Highly stereoselective asymmetric aldol routes to tert-butyl-2-(3,5-difluorophenyl)-1-oxiran-2-yl)ethyl)carbamates: Building blocks for novel protease inhibitors

Article abstract of DOI:10.1016/j.tetlet.2017.09.025

Enantioselective syntheses of tert-butyl ((S)-2-(3,5-difluorophenyl)-1-((S)-oxiran-2-yl)ethyl)carbamate and ((S)-2-(3,5-difluorophenyl)-1-((R)-oxiran-2-yl)ethyl)carbamate are described. We utilized asymmetric syn- and anti-aldol reactions to set both ster

Full text of DOI:10.1016/j.tetlet.2017.09.025

Accepted Manuscript
Highly Stereoselective Asymmetric Aldol Routes to tert-Butyl-2-(3,5-difluor-
ophenyl)-1-oxiran-2-yl)ethyl)carbamates: Building Blocks for Novel Protease
Inhibitors
Arun K. Ghosh, Emilio L. Cárdenas, Margherita Brindisi
PII:
S0040-4039(17)31152-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.09.025
TETL 49295
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
14 August 2017
6 September 2017
13 September 2017
Please cite this article as: Ghosh, A.K., Cárdenas, E.L., Brindisi, M., Highly Stereoselective Asymmetric Aldol
Routes to tert-Butyl-2-(3,5-difluorophenyl)-1-oxiran-2-yl)ethyl)carbamates: Building Blocks for Novel Protease
Inhibitors, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.09.025
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Highly Stereoselective Asymmetric Aldol Routes to tert-
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Butyl-2-(3,5-difluorophenyl)-1-(oxiran-2-yl)ethyl)
Carbamates: Building Blocks for Novel Protease Inhibitors
Arun K. Ghosh,* Emilio L. Cárdenas, and Margherita Brindisi
Department of Chemistry and Department of Medicinal Chemistry, Purdue University, 560 Oval Drive, West Lafayette,
Indiana 47907, USA.
O
O
O
OH
H
H
R
N
N
Xc
Boc
Boc
H
H
and
4
7
F
F
F
F
F
F
syn- or, anti-Aldol
Aminoalkyl oxiranes

2
Tetrahedron Letters
jour nal homepage: w ww.elsevier. com
Highly Stereoselective Asymmetric Aldol Routes to tert-Butyl-2-(3,5-
difluorophenyl)-1-oxiran-2-yl)ethyl)carbamates: Building Blocks for Novel Protease
Inhibitors
Arun K. Ghosh,∗ Emilio L. Cárdenas, and Margherita Brindisi
Department of Chemistry and Department of Medicinal Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
ARTICLE INFO
ABSTRACT
Article history:
Received
Enantioselective
syntheses
of
tert-butyl
((S)-2-(3,5-difluorophenyl)-1-((S)-oxiran-2-
yl)ethyl)carbamate and ((S)-2-(3,5-difluorophenyl)-1-((R)-oxiran-2-yl)ethyl)carbamate are
described. We utilized asymmetric syn- and anti-aldol reactions to set both stereogenic centers.
We investigated ester-derived Ti-enolate aldol reactions as well as Evans’ diastereoselective
syn-aldol reaction for these syntheses. We have converted optically active ((S)-2-(3,5-
difluorophenyl)-1-((S)-oxiran-2-yl)ethyl)carbamate to a potent β-secretase inhibitor.
Received in revised form
Accepted
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
Aldol reaction
Asymmetric
Protease Inhibitor
Fluoroisostere
Stereoselective
The design of aspartic acid protease inhibitors continues to
be an important area for drug development and discovery against
a variety of human diseases.1 These include renin inhibitors for
hypertension, HIV protease inhibitors for HIV/AIDS and β- and
γ-secretase inhibitors for Alzheimer’s disease.1-3 The β-secretase
inhibitors are particularly receiving much attention due to their
potential for the treatment of Alzheimer’s disease (AD).4 BACE1
is a membrane anchored aspartic acid protease, responsible for
the initial cleavage of amyloid precursor protein to neurotoxic
amyloid-β-peptides in the brain. The amyloid-β-peptide is the
main component of amyloid plaques, the neuropathological
hallmark of AD.5,6 Since the discovery of BACE1 in 1999,
extensive research efforts led to the evolution of a variety of
small-molecule peptidomimetic and non-peptide BACE1
inhibitors with therapeutic potentials.7,8 Over the years, many
potent and selective peptidomimetic BACE1 inhibitors have been
designed
based
upon
incorporation
of
traditional
hydroxyethylene and hydroxyethylamine transition-state isosteres
at the cleavage site of BACE1.4,9,10 The majority of early BACE1
inhibitors containing phenylalanine and leucine side chains as the
P1 ligand showed low nanomolar BACE1 Ki values and they also
exhibited good reduction of cellular Aβ production.4,7,8 At
present, there are at least three small molecule BACE1 inhibitors
advanced to clinical trials.11,12
Figure 1. Structures of BACE-1 inhibitors 1 and 2.
A clinically effective BACE1 inhibitor, however, needs to
have the ability to cross the blood-brain barrier (BBB) and the
neuronal membrane.13 In this context, traditional transition-state
isosteres were made more lipophilic by inserting fluorines in an
effort to improve membrane permeability, metabolic stability,
and enzyme-inhibitor interactions.4,7,8 As exemplified in Figure 1,
———
∗ Corresponding author. E-mail: akghosh@purdue.edu

peptidomimetic inhibitors 1 (BACE1 IC50 = 5 nM; Cell IC50 = 3
nM) and 2 (BACE1 IC50 = 30 nM; Cell IC50 = 3 µM)
incorporated hydroethylamine and hydroxyethylene dipeptide
isosteres, respectively, with a 3,5-difluorophenylmethyl group as
the P1 ligand.14,15 These difluoro-dipeptide isosteres have been
utilized in the design and synthesis of many other potent and
selective BACE1 inhibitors with BBB permeability.4,7,8
dipeptide isosteres.19,20,23 The synthesis of oxirane 4 using syn-
aldol reaction is shown in Scheme 1. 3,5-Difluorohydrocinnamic
acid 9 was prepared in multigram scale as described in the
literature.24,25 Reaction of this acid with commercially available
N-tosyl-1-aminoindan-2-ol 10 with DCC in the presence of
DMAP in CH2Cl2 at 23 °C for 18 h afforded ester 11 in 76%
yield. Treatment of ester 11 with 1M TiCl4 in the presence of
diisopropylethylamine (DIPEA) in CH2Cl2 at 0 °C to 23 °C for 2
h provided the corresponding Ti-enolate. Reaction of this Ti-
enolate with (benzyloxy)acetaldehyde precomplexed with TiCl4
in CH2Cl2 (1 M CH2Cl2) at -78 °C for 2 h provided the syn-aldol
In general, BACE1 inhibitors containing hydroxyethylamine
isosteres show potent BACE1 inhibitory activity and cellular
activity. The synthesis of inhibitors involves the opening of an
aminoalkyl epoxide with an appropriate amine followed by the
functionalization of the N-terminus with suitable P2 ligands.2,4 As
shown in Figure 2, BACE1 inhibitors containing a 3,5-
difluorophenylmethyl side chain as the P1 ligand in a general
inhibitor 3, can be synthesized from tert-butyl-((S)-1-(S)-oxiran-
2yl)-2-phenylethyl carbamate 4. Enantioselective synthesis of
such epoxide is typically carried out with optically active 3,5-
difluorophenyl alanine 5.16,17 In our continuing interest in the
design and synthesis of novel BACE1 inhibitors containing
fluorines at P1-ligand, we have investigated the synthesis of
difluoroepoxide 4 utilizing an asymmetric syn-aldol as the key
step.18,19 In principle, aldol product such as 6 would provide
access to epoxide by removal of chiral auxiliary (XC) followed by
Curtius rearrangement of the resulting acid to install the amine
functionality.20,21 Furthermore, anti-aldol product such as 8,
obtained from a diastereoselective anti-aldol reaction,22 could
provide access to diastereomeric oxirane derivative 7 for the
synthesis of BACE1 inhibitors with hydroxyethylene isosteres.
Herein, we report a highly diastereoselective synthesis of (2R,
Scheme 1. Synthesis of 3,5-difluorobenzyl epoxide 4
product 12 in 60% yield. The 1H-NMR analysis revealed the
presence of a single diastereomer. Saponification of aldol product
12 with aqueous lithium hydroperoxide at 0 °C to 23 °C for 12 h
provided the corresponding acid which was subjected to Curtius
rearrangement20,26 with diphenylphosphorazidate in the presence
of triethylamine in dry benzene at 90 °C for 12 h to afford
oxazolidinone derivative 13 in 70% yield over 2-steps.
Oxazolidinone was converted to Boc-derivative 14 in a two-step
sequence. Hydrolysis of 13 with aqueous KOH provided
aminoalcohol which was reacted with di-tert-butyl dicarbonate in
a mixture (1:1) of CH2Cl2 and water at 23 °C for 4 h to affod the
Boc-derivative. Catalytic hydrogenation of benzyl ether over
Pearlman’s catalyst using a hydrogen-filled balloon in ethyl
acetate removed the benzyl group and diol 14 was obtained in
77% yield over 2-steps. The diol 14 was converted to epoxide 4
Figure 2. Structures of epoxides 4 and 8 and their
respective aldol precursors
3S)-1,2-epoxy-3-(Boc-amino)-4-(3,5-difluorophenylmethyl)
butane and (2R, 3R)-1,2-epoxy-3-(Boc-amino)-4-(3,5-
difluorophenylmethyl)butane (Figure 2) utilizing an
-
4
7
asymmetric aldol reaction as the key step. The overall route is
amenable to quantities of difluoroepoxide cores 4 and 7 in high
optical purity.
We first investigated ester-derived titanium-enolate based
aldol reactions to afford both syn- and anti-aldol products for the
stereoselective synthesis of aminoalkyl oxiranes 4 and 7. These
reactions have been utilized in the synthesis of a number of

4
Tetrahedron
by a regioselective monotosylation of the primary alcohol with
20.27,28 Treatment of this chiral carboximide derivative with
dibutylboron trifluoromethanesulfonate (Bu2BOTf) in the
presence of N,N-diisopropylethylamine at -78 °C provided the
boron enolate. Reaction of this boron enolate with benzyloxy
acetaldehyde furnished aldol product 21 in 46% yield after
p-toluenesulfonylchloride and triethylamine in the presence of a
catalytic amount (25 mol %) of dibutyltin oxide at 23 °C for 4 h
to provide the corresponding tosylate. Exposure of this tosylate to
K2CO3 in MeOH at 0 °C to 23 °C for 1 h afforded (S)-2-(3,5-
difluorophenyl)-1(S)-oxiranyl ethyl carbamate 4 in 67% yield
over two-steps. The overall route is straightforward and provided
fluorine substituted chiral epoxide for the synthesis of BACE1
inhibitors.
1
standard work up and silica gel chromatography. The H- and
13C-NMR analysis showed the presence of a single diastereomer.
In an effort to improve the overall efficiency of this asymmetric
aldol route, we further explored the Evan’s asymmetric aldol
reaction of chiral carboximide 20 with relatively inexpensive
cinnamaldehyde. As shown in Scheme 3, formation of boron
enolate with Bu2BOTf and aldol reaction with cinnamaldehyde
proceeded very well providing syn-aldol product 22 in near
quantitative yield. Aldol product 22 was obtained as a single
diastereomer by 1H- and 13C-NMR analysis. This is a significant
improvement over aldol reaction with benzyloxyacetaldehyde for
aldol product 21. Exposure of aldol product 21 to lithium
hydroperoxide in aqueous THF at 0 °C to 23 °C for 10 h to
provide the corresponding carboxylic acid. This acid was
subjected to Curtius rearrangement20,21 with diphenyl
phosphorazidate in the presence of triethylamine in toluene at 90
°C for 16 h to provide the corresponding oxazolidinone
derivative. Catalytic hydrogenation of benzyl ether over
Pearlman’s catalyst provided alcohol 23 in 82% yield over 3-
steps. This was converted to epoxide 4 as described above.
O
O
O
t
F
1. -BuCOCl
F
HO
O
N
O
Et3N, Et2O
19
n
2.
, -BuLi, THF
9
20
F
- 78 ºC to 0 ºC
(85%)
F
Ph
O
NH
Bu2BOTf, DIPEA,
(46%) BnOCH2CHO
CH2Cl2, -78 ºC,
19
Bu2BOTf, DIPEA
cinnamaldehyde
CH2Cl2, -78 ºC
Bn
(99%)
O
O
OH
O
O
OH
OBn
F
N
O
N
Ph
21
O
Scheme 2. Synthesis of 3,5-difluorobenzyl epoxide 7
Ph
F
Ph
22
(68%) 1. LiOH, H2O2
For the synthesis of 3,5-difluorobenzyl oxirane 7, we carried
diastereoselective anti-aldol reaction of ester 11 with
cinnamaldehyde as shown in Scheme 2. Ester 11 was converted
to Ti-enolate with 1M TiCl4 in CH2Cl2 in the presence of DIPEA
as described above. This enolate was reacted with trans-
cinnamaldehyde precomplexed with 1.3 equivalents of TiCl4 at -
78 °C for 3 h to provide anti-aldol product 15 as a single isomer
(by 1H-NMR and 13C-NMR analysis). Saponification of ester 15
with aqueous lithium hydroperoxide followed by Curtius
rearrangement of the resulting acid as described above resulted in
oxazolidinone derivative 16 in 96% yield over 2-steps.
Ozonolytic cleavage of the double bond in compound 16 was
achieved by passing ozone in a mixture (4:1) of CH2Cl2 and
MeOH at -78 °C followed by reduction with NaBH4 providing
alcohol 17 in 90% yield. Exposure of alcohol 17 to aqueous KOH
followed by reaction of the resulting aminoalcohol with (Boc)2O
in CH2Cl2 afforded Boc-derivative 18 in 67% yield. Diol
derivative 18 was converted to (S,R)-3,5-difluorophenyl oxirane
7 by selective tosylation followed by treatment of the resulting
tosylate with K2CO3 in MeOH as described for compound 14 in
Scheme 1 to provide oxirane 7 in 67% yield over 2-steps.
F
1. LiOH, H2O2
2. DPPA, Et3N
3. H2, Pd(OH)2
F
(82%)
for 3-steps
for 2-steps
2. DPPA, Et3N
O
O
O
O
HN
HN
OH
Ph
O3, NaBH4
23
24
CH2Cl2, MeOH
F
F
F
F
O
H
N
Boc
4
F
F
Scheme 3. Synthesis of 3,5-difluorobenzyl epoxide 4
Aldol product 22 was also converted to epoxide 4. The
removal of the chiral auxiliary by exposure to lithium
hydroperoxide followed by Curtius rearrangement of the
resulting acid with DPPA as described above furnished
oxazolidinone derivative 24 in 68% yield after silica gel
chromatography. Ozonolysis of the double bond in a mixture
The synthesis of (S)-2-(3,5-difluorophenyl)-1-(S)-(oxiran-2-
yl)ethyl carbamate 4 using Evans’ syn-aldol reaction is shown in
Scheme 3. Difluorohydrocinnamic acid 9 was transformed into a
mixed anhydride with pivaloyl chloride and triethylamine at -78
°C. The resulting mixed anhydride was reacted with lithio
derivative of chiral oxazolidinone 19 to furnish carboximide

(4:1) of CH2Cl2 and methanol at -78 °C followed by a reductive
workup with NaBH4 furnished alcohol 23 in 90% yield. Alcohol
23 was readily converted to Boc-derivative 14 by exposure to
aqueous KOH in ethanol followed by protection of the resulting
crude amine as Boc derivative 14 as described previously.
We demonstrated the utility of difluorophenylethyl oxirane
derivative 4 in the synthesis of known BACE1 inhibitor 27.14 As
shown in Scheme 4, reaction of oxirane 4 with 3-methoxy-
benzylamine in isopropanol at 80 °C for 12 h provided the
corresponding aminoalcohol. Deprotection of the Boc-group by
exposure to trifluoroacetic acid (TFA) in CH2Cl2 at 23 °C for 2 h
afforded aminoalcohol 25. Coupling of the primary amine with
known14 isophthalic acid derivative 26 in the presence of HATU
and triethylamine in CH2Cl2 at 23 °C for 16 h furnished inhibitor
27 in 43% yield over three steps.
Supplementary Data
Supplementary data associated with this article can be found in
the online version.
References and notes
1. Ghosh, A. K. Ed. ‘Aspartic acid protease as therapeutic
targets’ Wiley-VCH, Weinheim, Germany, 2010.
2. Ghosh, A. K.; Osswald, H. L.; Prato, G. J. Med. Chem. 2016,
59, 5172-5208.
3. Ghosh, A. K.; Cárdenas, E. L.; Osswald, H. L. Top. Med.
Chem., 2016, ASAP; Ghosh, A. K.; Tang, J. ChemMedChem,
2015, 10, 1463-1466.
4. Ghosh, A. K.; Osswald, H. L. Chem. Soc. Rev. 2014, 43,
6765-6813.
5. Hardy, J.; Selkoe, D. J. Science 2002, 297, 353-356.
6. Selkoe, D. J.; Schenk, D. Annu. Rev. Pharacol. 2003, 43,
545-584.
7. Citron, M. Trends in Pharma. Sci. 2004, 25, 92-97.
8. Iserloh, U.; Cummins, J. N. "Peptidomimetic BACE1
Inhibitors for Treatment of Alzheimer's Disease: design and
evolution in Aspartic Acid proteases as Therapeutic Targets"
(ed A. K. Ghosh) Wiley-VCH, 2010, p. 441-479.
9. Cole, C. D.; Bursavich, M. Nonpeptide BACE1 inhibitors:
Design and Synthesis In: Ghosh AK, ed. Aspartic acid
proteases as therapeutic targets. Wiley-VCH, 2010, p. 481-
509.
10. Ghosh, A. K.; Brindisi, M.; Tang, J. J. Neurochem. 2012, 120
Suppl 1, 71-83.
11. Vassar, R.; Kovacs, D. M.; Yan, R.; Wong, P. C. J. Neurosci.
2009, 29, 12787-12794.
12. Vassar, R. Alzheimer's Res. Ther. 2014, 6, 89.
13. Ghosh, A. K.; Bilcer, G.; Hong, L.; Koelsch, G.; Tang, J.
Curr. Alzheimer Res. 2007, 4, 418-422.
Scheme 4. Synthesis of BACE1 inhibitor 27
14. Maillard, M. C.; Hom, R. K.; Benson, T. E.; Moon, J. B.;
Mamo, S.; Bienkowski, M.; Tomasselli, A. G.; Woods, D. D.;
Prince, D. B.; Paddock, D. J.; Emmons, T. L.; Tucker, J. A.;
Dappen, M. S.; Brogley, L.; Thorsett, E. D.; Jewett, N.;
Sinha, S.; John, V. J. Med. Chem. 2007, 50, 776-781.
15. Hom, R. K.; Gailunas, A. F.; Mamo, S.; Fang, L. Y.; Tung, J.
S.; Walker, D. E.; Davis, D.; Thorsett, E. D.; Jewett, N. E.;
Moon, J. B.; John, V. J. Med. Chem. 2004, 47, 158-164.
16. Greenlee, W. J. Med. Res. Rev. 1990, 10, 173-236.
17. Ghosh, A. K.; Bilcer, G.; Schiltz, G. Synthesis, 2001, 15,
2203-2229.
18. Evans, D. A.; Takacs, J. M.; McGee, L. R.; Ennis, M. D.;
Mathre, D. J.; Bartroli, J. Pure & Appl. Chem. 1981, 53,
1109-1127.
19. Ghosh, A. K.; Dawson, Z. Synthesis 2009, 17, 2992-3002.
20. Ghosh, A. K.; Fidanze, S. J. Org. Chem. 1998, 63, 6146-
6152.
In conclusion, we accomplished convenient syntheses of
aminoalkyl oxiranes 4 and 7 containing 3,5-difluorobenzyl side
chain using an asymmetric aldol reaction as the key step. The
stereochemistry of both stereogenic centers was set by highly
diastereoselective syn- and anti-aldol reactions. The removal of
the chiral auxiliary followed by Curtius rearrangement of the
resulting acid installed the amine functionality. This was readily
converted to epoxides 4 and 7 efficiently. These epoxides are
important building blocks for the synthesis of a variety of
BACE1 inhibitors incorporating hydroxyethylamine and
hydroxyethylene isosteres. The overall route is quite efficient,
scalable and provides facile access to diverse inhibitors. We have
converted epoxide 4 to BACE1 inhibitor 27. Further application
of these epoxides in the synthesis of novel protease inhibitors is
in progress in our laboratory.
21. am Ende, D. J.; DeVries, K. M.; Clifford, P. J.; Brenek, S. J.
Org. Proc. Res. & Dev. 1998, 2, 382-392.
22. Ghosh, A. K.; Onishi, M. J. Am. Chem. Soc. 1996, 118, 2527-
2528.
Acknowledgments
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4755-4763.
Financial support by the National Institutes of Health (GM53386)
is gratefully acknowledged. We would also like to thank the
Purdue University Center for Cancer Research, which supports
the shared NMR and mass spectrometry facilities.
24. Shi, Z.; et al. Bioorg. Med. Chem. Lett. 2005, 13, 4200-4208.
25. Jin, Y. Z.; et al. Green Chem. 2002, 4, 498-500.
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103, 2127-2129.
28. Gage, J. R.; Evans, D. A. Org. Synth. 1990, 68, 83-91.

6
Tetrahedron
Highlights
• Enantioselective syntheses of difluorinated aminoalkyl
epoxides are reported.
• The synthesis utilized Ti-enolate and boron-enolate
asymmetric aldol reactions.
• Aldol products were obtained with high
diastereoselectivites and in good yields.
• Curtius rearrangement of β-hydroxy acids installed the
amine functionalities.
• A fluorinated aminoalkyl oxirane was converted to a
potent BACE1 inhibitor.