Dehydrative synthesis of chiral oxazolidinones catalyzed by alkali metal carbonates under low pressure of CO2

Article abstract of DOI:10.1016/j.tetlet.2013.06.100

Dehydrative synthesis of oxazolidinones from amino alcohols and CO ₂ was achieved in the presence of alkali metal carbonates such as Cs₂CO₃ as catalyst. It is noteworthy that 1 atm of CO ₂ is enough for the reaction to proceed and no special dehydrating agent is required in this system. A mechanstic study showed that the OH of amino alcohol acts as nucleophile and the OH in the carbamic acid moiety is liberated during the cyclization process.

Full text of DOI:10.1016/j.tetlet.2013.06.100

Tetrahedron Letters 54 (2013) 4717–4720
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Tetrahedron Letters
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Dehydrative synthesis of chiral oxazolidinones catalyzed by alkali
metal carbonates under low pressure of CO2
Siong Wan Foo a, Yuki Takada a, Yusuke Yamazaki a, Susumu Saito a,b,
⇑
a Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
b Institute for Advanced Research, Nagoya University, Chikusa, Nagoya 464-8601, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Dehydrative synthesis of oxazolidinones from amino alcohols and CO2 was achieved in the presence of
alkali metal carbonates such as Cs2CO3 as catalyst. It is noteworthy that 1 atm of CO2 is enough for the
reaction to proceed and no special dehydrating agent is required in this system. A mechanstic study
showed that the OH of amino alcohol acts as nucleophile and the OH in the carbamic acid moiety is lib-
erated during the cyclization process.
Received 7 May 2013
Revised 18 June 2013
Accepted 21 June 2013
Available online 28 June 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Carbon dioxide
Amino alcohol
Dehydration
Cesium carbonate
Alkali metal carbonate
Oxazolidinone
In recent years, the expected depletion of petroleum has im-
pelled us to seek other carbon resources. Carbon dioxide (CO2) is
one of the potential candidates as it is the most abundant carbon
source in the Earth’s atmosphere—with around 300 billion tons.1
Increasing exploitation of CO2 as starting material in the chemical
industry is thus highly desirable. It is an abundant, cheap, and non-
toxic source of C1.2 In recent years, studies in CO2 transformation
have extended to the synthesis of polymers,3 carbonates,4 carba-
mates,5 formate,6 methanol,7 carbon monoxide,8 etc.
Oxazolidinones are in the family of carbamates, and are among
the most widely used chiral auxiliaries in asymmetric natural
product synthesis.9 Oxazolidinones are mainly manufactured by
phosgenation of b-amino alcohols.10,11 However, the use of phos-
gene (Cl2C@O) precludes widespread application in laboratory
and industry due to phosgene’s toxicity and adverse environmental
impact.5b,11 Replacement of phosgene with CO2 is an ideal alterna-
tive. In fact, research on the synthesis of oxazolidinones starting
from b-amino alcohol5b,11,12 and CO2 has been an important sub-
ject for many years.13–16 Nevertheless, simple and salt-free ap-
proach in oxazolidinone synthesis starting from b-amino alcohol
has yet to be established. This might be due to thermodynamic rea-
sons (vide infra) and catalyst deactivation caused by the copro-
duced water.5b,11 In order to overcome these drawbacks, many
attempts have been made to shift the equilibrium to the product
side. The most straightforward way is to use a dehydrating agent
to trap the coproduced water14a,b,15; other attempts include the
use of aziridine17 (a predehydrated form of b-amino alcohol) and
phosphorylating agents (similar to Mitsunobu’s reaction).16i–m Car-
bon monoxide (CO)5b,11,18 and supercritical CO2 are additional pop-
ular alternatives for combating the thermodynamic issues in
oxazolidinone synthesis.13,15 Although several groups have re-
ported catalytic synthesis of oxazolidinone using cyclic carbonate
as ‘carbonyl source’, it rarely attracted much attention because
the use of cyclic carbonate incurred higher production cost and
complexity in the reaction.19 Recently, CeO2 nanoparticles were
used as catalyst in the reaction of N-substituted-b-amino alcohols
or other
x-amino alcohols with CO2 (7 bar) at 160 °C but the con-
version was unsatisfactory.20 Therefore, simply dehydrative meth-
ods are rarely employed in the general synthesis of chiral
auxiliaries. We report here a chiral oxazolidinone synthesis via
the incorporation of CO2 under atmospheric pressure (1 atm) into
b-amino alcohols, with merely H2O as byproduct and without
using any special dehydrating agents (Scheme 1).
In order to assess a possibility of simple dehydrative method of
oxazolidinone synthesis from CO2, we started our considerations
with fundamentals, that is, the Gibbs free energy and enthalpy cal-
culations. The reaction with ethanolamine (R1 = H) is endergonic
and endothermic
(B3LYP/6–31+G⁄⁄) and
mol (M062X/6–311++G⁄⁄)]. Nonetheless, the thermodynamics are
[
D
G = +3.1 kcal/mol and
D
H = +1.6 kcal/mol
⇑
D
G = +2.63 kcal/mol and
D
H = +1.33 kcal/
Corresponding author. Tel./fax: +81 52 789 5945.
E-mail addresses: saito.susumu@f.mbox.nagoya-u.ac.jp, saito.susumu@me.com
(S. Saito).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.100

4718
S. W. Foo et al. / Tetrahedron Letters 54 (2013) 4717–4720
Table 1
more favorable when ethanolamine is replaced with
(R1 = iPr) [
G = +0.35 kcal/mol and H = –0.84 kcal/mol (B3LYP/
6–31+G⁄⁄
and G = +0.01 kcal/mol and H = –0.80 kcal/mol
L-valinol
Synthesis of oxazolidinone (S)-2a from (S)-1aa
D
D
)
D
D
(M062X/6–311++G⁄⁄)]. For this reason, amino alcohols derived
from natural amino acids were selected (R1 – H) (Scheme 1).
Based on the results obtained in the synthesis of dialkyl carba-
mates21a,b and of dialkyl carbonates21c,d from CH2Cl2 and CO2 at a
low pressure,21d we found, to our surprise, that an alkali metal salt
sufficed to promote the dehydration, when
L-valinol ((S)-1a:
Entry
Cat.
Yieldb (%)
[1a]0 = 0.90 M) and Cs2CO3 (10 mol %: [Cs2CO3]0 = 0.09 M) in
DMSO-d6 were exposed to a 1 atm of CO2 (purity = 99.9%) at
150 °C for 24 h; (S)-2a was obtained in 90% (isolated yield: 80%,22
Table 1, entry 10).
1
2
3
(NH4)2CO3
Li2CO3
Na2CO3
K2CO3
6
3
22
49
4
The yield was only 6% when Cs2CO3 was replaced with ammo-
nium carbonate (entry 1). Among the alkali metal (bi)carbonates
(e.g., Li2CO3, Na2CO3, K2CO3, Rb2CO3, KHCO3, and CsHCO3) that pro-
moted this reaction (entries 2–7 and 10), Cs2CO3 was the most
effective. The catalytic activity decreased in the order Cs+ ꢀ Rb+ >
K+ > Na+ > Li+ (entries 2–5 and 10). Based on this observed trend,
dissociation of the alkali metal cation from the carbonate anion ap-
peared to be playing an important role in the reactivity. Hence, in
the case of K2CO3, 18-crown-6 was used as an additive (10 mol %),
and in fact the product yield was improved to 64% (compared to
entry 4). Equivalent amount of CsHCO3 (10 mol %) gave a less
encouraging result (entries 7 and 10). Other Cs sources, for exam-
ple, CsOAc or Cs2SO4 (2a: 4% and 6%, respectively), were ineffective
compared to Cs2CO3 (entry 10). Different solvents such as DMF
(4%), NMP (8%), DMI (55%), DMA (9%), and MeCN (<1%) did not pro-
vide a satisfying yield of 2a. No significant difference in reactivity
was observed between a commercial DMSO directly used and
anhydrous DMSO-d6 distilled over CaH2 (2a: 91% and 90%, respec-
tively). Thus, the moisture present in a commercial DMSO did not
interfere with the action of Cs2CO3. A P1 mL of DMSO per 1 mmol
of 1a was needed to obtain satisfactory results (entry 8). When di-
methyl sulfone was used as solvent or no catalyst was present (en-
try 11), only a trace amount of the desired product was obtained
(2a: <1%).
With the optimized conditions in hand, various amino alcohols
have been screened (Table 2). Oxazolidinones, especially those
widely used as Evans auxiliaries,9 were obtained in satisfactory
yields (entries 4, 5, and 7). Regardless of whether the substituent
(R1) on the amino-alcohol skeleton was a primary, secondary, or
tertiary carbon, trapping of CO2 proceeded smoothly. Albeit some
of them needed slightly higher CO2 pressure (3–5 atm) compared
to the optimized conditions, moderate to good yields were ob-
tained. When R1 was the methyl group, the product yield was
not as good as when it was a larger substituent (entries 1–8).
The larger the R1, the better the yield of 2. This observation can
be rationalized by the Thorpe–Ingold effect.23 According to the ini-
tial calculations, the reaction with ethanolamine (R1 = H) faces a
thermodynamic disadvantage, and the experimental observation
is in accordance with the calculations (vide supra). Indeed, when
ethanolamine was used, only 7% of the desired product was ob-
tained under the optimized conditions. A higher catalyst load
(20 mol %) or an increased CO2 pressure (9 atm) failed to increase
5
6
7
8
9
10
11
Rb2CO3
KHCO3
CsHCO3
Cs2CO3
Cs2CO3
Cs2CO3
None
88
56
48
34c,d
62c
90e (80)
<1
a
The reaction was performed using (S)-1a (1 mmol) in DMSO (1 mL) with an
initial CO2 pressure of 1 atm at 25 °C.
Determined by 1H NMR (DMF as internal standard); the number in parentheses
b
is the isolated yield.
c
5 mol % of Cs2CO3.
0.5 mL of DMSO.
Anhydrous DMSO-d6 was used.
d
e
Table 2
Substrate scopea
Entry
Amino alcohol 1
Oxazolidinone 2
Yieldb,c (%)
1d
2d
3d
4d
5
(S)-1b (R = Me)
(S)-1c (R = Et)
(S)-1d (R = iBu)
(S)-1e (R = Bn)
(R)-1a (R = iPr)
(S)-1f (R = sBu)
(S)-1g (R = tBu)
(R)-1h (R = Ph)
(S)-2b (R = Me)
(S)-2c (R = Et)
(S)-2d (R = iBu)
(S)-2e (R = Bn)
(R)-2a (R = iPr)
(S)-2f (R = sBu)
(S)-2g (R = tBu)
(R)-2h (R = Ph)
42 (36)
63 (51)
57 (49)
88 (90)
89 (70)
81 (78)
— (80)
6
7
8e
61 (69)
a
Unless otherwise specified, the reaction conditions were: 1 (1 mmol), Cs2CO3
(10 mol %), CO2 (PCO2 = 1 atm; CO2 balloon) in anhydrous DMSO or DMSO-d6 (1 mL),
at 150 °C for 24 h.
Determined by 1H NMR (DMF as internal standard); the number in parentheses
b
is the isolated yield.
c
Conversion of 1:>99%. Other products: carbamic acid (RNHCO2H, major) and
carbamate (RNHCO2)–(RNH3)+ or ammonium bicarbonate RNH3þHCOꢁ3 were detec-
ted by 1H NMR (DMSO-d6) in a crude mixture (R = ꢁ(R1)CHCH2OH). See Refs. 24a,b
for the assignment.
d
PCO2 = 3 atm, adjusted using autoclave.
PCO2 = 5 atm, adjusted using autoclave.
e
the product yield. The retention of the absolute configuration at
the b-positions of amino alcohols 1a–h was consistently observed,
giving products 2a–h as single enantiomers (see Supplementary
data).
Carbamic acid (Scheme 1) can be formed from amino alcohol
when exposed to 1 atm of CO2 and was detected in DMSO-d6 as a
more favorable structure than the ammonium carbamate.24 Car-
bamic acid has two hydroxyl groups (the alcohol OH and the acid
OH). One acts as nucleophile and the other is liberated, as will be
shown in Scheme 3. Generally, the alcohol OH acts as a nucleophile
in the C–O bond formation.25 Nonetheless, selectivity of the leaving
OH can be altered by steric control.26,27 Thus, control experiments
Scheme 1. Synthesis of oxazolidinone from CO2 under atmospheric pressure.

S. W. Foo et al. / Tetrahedron Letters 54 (2013) 4717–4720
4719
Acknowledgments
This work was supported by funds from Asahi Glass, Asahi Kasei
Chemicals, Kuraray, Mitsubishi Chemical, Mitsui Chemicals, Nissan
Chemical Ind., and Sumitomo Chemical, Asahi Glass Foundation
and ACT-C, JST. S.W.F. and Y.T. were financially supported by the
GCOE in chemistry, Nagoya University (NU) and IGER in chemistry,
NU. The authors wish to thank Professor R. Noyori (NU & RIKEN)
for fruitful discussions and Professor P. Butko (NU) for technical
advice.
Scheme 2. Reaction with (R)-1-aminopropan-2-ol ((R)-1i).
were carried out to clarify which OH is liberated as water
(Scheme 2).
In contrast to the absolute retention at the b-carbon of b-ami-
Supplementary data
no alcohols, the chirality at the
a-position can provide useful
information on which OH group (the alcohol OH or the acid
OH) in the carbamic acid is liberated (Scheme 3). In this case,
(R)-1-aminopropan-2-ol ((R)-1i) was used under similar reaction
conditions (Scheme 2). The chiral GC analysis of the product
showed complete retention of the parent chiral center bearing
the methyl substituent, demonstrating the exclusive formation
of (R)-2i (see Supplementary data). In addition, when C18O2
was used upon reaction with 1a, one 18O was incorporated pre-
dominantly into 2a.21a,28 These results suggest that the alcohol
OH acts as a nucleophile and the acid OH is a leaving group.
The HCOꢁ3 anion seems one of the most critical species involved
in the reaction.29
In summary, oxazolidinones were synthesized from amino
alcohols and CO2 (1–5 atm) with the aid of catalytic alkali metal
(bi)carbonates represented by Cs2CO3. Special dehydrating
agents were not needed in this system. The reaction protocol
reported herein is very simple, straightforward, and can be rep-
licated in any laboratory. The preliminary mechanistic study re-
vealed that the OH of amino alcohol acts as nucleophile and the
OH at carbamic acid moiety is liberated during the cyclization
process.30
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
06.100.
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4720
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(ppm in DMSO-d6) d 159.0, 62.1, 57.6, 28.4, 19.7, 17.9] essentially can preserve
its structure (ꢂ50 °C) or can be formed reversibly in DMSO (ꢂ150 °C) under
1 atm of CO2. See also Supplementary Figures S1 and S2. On the other hand,
based on the low resolution FAB-MS analysis, the carbamic acid that
precipitated out from a less polar solvent such as MeCN at 25 °C was mainly
the ammonium carbamate; it is an intermolecular acid–base adduct comprised
of two molecules of 1a and one molecule of CO2 in the form of
(RNHCO2)–ꢃ(RNH3)+. DMSO is indeed an excellent solvent to maintain the
structure of carbamic acid, superior to (RNHCO2)–ꢃ(RNH3)+: (a) Ito, Y.; Ushitora,
H. Tetrahedron 2006, 62, 226–235; (b) Matsuda, K.; Ito, Y.; Horiguchi, M.; Fujita,
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M.; Rudkevich, D. M. Tetrahedron 2003, 59, 9619–9625.
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University Press Inc.: New York, 2001.
26. Mechanistic studies with chiral alcohol or C18O2 in Mitsunobu-like reaction
from other groups: the selectivity can depend on the starting amino alcohol, N-
substituted with hydrogen (acid OH leaves) or with carbon (alcohol OH leaves)
(Ref. 16j); it depended on the phosphine species, PBu3 (acid OH leaves) and
PPh3 (alcohol OH leaves) (Ref. 16i).
27. Bartoli, G.; Bosco, M.; Carlone, A.; Locatelli, M.; Melchiorre, P.; Sambri, L. Org.
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28. 2a-18O2 was detected in a negligible amount, in addition to a modest amount
of 2a without 18O-incorporation. The latter product may be due to fast
exchange of the two oxygens of the carbamic acid with oxygen of water or of
Cs2CO3.
29. A carbamic acid (RNHCO2H) derived from beta-amino alcohol 1 and CO2 has an
acid proton more susceptible to deprotonation by Cs2CO3 than that of the
alcohol OH. However, the reaction system is weakly acidic rather than basic,
since carbamic acid remains consistently in an excess quantity during the
course of the reaction (at least from the beginning to the later stage), compared
with the amount of Cs2CO3, CsHCO3 (<10 mol %), or RNHCO2Cs (<10 mol %) that
may not react as substrate. In fact, the yield of 2 has so far never exceeded 90%
(Tables 1 and 2). In addition, when a similar oxazolidinone synthesis was
carried out using a 2:1 molar ratio of 1a and Cs2CO3 or 1a and CsHCO3 except
that CO2 was absence, only more acidic CsHCO3 gave 2a but in a low yield (15%
based on 1a; 30% based on CsHCO3). The HCO2ꢁ, rather than a more basic CO23ꢁ
,
is capable of promoting the reaction. This sug3gests that a weak acid–weak base
cooperative catalysis as shown in Scheme 3 would be responsible.
30. Although the three-component coupling among alcohol, amine and CO2
(25 atm) was reported using catalytic Cs2CO3 at
a higher temperature
(200 °C), the mechanism disclosed therein is entirely different from ours: the
carbamate was formed from intermediate carbamide: (a) Ion, A.; Doorslaer, C.
V.; Parvulescu, V.; Jacobs, P.; Vos, D. D. Green Chem. 2008, 10, 111–116; (b) Ion,
A.; Parvulescu, V.; Jacobs, P.; Vos, D. D. Green Chem. 2007, 9, 158–161.
19. (a) Henkelmann, J.; Ruhl, T. 1998, US Patent 5783706; (b) Patil, Y. P.; Tambade,
P. J.; Jagtap, S. R.; Bhanage, B. M. J. Mol. Catal. A: Chem. 2008, 289, 14–21; (c)
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