Catalytic enantioselective 5-hydroxyisoxazolidine synthesis: An asymmetric entry to β-amino acids

Article abstract of DOI:10.1055/s-2007-990935

The highly chemo- and enantioselective organocatalytic tandem reaction between N-carbamate-protected hydroxylamines and α,β-unsaturated aldehydes is presented. The reaction represents a unique entry for the asymmetric synthesis of 5-hydroxyisoxazolidines, oxazolidin-5-ones or γ-hydroxyamino alcohols in high yields and 90-99% ee. A procedure for the conversion of the oxazolidin-5-ones into the corresponding β-amino acids is also described. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-990935

PRACTICAL SYNTHETIC PROCEDURES
1153
Catalytic Enantioselective 5-Hydroxyisoxazolidine Synthesis: An Asymmetric
Entry to b-Amino Acids
C
atalytic
s
E
nantios
m
elective 5-Hydroxyi
a
soxazolidin
i
e
Synt
l
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University,
106 91 Stockholm, Sweden
Fax +46(8)154908; E-mail: acordova@organ.su.se
PSP
Received 16 August 2007
No119
Abstract: The highly chemo- and enantioselective organocatalytic tandem reaction between N-carbamate-protected hydroxylamines and
a,b-unsaturated aldehydes is presented. The reaction represents a unique entry for the asymmetric synthesis of 5-hydroxyisoxazolidines,
oxazolidin-5-ones or g-hydroxyamino alcohols in high yields and 90–99% ee. A procedure for the conversion of the oxazolidin-5-ones into
the corresponding b-amino acids is also described.
Key words: organocatalysis, amination reactions, b-amino acid synthesis, isoxazolidines, asymmetric catalysis
Procedure 1
Ph
Ph
OTMS
Ph
– H+
N
R
+
N
Ph
OTMS
HO
HO
NH
H
N
Cbz
Cbz
R
1b
I
H+
O
Ph
R
2a
H2O
+
N
Ph
Ph
OTMS
Ph
N
H
OTMS
HO
4
N
R
O
R
Cbz
Cbz
R
O
N
HO
OH
N
Cbz
3b: R = Ph, 94% yield; 99% ee
O
1.
4
Cbz
Procedure 2
O
N
OH
Cbz
(20 mol%)
N
H
Ph
H
O
+
Ph
2. NaClO2
1b
2a
5b: 81% yield; 99% ee
O
1.
4
Boc
OH
Procedure 3
Procedure 4
N
OH
Boc
(20 mol%)
N
H
Ph
H
+
OH
7a: 87% yield, 99% ee
Ph
2. NaBH4
1a
2a
Cbz
Ph
Pd/C, H2
NH2
O
O
N
O
Ph
OH
6b: quantitative
5b
Scheme 1
SYNTHESIS 2007, No. 7, pp 1153–1157
x
x
.
x
x
.2
0
0
7
Advanced online publication: 28.11.2007
DOI: 10.1055/s-2007-990935; Art ID: Z20807SS
© Georg Thieme Verlag Stuttgart · New York

1154
I. Ibrahem et al.
PRACTICAL SYNTHETIC PROCEDURES
catalyzed reaction between N-protected hydroxylamines
and enals.8 The reaction is a simple asymmetric entry to
5-hydroxyisoxazolidines where the subsequent tandem
intramolecular hemiacetal formation is an important driv-
ing force for product formation (procedure 1, Scheme 1).
Introduction
5-Hydroxyisoxazolidines and isoxazolidin-5-ones are im-
portant chiral building blocks1 which are readily convert-
ed into the corresponding amino alcohols and b-amino
acids.2 Thus, asymmetric methods have been developed
for their preparation.1 For example, optically active 5-ac-
etoxydihydroisoxazoles can be converted in two steps into
the corresponding isoxazolidinones.1d,e Moreover, utiliza-
tion of chiral auxiliaries enables the asymmetric synthesis
of isoxazolidin-5-ones.1e–j Recently, isoxazolidin-5-ones
were prepared by Lewis acid catalyzed enantioselective
conjugate addition of hydroxylamines to a,b-unsaturated
amide derivatives.1b,c,3
Scope and Limitations
The mild reaction conditions for the 2-[diphenyl(trimeth-
ylsiloxy)methyl)pyrrolidine (4)9 catalyzed reactions are
compatible with several different types of enals. For ex-
ample, the reaction between cinnamaldehyde (2a) and
tert-butyl N-hydroxycarbamate (1a) in chloroform afford-
ed the desired product 3a in 90% yield and 99% ee at
room temperature. In fact, the a,b-unsaturated aldehydes
2 reacted with N-hydroxycarbamates 1 leading to 5-hy-
droxyisoxazolidines 3 in excellent yields and enantiose-
lectivities in the presence of chiral amine 4 (Table 1).
In the field of organocatalysis, amine-catalyzed reactions
that involve catalytic domino, tandem, or cascade reaction
pathways have recently been developed.4–6 In this context,
we have developed an asymmetric domino amine–conju-
gate/aldol reaction.7 Based on these lessons and retrosyn-
thetic analysis, we recently discovered a chiral-amine-
Table 1 Scope of the Organocatalytic Tandem Reaction
Ph
Ph
N
4
OTMS
H
R1
R2
O
(20 mol%)
O
N
R1
OH
R2
H
OH
N
H
CHCl3, 4 °C
+
1
2
3
Entry
R1
R2
Ph
Product
Time (h)
3
Yielda (%)a
90c
eeb (%)
99c
Boc
N
O
1
Boc
Cbz
OH
Ph
3a
Cbz
O
N
2
3
Ph
3
3
94
89
99
90
OH
N
Ph
3b
Boc
O
O
O
OH
OH
OH
Boc
Boc
Boc
4-ClC6H4
4-BrC6H4
4-NCC6H4
Cl
3c
Boc
Boc
N
4
5
3
3
80
90
97
97
Br
3d
N
NC
3e
Synthesis 2007, No. 7, 1153–1157 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Catalytic Enantioselective 5-Hydroxyisoxazolidine Synthesis
1155
Table 1 Scope of the Organocatalytic Tandem Reaction (continued)
Ph
Ph
N
4
OTMS
H
R1
R2
O
(20 mol%)
O
N
R1
OH
R2
H
OH
N
H
CHCl3, 4 °C
+
1
2
3
Entry
6
R1
R2
Product
Time (h)
Yielda (%)a
eeb (%)
Boc
O
O
N
N
OH
OH
Boc
Boc
4-O2NC6H4
2-naphthyl
3
75
98
O2N
3f
Boc
7
3
77
95
3g
Cbz
O
N
8
9
Cbz
Boc
Boc
Cbz
CO2Et
n-Bu
n-Pr
16
16
16
16
85
94
93
92
97
91
91
95
OH
EtO2C
3h
Boc
O
O
O
N
OH
OH
OH
n-Bu
3i
Boc
N
10
11
n-Pr
3j
Cbz
N
n-Pr
n-Pr
3k
a Isolated yield of the pure product 3 after silica gel chromatography.
b Determined by chiral-phase HPLC or GC analyses.
c Reaction performed at r.t.
Thus, the reactions work well with aliphatic and aromatic asymmetric synthesis of oxazolidin-5-ones 5 and g-hy-
a,b-unsaturated aldehydes. Moreover the reaction with N- droxyamino alcohols 7 (Scheme 2).
Cbz-protected hydroxylamine 1b gave the corresponding
products 3 in 85–94% yield and 95–99% ee (entries 2, 8
and 11). Moreover, products 3 were formed as single ano-
mers. The tandem reactions with a,b-unsaturated aliphatic
acceptor aldehydes 2 (entries 9–11) were slower as com-
pared to the aryl-substituted enals 2 (entries 1–7). In addi-
tion, the organocatalytic reaction is readily scaled up and
gives access to a variety of 5-hydroxyisoxazolidines. No-
Both aromatic and aliphatic enals are suitable as sub-
strates for these organocatalytic one-pot reactions. Thus,
in situ oxidation with sodium chlorite of the 5-hydroxy-
isoxazilidones 3 gave the corresponding N-protected
5-oxazolidinones 5 in high overall yield with 95–99% ee.
Moreover, in situ reduction with sodium borohydride
gave a direct access to g-amino alcohol 7a. Notably, hy-
drogenolysis of the isolated oxazolidinones 5b and 5c
with 10% palladium-on-carbon led to efficient N–O bond
cleavage and removal of the Cbz group to quantitatively
give the corresponding b-amino acids 6. Comparison with
the literature revealed that the absolute configuration of
tably, the pKa of the hydroxylamine is important since
changing the carbamate group of the N-hydroxycarbam-
ate 1 to an aryl group led to a complete change of
chemoselectivity and 1,3-dipolar addition with the enal.10
The chiral-amine-catalyzed reaction between N-hydroxy- 6b at C3 was S {[a]D25 –6.9 (c 1, H2O), Lit.11 [a]D25 –6.9
carbamates 1 and enals 2 is also suitable for the one-pot (c 0.8, H2O)}.
Synthesis 2007, No. 7, 1153–1157 © Thieme Stuttgart · New York

1156
I. Ibrahem et al.
PRACTICAL SYNTHETIC PROCEDURES
O
1.
4
R1
R2
Pd/C, H2
R1
(20 mol%)
NH2
O
O
N
OH
N
H
R2
H
+
O
R2
OH
2. NaClO2
1
2
5a: R1 = Boc, R2 = Ph, 74% yield, 99% ee
5b: R1 = Cbz, R2 = Ph, 81% yield, 99% ee
5c: R1 = Cbz, R2 = n-Pr, 80% yield, 95% ee
6b and 6c
quantitative
1. 4 (20 mol%)
2. NaBH4
R1
OH
N
R2
OH
7a: R1 = Boc, R2 = Ph, 87% yield, 99% ee
Scheme 2 One-pot organocatalytic enantioselective synthesis of oxazolidin-5-ones 5 and amino alcohol 7a and b-amino acid 6 synthesis
HRMS (ESI): m/z [M + H]+ calcd for C17H18NO4: 300.1230; found:
300.1233.
In summary, we report highly chemo- and enantioselec-
tive catalytic procedures for the synthesis of 5-hydroxy-
isoxazolidines, oxazolidin-5-ones, and g-amino alcohols,
(–)-(3S)-2-(Benzyloxycarbonyl)-3-phenylisoxazolidin-5-one
which are formed in high yields with 90–99% ee. More-
(5b); Typical Procedure
over, the organocatalytic tandem reaction represents a
versatile asymmetric entry to different b-amino acid and
g-amino alcohol derivatives.
To a stirred soln of catalyst 4 (20 mol%) in CHCl3 (0.5 mL) at 4 °C
was added a,b-unsaturated aldehyde 2a (33 mg, 0.25 mmol) and hy-
droxycarbamate 1b (55 mg, 0.3 mmol). The reaction was vigorous-
ly stirred for 3 h. Upon completion (a small aliquot was removed for
ee determination) The reaction temperature was increased to r.t.,
isobutene (0.1 mL), t-BuOH (0.4 mL), H2O (0.2mL), KH2PO4 (54.4
mg, 4 mmol), and NaClO2 (36 mg, 4mmol) were added sequential-
ly. After 16 h, the crude product 5b was purified by column chro-
matography (pentane–EtOAc mixtures) to afford isoxazolidin-5-
one 5b (60 mg, 81%) as a clear oil.
[a]D25 – 33.2 (c 1.0, CHCl3).
1H NMR (400 MHz, CDCl3): d = 2.85 (dd, J = 5.1 Hz, J¢ = 15.9 Hz,
1 H), 3.26 (dd, J = 10.5 Hz, J¢ = 15.9 Hz, 1 H), 5.20–5.10 (m, 2 H),
5.60 (dd, J = 5.1 Hz, J¢ = 10.5 Hz, 1 H), 7.20–7.42 (m, 10 H).
Procedures
Herein, we describe four typical experimental procedures demon-
strating the synthetic scope of the chiral amine-catalyzed tandem re-
action between hydroxylamines and enals. In Procedure 1, we
report the preparation of 5-hydroxyisoxazolidine 3b (94% yield;
99% ee) starting from commercially available hydroxylamine 1b
and enal 2a in the presence of chiral amine 4. The second procedure
(Procedure 2) describes the in situ conversion of 3b, which is gen-
erated by 4 catalyzed reaction between 1b and 2a in one-pot, into
the corresponding oxazolidin-5-one 5b in 81% yield and 99% ee. In
Procedure 3, we report the one-pot synthesis of g-hydroxyamino al-
cohol 7a (87% yield; 99% ee) starting from hydroxylamine 1a and
enal 2a in the presence of a catalytic amount of 4 followed by in situ
reduction with NaBH4. In the last procedure (Procedure 4), the Pd/
C promoted N–O bond cleavage of oxazolidin-5-one 5b, which
quantitatively gives the corresponding known b-amino acid 6b, is
described.
13C NMR (100 MHz, CDCl3): d = 36.6, 59.1, 68.3, 127.2, 127.9,
128.1, 128.2, 128.5, 128.6, 135.5, 138.1, 157.4, 175.3.
HRMS (ESI): m/z [M + H2O + Na]+ calcd for C17H17NNaO5:
338.0999: found: 338.1002.
(–)-(3S)-3-[(tert-Butoxycarbonyl)(hydroxy)amino]-3-phenyl-
propan-1-ol (7a)
(–)-(3S,5R)-2-(Benzyloxycarbonyl)-3-phenylisoxazolidin-5-ol
(3b); Typical Procedure
To a stirred soln of the catalyst 4 (16 mg, 20 mol%) in CHCl3 (1 mL)
was added a,b-unsaturated aldehyde 2a (33 mg, 0.25 mmol) and 1a
(40 mg, 0.3 mmol). The reaction was vigorously stirred at r.t. for 4
h. Next, the mixture was diluted with MeOH (1 mL) and cooled to
0 °C followed by addition of NaBH4 (19 mg, 0.5 mmol). The mix-
ture was then stirred for 10 min, quenched with 1 M HCl, and ex-
tracted with EtOAc. The organic layer was separated and dried
(Na2SO4) and the solvent was removed. The residue was purified by
column chromatography (silica gel, pentane–EtOAc, 4:1) to give 7a
(58 mg, 87%).
To a stirred soln of catalyst 4 (20 mol%) in CHCl3 (0.5 mL) at 4 °C
was added a,b-unsaturated aldehyde 2a (33 mg, 0.25 mmol) and hy-
droxycarbamate 1b (55 mg, 0.3 mmol). The reaction was vigorous-
ly stirred for 3 h. Next, the mixture was directly loaded upon a
column and immediate subjected to chromatography (silica gel,
pentane–EtOAc mixtures or toluene–EtOAc mixtures) to furnish
the pure 5-hydroxyisoxazolidine 3b (70 mg, 94%) as a clear oil.
The enantiomeric excess value was determined by HPLC with an
Daicel Chiralpack ODH column (n-hexene–i-PrOH, 98.0:2.0), 1.0
mL/min): tR = 35.9 (major isomer), 30.8 min (minor isomer).
[a]D25 –52.0 (c 0.5, CHCl3).
1H NMR (300 MHz, CDCl3): d = 1.43 (s, 9 H), 2.02–2.11 (m, 1 H),
2.36–2.47 (m, 1 H), 3.76–3.81 (m, 2 H), 5.21 (dd, J = 10.8, 2.1 Hz,
1 H), 6.91 (br s, 1 H), 7.27–7.42 (m, 5 H).
13C NMR (75 MHz, CDCl3): d = 28.4, 34.2, 60.1, 60.3, 82.0, 127.4,
127.6, 128.5, 140.3, 157.2.
HRMS (ESI): m/z [M + Na]+ calcd for C14H21NNaO4: 290.1363;
found: 290.1355.
[a]D25 –22.2 (c 1.0, CHCl3).
1H NMR (400 MHz, CDCl3): d = 2.28–2.32 (m, 1 H), 2.78 (dd,
J = 8.4 Hz, J¢ = 12.8 Hz, 1 H), 5.18 (s, 2 H), 5.39 (t, J = 8.4 Hz, 1
H), 5.84 (d, J = 4.4 Hz, 1 H), 7.20–7.40 (m, 10 H).
13C NMR (100 MHz, CDCl3): d = 45.2, 61.3, 68.1, 98.7, 126.0,
127.4, 127.7, 128.1, 128.4, 128.6, 135.6, 141.4, 159.3.
Synthesis 2007, No. 7, 1153–1157 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Catalytic Enantioselective 5-Hydroxyisoxazolidine Synthesis
1157
(–)-(3S)-3-Phenyl-2-aminopropanoic Acid (6b)11
Chem. Soc. 2004, 126, 9188. (g) Yamagiwa, N.; Qin, H.;
Matunaga, S.; Shibasaki, M. J. Am. Chem. Soc. 2005, 127,
13419; and references therein. (h) Xu, L.-W.; Xia, C.-G.
Eur. J. Org. Chem. 2005, 633.
To a stirred soln of Cbz-protected isoxazolidinone 5b (149 mg, 0.5
mmol) in MeOH (5 mL, 0.1 M), was added 10% (in weight) of
Pd/C (10%). The reaction was stirred under H2 (91 bar) overnight.
Next, the crude reaction was filtered through a plug of Celite. The
solvent was removed under reduced pressure to afford the pure
b-amino acid 6b (83 mg, 100%).
[a]D25 –6.9 (c 1, H2O) {Lit.11 [a]D25 –6.9 (c 0.8, H2O)}.
1H NMR (400 MHz, D2O/K2CO3): d = 2.45–2.60 (m, 2 H), 4.27 (t,
J = 7.2 Hz, 1 H), 7.30–7.40 (m, 5 H).
(4) (a) Dalko, P. I.; Moisan, L. Angew. Chem. Int. Ed. 2001, 40,
3726. (b) List, B. Chem. Commun. 2006, 819. (c) Duthaler,
R. O. Angew. Chem. Int. Ed. 2003, 42, 975. (d) Dalko, P. I.;
Moisan, L. Angew. Chem. Int. Ed. 2004, 43, 5138.
(e) Enders, D.; Grondal, C.; Hüttl, M. R. M. Angew. Chem.
Int. Ed. 2007, 46, 1545.
(5) Selected examples see: (a) Halland, N.; Aburell, P. S.;
Jørgensen, K. A. Angew. Chem. Int. Ed. 2004, 43, 1272.
(b) Huang, Y.; Walji, A. M.; Larsen, C. H.; MacMillan, D.
W. C. J. Am. Chem. Soc. 2005, 127, 15051. (c) Yang, J. W.;
Hechavarria Fonseca, M. T.; List, B. J. Am. Chem. Soc.
2005, 127, 15036. (d) Marigo, M.; Schulte, T.; Franén, J.;
Jørgensen, K. A. J. Am. Chem. Soc. 2005, 127, 15710.
(e) Yamamoto, Y.; Momiyama, N.; Yamamoto, H. J. Am.
Chem. Soc. 2004, 126, 5962. (f) Ramachary, D. B.;
Chowdari, N. S.; Barbas, C. F. III. Angew. Chem. Int. Ed.
2003, 42, 4233. (g) Sundén, H.; Ibrahem, I.; Eriksson, L.;
Córdova, A. Angew. Chem. Int. Ed. 2005, 44, 4877.
(h) Enders, D.; Hüttl, M. R. M.; Grondal, C.; Raabe, G.
Nature 2006, 441, 861. For organocatalytic b-amino acid
synthesis, see: (i) Yang, J. W.; Stadler, M.; List, B. Angew.
Chem. Int. Ed. 2007, 46, 609. (j) Vesely, J.; Ibrahem, I.;
Rios, R.; Córdova, A. Tetrahedron Lett. 2007, 48, 421.
(6) For excellent reviews on the concept of domino and tandem
reactions see: (a) Tietze, L. F. Chem. Rev. 1996, 96, 115.
(b) Wasilke, J. C.; Obrey, S. J.; Baker, R. T.; Bazan, G. C.
Chem. Rev. 2005, 105, 1001.
13C NMR (100 MHz, D2O/K2CO3): d = 46.7, 53.1, 127.6, 128.7,
128.8, 128.9, 179.4.
Acknowledgment
We thank the Swedish National Research council and Medivir AB
for financial support.
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Synthesis 2007, No. 7, 1153–1157 © Thieme Stuttgart · New York