Structure-guided design and synthesis of P1′ position 1-phenylcycloalkylamine-derived pentapeptidic BACE1 inhibitors

Article abstract of DOI:10.1016/j.bmc.2011.07.002

Previously, we reported potent pentapeptidic BACE1 inhibitors with the hydroxymethylcarbonyl isostere as a substrate transition-state mimic. To improve the in vitro potency, we further reported pentapeptidic inhibitors with carboxylic acid bioisosteres at the P₄ and P₁′ positions. In the current study, we screened new P₁′ position 1-phenylcycloalkylamine analogs to find non-acidic inhibitors that possess double-digit nanomolar range IC₅₀ values. An extensive structure-activity relationship study was performed with various amine derivatives at the P1′ position. The most potent inhibitor of this pentapeptide series, KMI-1830, possessing 1-phenylcyclopentylamine at the P ₁′ position had an IC₅₀ value of 11.6 nM against BACE1 in vitro enzymatic assay.

Full text of DOI:10.1016/j.bmc.2011.07.002

Bioorganic & Medicinal Chemistry 19 (2011) 5238–5246
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
0
Structure-guided design and synthesis of P1 position
1-phenylcycloalkylamine-derived pentapeptidic BACE1 inhibitors
Harichandra D. Tagad a, Yoshio Hamada a, Jeffrey-Tri Nguyen a, Koushi Hidaka a, Takashi Hamada a,
Youhei Sohma a, Tooru Kimura a, Yoshiaki Kiso a,b,
⇑
a Department of Medicinal Chemistry, Centre for Frontier Research in Medicinal Science, Kyoto Pharmaceutical University, Yamashina-Ku, Kyoto 607-8412, Japan
b Laboratory of Peptide Science, Nagahama Institute of Bio-Science and Technology, Nagahama, Shiga 526-0829, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Previously, we reported potent pentapeptidic BACE1 inhibitors with the hydroxymethylcarbonyl isostere
as a substrate transition-state mimic. To improve the in vitro potency, we further reported pentapeptidic
Received 9 June 2011
Revised 30 June 2011
Accepted 1 July 2011
Available online 13 July 2011
0
inhibitors with carboxylic acid bioisosteres at the P4 and P1 positions. In the current study, we screened
0
new P1 position 1-phenylcycloalkylamine analogs to find non-acidic inhibitors that possess double-digit
nanomolar range IC50 values. An extensive structure–activity relationship study was performed with var-
0
ious amine derivatives at the P1 position. The most potent inhibitor of this pentapeptide series, KMI-
Keywords:
Alzheimer’s disease
b-Secretase
Pentapeptides
Bioisosteres
0
1830, possessing 1-phenylcyclopentylamine at the P1 position had an IC50 value of 11.6 nM against
BACE1 in vitro enzymatic assay.
Ó 2011 Elsevier Ltd. All rights reserved.
Phenylcycloalkylamines
1. Introduction
of BACE1 is considered a promising therapeutic target for the
treatment and prevention of AD.16 Although numerous BACE1
Alzheimer’s disease (AD)1–3 is the most common chronic neuro-
degenerative disorder of the central nervous system. It is charac-
terized by a progressive loss of memory, cognitive deficits, and
psychiatric symptoms.4,5 The pathology of AD is exemplified by
the formation of insoluble extracellular amyloid plaques from
amyloid fibril tangles, which are derived from the aggregation of
amyloid peptides (Ab) in the brain.6,7 Ab are found from 38 to 43
residues in length, with Ab40 being most numerous and Ab42 the
most aggregative and pathogenic. Sequential cleavage of amyloid
precursor protein (APP) by the enzyme, b-secretase (BACE1) at
Met671-Asp672 (APP770 numbering) releases soluble sAPPb in
the pathogenic pathway. The remaining membrane-bound C99
inhibitors have been reported, most of the peptide-based struc-
tures have relatively high molecular weights and possess unfavor-
able properties for traversing the blood–brain barrier (BBB).17,18
In our research on pentapeptidic BACE1 inhibitors, extensive
structure–activity relationship (SAR) studies were used to derive
the pentapeptidic inhibitor KMI-360 (1)19 which has glutamic acid
0
at the P4 position and 3-aminophthalic acid at the P1 position
(Fig. 1). Acidic moieties at the prime and non-prime positions are
thought to be unfavorable for crossing the BBB.20,21 Indeed, more
than 98% of drugs with high in vitro potency against their respec-
tive enzymes do not actually cross the BBB in vivo due to physico-
chemical features such as acidity, molecular mass and lipid
solubility.22 In order to reduce acidity of the peptide, we identified
fragment is further cleaved by the enzyme, c-secretase, to generate
amyloid Ab and APP intracellular domain.8,9 Of these two prote-
ases, BACE1 catalyzes the initial proteolysis of APP, which is con-
sidered as a rate-limiting step in the cleavage cascade.10 First
identified in 1999, BACE1 belongs to the type-I class of aspartyl
proteases.11 BACE1 knockout mice with significantly reduced lev-
els of b-amyloid was shown to maintain a relatively normal pheno-
type.12 Moreover, deletion of the BACE1 gene in a Tg2576 mouse
model was shown to reduce cognitive deficit.13–15 Thus inhibition
L-2,3-diaminopropionic acid (DAP) as a novel P4 position linker
which then led us to identify highly potent P4 Dap(tetrazole) pen-
tapeptidic BACE1 inhibitors, KMI-420 (2a) and KMI-429 (2b).23
Further SAR studies yielded P4 2,5-dihydroxybenzoyl (3a) and 5-
0
fluoroorotyl (3b) inhibitors with a P1 position 3-(1-H-tetrazol-5-
yl)aniline moiety24 possessing potent BACE1 inhibitory activity
in vitro enzymatic assay. With 2,5-dihydroxybenzoyl and 5-fluoro-
0
orotyl at the P4 position, we turned our attention to the P1 position
3-(1-H-tetrazol-5-yl)aniline derivative of 3a–b. Recently, we re-
ported pentapeptidic inhibitors with 3-(1-H-tetrazol-5-yl)aniline
bioisosteres at the P1 position (4a–c).25 Although the inhibitors
0
⇑
Corresponding author. Tel.: +81 749 64 8113; fax: +81 749 64 8140.
E-mail address: y_kiso@nagahama-i-bio.ac.jp (Y. Kiso).
of type 4a–c showed excellent potency against BACE1 (IC50
=
0968-0896/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2011.07.002

H. D. Tagad et al. / Bioorg. Med. Chem. 19 (2011) 5238–5246
5239
X
OH
H-DAP-Val-Leu-Pns
HN
O
N
O
NH
N
N
2a. X = H, KMI-420, IC50 = 8.2 nM
2b. X = COOH, KMI-429, IC50 = 3.9 nM
'
P4
P3
P2
P1
P1
O
OH
H
N
O
O
O
H-DAP(R1)-Val-Leu-Pns
N
H
N
H-DAP(R1)-Val-Leu-Pns
HN
HN
H2N
OH
N
O
O
N
N
N
N
HN
N
H
H
H
O
OH
O
3a. R1 = 5-fluoroorotyl
(KMI-572, IC50 = 6.5 nM)
3b. R1 = 2,5-dihydroxybenzoyl
(KMI-573, BACE1 inhibitory activity
at 2 µM: 99%; at 0.2 µM: 93%)
4a. R1 = tetrazole-5-carbonyl
(KMI-696, IC50 = 6.6 nM)
4b. R1 = 5-fluoroorotyl
(KMI-707, IC50 = 7.9 nM)
4c. R1 = 2,5-dihydroxybenzoyl
(KMI-708, IC50 = 10 nM)
O
OH
1
KMI-360, IC50 = 55 nM
H-DAP(R1)-Val-(R2)-Pns
NH R3
13−36
(see Table 1 )
0
Figure 1. P1 position modifications of our lead pentapeptidic BACE1 inhibitor 1. (DAP:
L-2,3-diaminopropionic acid; Pns [phenylnorstatine: (2R,3S)-3-amino-2-hydroxy-4-
phenylbutyric acid]).
0
7–10 nM), on comparison of P1 residues of this inhibitors with
and cycloalkyl amine moieties (13–36). Because we could not
inhibitors 2a and 3a–b displays similar ionizability.
agree on the exact SAR for the P2 and P4 residues in our past stud-
ies,24,25 modifications at these positions with P2 leucine or
L-cyclo-
0
In the current study, we examined less ionizable P1 moieties in
our pentapeptidic BACE1 inhibitors. In the literature, substituted
hexyl-alanine (Cha) and P4 2,5-dihydroxybenzoic or 5-fluoroorotic
acid were also examined.
0
benzylamine derivatives have been used as P1 residues in several
hydroxyethylamine (HEA) type BACE1 inhibitors.26–28 In our inhib-
itors, phenylnorstatine (Pns) served as an hydroxymethylcarbonyl
transition-state mimic isostere. By using computer-assisted drug
design, we initially calculated that a benzylamine structure could
2. Chemistry
The P10 position amines were prepared by using a three-step pro-
cedure starting from iodobenzene and its corresponding ketones as
shown in Schemes 1 and 2. Commercially available cyclic ketones
(5a–b) were reacted with phenylmagnesium iodide generated from
iodobenzene and magnesium turnings in tetrahydrofuran (Grignard
reagent) to give 1-phenylcyclopentanol (6a) and 1-phenylcyclohex-
anol (6b) in good yields. These alcohols were then converted to their
respective azides (7a and 7b) by using sodium azide and trifluoro-
acetic acid (TFA) followed by reduction with lithium aluminum
hydride (LAH) to produce amines 8a and 8b.
0
serve as an alternative for the P1 residues found in inhibitors 1,
2a–b, 3a–b, and 4a–c.19 However, because the P1 benzylamine
0
would most likely be easily degraded in vivo,29 we examined more
0
conformationally rigid P1 benzylamine derivatives including alkyl
n
a
b
c
A Ritter reaction30,31 was used to prepare the 1-benzyl-4-phen-
ylpiperidin-4-amine (12) that was needed for the synthesis of pen-
tapeptides 33–36. The tertiary alcohol required for the Ritter
reaction was synthesized starting from 4-piperidone, as shown in
Scheme 2. First, benzyl protection of the amine functionality of
4-piperidone was carried out with benzyl bromide by using
O
HO
N3
H2N
n
n
n
n = 0, 5a
n = 1, 5b
n = 0, 6a
n = 1, 6b
n = 0, 7a
n = 1, 7b
n = 0, 8a
n = 1, 8b
Scheme 1. Synthesis of 1-phenylcycloalkylamines: reagents and conditions: (a)
PhMgI, 0 °C–rt, THF; (b) NaN3, TFA, CH2Cl2; (c) LAH, Et2O.
a
O
b
c
O
d
O
NH
HO
NBn
N
H2N
NBn
NBn
NBn
H
9
10
11
12
Scheme 2. Synthesis of 1-benzyl-4-phenylpiperidin-4-amine: reagents and conditions: (a) BnBr, K2CO3, CH3CN; (b) PhMgI, 0 °C–rt, Et2O; (c) MeCN, conc. H2SO4, 0 °C–rt; (d)
6 N HCl (aq), reflux.

5240
H. D. Tagad et al. / Bioorg. Med. Chem. 19 (2011) 5238–5246
K2CO3 in CH3CN, followed by a Grignard reaction similar to that
used for alcohol 6a–b. The obtained alcohol 10 was converted into
acetamide derivative 11 by using conc. H2SO4 and CH3CN (Ritter
reaction), which was further transformed into the required amine
12 under acidic hydrolysis conditions by using 6 N HCl (aq).32
attention towards the more stable and hydrophobic six-member
ring analog, 1-phenylcyclohexylamine, at the P1 position, and syn-
0
thesized pentapeptides 29–32. Even after an increase in ring size
from a five- to a six-member ring, there was no improvement in
inhibition potency with respect to the P2 and P4 variants (29 and
30: 5-fluoroortyl, IC50 = 25 and 41 nM, respectively; 31 and 32:
2,5-dihydroxybenzoyl, 43% and 46% inhibition, respectively, at
The a-methylbenzylamine and a,a-dimethylbenzylamine (cumyl-
amine) were commercially available and used directly for the
respective pentapeptide synthesis reactions.
0.2
lM concentration).
All pentapeptides were synthesized in solution phase by using
Boc chemistry as reported previously.24 Peptide bonds were
formed between an amine and the N-protected amino acid by using
1-ethyl-3(3-dimethylaminopropyl)carbodiimideꢀHCl (EDCꢀHCl)
and 1-hydroxybenzotriazole (HOBt) as coupling agents, with
triethylamine (NEt3) as a base in N,N-dimethylformamide (DMF).
Pentapeptide synthesis was carried out starting from the P1 posi-
tion amines and Boc–Pns–OH followed by the respective amino
acids couplings.
For a thorough analysis, we also performed computer-assisted
docking experiments and chemical property calculations to corre-
late structural properties with inhibition potency by using the
Chemical Computing Group’s Molecular Operating Environment
2010.10. The models were designed primarily from the X-ray dif-
fraction crystallography data of the classical peptidic inhibitor
OM99-2 (PDB ID: 1FKN), and secondarily, from the non-peptidic
inhibitor Z75 (PDB ID: 3LPJ), which possess similar non-prime
0
0
and P1 moieties that of our pentapeptides (Fig. S1 of Supplemen-
tary data). At the P4 side-chain, in agreement with a recent study
of a series of BACE1 inhibitors,24 the models indicate that 5-fluo-
roorotic acid is favored over 2,5-dihydroxybenzoic acid by the pro-
tease because of stronger hydrogen bond potentials. At the P2
position, although no appreciable potency difference was seen, leu-
cine was slightly favored over Cha, which is also in agreement with
the inhibition profiles of related inhibitors.25 Also our computer
models suggests that there is ample hydrophobic space and flexi-
bility at the S2 subsite to accommodate either residue, despite a
slight obstruction by Arg235 (Fig. S2 of Supplementary data).
Coinciding with the inhibition profiles of compounds 13–32, our
3. Results and discussion
The inhibition potency of pentapeptides against BACE1 is sum-
marized in Table 1. The synthesized peptides were screened for
BACE1 inhibition at 2
lM then 0.2 lM concentrations, and the
IC50 values were determined only for the very potent (>75% of inhi-
bition at 0.2
l
M concentration) BACE1 inhibitors.
0
In the case of the P1 residue, which is our main residue of inter-
est we selected benzylamine, in agreement with a study that used
a barely similar moiety,33 the phenyl group that resides in the S2
0
0
models propose that the S1 subsite is sufficiently flexible and large
subsite. HEA types BACE1 inhibitors with benzylamine at the
prime position suffer from low metabolic stability due to debenzy-
lation by rat and human microsomes.29 Therefore, we initially
to accommodate several hydrophobic bulky moieties up to a six-
member ring system (Fig. 2). The amino group of the piperidinyl
moiety can form a direct hydrogen bond interaction with the side-
chain of Thr72 as shown in Figure 2A for the 1-benzyl-4-phenylpi-
peridin-4-amine at the P10 position, which could offer an advantage
over inhibitors 13–32. Based on this observation, we synthesized
pentapeptides (33–36) with variations at the P2 and P4 positions.
However, the inhibition potency profiles of these inhibitors (33:
selected a commercially available benzylamine derivative,
a-
0
methylbenzylamine, for the P1 position and synthesized
pentapeptides by using leucine or Cha at the P2 position with
2,5-dihydroxybenzoic acid or 5-fluoroorotic acid at the P4 position.
The pentapeptides with both R and S isomers were prepared (13–
20) to examine the selectivity at the prime side position of the
59% and 34: 64% inhibition at 0.2
36: 57% inhibition at 2 M concentration) were relatively low
compared with those of compounds 13–32 (5-fluoroorotyl, 77–
lM concentration; 35: 59% and
pentapeptides.
0
l
Compounds with R and S
tion (13–20) showed similar BACE1 inhibition potency at 0.2 lM
concentration ranging from 79% to 83% for 5-fluoroorotyl (13, 14,
17, and 18), and 44% to 54% for 2,5-dihydroxybenzoyl (15, 16, 19,
and 20). With respect to IC50 values, inhibitors with the S isomer
(17 and 18) were approximately 1.5-fold more potent than those
with the corresponding R isomer (13 and 14). We then inserted
a
-methylbenzylamine at the P1 posi-
86% inhibition at 0.2
86–90% inhibition at 2
l
l
M concentration; 2,5-dihydroxybenzoyl,
M concentration). This is likely due to an
obstruction of the inhibitors large benzyl group by Thr329 (Fig. 2B)
relative to the exactly fitting cyclopentyl ring of most potent penta-
peptide KMI-1830 (25) (Fig. 2C). Hence, piperidinyl analogs with
smaller moieties on the nitrogen atom may be a more appropriate
size.
an additional methyl group at the benzylic position of
benzylamine, which is cumylamine, as a prime position analog
a-methyl-
and synthesized inhibitors (21–24). Inhibitors 21–24 (21: 83%
and 22: 78% inhibition at 0.2
lM concentration; 23: 44% and 24:
4. Conclusions
49% inhibition at 0.2 M concentration) showed similar inhibition
l
potency to that of compounds 13–20, but the IC50 values of inhib-
itor 21 (IC50 = 27 nM) and 22 (IC50 = 32 nM) indicated a slight de-
crease in inhibition potency compared with that of inhibitors 17
(IC50 = 20 nM) and 18 (IC50 = 24 nM). To further improve the inhi-
bition potency, we searched for new moieties at the prime position
In conclusion, by using a structure-based approach, we de-
signed and synthesized novel pentapeptidic BACE1 inhibitors mod-
ified at the P1 position. A structure–activity relationship study
0
revealed that the S4 subsite of BACE1 favors 5-fluoroorotyl over
2,5-dihydroxybenzoyl in the P4 side-chain, and that the S2 subsite
can accommodate either leucine or Cha, although leucine may be
slightly preferred. Through comparative analysis, we found the fol-
with increased acid stability.
0
Appending a cycloalkyl ring at the
a
-carbon of P1 position ben-
0
zylamine derivatives could play an important role in stability.
Moreover, the new cyclic analogs do not possess any acidic func-
tionality. With this in mind, we synthesized 1-phenylcyclopentyl-
amine and produced the corresponding pentapeptides 25–28 (25:
lowing general potency order from most to least for the P1 moiety:
cyclopentyl, S-methyl, cyclohexyl,
a,a-dimethyl, R-methyl, and N-
benzylpiperidinyl group at the -carbon of benzylamide derivative
a
with leucine at the P2 position of pentapeptide. This study identi-
86% and 26: 84% inhibition at 0.2
lM concentration; 27 and 28:
fied a stable, hydrophobic, and non-ionizable BACE1 inhibitor
possessing a 1-phenylclopentylamine moiety at the P1 position,
0
58% inhibition at 0.2 M concentration). Surprisingly, the IC50 val-
l
ues of inhibitors 25 and 26 indicated that inhibitor 25 was a more
potent inhibitor with an IC50 value of 12 nM. Then we turned our
KMI-1830 (25, IC50 = 12 nM), with a five-fold improvement in
potency over lead compound 1 (IC50 = 55 nM).

H. D. Tagad et al. / Bioorg. Med. Chem. 19 (2011) 5238–5246
5241
Table 1
0
BACE1 inhibitory activity of synthesized pentapeptides (13–36) with modification at the P1 position
O
O
O
A =
B =
H
H2N
N
R3
R2
N
N
N
H
H
H
O
OH
R2
HN
O
R1
R1
R2
R3
BACE1 inhibition (%)
at 2
IC50 (nM)
a
Compound (KMI No.)
l
M
at 0.2 lM
F
O
13 (KMI-1825)
14 (KMI-1827)
A
B
98
98
81
79
34.8 1.0
39.6 2.5
NH
O
HN
HO
15 (KMI-1826)
16 (KMI-1828)
A
B
87
88
44
51
—
—
OH
O
F
17 (KMI-1834)
18 (KMI-1836)
A
B
99
98
83
80
20.4 0.7
24.2 1.1
NH
O
HN
HO
19 (KMI-1835)
20 (KMI-1837)
A
B
88
89
47
54
—
—
OH
O
F
21 (KMI-1779)
22 (KMI-1781)
A
B
98
98
83
78
27.0 1.5
31.9 1.3
NH
O
HN
HO
23 (KMI-1780)
24 (KMI-1782)
A
B
88
89
44
49
—
—
OH
O
F
25 (KMI-1830)
26 (KMI-1832)
A
B
99
98
86
84
11.6 0.6
29.0 0.9
NH
O
HN
HO
27 (KMI-1831)
28 (KMI-1833)
A
B
90
89
58
58
—
—
OH
O
F
29 (KMI-1720)
30 (KMI-1838)
A
B
98
97
85
77
24.8 0.6
40.6 0.2
NH
O
HN
HO
31 (KMI-1771)
32 (KMI-1839)
A
B
86
87
43
46
—
—
OH
O
F
33 (KMI-1876)
34 (KMI-1829)
A
B
93
95
59
64
—
—
NH
O
HN
HO
35 (KMI-1877)
36 (KMI-1875)
A
B
59
57
—
—
—
—
NBn
OH
a
Errors expressed as standard error of the mean, n = 3. For less potent inhibitors, percent inhibition at a tested compound concentration is described.

5242
H. D. Tagad et al. / Bioorg. Med. Chem. 19 (2011) 5238–5246
Figure 2. A hydrogen bond might form with the flap’s Thr72 in KMI-1876 (33, A) while BACE1’s Thr329 obstructs the inhibitor’s benzyl group (B). The more potent inhibitor
0
KMI-1830 (25) has less steric strain than inhibitor 33 in the S1 subsite in the model (C).
The mixture was stirred at room temperature for 30 min, followed
by refluxed for 30 min. The solution was cooled in ice bath and
cyclopentanone 5a (0.53 mL, 5.95 mmol) in dry THF (1 mL) was
added dropwise over 5 min. The ice bath was removed and the
mixture was allowed to warm to ambient temperature over 3 h.
The reaction mixture was decanted into aqueous saturated NH4Cl
solution and combined with an ether wash of the residual magne-
sium turnings. The organic phase was washed twice more with the
saturated solution of aqueous NH4Cl solution, dried over anhy-
drous Na2SO4, filtered and concentrated to give the corresponding
alcohol. The resulting crude product was purified by column chro-
matography (hexane/EtOAc = 8:2) to afford compound 6a (646 mg,
67%) as colorless oil. TLC: Rf = 0.53 (hexane/EtOAc = 8:2); 1H NMR
(300 MHz, CDCl3) d (ppm): 1.57 (bs, 1H, OH), 1.83–1.85 (m, 2H,
CH2), 1.90–2.09 (m, 6H, (CH2)3), 7.21–7.37 (m, 3H, ArH), 7.47–
7.51 (m, 2H, ArH); 13C NMR (75 MHz, CDCl3) d (ppm): 23.85,
41.88, 83.50, 125.07, 126.80, 128.19, 147.11.
5. Experimental section
5.1. Materials
Reagents and solvents were purchased from Wako Pure Chem-
ical Ind. Ltd (Osaka, Japan), Nacalai Tesque (Kyoto, Japan), Aldrich
Chemical Co. Inc. (Milwaukee, WI, USA), and Tokyo Kasei Kogyo
Co. Ltd (Tokyo, Japan) and used without further purification. The
column chromatography was performed on Merck 107734 silica
gel 60 (70–230 mesh). Preparative HPLC was carried out on a C18
reverse phase column (250 ꢁ 20 mm I.D.; YMC-Pack-ODS-AM)
with a binary solvent system: a linear gradient of CH3CN in 0.1%
aqueous TFA with a flow rate of 5.0 mL/min and detection at
230 nm. NMR spectra’s were recorded on
a JEOL AL300
(300 MHz) and EX 270 (270 MHz) spectrometer for 1H with TMS
as an internal standard and at 75 MHz for 13C NMR. Mass spectra
(electrospray ionization, MeOH as the mobile phase) were ana-
lyzed with SHIMADZU LCMS-2010 spectrometer. MALDI-TOF MS
was performed on a Voyager-DE RP spectrometer (Per Septive Bio-
systems Inc.). Analytical HPLC was performed using a C18 reverse
phase column (4.6 ꢁ 150 mm; YMC-Pack-ODS AM-302) with a bin-
ary solvent system: linear gradient of CH3CN 0–100% in 0.1% aque-
ous TFA in 40 min at a flow rate of 0.9 mL/min, detected at 230 nm.
The purity of the desired compounds was >99% pure.
5.2.2. 1-Phenylcyclohexanol (6b)
Compound 6b was synthesized from cyclohexanone 5b and
iodobenzene in 61% yield as a white solid by following the same
procedure used for the synthesis of 6a. TLC: Rf = 0.49 (hexane/
EtOAc = 1:1); 1H NMR (300 MHz, CDCl3) d (ppm): 1.13–1.98 (m,
11H, (CH2)5, OH), 7.19–7.78 (m, 5H, ArH); 13C NMR (75 MHz,
CDCl3)
d (ppm): 21.97, 25.66, 38.72, 73.06, 124.52, 126.56,
128.10, 149.37.
5.2. Synthesis of 1-phenylcyclopentylamine (8a) and
1-phenylcyclohexylamine (8b)
5.2.3. 1-(1-Azidocyclopentyl)benzene (7a)
To 1-phenylcyclopentanol 6a (500 mg, 3.09 mmol) in CH2Cl2
(6 mL) under nitrogen was added sodium azide (441 mg,
6.79 mmol). The stirred suspension was cooled to ꢂ5 °C and a solu-
tion of TFA (1.94 mL, 25.3 mmol) in CH2Cl2 (4 mL) was added
5.2.1. 1-Phenylcyclopentanol (6a)
Magnesium turnings (217 mg, 8.93 mmol) in dry THF (5 mL)
was placed in ice cold water and iodobenzene (0.73 mL,
6.55 mmol) in dry THF (2 mL) was added dropwise over 10 min.

H. D. Tagad et al. / Bioorg. Med. Chem. 19 (2011) 5238–5246
5243
dropwise over 15 min. The resulting suspension was stirred at 0 °C
for an additional 1 h. Distilled water (10 mL) was added dropwise
to the cold, vigorously stirred mixture, followed by dropwise addi-
tion of a mixture of distilled water (5 mL) and 28% aqueous NH4OH
solution (5 mL). After 30 min the mixture was poured into a sepa-
rating funnel containing hexane and EtOAc (20 mL of 1:1 mixture),
and water (10 mL). The organic phase was washed with an addi-
tional portion of water, followed successively by 1 N KH2PO4
(aq), water, and finally brine. The organic phase was dried over
anhydrous Na2SO4, filtered, and concentrated. After workup, the
resulting crude product was purified by column chromatography
(in hexane) to afford azido compound 7a (409.8 mg, 71%) as color-
less oil. TLC: Rf = 0.57 (hexane); 1H NMR (300 MHz, CDCl3) d (ppm):
1.82–2.01 (m, 6H, (CH2)3), 2.21–2.27 (m, 2H, CH2), 7.26–7.44 (m,
5H, Ar–H); 13C NMR (75 MHz, CDCl3) d (ppm): 23.17, 37.58,
50.82, 126.13, 127.67, 128.51, 146.31.
5.3.2. 1-Benzyl-4-phenylpiperidin-4-ol (10)
Compound 10 was synthesized from 1-benzylpiperidin-4-one 9
using phenylmagnesium iodide in 73% (205 mg) yield as a white
solid following the procedure used for the synthesis of 6a–b.
TLC: Rf = 0.30 (hexane/EtOAc = 6:4); 1H NMR (300 MHz, CDCl3) d
(ppm): 1.73 (dd, J = 2.6 Hz, 2H), 2.18 (td, J = 12.8 Hz, 2H), 2.50 (td,
J = 12.3 Hz, 2H), 2.79 (d, J = 10.7 Hz, 2H), 3.59 (s, 2H), 7.22–7.38
(m, 8H), 7.51 (dd, J = 8 Hz, 2H); 13C NMR (75 MHz, CDCl3) d
(ppm): 38.35, 49.44, 63.24, 71.30, 124.53, 126.92, 126.99, 128.20,
128.29, 129.20, 138.41, 148.41; ESI-MS m/z 268.1 for [M+H]+.
5.3.3. N-(1-Benzyl-4-phenylpiperidin-4-yl)acetamide (11)
To
a
suspension of 1-benzyl-4-phenylpiperidin-4-ol 10
(105 mg, 0.39 mmol) in CH3CN (1 mL) at 0 °C was added conc.
H2SO4 (0.13 mL, 2.46 mmol) dropwise. The reaction mixture was
warmed to room temperature and stirred over 19 h, poured onto
ice and basified with 20% solution of NaOH (aq). The mixture
was extracted with EtOAc and the combined organic extracts were
dried over Na2SO4, filtered, and concentrated in vacuo. The residue
was purified by column chromatography to give acetamide 11
(86 mg, 71%) as colorless solid. TLC: Rf = 0.25 (EtOAc); 1H NMR
(300 MHz, CDCl3) d 2.00 (s, 3H), 2.08–2.17 (m, 2H), 2.24–2.36 (m,
4H), 2.75–2.79 (m, 2H), 3.54 (s, 2H), 5.61 (br s, 1H), 7.18–7.40
(m, 10H); 13C NMR (75 MHz, CDCl3): d 24.31, 35.60, 49.58, 56.48,
63.14, 125.06, 126.69, 127.10, 128.23, 128.29, 129.09, 138.18,
145.69, 169.09; ESI-MS m/z 309.1 for [M+H]+ and 331.1 for
[M+Na]+.
5.2.4. 1-(1-Azidocyclohexyl)benzene (7b)
Compound 7b was synthesized from 1-phenylcyclohexanol 6b
in 69% of yield as colorless liquid by following the similar proce-
dure used for synthesis of 7a. TLC: Rf = 0.82 (hexane/EtOAc = 8:2);
1H NMR (300 MHz, CDCl3) d (ppm): 1.42–1.99 (m, 10H, (CH2)5),
7.12–7.78 (m, 5H, Ar–H); 13C NMR (75 MHz, CDCl3) d (ppm):
22.37, 25.18, 35.87, 50.82, 66.56, 125.40, 127.10, 127.20, 127.47,
128.58, 128.70, 141.18, 144.29.
5.2.5. 1-Phenylcyclopentylamine (8a)
A solution of 1-phenylcyclopentylazide 7a (132 mg, 1.42 mmol)
in diethyl ether (Et2O) (1 mL) was added dropwise to a suspension
of lithium aluminum hydride (54 mg, 1.42 mmol) in Et2O (4 mL).
The mixture was stirred at room temperature under nitrogen for
3 h, whereupon the reaction was quenched with 1 N NaOH (aq).
The reaction mixture was then partitioned between Et2O (5 mL)
and 1 N HCl (aq) (5 mL). The aqueous layer was collected and basi-
fied with 28% NH4OH (aq) and extracted with CHCl3. The organic
layer was separated, dried over Na2SO4, filtered, and concentrated
under reduced pressure. The obtained crude product 8a (70.5 mg,
62%) was used directly for next step without further purification.
TLC: Rf = 0.19 (hexane/EtOAc = 1:1); ESI-MS m/z 162.10 for [M+H]+.
5.3.4. 1-Benzyl-4-phenylpiperidin-4-amine (12)
The N-(1-benzyl-4-phenylpiperidin-4-yl)acetamide 11 (86 mg,
0.28 mmol) was taken in 6 N HCl (aq) and refluxed for 10 h. Reac-
tion mixture was cooled, ice cold water was added followed by
alkalinization with 3 N NaOH (aq) and then extraction with EtOAc.
The organic layer was dried over Na2SO4, filtered, and concentrated
under reduced pressure. The obtained crude compound 12
(34.91 mg, 47%) was used directly for next step without further
purification. TLC: Rf = 0.27 (EtOAc); ESI-MS m/z 267.1 for [M+H]+.
5.4. Experimental data for pentapeptides (13–36)
5.4.1. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-3-hydroxy-4-oxo-
1-phenyl-4-(1-phenylethylamino)butan-2-ylamino)-4-methyl-
1-oxopentan-2-ylamino)-3-methyl-1-oxobutan-2-ylamino)-3-
oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-
carboxamide
5.2.6. 1-Phenylcyclohexylamine (8b)
Compound 8b was synthesized from 1-(1-azidocyclohexyl)ben-
zene 7b by similar procedure used for synthesis of compound 8a.
Yield: (98.5 mg, 67%); TLC: Rf = 0.21 (hexane/EtOAc = 1:1); ESI-MS
m/z 198.1 for [M+Na]+.
Compound 13: The title compound was prepared from (R)-(+)-a-
methylbenzylamine in following order: Boc–Pns–OH, Boc–Leu–OH,
Boc–Val–OH, Boc–DAP(Fmoc)–OH, and finally 5-fluoroorotic acid.
Yield: 39%; 1H NMR [270 MHz, CDCl3 + DMSO (9:1)] d 0.81–0.99
(m, 12H), 1.46 (d, J = 7 Hz, 3H), 1.51–1.70 (m, 3H), 2.19–2.31 (m,
1H), 2.77–2.94 (m, 2H), 3.66–3.72 (m, 1H), 3.90 (d, J = 2.6 Hz,
1H), 3.94–4.04 (m, 1H), 4.22–4.44 (m, 4H), 4.96 (q, J = 7.5 Hz,
1H), 7.13–7.22 (m, 4H), 7.28–7.31 (m, 3H), 7.53 (d, J = 9 Hz, 1H),
7.60–7.70 (m, 2H); TOF-MS (m/z): 776.1 for [M+Na]+.
5.3. Synthesis of 1-benzyl-4-phenylpiperidin-4-amine (12)
5.3.1. 1-Benzylpiperidin-4-one (9)
To a solution of 4-piperidone (307 mg, 1.99 mmol) in CH3CN
(5 mL), was added K2CO3 (690 mg, 4.99 mmol), followed by drop-
wise addition of benzylbromide (0.28 mL, 2.40 mmol) at room
temperature. This was refluxed at 85 °C for 16 h. The reaction mix-
ture was cooled and filtered to isolate the product. The remaining
solid was re-suspended in CH3CN and filtered again. The filtrates
were combined and evaporated in vacuo to give yellowish oily
product, which was further purified by column chromatography
(EtOAc/hexane = 3:7) to give compound 9 (294 mg, 78%) as color-
less oil. TLC: Rf = 0.15 (hexane/EtOAc = 8:2); 1H NMR (300 MHz,
CDCl3) d (ppm): 2.45 (t, J = 6.3 Hz, 4H, (CH2)2), 2.75 (t, J = 6.0 Hz,
4H, (CH2)2), 3.62 (s, 2H, Ar–CH2), 7.25–7.35 (m, 5H, ArH); 13C
NMR (75 MHz, CDCl3) d (ppm): 41.30, 52.91, 61.97, 127.31,
128.37, 128.88, 138.14, 209.17; ESI-MS m/z 190 for [M+H]+.
Compound 17: The title compound was prepared from (S)-(ꢂ)-
a
-methylbenzylamine in a manner similar to that described for
compound 13. Yield: 27%; 1H NMR [300 MHz, CDCl3 + DMSO
(9:1)] d 0.81–0.94 (m, 12H), 1.42 (d, J = 6.9 Hz, 3H), 1.48–1.59 (m,
3H), 2.18–2.24 (m, 1H), 2.70 (dd, J = 6.9 Hz, 1H), 2.85 (dd,
J = 7.8 Hz, 1H), 3.72–3.80 (m, 1H), 3.93 (d, J = 2.4 Hz, 1H), 3.97–
4.04 (m, 1H), 4.22–4.37 (m, 4H), 4.95 (q, J = 7.2 Hz, 1H), 7.12–
7.21 (m, 4H), 7.28–7.30 (m, 3H), 7.46 (d, J = 8.1 Hz, 1H), 7.72 (d,
J = 7.8 Hz, 1H), 7.82 (d, J = 6.6 Hz, 1H); TOF-MS (m/z): 753.8 for
[M+H]+.

5244
H. D. Tagad et al. / Bioorg. Med. Chem. 19 (2011) 5238–5246
5.4.2. N-((S)-2-Amino-3-((S)-1-((S)-3-cyclohexyl-1-((2S,3R)-3-
hydroxy-4-oxo-1-phenyl-4-(1-phenylethylamino)butan-2-
ylamino)-1-oxopropan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
tetrahydropyrimidine-4-carboxamide
7.15–7.20 (m, 5H), 7.27–7.32 (m, 5H), 7.49 (d, J = 7.3 Hz, 1H);
TOF-MS (m/z): 795.7 for [M+Na]+.
Compound 20: The title compound 20 was prepared from (S)-
(ꢂ)-
a-methylbenzylamine in a manner similar to that described
for compound 16. Yield: 43%; 1H NMR [300 MHz, CDCl3 + DMSO
(9:1)] d 0.71–0.96 (m, 8H), 1.06–1.25 (m, 5H), 1.40 (d, J = 7.2 Hz,
3H), 1.48–1.68 (m, 6H), 2.12–2.23 (m, 1H), 2.74 (dd, J = 7.2 Hz,
1H), 2.97 (dd, J = 8.1 Hz, 1H), 3.83–3.91 (m, 1H), 3.96–4.06 (m,
2H), 4.15 (t, J = 6.6 Hz, 1H), 4.27–4.36 (m, 2H), 4.48 (q, J = 7.8 Hz,
1H), 4.99 (q, J = 7.5 Hz, 1H), 6.79 (d, J = 9 Hz, 1H), 6.93 (dd,
J = 1.8 Hz, 1H), 7.12–7.22 (m, 5H), 7.25–7.32 (m, 4H), 7.36 (d,
J = 2.4 Hz, 1H), 7.45 (d, J = 8.5 Hz, 1H); TOF-MS (m/z): 796.1 for
[M+Na]+.
Compound 14: The title compound was prepared from (R)-(+)-a-
methylbenzylamine in following order: Boc–Pns–OH, Boc–Cha–OH,
Boc–Val–OH, Boc–DAP(Fmoc)–OH, and finally 5-fluoroorotic acid.
Yield: 48%; 1H NMR [270 MHz, CDCl3 + DMSO (9:1)] d 0.79–0.96
(m, 6 + 1H), 1.03–1.25 (m, 5H), 1.37–1.45 (m, 3 + 1H), 1.52–1.73
(m, 6H), 2.16–2.30 (m, 1H), 2.71 (dd, J = 8.1 Hz, 1H), 2.86–2.94 (m,
1H), 3.79–3.85 (m, 1H), 3.99–4.10 (m, 2H), 4.22–4.31 (m, 2H),
4.34–4.43 (m, 2H), 4.97 (q, J = 5.1 Hz, 1H), 7.13–7.22 (m, 4H), 7.24–
7.31 (m, 3H), 7.46 (d, J = 5.7 Hz, 1H), 7.53–7.60 (m, 1H), 7.65–7.70
(m, 1H); TOF-MS (m/z): 816.3 for [M+Na]+.
5.4.5. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-3-hydroxy-4-oxo-
1-phenyl-4-(2-phenylpropan-2-ylamino)butan-2-ylamino)-4-
methyl-1-oxopentan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
Compound 18: The title compound was prepared from (S)-(ꢂ)-
a
-methylbenzylamine in a manner similar to that described for
compound 14. Yield: 51%; 1H NMR [300 MHz, CDCl3 + DMSO
(9:1)] d 0.78–0.94 (m, 6 + 1H), 1.06–1.25 (m, 5H), 1.38–1.44 (m,
3 + 1H), 1.52–1.74 (m, 6H), 2.20–2.32 (m, 1H), 2.67–2.76 (m, 1H),
2.85–2.94 (m, 1H), 3.77–3.85 (m, 1H), 3.97–4.00 (m, 1H), 4.11–
4.20 (m, 1H), 4.25–4.34 (m, 2H), 4.36–4.44 (m, 2H), 4.98 (q,
J = 4.5 Hz, 1H), 7.17–7.23 (m, 4H), 7.26–7.29 (m, 3H), 7.43 (d,
J = 5 Hz, 1H), 7.60–7.67 (m, 2H); TOF-MS (m/z): 815.9 for [M+Na]+.
tetrahydropyrimidine-4-carboxamide (21)
Compound 21 was prepared from cumylamine in following or-
der: Boc–Pns–OH, Boc–Leu–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH,
and finally 5-fluoroorotic acid. Yield: 65%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.84–0.95 (m, 12H), 1.06–1.13 (m, 1H),
1.37–1.48 (m, 2H), 1.65 (s, 6H), 2.19–2.29 (m, 1H), 2.74 (dd,
J = 6.5 Hz, 1H), 2.89 (dd, J = 6.5 Hz, 1H), 3.86 (d, J = 4.2 Hz, 2H),
4.01–4.10 (m, 1H), 4.26–4.44 (m, 4H), 7.16–7.35 (m, 8H), 7.46 (d,
J = 10.2 Hz, 1H), 7.84 (d, J = 8.7 Hz, 1H); TOF-MS (m/z): 789.6 for
[M+Na]+.
5.4.3. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-3-hydroxy-4-oxo-
1-phenyl-4-(1-phenylethylamino)butan-2-ylamino)-4-methyl-
1-oxopentan-2-ylamino)-3-methyl-1-oxobutan-2-ylamino)-3-
oxopropyl)-2,5-dihydroxybenzamide
Compound 15: The title compound was prepared from (R)-(+)-
a
-
5.4.6. N-((S)-2-Amino-3-((S)-1-((S)-3-cyclohexyl-1-((2S,3R)-3-
hydroxy-4-oxo-1-phenyl-4-(2-phenylpropan-2-ylamino)butan-
2-ylamino)-1-oxopropan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
methylbenzylamine in following order: Boc–Pns–OH, Boc–Leu–OH,
Boc–Val–OH, Boc–DAP(Fmoc)–OH, and finally 2,5-dihydroxyben-
zoic acid. Yield: 61%; 1H NMR [270 MHz, CDCl3 + DMSO (9:1)] d
0.76 (d, J = 5.9 Hz, 3H), 0.78 (d, J = 5.9 Hz, 3H), 0.88 (d, J = 6.2 Hz,
6H), 1.23–1.36 (m, 3H + 3H), 2.0–2.07 (m, 1H), 2.62 (dd,
J = 6.5 Hz, 1H), 2.77 (dd, J = 5.1 Hz, 1H), 3.64–3.72 (m, 3H), 3.84
(d, J = 1.9 Hz, 1H), 4.13–4.32 (m, 3H), 4.86 (q, J = 6.7 Hz, 1H), 6.65
(d, J = 7.8 Hz, 1H), 6.89 (dd, J = 2.7 Hz, 1H), 7.14–7.30 (m, 10H),
7.54 (d, J = 8.3 Hz, 1H); TOF-MS (m/z): 755.9 for [M+Na]+.
tetrahydropyrimidine-4-carboxamide (22)
Compound 22 was prepared from cumylamine in following or-
der: Boc–Pns–OH, Boc–Cha–OH, Boc–Val–OH, Boc–DAP(Fmoc)–
OH, and finally 5-fluoroorotic acid. Yield: 67%; 1H NMR
[300 MHz, CDCl3 + DMSO (9:1)] d 0.79–0.94 (m, 6 + 2H), 1.06–
1.45 (m, 5H), 1.47–1.75 (m, 6 + 6H), 2.19–2.30 (m, 1H), 2.72–2.79
(m, 1H), 2.87–2.94 (m, 1H), 3.80–3.89 (m, 2H), 4.05–4.14 (m,
1H), 4.26–4.38 (m, 3H), 4.41–4.49 (m, 1H), 7.11–7.41 (m, 11H);
TOF-MS (m/z): 830.1 for [M+Na]+.
Compound 19: The title compound was prepared from (S)-(ꢂ)-
a
-methylbenzylamine in a manner similar to that described for
compound 15. Yield: 53%; 1H NMR [300 MHz, CDCl3 + DMSO
(9:1)] d 0.77 (d, J = 6.9 Hz, 3H), 0.83 (d, J = 5.1 Hz, 3H), 0.92 (d,
J = 6.6 Hz, 6H), 1.44 (d, J = 5.4 Hz, 3H), 1.47–1.57 (m, 3H), 2.10–
2.21 (m, 1H), 2.58 (dd, J = 9 Hz, 1H), 3.00 (dd, J = 8.4 Hz, 1H),
3.82–3.85 (m, 1H), 3.91–3.95 (m, 1H), 4.03 (d, J = 2.4 Hz, 1H),
4.15 (t, J = 6.3 Hz, 1H), 4.29–4.33 (m, 2H), 4.43–4.50 (m, 1H), 4.99
(q, J = 7.5 Hz, 1H), 6.79 (d, J = 8.1 Hz, 1H), 6.94 (dd, J = 3.3 Hz, 1H),
7.13–7.17 (m, 5H), 7.26–7.28 (m, 2H), 7.35 (d, J = 2.7 Hz, 1H),
7.45 (d, J = 8.1 Hz, 1H), 7.65 (dd, J = 8.1 Hz, 2H); TOF-MS (m/z):
756.2 for [M+Na]+.
5.4.7. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-3-hydroxy-4-oxo-
1-phenyl-4-(2-phenylpropan-2-ylamino)butan-2-ylamino)-4-
methyl-1-oxopentan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide (23)
Compound 23 was prepared from cumylamine in following or-
der: Boc–Pns–OH, Boc–Leu–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH,
and finally 2,5-dihydroxybenzoic acid. Yield: 57%; 1H NMR
[300 MHz, CDCl3 + DMSO (9:1)] d 0.79 (d, J = 6.3 Hz, 3H), 0.85 (d,
J = 6 Hz, 3H), 0.92–0.97 (m, 6H), 1.35–1.42 (m, 1H), 1.53–1.60 (m,
2H), 1.66 (s, 6H), 2.19–2.29 (m, 1H), 2.71 (dd, J = 6.9 Hz, 1H), 2.96
(dd, J = 8.1 Hz, 1H), 3.90–3.94 (m, 2H), 4.05–4.13 (m, 1H), 4.20–
4.31 (m, 3H), 4.45 (q, J = 8.1 Hz, 1H), 6.76 (d, J = 9 Hz, 1H), 6.90
(dd, J = 3.3 Hz, 1H), 7.13–7.19 (m, 7H), 7.25–7.30 (m, 2H), 7.34–
7.37 (m, 2H); TOF-MS (m/z): 770.1 for [M+Na]+.
5.4.4. N-((S)-2-Amino-3-((S)-1-((S)-3-cyclohexyl-1-((2S,3R)-3-
hydroxy-4-oxo-1-phenyl-4-(1-phenylethylamino)butan-2-
ylamino)-1-oxopropan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide
Compound 16: The title compound was prepared from (R)-(+)-a-
methylbenzylamine in following order: Boc–Pns–OH, Boc–Cha–
OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and finally 2,5-dihydroxy-
benzoic acid. Yield: 35%; 1H NMR [270 MHz, CDCl3 + DMSO (9:1)]
d 0.72–0.95 (m, 8H), 1.06–1.25 (m, 5H), 1.43 (d, J = 6.7 Hz, 3H),
1.54–1.66 (m, 6H), 2.12–2.20 (m, 1H), 2.74 (dd, J = 8.1 Hz, 1H),
2.95–3.03 (m, 1H), 3.71–3.81 (m, 1H), 3.89–4.05 (m, 2H), 4.16 (t,
J = 6.1 Hz, 1H), 4.29–4.38 (m, 2H), 4.44–4.47 (m, 1H), 4.98 (q,
J = 8.1 Hz, 1H), 6.79 (d, J = 8.6 Hz, 1H), 6.93 (dd, J = 2.2 Hz, 1H),
5.4.8. N-((S)-2-Amino-3-((S)-1-((S)-3-cyclohexyl-1-((2S,3R)-3-
hydroxy-4-oxo-1-phenyl-4-(2-phenylpropan-2-ylamino)butan-
2-ylamino)-1-oxopropan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide (24)
Compound 24 was prepared from cumylamine in following or-
der: Boc–Pns–OH, Boc–Cha–OH, Boc–Val–OH, Boc–DAP(Fmoc)–
OH, and finally 2,5-dihydroxybenzoic acid. Yield: 49%; 1H NMR

H. D. Tagad et al. / Bioorg. Med. Chem. 19 (2011) 5238–5246
5245
[300 MHz, CDCl3 + CD3OD (9:1)] d 0.67–0.83 (m, 2H), 0.88–0.90 (m,
5.4.13. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-3-hydroxy-4-
6H), 1.01–1.22 (m, 4H), 1.43–1.48 (m, 2H), 1.51–1.63 (m, 5 + 6H),
1.99–2.11 (m, 1H), 2.72 (dd, J = 8.4 Hz, 1H), 3.05 (dd, J = 7.5 Hz,
1H), 3.49 (dd, J = 4.8 Hz, 1H), 3.83 (dd, J = 3.9 Hz, 1H), 3.92–3.95
(m, 2H), 4.11–4.19 (m, 2H), 4.28–4.33 (m, 1H), 6.79 (d, J = 9.3 Hz,
1H), 6.90 (dd, J = 3 Hz, 1H), 7.12–7.19 (m, 5H), 7.21–7.29 (m, 6H);
TOF-MS (m/z): 809.8 for [M+Na]+.
oxo-1-phenyl-4-(1-phenylcyclohexylamino)butan-2-ylamino)-
4-methyl-1-oxopentan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
tetrahydropyrimidine-4-carboxamide (29)
Compound 29 was prepared from amine 8b in following order:
Boc–Pns–OH, Boc–Leu–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 5-fluoroorotic acid. Yield: 69%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.83–0.93 (m, 12H), 1.25–1.44 (m, 3H),
1.51–1.80 (m, 8H), 2.2–2.38 (m, 3H), 2.71 (dd, J = 6.9 Hz, 1H),
2.89 (dd, J = 7.5 Hz, 1H), 3.72–3.78 (m, 1H), 3.87 (d, J = 2.1 Hz,
1H), 3.97–4.06 (m, 1H), 4.23–4.42 (m, 4H), 7.12–7.16 (m, 2H),
7.21–7.35 (m, 6H), 7.45 (d, J = 8.7 Hz, 1H); TOF-MS (m/z): 829.4
for [M+Na]+.
5.4.9. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-3-hydroxy-4-oxo-
1-phenyl-4-(1-phenylcyclopentylamino)butan-2-ylamino)-4-
methyl-1-oxopentan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
tetrahydropyrimidine-4-carboxamide (25)
Compound 25 was prepared from amine 8a in following order:
Boc–Pns–OH, Boc–Leu–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 5-fluoroorotic acid. Yield: 35%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.84–0.94 (m, 12H), 1.36–1.44 (m, 1H),
1.55–1.63 (m, 2H), 1.71–1.83 (m, 4H), 1.97–2.03 (m, 2H), 2.21–
2.39 (m, 3H), 2.71 (dd, J = 7.2 Hz, 1H), 2.82 (dd, J = 8.1 Hz, 1H),
3.73–3.83 (m, 2H), 3.98–4.07 (m, 1H), 4.23–4.42 (m, 4H), 7.11–
7.16 (m, 2H), 7.21–7.26 (m, 6H), 7.34 (d, J = 6.9 Hz, 2H); TOF-MS
(m/z): 815.74 for [M+Na]+.
5.4.14. N-((S)-2-Amino-3-((S)-1-((S)-3-cyclohexyl-1-((2S,3R)-3-
hydroxy-4-oxo-1-phenyl-4-(1-phenylcyclohexylamino)butan-
2-ylamino)-1-oxopropan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
tetrahydropyrimidine-4-carboxamide (30)
Compound 30 was prepared from amine 8b in following order:
Boc–Pns–OH, Boc–Cha–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 5-fluoroorotic acid. Yield: 50%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.72–1.02 (m, 6 + 3H), 1.04–1.32 (m, 6H),
1.38–1.85 (m, 14H), 2.18–2.40 (m, 3H), 2.70 (dd, J = 7.6 Hz, 1H),
2.90 (dd, J = 4.6 Hz, 1H), 3.72–3.85 (m, 1H), 3.89 (s, 1H), 3.97–
4.12 (m, 1H), 4.19–4.5 (m, 2H), 7.15–7.45 (m, 10H); TOF-MS (m/
z): 870.4 for [M+Na]+.
5.4.10. N-((S)-2-Amino-3-((S)-1-((S)-3-cyclohexyl-1-((2S,3R)-3-
hydroxy-4-oxo-1-phenyl-4-(1-phenylcyclopentylamino)butan-
2-ylamino)-1-oxopropan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
tetrahydropyrimidine-4-carboxamide (26)
Compound 26 was prepared from amine 8a in following order:
Boc–Pns–OH, Boc–Cha–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 5-fluoroorotic acid. Yield: 57%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.79–0.94 (m, 6+2H), 1.07–1.18 (m, 3H),
1.25–1.48 (m, 2H), 1.50–1.80 (m, 10H), 1.95–2.09 (m, 2H), 2.22–
2.40 (m, 2 + 1H), 2.71 (dd, J = 7.5 Hz, 1H), 2.87 (dd, J = 6.3 Hz, 1H),
3.77–3.87 (m, 2H), 4.04–4.14 (m, 1H), 4.26–4.33 (m, 3H), 4.38–
4.46 (m, 1H), 7.11–7.27 (m, 7H), 7.34 (d, J = 4.2 Hz, 2H), 7.46 (d,
J = 8.1 Hz, 1H); TOF-MS (m/z): 856.1 for [M+Na]+.
5.4.15. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-3-hydroxy-4-
oxo-1-phenyl-4-(1-phenylcyclohexylamino)butan-2-ylamino)-
4-methyl-1-oxopentan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide (31)
Compound 31 was prepared from amine 8b in following order:
Boc–Pns–OH, Boc–Leu–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 2,5-dihydroxybenzoic acid. Yield: 54%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.76 (d, J = 5.1 Hz, 3H), 0.86–0.92 (m, 9H),
2.28 (d, J = 11.7 Hz, 1H), 1.48–1.55 (m, 6H), 1.64–1.86 (m, 6H),
2.06–2.12 (m, 1H), 2.20 (d, J = 13.2 Hz, 1H), 2.36 (d, J = 13.5 Hz,
1H), 2.67–2.74 (m, 1H), 3.11–3.18 (m, 1H), 3.39–3.40 (m, 1H),
3.77–3.83 (m, 1H), 3.99 (d, J = 3.3 Hz, 1H), 4.13–4.19 (m, 1H),
4.29–4.34 (m, 1H), 6.82 (d, J = 9 Hz, 1H), 6.93 (dd, J = 2.7 Hz, 1H),
7.15–7.34 (m, 11H); TOF-MS (m/z): 809.4 for [M+Na]+.
5.4.11. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-3-hydroxy-4-
oxo-1-phenyl-4-(1-phenylcyclopentylamino)butan-2-ylamino)-
4-methyl-1-oxopentan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide (27)
Compound 27 was prepared from amine 8a in following order:
Boc–Pns–OH, Boc–Leu–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 2,5-dihydroxybenzoic acid. Yield: 46%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.74 (d, J = 6.6 Hz, 3H), 0.85–0.92 (m, 9H),
1.39–1.53 (m, 4H), 1.75–1.83 (m, 4H), 2.04–2.12 (m, 3H), 2.23–
2.29 (m, 2H), 2.67 (dd, J = 9 Hz, 1H), 3.57 (dd, J = 6 Hz, 1H), 3.91–
4.00 (m, 3H), 4.04–4.29 (m, 3H), 6.81 (d, J = 9 Hz, 1H), 6.91 (dd,
J = 3 Hz, 1H), 7.16–7.34 (m, 11H); TOF-MS (m/z): 796.4 for
[M+Na]+.
5.4.16. N-((S)-2-Amino-3-((S)-1-((S)-3-cyclohexyl-1-((2S,3R)-3-
hydroxy-4-oxo-1-phenyl-4-(1-phenylcyclohexylamino)butan-
2-ylamino)-1-oxopropan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide (32)
Compound 32 was prepared from amine 8b in following order:
Boc–Pns–OH, Boc–Cha–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 2,5-dihydroxybenzoic acid. Yield: 41%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.73–0.93 (m, 6H), 1.05–1.25 (m, 6H),
1.40–1.83 (m, 17H), 2.12–2.18 (m, 1H), 2.26 (d, J = 13.5 Hz, 1H),
2.37 (d, J = 11.7 Hz, 1H), 2.66–2.73 (m, 1H), 3.03–3.10 (m, 1H),
3.69–3.74 (m, 1H), 3.98 (d, J = 2.7 Hz, 1H), 4.14 (t, J = 12.6 Hz,
1H), 4.22–4.37 (m, 2H), 6.78 (d, J = 9 Hz, 1H), 6.91 (dd, J = 2.4 Hz,
1H), 7.11–7.49 (m, 10H), 7.64 (d, J = 7.2 Hz, 1H); TOF-MS (m/z):
849.8 for [M+Na]+.
5.4.12. N-((S)-2-Amino-3-((S)-1-((S)-3-cyclohexyl-1-((2S,3R)-3-
hydroxy-4-oxo-1-phenyl-4-(1-phenylcyclopentylamino)butan-
2-ylamino)-1-oxopropan-2-ylamino)-3-methyl-1-oxobutan-2-
ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide (28)
Compound 28 was prepared from amine 8a in following order:
Boc–Pns–OH, Boc–Cha–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 2,5-dihydroxybenzoic acid. Yield: 59%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.91 (d, J = 6.6 Hz, 6H), 1.07–1.26 (m, 6H),
1.43–1.50 (m, 2H), 1.54–1.63 (m, 5H), 1.75–1.82 (m, 4H), 2.05–
2.12 (m, 2H), 2.19–2.29 (m, 2H), 2.67 (dd, J = 5.4 Hz, 1H), 3.06
(dd, J = 6.3 Hz, 2H), 3.56 (dd, J = 4.8 Hz, 1H), 3.89–3.99 (m, 2H),
4.16–4.22 (m, 2H), 4.27–4.32 (m, 2H), 6.81 (d, J = 8.7 Hz, 1H),
6.92 (dd, J = 3 Hz, 1H), 7.14–7.33 (m, 11H); TOF-MS (m/z): 836.4
for [M+Na]+.
5.4.17. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-4-(1-benzyl-4-
phenylpiperidin-4-ylamino)-3-hydroxy-4-oxo-1-phenylbutan-
2-ylamino)-4-methyl-1-oxopentan-2-ylamino)-3-methyl-1-
oxobutan-2-ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
tetrahydropyrimidine-4-carboxamide (33)
Compound 33 was prepared from amine 12 in following order:
Boc–Pns–OH, Boc–Leu–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and

5246
H. D. Tagad et al. / Bioorg. Med. Chem. 19 (2011) 5238–5246
finally 5-fluoroorotic acid. Yield: 44%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.79 (d, J = 5.7 Hz, 3H), 0.86 (d, J = 5.6 Hz,
3H), 0.91–0.96 (m, 8H), 1.19–1.39 (m, 4H), 1.45–1.51 (m, 2H),
2.02–2.09 (m, 2H), 2.64–2.89 (m, 4H), 3.57–3.75 (m, 3H), 3.87–
3.90 (m, 1H), 4.13–4.44 (m, 5H), 7.10–7.29 (m, 10H), 7.31–7.38
(m, 5H); TOF-MS (m/z): 898.4 for [M+H]+.
Supplementary data
Supplementary data (general experimental procedures, MALDI-
TOF mass and HPLC profiles of compounds 13–36, the docking
study, and assay protocol) associated with this article can be found,
in the online version, at doi:10.1016/j.bmc.2011.07.002.
5.4.18. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-4-(1-benzyl-4-
phenylpiperidin-4-ylamino)-3-hydroxy-4-oxo-1-phenylbutan-
2-ylamino)-3-cyclohexyl-1-oxopropan-2-ylamino)-3-methyl-1-
oxobutan-2-ylamino)-3-oxopropyl)-5-fluoro-2,6-dioxo-1,2,3,6-
tetrahydropyrimidine-4-carboxamide (34)
Compound 34 was prepared from amine 12 in following order:
Boc–Pns–OH, Boc–Cha–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 5-fluoroorotic acid. Yield: 74%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.71–0.92 (m, 2H), 0.98 (d, J = 7.3 Hz, 6H),
1.06–1.43 (m, 8H), 1.51–1.66 (m, 8H), 2.13–2.37 (m, 3H), 2.75–
2.88 (m, 2H), 3.22 (d, J = 11.7 Hz, 2H), 3.34–3.38 (m, 1H), 3.97–
4.13 (m, 2 + 2H), 4.32–4.43 (m, 2H), 4.51–4.61 (m, 1H), 7.13–7.31
(m, 10H), 7.39–7.41 (m, 3H), 7.59–7.62 (m, 2H); TOF-MS (m/z):
939.4 for [M+H]+.
References and notes
1. Nguyen, J.; Yamani, A.; Kiso, Y. Curr. Pharm. Des. 2006, 12, 4295.
2. Zimmermann, M.; Gardoni, F.; Di Luca, M. Drugs Aging 2005, 22, 27.
3. Citron, M. Nat. Rev. Neurosci. 2004, 5, 677.
4. Roberson, E. D.; Mucke, L. Science 2006, 314, 781.
5. Goedert, M.; Spillantini, M. G. Science 2006, 314, 777.
6. Selkoe, D. J. Physiol. Rev. 2001, 81, 741.
7. Sinha, S.; Lieberburg, I. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 11049.
8. Vassar, R. J. Mol. Neurosci. 2004, 23, 105.
9. Cole, S. L.; Vassar, R. Mol. Neurodegeneration 2007, 2, 22.
10. Lundkvist, J.; Naslund, J. Curr. Opin. Pharmacol. 2007, 7, 112.
11. Johnstone, J. A.; Liu, W. W.; Todd, S. A.; Coulson, D. T. R.; Murphy, S.; Irvine, G.
B.; Passmore, A. P. Biochem. Soc. Trans. 2005, 33, 1096.
12. Willem, M.; Garratt, A. N.; Novak, B.; Citron, M.; Kaufmann, S.; Rittger, A.;
DeStrooper, B.; Saftig, P.; Birchmeier, C.; Haass, C. Science 2006, 314, 664.
13. Nishitomi, K.; Sakaguchi, G.; Horikoshi, Y.; Gray, A. J.; Maeda, M.; Hirata-Fukae,
C.; Becker, A. G.; Hosono, M.; Sakaguchi, I.; Minami, S. S.; Nakajima, Y.; Li, H.-F.;
Takeyama, C.; Kihara, T.; Ota, A.; Wong, P. C.; Aisen, P. S.; Kato, A.; Kinoshita, N.;
Matsuoka, Y. Neurochemistry 2006, 99, 1555.
5.4.19. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-4-(1-benzyl-4-
phenylpiperidin-4-ylamino)-3-hydroxy-4-oxo-1-phenylbutan-
2-ylamino)-4-methyl-1-oxopentan-2-ylamino)-3-methyl-1-
oxobutan-2-ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide
(35)
Compound 35 was prepared from amine 12 in following order:
Boc–Pns–OH, Boc–Leu–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 2,5-dihydroxybenzoic acid. Yield: 35%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.79 (d, J = 6 Hz, 3H), 0.87 (d, J = 6 Hz, 3H),
0.95–0.98 (m, 6H), 1.25–1.33 (m, 1H), 1.41–1.50 (m, 2H), 2.09–
2.15 (m, 1H), 2.21–2.28 (m, 2H), 2.79–2.91 (m, 4H), 3.12–3.35
(m, 4H), 4.02–4.03 (m, 2H), 4.09 (s, 2H), 4.31–4.36 (m, 2H), 4.41–
4.50 (m, 2H), 4.62–4.64 (m, 1H), 6.78 (d, J = 8.8 Hz, 1H), 6.96 (dd,
J = 2.4 Hz, 1H), 7.12–7.34 (m, 14H), 7.55–7.61 (m, 2H); TOF-MS
(m/z): 878.7 for [M+H]+.
14. Roberds, S. L.; Anderson, J.; Basi, G.; Bienkowski, M. J.; Branstetter, D. G.; Chen,
K. S.; Freedman, S.; Frigon, N. L.; Games, D.; Hu, K.; Johnson-Wood, K.;
Kappenman, K. E.; Kawabe, T.; Kola, I.; Kuehn, R.; Lee, M.; Liu, W.; Motter, R.;
Nichols, N. F.; Power, M.; Robertson, D. W.; Schenk, D.; Schoor, M.; Shopp, G.
M.; Shuck, M. E.; Sihna, S.; Svensson, K. A.; Tatsuno, G.; Tintrup, H.; Wijsman, J.;
Wright, S.; McConlogue, L. Hum. Mol. Genet. 2001, 10, 1317.
15. Luo, Y.; Bolon, B.; Damore, M. A.; Fitzpatrick, D.; Liu, H.; Zhang, J.; Yan, Q.;
Vassar, R.; Citron, M. Neurobiol. Dis. 2003, 14, 81.
16. Ohno, M.; Sametsky, E. A.; Younkin, L. H.; Oakley, H.; Younkin, S. G.; Citron, M.;
Vassar, R.; Disterhoft, J. F. Neuron 2004, 41, 27.
17. John, V.; Beck, J. P.; Bienkowski, M. J.; Sinha, S.; Heinrikson, R. L. J. Med. Chem.
2003, 46, 4625.
18. McConlogue, L.; Buttini, M.; Anderson, J. P.; Brigham, E. F.; Chen, K. S.;
Freedman, S. B.; Games, D.; Johnson-Wood, K.; Lee, M.; Zeller, M.; Liu, W.;
Motter, R.; Sinha, S. J. Biol. Chem. 2007, 282, 26326.
19. Kimura, T.; Shuto, D.; Kasai, S.; Liu, P.; Hidaka, K.; Hamada, T.; Hayashi, Y.;
Hattori, C.; Asai, M.; Kitazume, S.; Saido, T. C.; Ishiura, S.; Kiso, Y. Bioorg. Med.
Chem. Lett. 2004, 14, 1527.
20. 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.
21. Ghosh, A. K.; Shin, D.; Downs, D.; Koelsch, G.; Lin, X.; Ermolieff, J.; Tang, J. J. Am.
Chem. Soc. 2000, 122, 3522.
22. Pardridge, W. M. Mol. Interv. 2003, 3, 90.
5.4.20. N-((S)-2-Amino-3-((S)-1-((S)-1-((2S,3R)-4-(1-benzyl-4-
phenylpiperidin-4-ylamino)-3-hydroxy-4-oxo-1-phenylbutan-
2-ylamino)-3-cyclohexyl-1-oxopropan-2-ylamino)-3-methyl-1-
oxobutan-2-ylamino)-3-oxopropyl)-2,5-dihydroxybenzamide
(36)
Compound 36 was prepared from amine 12 in following order:
Boc–Pns–OH, Boc–Cha–OH, Boc–Val–OH, Boc–DAP(Fmoc)–OH, and
finally 2,5-dihydroxybenzoic acid. Yield: 67%; 1H NMR [300 MHz,
CDCl3 + DMSO (9:1)] d 0.74–0.97 (m, 9H), 1.04–1.15 (m, 5H),
1.26–1.44 (m, 3H), 1.48–1.64 (m, 6H), 2.02–2.16 (m, 2H), 2.2–2.3
(m, 1H), 2.65 (d, J = 12 Hz, 1H), 2.77–2.93 (m, 3H), 3.29–3.42 (m,
3H), 3.63–3.77 (m, 1H), 3.92 (d, J = 6.6 Hz, 1H), 4.10 (bs, 1H), 4.18
(d, J = 5.7 Hz, 1H), 4.30–4.38 (m, 1H), 4.47–4.52 (m, 1H), 6.82 (d,
J = 8.7 Hz, 1H), 6.95 (dd, J = 2.7 Hz, 1H), 7.15–7.51 (m, 16H); TOF-
MS (m/z): 918.7 for [M+H]+.
23. Kimura, T.; Shuto, D.; Hamada, Y.; Igawa, N.; Kasai, S.; Liu, P.; Hidaka, K.;
Hamada, T.; Hayashi, Y.; Kiso, Y. Bioorg. Med. Chem. Lett. 2005, 15, 211.
24. Hamada, Y.; Igawa, N.; Ikari, H.; Ziora, Z.; Nguyen, J.-T.; Yamani, A.; Hidaka, K.;
Kimura, T.; Saito, K.; Hayashi, Y.; Ebina, M.; Ishiura, S.; Kiso, Y. Bioorg. Med.
Chem. Lett. 2006, 43, 4354.
25. Tagad, H. D.; Hamada, Y.; Nguyen, J.-T.; Hamada, T.; Abdel-Rahman, H.;
Yamani, A.; Nagamine, A.; Ikari, H.; Igawa, N.; Hidaka, K.; Sohma, Y.; Kimura, T.;
Kiso, Y. Bioorg. Med. Chem. 2010, 18, 3175.
26. Sealy, J. M.; Truong, A. P.; Tso, L.; Probst, G. D.; Aquino, J.; Hom, R. K.;
Jagodzinska, B. M.; Dressen, D.; Wone, D. W. G.; Brogley, L.; John, V.; Tung, J. S.;
Pleiss, M. A.; Tucker, J. A.; Konradi, A. W.; Dappen, M. S.; Toth, G.; Pan, H.;
Ruslim, L.; Miller, J. Bioorg. Med. Chem. Lett. 2009, 19, 6386.
27. Lerchner, A.; Machauer, R.; Betschart, C.; Veenstra, S.; Rueeger, H.; McCarthy,
C.; Tintelnot-Blomley, M.; Jaton, A. L.; Rabe, S.; Desrayaud, S.; Enz, A.;
Staufenbiel, M.; Paganetti, P.; Rondeau, J. M.; Neumann, U. Bioorg. Med. Chem.
Lett. 2010, 20, 603.
28. Hanessian, S.; Shao, Z.; Betschart, C.; Rondeau, J. M.; Neumann, U.; Tintelnot-
Blomley, M. Bioorg. Med. Chem. Lett. 2010, 20, 1924.
29. Kortum, S. W.; Benson, T. E.; Bienkowski, M. J.; Emmons, T. L.; Prince, D. B.;
Paddock, D. J.; Tomasselli, A. G.; Moon, J. B.; LaBordea, A.; TenBrink, R. E. Bioorg.
Med. Chem. Lett. 2007, 17, 3378.
Acknowledgements
This research was supported in part by the ‘Academic Frontier’
Project for Private Universities: a matching fund subsidy from
MEXT (Ministry of Education, Culture, Sports, Science and Technol-
ogy), and the 21st Century Centre of Excellence Program ‘Develop-
ment of Drug Discovery Frontier Integrated from Tradition to
Proteome’ from MEXT. We are grateful to H.-O. Kumada for mass
spectra measurements, and to Dr. Shunjiro Ogawa for valuable dis-
cussions regarding the NMR spectroscopy.
30. Theerthagirl, P.; Lalitha, A.; Arunachalam, P. N. Tetrahedron Lett. 2010, 51, 2813.
and references therein.
31. Krimen, L. I.; Cota, D. J. Org. React. 1969, 17, 213.
32. Nitta, A.; Fujii, H.; Sakami, S.; Nishimura, Y.; Ohyama, T.; Satoh, M.; Nakaki, J.;
Satoh, S.; Inada, C.; Kozono, H.; Kumagai, H.; Shimamura, M.; Fukazawa, T.;
Kawai, H. Bioorg. Med. Chem. Lett. 2008, 18, 5435.
33. Cumming, J.; Babu, S.; Huang, Y.; Carrol, C.; Chen, X.; Favreau, L.; Greenlee, W.;
Guo, T.; Kennedy, M.; Kuvelkar, R.; Le, T.; Li, G.; McHugh, N.; Orth, P.; Ozgur, L.;
Parker, E.; Saionz, K.; Stamford, A.; Strickland, C.; Tadesse, D.; Voigt, J.; Zhang,
L.; Zhang, Q. Bioorg. Med. Chem. Lett. 2010, 20, 2837.