Synthesis of β-secretase inhibitors containing a hydroxyethylene dipeptide isostere

Article abstract of DOI:10.1080/00397910600977509

β-Secretase inhibitors with a Leu* Ala hydroxyethylene dipeptide (HED) isostere have been an especially interesting topic in recent years. In this study, a template compound 17 was synthesized, featuring truncation at the P2′ position and changes at P2 an

Full text of DOI:10.1080/00397910600977509

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Synthesis of β‐Secretase Inhibitors Containing
a Hydroxyethylene Dipeptide Isostere
Xiaoming Yang a , Xiaomin Zou a , Yiqiu Fu a , Ke Mou a , Gang Fu a , Chao Ma
a & Ping Xu a
a Department of Medicinal Chemistry , School of Pharmaceutical Sciences,
State Key Laboratory of Natural and Biomimetic Drugs, Peking University ,
Beijing, China
Published online: 25 Jul 2007.
To cite this article: Xiaoming Yang , Xiaomin Zou , Yiqiu Fu , Ke Mou , Gang Fu , Chao Ma & Ping Xu
(2007) Synthesis of β‐Secretase Inhibitors Containing a Hydroxyethylene Dipeptide Isostere, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:1,
9-24, DOI: 10.1080/00397910600977509
To link to this article: http://dx.doi.org/10.1080/00397910600977509
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Synthetic Communicationsw, 37: 9–24, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600977509
Synthesis of b-Secretase Inhibitors
Containing a Hydroxyethylene Dipeptide
Isostere
Xiaoming Yang, Xiaomin Zou, Yiqiu Fu, Ke Mou, Gang Fu,
Chao Ma, and Ping Xu
Department of Medicinal Chemistry, School of Pharmaceutical Sciences,
State Key Laboratory of Natural and Biomimetic Drugs, Peking
University, Beijing, China
Abstract: b-Secretase inhibitors with a LeuꢀAla hydroxyethylene dipeptide (HED)
isostere have been an especially interesting topic in recent years. In this study, a
template compound 17 was synthesized, featuring truncation at the P20 position and
changes at P2 and P3, which differs from other reported potent inhibitors. The
purpose was to explore optimal reaction conditions and construct an inhibitor library
to investigate ideal protein–substrate interaction.
Keywords: hydroxyethylene dipeptide isostere, peptidomimetics, b-secretase,
synthesis
INTRODUCTION
b-Amyloid plaques, a primary pathological characteristic of Alzheimer’s
disease, are generated through a sequential hydrolysis of APP (amyloid
precursor protein), catalyzed by two proteases, b-secretase and g-secretase.
During this process, the hydrolysis via b-secretase was well known to be
upstream to the cleavage induced by g-secretase.[1,2] The inhibition of
b-secretase is believed to be a desirable approach to preventing the
formation of b-amyloid. Up to now, a great number of b-secretase inhibitors
Received in R.O.C. March 24, 2006
Address correspondence to Ping Xu, Department of Medicinal Chemistry, School of
Pharmaceutical Sciences, State Key Laboratory of Natural and Biomimetic Drugs,
Peking University, Beijing 100083, China. E-mail: pingxu@bjmu.edu.cn
9

10
X. Yang et al.
have been designed and synthesized, based on various transition-state analogs,
such as hydroxyethylene dipeptide (HED),[3] HEA (hydroxyethylamine
dipeptide),[4] statine,[5] and the like, of which the LeuꢀAla hydroxyethylene
dipeptide isostere was the most successful. Two potent inhibitors OM99-2
(EVNLꢀAAEF, Ki ¼ 1.4 nM) and OM00-3 (ELDLꢀAVEF, Ki ¼ 0.3 nM)
were synthesized and reported first by Gosh et al. in 2001 and 2002.[3] Impen-
etration through the brain–blood barrier limited the compounds’ further
application in vivo because of the high molecular length and weight.
Therefore, the emphasis was naturally directed to smaller molecular
entities, which were shorter in length but retained their binding capability.
The GT-1017 was a successful optimization, with a much shorter length
and a Ki of 2.5 nM.[3]
In view of these facts, a template peptidomimetic compound (Fig. 1) in
0
our project was designed and characterized by truncation of the P2 side
chain and variations at the P2 and P3 positions. The purpose was to
maintain the interaction with b-secretase in0 the S2 and S3 pocket while
removing the much weaker binding of the P2 side chain. At the P2 and P3
positions, versatile structures were introduced, including non–amino acid
scaffolds, especially P3 capping. Considering the importance of LeuꢀAla
(LꢀA), our group followed the LꢀA modality in this work to detail the
synthesis of such inhibitors and provide more information on the structural
optimization and modification, which might accelerate finding of promising
candidates.
RESULTS AND DISCUSSION
To common knowledge, the synthesis of HED has been published in more
than 20 references over the past two decades,[6] and strategies include, the
3S,4S-g-lactone strategy or using saccharides or other small molecules as a
starting point. Because of a certain hindrance in the route described in
Scheme 1, our group first adopted a relatively longer route to synthesize
Figure 1. Template structure of target peptidomimetics.

b-Secretase Inhibitors
11
Scheme 1. Reagents and conditions: (i) ethyl propiolate, LDA (n-BuLi and i-Pr2NH,
2238C in THF, 1 h), 2788C, N2, 4.5 h; and (ii) H2/Pd, 60 psi, EtOAc, then reflux in
toluene/acetic acid (97.5:2.5 v/v).
lactone 10 (Scheme 2).[7] All the amino acids used herein had an L
configuration.
Boc-Leu-OH 1 was initially reacted with CH3I to afford methyl ester
2.[8] Then 2 was reduced by NaBH4 to give Boc-Leucinal 3, which was
[9]
further oxidized by Py SO3 to furnish Boc-Leucinal 4. We also tried to
.
reduce 2 to 4 in one step by diisobutylaluminum hydride (DIBAL) as
reported[10]; however, the solvent toluene was hard to be completely
remove from the reaction mixture, and 4 would readily undergo racemization
during distillation under reduced pressure at a temperature of more than
308C.[11] The Boc-Leucinal 4 was then treated with anhydrous EtOAc to
Scheme 2. Reagents and conditions: (i) NaHCO3, CH3I, DMF; (ii) NaBH4, CaCl2,
.
THF/EtOH; (iii) Py SO3, TEA, DCM/DMSO; (iv) DIBAL, toluene, 08C, 10 min;
(v) n-BuLi, i-Pr2NH, ethyl acetate, 2788C; (vi) 2-methoxy-propene, POCl3, TEA,
CH2Cl2; (vii) DIBAL, THF, 08C; (viii) MsCl, TEA, CH2Cl2, then NaCN, DMSO,
508C; and (ix) (1) H2O2, NaOH (1 N), ethanol, 08C; (2) Na2S2O3 (20%), 2308C; (3)
water–acetic acid ¼ 35:65, 3 days, reflux in toluene.

12
X. Yang et al.
give the addition product SS hydroxyl ester 5 (less polar on thin-layer chrom-
atography, TLC, according to Rich) and its diastereomer under the aid of
lithium diisopropylamide (LDA).[10] However, because of lack of specificity,
the 3S,4S- and 3R,4S-diastereomers were obtained in amounts that were not
far from each other; therefore the low yield of the SS diastereomer rendered
this reaction unfavorable en route to lactone 10. Protection of 5 with
2-methoxy-propene allowed successful reduction of the ester group of 6 to
alcohol 7 by DIBAL in a yield of 80%. The capillary holding the catalyst
POCl3 should be quickly dipped into the mixture before the liquid within
turned brown (as a result of interaction of POCl3 with 2-methoxy-propene
vapor), which would otherwise result in unexpected side reactions and
obstacles for isolation. Besides, to remove the DIBAL left after the
reaction, Rochester salt (potassium sodium tartate, saturated in H2O)
should be added in excess and violently shaken with DIBAL for the
complex to be formed, or else the reaction mixture would be extremely
viscous and hard to extract. Then alcohol 7 was treated with MsCl to give
the sulfonate, followed by further substitution by the CN group to generate
nitrile 8. The formation of 3S,4S-lactone 10 involved three consecutive
steps: hydrolysis under NaOH with H2O2, removal of excessive H2O2 by
Na2S2O3, and deprotection followed by cyclization in benzene.[7] These
three steps took more than 4 days to finish, and the overall yield was far
from ideal, nearly the half of the procedure to be discussed in Scheme 1.
C4 of lactone 10 was identified as S configuration by lack of nuclear Over-
hauser effect (NOE) between protons of -NCH- and -OCH-.
Taking into account the unfavorable factors in Scheme 2, we made partial
revisions and retried the route in Scheme 1 until it was put through. This
approach was first reported by Fray et al. and modified by later researchers.[7]
In this article, we documented several potential problems that were implicated
(Scheme 1). Condensation of 4 with the lithium salt of ethyl propiolate,
formed in situ by LDA (prepared via n-BuLi with diisopropylamine) with
ethyl propiolate afforded hydroxyl acetylenic ester 9 as a stereo-mixture.
During the nucleophilic addition, the lithium salt was found to be extremely
sensitive to traces of O2 and H2O, which would turn the whole system
almost black, probably resulting from its degradation or a succession of
complex chain reactions, but it is interesting to note that sometimes the
yields under such circumstances were a bit higher than that in normally
treated reactions, up to approximately 47%. It remains to be determined
whether these black substances played a positive or negative role in this
step. Furthermore, because the aldehyde could be readily reduced by
LDA,[12] sufficient time should be allowed for complete formation of the
lithium salt of ethyl propiolate before 4 was added. Additionally, compound
9 was a light yellow solid after recrystallization in hexane, distinct from
what has been reported in literature as an oil.[7] The structure of 9
(mp ¼ 72–748C) was verified by 1H NMR and MS. Subsequent
reduction of ester 9 by H2 on Pd/C at 60 psi and further lactonization in

b-Secretase Inhibitors
13
toluene/acetic acid without separation of the reduced product gave lactone 10,
and the 3R,4S-diastereomer 11 was also obtained in a yield below 15%.
The methyl group was smoothly introduced at a-C of 10 in 80% yield
using methyl iodide and lithium hexamethyldisilazide (LiHMDs, Scheme 3).
On a larger scale, the reaction duration should be prolonged to 35–40 min
instead of 30 min as reported,[3a] for exhaustion of the starting materials.
However, longer duration would also in the meantime lead to a dimethylated
product (a white crystal, determined by 1H NMR and MS) in a small
amount. The a-C of compound 12 was identified as R configuration on
the basis of NOE between a-CH3 and C3-a-H, as well as nonexistence of
NOE between -OCH- and a-CH-. Subsequent hydrolysis of 2R,4S,5S-
lactone 12 gave the hydroxyl acid derivative, which was protected by tert-
butyldimethylsilyl chloride (TBDMSCl) to furnish 13, followed by
coupling with diisobutylamine. The TBDMS group was then removed to
afford hydroxyl amide 15 as an amorphous white solid, which proceeded
from 16 to the model compound 17 through classic peptide condensation
using N-Boc-L-Phenylalanine and then 3-(trifluoromethyl)cinnamic acid in
turn. The reagents for condensation involved N-ethyl-N0-(3-dimethylamino-
propyl)carbodiimide hydrochloride (EDCI) or N,N0-dicyclohexylcarbodii-
mide (DCC), accompanied by 1-hydroxybenzotriazole (HOBt) and
N,N0-diisopropyethylamine (DIPEA) or N-methylmorpholine (NMM) in
THF or DMF at room temperature. The reaction duration varied along
with different acids used.
Scheme 3. Reagents and conditions: (i) CH3I, LiHMDS, 2788C; (1) LiOH (1 N),
THF; (2) TBDMSCl, imidazole, DMF; (iii) isobutylamine, EDC, HOBt, DIPEA,
DMF/CH2Cl2; (iv) n-Bu4NþF2, THF; (v) N-Boc-L-phenylalanine, EDC, HOBt,
NMM, DMF; and (vi) 3-(trifluoromethyl)cinnamic acid, EDC, HOBt, NMM, DMF.

14
X. Yang et al.
CONCLUSION
A new series of potential peptidomimetic b-secretase inhibitors containing a
LeuꢀAla hydroxyethylene dipeptide isostere was designed, which is structu-
rally altered at the P2 side chain and P3 capping, for further structure–
activity relationship studies. In this work, compound 17 was selected as a
model and synthesized first to build up a practically universal preparation
method for the target molecules. Two synthetic pathways to compound 17
were performed, modified, and compared in reaction conditions, yields, and
stereoselectivity. With these methods, a number of target compounds are
being prepared and their bioactivity evaluation is under way.
EXPERIMENTAL
Unless otherwise mentioned, reagents were obtained commercially and used
without further purification. The petroleum ether used had a boiling point
range of 60–908C. Melting points were taken with an X-4 apparatus and were
1
uncorrected. IR (KBr), H NMR, MS, element data, and optical rotations
were taken with Perkin-Elmer 983, Inova-500, MDS SCIEX QSTAR, Flash
EA 1112, and AA-10R Automatic Polarimeter instruments respectively.
(S)-Methyl 2-(t-boc-amino)-4-methylpentanoate (N-Boc-L-Leucine
methyl ester) (2)
To a stirred solution of N-Boc-Leu-OH (30 g, 130 mmol) in DMF, 21.840 g
(260 mmol) of NaHCO3 and 13 mL (208 mmol) of CH3I were successively
added. After 48 h at room temperature, the reaction was quenched with
H2O (400 mL), and the mixture was extracted with ethyl acetate/benzene
(1:1) (100 mL ꢁ 4). The organic layers were collected and washed with
H2O (100 mL ꢁ 2), 10% aqueous Na2SO3 (100 mL ꢁ 2), and saturated
aqueous NaCl (100 mL) and dried over Na2SO4. Removal of ethyl acetate
afforded 29.219 g (91.7%) of ester 2 as a colorless viscous oil and was used
1
for the next step without further purification. H NMR (CDCl3, 300 MHz):
d 4.925 (d, 1H, BocNH-), 4.317 (m, 1H, N-CH-), 3.732 (s, 3H, -OCH3),
1.688 (m, 1H, -CH-), 1.656–1.487 (m, 2H, -CH2-), 1.444 (s, 9H, Boc),
0.959–0.929 (d, 6H, J ¼ 6.5 Hz, 2CH3-). MS (ESI): 246.20 (Mþ).
(S)-2-(t-Boc-amino)-4-methylpentan-1-ol (N-Boc-L-Leucinol) (3)
At 08C, 9.887 g (89.10 mmol) of CaCl2 and 6.731 g (178.20 mmol) of NaBH4
were successively put into a solution of 10.914 g (44.55 mmol) of ester 2 in
ethanol/THF (80 mL/48 mL). The ice bath was removed after NaBH4 was
added. After 6 h, the mixture was poured into 250 mL of citric acid/ice,

b-Secretase Inhibitors
15
followed by extraction with ethyl acetate (100 mL ꢁ 4), and was then washed
with saturated aqueous NaHCO3 (100 mL ꢁ 2) and NaCl (100 mL). The
extracts were dried over MgSO4, and removal of the solvent gave 3 as a
1
colorless oil in 98% crude yield (9.495 g). H NMR (CDCl3, 500 MHz): d
4.560 (s, 1H, BocNH-), 3.719 (m, 1H, N-CH-), 3.677–3.489 (dd, 2H,
J ¼ 6 Hz and 11 Hz, -CH2-O-), 2.326 (b-s, 1H, -OH), 1.668 (m, 1H, -CH-),
1.449 (s, 9H, Boc), 1.358–1.274 (m, 2H, -CH2-), 0.942–0.924 (d, 6H,
J ¼ 6.5 Hz, 2CH3-). MS (ESI): 218.20 (M+).
(S)-2-(t-Boc-amino)-4-methylpentanal (N-Boc-L-Leucinal) (4)
Method A
N-Boc-L-Leucinol 3 (651 mg, 3 mmol) and 1.27 mL (9 mmol) of TEA were
dissolved in 5 mL of CH2Cl2 and cooled to 08C, to which a solution of
.
1.440 g (9 mmol) of Py SO3 in 5 mL of DMSO was added, followed by
removal of the ice bath. After 35 min (prolonged accordingly on larger
scales and monitored by TLC), the mixture turned light brown and was
quenched with 25 mL of water/ice. Then the mixture was extracted with
ether (30 mL ꢁ 3), which was washed successively with 10% citric acid
(30 mL ꢁ 2), H2O (30 mL ꢁ 2), saturated NaHCO3 (30 mL), and brine
(30 mL) and dried over MgSO4 Crude aldehyde 4 (602 mg, 93%) was
obtained after removal of the solvent (below 308C), which was used for the
1
next step without isolation. H NMR (CDCl3, 300 MHz): d 9.589 (s, 1H, -
CHO), 5.047 (d, 1H, BocNH-), 4.251 (m, 1H, N-CH-), 1.449 (s, 9H, Boc),
0.979–0.919 (d, 6H, 2CH3-).
Method B
At 2788C under a nitrogen atmosphere, were added 2.07 mL of DIBAL
(2.5 mmol, 20 wt% in toluene) to a stirred solution of 245 mg (1 mmol) of
ester 2 in 3 mL of dry toluene. After 10 min, the reaction was quenched with
1.5 mL of CH3OH, followed by 10 mL of saturated aqueous Rochelle salt.
The mixture was allowed to warm to room temperature, and 10 mL of ether
was added. After violent shaking for 3 min, until the mixture turned transparent,
the organic layer was separated. The aqueous layer was extracted with ether
(10 mL ꢁ 3). The layers were combined and dried over MgSO4. Crude
aldehyde 4 (190 mg, 88%) was obtained under reduced pressure.
(3S,4S)-(N-Boc-4-amino)-3-hydroxy-6-methylheptanioc Acid
Ethyl Ester (5)
At 2238C under a N2 atmosphere, 8 mL (12.75 mmol, 1.6 M in THF)
n-BuLi were added to a stirred solution of 2.16 mL (15.3 mmol)

16
X. Yang et al.
diisopropylamine in 5 mL of anhydrous THF. After 1 h, the mixture was
cooled to 2788C and for 30 min, followed by the addition of 1.20 mL
anhydrous EtOAc in 1 mL of THF, which was then stirred for another
20 min. Then, 1.85 g of crude aldehyde 4 in 2 mL of THF was added,
and the mixture was stirred for 40 min before 36 mL of HCl (in excess,
1N) were added. The mixture was warmed to room temperature and
extracted with EtOAc (50 mL ꢁ 3). The extracts were washed with brine
and dried over MgSO4. After silica-gel column chromatography, 757 mg
(29%) of 3S,4S-N-Boc-Leu-statine-OEt (oil, less polar), 404 mg (15%) of
3R,4S-diastereomer (white solid, mp: 48–508C), and 274 mg of the
mixture were obtained. 1H NMR (CDCl3, 500 MHz) for 3S,4S-diastereomer:
d 4.708 (d, 1H, J ¼ 9.5 Hz, BocNH-), 4.172 (m, 2H, J ¼ 7 Hz, -CO2CH2-),
4.018 (d, 1H, -CH-O), 3.617 (m, 1H, N-CH-), 3.276 (b-s, 1H, -OH),
2.468–2.571 (m, 2H, -CH2CO2-), 1.661 (m, 1H, -CH-), 1.527 (m, 1H,
-CH2-), 1.440 (s, 9H, Boc), 1.348 (m, 1H, -CH2-), 1.274 (t, 3H, J ¼ 7 Hz,
-CO2-C-CH3),0.936 (d, 3H, J ¼ 1.5 Hz, -CH3), 0.922 (d, 3H, J ¼ 1 Hz,
-CH3). MS (ESI): 304.27 (Mþ). 1H NMR (CDCl3, 500 MHz) for 3R,
4S-diastereomer: d 4.577 (d, 1H, J ¼ 8 Hz, BocNH-), 4.170 (m, 2H, J¼
7 Hz, -OCH2-), 4.003 (m, 1H, -CH-O), 3.666 (m, 1H, N-CH-), 3.442 (b-s,
1H, -OH), 2.495–2.422 (m, 2H, -CH2CO2-), 1.669 (m, 1H, -CH-), 1.438
(s, 9H, Boc), 1.325 (t, 2H, -CH2-), 1.271 (t, 3H, J ¼ 7.5 Hz, -CO2-C-CH3),
0.934 (d, 3H, J ¼ 6.5 Hz, -CH3), 0.910 (d, 3H, J ¼ 6.5 Hz, -CH3). MS
(ESI): 304.24 (Mþ).
(4S,5S)-N-Boc-5-(carbethoxymethyl)-2,2-dimethyl-4-
(2-methylpropyl)-1,3-oxazolidine (6)
3S,4S-N-Boc-Leu-Statine-OEt (977 mg, 3.23 mmol) and 1.30 mL (13 mmol)
of 2-methoxy-propene were dissolved in 2 mL of CH2Cl2, to which was
added a trace of phosphorous oxychloride using a capillary, dipping
quickly into the mixture before POCl3 turned brown. The mixture was
allowed to react for 15 h, and 4 drops of Et3N was added to remove
POCl3, which was concentrated under reduced pressure, followed by
dilution with 25 mL of EtOAc. The mixture was washed with aqueous
10% citric acid (20 mL ꢁ 2), saturated aqueous NaHCO3 (20 mL ꢁ 2),
and brine (20 mL) and dried over MgSO4. Silica-gel column chromato-
graphy gave 1.01 g (91%) of protected statine 6 as a light yellow oil.
1H NMR (CDCl3, 500 MHz): d 4.324 (b-s, 1H, -OCH-), 4.164 (m, 2H,
J ¼ 7 Hz, -CH2-), 3.835–3.722 (m, 1H, -NCH-), 2.680–2.566 (m, 2H,
-CH2CO2-), 1.602 (m, 1H, -CH-), 1.511 (s, 6H, 2CH3-), 1.499 (s, 1H,
-CH2-), 1.484 (s, 9H, Boc), 1.441 (s, 1H, -CH2-), 1.273 (t, 3H, J ¼ 7 Hz,
-CH3), 0.946 (d, 3H, J ¼ 6.5 Hz, -CH3), 0.925 (d, 3H, J ¼ 6.5 Hz, -CH3).
MS (FAB): 344.30 (Mþ).

b-Secretase Inhibitors
17
(4S,5S)-N-Boc-2,2-dimethyl-5-(2-hydroxyethyl)-4-
(2-methylpropyl)-1,3-oxazolidine (7)
To a stirred solution of 1.01 g (2.94 mmol) of ester 6 in 3 mL of THF, 6.18 mL
(7.37 mmol) of DIBAL were added at 08C. After 45 min, the reaction was
quenched by pouring onto ice, which was then diluted with 50 mL of ether.
The mixture was warmed to 308C, and 50 mL of saturated aqueous
Rochester salt was added, which was shaken violently to remove excessive
aluminum in the system. After both the organic and aqueous layers turned
clear, the upper layer was separated and subjected to silica-gel column chrom-
atography to furnish 802 mg (91%) of 7 as a light yellow oil. 1H NMR (CDCl3,
500 MHz): d 3.910–3.809 (m, 3H, -OCH-, -CH2O-), 3.614 (m, 1H, -NCH-),
3.000–2.500 (b-s, 1H, -OH), 2.192–2.164 (m, 2H, -CH2-C-O-), 1.826–1.791
(m, 1H, -CH-), 1.687–1.604 (m, 2H, -CH2-), 1.482 (s, 6H, 2CH3-), 1.445
(s, 9H, Boc), 0.937–0.910 (d, 6H, J ¼ 6.5 Hz, 2CH3-). MS (FAB): 302.4 (Mþ).
(4S,5S)-N-Boc-5-(2-cyanoethyl)-2,2-dimethyl-4-(2-methylpropyl)-
1,3-oxazolidine (8)
At 08C, 0.31 mL (4.00 mmol) of MsCl was added to a solution of 802 mg
(2.66 mmol) of alcohol 7 and 0.56 mL (3.88 mmol) Et3N in 4 mL of
CH2Cl2. After 2.5 h, the mixture was washed successively with aqueous
10% citric acid (10 mL ꢁ 2), saturated aqueous NaHCO3 (10 mL ꢁ 2), and
brine (10 mL). The organic layer was dried over MgSO4 and evaporated to
846 mg of mesylate as a yellow oil, which was used for the next step
without further purification. Then the residue was dissolved in 8 mL of
DMSO and treated with 150 mg of NaCN (3.06 mmol). The reaction was
allowed to proceed at 508C for 10 h before another 150 mg of NaCN
was added, which was allowed to react for an additional 10 h. The mixture
was diluted with 25 mL of water and extracted with ether (25 mL ꢁ 4). The
combined organic layer was washed with water (25 mL ꢁ 3) and dried over
MgSO4. Silica-gel purification afforded 588 mg (71%) of nitrile 8 as an oil.
1H NMR (CDCl3, 500 MHz): d 3.948–3.914 (m, 1H, -OCH-), 3.740–3.650
(m, 1H, -NCH-), 2.553–2.440 (m, 2H, -CH2CN), 1.983–1.867 (m, 2H,
-CH2-C-CN), 1.581 (s, 6H, 2CH3-), 1.604–1.440 (m, 3H, -CH-, -CH2-),
1.481 (s, 9H, Boc), 0.955–0.942 (d, 6H, J ¼ 6.5 Hz, 2CH3-). IR (CH3Cl):
2247.46, 1696.94 cm21. MS (ESI): 311.27 (Mþ).
(4RS,5S)-N-Boc-5-amino-4-hydroxy-7-methyloct-2-ynoic Acid
Ethyl Ester (9)
To a stirred solution of 17.60 mL (67.74 mmol) diisopropylamine in 40 mL of
anhydrous THF, 35.52 mL (56.45 mmol, 1.6 M in THF) of n-BuLi were added

18
X. Yang et al.
at 2238C under an N2 atmosphere. After 1 h, the mixture was cooled to
2788C for 30 min, when 5.86 mL (56.45 mmol) of ethyl propiolate in 5 mL
of THF were added in a dropwise fashion and stirred for 1 h. Then, 8.164 g
of crude aldehyde 4 (precooled at 2788C) in 10 mL of THF was added,
and the mixture was stirred for 4.5 h before 16 mL of acetic acid (in excess)
was added. The mixture was warmed to room temperature, diluted with
250 mL of ether, washed sequentially with aqueous 10% citric acid
(100 mL ꢁ 2) and saturated aqueous NaHCO3 (100 mL ꢁ 2), and dried
over MgSO4. After silica-gel column chromatography, 6.200 g (47%) of
acetylenic alcohols 9 were obtained as a light yellow solid. Mp: 72–748C.
1H NMR (CDCl3, 500 MHz): d 4.665 (broad s, 1H, BocNH-), 4.500
(m, 1H, -OCH-), 4.267–4.216 (m, 2H, J ¼ 7 Hz, -CH2-), 3.847–3.790
(m, 1H, -NCH-), 3.221 (b-s, 1H, -OH), 1.730–1.626 (m, 1H, J ¼ 6.5 Hz,
-CH-), 1.514–1.477 (m, 1H, -CH2-), 1.449 (s, 9H, Boc), 1.379–1.342
(m, 1H, -CH2-), 1.310 (t, 3H, J ¼ 7 Hz, -CH3), 0.964–0.924 (d, 6H,
J ¼ 6.5 Hz, 2CH3-). MS (FAB): 313.8 (Mþ).
(5S)-5-[(10S)-1-(N-Boc-amino)-3-methylbutyl]dihydrofuran-
2(3H)-one (10) and (5R)-5-[(10S)-1-(N-Boc-amino)-
3-methylbutyl]dihydrofuran-2(3H)-one (11)
Method A
Acetylenic alcohol 9 (6.200 g, 19.79 mmol) dissolved in 100 mL of EtOAc,
which underwent hydrogenation at 60 psi over 3.72 g of 10% Pd/C for
2.5 h. After filtration of Pd/C, the solvent was evaporated, and the residue
was redissolved in 250 mL of toluene/acetic acid (97.5:2.5) for reflux at
1208C for 3 h. The mixture was concentrated and subjected to silica-gel
purification. The less polar lactone 10 (3.269 g, 61%) and 751 mg (14%) of
1
the more polar lactone 11 were obtained as white solids. H NMR (CDCl3,
500 MHz) for 10: d 4.511 (t, 1H, -OCH-), 4.432 (d, 1H, J ¼ 9.5 Hz,
BocNH-), 3.877–3.830 (m, 1H, -NCH-), 2.542–2.507 (m, 2H, -COCH2-),
2.255–2.200 (m, 1H, -CH2-C-CO-), 2.170–2.109 (m, 1H, -CH2-C-CO-),
1.696–1.642 (m, 1H, -CH-), 1.602–1.543 (m, 1H, -CH2-), 1.439 (s, 9H,
Boc), 1.384–1.329 (m, 1H, -CH2-), 0.940 (d, 3H, J ¼ 6.5 Hz, -CH3), 0.927
(d, 3H, J ¼ 6.5 Hz, -CH3). MS (FAB): 271.4 (Mþ). [a]D 2 328 (c 1.0
CH3OH). Mp: 77–798C. 1H NMR (CDCl3, 500 MHz) for 11: d 4.456–
4.432 (m, 2H, -OCH-, BocNH-), 3.759 (s, 1H, -NCH-), 2.611–2.418
(m, 2H, -COCH2-), 2.322–2.250 (m, 1H, -CH2-C-CO-), 2.102–2.024
(m, 1H, -CH2-C-CO-), 1.719–1.707 (m, 1H, -CH-), 1.442 (s, 9H, Boc),
1.423–1.283 (m, 2H, -CH2-), 0.944 (d, 3H, J ¼ 6.5 Hz, -CH3), 0.919
(d, 3H, J ¼ 6.0 Hz, -CH3). MS (ESI): 272.22 (Mþ). [a]D 244.88 (c 0.625
CH3OH). Mp: 119–1208C.

b-Secretase Inhibitors
19
Method B
Nitrile 8 (278 mg, 0.90 mmol) was dissolved in 6 mL of EtOH at 08C,
followed by the addition of 1.56 mL of aqueous NaOH (1 N) and 5.13 mL
of H2O2 (30%). The mixture was stirred at 08C overnight, which was then
cooled to 2308C, and 12 mL of aqueous Na2S2O3 (20%) were added. After
removal of ethanol under reduced pressure at 408C, the aqueous layer (with
small drops of oil on the surface) was extracted with EtOAc (20 mL ꢁ 4).
The layers were combined, washed with brine (20 mL), and dried over
MgSO4. The solvent was then evaporated, and the residue was stirred in
acetic acid/water (65:35) for 3 days at room temperature. Then the solvent
was removed, and 10 mL benzene and 10 mL CH2Cl2 were used to remove
the small amount of water left. The residue was refluxed in benzene for 1 h
and then purified by silica-gel column chromatography, which furnished
211 mg (86%) of lactone 10.
(3R,5S)-5-[(10S)-1-(N-Boc-amino)-3-methylbutyl]-3-methyl-
dihydrofuran-2(3H)-one (12)
To a stirred solution of 2.012 g (7.42 mmol) of lactone 10 in 25 mL of
anhydrous THF, 16.33 mL of LiHMDS (1.0 M in THF) were added and
stirred at 2788C under a nitrogen atmosphere for 1 h. CH3I (0.51 mL,
8.17 mmol) was then added, and the mixture was allowed to react for
35 min before 40 mL of saturated aqueous NH4Cl solution was added. The
mixture was warmed to room temperature and underwent evaporation to
remove THF. The residue was extracted with EtOAc (25 mL ꢁ 3) and the
combined organic layers were washed with 25 mL of brine and then dried
over MgSO4. Silica-gel chromatography gave 1.465 g (69%) of the
alkylated lactone 12 as a colorless solid. Dimethylated product (180 mg)
was also obtained. 1H NMR (CDCl3, 500 MHz) for 12: d 4.492 (t, 1H,
-OCH-), 4.367 (d, 1H, J ¼ 9.5 Hz, BocNH-), 3.839 (m, 1H, -NCH-), 2.684
(m, 1H, -CHCO-), 2.413 (m, 1H, b-H, -CH2-C-CO-), 1.922 (m, 1H, a-H,
-CH2-C-CO-), 1.655 (m, 1H, -CH-), 1.546 (m, 1H, -CH2-), 1.440 (s, 9H,
Boc), 1.359 (m, 1H, -CH2-), 1.272 (d, 3H, J ¼ 7.5 Hz, -CH3), 0.937–0.915
(d, 6H, J ¼ 6.5 Hz, 2CH3-). MS (ESI): 286.22 (Mþ). Mp: 90–918C. [a]D
-28.68 (c 0.7 CH3OH). 1H NMR (CDCl3, 500 MHz) for dimethylated
lactone: d 4.465–4.433 (m, 2H, BocNH-, -OCH-), 3.821–3.774 (m, 1H,
-NCH-), 2.060–2.021 (dd, 1H, J ¼ 13 Hz, 6.5 Hz, -CH2-C-CO), 1.959–1.914
(dd, 1H, J ¼ 9.5 Hz, 12.5 Hz, -CH2-C-CO-), 1.709–1.655 (m, 1H, -CH-),
1.629–1.571 (m, 1H, -CH2-), 1.432 (s, 9H,), 1.387–1.332 (m, 1H, -CH2-),
1.266 (s, 3H, -CH3), 1.263 (s, 3H, -CH3), 0.943–0.926 (dd, 6H, J ¼ 2 Hz,
6.5 Hz, 2CH3-). MS (EI): 300.1 (Mþ). Anal. calcd. for C15H27NO4: C, 64.18;
H, 9.76; N, 4.68. Found: C, 64.33; H, 9.48; N, 4.67. Mp: 154–1568C.

20
X. Yang et al.
(2R,4S,5S)-5-(N-Boc-amino)-4-[(tert-butyldimethylsilyl)oxy]-
2,7-dimethyloctanoic Acid (13)
Methylated lactone 12 (940 mg, 3.30 mmol) was dissolved in 15 mL of THF
and then treated with 16.5 mL of aqueous LiOH (1 N), which was stirred
overnight at room temperature. The mixture was concentrated under
reduced pressure and acidified with 25% aqueous citric acid to pH 3–4,
followed by extraction with EtOAc (20 mL ꢁ 4). The extracts were
combined, washed with 20 mL of brine, and dried over MgSO4. Evaporation
of the solvent gave 920 mg of hydroxyl acid intermediate as a white solid.
Then the intermediate was dissolved in 15 mL of anhydrous DMF, and
4.528 g (66.46 mmol) of imidazole and 5.012 g (33.23 mmol) of TBDMSCl
(tert-butyldimethylchlorosilane) were added. The mixture was stirred at
room temperature for 24 h and then quenched by 12 mL of MeOH, which
was allowed to be stirred for 1 h to remove the residual TBDMSCl. The
mixture was then acidified with excessive 25% aqueous citric acid and
extracted with EtOAc (40 mL ꢁ 4). The organic layers were collected,
washed with 50 mL of water and 50 mL of saturated aqueous NaCl
solution, and then dried over Na2SO4. Removal of the solvent and silica-gel
column chromatography afforded 1.250 g (91%) of the protected acid 13 as
1
a colorless solid, mp: 132–1348C. H NMR (CDCl3, 500 MHz): d 10.340
(b-s, 1H, -COOH), 4.576 (d, 1H, J ¼ 9.5 Hz, BocNH-), 3.736–3.719
(m, 1H, -OCH-), 3.707–3.696 (m, 1H, -NCH-), 2.665–2.596 (m, 1H,
J ¼ 7 Hz, -CHCO-), 1.927–1.864 (m, 1H, -CH-), 1.655–1.615 (m, 1H,
-CH2-C-CO-), 1.445 (s, 9H, Boc), 1.401–1.332 (m, 1H, -CH2-C-CO-),
1.299–1.256 (m, 2H, -CH2-), 1.190 (d, 3H, J ¼ 7 Hz, -CH3), 0.932 (d, 3H,
-CH3), 0.919 (d, 3H, -CH3), 0.896 [s, 9H, (CH3)3-], 0.082 (s, 6H, 2CH3Si-).
MS (EI): 418.2 (Mþ).
[(2R,4S,5S)-5-(N-Boc-amino)-4-[(tert-butyldimethylsilyl)oxy]-
2,7-dimethyloctal] N0-Isobutyl Amide (14)
HOBt (350 mg, 2.5 mmol), 485 mg (2.5 mmol) of EDC, 0.45 mL (4.40 mmol)
of isobutylamine, and 1.50 mL (13.2 mmol) of NMM were successively
added to a stirred solution of 918 mg (2.20 mmol) of acid 13 in DMF/CH2Cl2
(5 mL/30 mL) at room temperature. The mixture was stirred for 36 h and then
poured into 50 mL of saturated aqueous NaHCO3, followed by extraction with
EtOAc (40 mL ꢁ 4). The organic layer was washed with 50 mL of brine and
dried over Na2SO4. Silica-gel chromatography with an eluant of EtOAc/
petroleum ether (1/5) afforded 923 mg (89%) of amide 14 as a light yellow
1
oil. H NMR (CDCl3, 500 MHz): d 6.227 (s, 1H, -C-CONH-), 4.563 (d, 1H,
J ¼ 10 Hz, BocNH-), 3.804–3.755 (m, 1H, -OCH-), 3.648–3.621 (m, 1H,
BocN-CH-), 3.284–3.229 (m, 1H, -CON-CH2-), 2.769–2.719 (m, 1H,
-CON-CH2-), 2.447–2.406 (m, 1H, -CH-CO-), 1.767–1.688 (m, 2H, 2-CH-),

b-Secretase Inhibitors
21
1.659–1.604 (m, 1H, -CH2-C-CO), 1.518–1.468 (m, 1H, -CH2-C-CO), 1.446
(s, 9H, Boc), 1.287–1.186 (m, 2H, -CH2-), 1.120 (d, 3H, J ¼ 6.5 Hz, -CH3),
0.937–0.923 (s, 6H, 2CH3-), 0.898–0.895 (d, 6H, 2CH3-), 0.881 (s, 9H,
(CH3)3-), 0.070 (s, 3H, -SiCH3), 0.055 (s, 3H, -SiCH3). MS (ESI): 473.36 (Mþ).
[(2R,4S,5S)-5-(N-Boc-amino)-4-hydroxy-2,7-dimethyloctal]
N0-Isobutyl Amide (15)
At room temperature, 1.388 g (2.93 mmol) of amide 14 was dissolved in
15 mL of THF, which was treated with 8.80 mL (8.80 mmol) of
n-Bu4NþF2 (1 M in THF). The mixture was stirred for 12 h and then concen-
trated for separation. Silica-gel chromatography purification furnished 798 mg
(76%) of the deprotected amide 15 as a white solid, mp: 137–1388C. 1H NMR
(CDCl3, 500 MHz): d 5.937 (s, 1H, -CONH-), 4.696 (d, 1H, J ¼ 9.5 Hz,
BocNH-), 3.602 (s, 1H, -OCH-), 3.541 (m, 1H, -NCH-), 3.602–3.541
(s, 1H, -OH), 3.164–3.111 (m, 1H, -CONCH2-), 3.012–3.296 (m, 1H,
-CONCH2-), 2.608–2.570 (m, 1H, -CHCO-), 1.792–1.739 (m, 1H, -N-C-CH),
1.693–1.623 (m, 3H, -CH-, -CH2-C-CO-), 1.531–1.483 (m, 1H, -CH2-),
1.435 (s, 9H, Boc), 1.323–1.283 (m, 1H, -CH2-), 1.195 (d, 3H, J ¼ 7 Hz, -
CH3), 0.925–0.902 (d, 12H, 4CH3-). MS (ESI) (Mþ 359.28). Anal. calcd.
for C19H38N2O4: C, 63.65; H, 10.68; N, 7.81. Found: C, 63.97; H, 10.43; N,
7.65. [a]D 255.48 (c 0.65 CH3OH).
[(2R,4S,5S)-5-(N-Boc-L-phenylalanylamido)-4-hydroxy-
2,7-dimethyloctal] N0-Isobutyl Amide (16)
Amide 15 (294 mg, 0.82 mmol) was dissolved in 3 mL of CH2Cl2 at room
temperature, followed by the addition of 1 mL of CF3COOH, which was
stirred for 30 min to remove the Boc group. The solvent and TFA were
removed under reduced pressure. In another flask, 239 mg (0.90 mmol) of
N-Boc-L-phenylalanine, 174 mg (0.90 mmol) of EDC, and 123 mg
(0.90 mmol) of HOBt in 10 mL of anhydrous DMF was allowed to react for
10 min, to which the concentrated residue of amine (neutralized with
0.1 mL of NMM) and 0.2 mL (1.8 mmol) of NMM were added. The
mixture was stirred for 24 h at room temperature and then washed succes-
sively with 10% aqueous citric acid (10 mL ꢁ 2), saturated aqueous
NaHCO3 (10 mL ꢁ 2), and brine. Silica-gel chromatography gave 245 mg
(60%) of amide 16 as a white solid, mp: 200–2018C. The analytic sample
can be prepared by crystallization in EtOAc/petroleum ether (1:4) at 608C.
1H NMR (DMSO-d6, 500 MHz): d 7.615 (m, 1H, -CONH-), 7.247–7.191
(m, 5H, Ph-), 7.170 (d, 1H, -CONH-), 6.968 (d, 1H, BocNH-), 4.638
(d, 1H,-OH), 4.130–4.085 (m, 1H, -NCHCO-), 3.706 (b-s, 1H, -OCH-),
3.273–3.254 (d, 1H, -NCH-), 2.974–2.938 (dd, 1H, J ¼ 4 Hz and 14 Hz,

22
X. Yang et al.
PhCH2-), 2.832 (m, 2H, -CON-CH2-), 2.723–2.675 (dd, 1H, J ¼ 10.5 Hz and
13.5 Hz, PhCH2-), 2.446 (m, 1H, -CHCO-), 1.675–1.608 (m, 1H, -CON-C-
CH-), 1.535–1.369 (m, 3H, -CH-, -CH2-C-CO-), 1.295 (s, 9H, Boc),
1.191–1.081 (m, 2H, -CH2-), 0.949 (d, 3H, J ¼ 6.5 Hz, -CH3), 0.841–0.797
(d, 12H, 4CH3-). MS (ESI): 506.42 (Mþ). Anal. calcd. for C28H47N3O5:
C, 66.50; H, 9.37; N, 8.31. Found: C, 66.80; H, 9.28; N, 8.14. [a]D –51.28
(c 0.625 CH3OH).
Model Compound 17
To a stirred solution of 65 mg (0.13 mmol) of amide 16 in CH2Cl2, 1 mL of
TFA was added. After 30 min, the solvent and most of the TFA were
removed by evaporation. In another flask, 33 mg (0.15 mmol) of 3-(trifluoro-
methyl)cinnamic acid, 30 mg (0.15 mmol) of EDC, and 21 mg (0.15 mmol)
of HOBt were dissolved in 4 mL of DMF, to which were added the deprotected
amine (neutralized with NMM) and 0.1 mL (0.3 mmol) of NMM. The mixture
was stirred at room temperature for 48 h. After this period, the mixture was
poured into 10 mL of aqueous NaHCO3, extracted with EtOAc (30 mL ꢁ 3);
washed successively with 10% aqueous citric acid (10 mL ꢁ 2), saturated
aqueous NaHCO3 (10 mL ꢁ 2), and brine; and dried over MgSO4. After evap-
oration of the solvent, the residue was dissolved in MeOH at 658C, to which
ether and petroleum ether were added slowly. After 5 h at room temperature
(lower temperature is desirable), 20 mg (26%) of compound 17, a white
crystal, were obtained. Recrystallization can be repeated for several times.
Mp: 246–2478C. 1H NMR (DMSO-d6, 500 MHz): d 8.413 (d, 1H, -CONH-),
7.761–7.722 (m, 4H, CF3Ph-), 7.688–7.645 (m, 1H, -CONH-), 7.600 (d, 1H,
J ¼ 9.5 Hz, -CONH-), 7.402 (d, 1H, J ¼ 16 Hz, CF3PhCH55), 7.293–7.141
(m, 5H, Ph-), 6.818 (d, 1H, J ¼ 16 Hz, 55CHCO-), 4.708–4.663 (m, 1H,
-NCHCO-), 4.578 (d, 1H, J ¼ 5.5 Hz, -OCH-), 3.727 (m, 1H, -NCH-),
3.114–3.077 (dd, 1H, J ¼ 4.5 Hz and 14 Hz, PhCH2-), 2.844–2.814 (m, 2H,
-CON-CH2-), 2.797–2.749 (dd, 1H, J ¼ 10 Hz and 14 Hz, PhCH2-), 2.460
(m, 1H, -CHCO-), 1.672–1.618 (m, 1H, -N-C-CH-), 1.604–1.555 (m, 1H,
-CH-), 1.540–1.469 (m, 1H, -CH2-C-CO-), 1.379–1.322 (m, 1H, -CH2-C-
CO-), 1.253–1.199 (m, 1H, -CH2-), 1.124–1.069 (m, 1H, -CH2-), 0.966
(d, 3H, J ¼ 6.5 Hz, -CH3), 0.841–0.791 (d, 12H, 4CH3-). MS (ESI): 604.41
(Mþ). Anal. calcd. for C33H44F3N3O4: C, 65.65; H, 7.57; N, 6.86. Found: C,
65.65; H, 7.35; N, 6.96.
ACKNOWLEDGMENT
This work was supported by the National Natural Science Foundation of
China (Nos. 20272004 and 20572006) and the 985 program, Ministry of
Education of China.

b-Secretase Inhibitors
23
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