Stereoselective Synthesis of (R)-3-Methylthalidomide by Piperidin-2-one Ring Assembly Approach

Article abstract of DOI:10.1002/chir.22474

A simple and stereoselective synthesis of 3-methylthalidomide, a configurationally stable thalidomide analog, is presented. Herein we describe the synthesis of (R)-3-methylthalidomide starting from (S)-alanine by piperidin-2-one ring assembly approach in high yield and enantiomeric purity without using a chiral auxiliary or reagent. Starting from (R)-alanine, the corresponding (S)-3-methylthalidomide can be prepared using the same methodology. Chirality 27:619-624, 2015.

Full text of DOI:10.1002/chir.22474

CHIRALITY 27:619–624 (2015)
Stereoselective Synthesis of (R)-3-Methylthalidomide by Piperidin-2-
one Ring Assembly Approach
SHYAM RAJ YADAV, VINAY SHANKAR TIWARI, AND WAHAJUL HAQ1,2*
1
1,2
1
Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute, Lucknow, India
2
Academy of Scientific and Innovative Research, New Delhi, India
ABSTRACT
A simple and stereoselective synthesis of 3-methylthalidomide, a configurationally
stable thalidomide analog, is presented. Herein we describe the synthesis of (R)-3-methylthalidomide
starting from (S)-alanine by piperidin-2-one ring assembly approach in high yield and enantiomeric
purity without using a chiral auxiliary or reagent. Starting from (R)-alanine, the corresponding
(
S)-3-methylthalidomide can be prepared using the same methodology. Chirality 27:619–624,
2015 © 2015 Wiley Periodicals, Inc.
KEY WORDS: thalidomide; teratogenic; tumor necrosis factor-α; oxazolidinone; enantioretentive
alkylation
Thalidomide (1) was marketed in racemic form as a seda-
tive drug, but it was later withdrawn from the market after
the emergence of its serious teratogenic effects leading to
limb defects in newborns. Initially, thalidomide was given in
racemic form and therefore it was thought that teratogenicity
spectrophotometer. Mass spectra were obtained using JEOL (Tokyo,
Japan) SX-102 (ESI), Agilent 6520 Q-TOF (ESI-HRMS) instruments. Opti-
cal rotations were measured on an AUTOPOL III digital polarimeter.
Preparative Procedure and Characterization
1
(2S, 4S)-Benzyl-4-methyl-5-oxo-2-phenyloxazolidine-3-carboxylate
was associated with (S)-thalidomide only. Later studies have
(
3). Z-(S)-alanine was prepared initially by the treatment of (S)-alanine
shown that both the enantiomers of thalidomide are intercon-
vertible to one another at physiological pH in biophase be-
cause of the presence of an exchangeable acidic proton at
the asymmetric carbon (Fig. 1). Therefore, it was inferred
that attribution of a biological response associated with a par-
17
with benzyl chloformate using the standard procedure. Properly dried
Z-(S)-alanine (22.3 g, 100 mmol) and benzaldehyde dimethyl acetal
(14.4 mL, 95.9 mmol) in diethyl ether (500 mL) was cooled to –78°C
followed by dropwise addition of BF ·Et O (60.2 mL, 489 mmol) under ni-
3
2
trogen atmosphere. The reaction mixture was then allowed to warm to
room temperature and stirred till completion of the reaction as monitored
by TLC. The reaction mixture was treated with saturated aqueous
2
–4
ticular enantiomer is difficult.
Literature reports reveal a
basis of thalidomide teratogenicity but the precise mecha-
nism is still unclear.5
NaHCO
3
at 0°C, initially in small portions until strong bubbling ceased,
O (3 × 150 mL). The com-
and then the mixture was extracted with Et
2
In the last decades thalidomide has shown a resurgence for
its clinical potential towards the cure of various diseases be-
cause of its regulatory activity of tumor necrosis factor-alpha
bined organic phase was washed with brine and dried over anhydrous so-
dium sulfate. The solvent was removed under vacuum to obtain the
residue, which was a mixture of cis and trans isomers (~90:10) on TLC.
The predominant cis isomer was obtained by purification of the crude
product by silica gel column chromatography using ethyl acetate-hexane
(
TNF-α) release, a phenomenon associated with diseases
such as rheumatoid arthritis, Crohn’s disease, leprosy, AIDS,
and cancers.
6
–16
It is therefore desirable to develop pure
(1:4) as eluent. The pure fractions were pooled and after removal of the
enantiomers of configurationally stable thalidomide analogs
to prevent racemization at physiological pH and to provide
better insight on their enantiodependent biological activity.
solvent the residue was recrystallized from diethyl ether/hexane to yield
compound 3 (18.66 g, 60% yield) as a colorless white crystalline solid in
>98% diastereomeric purity as evident by NMR-spectroscopy; mp
1
5
3–54°C; H NMR (300 MHz, CDCl
3
, δ): 7.25-7.40 (m, 10H, ArH), 6.64
CH ), 4.44-4.51 (m, 1H, CH CH),
); C NMR (75 MHz, CDCl , δ): 172.5, 153.4,
36.9,135.4, 129.8, 128.8, 128.6, 128.5, 128.1, 126.3,89.1, 67.9, 52.1, 18.2;
(s, 1H, C
6
H
5
CH), 5.11-5.21 (m, 2H, C
6
H
5
2
3
MATERIALS AND METHODS
13
1
1
.56 (d, J = 6.9 Hz, 3H, CH
3
3
General Experimental
-
1
All the chemicals and reagents were purchased from Sigma Aldrich (St.
Louis, MO). The alkylation and hydroxylation reactions were carried out
in freshly distilled anhydrous tetrahydrofuran (THF). Progress of the re-
action was monitored by thin-layer chromatography (TLC) on ready-
made silica gel plates (Merck, Darmstadt, Germany; UV active). The
plates were either developed under iodine vapors or seen directly under
UV-light (254 nm). Wherever required, compounds were also detected
by spraying the TLC plates with 2N HBr/AcOH followed by 0.2 % ninhy-
drin solution in acetone and heating the plates at ~110°C in a hot air oven
FTIR (KBr): v = 3352, 1800, 1717, 763 cm . HRMS (ESI-TOF): m/z calcd
for C15 [M+H]: 312.1230, found: 312.1229.
H
22NO
5
(2S,4R)-benzyl 4-allyl-4-methyl-5-oxo-2-phenyloxazolidine-3-carboxylate
(4). A solution of compound 3 (7.78 g, 25 mmol) and allyl bromide (2.6 mL,
30 mmol) in freshly dried THF (30 mL) was cooled to –78°C under stirring. To
this solution, LHMDS (30 mL, 1M in THF) was added at –78°C under nitrogen
atmosphere. The reaction mixture was further stirred at –78°C for 3 h. Then
the reaction mixture was quenched with saturated ammonium chloride and ex-
tracted with ethyl acetate (3 × 150 mL). The organic layer was washed with
brine, dried over sodium sulfate, and concentrated at reduced pressure. The
crude product obtained was purified by column chromatography using ethyl
for 45 min. Column chromatography was performed over silica gel
1
(
60–120 mesh size). H spectra were recorded at 300 MHz or 400 MHz
1
3
and C spectra were recorded at 75 MHz or 100 MHz using a Bruker
Billerica, MA) DRX-300 or Bruker DRX-400 spectrophotometer, respec-
(
tively, and reported in parts per million (ppm) on the δ scale relative to
tetramethylsilane as an internal standard. Coupling constants (J) are
reported in Hz.. Melting points are reported uncorrected and were
determined on melting point apparatus containing silicon oil. IR spectra
were recorded using Agilent (Palo Alto, CA) Cary 630 FTIR-
*Correspondence to: W. Haq, Medicinal and Process Chemistry, CSIR-Central
Drug Research Institution, Lucknow 226031 India. E-mail: w_haq@cdri.res.in
Received for publication 22 September 2014; Accepted 6 May 2015
DOI: 10.1002/chir.22474
Published online 16 June 2015 in Wiley Online Library
(wileyonlinelibrary.com).
©
2015 Wiley Periodicals, Inc.

6
20
YADAV ET AL.
-1
3
015, 2949, 1720 cm ; HRMS (ESI-TOF): m/z calcd for C15
5
H22NO [M+H]:
296.1492, found: 296.1494.
(
(
(
(
R)-Methyl-5-azido-2-(benzyloxycarbonylamino)-2-methylpentanoate
7). To the stirred solution of compound 6 (1.86 g, 6.3 mmol) in THF
50 mL) was added Et
1.4 mL, 18 mmol) at 0°C. The reaction mixture was stirred at 0°C for
5 min then saturated sodium bicarbonate (30 mL) was added and the
3
N (2.5 mL, 18 mmol) followed by mesyl chloride
1
Fig. 1. Interconversion of thalidomide enantiomers at physiological pH.
aqueous layer was extracted with ethyl acetate (3 × 50 mL). The com-
bined organic layer was further washed with brine, dried over anhydrous
sodium sulfate, and evaporated under reduced pressure. The crude prod-
uct obtained was dissolved in DMF (35 mL) and sodium azide (780 mg,
12 mmol) was added to it. The reaction mixture was stirred for 5 h at
80°C. After completion of the reaction DMF was evaporated and the res-
idue obtained was suspended in water (30 mL), then the aqueous layer
was extracted with ethyl acetate (3 × 50 mL). The combined organic
phase was washed with brine, dried over anhydrous sodium sulfate, and
solvent was concentrated under reduced pressure to obtaine the crude
product, which was purified by column chromatography using ethyl
acetate-hexane (1:10) to give compound 7 (1.49 g, 74%) as a colorless
2
1
acetate-hexane (1:5) to obtain compound 4 (7.02 g, 80%) as a colorless oil. [α]
D
1
–
7
(
3
63.4° (c = 0.73, CHCl
.41 (m, 9H, ArH), 6.87 (d J = 6.3 Hz, 1H, ArH), 6.28 (s, 1H, C
m, 1H, CH=CH ), 5.10-5.26 (m, 2H, CH=CH ), 4.91-4.97 (m, 2H, C
.24-3.30 (m, 1H, CCH ), 2.52 (dd, J = 13.7, 6.4 Hz, 1H, CCH
H, CH ); (minor rotamer) 7.20-7.41 (m, 9H, ArH), 6.87 (d J = 6.3 Hz, 1H,
ArH), 6.35 (s, 1H, C CH), 5.61-5.69 (m, 1H, CH=CH ), 5.10-5.26 (m, 2H,
CH=CH ), 4.91-4.97 (m, 2H, C CH ) 2.89-2.91 (m, 1H, CCH ), 2.52 (dd,
J = 13.7, 6.4 Hz, 1H, CCH ), 1.72(s, 3H, CH ); C NMR (100 MHz, CDCl
δ): (major rotamer only), 174.2, 151.7, 137.1, 135.3, 131.0, 129.9, 128.7, 126.8,
3
); H NMR (400 MHz, CDCl
3
, δ): (major rotamer) 7.20-
CH), 5.61-5.69
CH ),
), 1.81(s,
6 5
H
2
2
6
H
5
2
a
H
b
a b
H
3
3
6
H
5
2
2
6
H
5
2
a b
H
13
a
H
b
3
3
,
2
1
1
oil. [α] –67.0° (c = 0.10, CHCl3); H NMR (300 MHz, CDCl , δ):
D
3
-1
1
21.4, 89.5, 67.3, 63.2, 39.5, 23.7; FTIR (neat) v = 3535, 1795, 1711, 1216 cm .
HRMS (ESI-TOF): m/z calcd for C15 [M+H]: 352.1543, found:
52.1549.
6 5 2
7.33-7.37 (m, 5H, ArH), 5.71 (bs, 1H, NHCOO), 5.1 (s, 2H, C H CH ),
H
22NO
5
3.77 (s, 3H, COOCH ), 3.21-3.29 (m, 2H, CH CH N ), 1.88-2.26 (m, 2H,
3 2 2 3
13
3
CCH H CH ), 1.60 (s, 3H, CCH ), 1.33-1.43 (m, 2H, CH CH CH );
C
a
b
2
3
2
2
2
NMR (75 MHz,CDCl
52.9, 59.8, 51.0, 33.6, 23.8, 23.7; FTIR (neat) v = 3431, 2953, 2097, 1712,
3
, δ): 174.5, 154.5, 136.4, 128.5, 128.2, 128.0, 66.5,
(
R)-Methyl 2-(benzyloxycarbonylamino)-2-methylpent-4-enoate (5).
Aqueous NaOH (1N, 20 mL) was added to the stirred solution of compound
(3.52 g, 10 mmol) in methanol (100 mL) and the reaction mixture was
-
1
1655 cm ; HRMS (ESI-TOF): m/z calcd for
321.1557, found: 321.1564.
15 22 4 4
C H N O [M+H]:
4
stirred at room temperature for 5–6 h. The solvent was removed under re-
duced pressure and the resulting residue was acidified with citric acid up
to pH 3. This mixture was extracted with ethyl acetate (3 × 50 mL) and
the combined organic layer was washed with brine, dried over sodium sul-
fate, and concentrated under vacuum. The crude product thus obtained was
dissolved in dimethylformamide (DMF) (50 mL) and potassium carbonate
(R)-Benzyl 3-methyl-2-oxopiperidin-3-ylcarbamate (8). To the
stirred solution of compound 7 (1.28 g, 4 mmol) in moist THF (50 mL)
was added Ph P (1.3 g, 5 mmol). The reaction mixture was stirred at
3
50°C for 12 h. After completion of the reaction, THF was evaporated
and the residue obtained was partitioned between 200 mL of a (1:1)
water–ethyl acetate mixture. The organic layer was separated and the
aqueous layer was further extracted with (2 × 100 mL) ethyl acetate.
The combined organic layer was washed with brine, dried over anhy-
drous sodium sulfate, and evaporated under reduced pressure to obtain
the crude product, which was purified by column chromatography using
ethyl acetate-hexane (1:10) as eluent to obtain compound 8 (860 mg, 82%)
(1.38 g, 10 mmol) was added followed by the addition of methyl iodide
(684 μL, 11 mmol) at 0°C. The reaction mixture was stirred for several
hours till completion of the reaction as indicated by TLC. After completion
of the reaction, DMF was evaporated under reduced pressure and the resi-
due was suspended in water (30 mL) and the aqueous layer was extracted
with ethyl acetate (3 × 30 mL). The combined organic phase was washed
with brine and dried over anhydrous sodium sulfate. The solvent was
removed under reduced pressure and the crude product obtained was puri-
fied by column chromatography using ethyl acetate-hexane (1:4) to give
2
3
1
as a white solid; mp 125–126°C; [α]D -14.3° (c = 0.31, CHCl ); H NMR
3
(300 MHz, CDCl δ): 7.29-7.35 (m, 5H, ArH), 6.10 (bs, 1H, NHCO), 5.60
3
(bs, 1H, NHCOO), 5.12 (dd, J = 16.5, 12.3 Hz, 2H, C H CH ), 3.27-3.41
6
5
2
2
2
compound 5 (2.5 g, 90%) as light yellow oil. [α]
D
–15.5° (c = 0.67, CHCl
, δ): 7.25–7.34 (m, 5H, ArH), 5.50–5.71 (m, 2H,
and NHCOO), 5.06–5.12 (m, 4H, CH=CH and C CH ), 3.73
s, 3H, COOCH ), 2.76–2.78 (m, 1H, CCH ), 2.54–2.61 (dd, J = 13.9, 7.3
Hz, 1H, CCH ); C NMR (75 MHz, CDCl , δ):
74.2, 154.7, 136.5, 132.1, 128.5, 128.1, 128.0, 119.6, 66.5, 59.5, 52.6, 41.4,
3
);
(m, 2H, CH NHCO), 2.31-2.42 (m, 1H, CCH H ), 2.09-2.14 (m, 1H,
2
a
b
1
13
H NMR (300 MHz, CDCl
CH=CH
3
CCH H ), 1.77-1.91 (m, 2H, CH CH CH ), 1.49 (s, 3H, CCH );
C
a
b
2
2
2
3
2
2
H
6 5
2
NMR (75 MHz, CDCl , δ): 174.5, 155.2, 136.5, 128.5, 128.0, 127.9, 66.4,
3
-1
(
3
a
H
b
55.6, 42.2, 33.2, 25.1, 20.3; FTIR (KBr) v = 3352, 3020, 1704, 1506 cm ;
1
3
a
H
b
), 1.58 (s, 3H, CCH
3
3
HRMS (ESI-TOF): m/z calcd for C14H19N O [M+H] : 263.1390, found:
2
3
1
2
263.1369.
-
1
3.2; FTIR (neat) v = 3415, 3033, 2943, 1725 cm ; HRMS (ESI-TOF): m/z
[M+H]: 278.1387, found: 278.1372.
calcd for C15H20NO
4
(
R)-2-(3-Methyl-2-oxopiperidin-3-yl)isoindoline-1,3-dione (9). To
the solution of compound 8 (700 mg, 2.67 mmol) in methanol (30 mL)
was added Pd/C (10%, 150 mg), then this reaction mixture was subjected
to hydrogenation at 30 psi for 2 h. The catalyst was removed by filtration
and the filtrate was evaporated under reduced pressure to obtain amine
as a gummy residue. The residue was taken in 1,4-dioxane (30 mL) and
to this stirred solution was added DIPEA (0.7 mL, 4 mmol) followed by
addition of phthalic anhydride (445 mg, 3 mmol) and the reaction mixture
was refluxed for 12 h. After completion of the reaction 1,4-dioxane was
evaporated under reduced pressure and the crude material thus obtained
was directly purified without workup by column chromatography using
ethyl acetate-hexane (1:3) as eluent to obtain compound 9 (582 mg,
(
(
(
R)-Methyl- 2-(benzyloxycarbonylamino)-5-hydroxy-2-methylpentanoate
6). To the stirred solution of compound 5 (2.5 g, 9 mmol) in dry THF
50 mL) was added 9-BBN (0.5M in THF, 36 mL) under nitrogen atmosphere
at room temperature. The reaction mixture was stirred for 20 h, then water
9 mL) was carefully added, followed by addition of H (35% in water,
6 mL) and sodium acetate solution (20% in water, 60 mL). After 1 h of vigorous
(
3
2 2
O
stirring, the two layers were separated and the aqueous phase was extracted
with ethyl acetate (3 × 100 mL). The combined organic phase was washed with
brine, dried over anhydrous sodium sulfate, and concentrated under reduced
pressure. The crude product obtained was purified by column chromatography
using methanol-chloroform (1:20) to give compound 6 (1.86 g, 70%) as a color-
2
4
85 %) as a white solid; mp 141–142°C; [α]
D
+ 4.3° (c = 0.32, 1,4-dioxane);
, δ): 7.69-7.78 (m, 4H, ArH) 5.77 (bs, 1H,
NHCO), 3.38-3.58 (m, 2H, CH NHCO), 2.32-2.40 (m, 1H, CCH ),
1.89-2.17 (m, 6H, CCH CH NMR
(75 MHz, CDCl , δ) 172.6, 168.7, 134.0, 131.9, 123.1, 60.0, 42.4, 34.8,
23.5, 20.1; FTIR (KBr) v = 3420, 2941, 1708, 1663 cm ; HRMS (ESI-
TOF): m/z calcd for C15 [M+H]: 259.1077, found: 259.1067.
2
3
1
1
less oil. [α]
H, ArH), 5.73 (bs, 1H, NHCOO), 5.07 (s, 2H, C
COOCH ), 3.58-3.62 (t, J = 6.6 Hz, 2H, CH CH OH), 2.08-2.20 (m, 1H,
CCH CH ), 1.85-1.97 (m, 1H, CCH CH ), 1.58(s, 3H, CCH ), 1.37-1.46
m, 2H, CH ); C NMR (75 MHz, CDCl , δ) 174.7, 154.8, 136.4,
D
–36.2° (c = 0.71, CHCl
3
); H NMR (300 MHz, CDCl
3
, δ): 7.35 (m,
H NMR (300 MHz, CDCl
3
5
6
H
5
CH ), 3.74 (s, 3H,
2
2
a b
H
13
3
2
2
H
a b
,
CH CH
2 2 2
and CCH
3
);
C
a
H
b
2
a
H
b
2
3
3
13
-1
(
1
2
CH
2
CH
2
3
28.5, 128.1, 128.0, 66.5, 62.2, 59.7, 52.7, 33.4, 27.1, 23.4; FTIR (neat) v = 3414,
H20NO
4
Chirality DOI 10.1002/chir

STEREOSELECTIVE SYNTHESIS OF (R)-3-METHYLTHALIDOMIDE
621
(
R)-3-Methylthalidomide (10). A mixture of H
5
IO
6
(1.37 g, 6 mmol)
by brine and the organic layer was dried over anhydrous sodium sulfate.
The solution was filtered and evaporated under reduced pressure to ob-
tain the crude product, which was purified by column chromatography
and CrO (20 mg, 0.20 mol) in acetonitrile (30 mL) was stirred at room tem-
3
perature for 30 min, then acetic anhydride (570 μL, 6 mmol) was added.
The reaction mixture was cooled to 0°C and compound 9 (259 mg, 1 mmol)
was added in one portion and the reaction mixture was further stirred for
using ethyl acetate–dichloromethane (20:80) as eluent to obtain com-
1
pound 12 (1.96 g, 80%) as a white solid.: H NMR (400 MHz, CDCl
3
δ):
3
0 min at room temperature. After completion of the reaction, ice-water
8.03 (bs,1H, CONHCO), 7.29-7.38 (m, 5H, ArH) 5.55 (bs, 1H, NHCOO),
5.09 (dd, 11.8, 2.9 Hz, 2H, CH ), 2.80-2.55 (m, 3H,
CCH CH CONH), 2.33-2.28 (m, 1H, CCH ), 1.56 (s, 3H, CCH );
C NMR (100 MHz, CDCl , δ): 174.0, 171.2, 154.9, 136.0, 128.6, 128.3,
128.1, 66.9, 55.4, 29.5, 29.0, 22.5; HRMS (ESI-TOF): m/z calcd for
[M+H] : 277.1182, found: 277.1183.
(15–20 mL) was added and the mixture was extracted with ethyl acetate
J
=
C
6
H
5
2
(3 × 50 mL). The combined organic layer was washed with saturated
a
H
b
2
a
H
b
3
1
3
NaHCO
3
solution, saturated Na S O
2 2 3
solution, and finally with brine. The
3
organic phase was dried over anhydrous sodium sulfate and the solvent
was removed under reduced pressure. The crude product was filtered
through silica gel column using ethyl acetate–hexane (1:4) as eluent to ob-
tain (R)-3-methylthalidomide 10 (260 mg, 95%) as a white crystalline solid;
14 17 2 4
C H N O
(S)-Tert-butyl 2-((R)-3-methyl-2,6-dioxopiperidin-3-ylcarbamoyl)p
yrrolidine-1-carboxylate (13a). The method for synthesis was the
same as reported for compound 11a, while the starting amine was de-
rived from hydrogenolysis of compound 12 and the amino acid used
2
3
mp 249–250°C; [α]
D
-35.6° (c = 0.25, 1,4-dioxane) (lit. -36.9°, c = 1,
NMR (300 MHz, DMSO-d δ) 11.03 (bs, 1H,
CONHCO), 7.92 (s, 4H, ArH), 2.60-2.78 (m, 3H, NHCOCH and CCH ),
); C NMR (75 MHz,
, δ): 172.7, 172.6, 168.4, 135.2, 131.5, 123.5, 59.3, 29.6, 29.1, 21.5;
1
8
1
1
,4-dioxane) ;
H
6
,
2
a b
H
13
2
.10-2.15 (m, 1H, CCH
DMSO-d
FTIR (KBr) v = 3366, 2928, 1715 cm1; HRMS (ESI-TOF): m/z calcd for
[M+H]: 273.0870, found: 273.0869.
a b
H
), 1.96 (s, 3H, CCH
3
was Boc-L-proline (215 mg, 1 mmol). Yield (230 mg, 68%) as a gummy res-
1
6
idue; H NMR (400 MHz, CDCl
3
δ): 7.95 (bs,1H, CONHCO), 4.27-4.19
-
(m, 1H, α-CH), 3.45-3.35 (m, 2H, NCH
& COCH ), 2.31-2.14 (m, 2H, CHCH CH
CH CH CH ), 1.66 (s, 3H, CCH ), 1.48 (s, 9H, C(CH
(100 MHz, CDCl , δ): 173.4, 171.5, 155.8,80.7, 59.8, 55.2, 47.1, 29.4, 28.8,
2
), 2.75-2.59 (m, 3H, CCH H CH
a b 2
C
15
H
20NO
4
2
2
2
), 1.90-1.88 (m, 3H, CCH
a b
H
CH
2
13
&
2
2
2
3
3 3
) ); C NMR
Tert-butyl (S)-3-(benzyloxy)-1-((R)-3-methyl-2-oxopiperidin-3-ylamino)-
-oxopropan-2-ylcarbamate (11a). To the solution of compound 8 (262
3
1
28.3, 27.8, 24.5, 22.4; HRMS (ESI-TOF): m/z calcd for C16
26 3 5
H N O [M
mg, 1 mmol) in methanol (20 mL) was added Pd/C (10%, 50 mg), then this
reaction mixture was subjected to hydrogenation at 30 psi for 2 h. The catalyst
was removed by filtration and the filtrate was evaporated under reduced pres-
sure to obtain amine. A solution of amine, thus obtained, in DMF (5 mL) was
added to the prestirred mixture of N-Boc-O-benzyl-L-Serine (296 mg, 1 mmol),
EDCI.HCl (230 mg, 1.2 mmol), and HOBt (168 mg, 1.2 mmol) in dry DCM at
+H] : 340.1865, found: 340.1867.
Benzyl (S)-1-((R)-3-methyl-2,6-dioxopiperidin-3-ylamino)-1-oxo-3-
phenylpropan-2-ylcarbamate (13b). The method for synthesis was
the same as reported for compound 13a, while the amino acid used was
Cbz-L-phenylalanine (300 mg, 1 mmol). Yield (308 mg, 73%) as a gummy
1
0°C under inert atmosphere followed by addition of DIEA (0.2 mL, 1.2 mmol).
residue,; H NMR (400 MHz, CDCl
3
δ): 8.30 (bs, 1H, CONHCO), 7.36-
CH ),
CH), 2.67-2.52 (m, 2H,
), 1.44 (s, 3H, CCH ); HRMS
26 3 5
H N O [M+H] : 424.1867, found: 424.1865.
The reaction mixture was further stirred for 12 h at room temperature. After
completion of the reaction DCM was evaporated under reduced pressure
and water (15–20 mL) was added. The aqueous mixture was extracted with
ethyl acetate (3 × 30 mL). The combined organic layer was washed with 1N
7.24 (m, 10H, ArH), 6.39 (bs, 1H, CONH), 5.34-5.47 (m, 2H, C
4.48-4.39 (m, 1H, α-CH), 3.11-3.00 (m, 2H, C CH
COCH CH ), 2.36-2.18 (m, 2H, CHCH CH
(ESI-TOF): m/z calcd for C23
6
H
5
2
6
H
5
2
2
2
2
2
3
3
HCl, 10% aqueous NaHCO solution and finally with brine. The organic phase
was dried over anhydrous sodium sulfate and the solvent was removed under
reduced pressure to obtain the crude product, which was purified by column
RESULTS AND DISCUSSION
As it has been well established that both enantiomers of
thalidomide have different biological activities and are
interconvertable at physiological pH, a number of approaches
have been employed to develop configurationally stable tha-
lidomide analogs (Fig. 2). These molecules will be important
pharmacological tools for the study of enantiodependent
pharmacological effects including the teratogenicity of thalid-
omides as well as development of configurationally stable an-
alogs of thalidomide with better activity and selectivity.
Conventional synthetic approaches include the exchange of
chromatography using methanol-chloroform (1:20) as eluent to give com-
1
pound 11a as a gummy residue (290 mg, 72%); H NMR (400 MHz, CDCl
3
δ): 7.34-7.27 (m, 5H, ArH), 7.17 (bs, 1H, CONH) 5.80 (bs, 1H, CONH), 5,42
bs, 1H, OCONH), 4.58-4.53 (m, 2H, C CH ), 4.26 (m, 1H, α-CH), 3.89-
.86 (m, 1H, α-CHCH O), 3.60-3.56 (m, 1H, α-CHCH O), 3.44-3.25 (m,
H, COCH CH ) 2.32-2.28 (m, 1H, CH CH CH ), 2.14-2.10 (m, 1H,
CH CH C), 1.88-1.82 (m, 2H, CH CH CH C ), 1.44 (s, 3H, CCH ),
.43 (s, 9H C(CH ); C NMR (100 MHz, CDCl , δ): 173.7, 169.9, 154.4,
37.6, 128.4, 127.9, 76.0, 73.5, 70.0, 55.9, 42.4, 32.8, 28.3, 24.8, 20.3; HRMS
ESI-TOF): m/z calcd for C21 [M+H] : 406.2336, found: 406.2347.
(
6
H
5
2
3
a
H
b
a b
H
2
2
2
2
a
H
b
2
2
a
H
b
2
a
H
b
a
H
b
3
13
1
1
3
)
3
3
(
32 3 5
H N O
19,20
C3 proton of thalidomide with an alkyl group,
fluoroalkyl
(
S)-Benzyl 3-(tert-butoxycarbonylamino)-4-((R)-3-methyl-2-oxopip
21
22
23
group, deuterium atom, fluorine atom, or introduction of
eridin-3-ylamino)-4-oxobutanoate (11b). The method for synthesis
was the same as reported for compound 11a, while the amino acid used
was N-Boc-L-Aspartic acid 4-benzyl ester (323 mg, 1 mmol). Yield (282
1
mg, 65%) as a gummy residue: H NMR (400 MHz, CDCl
3
δ): 7.37-7.31
(
m, 5H, ArH), 7.16 (bs, 1H, CONH), 5.80 (bs, 1H, CONH), 5,65 (bs, 1H,
OCONH), 5.19-5.13 (m, 2H, C CH ), 4.53 (m, 1H, α-CH), 3.48-3.28
), 3.06-3.01 (m, 1H, α-CHCH CO), 2.78-2.72
CH ), 2.14-2.06
CH C ), 1.75
6
H
5
2
(m, 2H, COCH
(m, 1H, α-CHCH
(m, 1H, CH CH
(s, 3H, CCH ), 1.46 (s, 9H C(CH
[M+H] : 434.2286, found: 434.2295.
2
CH
CO), 2.34-2.26 (m, 1H, CH
C), 1.92-1.85 (m, 2H, CH
2
a b
H
a
H
b
2
CH
CH
H
a b
2
2
a
H
b
2
a
H
b
a b
H
3
3 3
) ); HRMS (ESI-TOF): m/z calcd
32 3 6
for C22H N O
(
R)-Benzyl 3-methyl-2,6-dioxopiperidin-3-ylcarbamate (12).
solution of amide 8 (2.33 g, 8.9 mmol) in 40 mL of ethyl acetate was
added to the mixture of RuCl .xH O (190 mg, 0.9 mmol) and 10% aqueous
NaIO (90 mL). The biphasic reaction mixture was vigorously stirred at
0°C in a sealed flask until completion as indicated by TLC. After comple-
A
3
2
4
4
tion of the reaction, the two layers were separated and the aqueous layer
was extracted with two 50 mL portions of ethyl acetate. The combined
organic layer was washed with a saturated solution of Na
2
S
2
O
3
, followed
Fig. 2. Configurationally stable thalidomide analogs.
Chirality DOI 10.1002/chir

6
22
YADAV ET AL.
an alkyl group at C424 to prepare nonracemizable structural
variants.
matched data reported by Kapadia et al.33 for the same com-
pound. Enantioretentive alkylation of 3 using LHMDS and
34
3
-Methylthalidomide enantiomers have been extensively
allyl bromide resulted in the formation of compound 4.
studied and were found to have better TNF-α inhibition
Since it has been reported that the addition of an electrophile
to the preformed enolate leads to poor yield of the alkylated
product, presumably due to the slow alkylation rate of
oxazolidinone 3, LHMDS was added slowly to a mixture of al-
lyl bromide and oxazolidinone 3 to improve the yield of the
1
8,25
activity when compare to thalidomide.
It has also been
reported that regulation of TNF-α production by thalidomide
analogs is specific to cell type and the inducer. It is well
known that only (S)-3-methylthalidomide increases TNF-α
production in the 12-O-tetradecanoylphorbol 13 acetate/human
leukemia HL-60 assay system, while the (R)-isomer is more ac-
tive than the (S)-isomer of 3-methylthalidomides in the okadaic
3
3,35
alkylated product 4.
The alkylated oxazolidinone 4 exists
as Cbz-rotamers in a ratio of 2.5:1, as evident by NMR spec-
35
troscopy. Compound 4 was subsequently hydrolyzed with
aq. NaOH and then converted to methyl ester 5. Terminal hy-
droxylation of 5 was carried out with 9-BBN to obtain primary
26–28
acid/HL-60 assay system.
The methodology for the stereoselective synthesis of 3-
methylthalidomide enantiomers is not yet explored and the
major limitation associated with previously reported methods
is the formation of a racemic mixture which requires chiral
purification to obtain the particular enantiomer of 3-
36
alcohol 6 using a standard protocol. Mesylation of com-
pound 6 followed by reaction with sodium azide afforded
azide 7. When azide 7 was subjected to the Staudinger reduc-
tion, the expected amine product was not isolated, but
cyclized in situ to the amide product 8. Removal of the Cbz-
protecting group was accomplished by catalytic hydrogena-
tion, followed by reaction of the resulting amine product with
phthalic anhydride to yield amide 9. Oxidation of amide 9
18,20
methylthalidomide.
The biological activity including tera-
togenicity of 3-methylthalidomide derivatives are reported to
exhibit enantiodependent bidirectional regulatory effects on
26
TNF-α production. Therefore, efforts were made to develop
stereoselective synthesis of nonracemizable 4-trifluoromethyl
substituted and 2-oxetano thalidomides (Fig. 2) in the recent
37
with periodic acid yielded (R)-3-methylthalidomide 10.
Furthermore, we extended our synthetic methodology to
the synthesis of some amino acid conjugates of 3-amino-3-
methylpiperidin-2-one (11a and 11b) and 3-amino-3-
methylpiperidin-2,6-dione (13a and 13c), which are structur-
29,30
literature.
Furthermore, the stereoselective synthesis of a
structurally similar analog, (S)-3-trifluoromethyl thalidomide,
is reported by Rh-catalyzed direct enantioselective
3
1
38
alkynylation of α-ketiminoesters. These configurationally
stable thalidomide analogs will be of great help for the devel-
opment of highly potent biologically active molecules, includ-
ing TNF-α inhibitors.
ally close to the compounds reported in the patent literature,
with reported biological activity as shown in Scheme 2. The
N-Cbz protection of amide 8 was removed by hydrogenation
followed by coupling with N-Boc-O-benzyl-L-serine and
N-Boc-L-aspartic acid 4-benzyl ester in the presence of EDCI.
HCl/HOBt to provide amino acid conjugates 11a and 11b,
respectively, the cyclic amide variants. Furthermore,
compound 8 was successfully oxidized to the corresponding
imide derivative 12 using ruthenium catalyzed oxidation in
water-ethyl acetate biphasic medium using excess sodium
metaperiodate. Other oxidizing reagents, viz., Dess-Martin
periodinane, m-CPBA, chromium trioxide/periodic acid did
not give satisfactory results. The N-Cbz-3-methylpiperidin-
2,6-dione 12 was subjected to hydrogenation and the resultant
Herein we report the stereoselective synthesis of (R)-3-
methylthalidomide enantiomer, as illustrated in Scheme 1.
The key intermediate oxazolidinone 3 was synthesized via
the improved procedure for the synthesis of Karady’s
3
2
oxazolidinone reported by Cheng et al. The reaction prod-
ucts yielded predominantly cis-isomer (>90%) and both the
cis and trans isomers are easily separable by silica gel chro-
matography. The diastereomeric purity of cis oxazolidinone
3
was further improved to >98% by crystallization from
diethyl ether–hexane. The NMR-spectroscopy data exactly
Scheme 1. Stereoselective Synthesis of (R)-3-methylthalidomide.
Chirality DOI 10.1002/chir

STEREOSELECTIVE SYNTHESIS OF (R)-3-METHYLTHALIDOMIDE
623
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Chirality DOI 10.1002/chir