An efficient synthesis of (+)-epi-cytoxazone via asymmetric organocatalysis

Article abstract of DOI:10.1016/j.tetasy.2008.06.021

The catalytic enantioselective synthesis of (+)-epi-cytoxazone has been accomplished in four steps from p-methoxybenzaldehyde N-Boc-imine with good diastereo- and enantioselectivity. This approach involves asymmetric Mannich reaction of an aldehyde with N

Full text of DOI:10.1016/j.tetasy.2008.06.021

Tetrahedron: Asymmetry 19 (2008) 1626–1629
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An efficient synthesis of (+)-epi-cytoxazone via asymmetric organocatalysis
b
Sung-Gon Kim a, , Tae-Ho Park
*
a Department of Chemistry, College of Natural Science, Kyonggi University, San 94-6, Iui-dong, Yeongtong-gu, Suwon 443-760, Republic of Korea
b Medicinal Science Division, Korea Research Institute of Chemical Technology, PO Box 107, Yuseong-gu, Daejeon 305-600, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic enantioselective synthesis of (+)-epi-cytoxazone has been accomplished in four steps from
p-methoxybenzaldehyde N-Boc-imine with good diastereo- and enantioselectivity. This approach
involves asymmetric Mannich reaction of an aldehyde with N-Boc-imine using the novel 2-pyrrole-
derived imidazolidinone as an organocatalyst.
Received 21 May 2008
Accepted 17 June 2008
Available online 16 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
including the direct asymmetric synthesis of an
amino alcohol.
a-hydroxy b-
(ꢀ)-Cytoxazone 1, containing a novel 4,5-disubstituted-2-oxa-
zolidinone moiety, is a microbial metabolite isolated from Strepto-
myces, which shows cytokine-modulating activity by inhibiting the
signaling pathway of Th2 cells.1 Since Th2 cells play a role in medi-
ating the immune response to allergens, cytoxazone its stereoiso-
mers might be lead compounds for the development of
therapeutic agents for asthma and atopic dermatitis, and have at-
tracted considerable attention from synthetic chemists. A variety
of synthetic methods including (ꢀ)-cytoxazone and its stereoiso-
mers (+)-epi-cytoxazone and isocytoxazone have been reported
2. Results and discussion
Our synthetic approach toward the target molecule is based on
the organocatalytic asymmetric Mannich reaction of aldehydes
with N-Boc-imines, which have recently been developed by List
and Córdova independently (Scheme 1).10 This direct asymmetric
Mannich reaction using (S)-proline as an organocatalyst is a highly
effective carbon–carbon bond forming reaction that affords syn-
selective and enantioselective b-amino aldehydes. We employed
their organocatalytic Mannich reaction in the synthesis of (+)-
epi-cytoxazone.
(Fig. 1).2 In several of the synthetic approaches, the
a-hydroxy b-
amino functionalities are used as key intermediates, which can
be indirectly introduced in two ways: either by the enantioselec-
tive Sharpless epoxidation,3 dihydroxylation,4 and aminohydroxy-
lation5 into cinnamic acid derivatives or by the enantioselective
and diastereoselective C–C bond formation between building
blocks involving the Petasis reaction,6 aldol reaction,7 and imino-
1,2-Wittig rearrangement.8
In an initial use of (S)-proline as the organocatalyst,11 we found
that the Mannich reaction between p-methoxybenzaldehyde
N-Boc-imine 412 and benzyloxyacetaldehyde
9 afforded the
corresponding b-amino aldehyde 10 in high yield with excellent
enantioselectivity (Table 1). However, the product had poor
diastereoselectivity (2:1 syn:anti) and the diastereomers could
not be separated by column chromatography even after hydrolysis
to the b-amino alcohol. Hence, we decided to investigate various
organocatalysts in order to obtain high diastereoselectivity in this
Herein, as part of a program for the synthesis of biologically
active natural products based on asymmetric organocatalysis,9 we
report
a
short and efficient synthesis of (+)-epi-cytoxazone
O
NH
O
O
O
HN
O
HN
*
O
*
OH
OH
OH
MeO
MeO
(-)-cytoxazone 1
MeO
(+)-epi-cytoxazone 2
isocytoxazone 3
Figure 1. Cytoxazone and its stereoisomers.
* Corresponding author. Tel.: +82 31 249 9631; fax: +82 31 249 9639.
E-mail address: sgkim123@kgu.ac.kr (S.-G. Kim).
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.06.021

S.-G. Kim, T.-H. Park / Tetrahedron: Asymmetry 19 (2008) 1626–1629
1627
Boc
Boc
N
O
(S)-proline
NH
O
78% yield
>19:1 dr
99% ee
(20 mol%)
H
H
+
H
DMF, 4 ºC
Me
Me
MeO
MeO
MeO
MeO
4
5
6
Boc
H
Boc
N
O
(S)-proline
(20 mol%)
NH
O
80% yield
>19:1 dr
99% ee
H
+
H
CH3CN, 0 ºC
iPr
iPr
4
7
8
Scheme 1. Proline-catalyzed Mannich reaction of aldehydes with N-Boc-imine.
Table 1
Organocatalytic Mannich reaction of
a
-benzyloxyacetaldehyde with p-methoxybenzaldehyde N-Boc-imine
Boc
Boc
NH
O
H
N
O
Catalyst
(20 mol%)
H
H
+
OBn
OBn
MeO
MeO
9
10
4
Entry
Catalyst
Solvent
Temp (°C)
Yielda (%)
dr (syn:anti)b
ee (%) sync
ee (%) antic
1
2
3
4
5
6
11a
11a
11b
11c
11d
11e
DMF
DMF
DMF
DMF
CHCl3
DMF
rt
0
rt
rt
0
80
84
2:1
3:1
2:1
3:2
1:1
1:4
98
99
98
67
48
21
99
99
95
68
60
62
d
—
—
d
77
42
rt
a
Yield of isolated product.
Determined by 1H NMR analysis.
Determined by chiral HPLC analysis (Chiralcel AD-H).
Not determined.
b
c
d
TBSO
Ph
N
Ph
Bn
CO2H
CO2H
CO2H
N
N
H
N
N
H
N
N
H
H
H
HN
OTMS
11d
N
11a
11b
11e
11c
Mannich reaction. trans-4-tert-Butyldimethylsilyloxy-(S)-proline
11b showed the same level of diastereomeric ratio and ee’s with
(S)-proline (entry 3). In the cases of (S)-5-pyrrolidin-2-yl-1H-tetra-
zole 11c and (S)-2-(diphenyl-trimethylsilanyloxymethyl)pyrro-
lidine 11d, their diastereo- and enantioselectivity were lower
than the case of (S)-proline (entries 4 and 5). Even though diastereo-
selectivity was increased in the case of (2S,5R)-5-benzyl-pyrro-
of diastereo- and enantioselectivity maintaining a good yield (en-
try 5, 90% yield, 4:1 syn:anti, 82% ee). After extensive optimization
of reaction conditions, including screening reaction media, temper-
ature, and acid additives, we found that the use of CHCl3 as a sol-
vent and TCA as an additive at ꢀ30 °C gave the best result (entry
7, 78% yield, 8:1 syn:anti, 94% ee).
Having the requisite b-amino aldehyde 10 in hand, we carried
out the synthesis of (+)-epi-cytoxazone (Scheme 2). b-Amino alde-
hyde 10 was reduced with sodium borohydride to afford b-amino
alcohol 12 in 98 % yield. Deprotection of 12 by hydrogenolysis of
the benzyl group afforded the desired diol 13 in 91% yield. Finally,
treatment of compound 13 with sodium hydride in THF led to
regioselective cyclization to give (+)-epi-cytoxazone 2 in 85% yield
lidine-2-carboxylic acid 11e13 when used as
a catalyst, the
enantioselectivity was not satisfied.
Next, we turned our attention to the imidazolidinone catalyst as
the organocatalyst for the Mannich reaction between p-methoxy-
benzaldehyde N-Boc-imine 4 and benzyloxyacetaldehyde 9, which
has not been applied to the asymmetric Mannich reactions until
now (Table 2). This Mannich reaction was successful with imdazo-
lidinone 11fꢁTFA but gave poor diastereo- and enantioselectivity
(entry 1). In contrast, the imidazolidinone 11hꢁTFA slightly in-
creased the diastereoselectivity (entry 3, 3:1 dr) although with a
low enantioselectivity. Hence, a heteroaromatic group at the C(5)
position in the imidazolidinone was considered to effect the ste-
reocontrol in this Mannich reaction and new 2-pyrrole-derived
imidazolidinones 11i and 11j were elaborated. As expected, new
2-pyrrole-derived imidazolidinone 11j afforded satisfactory levels
{½a 2D3
ꢂ
¼ þ32:8 (c 0.6, MeOH)}.3b,7 The physical and spectroscopic
data of 2 are in full agreement with the literature data.
3. Conclusion
In conclusion, we have completed a short and efficient synthesis
of (+)-epi-cytoxazone, based on the organocatalytic asymmetric
Mannich reaction between p-methoxybenzaldehyde N-Boc-imine
and benzyloxyacetaldehyde using the novel 2-pyrrole-derived

1628
S.-G. Kim, T.-H. Park / Tetrahedron: Asymmetry 19 (2008) 1626–1629
Table 2
Organocatalytic Mannich reaction of
a
-benzyloxyacetaldehyde with p-methoxybenzaldehyde N-Boc-imine using imidazolidinone catalysts
Boc
Boc
NH
O
N
O
Catalyst
(20 mol%)
H
H
H
+
OBn
OBn
MeO
MeO
9
10
4
Entry
Catalyst
Solvent
Temp (°C)
Yielda (%)
dr (syn:anti)b
ee (%) sync
ee (%) antic
1
2
3
4
5
6
7
11f + TFA
11g + TFA
11h + TFA
11i + TFA
11j + TFA
11j + TFA
11j + TCA
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
rt
rt
rt
rt
91
72
84
78
90
80
78
2:1
2:1
3:1
5:2
4:1
5:1
8:1
43
43
45
26
82
89
94
44
0
0
10
45
rt
d
ꢀ30
—
—
d
ꢀ30
a
Yield of isolated product.
Determined by 1H NMR analysis.
Determined by chiral HPLC analysis (Chiralcel AD-H).
Not determined.
b
c
d
O
O
O
O
N
O
N
N
N
N
Bn
Bn
Ph Bn
Bn
N
Bn
N
N
N
N
H
O
N
HN
H
H
H
H
11g
11f
11h
11i
11j
O
TCA
N
Bn
N
Boc
Boc
H
HN
O
H
NH
O
N
NaBH4
(20 mol%)
H
H
+
MeOH
98%
CHCl3, -30 ºC
78%
OBn
OBn
10
MeO
MeO
4
9
O
Boc
Boc
NH
NH
HN
O
10% Pd/C, H2
NaH
OH
OH
EtOH
91%
THF
85%
OH
OBn
OH
MeO
MeO
13
12
MeO
(+)-epi-Cytoxazone 2
Scheme 2. Asymmetric synthesis of (+)-epi-cytoxazone.
imidazolidinone as an organocatalyst. The enantioselective syn-
thesis of (+)-epi-cytoxazone was achieved in four steps from
p-methoxybenzaldehyde N-Boc-imine (59% yield, 94% ee). The
applications of the other organocatalytic asymmetric reactions of
2-pyrrole-derived imidazolidinone are now in progress and will
be presented in due course.
EM reagents 0.25 mm Silica Gel 60-F plates. Visualization of the
developed chromatogram was performed by fluorescence quench-
ing and anisaldehyde stain. Melting points were determined on a
Tomas Hoover capillary melting point apparatus and are uncor-
rected. 1H and 13C NMR spectra were recorded on Mercury 300
(300 MHz and 75 MHz) as noted, and are internally referenced to
residual protio solvent signals. Data for 1H NMR are reported as fol-
lows: chemical shift (d ppm), multiplicity (s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet), integration, coupling con-
stant (Hz), and assignment. Data for 13C NMR are reported in terms
of chemical shift. IR spectra were recorded on a Jasco 610 FT-IR
spectrometer using KBr salt plates, and reported in terms of fre-
quency of absorption (cmꢀ1). Mass spectra and elemental analysis
data were obtained from the Korea Research Institute of Chemical
Technology Facility. Optical rotations were recorded on a Jasco
P-1010 polarimeter (WI lamp, 589 nm). HPLC analysis was per-
formed on a Shimadzu LC-20 A Prominence HPLC using the follow-
ing Chiralcel columns: AD (25 cm) and AD guard (5 cm), as noted.
4. Experimental
4.1. General methods
Commercial reagents were used without further purification.
Organic solutions were concentrated under reduced pressure using
a Büchi rotary evaporator. All organic solvents were distilled prior
to use. Chromatographic purification of the products was accom-
plished using forced-flow chromatography on ICN 60 32–64 mesh
Silica Gel 63. Thin layer chromatography (TLC) was performed on

S.-G. Kim, T.-H. Park / Tetrahedron: Asymmetry 19 (2008) 1626–1629
1629
4.2. (2S,5S)-5-Benzyl-3-methyl-2-pyrrole-imidazolidine-4-one
11j
½
a 2D3
ꢂ
¼ ꢀ36:1 (c 1.0, CHCl3); IR (KBr): 3385, 2976, 2934, 1688,
1613, 1512, 1367, 1298, 1248, 1170, 1035 cmꢀ1 1H NMR
;
(300 MHz, CDCl3) d 7.24 (d, J = 8.4 Hz, 2H), 6.90 (d, J = 8.4 Hz, 2H),
5.22 (d, J = 8.1 Hz, 1H), 4.76 (br s, 1H), 3.92 (m, 1H), 3.80 (s, 3H),
3.54 (dd, J = 6.3, 6.6 Hz, 2H), 2.75 (br s, 1H), 2.66 (br s, 1H), 1.44
(s, 9H); 13C NMR (75 MHz, CDCl3) d 158.0, 156.0, 132.2, 129.2,
114.4, 79.5, 76.3, 64.9, 55.9, 54.3, 28.5; Anal. Calcd for
To
a solution of (S)-phenylalanine methyl amide (3.00 g,
16.8 mmol) and pyrrol-2-carboxaldehyde (1.40 g, 14 mmol) in
THF (25 mL) was added samarium(III) trifluoromethanesulfonate
hydrate (410 mg, 0.84 mmol) and powdered 4 Å molecular sieves
(4.5 g). After stirring for 52 h at room temperature, the reaction
mixture was filtered, concentrated, and purified by flash column
chromatography (60% EtOAc/hexane) to afford the title compound
as a pale yellow oil in 42% yield (1.50 g, 5.88 mmol) and the faster
eluting (2R,5S)-isomer as a pale yellow oil in 35% yield (1.25 g,
C15H23NO5: C, 60.59; H, 7.80; N, 4.71. Found: C, 60.27; H, 8.05; N,
4.65.
4.5. (4R,5S)-5-(Hydroxymethyl)-4-(4-methoxyphenyl)-
oxazolidin-2-one [(+)-epi-cytoxazone] 2
4.9 mmol). ½a 2D5
¼ ꢀ145:1 (c 0.7, CHCl3); IR (KBr): 3412, 3293,
ꢂ
2923, 2857, 1686, 1478, 1450, 1402, 1334, 1273, 1098,
To a solution of (1R,2S)-[2,3-dihydroxy-1-(4-methoxyphenyl)-
propyl]-carbamic acid tert-butyl ester 13 (60 mg, 0.27 mmol) in
THF (1 mL) was added NaH (14 mg, 0.32 mmol, 60% in mineral
oil) at 0 °C. After stirring for 2 h at this atmosphere, the reaction
mixture was allowed to warm at room temperature and stirred
for an additional 2 h and quenched with H2O, and then extracted
with EtOAc. The combined organic layer was washed with brine,
dried over anhydrous MgSO4, and concentrated in vacuo. The crude
product was purified by flash column chromatography (30% EtOAc/
hexanes then) to afford the title compound as a white solid (45 mg,
1029 cmꢀ1 1H NMR (300 MHz, CDCl3) d 7.28–7.44 (m, 5H), 7.16
;
(br s, 1H), 6.54 (dd, J = 3.0, 4.2 Hz, 1H), 6.16 (dd, J = 4.2, 6.0 Hz,
1H), 6.06 (dd, J = 3.0, 6.0 Hz, 1H), 5.32 (s, 1H), 4.02 (t, J = 4.6 Hz,
1H), 3.31 (dd, J = 4.8, 13.8 Hz, 2H), 2.56 (s, 3H); 13C NMR
(75 MHz, CDCl3) d 172.6, 137.8, 130.6, 128.9, 128.7, 127.1, 119.6,
109.4, 108.2, 70.6, 59.7, 38.0, 30.0; HRMS (M+) calcd for
C
15H17N3O+: 255.1372; found, 255.1366.
4.3. (1R,2S)-[2-Benzyloxy-3-hydroxy-1-(4-methoxyphenyl)-
propyl]-carbamic acid tert-butyl ester 12
85%). mp 161–162 °C; ½a D23
ꢂ
¼ þ32:8 (c 0.6, MeOH); IR (KBr): 3285,
2944, 2930, 1752, 1723, 1510, 1255, 1026 cmꢀ1
;
1H NMR
To a solution of (4-methoxybenzylidene)-carbamic acid tert-
butyl ester 4 (59 mg, 0.25 mmol), (2S,5S)-5-benzyl-3-methyl-2-
pyrrole-imidazolidine-4-one 11j (10.2 mg, 0.050 mmol), and 1 M
(300 MHz, DMSO-D6) d 8.01 (s, 1H), 7.25 (d, J = 8.4 Hz, 2H), 6.92
(d, J = 8.4 Hz, 2H), 5.17 (br s, 1H), 4.60 (d, J = 6.6 Hz, 1H), 4.12 (m,
1H), 3.73 (s, 3H), 3.58 (m, 2H); 13C NMR (75 MHz, CDCl3) d 158.9,
158.0, 131.9, 129.1, 114.6, 82.5, 61.4, 57.3, 54.8.
trichloroacetic acid (50 lL, 0.050 mmol) in chloroform (0.5 mL) at
ꢀ30 °C was added benzyloxyacetaldehyde 9 (75 mg, 0.50 mmol).
After stirring for 36 h at this temperature, the reaction mixture
was allowed to warm 0 °C, after which were added methanol
(1 mL) and an excess of sodium borohydride. After 15 min, the
remaining sodium borohydride was quenched with saturated
aqueous NaHCO3, and the mixture was extracted with EtOAc. The
combined organic layer was washed with brine, dried over anhy-
drous MgSO4, and concentrated in vacuo. The crude product was
purified by flash column chromatography (10% EtOAc/Hexanes
then 30% EtOAc/hexanes) to afford the title compound as a white
solid (75 mg, 78%). The ratio of the syn- and anti- product was
determined by 1H-NMR spectra. The enantiomeric excess of prod-
Acknowledgments
This work was supported by the Korea Foundation for Interna-
tional Cooperation of Science and Technology (KICOS) through a
Grant provided by the Korean Ministry of Education, Science and
Technology (MEST) in 2007 (No. K20501000001).
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ucts was measured by HPLC analysis. mp 174–175 °C; ½a D23
¼
ꢂ
ꢀ18:6 (c 0.6, CHCl3); IR (KBr): 3374, 2977, 2934, 1682, 1513,
1456, 1296, 1249, 1169, 1061 cmꢀ1 1H NMR (300 MHz, CDCl3) d
;
7.24–7.41 (m, 5H), 7.13 (d, J = 8.4 Hz, 2H), 6.87 (d, J = 8.4 Hz, 2H),
5.36 (d, J = 9.3 Hz, 1H), 4.95 (br s, 1H), 4.28 (dd, J = 11.1, 30.3 Hz,
2H), 3.81 (s, 3H), 3.55–3.77 (m, 3H), 3.13 (br s, 1H), 1.43 (s, 9H);
13C NMR (75 MHz, CDCl3) d 159.1, 157.2, 138.0, 128.7, 128.6,
128.3, 128.1, 128.0, 114.1, 82.7, 80.3, 73.8, 61.7, 55.5, 53.6, 28.6;
Anal. Calcd for C22H29NO5: C, 68.20; H, 7.54; N, 3.61. Found: C,
67.98; H, 7.77; N, 3.54. HPLC (Chiralcel AD-H, 7.5% isopropanol/
hexanes, 1 mL/min); tminor = 16.7 min, tmajor = 20.0 min, 94% ee.
4.4. (1R,2S)-[2,3-Dihydroxy-1-(4-methoxyphenyl)-propyl]-
carbamic acid tert-butyl ester 13
To a solution of (1R,2S)-[2-benzyloxy-3-hydroxy-1-(4-methoxy-
phenyl)propyl]-carbamic acid tert-butyl ester 12 (395 mg,
1.02 mmol) in EtOH (60 mL) were added 10% Pd/C (0.2 w/w,
60 mg) and 1 drop of 4 M HCl in dioxane. After stirring for 2 h
under an H2 atmosphere, Pd/C was filtered out and the reaction
solvent was evaporated in vacuo. The residue was purified by flash
column chromatography (70% EtOAc in hexanes) to afford the title
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compound as
a white solid (275 mg, 91%). mp 141–142 °C;