A multigram chemical synthesis of the γ-secretase inhibitor LY411575 and its diastereoisomers

Article abstract of DOI:10.1016/j.bmcl.2007.07.062

An improved chemical synthesis of N-2((2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl)-N1-((7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl)-l-alaninamide (LY411,575, 9a), a known γ-secretase inhibitor, is described. The key synthetic steps, which

Full text of DOI:10.1016/j.bmcl.2007.07.062

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 17 (2007) 6392–6395
A multigram chemical synthesis of the c-secretase inhibitor
LY411575 and its diastereoisomers
Abdul H. Fauq,* Katherine Simpson, Ghulam M. Maharvi, Todd Golde and Pritam Das
Department of Neuroscience, Mayo Clinic Jacksonville, 4500 San Pablo Road, Jacksonville, FL 32224, USA
Received 31 May 2007; revised 18 July 2007; accepted 19 July 2007
Available online 30 August 2007
Abstract—An improved chemical synthesis of N-2((2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl)-N1-((7S)-5-methyl-6-oxo-6,7-
dihydro-5H-dibenzo[b,d]azepin-7-yl)-L-alaninamide (LY411,575, 9a), a known c-secretase inhibitor, is described. The key synthetic
steps, which used no chiral chromatography in the entire sequence, involved (1) improved microwave-assisted synthesis of a seven-
membered lactam ( )-(5,7-dihydro-6H-dibenz-[b,d]azepin-6-one 2, and (2) convenient isolation of pure LY411575 from a mixture of
four diastereomers by simple flash silica gel chromatography. Starting from the resolved aminolactams 5a and 5b, all four diaste-
reomers were produced in enantiomerically pure form.
Ó 2007 Elsevier Ltd. All rights reserved.
Alzheimer’s disease has claimed 27 million individuals
worldwide, more than four million in the United States
alone.1a,b The presence of amyloid-b-containing peptide
plaques in the brain is the hallmark of this senile disease.
It is known that, of the two Ab-peptides (1–40 and 1–
42), the longer form has a stronger tendency to deposit
insoluble plaques in the AD brain.1c,2 Considerable ge-
netic, biophysical, and toxicological data emerging from
studies of familial forms of Alzheimer’s disease (AD),
transgenic animal modeling of amyloid b-protein pre-
cursor (APP), and presenilin (PS) mutants, toxicological
and biophysical studies have strongly suggested a path-
ogenic role in AD for Ab42.3 This discovery immedi-
ately points to a possibility of developing drugs for
AD that may target Ab42. Several strategies can be pur-
sued to stop or slow down the progression of the disease
by preventing the formation or intracellular deposition
of 1–42 Ab-peptide. One of the prominent approaches
to do this is to develop inhibitors of c-secretase,4 an
intramembrane aspartic protease best represented by a
macromolecular complex comprised of Presenilin-1
(PS1), Pen-2, Aph-1, and Nicastrin, and is known to
endoproteolyze APP as well as several other transmem-
brane proteins.5 One of these inhibitors, LY411575 (9a,
Fig. 1), that has shown promise for significantly reduc-
ing brain and CSF levels of the Ab-peptides through
its c-secretase activity in recent years was developed by
Eli Lilly.6
To the best of our knowledge, LY411575 is not available
commercially. Details of its chemical synthesis were
reported in a patent.6 However, in our hands this synthe-
sis did not lend itself to generating multigram quantities
of LY411575 needed for our ongoing pharmacological
studies on the downregulation of Ab (1–42) peptide lev-
els in animal models. This was due to (1) difficulties in
achieving efficient synthetic quantities of the dibenzaz-
epinone 2 (Scheme 1), (2) reported necessity of generat-
ing enantiomerically pure (S)-3,5-difluoromandelic acid,
and (3) required chiral resolution of the two diastereo-
mers of the Boc-derivative of the dibenzazepinone-alan-
inamide amines 7 by preparative chiral HPLC which
worked for small scale preparations only. For our mul-
tigram drug needs, we needed to make the synthesis con-
cise by devising a short and efficient synthesis of the
F
O
H
N
O
N
N
H
F
OH
O
Keywords: LY411575; Diastereomer; Microwave; Chiral chromatog-
raphy; R-lactic acid.
9a
*
Corresponding author. Tel.: +1 904 953 8034; fax: +1 904 953
7117; e-mail: fauq.abdul@mayo.edu
Figure 1. LY411575.
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.07.062

A. H. Fauq et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6392–6395
6393
O
O
Cl
H2N
NH
NH
Aluminum choride
Chloroacetyl chloride
200 ºC, microwave,
20 min
TEA, 67%
2
1
TFMSA,
100 °C
NO2
NHOH
OH
N
Phenylacetyl chloride,
Zn dust, NH4Cl,
EtOH, H2O, 50%
O
NaHCO3, H2O, ether, 92%
11
10
12
O
O
HON
N
N
Butyl nitrite,
NaH, MeI, DMF,
r.t, 92%
10% Pd-C,
2
KHMDS, toluene
100%
70°C,
EtOH, 98%
3
4
NH2
O
O
F
F
H2N
O
N
( )-8, EDC
(L)-Boc-alanine,
HN
N
N
HOBT, THF,
EDC, HOBT, THF
O
O
HO
r.t, thenHCl/EtOAc
r.t, 80-95% over
two steps
r.t, 80-95%
NH
N
H
O
5a,b
7a,b
9a-d
9a
CN
HO
HO
COOH
F
CHO
6N HCl, 110 ºC, 80%
NaHSO3, NaCN, H2O,
81%
F
F
F
F
F
13
( )-8
14
Scheme 1. Synthesis of LY411575 from racemic diphenyl lactam 5.
dibenzazepinone 2 (Scheme 1), as well as adopting a syn-
thetic strategy by eliminating the need for resolutions on
chiral HPLC columns.
solvent.10 We prepared lactam 2 by this route (Scheme
1) and found that the reaction sequence could not be
easily scaled up and that the overall yields of the lactam
were low. More recently, Fuwa had reported a general
method of generating several macrocyclic lactams
(including the lactam 2) which were obtained in six
steps. In this scheme, 2-bromoaniline and 2-iodobenzyl
cyanide were coupled in a key step via borylation-Suzu-
ki–Miyaura protocol to a biphenyl methylene cyanide
that was subsequently carried to lactam 2 in five steps
with the final step being a Staudinger-aza-Wittig lactam
ring closure.11 A four-step synthesis of the lactam 2
beginning with 2-nitro-20-biphenylcarboxylic acid was
also known in the literature to give 4–6 g of the lactam
but requires the use of large quantities (ca. 1 mol) of
hazardous diazomethane.12
The synthesis of dibenzazepinone 2 (Scheme 1) was orig-
´
inally reported by Stolle cyclization of the 2-chloroacetyl
derivative of 2-phenylaniline with AlCl3 at elevated tem-
peratures.7 However, the authors reported that they had
´
prepared 7-phenyloxindole (a five-membered Stolle ring
closure to indole instead of seven-membered lactam).
This assignment of structure was revised by Brown to
dibenzazepinone 2.8 This reaction, in our hands, how-
ever, produced intractable black tars from which the
desired lactam was obtained by cumbersome silica gel
chromatography only in 18–20% yield. More recently,
alternative syntheses of the same lactam 2 have been re-
ported. Thus, Baudoin and collaborators have prepared
small quantities of the lactam 2 by a two-step reaction
sequence by palladium-catalyzed borylation-Suzuki
reaction (45% overall yield).9 A four-step sequence of
generating lactam 2 was reported in which nitrobenzene
was partially reduced by Zn to phenylhydroxylamine
that, after derivatizing to a hydroxylamide, was cyclized
by heating it with neat trifluoromethanesufonic acid as
Recently, a large number of high temperature reactions
have been carried out using affordable microwave tech-
nology.13 Compared to the thermal reaction conditions,
the microwave energy seems to facilitate high tempera-
ture reactions often selectively producing cleaner desired
products in higher yield by minimizing side products,
and considerably reducing reaction times. Since we were

6394
A. H. Fauq et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6392–6395
interested in producing multigram quantities of the lac-
tam 2, we focused on using the shortest and least expen-
sive method to produce large quantities of the lactam 2.
To this end, we chose to employ microwave methodol-
ogy to improve upon our poor yield of the lactam ob-
tained in our thermal reaction runs. Thus, the
intramolecular Friedel–Craft cyclization of the 2-chloro-
acetyl derivative of 2-phenylaniline was attempted under
AlCl3 catalysis with or without solvent under different
temperature and microwave wattage conditions. The
best yields of the seven-membered biphenyl lactam 2
were obtained by irradiating a dry mixture of the lactam
precursor with anhydrous AlCl3 for 20 min at 200 °C.
The black tars were still obtained as in the thermal reac-
tion but seemed to be more amenable to effective extrac-
tion with n-butanol. After a short path silica gel
chromatography, the desired lactam was readily crystal-
lized in 40% isolated yield. When an aprotic, high
boiling solvent, such as N,N-dimethylformamide and
N-methylpyrrolidone was used, only 7-phenyloxindole
be separated by silica gel. The chiral HPLC, as reporte-
d,6a with larger batches of mixtures met with limited suc-
cess. Despite these disappointing results, we decided to
go forward and complete the synthesis of LY411575
with a remote hope that silica gel chromatography of
the final four diastereomers of LY411575 might be of
some help in isolating the desired enantiomerically pure
diastereomer LY411575 in adequate quantities (Scheme
1). The butoxycarbonyl (Boc) group of the lactam alan-
inamide was removed under mild acidic conditions and
the resulting amine diastereomeric mixture 7 was cou-
pled to racemic 3,5-difluoromandelic acid to generate a
mixture of four diastereomers. The mixture could be
1
partially analyzed on the basis of its H NMR spectra,
which gave four clear resonance signals each for N-
methyl and C-methyl proton. More significantly, and
much to our delight, the desired enantiomer LY411575
was readily isolated from this complex mixture of four
diastereomers in a reasonable yield.
´
(Stolle condensation product) was obtained in high
The absolute configuration (S,S,S) of the major isolated
isomer was immediately suspected to be that of
LY411575 by its expected activity in well-characterized
c-secretase assays. However, to confirm its configuration
positively and to evaluate the other three diastereomers of
LY411575 for c-secretase activity, it was felt necessary to
synthesize and isolate all four diastereomers in enantio-
merically pure form. We had initially attempted to react
separately the resolved 3,5-difluoromandelic acid enanti-
omers with the diastereomeric amide 7 (Scheme 1) with
the hope of isolating LY411575 from the two resulting
diastereomers by silica gel chromatography. However,
these diastereomers could not be separated in pure form
either by flash chromatography or reverse phase HPLC.
This necessitated resolving the racemic amines 5 in order
yield. Next, the lactam 2 was N-methylated to lactam
3 in DMF at room temperature in 92% yield. Oximation
of 3 with butyl nitrite and potassium hexamethyldisilaz-
ide to 4 followed by catalytic hydrogenation at 70 °C
afforded the racemic aminobiphenyl lactam 5 in essen-
tially quantitative yield (Scheme 1).
Initially, we decided to carry the racemic aminolactam 5
through the end of LY411575 synthesis, hoping that the
desired isomer might be isolated from a mixture of the
four diastereomers by silica gel chromatography rather
than by two inconvenient chiral HPLC separations. To
this end, the racemic aminolactam 5 was coupled with
Boc-(L)-alanine, but the two diastereomers could not
O
OH
O
H
H2N
(-)-S-5a
N
N
(R)-mandelic acid,
EDC, HOBT/DMF
N
Ph
10%. HCl, 110 ºC,
60 h, 100%
+
O
(+)-R-5a
71%, separate
diastereomers
by flash chormatography
5a,b
6a + 6b
(separable by silica
gel chromatography
F
N
O
F
NH2
O
H
N
O
(L)-Boc-alanine,
EDC, HOBT, 8
N
EDC,HOBT, THF,
HO
(-)-S-5a
(+)-R-5a
THF, r.t, then flash
chromatography,
70-95%
NH
r.t, then HCl/EtOAc
r.t, 80-95%
O
N
H
9a/9b
O
over two steps
7a
(S,S,S) + (R,S,S) separable
by silica gel chormatography
NH2
O
H
N
F
F
N
(L)-Boc-alanine,
EDC, HOBT, 8
N
O
EDC, HOBT, THF
O
THF, r.t, then
HO
r.t, then HCl/EtOAc
r.t, 80-95%
flash chromatography
70-95%
NH
O
N
H
over two steps
7a
O
9c/9d
(S,S,R) + (R,S,R) separable
by silica gel chormatography
Scheme 2. Synthesis of diastereomers of LY411575.

A. H. Fauq et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6392–6395
6395
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to achieve silica gel separation of LY411575 from the
other diastereomer in the final step of synthesis. To this
end, chiral resolution of the racemic lactam amine 5 was
attempted by crystallization from its diastereomeric salts,
as well as by converting it to diastereomeric amides, with
some optically pure acids, such as tartaric acid, camphor
sulfonic acid, malic acid, lactic acid, and mandelic acid.
While diastereomeric salt formation was not significantly
successful with any of the chiral acids tried, the diastereo-
meric amides 6a, b with R-mandelic acid could be easily
separated by silica gel chromatography (Scheme 2). For-
tunately, the R-mandelic acid component of each diaste-
reomer was successfully and selectively removed from
the chirally pure amines in high yield without any evi-
dence of attendant hydrolysis of the lactam ring by a
60 h reflux with 10% hydrochloric acid. In this way, pure
S- and R-5-amino-7-methyl-5,7-dihydro-6H-dibenz-
[b,d]azepin-6-one 5a and 5b were isolated and character-
ized by converting them to their crystalline HCl-salt form.
The absolute configuration of each enantiomeric amine
was determined by comparing the c-secretase activity of
the final isomers of LY411575 (vide supra).
5. Li, Y.-M. Mol. Interven. 2001, 1, 198.
6. (a) Wu, J. T.; Tung, J. S.; Thorsett, E. D.; Pleiss, M.
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Harrison, T.; Shearman, M. S.; Williamson, T. L.; Atack,
J. J. Pharmacol. Exp. Ther. 2005, 313, 902.
In summary, a highly convenient and useful synthesis of
a well-known c-secretase inhibitor LY411575 is devised
that is capable of rapidly producing multigram quanti-
ties of the drug without resorting to chiral chromatogra-
phy or HPLC at any step of the synthetic sequence.
Starting from the aminolactam ( )-5, the overall yield
of isolated, enantiomerically pure, LY411575 was
13.6% (See experimental data for all compounds in the
Supplementary material).
10. Endo, Y.; Ohta, T.; Shudo, K.; Okamoto, T. Heterocycles
1977, 8, 367.
Acknowledgment
11. Fuwa, H.; Okamura, Y.; Morohashi, Y.; Tomita, T.;
Iwatsubo, T.; Kan, T.; Fukuyama, T.; Natsugari, H.
Tetrahedron Lett. 2004, 45, 2323.
12. Muth, C. W.; Sung, W.-L.; Papanastassiou, Z. B. J. Am.
Chem. Soc. 1955, 77, 3393.
The financial support provided for this synthetic project
by National Institutes of Health (NIH Grant No. P01
AG20206) is gratefully acknowledged.
13. (a) Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera,
L. Tetrahedron Lett. 1986, 27, 279; (b) Stuerga, D.;
Gonon, K.; Lallemant, M. Tetrahedron 1993, 49, 6229; (c)
Lidstro¨m, P.; Tinerney, J.; Wathey, B.; Westman, J.
Tetrahedron 2001, 57, 9225.
References and notes
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