Organocatalytic synthesis of α-hydroxy and α-aminophosphonates

Article abstract of DOI:10.1016/j.tetlet.2008.08.094

A new and highly flexible procedure is described for the synthesis of α-amino- and α-hydroxy phosphonates. In the presence of a catalytic amount of oxalic acid (10 mol %), trimethyl phosphite reacts with aldehydes or imines (generated in situ from an aldehyde and an amine) to yield the corresponding coupled products in good yield.

Full text of DOI:10.1016/j.tetlet.2008.08.094

Tetrahedron Letters 49 (2008) 6501–6504
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Organocatalytic synthesis of
a-hydroxy and
a-aminophosphonates
b
b
Seyed Mohammad Vahdat a, , Robabeh Baharfar , Mahmood Tajbakhsh ,
*
Akbar Heydari c, Seyed Meysam Baghbanian d, Samad Khaksar a
a Chemistry Department, Islamic Azad University, Ayatollah Amoli Branch, PO Box 678, Amol, Iran
b Department of Chemistry, Mazandaran University, Babolsar 47415, Iran
c Chemistry Department, Tarbiat Modares University, PO Box 14115-175, Tehran, Iran
d Chemistry Department, Islamic Azad University, Rasht Branch, Pole-taleshan, PO Box 41325-3516, Rasht, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A new and highly flexible procedure is described for the synthesis of a-amino- and a-hydroxy phospho-
Received 27 May 2008
Revised 13 August 2008
Accepted 26 August 2008
Available online 30 August 2008
nates. In the presence of a catalytic amount of oxalic acid (10 mol %), trimethyl phosphite reacts with
aldehydes or imines (generated in situ from an aldehyde and an amine) to yield the corresponding
coupled products in good yield.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Organocatalyst
a
-Hydroxy phosphonate
Solvent-free
-Aminophosphonates
a
Organocatalysis1 has emerged as an important area of research
tion. However, these reactions cannot be carried out in a one-pot
operation with a carbonyl compound, an amine and a dialkyl phos-
phite because the amine and water present during imine formation
can decompose or deactivate the Lewis acid.11 This drawback has
been overcome in some recent methods using lanthanide tri-
flates/MgSO4,9 InCl3,12 ZrCl4,13 TaCl5–SiO2,14 bismuth nitrate
over the last decade. Compared with biocatalysts and metal
catalysts, organocatalysts are usually more stable, more environ-
mentally friendly, more readily available, less expensive and can
be applied using less demanding reaction conditions, such as
rigorously anhydrous or anaerobic conditions. Although organic
molecules such as amino acids, peptides, carbenes, ureas, and
phase-transfer agents have been used as catalysts in carbon–car-
bon, carbon–heteroatom bond-forming reactions, and 1,4 and 1,2
additions of trivalent P to carbonyl compounds,2 they are still
limited to specific reactions. Thus, new, more versatile organic
catalysts are highly desirable.3
pentahydrate,15 MgClO4,16 TiO2,17 Amberlite-IR 120,18 sulfamic
20
acid,19 H3PW12O40
,
trimethylanilinium chloride,21 lithium per-
chlorate,22 and Amberlyst-15.23 However, these catalysts have
various drawbacks: reactions require a long time, and when the
starting materials contain aliphatic amines, the reactions usually
give uncharacterizable products. In addition, some of these cata-
lysts are expensive or are difficult to prepare. It is well known that
oxalic acid is a relatively stable, easy to handle solid that is insen-
sitive to small amounts of air and moisture. Herein, we describe a
Since
a-aminophosphonate derivatives are structural mimics
of
a-amino acids, some of these compounds exhibit very high
potency in inhibiting enzymes that are involved in the metabolism
of the corresponding amino acids. These compounds have already
been found to act as antibacterial agents, neuroactive compounds,
anticancer drugs, and pesticides, with some of them being com-
mercialized.4 The altered activity of such enzymes has been associ-
ated with HIV infections and several pathological disorders,
including cancer and cataracts.5 A number of synthetic methods
mild and efficient protocol for the synthesis of
a-aminophospho-
nates using a catalytic amount of oxalic acid under solvent-free
conditions at 50 °C. We investigated the reaction between tri-
methyl phosphite and the imine generated in situ from benzalde-
hyde and aniline in the presence of a catalytic amount of oxalic
acid (10 mol %) and isolated the desired amino phosphonate in a
98% yield within 40 min at 50 °C. After this success, reactions of
several aldehydes (aliphatic, aromatic, heterocyclic, and conju-
gated), amines (primary and secondary), and trimethyl phosphite
were examined in the presence of 10 mol % oxalic acid under
solvent-free conditions. Benzaldehyde and electron-deficient
aromatic aldehydes reacted with aromatic and aliphatic amines
for
a
-amino phosphonates have been developed.6 Of these, the
nucleophilic addition of phosphites to imines, catalyzed by a base
or an acid, is the most convenient. Lewis acids such as SnCl4,7
10
BF3.OEt2,8 ZnCl2,9 and ZrCl4 have been used for this transforma-
* Corresponding author. Fax: +98 121 2552136.
to give the corresponding
a-aminophosphonates in high yields.
E-mail address: vahdat_mohammad@yahoo.com (S. M. Vahdat).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.08.094

6502
S. M. Vahdat et al. / Tetrahedron Letters 49 (2008) 6501–6504
Sensitive functionalities such as CN, MeO, and Cl were unaffected
during the reaction. In all cases, the reactions proceeded smoothly
OH
O
Oxalic acid (10 mol%)
Solvent-free, 80 ºC, 3 h
MeO
OMe
O
P
R1
at 50 °C to afford
a-aminophosphonates exclusively without any
P
+
R1
H
MeO
OMe
undesired side products. This method is even effective with
aliphatic aldehydes, which normally produce low yields due to
their intrinsic lower reactivity. The present method does not
require any additives or promoters, and the results are summa-
rized in Scheme 1.
OMe
1
5
R1
Phenyl
Entry
5 (%)
a
b
c
98
In assessing the catalytic activity of oxalic acid, we were pleased
to find that addition of trimethyl phosphite to aldehydes gave the
91
91
2-Cl-C6H4
4-Cl-C6H4
4-CN-C6H4
corresponding
a-hydroxy phosphonates in excellent yields. a-Hy-
droxy phosphonates have received attention as substrates for the
preparation of other
a-substituted phosphonates and because of
their potential biological activity.24 These compounds show antivi-
ral,25a antibacterial,25b antivaccinia,25c anticancer,25d pesticide,25e
renin inhibitory,25f HIV-protease,25g anti-HIV activities,25h,25i and
d
94
83
94
89
87
e
f
Cinnamyl
2-Furyl
enzyme inhibitor properties.25j Furthermore,
a
-hydroxy phospho-
nates are useful precursors for the preparation of -functionalized
phosphonates, such as -acet-
-amino,26 -keto,27 -halo,28 and
oxyphosphonates.29 Recently, numerous activators have been
developed for the synthesis of
-hydroxy phosphonates.30 How-
ever, these methods often have disadvantages. For example, in
the strongly alkaline medium used, -hydroxy-alkanephosphonic
a
a
a
a
a
g
4-MeO-C6H4
4-OH-C6H4
a
h
a
i
j
i-Propyl
Cyclohexyl
n-Propyl
n-Pentyl
Tolyl
87
90
90
87
84
esters are cleaved to regenerate the starting carbonyl com-
pounds.31 In addition, the yields are not always good and mixtures
of products are sometimes obtained. Hence, there is a need to
k
l
O
R3
R2
R1
R1
H
N
m
MeO
OMe Oxalic acid (10 mol%)
1
P
O
+
P
Scheme 2.
H
N
OMe
Solvent-free, 50 ºC, 2 h
MeO
OMe
3
R2
R3
4
develop a convenient and environmentally benign method for
the synthesis of -hydroxy phosphonates. Herein, we report our
results on the preparation of -hydroxy phosphonates from alde-
hydes and trimethyl phosphite in the presence of oxalic acid under
solvent-free conditions at 80 °C. The results are summarized in
Scheme 2.
When we treated benzaldehyde with trimethyl phosphite in the
absence of oxalic acid, only a low yield (<20%) of dimethyl 1-hydro-
xy-1-phenylmethylphosphonate was obtained, implying the role of
oxalic acid in this reaction. As shown in Scheme 2, reaction of a
mixture of aliphatic or aromatic aldehyde and trimethyl phosphite
in the presence of oxalic acid at 80 °C afforded the desired products
2
a
R2
Phenyl
R1
Phenyl
R3
Entry
4 (%)
a
a
H
98
91
H
H
b
c
2-Cl-C6H4
4-Cl-C6H4
4-CN-C6H4
Phenyl
Phenyl
91
94
d
e
Phenyl
Phenyl
Phenyl
Phenyl
H
H
83
94
89
Cinnamyl
2-Furyl
in good to high yields.
selectively the corresponding
a
,b-Unsaturated aldehydes also afforded
f
H
H
H
a
-hydroxy phosphonates in good
yield and with no byproduct formation. This reaction was per-
formed in organic solvents such as diethyl ether, CH2Cl2, CHCl3,
MeCN, THF, dioxane, and methanol, however low yields (<30%) of
g
4-MeO-C6H4
4-HO-C6H4
87
87
90
h
Phenyl
Phenyl
Benzyl
the a-hydroxy phosphonates were obtained. Ketones and trimethyl
H
H
phosphite did not react under these reaction conditions.
i
i-Propyl
In conclusion, the present procedure using oxalic acid as catalyst
provides an efficient one-pot synthesis of a-amino and a-hydroxy
j
Phenyl
phosphonates under solvent-free conditions. The advantages of this
procedure are operational simplicity, wide substrate scope, and
high yields. In many cases, the products crystallized directly from
the reaction mixture in high purity. We believe that this method
presents a practical alternative to existing procedures for the syn-
90
87
k
4-Cl-C6H4
Phenyl
Benzyl
Benzyl
H
Benzyl
l
m
n
Benzyl
2-Cl-C6H4
Benzyl
84
thesis of a-amino- and a-hydroxy phosphonates.
Typical procedure I: Solvent-free: a mixture of benzaldehyde
(106 mg, 1 mmol), oxalic acid (20 mg, 10 mol %), aniline (93 mg,
1 mmol), and trimethyl phosphite (152 mg, 1 mmol) was stirred
vigorously at 50 °C for approximately 2 h. The reaction mixture
was then diluted with water and extracted with CH2Cl2. The CH2Cl2
87
91
Phenyl
Phenyl
4-MeO-C6H4
4-Cl-C6H4
Scheme 1.
H
H
o

S. M. Vahdat et al. / Tetrahedron Letters 49 (2008) 6501–6504
6503
extract, after being washed with brine and drying over sodium
sulfate, was evaporated. The crude product was purified by silica
gel column chromatography with EtOAc/hexane (1:6) as eluent
(90 MHz, CDCl3): d 1.0 (t, J = 7.4 Hz, 3H), 1.8–1.1 (m, 4H), 2.7 (m,
3
3
1H), 3.7 (d, JPH = 5.4 Hz, 3H), 3.8 (d, JPH = 5.4 Hz, 3H), 3.9 (s, 1H,
4
OH); 13C NMR (22.5 MHz, CDCl3): d 13.8 (d, JPC = 19.1 Hz, CH3),
3
2
to provide pure
a-aminophosphonate (293 mg, 98%).
19.9 (d, JPC = 11.8 Hz, CH2), 29.0 (d, JPC = 7.4 Hz, CH2), 51.1 (d,
2
Typical procedure II: Solvent-free:
a
mixture of aldehyde
2JPC = 7.3 Hz, OCH3), 52.2 (d, JPC = 7.3 Hz, OCH3), 56.8 (d,
(2 mmol), oxalic acid (10 mol %), and trimethyl phosphite
(2.2 mmol) was stirred at 80 °C for 3 h. After completion of the
reaction, as indicated by TLC, the reaction mixture was quenched
with aq satd NaHCO3 followed by brine solution and then extracted
with CH2Cl2. The organic extracts were combined, dried (MgSO4),
and concentrated. The residue was purified by column chromato-
graphy on silica gel using hexane/ethyl acetate (4:1) to afford the
1JPC = 136.8 Hz, OCH3).
Acknowledgment
This research is supported by the Islamic Azad University, Aya-
tollah Amoli Branch.
pure a-hydroxy phosphonate.
The 1H (500 MHz) and 13C (125 MHz) NMR data reported below
for selected products were determined on a Bruker Avance DRX
500 spectrometer. The spectroscopic and physical data for all com-
pounds corresponded to those given in the literature: 4a,16,20 4b,17
4c,23 4d,17 4e,16,20 4f,16 4g,16 4h,23 4i,23 4j,19 4k,19 4l,22 4m,22 4n,23
4o,23 5a,30e,j 5b,30j 5c,30j 5d,30j 5e,30j 5f,30e 5g,30j 5h,30j 5i,30i 5j,30j
5k,30i 5l,30i 5m.30j
References and notes
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874; (b) Laschat, S.; Kunz, H. Synthesis 1992, 90–94; (c) Yokomatsu, T.; Yoshida,
Y.; Shibuya, S. J. Org. Chem. 1994, 59, 7930–7933.
Spectral data for selected products: Compound (4a): White solid,
mp 87 °C; 1H NMR (500 MHz, CDCl3): d 3.51 (d, J = 10.5 Hz, 3H),
3.81 (d, J = 10.6 Hz, 3H), 4.82 (d, J = 24 Hz, 1H), 4.84 (br s, 1H),
6.64 (d, J = 8.0 Hz, 2H), 6.74 (t, J = 7.2 Hz, 1H), 7.1 (t, J = 7.7 Hz,
2H), 7.3 (t, J = 7.5 Hz, 1H), 7.39 (t, J = 7.4 Hz, 2H), 7.5 (d, J = 7.3 Hz,
2H); 13C NMR (125 MHz, CDCl3): d 54.1 (d, Jp–c = 7.0 Hz, OCH3),
2
54.2 (2Jp–c = 6.8 Hz, OCH3), 56.2 (d, Jp–c = 150 Hz, CH), 68.59 (CH),
1
3
3
114.3 (CH), 119.0 (CH), 128.2 (d, Jp–c = 5.8 Hz, CH), 128.4 (d, Jp–c
=
=
2
3.1 Hz, CH), 129.1 (CH), 131.2 (CH), 136.0 (C), 146.6 (d, Jp–c
14.5 Hz, C). Compound (4c): White solid, mp 60 °C; 1H NMR
(500 MHz, CDCl3): d 3.51 (m, 1H), 3.79 (d, J = 11.8 Hz, 3H), 3.83
(d, J = 10.1 Hz, 3H), 5.2 (d, J = 24 Hz, 1H), 6.8–7.28 (m, 5H), 7.3 (d,
J = 8.5 Hz, 2H), 7.5 (d, J = 8.5 Hz, 2H); 13C NMR (22.5 MHz, CDCl3):
2
2
d 56.1 (d, Jp–c = 7.0 Hz, OCH3), 56.2 (d, Jp–c = 6.8 Hz, OCH3), 57.2
1
3
(d, Jp–c = 150 Hz, CH), 114.3 (CH), 120.0 (CH), 128.2(d, Jp–c
=
3
5.8 Hz, CH), 128.4 (d, Jp–c = 3.1 Hz, CH), 130.1 (CH), 131.2 (C),
2
140.0 (C), 146.6 (d, Jp–c = 14.5 Hz, C). Compound (4f): Viscous yel-
lowish oil; 1H NMR (500 MHz, CDCl3): d 3.6 (d, J = 10.6 Hz, 3H), 3.8
(d, J = 10.6 Hz, 3H), 4.5 (br s, 1H), 5.0 (d, J = 23.8 Hz, 1H), 6.37–7.40
(m, 8H); 13C NMR (125 MHz, CDCl3): d 50 (d, Jp-c = 159.6 Hz, CH),
1
2
2
54.1 (d, Jp-c = 5.8 Hz, OCH3), 54.4 (d, Jp-c = 6.9 Hz, OCH3), 109.4
7. Laschat, S.; Kunz, H. Synthesis 1992, 90–94.
8. Ha, H. J.; Nam, G. S. Synth. Commun. 1992, 1143–1148.
9. Zon, J. Pol. J. Chem. 1981, 55, 643–646.
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2277–2280.
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3
(d, Jp-c = 6.8 Hz, CH), 111.2 (CH), 114.4 (CH), 119.5 (CH), 129.9
3
2
(d, Jp–c = 5.6 Hz, CH), 143.1 (CH), 146.3 (d, Jp-c = 13.3 Hz,
C),149.4 (C). Compound (4h): Viscous yellowish oil; 1H NMR
(90 MHz, CDCl3):
d 3.49 (d, 3H, J = 10.5 Hz), 3.71 (d, 3H,
1
J = 10.6 Hz), 4.71–4.79 (d, 1H, JP-H = 23.9 Hz), 6.5–7.25 (m, 9H);
13C NMR (22.5 MHz, CDCl3): d 51.9 (CH), 54.03 (OMe), 54.3
(OMe), 114.0 (CH), 121.9 (CH), 125.7 (CH), 129.3 (CH), 130.1
(CH), 131.2 (C), 135.0 (C), 149.2 (C).
Compound (5a): White solid, mp 86 °C; 1H NMR (500 MHz,
CDCl3): d 3.6 (d, J = 10.3 Hz, 3H), 3.6 (d, J = 10.3 Hz, 3H), 5.0 (d,
1H, J = 13.2 Hz), 6.0 (s, 1H, OH), 7.3–7.5 (m, 5H); 13C NMR
(125 MHz, CDCl3): d 53.7 (d, JCP = 7.5 Hz), 54.2 (d, JCP = 7.5 Hz),
69.1 (d, JCP = 164.0 Hz), 128.8, 129.4, 131.1, 133.8 (d, JCP = 2.9 Hz).
Compound (5c): White solid, mp 69 °C; 1H NMR (500 MHz, CDCl3):
d 3.6–3.7 (m, 6 H), 5.1 (d, J = 13.4 Hz, 1H), 6.2 (s, 1H, OH), 7.4 (d,
J = 8.5 Hz, 2H), 7.5 (d, J = 8.5 Hz, 2H); 13C NMR (125 MHz, CDCl3):
d 53.7 (d, JCP = 7.1 Hz), 54.2 (d, JCP = 7.5 Hz), 69.1 (d, JCP = 161.1 Hz),
128.8, 129.4, 133.1 (d, JCP = 3.9 Hz), 138.2. Compound (5e): Color-
less solid, mp 82.6–83.3 °C; 1H NMR (500 MHz, CDCl3): d 3.8 (d,
JPH = 10.3 Hz, 3H), 3.8 (d, JPH = 10.3 Hz, 3H), 4.7 (s, 1H, OH), 4.8
(dd, J = 6.4, 15.9 Hz, 1H), 6.4 (dd, J = 6.4, 14 Hz, 1H), 6.8 (d,
J = 14 Hz, 1H), 7.2–7.4 (m, 5H); 13C NMR (500 MHz, CDCl3): d
53.7 (d, JPC = 7.4 Hz), 53.9 (d, JPC = 7.1 Hz), 69.2 (d, JPC = 161.0 Hz),
128.4, 127.8, 126.5, 123.5 (d, JPC = 4.3 Hz), 132.2 (d, JPC = 13.0 Hz),
136.1 (d, JPC = 2.9 Hz). Compound (5k): Colorless oil; 1H NMR
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