Gold(I)-catalyzed cycloisomerization of 2-fluoroalk-3-yn-1-ones: Synthesis of 2,5-substituted 3-fluorofurans

Article abstract of DOI:10.1002/adsc.201000411

Fluorination of 1,4-disubstituted tert-butyldimethylsilyl but-1-en-3-yn-1-yl ethers with Selectfluor gives 2-mono-fluorobut-3-yn-1-ones. Subsequent 5-endo-dig cyclization in the presence of chlorotriphenylphosphine gold(I)/silver trifluoromethanesulfonate (both 5 mol%, dichloromethane) under ambient conditions, provides a facile method for the generation of mainly 2,5-diaryl-substituted 3-fluorofurans in high yields (89-96%). The structure of 2-(4-bromophenyl)-3-fluoro-5-(4-methylphenyl)furan was confirmed by X-ray crystallography.

Full text of DOI:10.1002/adsc.201000411

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DOI: 10.1002/adsc.201000411
Gold(I)-Catalyzed Cycloisomerization of 2-Fluoroalk-3-yn-1-ones:
Synthesis of 2,5-Substituted 3-Fluorofurans
Yan Li,a Kraig A. Wheeler,b and Roman Dembinskia,*
a
Department of Chemistry, Oakland University, 2200 N. Squirrel Rd., Rochester, Michigan 48309-4477, USA
Fax : (+1)-248-370-2321; e-mail: dembinsk@oakland.edu
Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, Illinois 61920-3099, USA
b
Received: May 25, 2010; Revised: September 11, 2010; Published online: October 26, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000411.
Abstract: Fluorination of 1,4-disubstituted tert-bu-
tyldimethylsilyl but-1-en-3-yn-1-yl ethers with Se-
lectfluor gives 2-mono-fluorobut-3-yn-1-ones. Sub-
sequent 5-endo-dig cyclization in the presence of
chlorotriphenylphosphine gold(I)/silver trifluorome-
thanesulfonate (both 5 mol%, dichloromethane)
under ambient conditions, provides a facile method
for the generation of mainly 2,5-diaryl-substituted
3-fluorofurans in high yields (89–96%). The struc-
ture of 2-(4-bromophenyl)-3-fluoro-5-(4-methylphe-
nyl)furan was confirmed by X-ray crystallography.
growth as an inhibitor of the p53-MDM2 interac-
tion.[2] Pentamidine (DB289, 1d) is a prodrug of fura-
midine (DB75, 1e) that has demonstrated good effica-
cy against African trypanosomiasis, pneumonia, as
well as malaria in clinical trials.[3]
Active pharmaceutical ingredients containing fluo-
rine have found a wide application in medicinal
chemistry.[4,5] Over the past half century, fluorinated
drugs represent 5–15% of the total number of
launched drugs worldwide with a noticeable increase
in recent years.[6] Since the furan ring constitutes a
submotif encountered in lead compounds, the corre-
sponding fluorinated molecules are potentially
sought-after building blocks. Indeed, fluorofuran or
perfluoroalkylfuran fragments have already been em-
bedded within structures possessing important phar-
macological properties.[7] For example, an analogue of
trovirdine (2, Figure 1) reveals high anti-HIV-1 activi-
Keywords: alkynes; cyclization; fluorine; furans;
gold; oxygen heterocycles
2,5-Diarylfurans exhibit interesting biological activity. ty.[8] Upon considering the pharmaceutical potential,
Compounds such as 1a or 1b (Figure 1) serve as DNA as well as the limitations of available synthetic meth-
intercalating agents and are known to bind to the ods for 2,5-diaryl-3-fluorofurans, we decided to
RNA duplex or stem regions.[1] 5-Hydroxymethyl-2- pursue the development of their synthesis.
thienyldiphenylfuran derivative, 1c, suppresses cell
Synthetic methods leading to b-fluorofurans include
limited options.[7,9] The lithiation of 3-bromofuran de-
rivative and its subsequent reaction with N-fluoro-N-
(phenylsulfonyl)benzenesulfonamide (NFSI) gives 3-
fluoro-2-octylfuran with poor yield.[10] Rearrangement
of gem-difluorocyclopropyl aryl ketones (1-aryloyl-
2,2-difluorocyclopropanes) offers a method for the
preparation of 3-fluoro-2,5-diarylfurans with good
yield.[11] However,
a high reaction temperature
(2168C) is required to effectively accomplish this pro-
cess. Analogously, acid-catalyzed hydrolysis of gem-di-
fluorocyclopropyl acetals and ketals such as 3 pro-
ceeds at 1108C to afford 3-fluoro-2,5-diphenylfurans
when the electron-donating group in the p-position of
the phenyl ring is present. Acetals with electron-with-
drawing groups in the p-position of the aromatic ring
give a mixture of fluorofurans and gem-difluorocyclo-
Figure 1. Examples of biologically-active furans.
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Yan Li et al.
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actions are mainly limited to activated alkenes.[23] For
alkynes, the leading reference presents a gold-cata-
lyzed cyclization/fluorination sequence that yields a
mixture of fluorinated and non-fluorinated com-
pounds, which may be difficult to separate.[24] More-
over, the exposure of furan to Selectfluor may facili-
tate an oxidative ring opening, as it has been reported
for attempted direct fluorination of furans.[25] Thus,
we decided to introduce fluorine into an acyclic skele-
ton, and to investigate the metal-catalyzed cycloiso-
merization of 2-fluoroalk-3-yn-1-ones.[26,27] The prepa-
rations of 2-fluoroalk-3-yn-1-ones that have so far
been reported include the oxidation of 2-fluorobut-3-
yn-1-ol, obtainable by a low-yielding ring opening of
an epoxide precursor by a fluoride anion,[28] or a zinc
Scheme 1. Acid-catalyzed hydrolysis of gem-difluorocyclo-
propyl ketal 3.[12]
propyl ketones, with a predominance of the latter. or indium
ACHTUNGTREN(NUNG III) chloride-catalyzed reaction of fluoro-
The hydrolysis of ketals containing electron-with- propargyl chlorides with carbonyl compounds.[29]
drawing groups yields ketones such as 4 as the sole or
Considering the availability of alk-3-yn-1-ones,[18]
major products.[12] For example, using this method, we envisioned their direct fluorination in a joint alpha
compound 5a was obtained in only 2% yield position to the alkyne and ketone. Although an array
(Scheme 1).[12]
of a-monofluorination procedures is available for reg-
Heterocyclizations reactions are more appealing.[13] ular ketones,[30] a reaction of alkynone 6a with NFSI
Quite recently silver nitrate-catalyzed cyclization of (1 equiv.), in the presence of potassium carbonate, led
gem-difluorohomopropargyl alcohols to 3,3-difluoro- to the difluoro derivative 7 (Scheme 2). Such results
2,3-dihydrofurans has been reported.[14] Subsequent may be attributed to further rapid fluorination of the
elimination of hydrogen fluoride via silica gel chroma- monofluoro derivative, which is more prone to H ab-
tography yields the corresponding 3-fluorofurans. straction/enolization and effectively competes with
However, non-aromatic 3,3-difluoro-2,3-dihydrofurans the starting ketone. Similarly Peng and Shreeve have
also remain as the final product, dependent on the observed formation of the difluoro product when
substituents.[15] Similarly, treatment of difluorohomo- electronically akin cyano keto derivative (a-benzoyl-
propargyl alcohols with potassium tert-butoxide or
DBU at an elevated temperature provides 3-fluoro- fluor (1 equiv.).[31]
furans.[16]
Thus, an alternative fluorination procedure in neu-
The starting materials for this method, gem- tral conditions that would avoid the presence of the
difluoro
homopropargyl alcohols, are prepared by a base was sought. Conversion of ketone 6a[18] to the
ACHTUNGTRENaNGNU cetonitrile) was treated with sodium hydride/Selec-
ACHTUNGTRENNUNG
synthesis involving chlorofluorocarbons such as bro- corresponding tert-butyldimethylsilyl enol ether 8a
mochlorodifluoromethane,[17] and similar to the gem- was accomplished by following the literature proto-
difluorocyclopropyl derivatives, the elimination of hy- col.[32] To our delight, reaction of 8a with Selectfluor
drogen fluoride is required. Thus, we sought to devel- at room temperature gave a monofluoro ketone 9 in
op a more effective fluorofuran synthesis in terms of almost quantitative yield (Scheme 3).
halogen atom economy. Our recent efforts to explore
Cycloisomerization of 9 was attempted next. Con-
the application of cycloisomerization reactions result- version of non-halogenated but-3-yn-1-ones to furans
ed in the efficient synthesis of furans and furanopyri- may be quantitatively accomplished within minutes at
midine nucleosides.[18,19] Relevant electrophilic halo- room temperature using simple Lewis acids such as
cyclization reactions from our laboratory have also zinc chloride; other transition metals are also effec-
been reported.[20] Here we present our attempts to es- tive.[18,33] Unfortunately, the electron-withdrawing
tablish a synthesis of b-fluoro-substituted furans from effect of fluorine significantly inhibits the cycloisome-
but-3-yn-1-ones. The potential of a biological applica- rization process of fluorinated butynones.[34] Treat-
tion moved the focus to model diaryl substituents.
Our initial effort to facilitate electrophilic flurocyc-
lization of butynone 6a with Selectfluor,[21] in a similar
manner as for iodo-, bromo-, and chlorocycliza-
tions,[20] was not successful. This result came as no sur-
prise, since the electrophilic character of fluorine is
the lowest within the halogens, and formal electro-
philic reactions of fluorine presumably involve alter-
native mechanisms.[22] Reports of fluorocyclization re- Scheme 2. Fluorination of butynone 6a.
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Gold(I)-Catalyzed Cycloisomerization of 2-Fluoroalk-3-yn-1-ones
Scheme 4. Synthesis of 3-fluorofurans 5 via the sequence of
fluorination/cycloisomerization of 8.
Due to the gradual decomposition of the monofluoro
ketones such as 9 over prolonged storage and our in-
ability to establish an effective purification procedure,
we decided to use a sequence of consecutive fluorina-
tion and cyclization reactions, starting from silyl enol
Scheme 3. Synthesis of 3-fluorofuran 5a.
ment of fluorobutynone 9 with zinc chloride, even in ethers 8 and proceeding in the same flask, without
an equimolar amount and at an elevated temperature, isolation of 2-fluorobutynones (Scheme 4).
did not lead to conversion (Table 1). Similarly, results
Preparation of but-3-yn-1-ones (propargyl ketones),
acquired using silver nitrate, an effective cycloisome- the necessary starting materials, was carried out as de-
rization catalyst frequently cited by Marshallꢁs scribed earlier.[18] Although we focused on aryl sub-
group,[35] did not warrant further attention (entry 2). stituents, we also examined compounds containing
When a small preparative scale amount of fluoro one alkyl, and one cycloalkyl group. The explored
ketone 9 was treated with the palladium catalyst, substituents of silyl enol ethers 8 include phenyl, p-
(PhCN)2PdCl2 (10 mol%),[26] in dichloromethane, for- tolyl, p-halophenyls, p-trifluoromethylphenyl, ethyl,
mation of 5a was noticed after 1 h. Fluorofuran 5a and cyclopropyl; detailed structures are provided in
was isolated, after work-up by a silica gel short Table 2.
1
column chromatography in 60% yield (Table 1,
entry 3).
New 3-fluorofurans 5 were characterized by H, 13C,
and 19F NMR, IR, mass, and UV-vis spectroscopy.
In a continued search for a more reactive cycloiso- The characteristic NMR features for aryl and alkyl
merization catalyst, we turned our attention to substituted furans 5a–g include the 1H H-4 signals
gold.[36] Gold compounds are successful in the alkyne (doublets 7.08–6.28 ppm with JH,F 0.7–1.2 Hz), 13C C-
activation and cyloisomerization processes,[37] and fa- 3/C-4 (doublets 153.1–150.2/100.0–99.0 ppm, JC,F
cilitate heterocycles formation via vinyl-gold inter- 243.4–255.6/20.1–20.6 Hz). Longer range JH,F and JC,F
mediates that can even be isolated.[38] When fluoro were observed.[28] Acquired 19F NMR spectra showed
ketone 9 was treated with goldACTHUNRTGNE(GNU III) chloride, the for- agreement with literature data (at C-3, 158.9–
mation of fluorofuran was observed by TLC, but 163.0 ppm).[39] Mass spectra for 5a–g exhibited intense
within minutes a complex post-reaction mixture was molecular ions. Most solid furans gave accurate ele-
obtained. Concluding that the reactivity of gold
ACHTUNGTRENNUNG
too high, we turned our attention to the chlorotriphe-
nylphosphine gold(I), an easy-to-handle (air-stable) was confirmed by X-ray crystallography. Crystalliza-
and commercially available complex. The chlorotri- tion of compound 5c from ether gave single crystals
phenylphosphine gold, in combination with silver tri- suitable for X-ray analysis.[40] Inspection of Figure 2
fluoromethanesulfonate (both 5 mol%), in dichloro- reveals the molecular structure of the expected 2,5-
methane, at room temperature, facilitated almost disubstituted 3-fluorofuran. No significant distortion
quantitative conversion of 9 into furan 5a (Scheme 3). of the furan ring due to the presence of fluorine was
Table 1. Catalyst optimization: cycloisomerization of 2-fluorobutynone 9.
Entry
Catalyst
Loading [mol%]
Solvent
CH2Cl2
Conditions
Yield [%]
1
2
3
4
5
6
ZnCl2
AgNO3
100
10
10
5
5
1
reflux, 3 h
r.t., 4 h
r.t., 1 h
r.t., 30 min
r.t., 10 min
r.t., 12 h
no reaction
no reaction
60[a]
ACHTUNGTRENNUNG
G
multiple products
Ph3PAuCl/AgOTf
Ph3PAuCl/AgOTf
95[a]
42[a,b]
[a]
Isolated product.
Incomplete reaction after 1 h; multiple products found after 12 h.
[b]
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Yan Li et al.
Yield [%]a
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Table 2. Preparation of 3-fluorofurans 5.
Entry
Enol Ether
R
R’
Furan
Reaction Time [min]
1
2
3
4
5
6
7
8a
8b
8c
8d
8e
8f
8g
C6H5
p-F-C6H4
p-Br-C6H4
p-Br-C6H4
p-CF3-C6H4
Et
p-Me-C6H4
p-Me-C6H4
p-Me-C6H4
p-t-Bu-C6H4
p-Me-C6H4
C6H5
5a[12]
5b
5c
60 + 10
60 + 10
60 + 10
60 + 10
60 + 60
60 + 30
60 + 10
95
96
94
92
89
52
92
5d
5e
5f[16]
5g
C6H5
c-C3H5
[a]
Reactions were carried out on a 0.50 mmol scale with 0.55 mmol of Selectfluor in acetonitrile, and 0.025 mmol of
Ph3PAuCl and AgOTf in CH2Cl2, at room temperature.
substituents is the subject of further investigations in
our laboratory.
Experimental Section
General Remarks
Selectfluor
{1-chloromethyl-4-fluoro-1,4-diazoniabicyclo-
AHCTUNGTREG[NNNU 2.2.2]octane bis(tetrafluoroborate), Air Products and
Chemicals}, chlorotriphenylphosphine gold(I) (Strem), and
silver trifluoromethanesulfonate (Aldrich), were used as re-
ceived and handled in the air. Column chromatography was
Figure 2. An ORTEP view of the 5c illustrating atom label-
ing scheme and thermal ellipsoids (50% probability level).
1
À
À
Selected interatomic distances (ꢂ): F C3 1.339(6), O1 C2
1.371(6), O1 C5 1.389(6), C2 C3 1.355(7), C2 C6 1.455(7),
carried out on silica gel (Dynamic Adsorbents, 32–63 m). H,
13C, and 19F NMR spectra were recorded on a Bruker
Avance III 400 spectrometer (400, 100, and 376 MHz, re-
spectively). The chemical shifts are reported in d (ppm)
values (1H and 13C relative to CDCl3/acetone-d6, 7.26/
2.05 ppm and 77.16/29.9 ppm, respectively), and 19F relative
to CFCl3 (external standard). IR spectra were recorded
using a Varian 3100 Excalibur spectrometer.
À
À
À
À
À
À
C3 C4 1.389(8), C4 C5 1.359(7), C5 C12 1.469(7). Key
angles (deg): C2 O1 C5 107.0(4), O1 C2 C3 107.5(4), O1
À
À
À
À
À
À
À
À
À
À
C2 C6 116.8(4), C3 C2 C6 135.7(5), C2 C3 C4 110.4(4),
À
À
À
À
À
À
F C3 C2 125.9(5), F C3 C4 123.7(5), C3 C4 C5 105.3(4),
O1 C5 C4 109.9(5), O1 C5 C12 116.2(4), C4 C5 C12
133.9(4).
À
À
À
À
À
À
noticed. The entire molecule of 5c is nearly planar
within 0.17 ꢂ. The maximum atom deviations from
the average plane are 0.33 and 0.32 ꢂ for C-10 and C-
14, respectively.
In summary, we have demonstrated that 2-fluoro-
but-3-yn-1-ones can be successfully obtained by mon-
ofluorination of the but-1-en-3-yn-1-yl silyl ethers
with Selectfluor, and that chlorotriphenylphosphine
gold/silver trifluoromethanesulfonate is an efficient
catalyst for the cycloisomerization of the 2-fluorobut-
3-yn-1-ones. The methods proved to be most effective
2-Fluoro-4-(4-methylphenyl)-1-phenylbut-3-yn-1-one
(9)
The flask was charged with silyl enol ether 8a (0.175 g,
0.500 mmol), Selectfluor (0.195 g, 0.550 mmol), and acetoni-
trile (10 mL). The mixture was stirred at ambient tempera-
ture for about 1 h and monitored by TLC (hexanes/EtOAc,
8:2). Solvent was removed by rotary evaporation and the
residue was kept under oil pump vacuum for 30 min. Di-
chloromethane (60 mL) was added and the mixture was
stirred for 10 min. The solid was filtered off (fritted funnel)
for aryl substituents (C-2 and C-5 of furan). The rela- and washed with dichloromethane (20 mL). Solvent was re-
moved from the combined filtrates by rotary evaporation to
give crude 9 as yellow oil that may slowly solidify in a freez-
er. IR (film): n=2956, 2928, 2222, 1713, 1510, 1450, 1218,
tively short reaction time and mild conditions (room
temperature) provide an appealing alternative to the
currently available methods, especially from the
standpoint of halogen atom economy. This method
avoids the loss of halogens in the synthetic pathway,
facilitates the regioselective introduction of fluorine
within two available beta positions, and also allows
for the introduction of substituents such as cycloalkyls
that are not easily carried out by other methods. De-
1066, 943, 845, 817, 688, 667 cmÀ1 1H NMR (CDCl3): d=
;
8.16 (d, J=7.4 Hz,), 7.67–7.61 (m, 1H), 7.55–7.49 (m, 2H),
7.33 (d, J=8.0 Hz, 2H), 7.12 (d, J=8.0 Hz, 2H), 6.26 (d, J=
49.0 Hz, 1H), 2.34 (s, 3H); 13C NMR (CDCl3): d=189.9 (d,
J=21.8 Hz), 140.2, 134.4, 133.0, 132.1 (d, J=2.9 Hz), 129.6
(d, J=2.1 Hz), 129.3, 128.9, 118.0 (d, J=4.1 Hz), 93.4 (d, J=
10.4 Hz), 83.5 (d, J=184.6 Hz), 80.5 (d, J=25.8 Hz), 21.7;
termination of scope of the reaction with alkyl-only 19F NMR (CDCl3): d= À179.2 (d, J=49.0 Hz).
2764
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Gold(I)-Catalyzed Cycloisomerization of 2-Fluoroalk-3-yn-1-ones
3-Fluoro-5-(4-methylphenyl)-2-phenylfuran (5a)[12]
[6] W. K. Hagmann, J. Med. Chem. 2008, 51, 4359–4369.
[7] Review: O. Serdyuk, A. Butin, V. Abaev, J. Fluorine
Chem. 2010, 131, 296–319.
[8] A. S. Cantrell, P. Engelhardt, M. Hçgberg, R. S. Jasku-
nas, N. G. Johansson, C. L. Jordan, J. Kangasmetsꢄ,
M. D. Kinnick, P. Lind, J. M. Morin Jr, M. A. Muesing,
R. Noreꢃn, B. ꢅberg, P. Pranc, C. Sahlberg, R. J. Ter-
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Med. Chem. 1996, 39, 4261–4274.
[9] See also: S. Arimitsu, G. B. Hammond, Synthesis of
gem-Difluorinated Heterocycles Using a Difluoropro-
pargyl Molecular Scaffold, in: Fluorinated Heterocycles,
(Eds.: A. A. Gakh, K. L. Kirk), ACS Symposium Series
1003; American Chemical Society, Washington, DC,
2009, pp 135–162.
To a solution of the crude 9 from the reaction above in di-
chloromethane (10 mL), chlorotriphenylphosphine gold(I)
(0.013 g, 0.025 mmol) and silver trifluoromethanesulfonate
(0.0065 g, 0.025 mmol) were added. The mixture was stirred
vigorously in the darkness (the flask was wrapped in Al foil)
for 10 min. The solvent was removed by rotary evaporation
the residue was purified by silica gel column chromatogra-
phy (hexanes) to give 5a as a white solid; yield: 0.120 g
(0.476 mmol, 95%), mp 93–958C; anal. calcd. for C17H13FO:
C 80.93, H 5.19; found: C 80.72, H 5.44; IR (KBr): n=2923,
1631, 1501, 1428, 1150, 821, 792, 668 cmÀ1; MS (EI): m/z=
252 (M+, 100%); 1H NMR (acetone-d6): d=7.80–7.70 (m,
4H), 7.52–7.45 (m, 2H), 7.35–7.25 (m, 3H), 7.00 (d, J=
0.7 Hz, 1H), 2.36 (s, 3H); 13C NMR (acetone-d6): d=151.7
(d, J=9.1 Hz), 151.6 (d, J=252.3 Hz), 139.3, 136.2 (d, J=
20.5 Hz), 130.5, 129.9, 129.8 (d, J=5.2 Hz), 128.4 (d, J=
2.2 Hz), 128.1 (d, J=1.2 Hz), 124.7, 124.2 (d, J=5.2 Hz),
99.7 (d, J=20.3 Hz), 21.4; 19F NMR (CDCl3): d=À162.0.
´
[10] E. Dvornikova, M. Bechcicka, K. Kamienska-Trela, A.
´
Krꢆwczynski, J. Fluorine Chem. 2003, 124, 159–168.
[11] M. Derenberg, P. Hodge, J. Chem. Soc. Chem.
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[12] W. Xu, Q.-Y. Chen, Org. Biomol. Chem. 2003, 1, 1151–
1156.
Supporting Information
[13] Silyloxy-substituted b-fluorofurans are obtained via
1 H, 13C, and 19F NMR spectra for new fluorofurans 5 and
crystallographic tables for 5c are available as Supporting In-
formation.
metallation-silylation
of
3-fluorofuran-2(5H)-ones
formed as a result of acid-catalyzed cyclization of 2-flu-
ˇ
oroalk-2-enoates: a) K. Pomeisl, J. Kvicala, O. Paleta, J.
Fluorine Chem. 2006, 127, 1390–1397; b) K. Pomeisl, J.
ˇ
ˇ
Cejka, J. Kvicala, O. Paleta, Eur. J. Org. Chem. 2007,
5917–5925.
[14] S. Arimitsu, G. B. Hammond, J. Org. Chem. 2007, 72,
8559–8561.
[15] S. Arimitsu, J. M. Jacobsen, G. B. Hammond, J. Org.
Chem. 2008, 73, 2886–2889.
[16] a) H. L. Sham, D. A. Batebenner, J. Chem. Soc. Chem.
Commun. 1991, 1134–1135; b) P. Li, Z. Chai, G. Zhao,
S.-Z. Zhu, Synlett 2008, 2547–2551.
Acknowledgements
We are indebted to the donors of the Petroleum Research
Fund (ACS-PRF#46094) administered by the American
Chemical Society and to Oakland University and its Research
Excellence Program in Biotechnology for the support of this
research. The National Science Foundation (NSF) awards
(CHE-0821487 and CHE-0722547) are also acknowledged.
Y.L. is grateful for the Provostꢀs Graduate Student Research
Award. We are grateful to Dr. Robert Syvret (Air Products
and Chemicals) for the generous supply of Selectfluor.
[17] S. Arimitsu, G. B. Hammond, J. Org. Chem. 2006, 71,
8665–8668.
[18] A. Sniady, A. Durham, M. S. Morreale, A. Marcinek, S.
Szafert, T. Lis, K. R. Brzezinska, T. Iwasaki, T. Ohshi-
ma, K. Mashima, R. Dembinski, J. Org. Chem. 2008,
73, 5881–5889, and references cited therein.
[19] a) A. Sniady, A. Durham, M. S. Morreale, K. A. Wheel-
er, R. Dembinski, Org. Lett. 2007, 9, 1175–1178; b) For
heterocylization reactions leading to pyrroles see: P.
Wyre˛bek, A. Sniady, N. Bewick, Y. Li, A. Mikus, K. A.
Wheeler, R. Dembinski, Tetrahedron 2009, 65, 1268–
1275.
[20] a) A. Sniady, M. S. Morreale, K. A. Wheeler, R. Dem-
binski, Eur. J. Org. Chem. 2008, 3449–3452; b) A.
Sniady, K. A. Wheeler, R. Dembinski, Org. Lett. 2005,
7, 1769–1772; c) M. S. Rao, N. Esho, C. Sergeant, R.
Dembinski, J. Org. Chem. 2003, 68, 6788–6790.
[21] G. S. Lal, G. P. Pez, R. G. Syvret, Chem. Rev. 1996, 96,
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