Self-condensation reactions of acyl phosphonates: Synthesis of tertiary O-protected α-hydroxyphosphonates

Article abstract of DOI:10.3906/kim-1306-7

The self-condensation reaction of benzoyl dialkyl phosphonates was developed using cyanide ion as catalyst, affording versatile tertiary O-protected α-hydroxy phosphonates in moderate yield. TUeBITAK.

Full text of DOI:10.3906/kim-1306-7

Turk J Chem
Turkish Journal of Chemistry
(2014) 38: 164 – 171
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1306-7
Research Article
Self-condensation reactions of acyl phosphonates: synthesis of tertiary
O-protected α-hydroxyphosphonates
2
2,∗∗
Serkan EYMUR1,2,∗, Mehmet GOLLU , Ayhan Sıtkı DEMIR
¨
¨
˙
1Department of Chemistry, Sel¸cuk University, Konya, Turkey
2Department of Chemistry, Middle East Technical University, Ankara, Turkey
In memory of Prof Dr Ayhan S Demir (1950–2012)
Received: 04.06.2013
•
Accepted: 25.07.2013
•
Published Online: 16.12.2013
•
Printed: 20.01.2014
Abstract: The self-condensation reaction of benzoyl dialkyl phosphonates was developed using cyanide ion as catalyst,
affording versatile tertiary O-protected α-hydroxy phosphonates in moderate yield.
Key words: Acyl phosphonates, tertiary α-hydroxy phosphonates, phosphonate-phosphate rearrangement
1. Introduction
Hydroxy phosphonic acids and their ester derivatives have gained considerable attention due to their inhibitory
activity towards various important groups of enzymes. For example, they have inhibitory effects on renin,1,2
HIV protease,3 farnesyl protein transferase,4,5 (FPTase), and 5-enolpyruvylshikimate-3-phosphate (EPSP)
synthase.6 Many hydroxy phosphonic acid derivatives have also been reported to have a broad range of biological
activities such as antiviral,7 antitumor,8,9 and antibiotic properties.10,11 Due to their biological activities, a
great effort has been devoted to synthesize α-hydroxy phosphonates and derivatives in the last decade.12−14
A
variety of ways have been developed to obtain α-hydroxy phosphonates, such as phospho-aldol reaction,15−20
reduction of α-keto phosphonates,21−23 and oxidation of benzyl and vinyl phosphonates.24,25 Although these
methods are able to produce secondary α-hydroxy phosphonates, only a few methods have been described
to synthesize tertiary α-hydroxy phosphonates. Wiemer et al. reported that the addition of allyl indium
reagents to acyl phosphonates could provide tertiary α-hydroxy phosphonates.26 The Zhao group reported the
enantioselective synthesis of tertiary α-hydroxy phosphonates via aldol reaction.27,28 Moreover, the Demir group
reported the reaction of acyl phosphonates with trimethylsilyl cyanide to furnish the trimethylsilyloxycyano-
phosphonates.29
The Demir group and others have found that acyl phosphonates are new potent acyl anion precursors
and undergo nucleophile-promoted phosphonate-phosphate rearrangement to afford the resultant acyl anion
equivalents as reactive intermediates.30−33 The proposed mechanism is similar to the benzoin reaction mecha-
nism and its congeners. Cyanide ion-promoted rearrangement gives the acyl anion equivalent 3, which reacts
with aldehyde to provide the intermediate adduct 4, undergoing a [1,4]-O,O-phosphate migration leading to
retrocyanates to produce the corresponding benzoin product 6.
∗Correspondence: eymur@selcuk.edu.tr
∗∗
˙
Prof Dr Ayhan Sıtkı DEMIR passed away on 24 June 2012.
164

EYMUR et al./Turk J Chem
O
O
OEt
OEt
Ar2
OPO(OEt)2
Ar1
P
Ar1
O
1
CN
6
O
NC
Ar1
O
PO(OEt)2
CN
Ar2
Ar1
OPO(OEt)2
2
5
OPO(OEt)2
Ar2
NC
Ar1
OPO(OEt)2
Ar1
CN
O
4
3
O
Ar2
H
Scheme 1. Mechanism of cross-benzoin reaction via cyanide ion-promoted generation of acyl anions from acylphospho-
nates.
1.1. Results and discussion
As a general extension of earlier works,30−33 we proposed that in the absence of an aldehyde the acyl anion
equivalent generated from acyl phosphonate attacks another acyl phosphonate and produces O-protected
tertiary α-hydroxy-phosphonates. As a model transformation the cyanide ion catalyzed self-condensation
reaction of benzoyl dialkyl phosphonate was chosen. The results are summarized in Table 1.
Table 1. The optimization of reaction conditions for the synthesis of tertiary O-protected α-hydroxyphosphonates.
O
O
OPO(OR)2
PO(OR)2
OR
OR
KCN (X mol%)
T (oC), time
P
O
Entry
R
Solvent KCN (mole %) T (◦C) Time (h) Yield (%)
1
2
3
4
5
6
7
8
9
10
Et DMF
Et THF
Et THF
Et DMF
Et DMF
Me DMF
Me THF
Me DMF
Me DMF
Me DMF
20
20
30
30
30
20
30
20
20
30
25
25
25
25
40
25
25
40
75
75
48
72
72
48
48
48
72
24
24
24
40
25
36
42
50
46
44
52
56
62
As can be seen in Table 1, the initial results showed that the cyanide ion effectively catalyzed the self-
condensation reaction of acyl phosphonates. Reaction times ranged from 24 to 72 h at 25–75 ◦ C. The use of
KCN in DMF was successful. Poorer reactivity was observed in THF. Increasing the catalyst loading caused
165

EYMUR et al./Turk J Chem
a slight increase in yield (Table 1, entry 2). Benzoyl dimethyl phosphonate gave a higher yield than benzoyl
diethyl phosphonate (Table 1, entry 6). The higher loading of the cyanide as a catalyst led to a higher yield
of the product. At 40 ◦ C and 75 ◦ C, the products were obtained in 52% and 55% yield, respectively (Table
1, entry 5). Moreover, at these temperatures, the reaction time was shortened. Finally, acceptable yields were
obtained at 75 ◦ C with 30% mole KCN (Table 1, entry 10).
With these optimized reaction conditions in hand, we then examined the scope of the corresponding
reaction with different benzoyl dimethylphosphonates. As shown in Table 2, the reaction proceeded smoothly
and was completed in 24 h with isolated yields ranging from 45% to 65%. It should be stated that all
examples engaged aryl-substituted keto phosphonates (aryl phosphonates). Electron-rich, -poor, and -neutral
aryl phosphonates were well tolerated. 4-Methoxy and 4-methyl benzoyl dimethylphosphonates furnished the
reaction in good yields (65% and 60% yield, respectively, Table 2, entries 2 and 3). On the other hand, 4-fluoro
benzoyl dimethylphosphonate gave only 45% yield (Table 2, entry 4). Meta-substituted aryl phosphonates also
gave good yields (Table 2, entries 6 and 7). The main difficulty encountered with this reaction is the reaction
of ortho-substituted aryl phosphonates and also alkyl phosphonates. No product formation was detected when
ortho-substituted aryl phosphonates were employed (Table 2, entry 9). Alkyl phosphonates also gave no reaction.
The structures of the corresponding products were established from their spectral (1 H, 13 C, and 31 P NMR)
and analytical data.
The proposed catalytic cycle for the formation of O-protected α-hydroxy phosphonates is based on the
classical route of benzoin condensation, which has common key steps for a variety of congeners of this reaction
like acylphosphonates (Scheme 2). In this context, acyl anion intermediate 10 generated from cyanide ion
promoted rearrangement reacts with another acyl phosphonate to afford intermediate 11, which undergoes a
1,4-phosphate migration yielding tertiary O−protected α-hydroxy phosphonate 8.
O
O
OMe
Ar1
Ar1
P
OMe
Ar1
PO(OMe)2
O
OPO(OMe)2
7
CN
8
O
O
PO(OMe)2
CN
NC
Ar1
PO(OMe)2
OPO(OMe)2
Ar1
Ar1
12
9
OPO(OMe)2
Ar1
NC
Ar1
OPO(OMe)2
PO(OMe)2
O
Ar1
CN
11
10
O
OMe
P
OMe
Ar1
O
Scheme 2. Proposed mechanism for O−protected α-hydroxy phosphonate formation.
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EYMUR et al./Turk J Chem
Table 2. Scope of the reaction with derivatives of benzoyl dimethylphosphonates.
O
O
30% mole KCN
DMF, 75 oC, 24h
OMe
OMe
OPO(OMe)2
PO(OMe)2
R
P
R
O
R
Yield
(%)
Entry
Product
R
O
OPO(OMe)2
PO(OMe)2
1
62
8a
O
OPO(OMe)2
PO(OMe)2
MeO
2
65
60
45
MeO
OMe
8b
8c
O
OPO(OMe)2
PO(OMe)2
Me
3
4
Me
Me
O
OPO(OMe)2
PO(OMe)2
F
F
F
8d
8e
O
OPO(OMe)2
PO(OMe)2
Cl
5
6
50
Cl
Cl
O
OPO(OMe)2
PO(OMe)2
Me
Me
59
61
Me
8f
O
OPO(OMe)2
PO(OMe)2
Cl
Cl
7
Cl
8g
O
OPO(OEt)2
PO(OEt)2
8a
54
nd
8h
Me
O
OPO(OMe)2
PO(OMe)2
Me
9
Me
8i
abenzoyl dimethylphosphonate was used
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EYMUR et al./Turk J Chem
In conclusion, we have described a new method for the synthesis of O−protected α-hydroxy phospho-
nates. This method works well with aryl-substituted keto phosphonates. The cyanide ion catalyzed formation
of acyl anion from aryl phosphonates and the reaction of this anion with another aryl phosphonates furnished
corresponding O-protected α-hydroxy phosphonates in 42%–65% yields.
2. Experimental section
2.1. General
All commercially available reagents were used without further purification. DMF was purified by distillation
˚
under vacuum and stored over 4-A molecular sieves. Purification of the products was carried out by flash
column chromatography using silica gel 60. Analytical thin layer chromatography was performed on aluminum
sheets precoated with silica gel 60F254. 1 H, 13 C, and 31 P NMR spectra were reported on a Bruker Spectrospin
Avance DPX-400 Ultrashield instrument at 400, 100, and 162 MHz, respectively, relative to TMS for 1 H and
13 C NMR and H3 PO4 for 31 P NMR. Data are reported as s = singlet, d = doublet, t = triplet, q = quartet, m
= multiplet; coupling constant(s) in Hz; integration. Elemental analyses were conducted at the METU Central
Laboratory using a LECO, CHNS-932.
2.2. General experimental procedure for the preparation of O-protected α-hydroxy phosphonates
First 13 mg of KCN was dried at 100 ◦ C under vacuum for 3 h and dissolved in dry DMF (1 mL). Then 1
mmol of acyl phosphonate was added. The reaction mixture was stirred under argon atmosphere for 24 h at 75
◦ C. The reaction was diluted with Et2 O and brine solution was added. The aqueous phase was extracted with
Et2 O 3 times. The collected organic phase was dried over MgSO4 and evaporated under reduced pressure.
The crude mixture was purified by flash column chromatography on silica gel with EtOAc as eluent.
2.3. 1-(Methoxyphosphono)-2-oxo-1,2-diphenylethyl dimethyl phosphate (8a)
Yield: 62%; colorless oil; 1 H NMR (CDCl3)δ 3.29 (d, J = 11.5 Hz, 3H); 3.67 (d, J = 10.8 Hz, 3H); 3.73 (d,
J = 11.6 Hz, 3H); 3.79 (d, J = 10.9 Hz, 3H); 7.16–7.20 (m, 3H); 7.30–7.36 (m, 3H); 7.52 (d, J = 7.2 Hz, 2H);
7.61 (d, J = 7.8 Hz, 2H); 13 C NMR (CDCl3) 53.2 (d, J = 7.1 Hz); 53.4 (d, J = 6.1 Hz); 53.7 (d, J = 6.6
Hz); 54.0 (d, J = 6.1 Hz); 90.2; 126.3 (d, J = 3.6 Hz); 127.8; 128.4; 128.6 (d, J = 2.7 Hz); 130.5; 132.5; 134.3
(d, J = 4.5 Hz); 134.8 (d, J = 7.1 Hz); 192.7; 31 P NMR (CDCl3) –5.62; 12.70. C18 H22 O8 P2 (428.08): Calcd
C, 50.48; H, 5.18; O, 29.88; P, 14.46; found C, 50.42; H, 5.01; O, 30.10; P, 14.50.
2.4. 1-(Methoxyphosphono)-1,2-bis(4-methoxyphenyl)-2-oxoethyl dimethyl phosphate (8b)
Yield: 65%; colorless oil; 1 H NMR (CDCl3)δ 3.40 (d, J = 11.5 Hz, 3H); 3.68 (d, J = 10.7 Hz, 3H); 3.71 (s,
3H); 3.72 (d, J = 11.1 Hz, 3H); 3.73 (s, 3H); 3.77 (d, J = 10.9 Hz, 3H); 6.68 (d, J = 8.9 Hz, 2H); 6.83 (d, J =
8.8 Hz, 2H); 7.41 (dd, J1 = 2.2 Hz, J2 = 8.9 Hz, 2H); 7.66 (d, J = 8.9 Hz, 2H); 13 C NMR (CDCl3) 53.1 (d,
J = 7.4 Hz); 53.3 (d, J = 6.0 Hz); 53.8 (d, J = 7.3 Hz); 54.1 (d, J = 6.1 Hz); 54.2; 54.3; 12.3; 113.0; 125.1;
125.8 (d, J = 6.2 Hz); 126.8 (d, J = 4.2 Hz); 132.0; 159.0 (d, J = 2.8 Hz); 162.2; 190.6; 31 P NMR (CDCl3)
–2.86; 15.9. C20 H26 O10 P2 (488.10): Calcd C, 49.19; H, 5.37; O, 32.76; P, 12.68; found C, 49.02; H, 5.45; O,
33.15; P, 12.57.
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EYMUR et al./Turk J Chem
2.5. 1-(Methoxyphosphono)-2-oxo-1,2-bis-tolylethyl dimethyl phosphate (8c)
Yield: 60%; colorless oil; 1 H NMR (CDCl3)δ 2.22 (s, 3H); 2.27 (s, 3H); 3.36 (d, J = 11.5 Hz, 3H); 3.67 (d,
J = 10.8 Hz, 3H); 3.72 (d, J = 11.6 Hz, 3H); 3.77 (d, J = 10.9 Hz, 3H); 6.99 (d, J = 8.1 Hz, 2H); 7.12 (d,
J = 8.0 Hz, 2H); 7.37 (d, J = 8.2 Hz, 2H); 7.54 (d, J = 8.2 Hz, 2H); 13 C NMR (CDCl3) 20.1; 20.6; 53.1
(d, J = 7.0 Hz); 53.3 (d, J = 6.0 Hz); 53.8 (d, J = 6.5 Hz); 54.1 (d, J = 6.2 Hz); 59.3; 125.1 (d, J = 4.2
Hz); 126.8; 127.0; 127.67; 128.2 (d, J = 2.7 Hz); 129.6, 130.5; 130.6; 191.8; 31 P NMR (CDCl3) –3.35; 15.69.
C20 H26 O8 P2 (456.11): calcd C, 52.64; H, 5.74; O, 28.05; P, 13.57; found C, 52.56; H, 5.67; O, 28.85; P, 13.50.
2.6. 1-(Methoxyphosphono)-1,2-bis(4-fluorophenyl)-2-oxoethyl dimethyl phosphate (8d)
Yield: 45%; colorless oil; 1 H NMR (CDCl3)δ 3.38 (d, J = 11.5 Hz, 3H); 3.68 (d, J = 10.8 Hz, 3H); 3.75 (d,
J = 11.6 Hz, 3H); 3.80 (d,J = 10.9 Hz, 3H); 6.89 (d, J = 8.6 Hz, 2H); 7.03 (d, J = 8.5 Hz, 2H); 7.47–7.51 (m,
2H); 7.66 (dd, J1 = 8.7 Hz,J2 = 5.5 Hz, 2H); 13 C NMR (CDCl3) 54.31 (d, J = 6.0 Hz); 55.23 (J = 6.2 Hz);
60.45; 115.18; 115.40; 115.78; 115.98; 128.31; 130.26; 133.16; 133.25; 191.44; 31 P NMR (CDCl3) –3.15; 15.02.
C18 H20 F2 O8 P2 (464.06): calcd C, 46.56; H, 4.34; F, 8.18; O, 27.57; P, 13.34; found C, 47.03; H, 4.45; F, 8.11;
O, 28.02; P, 13.29.
2.7. 1-(Methoxyphosphono)-1,2-bis(4-chlorophenyl)-2-oxoethyl dimethyl phosphate (8e)
Yield: 50%; colorless oil; 1 H NMR (CDCl3)δ 3.38 (d, J = 11.5 Hz, 3H); 3.69 (d, J = 10.9 Hz, 3H); 3.76 (d, J
= 11.6 Hz, 3H); 3.81 (d,J = 10.9 Hz, 3H); 7.19 (d, J = 8.7 Hz, 2H); 7.31 (d, J = 8.6 Hz, 2H); 7.41 (dd,J1 =
8.8 Hz,J2 = 2.3 Hz 2H); 7.56 (dd, J1 = 8.7 Hz, 2H); 13 C NMR (CDCl3) 54.3 (d, J = 3.2 Hz); 54.4 (d, J =
2.8 Hz); 55.6 (d, J = 1.7 Hz); 55.7 (d, J = 1.1 Hz); 96.1; 127.1 (d, J = 4.4 Hz); 127.5 (d, J = 4.4 Hz); 128.5;
128.6; 128.8; 129.1 (d, J = 5.6 Hz); 131.5; 131.8 (d, J = 4.5 Hz); 192.8; 31 P NMR (CDCl3) –2.80; 15.27.
C18 H20 Cl2 O8 P2 (496.00): calcd C, 43.48; H, 4.05; Cl, 14.26; O, 25.74; P, 12.46; found C, 43.45; H, 4.25; Cl,
14.32; O, 26.14; P, 12.40.
2.8. 1-(Methoxyphosphono)-2-oxo-1,2-di-m-tolylethyl dimethyl phosphate (8f)
Yield: 59%; colorless oil; 1 H NMR (CDCl3)δ 2.22 (s, 3H); 2.29 (s, 3H); 3.28 (d, J = 11.5 Hz, 3H); 3.66 (d, J
= 10.8 Hz, 3H); 3.73 (d, J = 11.6 Hz, 3H); 3.78 (d, J = 10.9 Hz, 3H); 7.00–7.33 (m, 7H); 7.55 (s, 1H); 13 C
NMR (CDCl3) 20.3; 20.6; 52.9 (d, J = 6.5 Hz); 53.0 (d, J = 5.8 Hz); 53.8 (d, J = 6.3 Hz); 53.9 (d, J = 6.2
Hz); 89.6; 122.2 (d, J = 3.8 Hz); 125.7 (d, J = 4.2 Hz); 126.5; 126.7; 127.4 (d, J = 2.6 Hz); 128.7 (d, J = 2.6
Hz); 130.1; 132.5; 132.8 (d, J = 4.4 Hz); 133.3 (d, J = 6.9 Hz); 136.7; 137.1 (d, J = 2.7 Hz); 191.2; 31 P NMR
(CDCl3) –3.12; 15.56. C20 H26 O8 P2 (456.11): calcd C, 52.64; H, 5.74; O, 28.05; P, 13.57; found C, 52.60; H,
5.70; O, 28.55; P, 13.51.
2.9. 1-(Methoxyphosphono)-1,2-bis(3-chlorophenyl)-2-oxoethyl dimethyl phosphate (8g)
Yield: 61%; colorless oil; 1 H NMR (CDCl3)δ 3.38 (d, J = 11.5 Hz, 3H); 3.71 (d, J = 10.9 Hz, 3H); 3.78 (d,
J = 11.6 Hz, 3H); 3.82 (d,J = 10.9 Hz, 3H); 7.24–7.37 (m, 5H); 7.47 (td, J1 = 8.1 Hz, J2 = 1.3 Hz, 1H);
7.53–7.55 (m, 1H); 7.64 (t, J = 1.8 Hz, 1H); 13 C NMR (CDCl3); 54.5 (d, J = 4.5 Hz); 54.7 (d, J = 2.2 Hz);
55.5 (d, J = 6.7 Hz); 55.7 (d, J = 6.2 Hz); 96.4; 124.7 (d; J = 4.5 Hz); 125.6 (d, J = 2.2 Hz); 126.3 (d, J
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EYMUR et al./Turk J Chem
= 4.0 Hz); 128.7; 129.7; 129.8 (d, J = 3.0 Hz); 130.3 (d, J = 2.6 Hz); 130.6; 133.4; 134.7; 192.9; 31 P NMR
(CDCl3) –2.43; 15.10. C18 H20 Cl2 O8 P2 (496.00): calcd C, 43.48; H, 4.05; Cl, 14.26; O, 25.74; P, 12.46; found
C, 43.54; H, 4.16; Cl, 14.30; O, 26.15; P, 12.38.
2.10. 1-(Ethoxyphosphono)-2-oxo-1,2-diphenylethyl diethyl phosphate (8h)
Yield: 54%; colorless oil; 1 H NMR (CDCl3) δ 1.02 (t, J = 7.0 Hz, 3H); 1.17 (t, J = 7.0 Hz, 3H); 1.19–1.26
(m, 6H); 3.50–3.66 (m, 2H); 3.93–4.22 (m, 6H); 7.13–7.19 (m, 3H); 7.25–7.35 (m, 3H); 7.50 (d, J = 7.2 Hz);
7.58 (d, J = 7.6 Hz); 13 C NMR (CDCl3) 15.8 (d, J = 8.2 Hz); 16.0 (d, J = 8.4 Hz); 16.3 (d, J = 5.7 Hz);
16.4 (d, J = 6.5 Hz); 63.3 (d, J = 7.4 Hz); 63.5 (d, J = 5.8 Hz); 64.1 (d, J = 7.8 Hz); 64.3 (d, J = 7.2 Hz);
90.4; 126.5 (d, J = 3.7 Hz); 127.7; 128.3; 128.7 (d, J = 2.8 Hz); 130.6; 132.4; 134.4 (d, J = 4.5 Hz); 134.7
(d, J = 7.2 Hz); 192.9; 31 P NMR (CDCl3) –5.58; 12.64. C18 H22 O8 P2 (428.08): calcd C, 50.48; H, 5.18; O,
29.88; P, 14.46; found C, 50.55; H, 5.23; O, 30.11; P, 14.40.
Acknowledgments
¨
The authors gratefully acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK),
˙
Sel¸cuk University, and Middle East Technical University (METU).
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