Synthesis of α-arylphosphonates using copper-catalyzed α-arylation and deacylative α-arylation of β-ketophosphonates

Article abstract of DOI:10.1002/adsc.201100605

Efficient methods for the direct arylation and deacylative arylation of β-ketophosphonates with iodoarenes in presence of a copper(I) or a copper(II) salt as the catalysts have been developed. The corresponding α-arylphosphonates were obtained in high yields. A tentative mechanism for the deacylative arylation reaction was proposed on the basis of the experimental data. Copyright

Full text of DOI:10.1002/adsc.201100605

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DOI: 10.1002/adsc.201100605
Synthesis of a-Arylphosphonates using Copper-Catalyzed
a-Arylation and Deacylative a-Arylation of b-Ketophosphonates
Laxmidhar Rout,a Sridhar Regati,a and Cong-Gui Zhaoa,*
a
Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA
Fax : (+1)-210-458-7428; phone: (+1)-210-458-5432; e-mail: cong.zhao@utsa.edu
Received: July 31, 2011; Published online: December 6, 2011
Abstract: Efficient methods for the direct arylation
and deacylative arylation of b-ketophosphonates
with iodoarenes in presence of a copper(I) or a cop-
per(II) salt as the catalysts have been developed.
The corresponding a-arylphosphonates were ob-
tained in high yields. A tentative mechanism for the
deacylative arylation reaction was proposed on the
basis of the experimental data.
diACTHNGUTERNNUG
alkyl phosphates.[3k] As summarized above, most of
these reported methods for the synthesis of these two
types of compounds require either drastic reaction
conditions or some special reagents. Moreover, to the
best of our knowledge, there is no general method for
the synthesis a-aryl-substituted methylphosphonates
and b-ketophosphonates from a common starting ma-
terial using readily available catalysts.
As part of our ongoing efforts in developing novel
synthetic methods for a-substituted phosphonates,[4]
we became interested in the synthesis of a-aryl-substi-
tuted phosphonates through the direct a-arylation of
the readily available b-ketophosphonates. Although
there is no such precedence in the literature, there
are a few reports on the direct a-arylation of active
Keywords: arylation; a-arylphosphonates; coupling;
deacylation; iodoarenes
a-Aryl-substituted methylphosphonate and b-keto- methylene compounds.[5] For example, Ma and co-
phosphonate derivatives are frequently used as anti- workers have reported a copper-catalyzed coupling of
inflammatory agents.[1] For example, they have been b-keto esters and aryl iodides.[5a–d] Inspired by these
shown to be potent dual inhibitors of neutrophil ca- reports, we envisioned that a-aryl-substituted b-keto-
thepsin G and mast cell chymase, which are associat- phosphonates could be synthesized from b-keto-
ed with asthma and chronic obstructive pulmonary phosphonates and aryl halides with a suitable catalyst.
diseases.[1a–c] They are also useful precursors for the Herein we report a copper-catalyzed direct synthesis
synthesis of other biologically active compounds.[1b] In of a-aryl-substituted b-ketophosphonates and methyl-
organic chemistry, these compounds are vital starting phosphonates from b-ketophosphonates and aryl io-
materials for the synthesis of alkene derivatives.[2] Tra- dides via a cross-coupling reaction and a cascade
ditional methods for the synthesis of a-arylmethyl- cross-coupling and deacylation reaction, respectively.
phosphonates involve the reaction of arylmethyl hal- We believe this is the first method where these two
ACHTUNGTRENNUNG
ides[3a] or arylmethylpyridinium iodides[3b] with trialkyl types of compounds are prepared from a common b-
phosphites and the Michaelis–Arbuzov rearrangement ketophosphonate derivative.
of arylmethylphosphites.[3c,d] Other reported methods
With diethyl (2-oxopropane)phosphonate (1a) and
include the rhodium-catalyzed coupling reaction of iodobenzene (2a) as the model substrates, we first
(EtO)2P(O)CH2ZnI and PhI,[3e] and the coupling reac- studied the a-arylation reaction using the reaction
tion of chloromethylphosphonates with aryl bromide conditions similar to those reported by Ma and co-
mediated by CuI and t-BuLi.[3f] As for the synthesis workers.[5a] The results of this study are summarized
of a-aryl-substituted b-ketophosphonates, only
a
in Table 1. As shown by the results, in presence of
handful of reports is available, namely, the addition of CuI (20 mol%), l-proline (40 mol%) and Cs2CO3
trialkyl phosphites in the presence of TiCl4,[3g] or di- (3 equiv.) in DMSO, the desired a-arylated product
ACHTUNGTRENNUNG
alkyl phosphites in the presence of DBU/TMSCl,[3h] 3a was isolated with 64% yield after reacting for 24 h
to nitroalkenes followed by m-CPBA oxidation; the at 508C (entry 1). While copper(I) bromide (entry 2),
reaction of diethyl phosphonocarboxylic acid chlor- copper(II) acetate monohydrate (entry 3), copper(II)
ides and organocuprates or Grignard reagents,[3i] the sulfate pentahydrate (entry 4), and copper(II) tosylate
rearrangement of substituted epoxyphosphonates,[3j] monohydrate (entry 5) all catalyze this coupling reac-
and the reaction of arylsulfonyl epoxides and sodium tion, their reactivity is much lower. Screening of the
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Adv. Synth. Catal. 2011, 353, 3340 – 3346

Synthesis of a-Arylphosphonates using Copper-Catalyzed a-Arylation and Deacylative a-Arylation
Table 1. Optimization of the reaction conditions for the ary-
lation of 1a.[a]
Table 2. Copper(I) iodide-catalyzed arylation of b-ketophos-
phonates.[a]
Entry Catalyst
Solvent Base
Yield [%][b]
1
2
3
4
5
6
7
8
CuI
CuBr
DMSO Cs2CO3
DMSO Cs2CO3
64
24
31
38
35
61
0
0
0
19
Entry R1
R2
Time [h] Product Yield [%][b]
Cu
CuSO4·5H2O
Cu(OTf)2·H2O DMSO Cs2CO3
DMF Cs2CO3
ACHTUNGTRENNUNG
1
2
Me
Me
H
F
24
26
26
24
3a
3b
3c
3d
3e
3f
3g
3h
3i
84
81
74
81
81
81
80
82
83
81
75
ACHTUNGTRENNUNG
3
4
Me Br
Me
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
I
dioxane Cs2CO3
toluene Cs2CO3
DMSO K2CO3
DMSO KOH
5
Me CO2Et 24
6
7
8
9
Me CN
Me NO2
Me Me
Me MeO
22
20
26
26
24
24
9
10
11
12[c]
13[d]
14[e]
15[f]
DMSO K3PO4·3H2O 24
DMSO Cs2CO3
DMSO Cs2CO3
DMSO Cs2CO3
DMSO Cs2CO3
0
0
84
61
10
11
Et
Ph
H
H
3j
3k
[a]
[b]
All reactions were conducted with 1a (1.2 mmol), 2
(1.0 mmol), CuI (20 mol%), l-proline (40 mol%), and
Cs2CO3 (3.0 mmol) in DMSO (1.0 mL) under an N2 at-
mosphere at 708C for the indicated reaction times.
Yield of the isolated product.
[a]
Unless otherwise indicated, all reactions were conducted
with 1a (1.2 mmol), 2a (1.0 mmol), the copper salt
(20 mol%), l-proline (40 mol%), and the specified base
(3.0 mmol) in the indicated solvent (1.0 mL) under an N2
atmosphere at 508C for 24 h.
Yield of the isolated product.
No l-proline was added.
PPh3 was used instead of l-proline.
The reaction was carried out 708C.
[b]
[c]
[d]
[e]
[f]
(entry 4), we only observed the monoarylated 3d. No
trace of the diarylated product was detected even
when the reaction was carried out in the presence of
6 equivalents of Cs2CO3 and 3 equivalents of 1a after
48 h (data not shown).
The reaction was carried out in bromobenzene at 908C.
solvents revealed that good reactivity was also ob-
During the course of the above study, we noted
tained in DMF (entry 6). However, no reaction was that Cu(II) salts also can catalyze the arylation reac-
observed in dioxane and toluene (entries 7 and 8). tion (Table 1, entries 3–5), albeit in lower efficiency as
Similarly, other bases tested in this reaction, such as compared to CuI. Because Cu(II) salts are cheaper
K2CO3 (entry 9), KOH (entry 10), and K3PO4·3H2O than CuI, it was our intention to optimize the reaction
(entry 11), are not effective. It is also noteworthy that conditions to improve the efficiency of the Cu(II)-cat-
the reaction did not proceed in absence of l-proline alyzed arylation. Since higher temperature works
(entry 12) or when l-proline was replaced with PPh3 better for CuI (Table 1, entry 14), we attempted the
(entry 13). Gratifyingly, the yield of this arylation re- copper(II) acetate monohydrate-catalyzed coupling
action may be improved to 84% by carrying out the reaction at higher temperature. Surprisingly, increas-
reaction at 708C (entry 14). Under these optimized ing the reaction temperature to 1108C led to the for-
reaction conditions, less reactive bromobenzene also mation of the deacylated coupling product 4a in 72%
affords compound 3a in 61% yield (entry 15).
yield; whereas the expected 3a was only isolated in
Next, we studied the scope of reaction with differ- 12% yield (Table 3, entry 1). This result represents
ent iodobenzenes bearing electron-withdrawing or the first catalytic method for the direct preparation of
electron-donating functional groups (Table 2). As arylmethylketophosphonates from readily available b-
shown by the results, this catalytic system tolerates io- ketophosphonates. In this regard, it should be pointed
dobenzenes with both an electron-withdrawing group out that palladium acetate-catalyzed deacylative cou-
and an electron-donating group at the para-position pling between b-keto esters and aryl halides[6] and
of the benzene ring and different b-ketophophonate copper(I) iodide-catalyzed deacylative coupling be-
starting materials to afford the desired a-arylated b- tween b-keto esters[7] or 1,3-diketones[8] and aryl hal-
ketophosphonates 3a–3k in good yields (74–84%, en- ides have been reported recently. Nevertheless, deacy-
tries 1–11). Interestingly, for 1,4-diiodobenzene lative coupling reaction of b-ketophosphonates is un-
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Laxmidhar Rout et al.
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Table 3. Optimization of the reaction conditions for the de-
acylative a-arylation of 1a.[a]
Replacing iodobenzene with bromobenzene afforded
4a in only 13% yield even at 1308C (entry 14).
Then we further investigated the scope of this de-
acylative arylation reaction (Table 4) under the opti-
mized conditions. Besides iodobenzene (entry 1), io-
dobenzene derivatives bearing an electron-withdraw-
ing group, such as, 4-fluoro (entry 2), 4-bromo
(entry 3), 4-iodo (entry 4), 4-ethoxycarbonyl (entry 5),
4-cyano (entry 6), and 4-nitro (entry 7), all undergo
the expected reaction smoothly to provide the corre-
sponding deacylated cross-coupling products 4b–4g in
high yields (85–91%). Again 1,4-diiodobenzene
(entry 4) gave only the monoarylated phosphonate
4d. No formation of diarylated product was observed.
Nonetheless, with iodobenzenes bearing an electron-
donating group, such as 4-methyl and 4-methoxy
groups, no deacylative arylation products, such as, 4h
and 4i, could be isolated. Only low yields of the aryla-
tion products 3h and 3i were obtained, respectively
(Table 4, entries 8 and 9). On the other hand, 1-iodo-
naphthalene underwent this reaction smoothly to pro-
vide corresponding deacylated cross-coupling product
4h in 92% yield (entry 10). This deacylative arylation
reaction may also be applied to other b-ketophospho-
nate substrates. When the terminal methyl group of
1a was changed to ethyl (1b, entry 11) and phenyl (1c,
entry 12) groups, reactions with iodobenzene under
similar conditions produced the expected deacylated
product 4a in 85% and 93% yields, respectively.
Entry Catalyst
Solvent Base
Yield [%][b]
1[c]
2[e]
3
Cu
Cu
Cu
N
72[d]
61
89
53
51
4[f]
5
Cu
6
A
41
7
ACHTUNGTRENNUNG(OAc)2·H2O DMSO K3PO4·3H2O 62
8
A
K2CO3
0
0
9
A
10
11
12
13
U
K2CO3
DMSO K2CO3
trace
49
57
52
13
Cu
Cu
148h] Cu
[a]
Unless otherwise indicated, all reactions were conducted
with 1a (1.2 mmol), 2a (1.0 mmol), the copper salt
(10 mol%), 2,2’-bipyridine (20 mol%), and the specified
base (5.0 mmol) in the indicated solvent (1.0 mL) at
1108C for 24 h.
Yield of the isolated product.
The reaction was carried out with l-proline (40 mmol%)
as the ligand.
Compound 3a was also isolated in 12% yield in this case.
No ligand was added.
Phenanthrene was used as the ligand.
EG=ethylene glycol.
[b]
[c]
[d]
[e]
[f]
Although the mechanism of the reported copper-
or palladium-catalyzed deacylative coupling reac-
[9]
tions[6–8] remains elusive, a C C bond activation was
À
[g]
[h]
proposed to be a possible mechanism in the deacyla-
tive arylation of 1,3-diketones.[8] In order to find out
the possible mechanism of the current reaction, some
Bromobenzene was used at 1308C.
precedented and copper(II) salts have never been additional experiments were carried out. Firstly, four
used for such a purpose. separate reactions were carried out with the authentic
When K2CO3 was used as the base, the reaction a-phenyl-substituted b-ketophosphonate 3a, which is
also proceeded without l-proline to yield 4a in 61% a potential intermediate in the deacylative arylation
yield (entry 2). When 2,2’-bipyridine was used as the reaction. As shown in Scheme 1, in the presence of
ligand, the yield was improved to 89% (entry 3). 2 equivalents of K2CO3, the reaction of 3a at 1108C in
However, using phenanthrene as the ligand showed DMSO for 3 h led to the full conversion of 3a, and 4a
no improvement in the yield of 4a (entry 4). It should was isolated in 79% yield. Similarly, in the presence
be pointed out that, although the deacylation is most of 2 equivalents of K2CO3 and 10 mol% of
favorable with 5 equivalents of K2CO3, it does pro- CuACTHUNTRGNEUNG(OAc)2·H2O, as well as 20 mol% of 2,2’-bipyridine,
ceed with a lower amount of base. For example, with 4a was isolated in 89% yield (Scheme 1). However,
0.5, 1.0, 2.0, 3.0, and 4.0 equivalents of K2CO3, prod- under these conditions, no reaction was observed
uct 4a was obtained in 12%, 23%, 38%, 51%, and without the addition of any of these reagents. Similar-
72% yields, respectively (data not shown). Other ly, CuACTHNUTRGEN(UNG OAc)2·H2O alone cannot catalyze the conver-
bases, such as Cs2CO3, KOH, and K3PO4·3H2O are sion of 3a under these conditions (data not shown). It
less effective in promoting the reaction (entries 5-7). is also noteworthy that we did not observe any de-
Similarly, this reaction does not work in solvents like phosphorylation product, such as PhCH2COMe, in all
ethylene glycol, toluene, and DMF (entries 8–10). these cases. These results indicate that, if 3a is
Under the optimized conditions, copper(I) iodide, formed, the deacylation is mainly due to the action of
copper(II) tosylate monohydrate, and copper(II) ni- K2CO3. However, CuACTHNURGTNEUNG(OAc)2 also promotes the deacy-
trate monohydrate also led to the deacylative a-aryla- lation reaction in the presence of the ligand (2,2’-bi-
tion product 4a, albeit in lower yields (entries 11–13). pyridine).
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Synthesis of a-Arylphosphonates using Copper-Catalyzed a-Arylation and Deacylative a-Arylation
(OAc)2·H2O.[a]
Table 4. Synthesis of 4 from reaction of 1 and 2 with CuCAHTUNGTRENNUNG
Entry
R1
Ar
Time [h]
Product
Yield [%][b]
1
2
3
4
5
6
7
8
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
C6H5
24
40
34
26
28
27
32
48
48
35
24
24
4a
4b
4c
4d
4e
4f
4g
4h
4i
89
85
86
88
87
90
91
0[c]
0[d]
92
85
93
4-F-C6H4
4-Br-C6H4
4-I-C6H4
4-EtO2C-C6H4
4-CN-C6H4
4-NO2-C6H4
4-Me-C6H4
4-MeO-C6H4
1-naphthyl
C6H5
9
10
11
12
4j
4a
4a
Ph
C6H5
[a]
All reactions were conducted with 1 (1.2 mmol), 2 (1.0 mmol), K2CO3 (5.0 mmol), CuACHTNUTRGNEUNG(OAc)2·H2O (0.10 mmol, 10 mol%),
and 2,2’-bipyridine (0.20 mmol, 20 mol% in DMSO (1.0 mL) at 708C for the indicated reaction times.
Yield of the isolated product.
Only the coupling product 3h was obtained in 28% yield.
The coupling compound 3i was obtained in 30% yield with a trace amount of 4i.
[b]
[c]
[d]
On the basis of these facts and the mechanism pro-
posed for deacylative arylation of 1,3-diketones,[8]
a
plausible mechanism (Scheme 3) is proposed to ac-
count for the above deacylation reaction. As shown in
Scheme 3, Cu(I) formed in situ from Cu(II) acetate
reacts with 1 in the presence of K2CO3 to give the
Cu(I) intermediate 5. Oxidative addition of iodoarene
Scheme 1. Deacylation of 3a under various conditions.
to 5 forms a CuACHTUNTRGNEUNG(III) intermediate 6. Intermediate 6
may undergo a reductive elimination to yield the cou-
pling reaction product 3 and simultaneously regener-
ate the Cu(I) species to complete the catalytic cycle.
As our data show, compound 3 may be decarboxylat-
ed by K2CO3 directly to generate the carbanion inter-
mediate 7 (pathway a), which produces the deacyla-
tive product 4 after protonation. On the other hand,
by acting as a Lewis acid, Cu salts may enhance the
efficiency of the deacylation of 3 (pathway b). Alter-
natively, intermediate 6 may react with K2CO3 direct-
Scheme 2. Deacylation of 3i.
ly to generate a copper
undergoes rearrangement and elimination to yield
(III) intermediate 8,[8] which
ACHTUNGTRENNUNG
Next, the deacylation of compound 3i was studied. product 4 and the copper(I) to complete the catalytic
As we have mentioned previously, the direct deacyla- cycle (pathway c). Both pathways b and c can account
tive coupling of 1-iodo-4-methoxybenzene and 1a fails for the observed acceleration of the deacylation reac-
to proceed (Table 4, entry 9). However, the deacyla- tion in the presence of the copper catalyst (Scheme 1
tion of 3i works well in the presence of Cu
K2CO3 or K2CO3 alone (Scheme 2). The data again
verify that, although the reaction takes place with to clarify the role of copper in the deacylation pro-
K2CO3 alone, Cu(OAc)2/2,2’-bipyridine is also a pro- cess, several additional experiments were carried out.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
moter of the deacylation reaction. These data also Firstly, the deacylation of compound 3a was studied
hint that the direct deacylative arylation reaction is a with K2CO3 (2 equiv.) and different Lewis acids
cascade reaction beginning with the arylation reac- (10 mol%), such as La
N
ACHTNUGERTN(NUGN NO3)3·6H2O,
tion.
Ni
N
ACHTUNGTRENNUNG
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3343

Laxmidhar Rout et al.
COMMUNICATIONS
Scheme 3. Proposed mechanism for the deacylative arylation reaction (Ln=ligand).
mized reaction conditions (DMSO, 1108C), and the instead, the rearranged product 9 was obtained in low
corresponding deacylative product 4a was obtained in yield.[10] Formation of 9 may be rationalized through a
59%, 51%, 51%, and 0% yields, respectively. Compar- rearrangement of the enolate intermediate formed
ing these data with those reported in Scheme 1, it is from 1a under the action of a base. Similarly, the at-
clear that these Lewis acids do not promote the de- tempted deacylation using a stoichiometric amount of
acylation of compound 3a since the obtained yields Cu
are worse than that obtained with K2CO3 alone tions also failed (Scheme 4). Moreover, compounds
(79%). Thus, it is unlikely that Cu
(OAc)2·H2O is be- 11 and 12[11] (Figure 1) failed to deacylate under the
ACHTUNGTERNUN(NG OAc)·H2O under the optimized reaction condi-
AHCTUNGTRENNUNG
having as a Lewis acid in promoting the deacylation optimized reaction conditions, too. These results
reaction and, therefore, pathway b in Scheme 3 is un- again demonstrate that the presence of an a-aryl
likely. Secondly, the deacylation reaction of b-keto- group is essential for the deacylation of b-keto-
phosphonate 1a, which is the substrate used in this de- phosphonates. These results are in agreement with
acylative coupling reaction, was carried out. As both pathways a and c. Most likely both pathways are
shown in Scheme 4, upon the treatment with K2CO3 working under the reaction conditions. For pathway a,
(5 equiv.) in DMSO at 1108C for 10 h, no formation the aryl group can stabilize the carbanion intermedi-
of the desired deaceylated product 10 was observed; ate 7 to facilitate the direct deacylation of 3 by
Figure 1. Compounds attempted for the deacylation reac-
tion.
Scheme 4. Attempted deacylation of 1a.
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Adv. Synth. Catal. 2011, 353, 3340 – 3346

Synthesis of a-Arylphosphonates using Copper-Catalyzed a-Arylation and Deacylative a-Arylation
K2CO3. For pathway c, the presence of an aryl group Acknowledgements
can facilitate the insertion of the copper(I) reagent
The generous financial support of this project from the NIH-
NIGMS (Grant no. SC1GM082718) and the Welch Founda-
tion (Grant No. AX-1593) is gratefully acknowledged.
into the carbon-aryl bond of 3 to form the intermedi-
ate 6, which may deacylate through 8. Copper(II) ace-
tate itself cannot insert into 3 to generate 6 and,
therefore, it alone cannot deacylate compound 3 (vide
supra).
In conclusion, we have described an efficient syn-
thesis of a-aryl substituted b-ketophosphonates and
methylphosphonates from b-ketophoxphonates and
aryl iodides via an arylation reaction and a tandem ar-
ylation and deacylation reaction, respectively, using
Cu(I) or Cu(II) salts as the catalysts. The correspond-
ing a-aryl-substituted b-ketophosphonates and a-aryl-
methylphosphonates were obtained in high yields. A
plausible mechanism of this reaction is proposed and
the role of the base and the Cu(II) salt in the deacyla-
tion reaction is discussed.
References
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Experimental Section
General Procedure for the a-Arylation of b-
Ketophophonates
A mixture of b-ketophosphonate 1 (1.2 mmol), iodobenzene
[2] For reviews, see: a) W. S. Wardworth Jr, Org. React.
1977, 25, 73; b) B. J. Walker, in: Organophosphorous
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2
(1.0 mmol), CuI (0.2 mmol, 20 mol%), l-proline
(0.4 mmol, 40 mol%) and Cs2CO3 (3.0 mmol) in DMSO
(1.0 mL) was stirred under N2 at 708C. Upon the completion
of the reaction (monitored by TLC) and after being cooled
to room temperature, the reaction mixture was quenched
with saturated aqueous NH4Cl (2.0 mL). The mixture was
extracted with ethyl acetate (3ꢁ5 mL). The organic layers
were combined and dried over sodium sulfate. The drying
agent and the solvent were then removed and the residue
was purified by flash column chromatography on silica gel
to afford product 3. All the known compounds (3a,[3i] 3j,[3i]
and 3k[12a]) gave satisfactory spectroscopic data that are in
agreement with the reported data in the literature. All the
new compounds were fully characterized.[13]
[3] a) I. L. Odinets, E. V. Matveeva, E. V. Sharova, O. I.
Artyushin, V. A. Kozlov, D. V. Vorob’eva, S. N. Osipov,
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General Procedure for the a-Arylation and
Deacylation Reaction
A mixture of b-ketophophonates 1 (1.2 mmol), iodoben-
zenes 2 (1.0 mmol), CuACHTNUTRGNE(NUG OAc)2·H2O (0.10 mmol, 10 mol%),
2,2’-bipyridine (0.20 mmol, 20 mol%) and K2CO3 (5.0 mmol)
in DMSO (1.0 mL) was stirred under N2 at 1108C. Upon the
completion of the reaction (monitored by TLC) and after
being cooled to room temperature, the reaction was
quenched with saturated aqueous NH4Cl (2.0 mL). The mix-
ture was extracted with ethyl acetate (3ꢁ5 mL). The organic
layers were combined and dried over sodium sulfate. The
drying agent and the solvent were then removed and the
residue was purified by flash column chromatography on
silica gel to afford compounds 4. All the known compounds
(4a,[3c] 4b,[12b] 4c,[12c] 4d,[12d] 4e,[12e] 4f,[12f] 4g,[12g] 4i,[3e,12b] and
4j[12h]) gave satisfactory spectroscopic data that are in agree-
ment with the reported data in the literature.
Adv. Synth. Catal. 2011, 353, 3340 – 3346
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3345

Laxmidhar Rout et al.
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3346
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 3340 – 3346