A new and convenient asymmetric synthesis of α-amino- and α-alkyl-α-aminophosphonic acids using N-tert-butylsulfinyl imines as chiral auxiliaries

Article abstract of DOI:10.1055/s-2007-990872

Nucleophilic addition of dialkyl phosphites to N-tert-butylsulfinyl aldimines or ketimines occurs successfully at room temperature with potassium carbonate as base to afford α-amino- and α-alkyl-α-amino-N- (tert-butylsulfinyl)phosphonates in good to excel

Full text of DOI:10.1055/s-2007-990872

PAPER
3779
A New and Convenient Asymmetric Synthesis of a-Amino- and a-Alkyl-
a-aminophosphonic Acids Using N-tert-Butylsulfinyl Imines as Chiral
Auxiliaries
New
Asymmetric
i
S
ynthe
a
a
-A
m
n
ino- and
a
-A
y
lkyl-
a
-amin
i
ophosphonic
C
A
cids hen, Chengye Yuan*
State Key Laboratory of Bio-organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 354 Lingling Lu, Shanghai 200032, P. R. of China
Fax +86(21)54925379; E-mail: yuancy@mail.sioc.ac.cn
Received 16 August 2007; revised 11 September 2007
sulfinyl imines have found wide application in the asym-
metric synthesis of chiral molecules.
Abstract: Nucleophilic addition of dialkyl phosphites to N-tert-bu-
tylsulfinyl aldimines or ketimines occurs successfully at room tem-
perature with potassium carbonate as base to afford a-amino- and a-
alkyl-a-amino-N-(tert-butylsulfinyl)phosphonates in good to excel-
lent chemical yield and diastereoselectivity. The major diastereo-
mers were separated and smoothly converted into enantiopure a-
amino- and a-alkyl-a-aminophosphonic acids.
More recently, Ellman introduced N-tert-butylsulfinyl
imines as nucleophilic addition acceptors. The N-tert-bu-
tylsulfinyl group activates the imines for the nucleophilic
addition, serves as a powerful chiral directing group, and
after the addition reaction is readily cleaved upon treat-
ment of the product with acid. Most importantly, compet-
itive nucleophilic attack at sulfur should be minimized for
addition to N-tert-butylsulfinyl versus N-p-tolylsulfinyl
imines due to the greater steric hindrance and reduced
electronegativity of the tert-butyl group relative to the p-
tolyl moiety.14
Key words: a-aminophosphonic acids, a-alkyl-a-aminophosphon-
ic acids, N-tert-butylsulfinyl imines, asymmetric synthesis, nucleo-
philic addition
Optically active a-aminophosphonic acids are compounds
with significant biological activities which have found
widespread use as surrogates for a-amino acids and en-
zyme inhibitors. They also show antibacterial and antifun-
gal activities. It is well known that the biological activities
of a-aminophosphonic acids depend strongly on the abso-
lute configuration of the stereogenic carbon atom bearing
the amino group.1 Therefore, investigations aimed at con-
venient and effective routes leading to enantiopure a-ami-
nophosphonic acids and their derivatives have received
considerable attention. The methods so far available for
the preparation of such molecules include direct resolu-
tions, both chemical and enzymatic,2–4 and asymmetric
synthesis using either a chiral auxiliary or an organocata-
lyst.5 In fact, the asymmetric addition of dialkyl or trialkyl
phosphites to chiral aldimines or ketimines has proved to
be one of the facile approaches. Gilmore and McBride re-
ported the first synthesis of optically active a-aminophos-
phonic acids in 1972 using a diastereoselective addition of
diethyl phosphite to chiral imines.6 As reported by our
group, such an asymmetric nucleophilic addition is re-
markably influenced by the electronic and steric effects of
the substrates.7 Recently, asymmetric synthesis using N-
tolylsulfinyl imines as chiral auxiliary has aroused the in-
terest of organic chemists.8 These enantiopure sulfin-
imines, pioneered by Davis, are versatile chiral imine
building blocks which have been employed in the asym-
metric synthesis of amines,9 a- and b-amino acids,10 a-
and b-aminophosphonates,11,12 and heterocycles.13 These
experimental data demonstrate that enantiopure N-tolyl-
Nevertheless, asymmetric synthesis of a- and b-amino-
phosphonic acids has been reported based on the nucleo-
philic addition of dialkyl phosphites to N-tolylsulfinyl
aldimines or ketimines;11b herein we describe our results
using N-tert-butylsulfinyl aldimines and ketimines as
chiral auxiliaries in order to compare the chemical behav-
ior of these two sulfinimine types during the nucleophilic
addition reactions of dialkyl phosphites.
The required (S)-N-tert-butylsulfinyl imine 1 or 3 was
readily prepared by adding aldehyde or ketone to (S)-(–)-
tert-butanesulfinamide in the presence of 2 equivalents of
titanium(IV) ethoxide at room temperature to 60 °C in tet-
rahydrofuran.15 Optically active a-aminophosphonates
were obtained by the reaction of imines 1 with dialkyl
phosphites 2 under various reaction conditions. For opti-
mization of the reaction, the structural effect of the corre-
sponding aldimine or ketimine and dialkyl phosphite, as
well as the influence of various bases on the diastereose-
lectivity of this nucleophilic addition was briefly exam-
ined (Table 1).
As indicated in Table 1, the base used in the addition has
a crucial impact on the diastereoselectivity of the reaction.
When dimethyl or diethyl phosphite was employed, the a-
(tert-butylsulfinylamino)phosphonates 4a were obtained
in excellent chemical yield but the diastereoselectivity
was poor (Table 1, entries 1–3). Since the reaction pro-
ceeded very rapidly even at –78 °C (<1 h) in the presence
of LiHMDS, it is obvious that LiHMDS is not a suitable
base for use in this reaction. To our delight, the reaction
was successful when potassium carbonate was employed;
the diastereoselectivity improved dramatically with the de
of the reaction increasing from 38.9% to 81.8% (Table 1,
SYNTHESIS 2007, No. 24, pp 3779–3786
x
x.
x
x
.
2
0
0
7
Advanced online publication: 31.10.2007
DOI: 10.1055/s-2007-990872; Art ID: F14907SS
© Georg Thieme Verlag Stuttgart · New York

3780
Q. Chen, C. Yuan
PAPER
Table 1 Screening Dialkyl Phosphites and Optimization of the Reaction Conditions
O
O
S
HN
S
base
N
H
OR
OR
OR
OR
+
P
Ph
P
solvent
O
Ph
O
(Ss)-1a
(Ss,R)-4a
2
Entry
R
Base
Temp (°C)
–78 °C
–78 °C
0 °C
0 °C
r.t.
Solvent
THF
Time (h)
Yielda (%)
deb (%)
1
2
Et
LiHMDS
LiHMDS
CaH2
1
1
85
37.6
38.9
40.4
–
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
THF
83
3
THF
3
80
4
Et3N
Et2O
10
30
30
30
30
15
20
30
0
5
KF
Et2O
<10% conversion
–
6
CsF
r.t.
Et2O
<30% conversion
–
7
Li2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
r.t.
Et2O
<10% conversion
–
8
r.t.
Et2O
<30% conversion
–
9
r.t.
Et2O
85
85
81
69.4
70.9
81.8
10
11
r.t.
benzene
CH2Cl2
r.t.
a Isolated total yield of two isomers.
b From the 31P NMR spectrum of the crude reaction mixture.
entry 2 vs 11). The solvent also has an influence on the re- the reaction mixtures, except for entry 14 (72.4% de)
action rate and the diastereoselectivity (Table 1, entries 9– probably because of a 6.5:1 ratio of E/Z-isomers in the
11). The best result was obtained when potassium carbon- starting (–)-3g (R1 = Et) where phosphite addition to both
ate was used and the reaction was carried out at room tem- isomers results in the lower diastereoselectivity.13
perature using dichloromethane as solvent.
During the course of the reaction, the potassium cation is
The optimized protocol was then expanded to a variety of probably chelated to the sulfinyl and phosphonate oxy-
sulfinimines derived from aldimines and ketimines. The gens. Nucleophilic attack of the phosphite anion takes
results are summarized in Table 2.
place from the least hindered n-face (anti to the tert-butyl
group at sulfur, TS-1) (Figure 1).12
The data in Table 2 indicate that the reaction of N-tert-bu-
tylsulfinyl ketimines generally results in a much higher
diastereoselectivity than reaction of the corresponding al-
dimines (Table 2, entries 8–16 vs 1–7). The electronic ef-
fect of the R group in either aldimine or ketimine has no
significant effect on either the chemical yield or diaste-
reoselectivity.
OMe
P
O
–
OMe
K
O
S
Me(H)
R
N
TS - 1
Thus, the optimized protocol was applied to the asymmet-
ric synthesis of quaternary a-aminophosphonates using N-
sulfinyl ketimines. The phosphorus analogues of quater-
nary a-amino acids are of considerable value because it
has been found that incorporation of such molecules into
peptides results in increased rigidity and resistance to pro-
tease enzymes and therefore enhanced bioactivity.15 Fair-
ly good diastereoselectivities were obtained not only for
aromatic but also for aliphatic N-sulfinyl ketimines in the
presence of potassium carbonate at room temperature
(Table 2, entries 8–16). The electronic effect of the R
group has no significant influence on the selectivity as
only one isomer was detected in the 31P NMR spectra of
Figure 1
The major diastereomers of the (tert-butylsulfinylami-
no)methylphosphonates 4a–g were isolated by flash col-
umn chromatography on silica gel. Then, the N-tert-
butylsulfinyl group in 4a, 4d, 4g, and 5a was selectively
removed in excellent yield to give the corresponding
enantiopure benzyloxycarbonyl-protected aminometh-
ylphosphonates (R)-6a–d.
Free aminomethylphosphonic acids (R)-7 were obtained
upon refluxing enantiopure phosphonates 4 and 5 in 10 N
Synthesis 2007, No. 24, 3779–3786 © Thieme Stuttgart · New York

PAPER
New Asymmetric Synthesis of a-Amino- and a-Alkyl-a-aminophosphonic Acids
3781
Table 2 Addition of Dimethyl Phosphite to N-tert-Butylsulfinyl
Imines (S)-1,(S)-3
Table 3 Removal of the N-tert-Butylsulfinyl Groupa
O
O
S
R2
R1
NH2
O
2 HN
O
HN
O
(a) R2
(b)
R
(c)
(d)
S
OH
OH
S
OMe
R2
R1
HN
P
OMe
OMe
R1
P
H
OMe
OMe
R1
P
N
K2CO3 , r.t.
O
OMe
OMe
OMe
+
P
O
O
P
R1
R2
O
O
(SS,R)-4,5
(R)-7
(R)-6
R2 = H, Me
2
(SS,R)-4 (R2 = H)
(SS,R)-5 (R2 = Me)
SS-1 (R2 = H)
SS-3 (R2 = Me)
>99% ee
Entry
R1
R2
Yieldb 31P (d) [a]D
(%)
Configc
20
Entry
R1
R2
Solvent Time Yielda deb
(h)
CH2Cl2 30
CH2Cl2 30
CH2Cl2 20
CH2Cl2 20
CH2Cl2 20
CH2Cl2 30
CH2Cl2 30
(%)
81
78
81
82
80
79
77
85
82
85
81
83
80
73
75
73
(%)
81.8
85.2
80.2
72.4
77.0
85.1
86.9
>95
>95
>95
>95
>95
>95
72.4
>95
>95
1
2
6a
6b
6c
6d
7a
7b
7c
7d
7e
7f
Ph
H
84
85
81
83
81
83
88
84
85
83
84
24.41 10.6d
R
R
R
R
R
R
R
R
R
R
R
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
Ph
H
4-ClC6H4
t-Bu
Ph
H
23.86 14.0d
27.87 20.3d
27.14 20.1d
18.06 20.9e
21.60 51.1e
21.25 53.9e
20.57 82.6e
18.59 50.7e
21.38 75.9e
4-MeOC6H4
4-MeC6H4
4-ClC6H4
Et
H
3
H
H
4
Me
H
H
5
Ph
H
6
Ph
Me
Me
i-Pr
H
7
4-ClC6H4
4g
5a
5b
t-Bu
H
8
4-NO2C6H4 Me
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
24
24
20
20
24
24
9
1-C10H7
4-PhC6H4
Et
Me
Me
Me
4-MeC6H4
4-ClC6H4
4-NO2C6H4
1-C10H7
4-PhC6H4
Et
10
11
10 5c
11 5d
12 5e
13 5f
14 5g
15 5h
16 5i
7g
16.71
0.1e
a Reaction conditions: (a) 4 N HCl, MeOH, r.t., 6 h; (b) 5% NaHCO3,
CH2Cl2, CbzCl, 0 °C, 4 h; (c) 10 N HCl, reflux, 18 h; (d) propylene
oxide, EtOH.
b Isolated yield.
c Established based on a comparison of the sign of the optical rotation
with that reported in the literature.
CH2Cl2 30
CH2Cl2 30
CH2Cl2 40
d Determined in CHCl3.
n-Bu
e Determined in 1 N NaOH.
t-Bu
a Isolated yield of the mixture of diastereomers.
b From the 31P NMR spectrum of the crude reaction mixture.
imines where such additions have to be carried out under
rather complicated reaction conditions, i.e. at –78 °C in
the presence of LiHMDS as base. Studies on the fields of
application of such a reaction system are now ongoing in
our laboratory.
hydrochloric acid and isolation in the usual manner by the
addition of propylene oxide (Table 3).
The absolute configuration of the aminomethylphosphon-
ic acids 7 thus obtained was established based on a com-
parison of the sign of the optical rotation with that
reported in the literature.14
All solvents used throughout these experiments were dried using
standard procedures. IR spectra were measured on a Shimadzu IR
440 spectrometer, 1H NMR spectra were recorded at 300 MHz with
CDCl3 as solvent (unless otherwise indicated) on a Bruker Avance
300 or a Varian Mercury 300 spectrometer, and 31P NMR spectra
were recorded at 120 MHz with CDCl3 as solvent on a Varian EM
390 or a Bruker AM 300 spectrometer (external 85% H3PO4). EI-
MS measurements were performed at 2010 eV on a Hewlett–Pack-
ard HP 5989A apparatus. HRMS data were recorded on a Finnigan
MAT 8430 spectrometer. Elemental analyses were conducted on a
Heraeus Rapid–CHNO apparatus. Optical rotation values were
measured on a Perkin–Elmer 241MC polarimeter.
In summary, we have shown that nucleophilic addition of
dialkyl phosphites to N-tert-butylsulfinyl aldimines or
ketimines, followed by subsequent deprotection and hy-
drolysis, constitutes a new synthetic route leading to
chiral a-amino- and a-alkyl-a-aminophosphonic acids in
good to excellent chemical yield and enantioselectivity.
As found by us, N-tert-butylsulfinyl imines behave much
better as chiral Michael addition acceptors than N-tolyl-
sulfinyl imines. Furthermore, nucleophilic addition takes
place under milder conditions, i.e. at room temperature
with potassium carbonate as base, than for Davis’ sulfin-
(S)-N-tert-Butylsulfinyl Ketimines (S)-3; General Procedure
In a 20-mL, single-necked, round-bottom flask equipped with a
magnetic stirrer bar was placed (S)-(–)-tert-butanesulfinamide
(1.212 g, 10.0 mmol) in THF (5 mL), and then a mixture of the cor-
responding ketone (12 mmol) and Ti(OEt)4 (4.565 g, 20 mmol) was
Synthesis 2007, No. 24, 3779–3786 © Thieme Stuttgart · New York

3782
Q. Chen, C. Yuan
PAPER
added into the solution. The reaction mixture was then stirred at
60 °C for 6 h and finally was quenched with H2O (5 mL). At this
time EtOAc (10 mL) was added to the mixture. The H2O phase was
extracted with EtOAc (2 × 5 mL). The combined organic layers
were washed with brine (20 mL) and dried (Na2SO4). The solvent
was removed and the residue was subjected to column chromatog-
raphy on silica gel (PE–CH2Cl2, 2:1).
(S)-(–)-N-[1-(1,1¢-Biphenyl-4-yl)ethylidene]-tert-butanesulfin-
amide (3f, R1 = 4-PhC6H4)
Yield: 83%; yellow solid; mp 138–149 °C.
[a]D20 –89.05 (c 1.0, CHCl3).
IR (KBr): 2978, 1591, 1364, 1087, 767 cm–1.
1H NMR (CDCl3): d = 1.34 (s, 9 H), 2.80 (s, 3 H), 7.26 (m, 3 H),
7.61 (m, 4 H), 7.96 (m, 2 H).
ESI-MS: m/z = 300.2 [M + Na+].
(S)-(–)-N-(1-Phenylethylidene)-tert-butanesulfinamide (3a,
R1 = Ph)
Anal. Calcd for C18H21NOS: C, 72.20; H, 4.68; N, 7.07. Found: C,
72.21; H, 4.43; N, 6.99.
Yield: 85%.
1H NMR (CDCl3): d = 1.32 (s, 9 H), 2.77 (s, 3 H), 7.45 (m, 3 H),
7.89 (d, J = 7.5 Hz, 2 H).
(S)-N-(sec-Butylidene)-tert-butanesulfinamide (3g, R1 = Et)
Yield: 74%.
(S)-(–)-N-[1-(4-Methylphenyl)ethylidene]-tert-butanesulfin-
amide (3b, R1 = 4-MeC6H4)
Yield: 87%; yellow solid; mp 51–54 °C.
1H NMR (CDCl3): d = 0.63 (t, J = 7.8 Hz, 3 H), 0.76 (s, 9 H), 1.84
(s, 3 H), 1.96 (m, 2 H).
[a]D20 –11.1 (c 1.0, CHCl3).
IR (KBr): 2962, 1596, 1567, 1277, 1186, 1068, 823 cm–1.
(S)-N-(Hex-2-ylidene)-tert-butanesulfinamide (3h, R1 = n-Bu)
Yield: 75%.
1H NMR (CDCl3): d = 1.30 (s, 9 H), 2.38 (s, 3 H), 2.73 (s, 3 H), 7.20
(d, J = 8.4 Hz, 2 H), 7.78 (d, J = 8.1 Hz, 2 H).
ESI-MS: m/z = 238.1 [M + H+].
1H NMR (CDCl3): d = 0.47 (m, 3 H), 0.78 (s, 9 H), 0.90 (m, 2 H),
1.13 (m, 2 H), 1.86 (s, 3 H), 1.95 (m, 2 H).
(S)-N-(3,3-Dimethylbut-2-ylidene)-tert-butanesulfinamide (3i,
Anal. Calcd for C13H19NOS: C, 65.78; H, 8.07; N, 5.90. Found: C,
65.40; H, 8.24; N, 5.70.
R1 = t-Bu)
Yield: 71%.
1H NMR (CDCl3): d = 1.32 (s, 9 H), 2.77 (s, 3 H), 2.81 (m, 3 H).
(S)-(–)-N-[1-(4-Chlorophenyl)ethylidene]-tert-butanesulfin-
amide (3c, R1 = 4-ClC6H4)
Yield: 86%; yellow solid; mp 84–86 °C.
[a]D20 –67.5 (c 0.5, CHCl3).
IR (KBr): 2964, 1687, 1589, 1262, 1095, 1013, 832 cm–1.
1H NMR (CDCl3): d = 1.26 (s, 9 H), 2.75 (s, 3 H), 7.96 (d, J = 8.4
Hz, 2 H), 8.19 (d, J = 8.7 Hz, 2 H).
ESI-MS: m/z = 258.2 [M + H+].
Dimethyl (SS,R)-(+)-(tert-Butylsulfinylamino)methylphos-
phonates 4,5; General Procedure
In a 15-mL round-bottom flask fitted with a magnetic stirrer bar was
placed a soln of dimethyl phosphite (0.44 g, 4 mmol) in CH2Cl2 (10
mL), and K2CO3 (0.690 g, 5 mmol) was added. The reaction mix-
ture was stirred for 0.25 h at r.t., then N-tert-butylsulfinyl imine (S)-
1 or (S)-3 (1.0 mmol) was added. After being stirred for 15–40 h at
r.t., the reaction mixture was quenched with sat. aq NH4Cl (5 mL),
which was followed by extraction with CH2Cl2 (2 × 5 mL). The or-
ganic layer was successively washed with H2O (5 mL) and brine (5
mL), then dried (Na2SO4). The solvent was removed and the residue
was subjected to column chromatography on silica gel (acetone–
EtOAc, 1:2).
Anal. Calcd for C12H16ClNOS: C, 55.91; H, 6.26; N, 5.43. Found:
C, 55.92; H, 6.22; N, 5.23.
(S)-(–)-N-[1-(4-Nitrophenyl)ethylidene]-tert-butanesulfin-
amide (3d, R1 = 4-NO2C6H4)
Yield: 83%; colorless oil.
[a]D20 –136.3 (c 1.4, CHCl3).
IR (KBr): 2958, 1672, 1582, 1256, 1085, 1015, 829 cm–1.
Dimethyl (SS,R)-(+)-(tert-Butylsulfinylamino)(phenyl)meth-
ylphosphonate (4a)
Yield: 81%; 81.8% de; colorless oil.
1H NMR (CDCl3): d = 1.23 (s, 9 H), 2.66 (s, 3 H), 7.30 (d, J = 8.4
Hz, 2 H), 7.72 (d, J = 8.7 Hz, 2 H).
ESI-MS: m/z = 269.3 [M + H+].
[a]D20 +99.5 (c 1.0, CHCl3).
IR (KBr): 3219, 2958, 1456, 1250, 1032, 834, 556 cm–1.
1H NMR (CDCl3): d = 1.17 (s, 9 H), 3.45 (d, JHP = 10.5 Hz, 3 H),
3.77 (d, JHP = 10.8 Hz, 3 H), 4.08 (m, 1 H), 4.65 (m, 1 H), 7.30–7.49
(m, 5 H).
Anal. Calcd for C12H16N2O3S: C, 53.71; H, 6.01; N, 10.44. Found:
C, 53.65; H, 6.18; N, 10.33.
31P NMR (CDCl3): d = 24.05.
ESI-MS: m/z = 342.1 [M + Na+].
(S)-(–)-N-[1-(1-Naphthyl)ethylidene]-tert-butanesulfinamide
(3e, R1 = 1-C10H7)
Yield: 88%; yellow solid; mp 120–122 °C.
[a]D20 –16.7 (c 1.0, CHCl3).
Anal. Calcd for C13H22NO4PS: C, 48.89; H, 6.94; N, 4.39. Found:
C, 48.71; H, 7.00; N, 4.07.
IR (KBr): 2956, 1689, 1592, 1258, 1091, 1019, 827 cm–1.
1H NMR (CDCl3): d = 1.36 (s, 9 H), 2.89 (s, 3 H), 7.56 (m, 2 H),
7.84–8.06 (m, 5 H).
Dimethyl (SS,R)-(+)-(tert-Butylsulfinylamino)(4-methoxyphe-
nyl)methylphosphonate (4b)
Yield: 78%; 85.2% de; colorless oil.
ESI-MS: m/z = 274.2 [M + H+].
[a]D20 +124.6 (c 0.8, CHCl3).
IR (KBr): 3186, 2958, 1447, 1243, 1018, 837, 578 cm–1.
Anal. Calcd for C16H19NOS: C, 70.29; H, 7.00; N, 5.12. Found: C,
70.18; H, 6.99; N, 4.79.
1H NMR (CDCl3): d = 1.14 (s, 9 H), 3.40 (d, JHP = 10.5 Hz, 3 H),
3.53 (d, JHP = 10.5 Hz, 3 H), 3.73 (s, 3 H), 4.00 (m, 1 H), 4.68 (d,
J = 18.0 Hz, 1 H), 6.83 (m, 2 H), 7.28 (m, 2 H).
Synthesis 2007, No. 24, 3779–3786 © Thieme Stuttgart · New York

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New Asymmetric Synthesis of a-Amino- and a-Alkyl-a-aminophosphonic Acids
3783
31P NMR (CDCl3): d = 23.79.
ESI-MS: m/z = 372.1 [M + Na+].
HRMS: m/z calcd for C14H24NO5PSNa [M + Na+]: 372.1015; found:
372.1005.
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)-2,2-dimethyl-
propylphosphonate (4g)
Yield: 77%; 86.9% de; colorless oil.
[a]D20 +63.19 (c 1.0, CHCl3).
IR (KBr): 3486, 2958, 1473, 1237, 1081, 830, 577 cm–1.
Dimethyl (SS,R)-(+)-(tert-Butylsulfinylamino)(4-methylphe-
nyl)methylphosphonate (4c)
Yield: 81%; 80.2% de; colorless oil.
[a]D20 +117.3 (c 1.0, CHCl3).
IR (KBr): 3186, 2958, 1447, 1243, 1018, 837, 578 cm–1.
1H NMR (CDCl3): d = 1.15 (s, 9 H), 2.26 (s, 3 H), 3.51 (d, JHP = 10.5
Hz, 3 H), 3.77 (d, JHP = 10.2 Hz, 3 H), 4.06 (m, 1 H), 4.68 (d,
J = 18.3 Hz, 1 H), 7.11 (m, 2 H), 7.25 (m, 2 H).
1H NMR (CDCl3): d = 0.77 (s, 9 H), 0.98 (s, 9 H), 3.00 (m, 1 H),
3.49 (d, JHP = 8.7 Hz, 3 H), 3.53 (d, JHP = 8.4 Hz, 3 H), 3.65 (m, 1
H).
31P NMR (CDCl3): d = 28.01.
ESI-MS: m/z = 322.1 [M + Na+].
HRMS: m/z calcd for C11H26NO4PSNa [M + Na+]: 322.1216; found:
322.1212.
31P NMR (CDCl3): d = 23.37.
ESI-MS: m/z = 356.1 [M + Na+].
Dimethyl (Ss,R)-(+)-1-(tert-Butylsulfinylamino)-1-phenylethyl-
phosphonate (5a)
Yield: 85%; >95% de; colorless oil.
[a]D20 +67.8 (c 1.0, CHCl3).
IR (KBr): 3480, 2958, 1458, 1244, 1032, 830, 570 cm–1.
Anal. Calcd for C14H24NO4PS: C, 50.44; H, 7.26; N, 4.20. Found:
C, 50.96; H, 7.58; N, 3.91.
Dimethyl (SS,R)-(+)-(tert-Butylsulfinylamino)(4-chlorophe-
nyl)methylphosphonate (4d)
Yield: 82%; 72.4% de; colorless oil.
[a]D20 +119.1 (c 1.0, CHCl3).
IR (KBr): 3219, 2958, 1456, 1250, 1032, 834, 556 cm–1.
1H NMR (CDCl3): d = 1.22 (s, 9 H), 3.57 (d, JHP = 10.5 Hz, 3 H),
3.74 (d, JHP = 11.7 Hz, 3 H), 4.05 (m, 1 H), 4.75 (m, 1 H), 7.25–7.47
(m, 4 H).
1H NMR (CDCl3): d = 1.26 (s, 9 H), 2.10 (d, J = 16.2 Hz, 3 H), 3.60
(d, JHP = 1.5 Hz, 3 H), 3.64 (d, JHP = 1.8 Hz, 3 H), 4.08 (m, 1 H),
7.38 (m, 3 H), 7.61 (m, 2 H).
31P NMR (CDCl3): d = 26.28.
ESI-MS: m/z = 356.1 [M + Na+].
Anal. Calcd for C14H24NO4PS: C, 50.44; H, 7.26; N, 4.20. Found:
C, 50.81; H, 7.32; N, 3.93.
31P NMR (CDCl3): d = 23.32.
ESI-MS: m/z = 376.2 [M + Na+].
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)-1-(4-methyl-
phenyl)ethylphosphonate (5b)
Yield: 82%; >95% de; colorless oil.
[a]D20 +85.2 (c 1.0, CHCl3).
IR (KBr): 3477, 2984, 1458, 1245, 1031, 833, 751, 564 cm–1.
Anal. Calcd for C13H21ClNO4PS: C, 44.13; H, 5.98; N, 3.96. Found:
C, 45.08; H, 6.19; N, 3.58.
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)propylphos-
phonate (4e)
Yield: 80%; 77.0% de; colorless oil.
[a]D20 +65.9 (c 1.0, CHCl3).
IR (KBr): 3443, 2959, 1461, 1241, 1055, 830, 565 cm–1.
1H NMR (CDCl3): d = 0.98 (m, 3 H), 1.18 (s, 9 H), 1.58–1.94 (m, 2
H), 3.40 (m, 1 H), 3.73 (d, JHP = 10.8 Hz, 3 H), 3.78 (d, JHP = 10.2
Hz, 3 H).
1H NMR (CDCl3): d = 1.25 (s, 9 H), 2.06 (d, J = 16.2 Hz, 3 H), 2.35
(d, J = 2.1 Hz, 3 H), 3.61 (s, 3 H), 3.65 (s, 3 H), 4.01 (m, 1 H), 7.18
(m, 2 H), 7.46 (m, 2 H).
31P NMR (CDCl3): d = 26.21.
MS (EI): m/z = 348 [M + H+].
HRMS: m/z calcd for C15H26NO4PSNa [M + Na+]: 370.1210; found:
370.1212.
31P NMR (CDCl3): d = 28.51.
ESI-MS: m/z = 294.1 [M + Na+].
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)-1-(4-chloro-
phenyl)ethylphosphonate (5c)
Yield: 85%; >95% de; colorless oil.
[a]D20 +85.1 (c 1.0, CHCl3).
IR (KBr): 3478, 2959, 1466, 1245, 1060, 840, 562 cm–1.
Anal. Calcd for C9H22NO4PS: C, 39.84; H, 8.17; N, 5.16. Found: C,
39.63; H, 8.06; N, 4.29.
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)-2-methylpro-
pylphosphonate (4f)
Yield: 79%; 85.1% de; colorless oil.
[a]D20 +66.0 (c 1.0, CHCl3).
IR (KBr): 3450, 2964, 1468, 1241, 1041, 831, 563 cm–1.
1H NMR (CDCl3): d = 0.92 (m, 6 H), 1.21 (s, 9 H), 2.18 (m, 1 H),
3.40 (m, 1 H), 3.72 (d, JHP = 10.8 Hz, 3 H), 3.80 (d, JHP = 10.8 Hz,
3 H).
1H NMR (CDCl3): d = 1.16 (s, 9 H), 2.00 (d, J = 16.2 Hz, 3 H), 3.55
(d, JHP = 3.9 Hz, 3 H), 3.59 (d, JHP = 3.6 Hz, 3 H), 4.00 (m, 1 H),
7.28 (m, 2 H), 7.45 (m, 2 H).
31P NMR (CDCl3): d = 25.42.
MS (EI): m/z = 368 [M + H+].
Anal. Calcd for C14H23ClNO4PS: C, 45.72; H, 6.30; N, 3.81. Found:
C, 45.69; H, 6.43; N, 3.37.
31P NMR (CDCl3): d = 28.06.
ESI-MS: m/z = 308.2 [M + Na+].
HRMS: m/z calcd for C10H24NO4PSNa [M + Na+]: 308.1059; found:
308.1056.
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)-1-(4-nitrophe-
nyl)ethylphosphonate (5d)
Yield: 81%; >95% de; colorless oil.
[a]D20 +65.3 (c 1.0, CHCl3).
IR (KBr): 3478, 2959, 1521, 1245, 1028, 857, 560 cm–1.
Synthesis 2007, No. 24, 3779–3786 © Thieme Stuttgart · New York

3784
Q. Chen, C. Yuan
PAPER
1H NMR (CDCl3): d = 1.20 (s, 9 H), 2.06 (d, J = 15.9 Hz, 3 H), 3.62
(d, JHP = 8.4 Hz, 3 H), 3.66 (d, JHP = 8.7 Hz, 3 H), 4.07 (m, 1 H),
7.75 (m, 2 H), 8.18 (m, 2 H).
31P NMR (CDCl3): d = 24.95.
MS (EI): m/z = 379 [M + H+].
1H NMR (CDCl3): d = 0.83 (m, 3 H), 1.15 (s, 9 H), 1.24–1.43 (m, 7
H), 1.70 (m, 2 H), 3.52 (m, 1 H), 3.75 (m, 6 H).
31P NMR (CDCl3): d = 29.80.
ESI-MS: m/z = 336.2 [M + Na+].
HRMS: m/z calcd for C12H29NO4PS [M + H+]: 314.1558; found:
314.1549.
HRMS: m/z calcd for C14H23N2O6PSNa [M + Na+]: 401.0905;
found: 401.0906.
Anal. Calcd for C12H28NO4PS: C, 45.99; H, 9.01; N, 4.47. Found:
C, 46.79; H, 9.22; N, 3.84.
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)-1-(1-naph-
thyl)ethylphosphonate (5e)
Yield: 83%; >95% de; colorless oil.
[a]D20 +76.8 (c 1.3, CHCl3).
Dimethyl (Ss,R)-(+)-1-(tert-Butylsulfinylamino)-1,2,2-tri-
methylpropylphosphonate (5i)
Yield: 73%; >95% de; colorless oil.
IR (KBr): 3450, 2958, 1702, 1459, 1365, 1242, 1056, 827, 561
cm–1.
1H NMR (CDCl3): d = 1.19 (s, 9 H), 2.13 (d, J = 15.9 Hz, 3 H), 3.56
(d, JHP = 10.5 Hz, 6 H), 4.02 (s, 1 H), 7.41 (m, 2 H), 7.65–7.80 (m,
4 H), 7.94 (s, 1 H).
31P NMR (CDCl3): d = 25.71.
ESI-MS: m/z = 406.2 [M + Na+].
[a]D20 +39.7 (c 1.0, CHCl3).
IR (KBr): 3484, 2958, 1470, 1236, 1071, 813, 759, 567 cm–1.
1H NMR (CDCl3): d = 1.01 (s, 9 H), 1.18 (s, 9 H), 1.49 (d, J = 16.8
Hz, 3 H), 3.73 (d, JHP = 10.5 Hz, 3 H), 3.79 (d, JHP = 10.2 Hz, 3 H).
31P NMR (CDCl3): d = 30.23.
MS (EI): m/z = 314 [M + H+].
HRMS: m/z calcd for C12H28NO4PSNa [M + Na+]: 336.1370; found:
336.1369.
HRMS: m/z calcd for C18H26NO4PSNa [M + Na+]: 406.1226; found:
406.1226.
Anal. Calcd for C18H26NO4PS: C, 56.38; H, 6.83; N, 3.65. Found:
C, 56.45; H, 7.43; N, 2.94.
Dimethyl (R)-(+)-(Benzyloxycarbonylamino)methylphos-
phonates 6; General Procedure
In a 20-mL, single-necked, round-bottom flask equipped with a
magnetic stirrer bar was placed phosphonate 4 or 5 (1.0 mmol) in
MeOH (5 mL), then 4 N HCl (5 mL) was added into the solution.
The reaction mixture was then stirred at r.t. for 6 h. After that, the
resulting mixture was concentrated under reduced pressure for 0.5
h. At this time, CH2Cl2 (10 mL) was added to the flask and the so-
lution was cooled to 0 °C in an ice bath. This was followed by the
addition of Et3N until pH 7. Then, 5% aq NaHCO3 (5 mL) and
CbzCl (0.20 g, 1.2 mmol) were added in sequence. The reaction
mixture was stirred at 0 °C for 4 h, which was followed by the ad-
dition of H2O (5 mL). The aqueous phase was extracted successive-
ly with Et2O (5 mL) and EtOAc (2 × 5 mL). The combined organic
layers were washed with brine (20 mL) and dried (Na2SO4). The
solvent was removed and the residue was subjected to column chro-
matography on silica gel (PE–EtOAc, 1:1).
Dimethyl (SS,R)-(+)-1-(1,1¢-Biphenyl-4-yl)-1-(tert-butylsulfinyl-
amino)ethylphosphonate (5f)
Yield: 80%; >95% de; colorless oil.
[a]D20 +97.2 (c 1.0, CHCl3).
IR (KBr): 3480, 2958, 1489, 1365, 1243, 1031, 847, 577 cm–1.
1H NMR (CDCl3): d = 1.19 (s, 9 H), 2.06 (d, J = 15.6 Hz, 3 H), 3.58
(d, JHP = 10.5 Hz, 6 H), 4.01 (s, 1 H), 7.30 (m, 3 H), 7.53 (m, 6 H).
31P NMR (CDCl3): d = 24.74.
ESI-MS: m/z = 432.3 [M + Na+].
HRMS: m/z calcd for C20H28NO4PSNa [M + Na+]: 432.1371; found:
432.1369.
Anal. Calcd for C20H28NO4PS: C, 58.66; H, 6.89; N, 3.42. Found:
C, 58.89; H, 6.18; N, 3.51.
Dimethyl (R)-(+)-(Benzyloxycarbonylamino)(phenyl)methyl-
phosphonate (6a)
Yield: 84%; >99% ee; white powder; mp 129–130 °C.
[a]D20 +10.6 (c 1.0, CHCl3).
IR (KBr): 3222, 3033, 2955, 1714, 1552, 1254, 1041, 698 cm–1.
1H NMR (CDCl3): d = 3.49 (d, JHP = 10.5 Hz, 3 H), 3.73 (d,
JHP = 11.1 Hz, 3 H), 5.08–5.17 (m, 3 H), 5.72 (m, 1 H), 7.26–7.41
(m, 10 H).
31P NMR (CDCl3): d = 24.41.
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)-1-methylpro-
pylphosphonate (5g)
Yield: 73%; 72.4% de; colorless oil.
[a]D20 +22.6 (c 1.0, CHCl3).
IR (KBr): 3467, 2957, 1461, 1238, 1035, 827, 560 cm–1.
1H NMR (CDCl3): d = 0.95 (t, J = 7.5 Hz, 3 H), 1.16 (s, 9 H), 1.42
(d, J = 15.6 Hz, 3 H), 1.80 (m, 2 H), 3.58 (m, 1 H), 3.73 (d,
JHP = 13.5 Hz, 3 H), 3.78 (d, JHP = 10.5 Hz, 3 H).
31P NMR (CDCl3): d = 30.25.
MS (EI): m/z = 286 [M + H+].
ESI-MS: m/z = 372.1 [M + Na+].
Anal. Calcd for C17H20NO5P: C, 58.45; H, 5.77; N, 4.01. Found: C,
58.72; H, 5.93; N, 3.61.
Anal. Calcd for C10H24NO4PS: C, 42.09; H, 8.48; N, 4.91. Found:
C, 42.26; H, 8.28; N, 4.51.
Dimethyl (R)-(+)-(Benzyloxycarbonylamino)(4-chloro-
phenyl)methylphosphonate (6b)
Yield: 85%; >99% ee; white powder; mp 119–120 °C.
[a]D20 +14.0 (c 1.0, CHCl3).
IR (KBr): 3242, 3038, 2957, 1720, 1547, 1250, 1031, 564 cm–1.
1H NMR (CDCl3): d = 3.55 (d, JHP = 10.5 Hz, 3 H), 3.73 (d,
JHP = 10.8 Hz, 3 H), 5.00–5.21 (m, 3 H), 5.68 (m, 1 H), 7.20–7.40
(m, 9 H).
Dimethyl (SS,R)-(+)-1-(tert-Butylsulfinylamino)-1-methyl-
pentylphosphonate (5h)
Yield: 75%; >95% de; colorless oil.
[a]D20 +45.9 (c 1.0, CHCl3).
IR (KBr): 3467, 2957, 1461, 1238, 1035, 827, 560 cm–1.
Synthesis 2007, No. 24, 3779–3786 © Thieme Stuttgart · New York

PAPER
New Asymmetric Synthesis of a-Amino- and a-Alkyl-a-aminophosphonic Acids
3785
31P NMR (CDCl3): d = 23.86.
ESI-MS: m/z = 406.0 [M + Na+].
[a]D20 +51.1 (c 1.0, 1 N NaOH).
IR (KBr): 3263, 2872, 1618, 1543, 1188, 1062, 927 cm–1.
Anal. Calcd for C17H19ClNO5P: C, 53.21; H, 4.99; N, 3.65. Found:
C, 53.55; H, 5.14; N, 3.31.
1H NMR (D2O, 0.5 N NaOH): d = 1.43 (d, J = 12.9 Hz, 3 H), 7.12–
7.39 (m, 5 H).
31P NMR (D2O, 0.5 N NaOH): d = 21.60.
Dimethyl (R)-(+)-1-(Benzyloxycarbonylamino)-2,2-dimethyl-
propylphosphonate (6c)
Yield: 81%; >99% ee; colorless oil.
ESI-MS: m/z = 200.2 [M – 1].
HRMS: m/z calcd for C8H12NO3PNa [M + Na+]: 224.0455; found:
[a]D20 +20.3 (c 1.0, CHCl3).
224.0447.
IR (KBr): 3285, 2960, 1714, 1534, 1239, 1035, 821, 700, 560 cm–1.
(R)-(+)-1-Amino-1-(4-chlorophenyl)ethylphosphonic Acid (7c)
Yield: 88%; white solid; mp 225–230 °C.
[a]D20 +53.9 (c 0.6, 1 N NaOH).
IR (KBr): 3273, 2868, 1615, 1541, 1189, 1060, 922 cm–1.
1H NMR (D2O, 0.5 N NaOH): d = 1.35 (d, J = 12.6 Hz, 3 H), 7.16
(s, 2 H), 7.25 (s, 2 H).
1H NMR (CDCl3): d = 1.06 (s, 9 H), 3.65 (d, JHP = 10.2 Hz, 3 H),
3.71 (d, JHP = 11.1 Hz, 3 H), 5.03–5.21 (m, 3 H), 5.32 (m, 1 H), 7.35
(m, 5 H).
31P NMR (CDCl3): d = 27.87.
ESI-MS: m/z = 352.1 [M + Na+].
Anal. Calcd for C15H24NO5P: C, 54.71; H, 7.35; N, 4.25. Found: C,
54.46; H, 7.31; N, 4.13.
31P NMR (D2O, 0.5 N NaOH): d = 21.25.
ESI-MS: m/z = 234.6 [M – 1].
HRMS: m/z calcd for C8H11ClNO3PNa [M + Na+]: 258.0058;
found: 258.0057.
Dimethyl (R)-(+)-1-(Benzyloxycarbonylamino)-1-phenyl-
ethylphosphonate (6d)
Yield: 83%; >99% ee; colorless oil.
[a]D20 +20.1 (c 1.0, CHCl3).
(R)-(+)-1-Amino-1-(4-nitrophenyl)ethylphosphonic Acid (7d)
Yield: 84%; white solid; mp 230–235 °C.
[a]D20 +82.6 (c 0.45, 1 N NaOH).
IR (KBr): 3282, 2834, 1606, 1516, 1191, 1064, 926 cm–1.
1H NMR (D2O, 0.5 N NaOH): d = 1.42 (d, J = 12.3 Hz, 3 H), 7.21
(s, 2 H), 7.33 (s, 2 H).
IR (KBr): 3240, 3036, 2957, 1715, 1548, 1251, 1029, 849, 564
cm–1.
1H NMR (CDCl3): d = 2.19 (d, J = 16.8 Hz, 3 H), 3.48 (d, JHP = 10.8
Hz, 3 H), 3.57 (d, JHP = 11.1 Hz, 3 H), 5.02 (s, 2 H), 5.17 (s, 1 H),
5.98 (m, 1 H), 7.31 (m, 7 H), 7.50 (m, 2 H).
31P NMR (CDCl3): d = 27.14.
ESI-MS: m/z = 386.1 [M + Na+].
31P NMR (D2O, 0.5 N NaOH): d = 20.57.
ESI-MS: m/z = 245.2 [M – 1].
Anal. Calcd for C18H22NO5P: C, 59.50; H, 6.10; N, 3.85. Found: C,
59.20; H, 5.94; N, 3.60.
(R)-(+)-1-Amino-1-(1-naphthyl)ethylphosphonic Acid (7e)
Yield: 85%; white solid; mp 244–247 °C.
[a]D20 +50.7 (c 0.75, 1 N NaOH).
IR (KBr): 3114, 2907, 1614, 1538, 1184, 1063, 928 cm–1.
1H NMR (D2O, 0.5 N NaOH): d = 1.49 (d, J = 12.6 Hz, 3 H), 7.35
(m, 2 H), 7.62–7.76 (m, 5 H).
(R)-(+)-Aminomethylphosphonic Acids 7; General Procedure
In a 25-mL, single-necked, round-bottom flask equipped with a
magnetic stirrer bar and a reflux condenser was placed phosphonate
(+)-4 or (+)-5 (0.3 mmol) in 10 N HCl (5 mL). The solution was re-
fluxed for 18 h, which was followed by removal of the low-boiling
fraction under high vacuum for 3 h. The residue was dissolved in a
minimum amount of hot EtOH (2 mL). The solution was cooled to
r.t., excess propylene oxide (1 mL) was then introduced, and the
mixture was stirred for 3 h. The white solid was collected by filtra-
tion to afford (+)-7.
31P NMR (D2O, 0.5 N NaOH): d = 18.59.
ESI-MS: m/z = 250.1 [M – 1].
HRMS: m/z calcd for C12H14NO3PNa [M + Na+]: 274.0616; found:
274.0603.
(R)-(+)-Amino(phenyl)methylphosphonic Acid (7a)
Yield: 81%; white solid; mp 274–275 °C.
(R)-(+)-1-Amino-1-(1,1¢-biphenyl-4-yl)ethylphosphonic Acid
(7f)
[a]D20 +20.9 (c 1.0, 1 N NaOH).
Yield: 83%; white solid; mp 260–262 °C.
[a]D20 +75.9 (c 0.75, 1 N NaOH).
IR (KBr): 2972, 1614, 1488, 1147, 1091, 765 cm–1.
IR (KBr): 3141, 2924, 2612, 2307, 1622, 1540, 1269, 1188, 1066,
917 cm–1.
1H NMR (D2O, 0.5 N NaOH): d = 3.69 (d, J = 16.5 Hz, 1 H), 7.25
(m, 5 H).
1H NMR (D2O, 0.5 N NaOH): d = 1.48 (d, J = 12.9 Hz, 3 H), 7.30–
7.64 (m, 9 H).
31P NMR (D2O, 0.5 N NaOH): d = 18.06.
31P NMR (D2O, 0.5 N NaOH): d = 21.38.
ESI-MS: m/z = 186.1 [M – 1].
ESI-MS: m/z = 276.2 [M – 1].
HRMS: m/z calcd for C7H10NO3PNa [M + Na+]: 210.0297; found:
210.0290.
(R)-(+)-1-Amino-1-methylpropylphosphonic Acid (7g)
Yield: 84%; white solid; mp 249–253 °C.
[a]D20 +0.1 (c 0.5, 1 N NaOH).
IR (KBr): 3238, 2837, 1544, 1185, 1064, 925 cm–1.
1H NMR (D2O, 0.5 N NaOH): d = 0.85 (t, J = 7.5 Hz, 3 H), 1.25 (d,
J = 13.2 Hz, 3 H), 1.69 (m, 2 H).
Anal. Calcd for C7H10NO3P: C, 44.93; H, 5.39; N, 7.48. Found: C,
44.33; H, 5.41; N, 7.13.
(R)-(+)-1-Amino-1-phenylethylphosphonic Acid (7b)
Yield: 83%; white solid; mp 250–255 °C.
Synthesis 2007, No. 24, 3779–3786 © Thieme Stuttgart · New York

3786
Q. Chen, C. Yuan
PAPER
31P NMR (D2O, 0.5 N NaOH): d = 16.71.
ESI-MS: m/z = 152.2 [M – 1].
HRMS: m/z calcd for C4H12NO3PNa [M + Na+]: 176.0453; found:
(6) Gilmore, W. F.; McBride, H. A. J. Am. Chem. Soc. 1972, 94,
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(8) Davis, F. A.; Chen, B.-C. Chem. Soc. Rev. 1998, 27, 13.
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119, 9913.
(10) (a) Davis, F. A.; Fanelli, D. L. J. Org. Chem. 1998, 63,
1981. (b) Davis, F. A.; Szewczyk, J. M. Tetrahedron Lett.
1998, 39, 5951.
176.0447.
Acknowledgment
This project was supported by the National Natural Science Found-
ation of China (Grant No. 20372706, 20672132).
(11) (a) Lefebvre, I. M.; Evans, S. A. J. Org. Chem. 1997, 62,
7532. (b) Mikołajczyk, M.; Łyżwa, P.; Drabowicz, J.
Tetrahedron: Asymmetry 1997, 8, 3991. (c) Davis, F. A.;
Lee, S.; Yan, H.; Titus, D. D. Org. Lett. 2001, 3, 1757.
(d) Mikołajczyk, M.; Łyżwa, P.; Drabowicz, J. Tetrahedron:
Asymmetry 2002, 13, 2571. (e) Davis, F. A.; Prasad, K. R. J.
Org. Chem. 2003, 68, 7249.
(12) Mikołajczyk, M.; Łyżwa, P.; Drabowicz, J.; Wieczorek, M.
W.; Błaszczyk, J. Chem. Commun. 1996, 1503.
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