A concise enantioselective synthesis of (R)-selegiline, (S)-benzphetamine and formal synthesis of (R)-sitagliptin via electrophilic azidation of chiral imide enolates

Article abstract of DOI:10.1016/j.tetasy.2014.11.014

A concise and high yielding enantioselective synthesis of (R)-selegiline, an anti-Parkinson's drug, (S)-benzphetamine, an anti-obesity agent, and (S)-sitagliptin, an anti-diabetic drug has been described starting from commercially available starting materials employing Evans' electrophilic azidation of chiral imide enolates as a key chiral inducing step, which proceeds in a highly diastereoselective manner (>99%).

Full text of DOI:10.1016/j.tetasy.2014.11.014

Tetrahedron: Asymmetry 26 (2015) 67–72
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
A concise enantioselective synthesis of (R)-selegiline,
(S)-benzphetamine and formal synthesis of (R)-sitagliptin
via electrophilic azidation of chiral imide enolates
⇑
Soumen Dey, Arumugam Sudalai
Chemical Engineering and Process Development Division, CSIR-National Chemical Laboratory, Pashan Road, Pune 411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 November 2014
Accepted 20 November 2014
A concise and high yielding enantioselective synthesis of (R)-selegiline, an anti-Parkinson’s drug,
(S)-benzphetamine, an anti-obesity agent, and (S)-sitagliptin, an anti-diabetic drug has been described
starting from commercially available starting materials employing Evans’ electrophilic azidation of chiral
imide enolates as a key chiral inducing step, which proceeds in a highly diastereoselective manner
(>99%).
Ó 2014 Elsevier Ltd. All rights reserved.
CH3
1. Introduction
CH3
N
N
H3C
Chiral homobenzylic amines are subunits widely found in a
range of biologically active compounds, for example, 1–3. Further-
more, they serve as extremely useful synthetic intermediates, since
they can be transformed into an array of highly functionalized het-
erocycles. In particular, pharmaceutical substances belonging to
this category such as (R)-selegiline 1, (S)-benzphetamine 2, and
(S)-sitagliptin 3 are currently used in the treatment of a variety
of diseases. More importantly, (R)-selegiline 1 is a selective irre-
versible MAO-B inhibitor1 that works by slowing the breakdown
of certain natural substances in the brain (e.g. dopamine, norepi-
nephrine, and serotonin). It is usually used in combination with
L-DOPA or carbidopa for the treatment of early-stage Parkinson’s
disease, depression, and senile dementia, while (S)-benzphetamine
2, an anorectic drug, is an amphetamine derivative that exhibits
appetite suppressant activity and is utilized in the long-term man-
agement of obesity.2 In addition, compound 2 has been found to be
a superior bronchodilator and a CNS stimulator, which increases
heart rate and blood pressure. It has been established that dipepti-
dyl peptidase IV (DPP-IV) inhibitors are known to stimulate insulin
secretion indirectly by enhancing the action of the incretin hor-
mones glucagen-like peptide I (GLP-I) and glucose-dependent
insulinotropic polypeptide (GIP).3 (R)-Sitagliptin 3, a b-amino acid
derivative, is a potent DPP-IV inhibitor enzyme, which offers a new
mechanism in achieving glycemic control for the treatment of type
2 diabetes (Fig. 1).
H3C
2
(S)-benzphetamine
(R)-selegiline 1
CF3
N
F
F
COOH
F
F
N
N
N
NHBoc
F
NH2
O
F
(R)-sitagliptin 3
4
Figure 1. Structures of (R)-selegiline 1, (S)-benzphetamine 2, and (R)-sitagliptin 3.
Due to their highly potent biological activity, considerable
efforts have been devoted to their asymmetric synthesis. Although
many strategies for the synthesis of (R)-selegiline 1 have been
reported in recent years, most of them have relied on racemic
approaches,4 chiral pool starting materials,5 asymmetric hydroge-
nation, classical/kinetic resolutions,6 regioselective aziridine ring
openings with organocuprates,7 OsO4-catalyzed asymmetric
dihydroxylations8a as well as proline based
a-functionalization
of aldehydes.8b However, for (S)-benzphetamine 2, a recent report9
used a chiral pool approach for its synthesis. In the case of (R)-
sitagliptin 3, the coupling of b-amino acid derivative 4 with a
triazolopyrazine unit is generally employed and its asymmetric
synthesis mainly focuses on asymmetric reductions: (i) Ru-cata-
lyzed chemo- and stereoselective hydrogenations of a b-ketoester
followed by an EDC coupling/Mitsunobu reaction sequence;10 (ii)
⇑
Corresponding author. Tel.: +91 20 25902565; fax: +91 20 25902676.
E-mail address: a.sudalai@ncl.res.in (A. Sudalai).
http://dx.doi.org/10.1016/j.tetasy.2014.11.014
0957-4166/Ó 2014 Elsevier Ltd. All rights reserved.

68
S. Dey, A. Sudalai / Tetrahedron: Asymmetry 26 (2015) 67–72
Rh-catalyzed asymmetric hydrogenations of an enamine under
high pressure;11 (iii) biocatalytic or Ru-catalyzed reductive amin-
ations of a b-ketoamide.12 Quite recently, Davies et al.13 reported
on a highly diastereoselective conjugate addition of enantiopure
this area is to prepare enantioselectively the stereocenter present
in the b-amino acid unit.14 However, most of the reported meth-
ods for the synthesis of these drugs suffer from long reaction
sequences, low yields, and diastereoselectivity, and the use of
transition metals as catalysts for the introduction of the chiral
amine functionality at the homobenzylic position.
secondary lithium amides to
a,b-unsaturated esters to facilitate
an efficient synthesis of (R)-sitagliptin 3. The main challenge in
The use of Evans’ chiral N-acyloxazolidinone auxiliaries to con-
trol absolute stereoinduction has found wide application in a vari-
ety of reactions over the last two decades.15 The ready availability
of the starting materials, the ease of cleavage, and application to a
broad range of stereoselective reactions allow oxazolidinone auxil-
iaries to endure as ideal intermediates for asymmetric synthesis.
We envisioned that the chiral amine functionality could be intro-
duced via Evans’ electrophilic azidation of chiral imide enolates
using a chiral auxiliary, followed by reduction. Herein we report
a short, enantioselective synthesis of drug molecules 1, 2 and the
formal synthesis of 3 based on an Evans’ chiral azidation approach
(Schemes 1–3).
OH
O
O
iii
i
Ph
Ph
N
CO2H
O
Ph
N3
R
Bn
8
5
6
7
, R = H
ii
, R = N3
OR
CH3
R
iv
vi
Ph
H3C
11, R = H
Ph
N
NHBoc
9, R = H
10
2. Results and discussion
v
vii
, R = Ts
1
, R = propargyl
The synthetic sequences of these bioactive molecules are shown
in Schemes 1–3, which commenced from commercially available
hydrocinnamic acid employing an Evans’ chiral auxiliary protocol.
Thus the condensation of (S)-4-benzyloxazolidin-2-one, the chiral
auxiliary, with hydrocinnamic acid 5 via the formation of a
pivolyl ester (pivalolyl chloride, Et3N, À20 °C, THF, 3 h followed
by (S)-4-benzyloxazolidin-2-one, LiCl, À20 to 25 °C, 8 h) gave
oxazolidinone 6 (90%). The electrophilic azidation of chiral imide
Scheme 1. Reagents and conditions: (i) pivalolyl chloride, Et3N, dry THF, À20 °C,
3 h then (S)-4-benzyloxazolidin-2-one, LiCl, À20 to 25 °C, 8 h, 90%; (ii) KHMDS,
À78 °C, dry THF, 45 min, then 2,4,6-triisopropylbenzenesulfonyl azide, 5 min, then
AcOH, À78 to 25 °C, 12 h, 85%; (iii) NaBH4, THF/ H2O (3:1), 0–25 °C, 2 h, 95%; (iv) H2
(1 atm), 10% Pd/C, Boc2O, MeOH, 5 h, 90%; (v) TsCl, Et3N, CH2Cl2, 0–25 °C, 3 h; (vi)
LiAlH4, THF, reflux, 4 h, 65% (over two steps); (vii) propargyl bromide, K2CO3,
CH3CN, 3 h, 25 °C, 71%.
enolate
6
at
the
a
-position
(KHMDS,
2,4,6,-tri-
O
O
isopropylbenzenesulfonyl azide, THF, À78 °C; quenching with
CH3
ii - vi
Ph
H3C
Ph
N
AcOH) was carried out to produce azido oxazolidinone 7 (85%)
O
N
R
{[a]
25 = + 67.6 (c 1.0, CH2Cl2} (dr >99%).15a The reductive removal
D
R
Bn
of the chiral auxiliary was then achieved using NaBH4 in THF/
H2O to give the free b-azido alcohol 8 {97% ee as determined by
HPLC analysis (Chiracel AD-H column, Hex/i-PrOH 95:05, 0.3 mL/
min, 220 nm, retention time: tmajor = 36.12 min and tminor = 38.38 -
min.) and 95% yield}. The catalytic hydrogenation [10% Pd/C, H2
(1 atm), Boc2O, MeOH] of azide 8 furnished the corresponding
amino alcohol (90%) in which the amine function was protected
as carbamate 9 followed by formation of tosylate 10. Reduction
of 10 with LiAlH4 gave secondary methyl amine 11, which was
readily N-alkylated with propargyl bromide affording (R)-selegiline
ent 11, R = H
2, R = Bn
ent 6, R = H
ent 7, R = N3
vii
i
Scheme 2. Reagents and conditions: (i) pivalolyl chloride, Et3N, dry THF, À20 °C,
3 h then (S)-4-benzyloxazolidin-2-one, LiCl, À20 to 25 °C, 8 h, 90%; (ii) KHMDS,
À78 °C, dry THF, 45 min, then 2,4,6-triisopropylbenzenesulfonyl azide, 5 min, then
AcOH, À78 to 25 °C, 12 h, 85%; (iii) NaBH4, THF/ H2O (3:1), 0–25 °C, 2 h, 95%; (iv) H2
(1 atm), 10% Pd/C, Boc2O, MeOH, 5 h, 90%; (v) TsCl, Et3N, CH2Cl2, 0–25 °C, 3 h; (vi)
LiAlH4, THF, reflux, 4 h, 65% (over two steps); (vii) benzyl bromide, K2CO3, CH3CN,
2 h, 25 °C, 73%.
O
O
OH
iv
i - iii
vi
Ar
N
O
CO2H
Ar
Ar-CHO
Ar
R
N3
Bn
14, R = H
15, R = N3
16
12
13
v
Ar = 2,4,5-trifluorophenyl
R
x
vii
Ar
ref:22
(R)-sitagliptin
Ar
CO2H
3
NHBoc
NHBoc
4
17, R = OH
18, R = OTs
19, R = CN
viii
ix
Scheme 3. Reagents and conditions: (i) Ph3P@CHCO2Et, benzene, reflux, 4 h, 98%; (ii) H2 (1 atm), 10% Pd/C, MeOH, 1 h, 98%; (iii) LiOH, THF/MeOH/H2O (3:1:1), 2 h, 96%; (iv)
pivolyl chloride, Et3N, dry THF, À20 °C, 3 h then (R)-4-benzyloxazolidin-2-one, LiCl, À20 to 25 °C, 8 h, 94%; (v) KHMDS, À78 °C, dry THF, 45 min, then 2,4,6-
triisopropylbenzenesulfonyl azide, 5 min, then AcOH, À78 to 25 °C, 12 h, 88%; (vi) NaBH4, THF/ H2O (3:1), 0–25 °C, 2 h, 98%; (vii) H2 (1 atm), 10% Pd/C, Boc2O, MeOH, 3 h, 98%;
(viii) TsCl, Et3N, CH2Cl2, 1 h then (ix) NaCN, DMF, 80 °C, 4 h, 65% (over two steps); (x) 3 M NaOH, H2O2, 100 °C, 3 h, 75%.

S. Dey, A. Sudalai / Tetrahedron: Asymmetry 26 (2015) 67–72
69
1 in 30% overall yield. The enantiomeric purity of 1 was deter-
ysis was carried out on a Carlo Erba CHNS-O analyzer. Purification
was carried out using column chromatography (60–120 mesh).
Enantiomeric excesses were determined on Agilent HPLC instru-
ment equipped with a suitable chiral column.
mined to be 97% ee based on the comparison of its specific rotation
with the reported values
[
a
]
25 = À10.5 (c 6.3, EtOH) {lit.16
D
[
a]
D
25 = À10.8 (c 6.4, EtOH)}. As the ee values of all these com-
pounds were 97%, it indicates the enantiomeric purity of starting
oxazolidinone.
4.2. Experimental procedures
The synthesis of (S)-benzphetamine 2 was achieved by follow-
ing a similar sequence of reactions except that the chiral auxiliary
used was (R)-4-benzyloxazolidin-2-one (Scheme 2). Excellent
yields and ees were obtained in each step. Benzylation of ent-11
was the final step and gave (S)-benzphetamine 2 in 31% overall
yield. The enantiomeric purity of 2 was determined to be 97% ee
based on the comparison of its specific rotation with the reported
4.2.1. (S)-4-Benzyl-3-(3-phenylpropanoyl)oxazolidin-2-one 6
To a stirred solution of hydrocinnamic acid 5 (5 g, 33.2 mmol) in
dry THF (100 mL) was added pivolyl chloride (4 g, 33.2 mmol) and
Et3N (10 g, 99.6 mmol) at À20 °C and the mixture was stirred at the
same temperature for 4 h. To this stirred suspension, (S)-4-benzyl-
oxazolidin-2-one (6.5 g, 36.52 mmol) in dry THF (20 mL) was
added dropwise followed by the addition of LiCl (1.5 g, 33.2 mmol)
after which it was stirred for an additional 15 min at À20 °C and
stirring continued at 25 °C for 8 h until the complete consumption
of the starting materials (the progress of the reaction was moni-
tored by TLC). The product was then extracted with diethyl ether
and the combined organic layer was washed with water, brine,
and dried over anhydrous Na2SO4. Removal of the solvent under
reduced pressure gave the crude product, which upon column
chromatographic purification with petroleum ether/ethyl acetate
values [a] [a]
25 = +52.3 (c 0.28, CHCl3) {lit.9 25 = +53.9 (c 1, CHCl3}.
D
D
The spectroscopic data of compounds 1 and 2 were found to be
in good agreement with the reported values.16,9
The synthetic sequence for b-amino acid 4 is shown in Scheme 3
starting from 2,4,5-trifluorobenzaldehyde 12. Dihydrocinnamic
acid 13 was obtained from aldehyde 12 by simple functional group
manipulations: (i) two carbon homologation with a stabilized
Wittig ylide; (ii) hydrogenation of the benzylic C@C bond by 10%
Pd/C over H2 (1 atm); and (iii) conversion of the ester group into
an acid by LiOH-mediated hydrolysis. Azido alcohol 16 {98% ee
determined by HPLC (Chiracel AD-H column, Hex/i-PrOH 95:05,
(4:1) gave 6 as a colorless solid. Yield: 9.07 g, 90%; [
a]
25 = +66.6
D
(c 1.0, CHCl3) {lit.18 25 = +67.4 (c 0.98, CHCl3)}; mp: 101–
[a]
D
0.5 mL/min, 220 nm, retention time: tmajor = 21.73 min and tminor
=
102 °C; IR (CHCl3, cmÀ1): tmax 3065, 3030, 1781, 1697, 1387,
1212; 1H NMR (200 MHz, CDCl3): d 7.15–7.32 (m, 10 H), 4.63–
4.66 (m, 1H), 4.14–4.17 (m, 2H), 3.23–3.29 (m, 3H), 3.02–3.05
(m, 2H), 2.72 (dd, J = 9.7, 13.3 Hz, 1H); 13C NMR (50 MHz, CDCl3):
d 172.2, 153.2, 140.4, 135.2, 129.4, 128.9, 128.6, 127.3, 126.3,
66.0, 55.1, 37.8, 37.1, 30.3; Anal. Calcd for C19H19NO3 requires C,
73.77; H, 6.19; N, 4.53%; found C 73.79; H 6.15; N 4.55%.
20.03 min.) and 98% yield} was prepared by following a similar
reaction sequence using the Evans’ chiral auxiliary (R)-4-benzylox-
azolidin-2-one. Here, compound 15 was obtained with high diaste-
reoselectivity (dr >99%). Consequently, carbamate 17 was obtained
from 16 in a single step using catalytic hydrogenation [10% Pd/C,
H2 (1 atm), Boc2O, MeOH] in 98% yield. The alcohol functionality
in 17 was readily transformed into cyanide 19 via SN2 displace-
ment of its tosylate 18. Subsequently, the cyanide functionality
4.2.2. (2R,4S)-3-(2-Azido-3-phenyl-1-oxopropyl)-4-(phenylme-
thy1)-2-oxazolidinone 7
was converted into the corresponding carboxylic acid (3 M NaOH,
17
H2O2, reflux)
to give the known intermediate 4 in 75% yield,
To a stirred solution of 6 (8.5 g, 28.02 mmol) in dry THF (90 ml),
61.55 ml of 0.5 M in toluene (30.82 mmol) of potassium hexameth-
yldisilazide (KHMDS) was added under N2 at À78 °C and the
mixture was stirred for 45 min. To this suspension of potassium
enolate, stirred at À78 °C, was added 2,4,6-triisopropyl azide
(11.2 g, 36.42 mmol) in dry THF (30 mL). After 5 min, the reaction
was quenched with 8 ml (140.1 mmol) of glacial acetic acid and
stirred at 25 °C for 12 h. The solution was then partitioned between
CH2Cl2 and brine solution. The organic phase was washed with
aqueous NaHCO3, dried over Na2SO4, and evaporated in vacuo. Col-
umn chromatographic purification of the crude product with
petroleum ether/ethyl acetate (4:1) gave 7 as a yellow solid. Yield:
thereby constituting a formal synthesis of 322. The enantiomeric
purity of 4 was determined to be 98% ee based on the comparison
of its specific rotation with the reported values [
a]
25 = +31.8 (c 1,
D
CHCl3) {lit.21 25 = +32.3 (c 1, CHCl3)}.
[a]
D
3. Conclusion
In conclusion, we have specifically provided an efficient proce-
dure for the enantioselective synthesis of two important drugs,
(R)-selegiline 1 (30% overall yield; 97% ee) and (S)-benzphetamine
2 (31% overall yield; 97% ee), and 4 (36% overall yield; 98% ee), an
advanced intermediate in the formal synthesis of (R)-sitagliptin 3
from commercially available starting materials. In this approach,
the key intermediates were readily prepared with high diastere-
oselectivity from the corresponding carboxylic acids by employing
an Evans’ asymmetric direct azidation reaction. This methodology
should find wide applicability for the synthesis of many drug can-
didates with homobenzylic amine units with high enantioselectiv-
ity and diastereoselectivity.
8.3 g, 85%; [
a] [a]
25 = + 67.6 (c 1.0, CH2Cl2); {lit.15a 25 = +68 (c 1.0,
D
D
CH2Cl2)} mp 116–120 °C; IR (CHCl3, cmÀ1
)
tmax 3065, 3030, 2987,
2111, 1781, 1701, 1389; 1H NMR (200 MHz, CDCl3): d 7.13–7.36
(m, 10 H), 5.18 (q, J = 5.0 Hz, 1H), 4.65–4.77 (m, 1H), 4.19–4.29
(m, 2H), 3.34 (dd, J = 5.0 Hz and 13.5 Hz, 1H), 3.18 (dd, J = 3.1,
13.1 Hz, 1H), 3.03 (dd, J = 9.3, 13.6 Hz, 1H), 2.67 (dd, J = 9.4,
13.3 Hz, 1H); 13C NMR (50 MHz, CDCl3): d 170.3, 152.7, 135.7,
134.6, 129.4, 129.1, 128.7, 127.6, 127.3, 66.6, 61.3, 55.1, 37.7,
37.4; Anal. Calcd for C19H18N4O3 requires C 65.13; H 5.18; N
15.99%; found C 65.15; H 5.20; N 15.95%.
4. Experimental
4.1. General
4.2.3. (S)-2-Azido-3-phenylpropan-1-ol 8
To a stirred solution of 7 (5 g, 14.27 mmol) in THF (30 mL) was
added a solution of sodium borohydride (1.0 g, 28.54 mmol) in
water (10 mL) dropwise at 0 °C. After the addition, it was left to stir
at 25 °C for 2 h. Upon completion of the reaction (monitored by
checking TLC), 2 M HCl (20 mL) was added slowly so that the tem-
perature was maintained at 25 °C. The reaction mixture was then
extracted with ethyl acetate and washed with brine. The organic
phase was concentrated and upon column chromatographic
Solvents were purified and dried by standard procedures prior
to use. Optical rotations were measured using sodium D line on a
JASCO-181 digital polarimeter. IR spectra were recorded on a
Thermo Scientific–Nicolet 380 FT-IR and absorption is expressed
in cmÀ1 1H NMR and13C NMR spectra were recorded on a Bruker
.
AC-200 spectrometer unless mentioned otherwise. Elemental anal-

70
S. Dey, A. Sudalai / Tetrahedron: Asymmetry 26 (2015) 67–72
purification with petroleum ether/ethyl acetate (3:7) gave 8 as a
petroleum ether/ethyl acetate (4:1) to give pure (R)-selegiline 1.
colorless liquid. Yield: 2.4 g, 95%; [
a
]
D
25 = À2.3 (c 1, CHCl3) {lit.19
Yield: 41 mg, 71%, gum;
[
a
]
25 = À10.7 (c 6.5, EtOH); {lit.16
D
[a
]
D
25 = À2.4 (c 1.0, CHCl3)}; IR (CHCl3, cmÀ1) 3439, 2108, 1092,
[a]
25 = À10.8 (c 6.4, EtOH)}; 1H NMR (200 MHz, CDCl3): d 7.26–
D
1046; 1H NMR (200 MHz, CDCl3): d 7.21–7.36 (m, 5H), 3.68 (d,
J = 11.1 Hz, 2H), 3.54–3.58 (m, 1H), 2.76–2.87 (m, 2H), 2.30 (br, s,
1H); 13C NMR (50 MHz, CDCl3): d 136.9, 129.2, 128.6, 126.8, 65.2,
64.2, 36.9; Anal. Calcd for C9H11N3O requires C 61.00; H 6.26; N
23.71%; found C 61.03; H 6.24; N 23.73%; Enantiomeric purity:
97% ee determined by HPLC analysis (Chiracel AD-H column,
Hex/i-PrOH 95:05, 0.3 mL/min, 220 nm). Retention time:
tmajor = 36.12 min and tminor = 38.38 min.
7.23 (m, 2H), 7.19–7.14 (m, 3H), 3.42 (d, J = 2.4 Hz, 2H), 2.92–
3.08 (m, 2H), 2.37–2.42 (m, 4H), 2.21 (t, J = 2.4 Hz, 1H), 0.98 (d,
J = 6.6 Hz, 3H); 13C NMR (50 MHz, CDCl3): d 140.1, 129.3, 128.3,
125.9, 80.2, 72.6, 59.4, 43.2, 38.8, 37.4, 15.2; Anal. Calcd for
C13H17N requires C 83.37; H 9.15; N 7.48%; found C 83.35; H
9.16; N 7.50%.
4.2.7. (S)-N-Benzyl-N-methyl-1-phenylpropan-2-amine: (S)-
benzphetamine 2
4.2.4. (S)-tert-Butyl-1-hydroxy-3-phenylpropan-2-yl-carbamate 9
A mixture of azido alcohol 8 (2.5 g, 10.8 mmol), 10% Pd/C, and
di-tert-butyl dicarbonate (2.35 g, 10.8 mmol) in dry MeOH
(20 mL) was stirred under H2 (1 atm) at 25 °C for 5 h. After comple-
tion of reaction (monitored by TLC), it was filtered through Celite
(MeOH eluent) and the solvent was evaporated under reduced
pressure to afford 9 as a colorless solid. Yield: 2.5 g, 90%; mp:
To a stirred solution of (S)-2-(methylamino)-1-phenylpropane
ent-11 (0.080 g, 0.67 mmol) in CH3CN (3 mL) were added anhydrous
K2CO3 (0.146 g, 1.06 mmol) and benzyl bromide (0.1 mL,
0.79 mmol). The reaction mixture was then stirred for 2 h at 25 °C,
and then the solvent was evaporated under reduced pressure to pro-
vide the crude product, which was purified by column chromatogra-
phy over silica gel using petroleum ether/ethyl acetate (4:1) to give
96–98 °C;
[
a]
25 = À26.4 (c 1, MeOH) {lit.20
[
a]
25 = À27 (c 1,
pure (S)-benzphetamine 2. Yield: 98 mg, 79%; [
a]
25 = +52.3 (c 0.28,
D
D
D
MeOH)}; IR (CHCl3, cmÀ1) 3353, 2978, 2933, 1685, 1526, 1390,
1268; 1H NMR (200 MHz, CDCl3): d 7.19–7.34 (m, 5H), 4.69 (br s,
1H), 3.66–3.83 (m, 1H), 3.51–3.63 (m, 2H), 2.82 (d, J = 7.2 Hz,
2H), 2.32 (br s, 1H), 1.42 (s, 9H); 13C NMR (50 MHz, CDCl3): d
28.3, 37.4, 53.5, 63.7, 79.5, 126.4, 128.4, 129.3, 137.9, 156.0; Anal.
Calcd for C14H21NO3 requires C 66.91; H 8.42; N 5.57%; found C
66.93; H 8.40; N 5.55%.
CHCl3); {lit.9 25 = +53.9 (c 1, CHCl3}; 1H NMR (200 MHz, CDCl3):
[a]
D
d 7.12–7.30 (m, 10H), 3.6 (d, J = 2.4 Hz, 2H), 2.96–3.04 (m, 2 H),
2.42–2.54 (m, 1H), 2.24 (s, 3H), 0.99 (d, J = 6.3 Hz, 3H); 13C NMR
(50 MHz, CDCl3): d 140.7, 140.0, 129.2, 128.6, 128.1, 126.7, 125.7,
59.7, 57.8, 39.5, 36.8, 14.0; Anal. Calcd for C17H21N requires C
85.30; H 8.84; N 5.85%; found C 85.30; H 8.86; N 5.83%.
4.2.8. 3-(2,4,5-Trifluorophenyl)propanoic acid 13
4.2.5. (R)-N-Methyl-1-phenylpropan-2-amine 11
To a stirred solution of 2,4,5-trifluorobenzalkdehyde 12 (5 g,
31.23 mmol) in benzene (100 mL) stabilized Wittig salt
Ph3P@CHCO2Et (21.7 g, 62.46 mmol) was added and refluxed
overnight. After completion of the reaction (checked by TLC), the
solvent was evaporated and pure adduct (4.9 g) was obtained by
column chromatographic separation using petroleum ether/ethyl
acetate (9:1). The product was then hydrogenated using 10% Pd/
C, H2 (1 atm) for 1 h in MeOH. After completion of the reaction
(as monitored by TLC), it was filtered through Celite (MeOH eluent)
and the solvent was evaporated off under reduced pressure to
afford 3-(2,4,5-trifluorophenyl)ethylpropanoate (4.8 g), which
was then hydrolyzed using LiOH (1.3 g, 56.1 mmol) in THF/
MeOH/H2O (3:1:1) to give 13 as a colorless gum. Yield: 4.6 g,
96%; IR (CHCl3, cmÀ1): 3105, 2903, 1772, 1052, 1016; 1H NMR
(200 MHz, CDCl3): d 6.91–7.02 (m, 1H), 7.04–7.26 (m, 1H), 2.92
(t, J = 7.34 Hz, 2H), 2.67 (t, J = 7.34 Hz, 2H); 13C NMR (50 MHz,
CDCl3): d 23.7, 33.85, 105.4 (dd, J = 20.2, 27.1 Hz), 118.3 (dd,
J = 19.5, 6.4 Hz), 123.2 (ddd, J = 4.3, 9.2, 17.7 Hz), 145.5 (ddd,
J = 4.2, 5.7, 237.9 Hz), 147.5 (ddd, J = 250.5, 11.6, 3.5 Hz), 157.1
(ddd, J = 239.8, 10.2, 7.6 Hz); Anal. Calcd for C9H7F3O2 requires C
52.95; H 3.46%; found C 52.90; H 3.48%.
To a stirred solution of N-Boc protected amino alcohol 9
(0.5 mg, 1.98 mmol) in CH2Cl2 (5 mL) were added dry triethyl-
amine (0.3 mL, 2.37 mmol) and p-toluenesulfonyl chloride
(0.452 g, 2.37 mmol) in the presence of a catalytic amount of
4-dimethylaminopyridine (0.024 g,10 mol%) at 0 °C. The reaction
mixture was stirred at 25 °C for 3 h and then quenched by addition
of 10% NaHCO3. The aqueous layer was extracted with CH2Cl2
(3 Â 20 mL) and the combined organic layers were dried over
anhydrous Na2SO4, and concentrated to give the crude tosylate,
which was then dissolved in dry THF (5 mL), and added dropwise
to a suspension of LiAlH4 (0.225 g, 3 mmol) in dry THF (10 mL).
The mixture was refluxed for 4 h and then cooled to 0 °C after
which the excess LiAlH4 was quenched by the addition of EtOAc.
It was then treated with aq. 20% NaOH (0.5 mL). The white precip-
itate which formed was filtered off, and the residue was washed
with EtOAc (3 Â 10 mL). The combined EtOAc layers were dried
over anhydrous Na2SO4, and the solvent was evaporated under
reduced pressure. The crude product was purified by column chro-
matography over silica gel using CHCl3 as the eluent to afford the
corresponding pure N-methyl amine 11. Yield: 0.192 g, 65%;
[
a
]
25 = –10.8 (c 4.2, EtOH); {lit.20
[
a
]
25 = À10.9 (c 4.2, EtOH)}; IR
D
D
(CHCl3, cmÀ1) 3274, 2917, 2839, 1614,1438; 1H NMR (200 MHz,
CDCl3): d 7.15–7.28 (m, 5H), 2.63–2.82 (m, 3H), 2.4 (s, 3H), 1.67
(br s, 1H), 1.08 (d, J = 5.9 Hz, 3H); 13C NMR (50 MHz, CDCl3): d
139.2, 129.2, 128.4, 126.2, 56.3, 43.3, 33.8, 13.6; Anal. Calcd for
4.2.9. (R)-4-Benzyl-3-(3-(2,4,5-trifluorophenyl)propanoyl)oxaz-
olidin-2-one 14
To a stirred solution of hydrocinnamic acid 13 (4 g, 19.5 mmol)
in dry THF (100 mL) were added pivolyl chloride (2.36 g,
19.5 mmol) and Et3N (10 mL, 78 mmol) at À20 °C and the mixture
was stirred at the same temperature for 4 h. To this stirred suspen-
sion, (R)-4-benzyloxazolidin-2-one (3.8 g, 21.5 mmol) in dry THF
(20 mL) was added dropwise followed by the addition of LiCl
(0.9 g, 19.5 mmol) and then stirred for an additional 15 min at
À20 °C and stirring continued at 25 °C for 8 h until complete con-
sumption of the starting materials (the progress of the reaction
was monitored by TLC). The product was then extracted with
diethyl ether and the combined organic layer was washed with
water, brine, and dried over anhydrous Na2SO4. Removal of the sol-
vent under reduced pressure gave the crude product which upon
column chromatographic purification with petroleum ether/ethyl
C10H15N requires C 80.48; H 10.13; N 9.39%; found C 80.50; H
10.11; N 9.35%.
4.2.6. (R)-N-Methyl-N-(-1-phenylpropan-2-yl)prop-2-yn-1-amine:
(R)-selegiline 1
To a stirred solution of (R)-2-(methylamino)-1-phenylpropane
11 (0.1 g, 0.67 mmol) in CH3CN (3 mL) were added anhydrous
K2CO3 (0.185 g, 1.34 mmol) and propargyl bromide (0.1 mL,
0.73 mmol) (80 wt % solution in toluene). The reaction mixture
was then stirred for 3 h at 25 °C, after which the solvent evapo-
rated under reduced pressure to provide the crude product, which
was purified by column chromatography over silica gel using

S. Dey, A. Sudalai / Tetrahedron: Asymmetry 26 (2015) 67–72
71
acetate (4:1) gave 14 as a colorless solid. Yield: 3.7 g, 94%; mp:
128–130 °C; [
25 = +62.9 (c 1.0, CHCl3); IR (CHCl3, cmÀ1): 3065,
was stirred under H2 (1 atm) at 25 °C for 3 h. After completion of
the reaction (monitored by TLC), it was filtered through Celite
(MeOH eluent) and the solvent was evaporated under reduced
pressure to afford 17 as a colorless solid. Yield: 1.96 g, 98%; mp:
a]
D
3030, 1781, 1700, 1387, 1212; 1H NMR (200 MHz, CDCl3): d
7.26–7.33 (m, 4H), 7.12–7.19 (m, 3H), 6.91–6.93 (m, 1H), 4.61–
4.67 (m, 1H), 4.17–4.21 (m, 2H), 3.19–3.27 (m, 3H), 3.03
(t, J = 7.3 Hz, 2H), 2.80 (dd, J = 9.7, 13.3 Hz, 1H); 13C NMR
(50 MHz, CDCl3): d 23.0, 35.5, 37.8, 55.0, 66.2, 105.4 (dd, J = 20.8,
28.1 Hz), 118.5 (dd, J = 10.5, 6.4 Hz), 123.7 (ddd, J = 4.3, 9.5,
17.5 Hz), 127.4, 128.3, 129.3, 135.0, 146.4 (ddd, J = 4.4, 5.1,
227.8 Hz), 148.9 (ddd, J = 255.5, 12.5, 2.9 Hz), 158.2 (ddd,
J = 244.4, 11.2, 9.7 Hz); Anal. Calcd for C19H16F3NO3 requires C
62.81; H 4.4; N 3.86%; found C 62.80; H 4.42; N 3.82%.
98–100 °C; [a]
25 = +16.8 (c 1, CHCl3); IR (CHCl3, cmÀ1): 3401,
D
2908, 2853, 1682, 1526, 1410, 1128; 1H NMR (200 MHz, CDCl3):
d 6.88–6.94 (m, 1H), 7.09–7.11 (m, 1H), 4.77–4.84 (m, 1H), 3.78–
3.84 (m, 1H), 3.59–3.67 (m, 2H), 2.80–2.86 (t, 2H, J = 7.3 Hz), 2.1
(br, s, 1H), 1.40 (s, 9H); 13C NMR (50 MHz, CDCl3): d 28.3, 30.2,
52.7, 64.2, 105.3 (dd, J = 20.2, 27.6 Hz), 119.0 (dd, J = 10.2,
6.6 Hz), Anal. Calcd for C14H18F3NO3 requires C 55.08; H 5.94; N
4.59%; found C 55.06; H 5.96; N 4.54%.
4.2.10. (R)-3-((R)-2-Azido-3-(2,4,5-trifluorophenyl)propanoyl)-
4-benzyloxazolidin-2-one 15
4.2.13. (R)-3-(tert-Butyl-1-cyano-(2,4,5-trifluorophenyl)propan-
2yl)carbamate 19
To a stirred solution of 14 (3.8 g, 10.4 mmol) in dry THF (30 mL),
25 mL of 0.5 M in toluene (12.48 mmol) of potassium hexamethyl-
disilazide (KHMDS) was added under N2 at À78 °C and the mixture
was stirred for 45 min. To this suspension of potassium enolate,
stirred at À78 °C, was added 2,4,6-triisopropyl azide (4.27 g,
13.83 mmol) in dry THF (15 mL). After 5 min, the reaction was
quenched with 3 mL (52 mmol) of glacial acetic acid and stirred
at 25 °C for 12 h. The solution was then partitioned between
CH2Cl2 and brine solution. The organic phase was washed with
aqueous NaHCO3, dried over Na2SO4, and evaporated in vacuo.
Column chromatographic purification of the crude product with
petroleum ether/ethyl acetate (4:1) gave 15 as a gum. Yield:
To a stirred solution of N-Boc protected amino alcohol 17 (1.5 g,
4.9 mmol) in CH2Cl2 (5 mL) were added dry triethylamine (1.3 mL,
9.8 mmol) and p-toluenesulfonyl chloride (1.12 g, 5.88 mmol) in
the presence of a catalytic amount of 4-dimethylaminopyridine
(0.059 g,10 mol %) at 0 °C. The reaction mixture was stirred at
25 °C for 3 h and then quenched by the addition of 10% NaHCO3.
The aqueous layer was extracted with CH2Cl2 (3 Â 20 mL) and
the combined organic layers were dried over anhydrous Na2SO4,
concentrated to give the crude tosylate 18, which was then dis-
solved in DMF (5 mL), and NaCN (1.4 g, 29.4 mmol) carefully
added. The mixture was refluxed for 4 h and then cooled to RT
and extracted with EtOAc (3 Â 10 mL). The combined EtOAc layers
were dried over anhydrous Na2SO4, the solvent was evaporated
under reduced pressure, and the crude product was purified by col-
umn chromatography over silica gel using petroleum ether/ethyl
acetate (4:1) to afford the corresponding pure cyano compound
19 as a colorless solid. Yield: 0.97 g, 65% (over two steps); mp:
3.3 g, 88%; [a]
25 = +57.2 (c 1.0, CHCl3); IR (CHCl3, cmÀ1): 3065,
D
3030, 2987, 2111, 1781, 1701, 1389; 1H NMR (200 MHz, CDCl3):
d 7.33–7.20 (m, 4H), 7.02–7.22 (m, 3H), 6.84–6.97 (m, 1H), 5.2 (q,
J = 4.5 Hz, 1H), 4.61–4.74 (m, 1H), 3.34 (dd, J = 5.0, 13.1 Hz, 2H),
3.01 (dd, J = 4.1, 13.5 Hz, 2H), 2.8 (dd, J = 9.3, 13.3 Hz, 2H); 13C
NMR (50 MHz, CDCl3): d 35.4, 37.6, 55.0, 66.2, 71.5, 105.2 (dd,
J = 20.2, 28.7 Hz), 118.5 (dd, J = 10.5, 6.4 Hz), 123.7 (ddd, J = 4.7,
9.8, 16.5 Hz), 127.4, 128.3, 129.3, 135.0, 146.4 (ddd, J = 4.6, 5.3,
237.8 Hz), 148.9 (ddd, J = 253.5, 12.5, 2.7 Hz), 158.2 (ddd,
J = 244.4, 11.2, 9.7 Hz); Anal. Calcd for C19H16F3N4O3 requires C
56.44; H 3.74; N 13.8%; found C 56.42; H 3.76; N 13.8%.
110–112 °C; [a]
25 = +22.2 (c 0.6, CHCl3); mp: 110–112 °C; IR
D
(CHCl3, cmÀ1): 3101, 2956, 2105, 1685, 1456, 1128; 1H NMR
(200 MHz, CDCl3): d 6.62–6.98 (m, 1H), 7.05–7.12, 9 m, 1H), 4.87
(m, 1H), 4.05 (m, 1H), 2.90–2.98 (m, 2H), 2.73–2.77 (m, 1H),
2.54–2.58 (m, 1H), 1.41 (s, 9H); 13C NMR (50 MHz, CDCl3): d
23.48, 28.57, 32.91, 47.95, 80.73, 105.4 (dd, J = 20.8, 28.1 Hz),
117.0, 118.5 (dd, J = 10.5, 6.4 Hz), 123.7 (ddd, J = 4.3, 9.5,
17.5 Hz), 127.4, 128.3, 129.3, 135.0, 146.4 (ddd, J = 4.8, 4.8,
230.8 Hz), 148.7 (ddd, J = 239.5, 12.5, 2.6 Hz), 158.2 (ddd,
J = 255.4, 11.2, 9.3 Hz); Anal. Calcd for C15H17F3N2O2 requires C
57.32; H 5.45; N 8.91%; found C 57.33; H 5.46; N 8.94%.
4.2.11. (R)-2-Azido-3-(2,4,5-trifluorophenyl)propan-1-ol 16
To a stirred solution of 15 (3 g, 7.42 mmol) in THF (20 mL) was
added dropwise
a solution of sodium borohydride (0.42 g,
11.13 mmol) in water (2 mL) at 0 °C. After the addition, it was stir-
red at 25 °C for 2 h. After completion of the reaction (as monitored
by TLC), 2 M HCl (15 mL) was added slowly so that the temperature
was maintained at 25 °C. The reaction mixture was then extracted
with ethyl acetate and washed with brine. The organic phase was
concentrated and column chromatographic purification with
petroleum ether/ethyl acetate (7:3) gave 16 as a colorless gum.
4.2.14. (R)-3-((tert-Butoxycarbonyl)amino)-4-(2,4,5-trifluoro-
phenyl)butanoic acid 4
To a stirred solution of N-Boc protected cyano compound 19
(0.5 g, 1.5 mmol) were added 3 M NaOH (10 mL), H2O2 (35%,
6 mL, 22 mmol) and refluxed at 100 °C for 3 h. After the reaction
was completed, the reaction mixture was cooled to 0 °C. To remove
organic impurities, Et2O (50 mL) was added and the ether phase
was removed. The aqueous phase was acidified with 6 M HCl to
neutralize pH and was extracted with Et2O (50 mL), and dried over
Na2SO4. Filtration and evaporation of the solvent gave the crude
product, which was purified by column chromatography over silica
gel using petroleum ether/ethyl acetate (4:1) to afford the corre-
sponding pure compound 4 as a colorless solid. Yield: 0.375 g,
Yield: 2.94 g, 98%; [a]
25 = +4.2 (c 1, CHCl3)); IR (CHCl3, cmÀ1):
D
3439, 2903, 2100, 1152, 1016; 1H NMR (200 MHz, CDCl3): d 7.0–
7.16 (m, 1H), 6.87–7.0 (m, 1H), 3.55–3.78 (m, 3H), 2.71–2.84 (m,
2H), 1.82 (br s, 1H); 13C NMR (50 MHz, CDCl3): d 29.8, 63.5, 64.3,
105.4 (dd, J = 20.2, 27.1 Hz), 118.5 (dd, J = 10.5, 6.4 Hz), 123.7
(ddd, J = 4.3, 9.5, 17.5 Hz), 146.4 (ddd, J = 4.4, 5.1, 241.8 Hz),
148.9 (ddd, J = 255.5, 11.6, 3.1 Hz), 158.2 (ddd, J = 239.4, 10.2,
8.7 Hz); Anal. Calcd for C9H8F3N3O requires C 46.76; H 3.49; N
18.18%; found C 46.78; H 3.46; N 18.15%; Enantiomeric purity:
98% ee determined by HPLC analysis (Chiracel AD-H column,
Hex/i-PrOH 95:05, 0.5 mL/min, 220 nm). Retention time:
tmajor = 21.73 min and tminor = 20.03 min.
75%; mp: 122–125 °C; {lit.21 mp: 124–125 °C}; [
a]
25 = +31.8 (c 1,
D
CHCl3) {lit.21 25 = +32.3 (c 1, CHCl3)}; IR (CHCl3, cmÀ1): 3101,
[a]
D
2956, 1770, 1685, 1366, 1095; 1H NMR (200 MHz, CDCl3): d 1.35
(s, 9H), 2.62–2.56 (m, 2H), 2.88 (d, J = 4.9 Hz, 2H), 4.10 (br s, 1H),
5.07 (br s, 1H), 6.94–6.87 (m, 1H), 7.09–7.03 (m, 1H; 13C NMR
(50 MHz, CDCl3): d 14.1, 28.57, 32.71, 47.95, 61.5, 80.73, 105.8
(dd, J = 20.6, 28.5 Hz), 118.2(dd, J = 10.7, 6.2 Hz), 124.5 (ddd,
J = 4.5, 9.5, 18.2 Hz), 146.7 (ddd, J = 4.1, 5.5, 229.9 Hz), 148.9
(ddd, J = 255.5, 12.5, 2.9 Hz), 158.2 (ddd, J = 244.4, 11.2, 9.7 Hz),
4.2.12. (R)-tert-Butyl-1-hydroxy-3-(2,4,5-trifluorophenyl)propan-
2-yl)carbamate 17
A mixture of azido alcohol 16 (2 g, 9.28 mmol), 10% Pd/C, and
di-tert-butyl dicarbonate (1.2 g, 9.28 mmol) in dry MeOH (20 mL)

72
S. Dey, A. Sudalai / Tetrahedron: Asymmetry 26 (2015) 67–72
7. Bornholdt, J.; Felding, J.; Clausen, R. P.; Kristensen, J. L. Chem. Eur. J. 2010, 16,
12474–12480.
175.1, 177.3; Anal. Calcd for C17H22F3NO4 requires C 56.50; H 6.14;
N 3.88%; found C 56.53; H 6.16; N 3.89%.
8. (a) Sayyed, I. A.; Sudalai, A. Tetrahedron: Asymmetry 2004, 15, 3111–3116; (b)
Talluri, S. K.; Sudalai, A. Tetrahedron 2007, 63, 9758–9763.
9. (a) Pramanik, C.; Bapat, K.; Chaudhari, A.; Kulkarni, M. G.; Kolla, R.; Sompalli, S.;
Tripathy, N.; Gurjar, M. K. Org. Process Res. Dev. 2014, 18, 495–500; (b) Gurjar,
M. K.; Tripathy, N. K.; Bapat, K. A.; Rao, V. P. K. V.; Biswas, S. B.; Mehta, S. S. U.S.
Patent 0046416, 2011.
10. Hansen, K. B.; Balsells, J.; Dreher, S.; Hsiao, Y.; Kubryk, M.; Palucki, M.; Rivera,
N.; Steinhuebel, D.; Armstrong, J. D., III; Askin, D.; Grabowski, E. J. J. Org. Process
Res. Dev. 2005, 9, 634–639.
11. (a) Clausen, A. M.; Dziadul, B.; Cappuccio, K. L.; Kaba, M.; Starbuck, C.; Hsiao, Y.;
Dowling, T. M. Org. Process Res. Dev. 2006, 10, 723–726; (b) Hansen, K. B.; Hsiao,
Y.; Xu, F.; Rivera, N.; Clausen, A.; Kubryk, M.; Krska, S.; Rosner, T.; Simmons, B.;
Balsells, J.; Ikemoto, N.; Sun, Y.; Spindler, F.; Malan, C.; Grabowski, E. J. J.;
Armstrong, J. D., III J. Am. Chem. Soc. 2009, 131, 8798–8804.
12. Steinhuebel, D.; Sun, Y.; Matsumura, K.; Sayo, N.; Saito, T. J. Am. Chem. Soc.
2009, 131, 11316–11317.
Acknowledgements
S.D. thanks CSIR, New Delhi for the award of a Senior Research
Fellowship and DST, New Delhi (sanction No. CSC 0123). The
authors are also thankful to Dr. V.V.Ranade, Head, Chemical Engi-
neering and Process Development Division for his encouragement
and support.
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