Parallel kinetic resolution of 2-methoxy and 2-phenoxy-substituted carboxylic acids using a combination of quasi-enantiomeric oxazolidinones

Article abstract of DOI:10.1016/j.tetasy.2007.01.018

2-Methoxy-2-phenyl acetic acid and 2-phenoxy-2-phenylpropionic acid were resolved by the parallel kinetic resolution of their corresponding pentafluorophenyl active ester using a quasi-enantiomeric combination of lithiated oxazolidinones.

Full text of DOI:10.1016/j.tetasy.2007.01.018

Tetrahedron: Asymmetry 18 (2007) 476–482
Parallel kinetic resolution of 2-methoxy and
2-phenoxy-substituted carboxylic acids using a combination
of quasi-enantiomeric oxazolidinones
Ewan Boyd,a Sameer Chavda,b Jason Eamesb,* and Yonas Yohannesc
aSyngenta, Grangemouth Manufacturing Centre, Earls Road, Grangemouth, Scotland FX3 8X, UK
bDepartment of Chemistry, University of Hull, Cottingham Road, Kingston upon Hull HU6 7RX, UK
cDepartment of Chemistry, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
Received 4 January 2007; accepted 8 January 2007
Available online 21 February 2007
Abstract—2-Methoxy-2-phenyl acetic acid and 2-phenoxy-2-phenylpropionic acid were resolved by the parallel kinetic resolution of their
corresponding pentafluorophenyl active ester using a quasi-enantiomeric combination of lithiated oxazolidinones.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
(rac)-8 can be achieved using two lithiated quasi-enantio-
meric Evans oxazolidinones [derived from oxazolidinone
(R)-6 and (S)-7] to give the corresponding syn-adducts 9
and 10 in good yield with high levels of diastereocontrol
(Scheme 2). We have also demonstrated12 that complemen-
tary parallel kinetic resolution of oxazolidinone (rac)-6
can occur efficiently by using two complementary quasi-
enantiomeric pentafluorophenyl active esters, (S)-11 and
(R)-12, to give the corresponding syn-adducts 13 and 14
respectively, with excellent levels of diastereoselectivity
(Scheme 3).
The synthesis of enantiomerically pure carboxylic acids1
based on the carbon-skeleton of phenylacetic acid, such
as 2-phenylpropionic acid,2 mandelic acid3 and phenylgly-
cine4 is well documented. Many of these derivatives have
found widespread use as synthetic building blocks5 and
many of these derivatives have been shown to have biolog-
ical importance.6
2. Results and discussion
We now report an extension to our methodology for the
resolution of 2-methoxy-2-phenyl acetic acid (rac)-15 and
2-phenoxy-2-phenylpropionic acid (rac)-16 using a quasi-
enantiomeric combination of Evans’ oxazolidinones
(Scheme 4). We chose to focus our attention on two penta-
fluorophenyl active esters (rac)-17 and (rac)-18 derived
from the corresponding 2-methoxy-2-phenyl acetic acid
(rac)-15 and 2-phenoxy-2-phenylpropionic acid (rac)-16
as these were structurally related to our original11,12 2-phe-
nylpropionic acid substrate (Scheme 4). These active esters
(rac)-17 and (rac)-18 were synthesised in moderate to good
yield by the addition of DCC to a stirred solution of
pentafluorophenol and the corresponding carboxylic acids
(rac)-15 and (rac)-16, respectively, in dichloromethane.
Over the last decade, the parallel separation of enantiomers
by kinetic resolution has attracted some attention.7 This
particular concept was developed by Davies8 and Vedejs.9
More recently, Davies10 has shown elegantly the parallel
separation of a racemic enoate (rac)-1 using an equimolar
combination of two quasi-enantiomeric lithium amides
(S)-2 and (R)-3 to give two separable b-amino esters syn,
syn,anti-4 and syn,syn,anti-5, respectively, in good yields
(35–39% out of a maximum of 50%) with high levels of
diastereoselectivity (95–99% de) (Scheme 1).
From our laboratory,11,12 we have reported11 the parallel
kinetic resolution of pentafluorophenyl 2-phenylpropionate
In order to determine the levels of mutual recognition be-
tween active esters (rac)-17 and (rac)-18, and the associated
oxazolidinones (e.g., 6 and 7), we first screened their
*
Corresponding author. Tel.: +44 1482 466401; fax: +44 1482 466410;
e-mail: j.eames@hull.ac.uk
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.01.018

E. Boyd et al. / Tetrahedron: Asymmetry 18 (2007) 476–482
Ar = 3,4-dimethoxyphenyl
477
Me
Ph
Me
Ar
Me
Ph
Ph
N
Li
CO2Me
t-Bu
CO2Me
t-Bu
CO2Me
t-Bu
N
N
(S)-2
Ph
Ph
Me
Ar
(R)-3
Ph
N
Li
syn,syn,anti-5
35%, 97-99% d.e.
syn,syn,anti-4
39%, 95-97% d.e.
(rac)-1
THF, -78oC
Scheme 1. Parallel kinetic resolution of enoate (rac)-1.
O
O
O
O
O
O
1. n-BuLi
THF, -78 oC
Ph
N
Ph
O
N
O
HN
Ph
O
HN
O
O
2.
Me
H
H
Me
i-Pr
Ph
Ph
i-Pr
(S)-7
OC6F5
Me
H
syn-: anti-9
95:5 (60%)
syn-: anti-10
(R)-6
2 equiv.
88:12 (60%)
(rac)-8
Scheme 2. Parallel kinetic resolution of active ester (rac)-8.
MeO
O
O
O
O
DCC, C6F5OH
CH2Cl2
Ph
Ph
OH
OC6F5
OC5F5
OC5F5
OMe
OMe
H
Me
Me
H
(rac)-15
(S)-11
(
R
)-12
(rac)-17; 57%
O
O
O
PhO
PhO
DCC, C6F5OH
CH2Cl2
1. n-BuLi
THF, -78oC
OH
OC6F5
HN
Ph
O
Me
(rac)-16
Me
2.
(S)-11
(R)-12
(rac)-18; 64%
Scheme 4. Synthesis of active esters (rac)-17 and (rac)-18.
(rac)-6
MeO
O
O
O
O
1. n-BuLi
O
O
O
THF, -78 oC
Ph
N
O
HN
Ph
O
N
O
N
O
+
2.
O
MeO
H
Me
H
H
Me
Ph
Ph
Ph
Ph
OC6F5
(rac)-6
syn-19
OMe
syn-13
syn-: anti-; 95:5 (49%)
syn-14
syn-: anti-; >95:<5 (52%)
syn-:anti-; 86:14 (51%)
(rac)-17
Scheme 3. Parallel kinetic resolution of oxazolidinone (rac)-6.
O
O
O
1. n-BuLi
THF, -78 oC
PhO
Me
N
O
HN
O
mutual kinetic resolution using racemic oxazolidinone
(rac)-6 (derived from phenylglycine) as our model sub-
strate (Scheme 5).13 Deprotonation of oxazolidinone (rac)-6
(by the addition of n-butyl lithium in THF at ꢀ78 ꢁC),
followed by the addition of pentafluorophenyl 2-meth-
oxy-2-phenyl acetate (rac)-17 and pentafluorophenyl 2-
phenoxypropionate (rac)-18, gave the corresponding
diastereoisomeric adducts 19 and 20 in good yield (Scheme
5). The levels of diastereocontrol were excellent favouring
formation of the syn-diastereoisomeric adducts 19 and
20, respectively (Scheme 5). The relative stereochemical
outcome of these mutual kinetic resolutions appear to be
2.
O
H
PhO
Ph
Ph
(rac)-6
OC6F5
syn-20
Me
syn-:anti-; 89:11 (62%)
(rac)-18
Scheme 5. Mutual kinetic resolution of active esters (rac)-17 and (rac)-18.
similar for both active esters, (rac)-17 and (rac)-18. How-
ever, it is interesting to note, their mutual enantiomer
recognition processes were evidently different;14 that is,
oxazolidinone (R)-6 appears to recognise (S)-enantiomer

478
E. Boyd et al. / Tetrahedron: Asymmetry 18 (2007) 476–482
of 17 but (R)-enantiomer of 18, and vice versa. This is also
true for their complementary mirror image combinations.
From these mutual resolutions, it appears that the Lewis
basicity of the oxygen atom at the C(2) position [within
both active esters (rac)-17 and (rac)-18] mediates the diaste-
reoselection; the more Lewis basic OMe group promotes
formation of syn-19, whereas the less Lewis basic OPh
group favours the formation of syn-20 (Scheme 5). It is
worthy of note, that this situation is additional complicated
due to the competitive Lewis basicity of the pentafluoro-
phenyl pro-leaving group.
O
O
O
Ph
LiOH, H2O2
CH2Cl2
Ph
OH
OMe
N
O
H
H
OMe
Ph
syn-19
(S)-15; 96%
O
O
O
Ph
LiOH, H2O2
CH2Cl2
Ph
OH
H
N
H
O
MeO
MeO
Ph
anti-19
(R)-15; 95%
With this information in hand, we next probed the parallel
kinetic resolution of these racemic active esters (rac)-17 and
(rac)-18 using an equimolar combination of lithiated ox-
azolidinones. For this study, we chose to use the combina-
tion of oxazolidinone, (R)-6, and its complementary
quasi-enantiomer, (S)-7, as our parallel resolving agents
(as outlined in Scheme 6), as we believed these would
mimic our original mutual kinetic resolution involving
racemic oxazolidinone (rac)-6 (in Scheme 5). Treatment
of an equimolar amount of oxazoldinones (R)-6 and
(S)-7 with n-butyl lithium, followed by the addition of a
solution of racemic active ester (rac)-17 and (rac)-18 in
THF at ꢀ78 ꢁC, gave a pair of separable quasi-enantio-
meric adducts syn-19 and syn-21, and syn-20 and syn-22,
respectively, in good yield (Scheme 6). The levels of diaste-
reocontrol were good (64–78% de), and most importantly
were comparable to those shown for their corresponding
mutual recognitions.
O
O
O
LiOH, H2O2
CH2Cl2
PhO
PhO
H
OH
Me
N
Me
O
H
Ph
syn-20
(S)-16; 91%
O
O
O
PhO
Me
LiOH, H2O2
CH2Cl2
PhO
Me
OH
N
H
O
H
Ph
anti-20
(R)-16; 85%
Scheme 7. Hydrolysis of oxazolidinones syn-19, anti-19, syn-20 and anti-
20.
The required resolutions of 2-methoxy-2-phenyl acetic acid
(rac)-15 and 2-phenoxy-2-phenylpropionic acid (rac)-16
were achieved through simple hydrolysis of adducts syn-
19, anti-19, syn-20 and anti-20 using a combination of
LiOH and H2O2 to give the corresponding 2-methoxy-2-
phenyl acetic acids (S)-15 and (R)-15, and 2-phenoxy-2-
phenylpropionic acid (S)-16 and (R)-16 in good yield
(Scheme 7). The absolute stereochemistry was assigned by
comparison with the specific rotation of known literature
values. The enantiomeric purity was determined by simple
re-derivatisation by the formation of the corresponding
pentafluorophenyl active esters, followed by sequential
addition to the lithiated oxazolidinone derived from (R)-6.
3. Conclusion
In conclusion, we report an efficient parallel kinetic resolu-
tion of pentafluorophenyl 2-methoxy-2-phenyl acetate
(rac)-17 and pentafluorophenyl 2-phenoxypropionate
(rac)-18 (derived from (rac)-15 and (rac)-16, respectively)
O
O
O
O
O
O
1. n-BuLi
THF, -78 oC
Ph
Ph
N
O
N
O
+
O
HN
Ph
HN
O
+
O
MeO
H
H
OMe
Ph
2.
Ph
OC6F5
OMe
syn-19
syn-: anti-; 89:11 (40%)
syn-21
syn-: anti-; 85:15 (50%)
(R)-6
(S)-7
(rac)-17
O
O
O
O
O
O
1. n-BuLi
THF, -78 oC
PhO
PhO
N
O
N
O
+
HN
O
HN
O
+
Me
H
H
Me
Ph
O
2.
PhO
Ph
OC6F5
Me
syn-20
syn-: anti-; 83:17 (62%)
syn-22
syn-: anti-; 82:18 (54%)
(R)-6
(S)-7
(rac)-18
Scheme 6. Parallel kinetic resolution of active esters (rac)-17 and (rac)-18 using a combination of oxazolidinones (R)-6 and (S)-7.

E. Boyd et al. / Tetrahedron: Asymmetry 18 (2007) 476–482
479
using an equimolar amount of quasi-enantiomeric oxazo-
lidinones (R)-6 and (S)-7. This resolution methodology
appears to be efficient for a range of substituted 2-hydroxy
carboxylic acid derivatives, giving access to both enantio-
merically pure forms of 2-methoxy-2-phenyl acetic acid
15 and 2-phenoxypropionic acid (rac)-16 in good yield.
The nearest analogy to this work is that reported by
Fox17 and Davies.18 Fox17 has shown the use of a combi-
nation of designer oxazolidinones to resolve racemic
anhydrides, and has recently desymmetrised a series of
meso-esters using methodology related to our parallel
kinetic resolution.19 By comparison, Davies18 has kineti-
cally resolved a series of 2-acetoxy acid chlorides using a
SuperQuat20 oxazolidinone to give adducts in high diaste-
reoselectivity. We are currently examining the effect of
competitive Lewis basicity on the mechanistic outcome of
these mutual/parallel kinetic resolutions, and this study
will be reported in due course.
CHO) and 3.51 (3H, s, CH3); dC (100 MHz; CDCl3)
167.0 (OC@O), 141.1 (142.19 and 139.68, 2C, ddt,
2
3
1JC,F = 250.6 Hz, JC,F = 12.2 Hz and JC,F = 3.8 Hz,
C(2)–F), 139.6 (140.83 and 138.31, 1C, dtt,
2
3
1JC,F = 252.1 Hz, JC,F = 12.9 Hz and JC,F = 3.9 = 8 Hz,
C(4)–F), 138.7 (i-C; Ph), 137.8 (139.03 and 136.52, 2 C,
1
2
3
dtdd, JC,F = 249.8 Hz, JC,F = 14.5 Hz, JC,F = 5.7 Hz
and JC,F = 2.3 Hz, C(3)–F), 134.5 (i-C; Ph), 129.4,1
4
128.82 and 127.22 (3 · CH; Ph), 82.0 (CHO) and 57.5
3
(OCH3); dF (378 MHz; CDCl3) ꢀ152.2 (2F, d, JF,F 20.9,
3
Fortho), ꢀ157.9 (1F, t, JF,F 20.9, Fpara) and ꢀ161.9 (2F,
t, 3JF,F 20þ.9, Fmeta
)
(Found MNH4
350.0810.
þ
þ
C15H13F5NO3 requires MNH4 350.0810).
4.2.2. Pentafluorophenyl 2-phenoxypropionate (rac)-18. In
the same way as active ester (rac)-17, 2-phenoxypropionic
acid (rac)-16 (1.0 g, 6.02 mmol), DCC (1.25 g, 6.62 mmol)
and pentafluorophenol (1.10 g, 6.02 mmol) gave active
ester (rac)-18 (1.28 g, 64%) as a colourless oil; RF [light
petroleum bp (40–60 ꢁC)–diethyl ether (9:1)] 0.50;
mp = 78–79 ꢁC; mmax (CHCl3); cmꢀ1 1788 (C@O); dH
(400 MHz; CDCl3) 7.34–7.29 (2H, m, 2 · CH; OPh), 7.03
(1H, t, J 7.3, CH; OPh), 6.94 (2H, d, J 8.5, 2 · CH;
OPh), 5.08 (1H, q, J 6.9, CHCH3) and 1.82 (3H, d, J 6.9,
4. Experimental
4.1. General
All solvents were distilled before use. All reactions were
carried out under nitrogen using oven-dried glassware.
Flash column chromatography was carried out using
Merck Kieselgel 60 (230–400 mesh). Thin layer chromatog-
raphy (TLC) was carried out on commercially available
pre-coated plates (Merck Kieselgel 60F254 silica). Proton
and carbon NMR spectra were recorded on a Bruker
250 MHz and 400 MHz Fourier transform spectrometers
using an internal deuterium lock. Chemical shifts are
quoted in parts per million downfield from tetramethyl-
silane. Carbon NMR spectra were recorded with broad
proton decoupling. Infrared spectra were recorded on a
Shimadzu 8300 FTIR spectrometer. Optical rotation was
measured using an automatic AA-10 Optical Activity Ltd
polarimeter.
CH3CH); dC (100.6 MHz; CDCl3) 168.5 (C@O), 157.1 (i-
1
CO; Ph), 141.1 (142.36 and 139.85, 2C, ddt, JC,F
=
2
3
251.3 Hz, JC,F = 12.2 Hz and JC,F = 3.8 Hz, C(2)–F),
1
139.8 (141.03 and 138.50, 1C, dtt, JC,F = 252.2 Hz,
2JC,F = 12.9 Hz and 3JC,F = 3.8 Hz, C(4)–F), 137.9
1
2
(139.17 and 136.65, 2C, dtdd, JC,F = 251.3 Hz, JC,F
=
3
4
14.5 Hz, JC,F = 5.7 Hz and JC,F = 2.2 Hz, C(3)–F),
2
3
124.6 (1C, tt, JC,F = 12.2 Hz and JC,F = 4.0 Hz, i-C;
C6F5), 129.7, 122.3 and 115.0 (3 · CH; OPh), 72.1 (PhCH)
and 18.7 (CH3); dF (378 MHz; CDCl3) ꢀ152.5 (2F, d, 3JF,F
3
21.9, Fortho), ꢀ156.9 (1F, t, JF,F 21.9, Fpara) and ꢀ161.7
3
(2F, t, JF,F 21.9, Fmeta
)
(Found M+, 332.0470;
þ
C15H9F5O3 requires 332.0466).
4.3. Mutual kinetic resolutions
4.2. Synthesis of active esters
4.3.1.
(4RS,2SR)-(2-Methoxy-2-phenylacetyl)-4-phenyl-
oxazolidin-2-one syn-20 and (4RS,2RS)-(2-methoxy-2-
phenylacetyl)-4-phenyl-oxazolidin-2-one anti-20. n-BuLi
(0.52 ml, 2.5 M in hexanes, 1.28 mmol) was added to a stir-
red solution of oxazolidinone (rac)-6 (0.20 g, 1.20 mmol) in
THF at ꢀ78 ꢁC. After stirring for 1 h, a solution of active
ester (rac)-17 (0.40 g, 1.28 mmol) in THF (5.0 ml) was
added. The resulting mixture was stirred for 2 h at
ꢀ78 ꢁC. The reaction was quenched with water (10 ml).
The organic layer was extracted with diethyl ether
(2 · 10 ml), dried (over MgSO4) and evaporated under re-
duced pressure to give a separable mixture of two diastereo-
isomers (ratio: anti:syn 86:14) of oxazolidinones anti- and
syn-19. The residue was purified by flash column chroma-
tography on silica gel eluting with light petroleum (bp
40–60 ꢁC)–diethyl ether (7:3) to give oxazolidinone syn-19
(0.15 g, 38%) as a colourless oil; RF [light petroleum (bp
40–60 ꢁC)–diethyl ether (1:1)] 0.14; vmax (film); cmꢀ1 1782
(C@O) and 1715 (C@O); dH (400 MHz, CDCl3) 7.36–7.12
(8 H, m, 8 · CH; PhA and PhB), 6.83 (2H, d, J 7.2,
2 · CH; PhA), 6.05 (1H, s, CHO), 5.47 (1H, dd, J 8.9 and
5.2, CHN), 4.67 (1H, t, J 8.9, CHAHBO), 4.10 (1H, dd, J
4.2.1. Pentafluorophenyl 2-methoxyphenylacetate (rac)-
17. N,N0-Dicyclohexylcarbodiimide (1.37 g, 6.61 mmol)
was slowly added to a stirred solution of 2-methoxy-phen-
ylacetic acid (rac)-15 (1.0 g, 6.01 mmol) in dichloromethane
(10 ml) at room temperature. The resulting solution was
stirred for 15 min. A solution of pentafluorophenol
(1.11 g, 6.01 mmol) in dichloromethane (3 ml) was slowly
added and the solution was stirred for a further 12 h. The
resulting precipitate (dicyclohexylurea) was removed
through filtration (using a sintered funnel). The reaction
was quenched with water (20 ml) and extracted with
dichloromethane (3 · 10 ml). The combined organic layers
were dried (over MgSO4) and evaporated under reduced
pressure. The residue was purified by flash column chroma-
tography on silica gel eluting with hexane–diethyl ether
(9:1) to give active ester (rac)-17 (1.13 g, 57%) as a colour-
less oil; RF [light petroleum bp (40–60 ꢁC)–diethyl ether
(8:2)] 0.66; tmax (CH2Cl2); cmꢀ1 1793 (C@O) and 732
(CH; Ph); dH (270 MHz, CDCl3) 7.53–7.49 (2H, m,
2 · CH; Ph), 7.44–7.40 (3H, m, 3 · CH; Ph), 5.10 (1H, s,

480
E. Boyd et al. / Tetrahedron: Asymmetry 18 (2007) 476–482
8.9 and 5.2, CHAHBO) and 3.35 (3H, s, CH3); dC
(100 MHz, CDCl3) 170.4 (C@O), 153.6 (C@O), 138.0 (i-
C; PhA), 135.3 (i-C; PhB), 129.6, 129.3, 129.2, 129.1,
128.9 and 126.1 (6 · C–H; PhA and PhB), 81.6 (CHO),
70.6 (CH2O), 57.9 (NCH) and þ 57.6 (CH3) (Found
oxazolidinonessyn-19 and anti-19 (ratio 89:11) and syn-21
and anti-21 (ratio 85:15). The residue was purified by flash
column chromatography on silica gel eluting with light
petroleum (bp 40–60 ꢁC)–diethyl ether (7:3) to give oxazo-
lidinone syn-19 (65 mg, 33%) as white needle-like crystals;
þ
þ
MNH4
329.1496. C18H21N2O4
requires MNH4
RF [light petroleum (40–60 ꢁC)–diethyl ether (1:1)] 0.37;
23
329.1496); and oxazolidinone anti-19 (52 mg, 13%) as white
needle-like crystals; RF [light petroleum (40–60 ꢁC)–diethyl
ether (1:1)] 0.37; mp 104–106 ꢁC; tmax (CH2Cl2)/cmꢀ1 1782
(C@O) and 1718 (C@O); dH (270 MHz, CDCl3) 7.53–7.49
(2H, m, 2 · CH; PhA), 7.36–7.31 (8H, m, 8 · CH; PhA
and PhB), 6.00 (1H, s, CHCO), 5.32 (1H, dd, J 8.6 and
3.2, CHN), 4.56 (1H, t, J 8.6, CHAHBO), 4.26 (1H, dd, J
8.6 and 3.2, CHAHBO) and 3.25 (3H, s, CH3); dC
(100 MHz, CDCl3) 170.1 (C@O), 153.0 (C@O), 138.8 (i-
C; PhA), 135.4 (i-C; PhB), 129.21, 129.01, 128.82, 128.61
and 125.91 (6 · C–H; PhA and PhB), 80.6 (CHCO), 70.3
(CH2O), 58.0 (NCH) and 57.0 (CH3) (Found M+
mp 92–98 ꢁC; ½aꢁD ¼ ꢀ14:4 (c 0.22, CHCl3); tmax
(CH2Cl2)/cmꢀ1 1782 (C@O) and 1718 (C@O); dH
(270 MHz, CDCl3) 7.36–7.12 (8H, m, 8 · CH; PhA and
PhB), 6.83 (2H, d, J 7.2, 2 · CH; PhA), 6.05 (1H, s,
CHO), 5.47 (1H, dd, J 8.9 and 5.2, CHN), 4.67 (1H, t, J
8.9, CHAHBO), 4.10 (1H, dd, J 8.9 and 5.2, CHAHBO)
and 3.35 (3H, s, CH3); dC (100 MHz, CDCl3) 170.4
(C@O), 153.6 (C@O), 138.0 (i-C; PhA), 135.3 (i-C; PhB),
129.6, 129.3, 129.2, 129.1, 128.9 and 126.1 (6 · C–H; PhA
and PhB), 81.6 (CHO), 70.6 þ(CH2O), 57.9 (NCH) and
þ
57.6 (CH3) (Found MNH4 329.1496. C18H21N2O4
þ
requires MNH4 329.1496); anti-19 (13 mg, 7%); RF [light
311.1152. C18H17NO requires M+ 311.1152).
petroleum (40–60 ꢁC)–diethyl ether (1:1)] 0.37; mp 164–
þ
4
23
165 ꢁC; ½aꢁD ꢀ 194:7 (c 0.72, CHCl3); tmax (CH2Cl2)/cmꢀ1
4.3.2.
(4RS,2RS)-(2-Phenoxypropionyl)-4-phenyl-oxazol-
1782 (C@O) and 1718 (C@O); dH (270 MHz, CDCl3)
7.53–7.49 (2H, m, 2 · CH; PhA), 7.36–7.31 (8H, m,
8 · CH; PhA and PhB), 6.00 (1H, s, CHCO), 5.32 (1H,
dd, J 8.6 and 3.2, CHN), 4.56 (1H, t, J 8.6, CHAHBO),
4.26 (1H, dd, J 8.6 and 3.2, CHAHBO) and 3.25 (3H, s,
CH3); dC (100 MHz, CDCl3) 170.1 (C@O), 153.0 (C@O),
138.8 (i-C; PhA), 135.4 (i-C; PhB), 129.21, 129.01, 128.82,
128.61 and 125.91 (6 · C–H; PhA and PhB), 80.6 (CHCO),
70.3 (CH2O), 58.0 (NCH) and 57.0 (CH3) (Found M+
idin-2-one syn-20 and (4RS,2SR)-(2-phenoxypropionyl)-4-
phenyl-oxazolidin-2-one anti-20. In the same way as the
above, n-butyl lithium (0.52 ml, 2.5 M in hexane,
1.28 mmol), oxazolidinone (rac)-6 (0.20 g, 1.22 mmol) and
active ester (rac)-18 (0.40 g, 1.28 mmol), gave a crude mix-
ture of oxazolidinones syn-20 and anti-20 (ratio 89:11).
The residue was purified by flash column chromatography
on silica gel eluting with light petroleum (bp 40–60 ꢁC)–
diethyl ether (7:3) to give oxazolidinone syn-20 (0.21 g,
52%) as a white solid; mp = 73–76 ꢁC; RF [light petroleum
(40–60 ꢁC)–diethyl ether (1:1)] 0.20; tmax/cmꢀ1 (CHCl2)
1782 (C@O) and 1715 (C@O); dH (270 MHz, CDCl3)
7.34–7.24 (5H, m, 5 · CH, Ph), 7.11 (2H, t, J 7.7, 2 · CH;
Ph), 6.86 (1H, t, J 7.4, 1 · CH; Ph), 5.88 (1H, q, J 6.4,
CHCH3), 5.44 (1H, dd, J 8.9 and 3.5, CHN), 4.76 (1H, t,
J 8.9, CHAHBO), 4.36 (1H, dd, J 8.9 and 3.5, CHAHBO),
1.63 (3H, d, J 6.4, CHCH3); dC (100 MHz, CDCl3) 171.6
(C@O), 157.1 (C@O), 153.5 (i-CO; OPh), 136.4 (i-C; Ph),
129.4, 129.2, 128.9, 126.0, 121.5 and 115.3 (6 · CH; Ph
and OPh), 71.9 (CH2O), 70.7 (CHCH3), 57.8 (CHN) and
18.2 (CH3) (Found MH+, 312.1244; C18H18NO4 requires
MH+, 312.1236); and the oxazolidinone anti-20 (39 mg,
10%) as an oil; RF [light petroleum (bp 40–60 ꢁC)–ether
(1:1)] 0.39; tmax/cmꢀ1 (CHCl2) 1780 (C@O) and 1718
(C@O); dH (270 MHz, CDCl3) 7.37–7.21 (7H, m, 7 · CH;
2 · Ph), 6.94 (1H, t, J 7.8, 1 · CH; Ph), 6.86 (2H, d, J 7.9,
2 · CH; Ph), 5.99 (1H, q, J 6.7, CHCH3), 5.44 (1H, dd, J
8.9 and 4.5, CHN), 4.76 (1H, t, J 8.9, CHACHBO), 4.35
(1H, dd, J 8.9 and 4.5; CHACHBO) and 1.54 (3H, d, J
6.7, CHCH3); dC (100 MHz, CDCl3) 171.4 (C@O), 157.2
(C@O), 153.3 (i-CO; OPh), 138.2 (i-C; Ph), 129.5, 129.2,
128.9, 126.0, 121.6 and 115.2 (6 · CH; Ph and OPh), 71.5
(CH2O), 70.6 (CHMe), 57.5 (CHN) and 18.0 (CH3) (Found
MH+ 312.1244; C18H18NO4 requires MH+, 312.1236).
311.1152. C18H17NO4 requires M+ 311.1152); syn-21
þ
(76 mg, 43%) as an oil; RF [light petroleum (40–60 ꢁC)–
23
diethyl ether (1:1)] 0.19; ½aꢁD ¼ ꢀ10:8 (c 2.71, CHCl3); tmax
(CH2Cl2)/cmꢀ1 1782 (C@O) and 1717 (C@O); dH
(270 MHz, CDCl3) 7.51–7.47 (2H, m, 2 · CH; Ph), 7.33–
7.28 (3H, m, 3 · CH; Ph), 6.06 (1H, s, CHO), 4.53–4.47
(1H, m, CHN), 4.27 (1H, t, J 9.1, CHAHBO), 4.09 (1H,
dd, J 9.1 and 3.7, CHAHBO), 3.34 (3H, s, OCH3), 2.14–
2.03 (1H, m, CH(CH3)2), 0.72 (3H, d, J 6.9, CH3) and
0.34 (3H, d, J 6.9, CH3); dC (100 MHz, CDCl3) 171.1
(C@O), 153.9 (C@O), 136.0 (i-C; Ph), 129.3, 128.9 and
128.9, (3 · CH; Ph), 81.1 (CHO), 64.0 (CH2O), 58.3
(NCH), 57.5 (CH3O), 29.4 (CþH(CH3)2), 17.9 (CH3) and
15.6 (CH3) (Found MNH4
requires MNH4 295.1652); anti-21 (12 mg, 7%) as an oil;
295.1654. C15H23N2O4
þ
RF [light petroleum (40–60 ꢁC)–diethyl ether (7:3)] 0.51;
23
½aꢁD ¼ þ138:0 (c 0.28, CHCl3); tmax (CH2Cl2)/cmꢀ1
1782 (C@O) and 1717 (C@O); dH (270 MHz, CDCl3)
7.51–7.47 (2H, m, 2 · CH; Ph), 7.36–7.32 (3H, m,
3 · CH; Ph), 6.04 (1H, s, CHO), 4.36–4.29 (1H, m,
CHN), 4.20–4.09 (2H, m, CH2O), 3.35 (3H, s, CH3),
2.57–2.45 (1H, m, CH(CH3)2), 0.92 (3H, d, J 6.9,
CHACHCHBÞ and 0.91 (3H, d, J 6.9, CHACHCHBÞ; dC
3
3
3
3
(100 MHz, CDCl3) 171.0 (C@O), 153.9 (C@O), 135.8 (i-
C; Ph), 129.4, 129.1 and 129.0 (3 · CH; Ph), 80.9 (CHCO),
64.0 (CH2O), 59.5 (NCH), 57.4 (CH3O), 28.7 (þCH(CH3)2),
18.3 (CH3) and 15.6 (CH3) (Found MNH4 295.1657.
þ
4.3.3. Parallel kinetic resolution of active ester (rac)-17 using
oxazolidinones (R)-6 and (S)-7. In the same way as the
above, n-butyl lithium (0.62 ml, 2.5 M in hexane,
1.55 mmol), oxazolidinone (R)-6 (0.105 g, 0.64 mmol),
oxazolidinone (S)-7 (83 mg, 0.64 mmol) and active ester
(rac)-17 (0.50 g, 1.55 mmol), gave a crude mixture of
C15H23N2O4 requires MNH4 295.1652).
4.3.4. Parallel kinetic resolution of active ester (rac)-18 using
oxazolidinones (R)-6 and (S)-7. In the same way as the
above, n-butyl lithium (0.54 ml, 2.5 M in hexane,
1.34 mmol), oxazolidinone (R)-6 (98 mg, 0.61 mmol),

E. Boyd et al. / Tetrahedron: Asymmetry 18 (2007) 476–482
481
oxazolidinone (S)-7 (78 mg, 0.61 mmol) and active ester
(rac)-18 (0.40 g, 1.23 mmol), gave a crude mixture of oxa-
zolidinones syn-20 and anti-20 (ratio 83:17) and syn-22
and anti-22 (ratio 85:15). The residue was purified by flash
column chromatography on silica gel eluting with light
petroleum (bp 40–60 ꢁC)–diethyl ether (7:3) to give oxazo-
lidinone syn-20 (91 mg, 48%) as white needle-like crystals;
(CHMe2), 18.7 (CH3), 17.7 þ(CH3) and 14.8 (CH3) (Found
MH+ 278.1400; C15H19NO requires 278.1392).
4
4.3.5. Hydrolysis of oxazolidinone adducts anti- and syn-19
(+)-2-methoxy-2-phenylacetic
acid
(S)-15. Lithium
hydroxide monohydrate (34 mg, 0.82 mmol) was slowly
added to a stirred solution of oxazolidinone syn-21
(127 mg, 0.41 mmol) and hydrogen peroxide (27 mg,
0.82 mmol, 30% v/w) in THF/water (1:1; 5 ml). The reac-
tion mixture was stirred at room temperature for 12 h.
The reaction was quenched with water (10 ml) and
extracted with dichloromethane (3 · 10 ml). The combined
organic layers were dried (over MgSO4) and evaporated
under reduced pressure to give the recovered oxazolidinone
(R)-6 (55 mg, 83%) as a white solid. The aqueous phase was
acidified using HCl (3 M HCl) until the pH = 3, and ex-
tracted with diethyl ether (3 · 10 ml). The combined organ-
ic phases were dried (over MgSO4) and evaporated under
reduced pressure to give (S)-2-methoxy-2-phenylacetic acid
RF [light petroleum (bp 40–60 ꢁC)–ether (1:1)] 0.20; mp
23
121–123 ꢁC; ½aꢁD ¼ þ24:2 (c 0.45, CHCl3); tmax/cmꢀ1
(CHCl2) 1782 (C@O) and 1715 (C@O); dH (270 MHz,
CDCl3) 7.34–7.24 (5H, m, 5 · CH, Ph), 7.11 (2H, t, J 7.7,
2 · CH; Ph), 6.86 (1H, t, J 7.4, 1 · CH; Ph), 5.88 (1H, q,
J 6.4, CHCH3), 5.44 (1H, dd, J 8.9 and 3.5, CHN), 4.76
(1H, t, J 8.9, CHAHBO), 4.36 (1H, dd, J 8.9 and 3.5,
CHAHBO) and 1.63 (3H, d,
J 6.4, CHCH3); dC
(100 MHz, CDCl3) 171.6 (C@O), 157.1 (C@O), 153.5 (i-
CO; OPh), 136.4 (i-C; Ph), 129.4, 129.2, 128.9, 126.0,
121.5 and 115.3 (6 · CH; Ph and OPh), 71.9 (CH2O),
70.7 (CHCH3), 57.8 (CHN) and 18.2 (CH3) (Found
þ
þ
MNH4
,
312.1244; C18H21N2O4 requires MNH4
,
(S)-15 (65 mg, 96%) as an oil; RF [diethyl ether] 0.54;
23
312.1236); oxazolidinone anti-20 (27 mg, 14%) as a white
½aꢁD ¼ 141:0 (c 4.7, CHCl3), {lit.21 for (R)-; [a]D = ꢀ141.8
solid; RF [light petroleum (bp 40–60 ꢁC)–ether (1:1)] 0.39;
(c 0.13, CHCl3)}; mmax(CHCl3); cmꢀ1 1725 (C@O); dH
(400 MHz; CDCl3) 8.60 (1H, br s, OH), 7.37–7.35 (2H,
m, 2 · CH; Ph), 7.30–7.25 (3H, m, 3 · CH; Ph), 4.70
(1H, s, CH) and 3.33 (3H, s, OCH3); dC (100 MHz; CDCl3)
175.3 (C@O), 135.3 (i-C; Ph), 129.0, 128.7 and 127.2
(3 · CH; Ph), 81.9þ (CH) and 57.2 (OCH3) (Found M+
166.0622. C9H10O3 requires 166.0624).
23
mp 151–153 ꢁC; ½aꢁD ¼ ꢀ171:1 (c 0.54, CHCl3); tmax
/
cmꢀ1 (CHCl2) 1780 (C@O) and 1718 (C@O); dH
(270 MHz, CDCl3) 7.37–7.21 (7H, m, 7 · CH; 2 · Ph),
6.94 (1H, t, J 7.8, 1 · CH; Ph), 6.86 (2H, d, J 7.9,
2 · CH; Ph), 5.99 (1H, q, J 6.7, CHCH3), 5.44 (1H, dd, J
8.9 and 4.5, CHN), 4.76 (1H, t, J 8.9, CHACHBO), 4.35
(1H, dd, J 8.9 and 4.5; CHACHBO), 1.54 (3H, d, J 6.7,
CHCH3); dC (100 MHz, CDCl3) 171.4 (C@O), 157.2
(C@O), 153.3 (i-CO; OPh), 138.2 (i-C; Ph), 129.5, 129.2,
128.9, 126.0, 121.6 and 115.2 (6 · CH; Ph and OPh), 71.5
4.3.6. (ꢀ)-2-Methoxy-2-phenylacetic acid (R)-15. In the
same way as for oxazolidinone syn-19, oxazolidinone
anti-19 (218 mg, 0.70 mmol), lithium hydroxide mono-
hydrate (59 mg, 1.40 mmol) and hydrogen peroxide (48 mg,
1.40 mmol, 30% v/w), gave after extraction the recovered
oxazolidinone (R)-6 (94 mg, 82%) as a white solid; and
(CH2Oþ), 70.6 (CHMe), 57.5 (CHN) and 18.0 (CH3) (Found
þ
MNH4
312.1244; C18H21N2O4 requires MNH4
,
312.1236); syn-22 (74 mg, 44%) as a white solid; RF [light
petroleum (bp 40–60 ꢁC)–ether (1:1)] 0.36; mp 72–74 ꢁC;
(R)-2-methoxy-2-phenylacetic acid (R)-15 (110 mg, 95%)
23
23
½aꢁD ¼ þ13:4 (c 2.2, CHCl3); tmax/cmꢀ1 (CHCl2) 1780
as an oil; RF [diethyl ether] 0.54; ½aꢁD ¼ ꢀ140:2 (c 4.4,
(C@O) and 1714 (C@O); dH (400 MHz, CDCl3) 7.20–7.15
(2H, m, 2 · CH, Ph), 6.90–6.85 (1H, t, J 7.3, 1 · CH;
Ph), 6.82–6.79 (2H, m, 2 · CH; Ph), 5.94 (1H, q, J 6.6,
CHCH3), 4.39–4.35 (1H, m, CHN), 4.27 (1H, t, J 9.1,
CHAHBO), 4.20 (1H, dd, J 9.1 and 3.0, CHAHBO), 2.31–
2.23 (1H, m, CH(CH3)2), 1.54 (3H, d, J 6.6, CHCH3),
0.80 (3H, d, J 6.9, CH3ACHCHB3 ) and 0.80 (3H, d, J 6.9,
CHACHCHB); dC (100 MHz, CDCl3) 172.1 (C@O), 157.1
CHCl3), {lit.21 [a]D = ꢀ141.8 (c 0.13, CHCl3)};
m
max(CHCl3); cmꢀ1 1724 (C@O); dH (400 MHz; CDCl3)
9.29 (1H, br s, OH), 7.37–7.35 (2H, m, 2 · CH; Ph),
7.32–7.26 (3H, m, 3 · CH; Ph), 4.70 (1H, s, CH) and 3.32
(3H, s, OCH3); dC (100 MHz; CDCl3) 175.6 (C@O),
135.3 (i-C; Ph), 128.9, 128.7 and 127.2 (3 · CþH; Ph), 81.9
(CH) andþ 57.2 (OCH3) (Found MNH4
C9H14NO3 requires 184.0968).
184.0968.
3
3
(C@O), 153.7 (i-CO; OPh), 129.5, 121.4 and 114.9
(3 · CH; Ph), 71.2 (CH2O), 63.8 (CHCH3), 58.6 (CHN),
27.9 (CH(CH3)2), 18.1 (CH3), 17.8 (CH3) and 14.3 (CH3)
(Found MH+ 278.1400; C15H19NO4 requires 278.1392);
4.3.7. (ꢀ)-2-Phenoxypropionic acid (S)-16. In the same
way as oxazolidinone syn-19, oxazolidinone syn-20
(0.701 g, 2.25 mmol), lithium hydroxide monohydrate
(0.319 g, 4.50 mmol) and hydrogen peroxide (0.15 g,
4.50 mmol), gave after extraction the recovered oxazolidi-
none (R)-6 (0.33 g, 90%) as a white solid and 2-phenoxy-
anti-22 (17 mg, 10%) as a white solid; RF [light petroleum
23
(bp 40–60 ꢁC)–ether (1:1)] 0.39; mp 75–78 ꢁC; ½aꢁD
¼
þ79:5 (c 2.3, CHCl3); tmax/cmꢀ1 (CHCl2) 1780 (C@O)
and 1718 (C@O); dH (270 MHz, CDCl3) 7.25 (2H, t, J
7.4, 2 · CH, Ph), 6.94 (1H, t, J 7.4, 1 · CH; Ph), 6.86
(2H, d, J 7.4, 2 · CH; Ph), 5.97 (1H, q, J 6.7, CHO),
4.51–4.45 (1H, m, CHN), 4.35 (1H, t, J 9.1, CHAHBO),
4.25 (1H, dd, J 9.1 and 3.5, CHAHBO), 2.40–2.26 (1H,
m, CH(CH3)2), 1.67 (3H, d, J 6.7, CHCH3), 0.89 (3H, d,
J 6.9, (CH3)ACH(CH3)B) and 0.88 (3H, d, J 6.9, (CH3)A
CH(CH3)B); dC (100 MHz, CDCl3) 172.1 (C@O), 157.3
(C@O), 153.7 (i-CO; Ph), 129.5, 121.6, 115.2 (3 · CH;
Ph), 71.7 (CH2O), 64.3 (CHCH3), 58.2 (CHN), 28.3
propionic acid (S)-16 (0.34 g, 91%) as a white solid; RF
23
[diethyl ether] 0.89; mp 82–84 ꢁC; ½aꢁD ¼ ꢀ23:4 (c 3.7,
CHCl3); tmax/cmꢀ1 (CH2Cl2) 1728 (C@O); dH (270 MHz,
CDCl3); 7.32–7.24 (2H, m, 3 · CH; Ph), 6.99 (1H, t, J
7.1, CH; Ph), 6.89 (2H, d, J 8.6, 2 · CH; Ph), 4.79 (1H,
q, J 6.7, CHO) and 1.65 (3H, d, J 6.7, Me); dC
(100 MHz, CDCl3) 177.4 (C@O), 157.1 (i-CO; Ph), 129.7,
122.0 and 115.1 (3 · CH; Ph), 72.0 (PhOCH)þ and 18.4
þ
(CH3) (Found MNH4 184.0965. C9H14NO3 requires
184.0968).

482
E. Boyd et al. / Tetrahedron: Asymmetry 18 (2007) 476–482
hedron 1997, 53, 1411–1416; (d) Ghosez, L.; Gouverneur, V.
Tetrahedron 1996, 52, 7585–7598; Chakraborty, T. K.;
Hussain, K. A.; Reddy, G. V. Tetrahedron 1995, 51, 9179–
9190; (e) Youshko, M. I.; van Langen, L. M.; Sheldon, R. A.;
Svedas, V. K. Tetrahedron: Asymmetry 2004, 15, 1933–1936;
(f) Keil, O.; Schneider, M. P.; Rasor, J. P. Tetrahedron:
Asymmetry 1995, 6, 1257–1260; (g) Caron, M.; Carlier, P. R.;
Sharpless, K. B. J. Org. Chem. 1988, 53, 5185–5187; (h)
Williams, R. M.; Hendrix, J. A. J. Org. Chem. 1990, 55, 3723–
3728; (i) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem.
1994, 59, 4194–4201.
4.3.8. (+)-2-Phenoxypropionic acid (R)-16. In the same
way as for oxazolidinone syn-19, oxazolidinone anti-20
(0.72 g, 2.31 mmol), lithium hydroxide monohydrate
(0.19 g, 4.63 mmol) and hydrogen peroxide (0.157 g,
0.14 ml, 4.63 mmol), gave after extraction the recovered
oxazolidinone (R)-6 (0.35 g, 93%) as a white solid and
2-phenoxypropionic acid (R)-16 (0.32 g, 85%) as a white
23
solid; RF [diethyl ether] 0.89; mp 84–86 ꢁC; ½aꢁD ¼ þ23:6
(c 3.6, CHCl3); tmax/cmꢀ1 (CH2Cl2) 1729 (C@O); dH
(270 MHz, CDCl3) 7.31–7.25 (2H, m, 3 · CH; Ph), 6.99
(1H, t, J 7.1, 1 · CH; Ph), 6.89 (2H, d, J 8.6, 2 · CH;
Ph), 4.79 (1H, q, J 6.7, CHO) and 1.65 (3H, d, J 6.7,
CH3); dC (100 MHz, CDCl3) 177.5 (C@O), 157.0 (i-CO;
OPh), 129.7, 122.0 and 115.2 (3 · CH; Ph), 72.1 (CH)
and 18.3 (CH3) (Found MNH4þ, 184.0967. C9H18N2O3
requires MNH4þ, 184.0968).
5. For some recent examples, see: (a) Blay, G.; Fernandez, I.;
Monje, B.; Munoz, M. C.; Pedro, J. R.; Vila, C. Tetrahedron
2006, 62, 9174–9182; (b) Blay, G.; Fernandez, I.; Monje, B.;
Munoz, M. C.; Pedro, J. R.; Vila, C. Tetrahedron 2006, 62,
8069–8076; (c) Lee, Y. J.; Lee, K.; Jung, S. I.; Jeon, H. B.;
Kim, K. S. Tetrahedron 2005, 61, 1987–2001.
6. Nerurkar, S. G.; Dighe, S. V.; Williams, R. L. J. Clin.
Pharmacol. 1992, 32, 935–943.
7. (a) Eames, J. Angew. Chem., Int. Ed. 2000, 39, 885–890; (b)
Eames, J. In Parallel Kinetic Resolutions. In Organic Synthesis
Highlights; VCH-Wiley: Weinheim, 2003; Vol. V, Chapter 17,
pp 151–164 (ISBN 3-527-30611-0); (c) Dehli, J. R.; Gotor, V.
Chem. Soc. Rev. 2002, 31, 365–370.
8. Reported in Asymmetric Synthesis via Iron Acyl Complexes;
Preston S. C.; D.Phil. Dissertation, Oxford University, 1989;
Diss. Abstr. Int. B, 1990, 51, 2896.
9. Vedejs, E.; Chen, X. J. Am. Chem. Soc. 1997, 119, 2584–2585;
For related examples, see: (a) Vedejs, E.; Rozners, E. J. Am.
Chem. Soc. 2001, 123, 2428–2429; (b) Vedejs, E.; Daugulis, O.
J. Am. Chem. Soc. 2003, 125, 4166–4173.
Acknowledgements
We are grateful to Queen Mary, University of London for
their financial support (to S.C.), The Royal Society and
The University of London Central Research Fund for their
financial support (to J.E.), the EPSRC National Mass
Spectrometry Service (Swansea) for accurate mass determi-
nations, and Marco Dingjan for his technical assistance.
10. Davies, S. G.; Diez, D.; El Hammouni, M. M.; Garner, A. C.;
Garrido, N. M.; Long, M. J.; Morrison, R. M.; Smith, A. D.;
Sweet, M. J.; Withey, J. M. Chem. Commum. 2003, 2410–
2411; For related examples, see: (a) Davies, S. G.; Garner, A.
C.; Long, M. J.; Smith, A. D.; Sweet, M. J.; Withey, J. M.
Org. Biomol. Chem. 2004, 2, 3355–3362; (b) Davies, S. G.;
Garner, A. C.; Long, M. J.; Morrison, R. M.; Roberts, P. M.;
Savory, E. D.; Smith, A. D.; Sweet, M. J.; Withey, J. M. Org.
Biomol. Chem. 2005, 3, 2762–2775.
11. Coumbarides, G. S.; Dingjan, M.; Eames, J.; Flinn, A.;
Northen, J.; Yohannes, Y. Tetrahedron Lett. 2005, 46, 2897–
2902.
12. Coumbarides, G. S.; Dingjan, M.; Eames, J.; Flinn, A.;
Motevalli, M.; Northen, J.; Yohannes, Y. Synlett 2006, 101–
105.
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