Cascade reactions: Catalytic synthesis of functionalized 1,3-dihydroisobenzofuran and tetrahydrofuran derivatives by sequential nucleophilic ring opening-heterocyclization-oxidative carbonylation of alkynyloxiranes

Article abstract of DOI:10.1002/adsc.200900436

Differently substituted alkynyloxiranes were efficiently converted into functionalized 1,3-dihydroisobenzofurans and tetrahydrofuran derivatives in fair to good yields by a new cascade reaction, consisting of a sequential nucleophilic ring opening-heteroc

Full text of DOI:10.1002/adsc.200900436

FULL PAPERS
DOI: 10.1002/adsc.200900436
Cascade Reactions: Catalytic Synthesis of Functionalized
1,3-Dihydroisobenzofuran and Tetrahydrofuran Derivatives by
Sequential Nucleophilic Ring Opening–Heterocyclization–
Oxidative Carbonylation of Alkynyloxiranes
Nicola Della Ca’,a Fabio Campanini,a Bartolo Gabriele,b Giuseppe Salerno,c
Chiara Massera,d and Mirco Costaa,*
a
Dipartimento di Chimica Organica e Industriale, Universitꢀ di Parma, V. le G. P. Usberti 17/a, 43100 Parma, Italy
Fax : (+39)-0521-905-472; e-mail: mirco.costa@unipr.it
Dipartimento di Scienze Farmaceutiche, Universitꢀ della Calabria, 87036 Arcavacata di Rende, Cosenza, Italy
Dipartimento di Chimica, Universitꢀ della Calabria, 87036 Arcavacata di Rende, Cosenza, Italy
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universitꢀ di Parma, V. le G. P.
b
c
d
Usberti 17/a, 43100 Parma, Italy
Received: June 24, 2009; Published online: September 23, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900436.
Abstract: Differently substituted alkynyloxiranes trile/methanol mixtures in the presence of catalytic
were efficiently converted into functionalized 1,3-di- amounts of palladium diiodide in conjunction with
hydroisobenzofurans and tetrahydrofuran derivatives an excess of potassium iodide. The nucleophilic spe-
in fair to good yields by a new cascade reaction, con- cies beginning the cascade process by regioselective
sisting of a sequential nucleophilic ring opening–het- attack to the less hindered carbon of the oxirane ring
erocyclization–oxidative carbonylation process. Re- can be methanol itself or iodide anions.
actions were carried out at 80–1008C and under a
3:1 mixture of carbon monoxide and air (total pres- Keywords: carbonylation; cyclization; heterocycles;
sure=32–42 atm at 258C) in methanol or acetoni- oxiranes; palladium
Introduction
amine), and a catalytic system based on PdI2 in con-
junction with an excess of KI.[2f,p,3]
Sequential catalytic reactions offer considerable ad-
vantages compared with the conventional synthetic
procedures in terms of efficiency, selectivity, atom and
energy economy.[1] The application of such processes
has proved to be very valuable for performing the
synthesis of functionalized five- and six-membered
heterocyclic compounds.[2] In particular, we have pre-
viously reported the synthesis of a variety of carbony-
lated heterocycles by a sequential process involving
the intramolecular nucleophilic attack to a triple bond
coordinated to Pd(II) followed by carbonylation
(Scheme 1).[2f,p,3] These reactions are carried out with
an acetylenic substrate, bearing a suitably placed
oxygen or nitrogen nucleophilic group (such as an al-
coholic, amino, carbonyl, amido, or carboxylic group)
in the presence of a carbon monoxide/air mixture, an
external nucleophile NuH (such as an alcohol or an
Scheme 1. Intramolecular nucleophilic attack to a triple
bond followed by a carbonylation step in the presence of
PdI2/KI as catalytic system.
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It is well known that a simple ethereal group is not affording a heterocyclic organopalladium intermedi-
reactive enough to act as a nucleophile in this kind of ate. At this point a carbonylation step would com-
reactions, because of its high stability under a wide plete the process, with formation of the final carbony-
range of reaction conditions. Accordingly, very few lated heterocyclic derivative (Scheme 2).
examples are known in the literature regarding their
According to the general mechanistic pathways
use as nucleophiles in metal-catalyzed heterocycliza- shown in Scheme 1, the nucleophilic attack by the ep-
tion reactions, limited to substrates of particular reac- oxide oxygen may occur in an anti fashion, when PdI2
tivity. For example, the higher reactivity of the acetal is coordinated on the opposite site with respect to the
À
C O bond in comparison with that of simple ethers epoxide moiety (Scheme 2, path a). This would lead
was favorably exploited in the palladium-catalyzed to either an anti-exo-dig or an anti-endo-dig cycliza-
synthesis of indenol ethers from arylalkynes bearing tion mode (Scheme 2, paths c and d, respectively). On
ortho-acetals.[4] Similarly, alkynyl derivatives contain- the other hand, PdI2, a Lewis acidic component of our
ing an allyl ether group showed good reactivity in the catalytic system, could also assist the epoxide ring
platinum-catalyzed rearrangement of enynes.[5] Select- cleavage through the formation of a Pd(II)-chelate
ed o-alkynylallyloxybenzene derivatives were also ad- complex;[10] in this case, a syn-exo-dig cyclization
vantageously used for palladium-catalyzed allylating– would occur (Scheme 2, path b).
heteroannulation reactions[6] and for sequential homo-
bimetallic palladium-catalyzed heterocyclization reac-
tions.[3a,b]
Results and Discussion
In this work, we have investigated the possibility to
use the epoxide function as a potential nucleophile The first experiments were carried out using 2-(2-
for the PdI2/KI-catalyzed oxidative cyclization–alk- ethynylphenyl)-2-methyloxirane 1a, easily prepared in
ACHTUNGTRENNUNG
oxycarbonylation reaction of acetylenic substrates.[7] three steps starting from commercially available 1-(2-
Alkynyloxiranes derivatives have been recently used bromophenyl)ethanone (see the Supporting Informa-
in reactions affording heterocyclic compounds in the tion for details). The PdI2-catalyzed carbonylation of
presence of gold and ruthenium catalysts.[8]
1a, carried out in MeOH in the presence of an excess
The cleavage of an epoxide ring by attack of a nu- of KI (KI/PdI2 molar ratio=10), under 32 bar (at
cleophile at the less substituted carbon[9] could favor 258C) of 3/1 mixture of CO-air at 808C for 24 h led
the intramolecular nucleophilic attack by the epoxide to the product 2a-Z in 71% isolated yield (79% by
oxygen to the triple bond coordinated to Pd(II), thus GLC, Scheme 3). The structure of 2a-Z was unequivo-
Scheme 2. Alternative pathways for the oxidative cyclization–alkoxycarbonylation reaction of acetylenic substrates bearing
an oxirane moiety.
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Cascade Reactions: Catalytic Synthesis of Functionalized 1,3-Dihydroisobenzofuran
FULL PAPERS
Figure 1. Possible isomers of 2a-Z in the PdI2/KI-catalyzed
oxidative carbonylation of 1a. See text for details.
Scheme 3. Oxidative cyclization–alkoxycarbonylation reac-
tion of 2-(2-ethynylphenyl)-2-methyloxirane 1a.
(Figure 1), either deriving from a 6-endo-dig pathway
(Scheme 2, path d) or from an initial nucleophilic
attack of MeOH at the more substituted carbon of
the epoxide moiety of 1a, was observed.
cally established by spectroscopic techniques, includ-
Different conditions of temperatures and solvent
ing NOESY experiments. Small amounts (ca. 2%) of mixtures for the carbonylation reaction of 1a were
the E stereoisomer, 2a-E, were also detected by GLC then tested, and the results are shown in Table 1 (en-
and GLC-MS analysis (Table 1, entry 2). The assign- tries 3–6). As can be seen, slightly higher yields of 2a-
ment of the geometry around the double bond of the Z and 2a-E (86% and 4% respectively) were ob-
two isomers 2a-Z and 2a-E was based on the chemical served working at 1008C in a 4:1 mixture of MeCN-
shift of the aromatic proton in ortho to the double MeOH (entry 6). On the other hand, an increase of
bond (7-H). In the case of the (Z) isomer the chemi- the total pressure to 42 atm did not cause a significant
cal shift is 7.62 ppm while for (E) isomer this proton effect on yields and selectivities.
undergo a significant influence by CO2Me group and
In order to expand the synthetic scope of the reac-
1
the chemical shift is above 8 ppm. Furthermore H-1H tion, a variety of substrates containing different sub-
NOESY experiments show a clear dipolar interaction stituents on the triple bond and on the aromatic ring
between the olefinic proton and 7-H only in the case were subjected to the carbonylation conditions opti-
of the (Z) isomer.
mized for 1a, either in MeOH or in MeCN-MeOH
Formation of 2a-Z clearly corresponds to a syn-5- mixtures, at 80–1008C. The results obtained are
exo-dig cyclization pathway, which means that, under shown in Table 2.
the conditions mentioned above, the mechanism
In most cases substrates 1 were converted into the
shown in Scheme 2, path b, is preferentially followed. corresponding (Z)-1-methoxymethyl-1-methyl-3-(me-
No appreciable interconversion of product 2a-Z to thoxycarbonyl)methylene-1,3-dihydroisobenzofuran
2a-E occurred under the reaction conditions. More- derivatives 2-Z along with 1-iodomethyl-1-methyl-3-
over, no formation of isomeric 6-membered products (methoxycarbonyl)methylene-1,3-dihydroisobenzofur-
Table 1. Carbonylation reactions of 2-(2-ethynylphenyl)-2-methyloxirane 1a under different conditions.[a]
Entry
Temperature [8C]
Solvent
Conversion of 1a [%][b]
Yield of 2a-Z [%][c]
Yield of 2a-E [%][c]
1[d]
2
3
4
5
80
80
100
80
80
MeOH
MeOH
MeOH
MeCN-MeOH (17:1)
MeCN-MeOH (4:1)
MeCN-MeOH (4:1)
36
94
99
41
77
99
28
79 (71)
52
25
63
2
2
6
100
86
4
[a]
All reactions were carried out in 4 mL of solvent in the presence of PdI2 (0.067 mmol), KI (0.67 mmol), 1a (2.0 mmol)
under 32 atm total pressure (at 258C) of a 3:1 mixture CO-air for 24 h.
Determined by GLC.
GLC yield (isolated yield) based on starting 1a.
Reaction time was 8 h.
[b]
[c]
[d]
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Table 2. Synthesis of functionalized 1,3-dihydroisobenzofurans via PdI2/KI-catalyzed sequential ring opening–heterocycliza-
tion–alkoxycarbonylation of 2-(2-alkynylphenyl)oxiranes 1b–i.[a]
Entry
1
R1 R2
R3
R4
Solvent
MeOH
MeCN-MeOH 100
MeCN-MeOH 100
MeOH
MeCN-MeOH 100
MeCN-MeOH 100
Temp. Conversion 2-Z
[8C]
Yield
of 2-Z
[%][c]
3-Z Yield
of 3-Z
3-E Yield
of 3-E
of 1 [%][b]
[%][c]
[%][c]
1
1b
1b
1b
1c
1c
1c
1d
1d
1d
1d
1e
1f
1g
1h
1h
1h
1i
H
H
H
H
H
H
H
H
H
H
F
H
H
H
OMe
OMe
OMe
F
F
F
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
80
81
83
82
91
92
90
80
83
84
70
77
81
81
85
76
80
82
83
81
2b-Z
2b-Z
2b-Z
2c-Z
2c-Z
2c-Z
2d-Z
2d-Z
2d-Z
2d-Z
2e-Z
2f-Z
58 (50) 3b-Z 11
45
47
77 (70) 3c-Z
46
34
35 (28) 3d-Z 25
30
24
17
55 (47)
69 (60)
2
3b-Z 16
3b-Z 20 (12)
3[d]
4
80
5
5
3c-Z 23
3c-Z 41 (33)
6[d]
7
MeOH
80
8
MeCN-MeOH 100
MeCN-MeOH 100
MeCN-MeOH 100
MeOH
MeCN-MeOH 100
3d-Z 31
3d-Z 40 (31)
3d-Z 38
9[d]
10[e]
11
12
13[f]
14 h]
15
16[d]
17
18
19[d]
F
H
H
80
H
H
H
H
H
H
H
H
OMe OMe
H
H
H
H
H
H
H
H
H
H
H
H
H
H
TMS MeCN-MeOH 100
2a-Z[g] 69
Bu
Bu
Bu
Ph
Ph
Ph
MeOH
MeCN-MeOH 100
MeCN-MeOH 100
MeOH
MeCN-MeOH 100
MeCN-MeOH 100
80
2h-Z
2h-Z
2h-Z
2i-Z
2i-Z
2i-Z
36 (27)
35
35
25 (19)
24
3h-E 6
3h-E 20
3h-E 28 (21)
3i-E 17
3i-E 23
80
1i
1i
23
3i-E 31 (25)
[a]
Unless otherwise noted, all reactions were carried out in 4 mL of solvent (pure MeOH or a 4:1 mixture MeCN-MeOH)
in the presence of PdI2 (0.067 mmol), KI (0.67 mmol), 1 (2.0 mmol) under 32 atm total pressure (at 258C) of a 3:1 mix-
ture CO-air for 24 h.
Determined by GLC.
GLC yield (isolated yield) based on starting 1.
KI/PdI2 molar ratio was 20.
KI/PdI2 molar ratio was 30.
Reaction time was 36 h.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
Desilylation occurred under the reaction conditions, so the product formed was 2a-Z.
The reaction also led to the formation of 2-(2-hex-1-ynylphenyl)-2-methoxypropan-1-ol 4h in 35% GLC yield (25% iso-
lated).
an derivatives 3 (Table 2). These latter products clear- (R4 =TMS), 1h (R4 =n-Bu), and 1i (R4 =Ph), led to
ly derive from an initial nucleophilic attack by the less satisfactory results with respect to substrates bear-
iodide anion on the epoxide ring.[11] It was proved ing a terminal triple bond 1a–f (R4 =H) (Table 1, en-
that the formation of methoxylated and iodinated tries 2 and 6, and Table 2, entries 1–12). This can be
products derive from two competing pathways which explained on the basis of steric effects lowering coor-
predominate over each other depending on the reac- dination ability to PdI2 of an internal triple bond com-
tion conditions and substrate structures. As expected, pared to a terminal triple bond.[2f,p,q,3] As we already
formation of 3 became more favored by increasing observed in other PdI2-catalyzed oxidative carbonyla-
the KI concentration (entries 3, 6, 9, 16, and 19). Thus tions,[2f,p,q,3] desilylation occurred in the case of trime-
an increase of the KI/PdI2 molar ratio to 30:1, led to thylsilyl-substituted substrate 1g (R4 =TMS), with se-
higher selectivity towards product 3, but depressed lective formation of the dihydroisobenzofuran deriva-
the reaction conversion as shown in entries 9 and 10. tive 2a-Z (R4 =H) in 69% yield (entry 13).
As can be seen from Table 2, entries 13–19, sub-
Interestingly, substrates such as 1h and 1i, substitut-
strates bearing an internal triple bond, such as 1g ed with an alkyl or a phenyl group in the triple bond,
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Cascade Reactions: Catalytic Synthesis of Functionalized 1,3-Dihydroisobenzofuran
FULL PAPERS
Scheme 4. Attack of MeOH (path a) or IÀ anion (path b) in the presence of a substituent on the triple bond (R4 =n-Bu or
Ph).
afforded the corresponding iodinated products with E
In the case of substrate 1h (R4 =n-Bu), under the
rather than Z stereochemistry (3h-E and 3i-E, respec- conditions of entry 14 (Table 2), a non-cyclized prod-
tively). This could be related to the remarkable steric uct 4h, corresponding to MeOH attack on the more
hindrance of the chelate complex A; in case of IÀ substituted carbon of the epoxide moiety (Scheme 6),
anion attack to the epoxide ring (Scheme 4, path b), was also isolated from the reaction mixture in 25%
PdI2 is forced to rotate around the triple bond in an yield. This result shows that neither a 6-exo-dig cycli-
anti mode to the oxirane moiety. As a consequence, a zation nor a 7-endo-dig cyclization is favored under
benzyl alkoxide intermediate is formed, which, as ex- the reaction conditions.
pected,[3e] selectively undergoes anti-5-exo-dig cycliza-
With the aim of expanding the scope of the reac-
tion to give the final product with E geometry. If tion, we also tested the reactivity of a non-cyclic ep-
MeOH attacks the oxirane unit, HI elimination can oxyalkyne substrate, such as 2-ethyl-2-(2-methyloxira-
reduce the steric hindrance in complex A thus leading nyl)pent-4-ynoic acid methyl ester 5. Thus, the oxida-
to the Z isomer (Scheme 4, path a). On the other tive carbonylation of 5 (1 mmol), carried out at 858C
hand, in the absence of a substituent on the triple in a 1:1 mixture of MeCN-MeOH (2 mL total) in the
bond (R4 =H), iodide attack occurs after chelation to presence of PdI2 (0.033 mmol) and KI (0.66 mmol)
PdI2, thus leading to the product with Z geometry under 42 atm (at 258C) of a 17:4 mixture of CO-air,
(Scheme 5). This is confirmed by the structure of led to the desired 5-[(methoxycarbonyl)methylene]te-
product 3d-Z determined by single crystal X-ray dif- trahydrofuran derivatives 6 and 7 in fairly good total
fraction analysis (Figure 2).[12]
isolated yield (53%, Scheme 7).
This result is noteworthy, if one considers the signif-
icantly higher conformational mobility of 5 as com-
pared to that of substrates 1. The higher conforma-
tional mobility of 5 with respect to 1 may, however,
be responsible for the observed E stereochemistry
around the exocyclic double bond of both 6 and 7. In
fact, this mobility may prevent chelation of the oxir-
ane oxygen and the triple bond to PdI2, thus hinder-
ing a 5-endo-dig mechanism. As a consequence, the
anti-5-exo-dig pathway becomes favored, leading to
the final products with E geometry (Scheme 8).
Finally, we tested amines instead of alcohols as nu-
cleophiles in our new cascade reaction. When sub-
strate 1a was allowed to react in MeCN at 1008C for
24 h, in the presence of piperidine (piperidine:1a
molar ratio=3:1), PdI2 (3.3 mol%) and KI (KI/PdI2
molar ratio=20), under 40 atm (at 258C) of a 4:1
mixture CO-air, [2-(3-iodomethyl-3-methyl-3H-iso-
benzofuran-1-ylidene)-1-piperidin-1-yl]ethanone
8
Scheme 5. Formation pathway of 3b–d-Z compounds.
was selectively obtained in 49% isolated yield
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Figure 2. ORTEP view and labelling scheme for compound 3d-Z.
Scheme 6. MeOH attack to the more substituted carbon of
1h.
Scheme 8. Formation pathway of 6 and 7.
Scheme 7. Oxidative cyclization–alkoxycarbonylation reac-
tion of 5.
Scheme 9. Oxidative cyclization–aminocarbonylation reac-
tion of 1a.
(Scheme 9). This result shows that, under these condi- Conclusions
tions, ring opening of the epoxide ring is initiated by
the iodide anion, while the amine intervenes in the Summing up, we have demonstrated that highly func-
final aminocarbonylation step.
tionalized 1,3-dihydroisobenzofuran and tetrahydro-
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Cascade Reactions: Catalytic Synthesis of Functionalized 1,3-Dihydroisobenzofuran
FULL PAPERS
furan derivatives can be successfully obtained in one General Procedure for the Catalytic Synthesis of
step from alkynyloxiranes through a novel sequential Tetrahydrofuran Derivatives 6 and 7 by Sequential
nucleophilic ring opening–heterocyclization–oxidative Nucleophilic Ring Opening–Heterocyclization–
carbonylation process, catalyzed by PdI2 in conjunc- Oxidative Methoxycarbonylation of 2-Ethyl-2-(2-
tion with an excess of KI. Different mechanistic path- methyloxiranyl)pent-4-ynoic Acid Methyl Ester 5
ways may be at work, depending on the nature of the
A 45-mL stainless steel autoclave was charged with PdI2
substrate and the nucleophile, and on reaction condi-
(12.0 mg, 0.033 mmol), KI (111 mg, 0.67 mmol) and a solu-
tions. This new cascade reaction allows a direct entry
to these important classes of heterocyclic derivatives,
which would be difficult to synthesize by “classical”
approaches not involving organometallic catalysis.
tion of 5 (196 mg, 1 mmol) in a 1:1 MeCN/MeOH mixture
(2 mL of solvent). The autoclave was then sealed. While the
mixture was stirred, the autoclave was charged with CO
(34 atm) and air (up to 42 atm), and then heated at 858C)
for 24 h. After cooling, the autoclave was degassed and
opened. The solvent was evaporated under reduced pressure
and products 6 (pale yellow oil, yield: 63 mg, 22%) and 7
(pale yellow oil, yield: 119 mg, 31%) were separated by
column chromatography (SiO2) using 40:60 hexane-EtOAc
as eluent.
Experimental Section
General Remarks
Solvents and chemicals were reagent grade and were used
without further purification. MeCN was dried over 3 ꢂ mo-
lecular sieves and stored under nitrogen. All reactions were
analyzed by TLC and by GLC using a 30 m SE-30 capillary
column. Column chromatography was performed on silica
gel 60 (70–230 mesh). Melting points were measured with
an Electrothermal apparatus and are uncorrected. Electron
impact mass spectra [m/z, relative intensity (%)] were deter-
mined with a GC-MS apparatus at 70 eV ionization energy.
Infrared (IR) spectra were recorded on an FT-IR 5700 spec-
trophotometer. Elemental analyses were performed at our
General Procedure for the Catalytic Synthesis of 2-(3-
Iodomethyl-3-methyl-3H-isobenzofuran-1-ylidene)-1-
piperidin-1-ylethanone 8 by Sequential Nucleophilic
Ring Opening–Heterocyclization–Oxidative
Aminocarbonylation of 2-(2-Ethynylphenyl)-2-
methyloxirane 1a
A 45-mL stainless steel autoclave was charged with PdI2
(24.0 mg, 0.067 mmol), KI (222 mg, 1.34 mmol) and a solu-
tion of 1a (316 mg, 2 mmol) and piperidine (510 mg,
6 mmol) in pure MeCN (4 mL). The autoclave was then
sealed. While the mixture was stirred, the autoclave was
charged with CO (32 atm) and air (up to 40 atm), and then
heated at 1008C for 24 h. After cooling, the autoclave was
degassed and opened. The solvent was evaporated under re-
duced pressure and product 8 was separated by column
chromatography (SiO2) using 85:15 hexane-EtOAc as
eluent: yield: 390 mg (49%).
1
analytical laboratory. H NMR and 13C NMR spectra were
recorded in CDCl3 at 300 and 75 MHz, respectively, using
1
the solvent as internal standard (7.26 ppm for H NMR and
77.0 ppm for 13C NMR). Chemical shifts (d) and coupling
constants (J) are given in ppm and in Hz, respectively. The
reported configurations (E or Z) were assigned on the basis
of decoupling, COSY, and NOESY correlation experiments,
and of X-ray diffraction analysis for compound 3d-Z.
(Z)-3-Methoxycarbonylmethylene-1-methoxymethyl-1-
methyl-3H-isobenzofuran (2a-Z): Yield: 71% (Table 1,
entry 2); eluent: hexane-EtOAc 90:10; pale yellow oil. IR
(film): n=2941 (m), 1715 (s), 1621 (s), 1463 (m), 1100 (s),
General Procedure for the Catalytic Synthesis of 1,3-
Dihydroisobenzofuran Derivatives 2 and 3 by
Sequential Nucleophilic Ring Opening–
Heterocyclization–Oxidative Methoxycarbonylation
of 2-(2-Alkynylphenyl)oxiranes 1a–i (Tables 1 and 2)
1
810 (m), 772 (m) cmÀ1; H NMR (CDCl3): d=7.62 (further
split d, J=7.9 Hz, 1H, CH aromatic), 7.50–7.42 (m, 2H, CH
aromatic), 7.33 (ddd, J=7.9, 7.0, 1.9 Hz, 1H, CH aromatic),
5.64 (s, 1H, C=CH), 4.34 (d, J=11.1 Hz, 1H, CHH), 4.10 (d,
J=11.1 Hz, 1H, CHH), 3.70 (s, 3H, CO2CH3), 3.16 (s, 3H,
OCH3), 1.50 (s, 3H, CH3); 13C NMR (CDCl3): d=166.0,
161.2, 138.1, 130.7, 128.4, 127.8, 125.5, 124.8, 92.7, 71.5, 71.1,
51.1, 50.7, 20.6; MS: m/z=248 (M+, 85), 233 (38), 217 (57),
203 (80), 185 (41), 159 (95), 144 (50), 129 (100), 115 (91),
101 (33), 79 (27), 59 (25); anal. calcd. for C14H16O4: C 67.73,
H 6.50; found: C 67.84, H 6.56.
A 45-mL stainless steel autoclave was charged with PdI2
(24.0 mg, 0.067 mmol), KI (111 mg, 0.67 mmol, or 222 mg,
1.34 mmol - see Tables 1 and 2) and a solution of 1
(2 mmol) in pure MeOH or in a 4:1 MeCN/MeOH mixture
(4 mL of solvent – see Tables 1 and 2). The autoclave was
then sealed. While the mixture was stirred, the autoclave
was charged with CO (24 atm) and air (up to 32 atm), and
then heated at the required temperature (80 or 1008C) for
24 h–36 h (see Tables 1 and 2). After cooling, the autoclave
was degassed and opened. The solvent was evaporated
under reduced pressure, and products 2 and 3 were separat-
ed by column chromatography (SiO2) using suitable mix-
tures of hexane-EtOAc as eluent (see the characterization
data for products for details regarding the eluent and for the
yields obtained in each case).
(Z)-3-Methoxycarbonylmethylene-1-methoxymethyl-1,5-
dimethyl-3H-isobenzofuran (2b-Z): Yield: 50% (Table 2,
entry 1); eluent: hexane-EtOAc 90:10; pale yellow oil. IR
(film): n=2945 (m), 1715 (s), 1625 (s), 1607 (s), 1434 (m),
1102 (s), 821 (m) cmÀ1 1H NMR (CDCl3): d=7.45 (br s,
;
1H, CH aromatic), 7.37 (d, J=7.9 Hz, 1H, CH aromatic),
7.27 (further split d, J=7.9 Hz, 1H, CH aromatic), 5.63 (s,
1H, C=CH), 4.34 (d, J=11.0 Hz, 1H, CHH), 4.07 (d, J=
11.0 Hz, 1H, CHH), 3.70 (s, 3H, CO2CH3), 3.14 (s, 3H,
OCH3), 2.36 (s, 3H, CH3), 1.48 (s, 3H, CH3); 13C NMR
Adv. Synth. Catal. 2009, 351, 2423 – 2432
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Nicola Della Ca’ et al.
FULL PAPERS
(CDCl3): d=166.1, 161.5, 138.3, 134.9, 131.6, 127.6, 126.0,
124.9, 92.4, 71.8, 70.9, 51.1, 50.8, 21.2, 20.4; MS: m/z=262
(M+, 100), 247 (61), 231 (63), 217 (65), 199 (52), 185 (28),
173 (96), 158 (50), 143 (73), 129 (65), 128 (98), 115 (85), 91
(21), 59 (20); anal. calcd. for C15H18O4: C 68.68, H 6.92;
found: C 68.83, H 6.99.
entry 9); eluent: hexane-EtOAc 90:10. 1H NMR (CDCl3):
d=7.53 (dd, J=8.5, 4.6 Hz, 1H, CH aromatic), 7.15 (td, J=
8.4, 2.3 Hz, 1H, CH aromatic), 7.07 (dd, J=7.8, 2.1 Hz, 1H,
CH aromatic), 5.43 (s, 1H, C=CH), 3.74 (s, 3H, CO2CH3),
3.66 (d, J=10.9 Hz, 1H, CHH), 3.60 (d, J=10.9 Hz, 1H,
CHH), 1.87 (s, 3H, CH3); MS: m/z=362 (M+, 21), 331 (11),
221 (100), 189 (15), 133 (19); anal. calcd. for C13H12FIO3: C
43.12, H 3.34; found: C 42.97, H 3.29.
(Z)-7-Fluoro-3-methoxycarbonylmethylene-1-methoxy-
methyl-1-methyl-3H-isobenzofuran (2e-Z): Yield: 47%;
eluent: hexane-EtOAc 90:10 (Table 2, entry 11). 1H NMR
(CDCl3): d=7.50 (dd, J=8.0, 1.0 Hz, 1H, CH aromatic),
7.36 (td, J=8.1, 5.2 Hz, 1H, CH aromatic), 7.16 (ddd, J=
10.9, 8.1, 1.0 Hz, 1H, CH aromatic), 5.66 (s, 1H, C=CH),
4.37 (d, J=11.6 Hz, 1H, CHH), 4.02 (d, J=11.6 Hz, 1H,
CHH), 3.73 (s, 3H, CO2CH3), 3.22 (d, J=0.9 Hz, 3H,
OCH3), 1.64 (d, J=3.8 Hz, 3H, CH3); 13C NMR (CDCl3):
d=165.9, 160.8 (d, JC,F =249.4 Hz), 130.0 (d, JC,F =8.9 Hz),
129.7 (d, JC,F =9.1 Hz), 121.7 (d, JC,F =3.4 Hz), 121.6 (d,
(Z)-1-(Iodomethyl)-3-methoxycarbonylmethylene-1,5-di-
methyl-3H-isobenzofuran (3b-Z): Yield: 12% (Table 2,
entry 3); eluent: hexane-EtOAc 90:10; pale yellow oil. IR
(film): n=2930 (m), 1719 (s), 1621 (s), 1603 (s), 1421 (m),
1
1100 (s), 794 (m) cmÀ1; H NMR (CDCl3): d=7.42 (s further
split, 1H, CH aromatic), 7.33 (d, J=7.8 Hz, 1H, CH aro-
matic), 7.26 (d further split, J=7.8 Hz, 1H, CH aromatic),
5.61 (s, 1H, C=CH), 3.79 (s, 3H, CO2CH3), 3.58 (d, J=
10.6 Hz, 1H, CHHI), 3.45 (d, J=10.6 Hz, 1H, CHHI), 2.34
(s, 3H, CH3), 1.71 (s, 3H, CH3); MS: m/z=358 (M+, 8), 217
(100), 199 (32), 185 (18), 172 (26), 141 (14), 128 (35), 115
(26), 59 (7); anal. calcd. for C14H15IO3: C 46.95, H 4.22;
found: C 46.83, H 4.17.
(Z)-6-Methoxy-3-methoxycarbonylmethylene-1-methoxy-
methyl-1-methyl-3H-isobenzofuran (2c-Z): Yield: 70%
(Table 2, entry 4); eluent: hexane-EtOAc 85:15; pale yellow
oil. IR (film): n=2946 (m), 2837 (w), 1713 (s), 1601 (s), 1492
JC,F =2.6 Hz), 118.3 (d, JC,F =23.7 Hz), 116.2 (d, JC,F =
23.3 Hz), 94.3, 72.6, 68.1, 51.5, 50.9, 20.2 (d, JC,F =7.9 Hz);
MS: m/z=266 (M+, 63), 251 (24), 235 (42), 221 (100), 203
(33), 189 (27), 177 (52), 165 (29), 147 (52), 133 (46), 88 (20),
59 (12); anal. calcd. for C14H15FO4: C 63.15, H 5.58; found:
C 63.31, H 5.61.
(m), 1288 (m), 1253 (m), 1160 (s), 1101 (s), 808 (m) cmÀ1
;
1H NMR (CDCl3): d=7.56 (d, J=8.7 Hz, 1H, CH aromatic),
6.97 (d, J=2.6 Hz, 1H, CH aromatic), 6.84 (dd, J=8.7,
2.6 Hz, 1H, CH aromatic), 5.51 (s, 1H, C=CH), 4.28 (d, J=
10.9 Hz, 1H, CHH), 4.11 (d, J=10.9 Hz, 1H, CHH), 3.83 (s,
3H, OCH3), 3.67 (s, 3H, OCH3), 3.18 (s, 3H, OCH3), 1.47
(s, 3H, CH3); 13C NMR (CDCl3): d=166.2, 161.8, 140.5,
131.8, 127.5, 120.0, 114.3, 109.5, 90.8, 71.3, 71.2, 55.4, 51.2,
50.7, 21.3; MS: m/z=278 (M+, 100), 263 (9), 247 (45), 233
(31), 220 (30), 201 (14), 189 (31), 175 (25), 159 (33), 145
(15), 115 (15), 59 (13); anal. calcd. for C15H18O5: C 64.74, H
6.52; found C 64.86, H 6.57.
(Z)-5,6-Dimethoxy-3-methoxycarbonylmethylene-1-me-
thoxymethyl-1-methyl-3H-isobenzofuran
(2f-Z):
Yield:
60%; eluent: hexane-EtOAc 85:15 (Table 2, entry 12): pale
yellow oil. IR (film): n=2940 (m), 1710 (s), 1600 (s), 1512
(s), 1465 (m), 1274 (s), 1159 (s), 1102 (s) cmÀ1 1H NMR
;
(CDCl3): d=7.02 (s, 1H, CH aromatic), 6.92 (s, 1H, CH aro-
matic), 5.50 (s, 1H, C=CH), 4.29 (d, J=11.0 Hz, 1H, CHH),
4.11 (d, J=11.0 Hz, 1H, CHH), 3.91 (s, 3H, OCH3), 3.87 (s,
3H, OCH3), 3.68 (s, 3H, OCH3), 3.18 (s, 3H, OCH3), 1.47
(s, 3H, CH3); 13C NMR (CDCl3): d=166.1, 161.4, 151.8,
148.8, 132.4, 120.0, 107.4, 106.8, 91.1, 71.4, 71.0, 56.0, 55.9,
51.3, 50.8, 21.8; MS: m/z=308 (M+, 100), 293 (55), 277 (75),
245 (73), 219 (33), 189 (41), 175 (20), 59 (12); anal. calcd.
for C16H20O6: C 62.33, H 6.54; found: C 62.48, H 6.58.
(Z)-1-Iodomethyl-6-methoxy-3-methoxycarbonylmethy-
lene-1-methyl-3H-isobenzofuran
(3c-Z):
Yield:
33%
(Table 2, entry 6); eluent: hexane-EtOAc 85:15: pale yellow
oil. IR (film): n=2945 (m), 1718 (s), 1607 (s), 1497 (m),
1279 (m), 1250 (m), 1168 (s), 1104 (s), 803 (m) cmÀ1
;
(Z)-3-[(2-Butyl-2-methoxycarbonyl)methylene]-1-meth-
1H NMR (CDCl3): d=7.53 (d, J=8.6 Hz, 1H, CH aromatic),
6.95 (d, J=2.6 Hz, 1H, CH aromatic), 6.83 (dd, J=8.6,
2.6 Hz, 1H, CH aromatic), 5.49 (s, 1H, C=CH), 3.81 (s, 3H,
CO2CH3), 3.68 (s, 3H, OCH3), 3.55 (d, J=10.7 Hz, 1H,
CHHI), 3.43 (d, J=10.7 Hz, 1H, CHHI), 1.71 (s, 3H, CH3);
MS: m/z=374 (M+, 25), 343 (9), 233 (100), 215 (10), 201
(14), 191 (18), 159 (14), 145 (15), 115 (19); anal. calcd for
C14H15IO4: C 44.94, H 4.04; found: C 45.12, H 4.09.
(Z)-6-Fluoro-3-methoxycarbonylmethylene-1-(methoxy-
methyl)-1-methyl-3H-isobenzofuran (2d-Z): Yield: 28%
(Table 2, entry 7); eluent: hexane-EtOAc 90:10. 1H NMR
(CDCl3): d=7.65 (dd, J=8.9, 5.2 Hz, 1H, CH aromatic),
7.21 (dd, J=9.0, 2.7 Hz, 1H, CH aromatic), 7.05 (td, J=8.7,
2.8 Hz, 1H, CH aromatic), 5.59 (s, 1H, C=CH), 4.31 (d, J=
11.0 Hz, 1H, CHH), 4.16 (d, J=11.0 Hz, 1H, CHH), 3.72 (s,
3H, CO2CH3), 3.22 (s, 3H, OCH3), 1.51 (s, 3H, CH3); MS:
m/z=266 (M+, 100), 251 (31), 235 (68), 221 (77), 203 (36),
189 (32), 177 (69), 162 (48), 147 (78), 133 (67), 59 (25); anal.
calcd. for C14H15FO4: C 63.15, H 5.58; found: C 63.26, H
5.53.
ACHTUNGTRENoNUNG xymethyl-1-methyl-3H-isobenzofuran (2h-Z): Yield: 27%;
eluent: hexane-EtOAc 90:10 (Table 2, entry 14); pale yellow
oil. IR (film): n=2955 (s), 2930 (s), 2871 (s), 1711 (s), 1619
(m), 1460 (s), 1283 (m), 1117 (s), 1082 (s), 763 (s) cmÀ1
;
1H NMR (CDCl3): d=7.44–7.41 (m, 1H, CH aromatic),
7.38–7.31 (m, 2H, CH aromatic), 7.27–7.24 (m, 1H, CH aro-
matic), 4.17 (d, J=10.8 Hz, 1H, CHH), 3.84 (d, J=10.8 Hz,
1H, CHH), 3.57 (s, 3H, CO2CH3), 3.16 (s, 3H, OCH3), 2.50
(t, J=7.6 Hz, 2H, C=CCH2), 1.51 (s, 3H, CH3), 1.51–1.35
(m, 4H, CH2CH2), 0.92 (t, J=7.2 Hz, 3H, CH2CH3);
13C NMR (CDCl3): d=171.1, 160.9, 137.7, 129.2, 128.9,
128.8, 127.0, 123.8, 105.9, 72.3, 71.6, 51.2, 51.1, 30.6, 27.5,
22.3, 19.0, 13.9; MS: m/z=304 (M+, 10), 272 (14), 229 (100),
201 (30), 186 (20), 171 (22), 156 (16), 143 (30), 128 (21), 115
(35), 59 (55); anal. calcd. for C18H24O4: C 71.03, H 7.95;
found: C 71.11, H 8.01.
(E)-3-[(2-Butyl-2-methoxycarbonyl)methylene]-1-iodo-
methyl-1-methyl-3H-isobenzofuran (3h-E): Yield: 21%
(Table 2, entry 16); eluent: hexane-EtOAc 90:10; pale
yellow oil. IR (film): n=2953 (s), 2928 (s), 2868 (m), 1703
(s), 1613 (s), 1463 (s), 1444 (m), 1096 (s), 1054 (m), 952 (m),
(Z)-6-Fluoro-1-iodomethyl-3-methoxycarbonylmethylene-
1-methyl-3H-isobenzofuran (3d-Z): Yield: 31% (Table 2,
1
763 (s) cmÀ1; H NMR (CDCl3): d=8.66–8.63 (m, 1H, CH
2430
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Cascade Reactions: Catalytic Synthesis of Functionalized 1,3-Dihydroisobenzofuran
FULL PAPERS
1
aromatic), 7.44–7.38 (m, 2H, CH aromatic), 7.26–7.22 (m,
1H, CH aromatic), 3.81 (s, 3H, CO2CH3), 3.58 (d, J=
10.6 Hz, 1H, CHHI), 3.45 (d, J=10.6 Hz, 1H, CHHI), 2.55
(t, J=7.0 Hz, 2H, C=CCH2), 1.75 (s, 3H, CH3), 1.58–1.35
(m, 4H, CH2CH2), 0.94 (t, J=7.2 Hz, 3H, CH2CH3);
13C NMR (CDCl3): d=169.2, 162.0, 147.4, 131.5, 130.1,
128.8, 127.2, 120.3, 106.7, 85.6, 51.1, 31.0, 27.9, 25.6, 22.6,
14.2, 14.0; MS: m/z=400 (M+, 10), 369 (5), 357 (25), 259
(100), 212 (40), 170 (18), 144 (22), 115 (41), 59 (10); anal.
calcd. for C17H21O3I: C 51.01, H 5.29; found: C 51.17, H
5.34.
(s), 1024 (s), 798 (m) cmÀ1; H NMR (CDCl3): d=5.25 (t,
J=1.5 Hz, 1H, C=CH), 3.69 (s, 3H, OCH3), 3.66 (s, 3H,
OCH3), 3.63–3.55 (m, 2H), 3.41–3.33 (m, 2H), 3.23 (s, 3H,
OCH3), 1.95–1.86 (m, 1H, CHHCH3), 1.47–1.34 (m, 1H,
CHHCH3), 1.37 (s, 3H, CH3), 0.86 (t, J=7.4 Hz, 3H,
CH2CH3); MS: m/z=286 (M+, 14), 255 (16), 241 (28), 227
(14), 209 (100), 181 (18), 69 (11), 59 (10); anal. calcd. for
C14H22O6: C 58.73, H 7.74; found: C 58.87, H 7.69.
(E)-3-Ethyl-2-iodomethyl-5-(methoxycarbonylmethylene)-
2-methyltetrahydrofuran-3-carboxylic acid methyl ester (7):
Yield: 31% (Scheme 7); eluent: hexane-CH2Cl2 40:60; pale
yellow oil. IR (film): n=2948 (s), 2880 (s), 1735 (s), 1706
(s), 1650 (s), 1435 (s), 1358 (s), 1241 (m), 1125 (s), 825 (m),
2-[2-(Hex-1-ynyl)phenyl]-2-methoxypropan-1-ol
(4h):
Yield: 25% (Table 2, entry 14); eluent: hexane-EtOAc
90:10; pale yellow oil. IR (film): n=3448 (m), 2929 (s), 2871
(s), 2225 (w), 1461 (m), 1259 (m), 1075 (s), 1032 (s), 760 (s)
730 (s) cmÀ1; H NMR (CDCl3): d=5.35 (t, J=1.6 Hz, 1H,
1
C=CH), 3.73 (s, 3H, OCH3), 3.66 (s, 3H, OCH3), 3.58 (fur-
ther split d, J=13.6 Hz, 1H), 3.37 (dd, J=13.6, 1.4 Hz, 1H),
3.30 (further split d, J=10.9 Hz, 1H, CHHI), 3.15 (d, J=
10.9 Hz, 1H, CHHI), 2.08–1.94 (m, 1H, CHHCH3), 1.55 (s,
3H, CH3), 1.51–1.38 (m, 1H, CHHCH3), 0.84 (t, J=7.4 Hz,
3H, CH2CH3); 13C NMR (CDCl3): d=172.1, 171.4, 168.5,
91.7, 87.0, 57.0, 52.4, 50.7, 36.4, 26.9, 21.1, 11.9, 9.4; MS:
m/z=382 (M+, 30), 353 (56), 323 (100), 291 (22), 209 (14),
163 (22), 69 (18), 59 (15); anal. calcd. for C13H19IO5: C
40.85, H 5.01; found: C 40.73, H 4.96.
1
cmÀ1; H NMR (DMSO-d6): d=7.42 (dd, J=7.8, 1.5 Hz, 1H,
CH aromatic), 7.36 (dd, J=7.8, 1.7 Hz, 1H, CH aromatic),
7.28 (td, J=7.8, 1.7 Hz, 1H, CH aromatic), 7.20 (td, J=7.8,
1.5 Hz, 1H, CH aromatic), 4.55 (t, J=5.6 Hz, 1H, OH), 3.83
(dd, J=11.2, 5.6 Hz, 1H, CHH-OH), 3.67 (dd, J=11.2,
5.6 Hz, 1H, CHH-OH), 3.10 (s, 3H, OCH3), 2.43 (t, J=
6.8 Hz, 2H, ꢀC-CH2), 1.60 (s, 3H, CH3), 1.56–1.41 (m, 4H,
CH2CH2), 0.91 (t, J=7.2 Hz, 3H, CH3); 13C NMR (DMSO-
d6): d=144.1, 134.3, 127.5, 127.2, 126.5, 121.1, 94.5, 80.7,
80.0, 66.1, 49.7, 30.0, 21.3, 20.0, 18.5, 13.3; MS: m/z=246
(M+, 2), 215 (100), 159 (41), 128 (20), 115 (15); anal. calcd.
for C16H22O2: C 78.01, H 9.00; found: C 77.88, H 8.96.
(Z)-[2-(3-Iodomethyl-3-methyl-3H-isobenzofuran-1-yli-
dene)-1-piperidin-1-yl]ethanone
(Scheme 8); eluent: hexane-EtOAc 85:15; pale yellow oil.
(8):
Yield:
49%
IR (film): n=2934 (s), 2855 (s), 1655 (s), 1604 (s), 1442 (s),
(Z)-3-[(2-Methoxycarbonyl-2-phenyl)methylene]-1-me-
thoxymethyl-1-methyl-3H-isobenzofuran (2i-Z): Yield: 19%
(Table 2, entry 17); eluent: hexane-EtOAc 90:10; pale
yellow oil. IR (film): n=2971 (s), 2944 (s), 1703 (s), 1611
1253 (m), 1019 (m), 948 (w), 761 (w) cmÀ1 1H NMR
;
(CDCl3): d=7.54–7.35 (m, 3H, CH aromatic), 7.29–7.25 (m,
1H, CH aromatic), 5.54 (s, 1H, =CH), 3.69–3.50 (m, 6H),
1.79 (s, 3H, CH3), 1.73–1.51 (m, 6H, 3 CH2); 13C NMR
(CDCl3, 230 K): d=165.6, 158.0, 145.4, 133.0, 130.6, 129.2,
121.0, 120.9, 89.5, 88.4, 48.5, 42.6, 26.7, 25.7, 25.6, 24.7, 15.0;
MS: m/z=397 (M+, 25), 313 (100), 286 (18), 270 (26), 256
(32), 242 (20), 186 (24), 158 (80), 145 (52), 129 (28), 115
(50), 84 (60), 69 (12); anal. calcd. for C17H20INO2: C 51.40,
H 5.07; found: C 51.51, H 5.09.
(m), 1442 (s), 1272 (m), 1083 (s), 775 (s) cmÀ1 1H NMR
;
(CDCl3): d=7.47–7.32 (m, 3H, CH aromatic), 7.29–7.23 (m,
2H, CH aromatic), 7.20–7.03 (m, 4H, CH aromatic), 4.13
(d, J=10.9 Hz, 1H, CHH), 3.88 (d, J=10.9 Hz, 1H, CHH),
3.78 (s, 3H, CO2CH3), 3.21 (s, 3H, OCH3), 1.70 (s, 3H,
CH3); 13C NMR (CDCl3): d=168.7, 159.1, 141.8, 132.6,
128.7, 128.0, 127.7, 126.4, 126.3, 126.2, 126.0, 120.6, 104.8,
73.1, 71.9, 52.2, 51.9, 21.5; MS: m/z=324 (M+, 45), 279
(100), 233 (15), 205 (36), 191 (24), 175 (39), 115 (25), 77
(20), 59 (15); anal. calcd. for C20H20O4: C 74.06, H 6.21;
found: C 73.95, H 6.23.
Acknowledgements
(E)-1-(Iodomethyl)-3-[(1-methoxycarbonyl-2-phenyl)-
methylene]-1-methyl-3H-isobenzofuran (3i-E): Yield: 25%
(Table 2, entry 19); eluent: hexane-EtOAc 90:10; pale
yellow oil. IR (film): n=2962 (s), 1700 (s), 1596 (m), 1413
(m), 1263 (s), 1060 (s), 1027 (s), 864 (m), 793 (s), 742 (s)
Financial support from The Ministero dell’Universitꢀ e della
Ricerca Scientifica e Tecnologica (Progetto d’Interesse Nazio-
nale PRIN 2006031888) is acknowledged. The facilities of
Centro Interfacoltꢀ di Misure (Universitꢀ di Parma) were
used for recording NMR spectra.
1
cmÀ1; H NMR (CDCl3): d=8.43–8.40 (m, 1H, CH aromat-
ic), 7.51–7.43 (m, 4H, CH aromatic), 7.40–7.35 (m, 2H, CH
aromatic), 7.30–7.26 (m, 2H, CH aromatic), 3.80 (s, 3H,
CO2CH3), 3.57 (d, J=10.7 Hz, 1H, CHHI), 3.49 (d, J=
10.7 Hz, 1H, CHHI), 1.72 (s, 3H, CH3); 13C NMR (CDCl3):
d=169.0, 160.3, 147.5, 135.9, 131.6, 130.6, 129.8, 129.0, 127.8,
126.6, 126.3, 120.4, 106.8, 86.9, 51.8, 25.3, 13.8; MS: m/z=
420 (M+, 33), 279 (100), 234 (10), 205 (17), 191 (15), 115
(20), 77 (10), 59 (9); anal. calcd. for C19H17O3I: C 54.30, H
4.08; found: C 54.21, H 4.12.
(E)-3-Ethyl-5-(methoxycarbonylmethylene)-2-methoxy-
methyl-2-methyltetrahydrofuran-3-carboxylic acid methyl
ester (6): Yield: 22% (Scheme 7); eluent: hexane-CH2Cl2
40:60; pale yellow oil. IR (film): n=2954 (m), 1730 (s), 1704
(s), 1644 (s), 1434 (m), 1359 (m), 1258 (m), 1122 (s), 1094
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asc.wiley-vch.de
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Nicola Della Ca’ et al.
FULL PAPERS
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Adv. Synth. Catal. 2009, 351, 2423 – 2432