. Rojo, I.; Martin, J. A.; Broughton, H.; Timm, D.; Erickson, J.; Yang, H. C.;
McCarthy, J. R. Bioorg. Med. Chem. Lett. 2006, 16, 191.
20. Cumming, J. N.; Iserloh, U.; Kennedy, M. E. Curr. Opin. Drug Discovery Dev. 2004,
7, 536.
21. Barazza, A.; Gotz, M.; Cadamuro, S. A.; Goettig, P.; Willem, M.; Steuber, H.;
Kohler, T.; Jestel, A.; Reinemer, P.; Renner, C.; Bode, W.; Moroder, L. Chem.
Biochem. 2007, 8, 2078.
33. Boyd, E. A.; Regan, A. C.; James, K. Tetrahedron Lett. 1992, 33, 813.
34. Matziari, M.; Yiotakis, A. Org. Lett. 2005, 7, 4049.
35. Oleksyszyn, J.; Gruszecka, E. Terahedron Lett. 1981, 22, 3537.
36. Yiotakis, A.; Vassiliou, S.; Jiracek, J.; Dive, V. J. Org. Chem. 1996, 61, 6601.
37. Nasopoulou, M.; Matziari, M.; Dive, V.; Yiotakis, A. J. Org. Chem. 2006, 71, 9525.
38. Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem., Int. Ed. Engl.
1995, 34, 2039.
22. Hanessian, S.; Yang, G. Q.; Rondeau, J. M.; Neumann, U.; Betschart, C.;
Tintelnot-Blomley, M. J. Med. Chem. 2006, 49, 4544.
39. Chatterjee, A. K.; Choi, T. L.; Sanders, D. P.; Grubbs, R. H. J. Am. Chem. Soc. 2003,
125, 11360.
23. Ghosh, A. K.; Devasamudram, T.; Hong, L.; De-Zutter, C.; Xu, X. M.; Weerasena,
V.; Koelsch, G.; Bilcer, G.; Tang, J. Bioorg. Med. Chem. Lett. 2005, 15, 15.
24. Manzenrieder, F.; Huber, T.; Kuttruff, C. A.; Dorner-Ciossek, C.; Kessler, H. J.
Pept. Sci. 2008, 14, 121.
25. Schmiedeberg, N.; Kessler, H. Org. Lett. 2002, 4, 59.
26. Martin, W. H.; Blechert, S. Curr. Top. Med. Chem. 2005, 515, 1521.
27. Dekker, F. J.; de Mol, N. J.; Fischer, M. J. E.; Kemmink, J.; Liskamp, R. M. J. Org.
Biomol. Chem. 2003, 1, 3297.
40. Tugyi, R.; Mezö, G.; Fellinger, E.; Andreu, D.; Hudecz, F. J. Pept. Sci. 2005, 11,
642.
41. Biron, E.; Chatterjee, J.; Ovadia, O.; Langenegger, D.; Brueggen, J.; Hoyer, . D.;
Schmid, H. A.; Jelinek, R.; Gilon, C.; Hoffman, A.; Kessler, H. Angew. Chem., Int.
Ed. 2008, 4714, 2595.
42. Chatterjee, J.; Gilon, C.; Hoffman, A.; Kessler, H. Acc. Chem. Res. 2008, 4110,
1331.
43. Jemal, M.; Ivashkiv, E.; Ribick, M.; Cohen, A. J. Chromatogr. 1985, 3452, 299.