Synthesis of dihydrooxazole analogues derived from linezolid

Article abstract of DOI:10.1016/S0040-4020(03)00490-3

Starting from (S)-serine, the linezolid analogue 3 with dihydrooxazole partial structure was synthesized and pharmacologically investigated. With the help of MEP comparisons structural requirements for antibacterial activity were evaluated.

Full text of DOI:10.1016/S0040-4020(03)00490-3

Tetrahedron 59 (2003) 3403–3407
Synthesis of dihydrooxazole analogues derived from linezolid
a
Jurgen Einsiedel, Christoph Schoerner and Peter Gmeiner
b
a
,
*
¨
aDepartment of Medicinal Chemistry, Emil Fischer Center, Friedrich Alexander University, Schuhstraße 19, D-91052 Erlangen, Germany
bInstitute of Clinical Microbiology, Immunology and Hygiene, Friedrich Alexander University, Wasserturmstraße 3,
D-91054 Erlangen, Germany
Received 24 February 2003; accepted 25 March 2003
Dedicated to Professor H.-D. Stachel on the occasion of his 75th birthday
Abstract—Starting from (S)-serine, the linezolid analogue 3 with dihydrooxazole partial structure was synthesized and pharmacologically
investigated. With the help of MEP comparisons structural requirements for antibacterial activity were evaluated. q 2003 Elsevier Science
Ltd. All rights reserved.
1. Introduction
probability, the H-accepting function of the oxazolidinone
carbonyl group was adopted by the dihydroisoxazole
nitrogen.
The bioisosteric replacement of functional groups in
receptor ligands, enzyme inhibitors or antimetabolites has
become a promising strategy for the design of new drug
candidates.1 Thus, a modification of target affinity, activity,
selectivity and lipophilicity can be beneficial for the
reduction of side effects and an increase of bioavailability.
The value of this principle was also demonstrated by our
recent investigations in the field of dopamine receptor
ligands, when bioisosteric exchanges led to highly selective
dopamine D4 antagonists,2 D3 agonists3 and partial
agonists.4
To exploit our ex-chiral pool approach to aminomethyl
substituted dihydrooxazoles5 for structure activity relation-
ship studies in the field of antibiotics, we chose linezolid
(1),6 a recently introduced antibiotic incorporating an
oxazolidinone substructure, as a pharmacological lead.
SAR studies on linezolid, being employed as the more
active (S)-enantiomer, were chiefly carried out by modifi-
cation of the 5-acetamidomethyl side chain7 and exchange
of the morpholine moiety.8 With respect to structural
variations of the heterocyclic core unit, the bioactive
butenolid derivative 4 is described,9 demonstrating, that
the carbamate nitrogen can be replaced by the bioisosteric
sp2-carbon without losing the antibacterial activity com-
pared to the oxazolidinone analogue DUP 721 (2).10
Furthermore, the dihydroisoxazole derivative 5 exhibited a
strong activity against Gram-positive bacteria.11 In all
We were intrigued by the question of whether a dihydro-
oxazole substructure, which looks quite similar to the
dihydroisoxazole moiety, could serve as a suitable bio-
isostere for the oxazolidinone moiety. In this paper, we
describe an ex-chiral pool synthesis and pharmacological
investigation of the target compound 3 in both stereo-
isomeric forms. Furthermore, computational studies of the
heterocyclic core structures are presented.
Our strategy for a stereocontrolled synthesis of the target
compound 3 was to build up the dihydrooxazole scaffold by
condensation of a suitable benzoic acid derivative and (S)-
serine methyl ester and subsequent reductive functionaliza-
tion of the side chain.
Keywords: serine; dihydrooxazoles; antibiotics.
*
Corresponding author. Tel.: þ49-9131-8529383; fax: þ49-9131-
8522585; e-mail: gmeiner@pharmazie.uni-erlangen.de
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00490-3

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J. Einsiedel et al. / Tetrahedron 59 (2003) 3403–3407
2. Results and discussion
be improved by the application of NaBH4 in methanol
(yield: 57%). In order to investigate the optical purity of the
primary alcohols 13 (synthesized using LiAlH4) and ent13
(analogously prepared from (R)-serine via NaBH4
reduction), HPLC separation on a chiral stationary phase
was performed indicating 92.8% ee for 13 and 92.4% ee for
the optical antipode ent13 (see Section 3).
The readily available nitrobenzene derivative 612 should
give rise to the synthetic intermediate 9 (Scheme 1). Since
the reduction of the nitro group using ammonium formate in
presence of Pd/C12 did not proceed quantitatively and
therefore required an awkward workup procedure, we
performed a low pressure hydrogenation to furnish the
aniline derivative 7 in 100% yield. Thus, the aromatic amine
7, being also the central intermediate for the technical
synthesis of linezolid (1),12 is now available by a
convenient, simplified methodology. Sandmeyer reaction13
allowed the introduction of the cyano group to give the
nitrile derivative 8, that was subsequently hydrolyzed to
furnish the carboxylic acid derivative 9.
The transformation of the primary alcohol 13 into the azide
14 was performed by sulfonylation and subsequent SN2-
reaction with NaN3. Finally, the target compound 3 was
synthesized by catalytic hydrogenation in presence of
Ac2O. The enantiomer ent3 was prepared analogously
starting from (R)-serine methyl ester.
The test compounds 3 and ent3 were microbiologically
screened for their antibacterial properties using the disk
diffusion technique according to National Committee for
Clinical Laboratory Standards (NCCLS).15 Antibiotic free
test disks (Sensidisce) were loaded with 1, 10, 30 and
100 mg of the compounds dissolved in EtOH and dried
under aseptic conditions. Suspensions of three clinically
significant Gram-positive bacteria (Staphylococcus aureus
ATCC 29212, hemolytic Streptococci Gr. A (Streptococcus
pyogenes VA 27659), Enterococcus faecalis ATCC 29212)
were adjusted photometrically to Mc Farland turbidity
standard 0.5 and streaked out on Mueller–Hinton–Agar
plates (Oxoide) supplemented with 5% sheep blood. The
disks were put on the agar plates, when the plates were
incubated for 18 h at 368C and then examined for inhibiting
zones around the disks. Disks containing vancomycin and
linezolid were used as the control. The screened test
substances indicated no antibacterial activity.
Scheme 1. (a) H2, Pd(OH)2/C, MeOH, room temperature, 1 h (100%
crude); (b) (1) NaNO2, H2SO4, 08C, 30 min, (2) Na2CO3, 08C to room
temperature, (3) CuCN, KCN, room temperature to 508C, 2 h (51%);
(c) HCl conc., refl., 1 h (99% crude).
Starting from 9, peptide coupling with (S)-serine methyl
ester hydrochloride (10) afforded the carboxamide 11, that
underwent a smooth cyclization by the reaction with
Burgess-reagent14 to give the dihydrooxazole derivative
12 (Scheme 2).
In order to understand the structural requirements for target
recognition, we decided to take a closer look at the
molecular electrostatic potentials (MEPs) of the hetero-
cyclic pharmacophore. To allow a direct comparability of
the different compounds, linezolid (1), dihydrooxazole 3,
the butenolid 4 and the dihydroisoxazole 5 were formally
defunctionalized to give the respective core scaffolds A–D.
The structures were preminimized at the MM-level with
MAXIMIN2 applying the Tripos force field implemented in
Sybyl 6.81. Subsequently, we performed ab initio calcu-
lations (Gaussian98) using a 6-31G(d) basis set. The charges
used to contour the MEPs were calculated on the resulting
structures applying Breneman’s CHelpG charge distribution
scheme. After reimporting the results into Sybyl 6.81, the
positive and negative isopotential surfaces were contoured
with MOLCAD Plus at 5.0 or 25.0 kcal/mol, respectively.
Qualitatively, the negative isopotential surfaces (light gray
marked MEPs) of the oxazolidinone A and the cardenolide
C, which mainly consist of two extended bulbs surrounding
both the heterocyclic and the acetamido carbonyl oxygens,
are very similar. Analogously, but to a lesser extend, these
potentials can be found in the dihydroisoxazole D,
obviously due to the sterically less demanding oxime
ether substructure. However, the dihydrooxazole substruc-
ture B shows quite different properties: the negative
isopotential surface is mainly localized on the bottom of
the molecule. Obviously, this significant change is due to
the shifted heteroatoms in the ring and might lead to
unfavourable interactions at the bacterial ribosome (Fig. 1).
Scheme 2. (a) DCC, HOBt; NMM, DMF/CH2Cl2 9:1, 08C, 1 h, 08C to room
temperature, 18 h (98%); (b) MeO2CNSO2NEt3 (Burgess reagent), THF,
708C, 1 h (88%); (c) LiAlH4, Et2O, 2308C, 1 h (26%) or NaBH4, MeOH,
room temperature, 3.5 h (57%); (d) (1) MesCl, NEt3, THF, 2238C, 30 min;
(2) NaN3, DMSO, 658C, 12 h (82%); (e) H2, Pd(OH)2/C, Ac2O/
EtOAc¼1:1, room temperature, 1 h (79%).
Low temperature LiAlH4-reduction afforded the primary
alcohol 13 in only 26% yield. However, the reaction could

J. Einsiedel et al. / Tetrahedron 59 (2003) 3403–3407
3405
as a gray–white solid, that was used without further
purification in the next step. All spectra data were in
accordance to those reported in the literature.8
3.1.2. 3-Fluoro-4-morpholinylbenzonitrile (8). To a
stirred solution of 3-fluoro-4-morpholinylaniline (7)
(1500.0 mg, 7.65 mmol) in H2SO4 30% (10 ml) was added
dropwise a solution of 528.1 mg (7.65 mmol) sodium nitrite
in 2 ml of water at 08C. When an excess of nitrous acid
could be detected with KI–starch paper, the mixture was
adjusted to pH 7 with a saturated solution of Na2CO3 in
H2O, then poured into a solution of 2741.0 mg Cu(I)CN
(30.61 mmol) and 2491 mg (28.27 mmol) KCN in 15 ml
H2O and heated to 508C for 2 h. After being cooled, the
solution was extracted with CH2Cl2 (3£15 ml) and dried
over MgSO4. After evaporation, the residue was purified by
flash chromatography (petroleum ether–EtOAc, 9:1) to give
8 (819.0 mg, 51%) as a yellow–orange solid (mp 948C).
TLC Rf 0.19 (petroleum ether–EtOAc, 1:1); IR (NaCl)
3072–2831, 2225, 1612, 1240, 1119, 1068 cm21; 1H NMR
(CHCl3, 360 MHz): d (ppm)¼3.19–3.22 (m, 4H, CH2N),
3.85–3.98 (m, 4H CH2O), 6.92 (dd, 1H, J¼8.5, 8.5 Hz,
5-Ph), 7.29 (dd, J¼12.8, 1.7 Hz, 1H, 2-Ph), 7.38 (ddd,
J¼8.5, 1.7, 0.6 Hz, 1H, 6-Ph). Analysis calcd for
C11H11FN2O: C, 64.07 H, 5.38 N, 13.58. Found: C, 63.93
H, 5.29 N, 13.86. EIMS (m/z): 206 (Mþ).
Figure 1. Isopotential surfaces of the core scaffolds of the antiinfective
linezolid analogues: A: phenyloxazolidinone; C: phenylbutenolid; D:
phenyldihydroisoxazole and the non-active phenyldihydrooxazole B
contouring positive (dark gray, þ5 kcal/mol) and negative (light gray,
25 kcal/mol) electrostatic potentials.
In conclusion, we were able to develop an EPC-synthesis
leading to a new class of linezolid analogues incorporating a
dihydrooxazole substructure.
3.1.3. 3-Fluoro-4-morpholinylbenzoic acid HCl (9). A
solution
of
3-fluoro-4-morpholinylbenzonitrile
(8)
The test compounds did not show antibacterial activity.
However, with the help of 3, MEP based structure activity
relationship studies indicated structural requirements of the
heterocyclic moiety to serve as a pharmacophoric unit,
which might be of interest for future drug development.
(592.6 mg, 2.88 mmol) in HCl 38% (7 ml) was refluxed
for 1 h. After being cooled, the solution was evaporated to
give 9 (746.0 mg, 99% crude) as an orange solid, that was
taken ‘as is’ for the next reaction step. For the analytical
data, a small sample was purified by adjusting to pH 3 with
NaHCO3 to give the free base, extracting with EtOAc,
drying with MgSO4, evaporating and performing a flash
chromatography (CH2Cl2–MeOH, 97:3) to afford an orange
solid (mp. 226–2278C). TLC Rf 0.3 (CH2Cl2–MeOH, 9:1);
IR (NaCl) 3120–2576, 1690, 1666, 1616, 1450, 1404, 1304,
3. Experimental
3.1. General
; d
1250, 1122 cm21 1H NMR (CHCl3, 360 MHz):
Solvents and reagents were purified and dried by standard
procedures or purchased in pure, dry quality from Fluka or
Aldrich. All reactions were performed under dry N2.
Evaporations of final product solutions were done in
vacuo using a rotary evaporator. Flash chromatography
was carried out with 230–400 mesh silica gel. Melting
points were determined on a Bu¨chi apparatus and are
uncorrected. If not otherwise stated, MS were run by EI
(ppm)¼3.11–3.13 (m, 4H, CH2N), 3.73–3.76 (m, 4H
CH2O), 7.08 (dd, 1H, J¼8.8, 8.7 Hz, 5-Ph), 7.58 (dd,
J¼14.0, 2.0 Hz, 1H, 2-Ph), 7.69 (dd, J¼8.7, 2.0 Hz, 1H,
6-Ph). Analysis calcd for C11H12FNO3£0.3H2O: C, 57.29
H, 5.51 N, 6.07. Found: C, 57.33 H, 5.46 N, 5.92. EIMS
(m/z): 225 (Mþ).
1
3.1.4. (S)-N-(3-Fluoro-4-morpholinylbenzoyl)-serine
methyl ester (11). To a solution of crude 3-fluoro-4-
morpholinylbenzoic acid HCl (9) (1000.0 mg, 3.82 mmol)
and (S)-serine methyl ester hydrochloride (10, 595.0 mg,
3.82 mmol) in CH2Cl2/DMF 8:1 (45 ml) was added NMM
(1.28 ml, 11.5 mmol) at 08C and then a solution of HOBt
(722.5 mg, 5.35 mmol) and DCC (947.5 mg, 4.59 mmol) in
CH2Cl2 (15 ml). The mixture was stirred for 1 h at 08C and
then for 18 h at room temperature. The solution was
evaporated and the resulting solid was extracted with
CH2Cl2 (10 ml) and filtered. The filtrate was evaporated and
purified by flash chromatography (gradient: petroleum
ether–EtOAc 4:1–3:1) to give 11 (1116.0 mg, 98%) as a
yellowish solid (mp 158–1598C). ent11 was prepared under
the same reaction conditions, starting from (R)-serine
methyl ester hydrochloride. TLC Rf 0.25 (petroleum
ionization (70 eV) with solid inlet. H NMR spectra were
recorded on BrukerAC 250 (250 MHz) and AM 360
(360 MHz) spectrometers, if not otherwise stated, in
CDCl3 relative to TMS (J values are given in Hertz).
Optical rotation was measured on a Perkin–Elmer polari-
meter 241 at 208C. IR spectra were recorded on a Jasco
FT/IR 410 spectrometer. Micro analyses were performed by
the Institute of Organic Chemistry of the Friedrich-
Alexander University Erlangen-Nu¨rnberg.
3.1.1. 3-Fluoro-4-morpholinylaniline (7).8 To a solution of
3-fluoro-4-morpholinylnitrobenzene
(6)8
(4000.0 mg,
17.69 mmol) in MeOH (250 ml) was added Pd(OH)2/C
(850.0 mg) and the mixture was stirred under a balloon with
H2 at room temperature. After 1 h, the mixture was filtered
through Celite and evaporated to give 7 (3570.0 mg, 100%)

3406
J. Einsiedel et al. / Tetrahedron 59 (2003) 3403–3407
ether–acetone 1:1); 11: [a]2D0¼þ40.58 (c¼0.7, CHCl3);
ent11: [a]2D0¼240.28 (c¼1.0, CHCl3); IR (NaCl) 3375,
2958, 2854, 1743, 1643, 1620, 1543, 1504, 1450, 1246,
TLC Rf 0.15 (petroleum ether–acetone 1:1); 13:
[a]D20¼þ15.98 (c¼0.68, CHCl3); ent13: [a]2D0¼216.08
(c¼0.025, CHCl3); IR (NaCl) 3178, 2962–2858, 1647,
1616, 1516, 1439, 1373, 1265, 1250, 1192, 1119 cm21; 1H
NMR (CHCl3, 360 MHz): d (ppm)¼2.52 (brs, 1H, –OH),
3.16–3.18 (m, 4H, CH2N), 3.66 (dd, J¼11.4, 3.9 Hz, 1H,
–CH2OH), 3.86–3.89 (m, 4H CH2O), 3.96 (dd, J¼11.4,
3.4 Hz, 1H, –CH2OH), 4.35 (dd, J¼7.2, 7.1 Hz, 1H, H-5a),
4.43 (dd, J¼9.4, 7.2, 3.9, 3.4 Hz, 1H, H-4), 4.50 (dd, J¼9.4,
7.1 Hz, 1H, H-5b), 6.87 (dd, 1H, J¼8.5, 8.4 Hz, 5-Ph), 7.55
(dd, J¼13.7, 2.1 Hz, 1H, 2-Ph), 7.64 (ddd, J¼8.4, 2.1,
0.4 Hz, 1H, 6-Ph). Analysis calcd for C14H17FN2O3: C,
59.99 H, 6.11 N, 9.99. Found: C, 59.93 H, 6.13 N, 9.84.
EIMS (m/z): 280 (Mþ).
1211, 1119 cm21
; d
1H NMR (CHCl3, 360 MHz):
(ppm)¼3.15–3.17 (m, 4H, CH2N), 3.27 (brs, 1H,
CH2OH), 3.82 (s, 3H, COOCH3), 3.86–3.88 (m, 4H
CH2O), 4.03 (dd, J¼11.3, 3.5 Hz, 1H, CH2OH), 4.08 (dd,
J¼11.3, 3.8 Hz, 1H, CH2OH), 4.84 (ddd, J¼7.3, 3.8,
3.5 Hz, 1H, CH2(NHCH)CH2), 6.90 (dd, 1H, J¼8.6,
8.4 Hz, 5-Ph), 7.07 (d, J¼7.3 Hz, 1H, CONH), 7.52 (dd,
J¼13.2, 2.2 Hz, 1H, 2-Ph), 7.54 (ddd, J¼8.6, 2.2, 0.7 Hz,
1H, 6-Ph). Analysis calcd for C15H19FN2O5: C, 55.21 H,
5.87 N, 8.58. Found: C, 54.87 H, 5.81 N, 8.22. EIMS (m/z):
326 (Mþ).
3.1.5. (S)-2-(3-Fluoro-4-morpholinylphenyl)-4,5-di-
hydrooxazole-4-carboxylic acid methyl ester (12). To a
solution of (S)-N-(3-fluoro-4-morpholinyl-benzoyl)-serine
methyl ester (11) (582.4 mg, 1.79 mmol) in THF (16 ml)
13 (synthesized by method 2) and ent13 (synthesized by
method 1) were investigated by performing chiral HPLC
(Chiralcel OD; petroleum ether/iPrOH 9:1; 1 ml/min;
215 nm): 13: R (rt¼19.57 min): S (rt¼21.40 min)¼96.4:
3.6; ent13: S/R¼96.2:3.8.
was
added
(methoxycar-bonylsulfamoyl)-triethylam-
monium-N-betaine (492.5 mg, 2.07 mmol) at room tem-
perature. The solution was transferred to a sealed tube and
heated to 708C. After 1 h, the solution was evaporated and
the residue was purified by flash chromatography (gradient:
petroleum ether–EtOAc 4:1–3:1–2:1) to give 12
(487.2 mg, 88%) as a yellowish solid (mp 868C). ent12
was prepared under the same reaction conditions, starting
from ent11. TLC Rf 0.5 (petroleum ether–acetone 1:1); 12:
[a]D20¼þ55.88 (c¼1.0, CHCl3); ent12: [a]2D0¼254.48
(c¼0.23, CHCl3); IR (NaCl) 2954–2854, 1743, 1639,
3.1.7. (R)-4-[4-(4-Azidomethyl-4,5-dihydrooxazol-2-yl)-
2-fluorophenyl]-morpholine (14). To a solution of (R)-
[2-(3-fluoro-4-morpho-linylphenyl)-4,5-dihydrooxazol-4-yl]-
methanol (13) (200.0 mg, 0.71 mmol) and NEt3 (323 ml,
2.5 mmol) in THF (7 ml) was added methansulfonyl chloride
(195 ml, 2.5 mmol) at 2238C. After 30 min, the mixture was
filtered and the filtrate was evaporated. The residue was
dissolved in Et2O (30 ml) and the solution was washed with
sat. aqueous NaHCO3. After drying with MgSO4, the solution
was evaporated and NaN3 (232.2 mg, 3.57 mmol) and DMSO
(7 ml) were added. The mixture was stirred for 12 h at 658C
and then diluted with sat. aqueous NaHCO3 and Et2O. The
organic layer was washed with brine, dried with MgSO4 and
evaporated. The residue was purified by flash chromatography
(petroleumether–acetone4:1)togive14(178.2 mg, 82%)asa
colorless solid (mp 798C). ent14 was prepared under the same
reaction conditions, starting from ent13. TLC Rf 0.4
(petroleum ether–acetone 1:1); 14: [a]2D0¼þ110.38 (c¼0.53,
CHCl3); IR (NaCl) 2962–2854, 2102, 1643, 1616, 1516,
1620, 1516, 1442, 1362, 1269, 1250, 1207, 1119 cm21
;
1H NMR (CHCl3, 360 MHz): d (ppm)¼3.16–3.18 (m, 4H,
CH2N), 3.82 (s, 3H, COOCH3), 3.85–3.88 (m, 4H CH2O),
4.57 (dd, J¼10.5, 8.6 Hz, 1H, H-5a), 4.67 (dd, J¼8.6,
8.0 Hz, 1H, H-5b), 4.93 (dd, J¼10.5, 8.0 Hz, 1H, H-4), 6.90
(dd, 1H, J¼8.5, 8.4 Hz, 5-Ph), 7.65 (dd, J¼13.7, 2.0 Hz, 1H,
2-Ph), 7.69 (dd, J¼8.4, 2.0 Hz, 1H, 6-Ph). Analysis calcd
for C15H17FN2O4: C, 58.44 H, 5.56 N, 9.09. Found: C, 58.28
H, 5.63 N, 8.93. EIMS (m/z): 308 (Mþ).
3.1.6. (R)-[2-(3-Fluoro-4-morpholinylphenyl)-4,5-di-
hydrooxazol-4-yl]-methanol (13). Method 1. To a solution
1446, 1361, 1254, 1188, 1119 cm21 1H NMR (CHCl3,
;
360 MHz): d (ppm)¼3.15–3.18 (m, 4H, CH2N), 3.42–3.47
(m, 1H, –CH2N3), 3.51–3.56 (m, 1H, –CH2N3), 3.85–3.88
(m, 4H CH2O), 4.20–4.28 (m, 1H, H-5a or H-4), 4.43–4.52
(m, 1H, H-5b and H-4 or H-5a), 6.91 (dd, 1H, J¼8.5, 8.4 Hz,
6-Ph), 7.61 (dd, J¼13.7, 2.0 Hz, 1H, 3-Ph), 7.66 (ddd, J¼8.4,
2.0, 0.5 Hz, 1H, 5-Ph). Analysis calcd for C14H16FN5O2: C,
55.08 H, 5.28 N, 22.49. Found: C, 55.12 H, 5.32 N, 22.53.
EIMS (m/z): 305 (Mþ).
of
(S)-2-(3-fluoro-4-morpholinylphenyl)-4,5-dihydro-
oxazole-4-carboxylic acid methyl ester (12) (487.2 mg,
1.58 mmol) in Et2O (30 ml) was added a 1 M solution of
LiAlH4 in Et2O (3.36 ml, 3.36 mmol) at 2308C. After 1 h,
the solution was quenched with H2O (1 ml) and warmed to
room temperature. The solution was filtered through Celite,
the filtrate was evaporated and the residue was purified by
flash chromatography (petroleum ether–acetone 4:1) to
give 13 (116.9 mg, 26%) as a gray–white solid (mp 148–
1498C). ent13 was prepared under the same reaction
conditions, starting from ent12.
3.1.8. (R)-N-[2-(3-Fluoro-4-morpholin-4-yl-phenyl)-4,5-
dihydro-oxazol-4-ylmethyl]-acetamide (3). To a solution
of
(R)-4-[4-(4-azidomethyl-4,5-dihydro-oxazol-2-yl)-2-
fluorophenyl]-morpholine (14) (100.0 mg, 0.33 mmol) in
EtOAc/Ac2O (1:1; 10 ml) was added Pd(OH)2/C (80 mg)
and the mixture was stirred under a balloon with H2 at room
temperature. After 1 h, the mixture was filtered through
Celite and evaporated. The residue was purified by flash
chromatography (gradient: petroleum ether–acetone 3:1–
2:1–1:1) to give 3 (83.3 mg, 79%) as a yellowish solid (mp
169–1708C). ent3 was prepared under the same reaction
conditions, starting from ent14. TLC Rf 0.05 (petroleum
ether–acetone 2:1); 3: [a]2D0¼þ19.38 (c¼0.57, CHCl3);
Method 2. To a solution of (S)-2-(3-fluoro-4-morpholinyl-
phenyl)-4,5-dihydrooxazole-4-carboxylic acid methyl ester
(12) (10.0 mg, 0.032 mmol) in MeOH (3 ml) was added
NaBH4 (6.5 mg, 0.17 mmol) at room temperature. After 4 h,
the solution was quenched with a saturated aqueous NH4Cl
solution (0.3 ml) and subsequently evaporated. The residue
was purified by flash chromatography (petroleum ether–
acetone 2:1) to give 13 (5.1 mg, 57%). ent13 was prepared
under the same reaction conditions, starting from ent12.

J. Einsiedel et al. / Tetrahedron 59 (2003) 3403–3407
3407
ent3: [a]2D0¼219.38 (c¼0.72, CHCl3); IR (NaCl) 3317,
5. Einsiedel, J.; Hu¨bner, H.; Gmeiner, P. Bioorg. Med. Chem.
Lett. 2001, 11, 2533. and references cited therein.
3078, 2966–2854, 2102, 1647, 1543, 1516, 1439, 1373,
1265, 1119 cm21
; d
1H NMR (CHCl3, 360 MHz):
6. Champney, W. S.; Miller, M. Curr. Microbiol. 2002, 44, 350.
7. For example see: Tokuyama, R.; Takahashi, Y.; Tomita, Y.;
Tsubouchi, M.; Yoshida, T.; Iwasaki, N.; Kado, N.; Okezaki,
E.; Nagata, O. Chem. Pharm. Bull. 2001, 49, 353.
(ppm)¼3.16–3.19 (m, 4H, CH2N), 3.37 (ddd, J¼13.7,
6.2, 6.2 Hz, 1H, –CH2NHAc), 3.66 (ddd, J¼13.7, 6.2,
3.6 Hz, 1H, –CH2NHAc), 3.86–3.89 (m, 4H CH2O), 4.09
(dd, J¼7.8, 7.8 Hz, 1H, H-5a), 4.39 (dd, J¼9.9, 7.9, 6.2,
3.6 Hz, 1H, H-4), 4.47 (dd, J¼9.9, 7.8 Hz, 1H, H-5b), 6.91
(dd, 1H, J¼8.5, 8.5 Hz, 5-Ph), 7.61 (dd, J¼13.7, 2.0 Hz, 1H,
2-Ph), 7.64 (dd, J¼8.5, 2.0 Hz, 1H, 6-Ph). Analysis calcd
for C16H20FN3O3: C, 59.80 H, 6.27 N, 13.08. Found: C,
59.35 H, 6.33 N, 12.87. EIMS (m/z): 321 (Mþ).
8. For example see: Genin, M. J.; Hutchinson, K. H.; Allwine,
D. A.; Hester, J. B.; Emmert, D. E.; Garmon, S. A.; Ford,
C. W.; Zurenko, G. E.; Hamel, J. C.; Schaadt, R. D.; Stapert,
D.; Yagi, B. H.; Friis, J. M.; Shobe, E. M.; Adams, W. J.
J. Med. Chem. 1998, 41, 5144.
9. Denis, A.; Villette, T. Bioorg. Med. Chem. Lett. 1994, 4, 1925.
10. Gregory, W. A.; Brittelli, D. R.; Wang, C.-L. J.; Kezar, H. S.,
III.; Carlson, R. K.; Park, C.-H.; Corless, P. F.; Miller, S. J.;
Rajagopalan, P.; Wuonola, M. A.; McRipley, R. J.; Eberly,
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Acknowledgements
The BMBF and the Fonds der Chemischen Industrie is
acknowledged for financial support. We thank Dr W. Utz
¨
and F. Bockler for the computational studies and helpful
discussions. Special thanks are due to Mr H. Bittermann and
Mrs I. Torres for their skillful assistance during the
experimental work. Dr R. Waibel is acknowledged for the
chiral HPLC investigations. We would also like to thank
12. Brickner, S. J.; Hutchinson, D. K.; Barbachyn, M. R.;
Manninen, P. R.; Ulanowicz, D. A.; Garmon, S. A.; Grega,
K. C.; Hendges, S. K.; Toops, D. S.; Ford, C. W.; Zurenko,
G. E. J. Med. Chem. 1996, 39, 673.
¨
Professor Dr M. Rollinghoff of the Institute of Clinical
13. Qui, J.; Stevenson, S. H.; O’Beirne, M. J.; Silverman, R. B.
J. Med. Chem. 1999, 42, 329.
Microbiology, Immunology and Hygiene for offering the
opportunity of performing the biological tests.
14. (a) Atkins, G. M.; Burgess, E. M. J. Am. Chem. Soc. 1968, 90,
4744. (b) Wipf, P.; Miller, C. P. Tetrahedron Lett. 1992, 33,
907.
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