Efficient AgOTf or Ph3PAuCl-AgSbF6 catalyzed cyclization of 1-hydroxy-2-alkynylallylphosphonates/2-alkynylallyl alcohols to 2-furylphosphonates/2,3,5-trisubstituted furans

Article abstract of DOI:10.1016/j.tetlet.2012.04.060

The reaction of 1-hydroxy-2-alkynylallylphosphonates, synthesized by the addition of the corresponding phosphites to 2-alkynylcinnamaldehydes, under AgOTf or Ph₃PAuCl-AgSbF₆ catalyzed cycloisomerization afforded 2-furylphosphonates i

Full text of DOI:10.1016/j.tetlet.2012.04.060

Tetrahedron Letters 53 (2012) 3831–3834
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Efficient AgOTf or Ph3PAuCl–AgSbF6 catalyzed cyclization of
1-hydroxy-2-alkynylallylphosphonates/2-alkynylallyl alcohols to
2-f urylphosphonates/2,3,5-trisubstituted furans
⇑
Ramesh Kotikalapudi, K. C. Kumara Swamy
School of Chemistry, University of Hyderabad, Hyderabad 500 046, AP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of 1-hydroxy-2-alkynylallylphosphonates, synthesized by the addition of the corresponding
phosphites to 2-alkynylcinnamaldehydes, under AgOTf or Ph3PAuCl–AgSbF6 catalyzed cycloisomerization
afforded 2-furylphosphonates in good to excellent yields. These cyclization reactions were compared
with those of 2-alkynylallyl alcohols that led to multisubstituted furans.
Received 28 February 2012
Revised 12 April 2012
Accepted 15 April 2012
Available online 15 May 2012
Ó 2012 Published by Elsevier Ltd.
Keywords:
2-Alkynylallyl alcohol
Allylphosphonate
2-Furylphosphonate
Gold(I) catalysis
Silver triflate
Cycloisomerization
Furan ring system is an important structural unit that appears
in many natural products1 and pharmaceuticals.2 Functionalized
furans are also useful intermediates in organic synthesis.3 Many
heteroarylphosphonates,4,5 in particular furylphosphonates and
their analogues, have found applications in drug discovery.6 Two
drugs involving furylphosphonic acid moiety which are useful as
therapeutic agents in treating type 2 diabetes mellitus7 are shown
in Figure 1. Organophosphonates themselves have a diverse range
of bio-related functions (e.g., glyphosate, fosfomycin).8 Thus, there
is a twofold interest in furylphosphonates. Despite the synthesis of
the similar structural units reported previously by introducing a
phosphonate group directly on furan,7a,9 many of these methods
are not amenable to synthesize poly-substituted furylphospho-
nates. Although there are reports on gold,10 silver,11 and palla-
dium12 catalyzed cyclization of 2-alkynylallylic alcohols leading
to multisubstituted furans in addition to a t-BuOK13 activated
route, there is no precedence for the cycloisomerization of 1-hy-
droxy-2-alkynylallylphosphonates leading to polysubstituted fur-
ylphosphonates. Cyclization of phosphonoalkynols leading to
phosphonylated isochromenes14/isobenzofurans15 has been re-
ported but the synthetic routes involve an entirely different reac-
tion sequence. Because of the pivotal role of gold catalysts10,16 in
cycloisomerization of alkynyl compounds, our interest in cyclizing
the phosphonoalkynols has been oriented toward gold catalysis
which is reported in this Letter; we have discovered that simple
AgOTf also catalyzes such reactions. 2-Alkynyl cinnamaldehydes
2 and their derivatives are functionalized molecules which have
been proved as elegant precursors in synthesizing polysubstituted
furans,17 bicyclic furoazepines,18 furooxazine,19 allenes,20 pyrans,21
and isoxazoles.22 Hence as an extension to our methodology, we
have included these reactions.
Initially, various new 1-hydroxy-2-alkynylallylphosphonates 3–
13 were synthesized from the corresponding H-phosphonates 1a–c
and 2-alkynylcinnamaldehydes 2a–i via the Pudovik reaction
(Scheme 1).23 Yields in these reactions were in the range of 88–
94%.
To accomplish the cyclization, we started with the alcohols de-
picted in Scheme 1. Initially, we performed AgOTf catalyzed cycli-
zation of 3 (Scheme 2) in 1,2-dichloroethane and obtained 58% of
NH2
F
NH2
O
N
O
N
O
O
HO
P
S
P
OH
OH
N
HO
Et
[5-(4-Amino-7-ethyl-5-fluoro-1-isobutyl-1H-
benzoimidazol-2-yl)-furan-2-yl]-phosphonic Acid
MB05032
⇑
Corresponding author. Tel.: +91 40 2313 4856; fax: +91 40 2301 2460.
E-mail address: kckssc@uohyd.ernet.in (K.C. Kumara Swamy).
Figure 1. Drugs containing furylphosphonic acid moiety.
0040-4039/$ - see front matter Ó 2012 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2012.04.060

3832
R. Kotikalapudi, K. C. Kumara Swamy / Tetrahedron Letters 53 (2012) 3831–3834
O
OR
OR
OH
P
CHO
NEt3, THF
O
H
OR
P
+
3 h, rt
R1
OR
R1
R2
3-13
R2
2 a-i
1 a-c
R, R =
R1 = H, R2 = Ph
[3, 94%]
[4, 92%]
C6H4-4-Me
C6H4-4-OMe
[5, 90%]
1-cyclohexenyl [6, 88%]
n-butyl
n-pentyl
n-hexyl
n-octyl
[7, 92%]
[8, 91%]
[9, 88%]
[10, 92%]
R1 = Me, R2 = 1-cyclohexenyl [11, 92%]
Figure 2. ORTEP diagram for compound 14. Selected bond lengths [Å] with esd’s in
parentheses: O4–C9 1.358(3), C6–C7 1.357(3), C7–C10 1.497(3), C8–C9 1.349(3),
and C8–H8 0.9300.
R = Et, R1 = H, R2 = Ph
R = i-Pr, R1 = H, R2 = Ph
[12, 93%]
[13, 93%]
Scheme 1. Synthesis of 1-hydroxyphosphonates 3–13.
lated yield (Table 1, entry 4). Gratifyingly, the use of 3%
Ph3PAuCl/AgSbF6 led to 90% isolated yield (Table 2, entry 5). At a
lower loading of the catalyst (1%), the yield decreased to 62% (Ta-
ble 1, entry 6); AgSbF6 alone could not effect the cyclization (Ta-
ble 1, entry 7). Different solvents like THF, toluene, acetonitrile,
and CH2Cl2 were examined in the presence of 3% Ph3PAuCl/AgSbF6,
but these gave a poor yield of the furan (entries 8–11); there was
no reaction in nitromethane (entry 12). Thus our studies indicated
that either 10% AgOTf or 3% Ph3PAuCl/AgSbF6 was the most suit-
able catalytic system for this conversion.24 The efficacy of the
above catalytic conditions was verified with various phospho-
noalkynols which were efficiently cycloisomerized to 2-fur-
ylphosphonates in excellent yields as summarized in Table 2.
The role of gold(I) catalyst in the above cyclization was further
explored by applying the catalyst in the cycloisomerization of 2-
alkynylallyl alcohols (Scheme 3 see Supplementary data for the
synthesis of precursors) also. Interestingly, alcohol 25 upon treat-
ment with even 1% Ph3PAuCl/AgSbF6 in dichloromethane at room
temperature, led to 31 in 91% yield. By varying the catalyst to 1%
Ph3PAuCl/AgOTf, the yield decreased to 72%. AuCl3 in DCE10a at
70 °C afforded furan in only 10% yield. The compounds Ph3PAuCl,
AgOTf, and AgSbF6 individually were not effective in the cyclization
process. These alkynols when treated with DBU did not form the
furan whereas t-BuOK led to furan in 85% yield; however, 1 mol e-
quiv of the base was required. These data are presented in Table 3.
Hence it was concluded that Ph3PAuCl/AgSbF6 was the most effi-
cient catalyst for this cycloisomerization (cf. Table 4).
O
O
O
P
Catalyst/ solvent
OH
O
Temperature/ time
Ph
P
O
O
O
Ph
14
3
Scheme 2. Reaction of 1-hydroxyphosphonate 3 leading to furylphosphonate 14.
the product 14 when 5 mol % catalyst was used (Table 1, entry 1;
Fig. 2); increasing the amount of catalyst increased the yield of
the isolated product to 88% (entry 2). The 31P NMR monitoring of
the reaction mixture showed a single product with the entire phos-
phorus precursor being consumed. We were able to utilize this
condition to isolate cyclized products 14–24 in 85–94% yield. Un-
der the conditions employed, Cu(OTf)2 (entry 3), Zn(OTf)2, and
Sc(OTf)3 were ineffective. At the same time, we were allured by
the prospect of using gold(I) complexes, since it is known that they
can effect activation of C„C bonds at low catalyst loading. Thus by
using 3% Ph3PAuCl/AgOTf, the furan 14 was obtained in 81% iso-
Table 1
Effect of catalyst/solvent in cycloisomerization of 3 (cf. Scheme 2)
Entry
Catalyst (mol %)
Temp. (°C)/Time (h)
Solvent
Yielda (%)
1
2
5% AgOTf
10% AgOTf
70/12
70/3
DCE
DCE
58b
88c
3
4
5
6
7
8
9
10
11
12
5% Cu(OTf)2
70/12
70/3
70/3
70/12
70/12
70/12
70/12
70/12
rt/12
70/12
DCE
DCE
DCE
DCE
DCE
THF
Toluene
CH3CN
DCM
No reaction
81
3% Ph3PAuCl/AgOTf
3% Ph3PAuCl/AgSbF6
1% Ph3PAuCl/AgSbF6
5% AgSbF6
3% Ph3PAuCl/AgSbF6
3% Ph3PAuCl/AgSbF6
3% Ph3PAuCl/AgSbF6
3% Ph3PAuCl/AgSbF6
3% Ph3PAuCl/AgSbF6
90
62b
Trace
20
42
32
15
Nitro-methane
No reaction
a
b
c
Isolated yield.
Starting material remained.
This condition could also be used to obtain furans 14–24 in excellent yields of 85–94%.

R. Kotikalapudi, K. C. Kumara Swamy / Tetrahedron Letters 53 (2012) 3831–3834
3833
Table 2
Synthesis of 2-furylphosphonatesa (cf. Scheme 2)
OH
Yieldb (%)
Catalyst (or) base/ solvent
Temperature/ time
Entry
Alcohol
Product
Ph
O
Ph
25
1
3
90
31
O
O
Ph
P
O
14
O
Scheme 3. Reaction of compound 25 leading to furan 31.
Table 3
Effect of catalyst/solvent in cycloisomerization of 25 (cf. Scheme 3)
2
3
4
5
4
5
6
7
90
93
86
88
O
O
P
O
O
15
Entry Catalyst (mol %)
Temp. (°C)/Time (h) Solvent Yielda (%)
1
2
3
4
5
6
7
8
1% Ph3PAuCl/AgSbF6 rt/1
DCM
DCM
DCE
DCE
DCE
DCE
DMSO
DMSO
91
72
10
1% Ph3PAuCl/AgOTf
1% AuCl3
rt/1
O
70/12
70/12
70/12
70/12
70/12
70/3
P
O
1% Ph3PAuCl
5% AgOTf
No product
No product
No product
No reaction
85
O
O
MeO
16
5% AgSbF6
DBUb
t-BuOKb
O
P
O
a
Isolated yield.
1 Equiv of base was used.
O
O
O
b
17
O
O
Table 4
Gold(I) catalyzed synthesis of various furansa (cf. Scheme 3)
n-butyl
P
O
18
Entry
Alcohol
Product
Yieldb (%)
91
6
7
8
8
92
85
91
O
1
25
P
n-pentyl
n-hexyl
O
31
O
Ph
O
19
O
9
2
3
26
27
52
92
O
32
P
O
O
O
O
O
20
21
10
O
O
33
n-hexyl
Ph
n-octyl
O
P
O
4
28
84
O
9
11
88
O
P
O
34
O
O
22
5
6
29
30
88
90
10
11
12
13
92
93
Ph
Ph
Ph
35
36
OEt
OEt
O
O
Ph
O
P
23
O
O
OiPr
OiPr
Ph
O
a
P
24
Conditions: Alkynol (0.4 mmol), Ph3PAuCl/AgSbF6 (1 mol %), DCM (2 mL), rt, 1 h.
Yield of the isolated product.
b
a
Conditions: Phosphonoalkynol (0.4 mmol), Ph3PAuCl/AgSbF6 (3 mol %), DCE
(2 mL), 70 °C, 3 h.
b
Yield of the isolated product.
work in this case is still to be answered. The likely role of phos-
phoryl P@O in these reactions needs further investigation since
simple AgOTf worked well in the cyclization reactions leading to
phosphonofurans 14–24.
To summarize, a new route to phosphonofurans using AgOTf or
Ph3PAuCl/AgSbF6 as the catalytic system has been demonstrated.
The latter system can also efficiently catalyze the cyclization of
nonphosphorylated 2-alkynylallyl alcohols. Phosphonoalkynols re-
quired an elevated temperature for cyclization (hence the solvent
DCE was used) when compared to other alkynols.
A possible pathway for the formation of phosphonofurans based
on the available literature10a is presented in Scheme 4. Since we
found that AgOTf also works well, it is likely that silver(I) also coor-
dinates in a manner analogous to gold(I) in these reactions.25 In the
case of non-phosphorylated alkynols 25–30 leading to the cyclized
products 31–36, the pathway is analogous, but the reaction is more
facile using the gold(I) catalytic system. Why AgOTf alone did not

3834
R. Kotikalapudi, K. C. Kumara Swamy / Tetrahedron Letters 53 (2012) 3831–3834
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O
=
P
P
P
O
O
Ph
O
Ph3PAuCl/ AgSbF6
=
[M]
H
or AgOTf
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Ph
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Acknowledgments
We thank the Department of Science and Technology (DST, New
Delhi) for financial support and for the single crystal X-ray diffrac-
tometer facility at the University of Hyderabad and UGC (New Del-
hi) for equipment under UPE and CAS programs. KCK thanks DST
for a J. C. Bose fellowship. RK thanks CSIR (New Delhi) for a
fellowship.
13. Marshall, J. A.; DuBay, W. J. J. Org. Chem. 1993, 58, 3435.
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Supplementary data
Supplementary data (experimental data for all the new com-
pounds reported and crystal data (CIF file) for compound 14) asso-
ciated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2012.04.060.
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24. General procedure for the synthesis of 2-furylphosphonates: To a solution of
phosphonoalkynol 3 (153 mg, 0.4 mmol) in dry DCE (2 mL) were added a
solution of Ph3PAuCl (0.03 equiv) and AgSbF6 (0.03 equiv) in dichloroethane
(DCE). The contents were stirred for 3 h at 70 °C. The solvent was removed
under vacuum and the crude product was purified by column chromatography
using silica gel with acetone/hexane (1:3) mixture as the eluent. X-ray data for
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14 were collected on
a Bruker AXS SMART diffractometer using Mo-K
a
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(k = 0.71073 Å) radiation. The structures were solved and refined by standard
methods. Crystal data: C22H23O4P, M = 382.37, Monoclinic, Space group P2(1)/c,
a = 10.342(1), b = 11.184(1), c = 19.725(1) Å, b = 121.16(1)o, V = 1952.5(3) Å3,
Z = 4,
(I > 2
l
= 0.165 mmÀ1
,
data/restraints/parameters: 3431/0/246,
R indices
r
(I)): R1 = 0.0596, wR2 (all data) = 0.1385. CCDC no. 867562.
25. Gold(I) is soft and readily coordinates with the C„C (triple) bonds. The
presence of AgSbF6 or AgOTf with the Ph3PAuCl perhaps enhances the softness
of the metal and allows for better activation of the triple bond. See: Lipshutz, B.
H.; Yamamoto, Y. Chem. Rev. 2008, 108, 2793.
4. (a) Seto, H.; Kuzuyama, T. Nat. Prod. Rep. 1999, 16, 589; (b) Robbins, B. L.;
Srinivas, R. V.; Kim, C.; Bischofberger, N.; Fridland, A. Antimicrob. Agents