Iodocyclization versus diiodination in the reaction of 3-alkynyl-4- methoxycoumarins with iodine: Synthesis of 3-iodofuro[2,3-b]chromones

Article abstract of DOI:10.1039/c0ob00935k

The reaction of 3-alkynyl-4-methoxycoumarins with molecular iodine in chlorinated solvents allows access to 3-iodofurochromones in good to excellent yields as the result of a iodocyclization-demethylation process. Competitive diiodination of the coumarin acetylene moiety could be eliminated by simply performing the reactions in refluxing 1,2-dichloroethane, owing to the thermal instability of the resulting (E)-1,2-diiodoethenylcoumarins. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0ob00935k

View Article Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 1474
www.rsc.org/obc
PAPER
Iodocyclization versus diiodination in the reaction of
3-alkynyl-4-methoxycoumarins with iodine: Synthesis of
3-iodofuro[2,3-b]chromones†
Guillaume Raffa, Se´bastien Belot, Genevie`ve Balme and Nuno Monteiro*
Received 26th October 2010, Accepted 2nd December 2010
DOI: 10.1039/c0ob00935k
The reaction of 3-alkynyl-4-methoxycoumarins with molecular iodine in chlorinated
solvents allows access to 3-iodofurochromones in good to excellent yields as the result of a
iodocyclization–demethylation process. Competitive diiodination of the coumarin acetylene
moiety could be eliminated by simply performing the reactions in refluxing 1,2-dichloroethane,
owing to the thermal instability of the resulting (E)-1,2-diiodoethenylcoumarins.
Introduction
The iodine-based activation of alkynes toward intramolecular ad-
dition of heteronucleophiles has proven to be an extremely effective
and broadly applicable method to access a variety of heterocyclic
compounds,1 notably furan derivatives.2 The introduction of a
iodide functional group on the newly formed ring is of special
interest as it can act as an effective chemical handle for further
derivatization. Clearly, the main drawback of this method is the
occasional difficulty in avoiding the competitive trans-addition of
iodine across the triple bond, a relatively well-documented process
Scheme
chromones.
1
An iodocyclization strategy toward 3-iodofuro[2,3-b]-
that leads to the formation of (E)-1,2-diiodoalkenes.3
Recently, we became interested in developing a general and con-
venient method for the synthesis of 4H-furo[2,3-b][1]benzopyran-
4-ones (furochromones).4 The latter compounds have received
little attention to date,5 possibly because this heterocyclic system is
not commonly encountered in nature.6 Preliminary investigations
based on previous work from this laboratory7 suggested that
these heterocycles (i.e. 2) would be accessible via iodocyclization
of readily available 3-alkynyl-4-methoxycoumarins (1) followed
by the in situ demethylation of the methoxyl group (Scheme 1).
We herein report on the scope and limitations of the method,
and provide some mechanistic insight into plausible reaction
pathways.
Results and discussion
Initially, we screened the reaction conditions for the electrophilic
cyclization of substrate 1a. As a preliminary experiment, 1a was
treated with two equivalents of I2 in dichloromethane at room
temperature. Disappointingly, the reaction essentially resulted in
the formation of diiodoethenylcoumarin 3a after stirring for 5 h
(90% conversion as determined by 1H NMR, 60% isolated yield),
which was accompanied by only small amounts of the expected 3-
iodofuran 2a (ca. 6%) and remaining starting material. However,
after further experimentation, we were pleased to discover that 1a
could be smoothly converted into the desired iodofurochromone
by simply performing the reaction at higher temperatures. It
is also interesting to note that the formation of the putative
furochromenylium salt intermediate could not be observed under
the reaction conditions. The best conditions consisted of heating
1a in refluxing 1,2-dichloroethane (DCE) for 5 h, which furnished
furan 2a in 85% isolated yield (Scheme 2).
Universite´ Lyon 1, Institut de Chimie et Biochimie Mole´culaires et
Supramole´culaires (ICBMS, UMR 5246 du CNRS), CPE Lyon, 43 Bd du
11 Novembre 1918, 69622, Villeurbanne Cedex, France. E-mail: monteiro@
univ-lyon1.fr
Although NMR and FT-IR spectroscopic data supported the
formation of a linearly-fused furochromone 2a, its structure was
unambiguously secured by X-ray analysis. On the other hand,
a crystal structure analysis of 3a allowed us to establish the E-
configuration of the diiodoalkene fragment (Fig. 1).
† Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for new alkynylcoumarins 1, copies
1
of H and 13C NMR spectra of 3-iodofurans 2a–i and 5, and X-ray data
for compounds 2a and 3a (CIF). CCDC reference numbers 798397 and
798398. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0ob00935k
1474 | Org. Biomol. Chem., 2011, 9, 1474–1478
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 1 Synthesis of diversely substituted 3-iodofurochromones 2 and
related furopyranone 5a
Entry
Starting alkyne
t/h
Product; yield (%)b
1
2
3
4
5
6
1a; Ar = C6H5
5
5
96
0.5
0.5
72
2a; 85
2b; 89
2c; 70c
2d; 92
2e; 98
1b; Ar = 4-FC6H4
1c; Ar = 4-MeCO2C6H4
1d; Ar = 4-MeOC6H4
1e; Ar = 3,4,5-(MeO)3C6H2
Scheme 2 Reaction of 1a with iodine at different temperatures.
7
6
8
9
6
72
10
6
a All reactions were run on 0.1 mmol of the acetylene and 0.2 mmol
of iodine in refluxing 1,2-dichloroethane. The synthesis of 2a was also
performed on a 2.1 mmol preparative scale (85% isolated yield). b Isolated
yields (single runs). c 50% yield after 48 h.
Fig. 1 X-Ray analyses of 3a and 2a.
The generality of the electrophilic cyclization process was
then explored under the optimized reaction conditions. Aryl-
substituted alkynylcoumarins 1a–h (Table 1, entries 1–8) provided
the corresponding 2-aryl-3-iodofurans in good to excellent yields.
The nature of the arene attached to the triple bond had a significant
impact on the rate of the reaction. As expected, electron-
withdrawing groups, which are less capable of stabilizing the devel-
oping positive charge, decreased the reactivity of the triple bond
dramatically, extending the reaction time up to several days, as
illustrated by the reaction of (4-methoxycarbonyl)phenylacetylene
1c (up to 4 days, 70% isolated yield). When the latter reaction
was performed at room temperature, the product of the 1,2-
diiodination of the triple bond (not shown) was formed in
81% yield, with only traces of the corresponding iodocyclization
product (2c) being formed. In contrast, electron-rich arenes had
a positive effect on the rate of the reaction, allowing the full
and clean conversion of the starting materials within a few
minutes (Table 1, entries 4 and 5). It is interesting to note that
the iodocyclization of (3,4,5-trimethoxy)phenylacetylene 1e also
took place at room temperature. However, the reaction proved
sluggish (85% conversion after 24 h) and led to the formation of
some unidentified by-products. Alkyl-substituted alkyne 1i also
participated efficiently in the cyclization process (Table 1, entry 9).
Interestingly, the same procedure applied well to the synthesis of
3-iodofuropyrone 5 (73% yield) (Table 1, entry 10).
From a mechanistic point of view, the cyclization process is
supposed to proceed through iodonium-promoted nucleophilic
attack of the coumarin carbonyl oxygen atom onto the triple bond
to form furochromenylium A.8 The latter would be unstable under
the reaction conditions and thus would eliminate methyl iodide
via SN2 displacement to generate the furochromone ring system
(Scheme 3).
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 1474–1478 | 1475
©

View Article Online
reflux for the indicated time (TLC). Unless otherwise stated,
the reaction mixture was then diluted with dichloromethane and
washed three times with aqueous Na2S2O3. The aqueous layer
was extracted twice with dichloromethane, and the combined
organic layers were dried over MgSO4 and concentrated in vacuo.
The residue was purified by flash chromatography (silica gel,
appropriate mixture of cyclohexane and ethyl acetate) to afford
the desired 3-iodofuran.
Scheme 3 Diiodination as a ‘reversible’ competing process in the
iodocyclization of 1a.
3-Iodo-2-phenyl-4H-furo[2,3-b]benzopyran-4-one (2a)
1H NMR monitoring of the iodocyclization reaction of 1a by
periodic aliquot removal9 showed that the product of the 1,2-
diiodination (3a) also formed at the early stage of the reaction
and then gradually disappeared as the desired iodocyclization
product 2a formed. For instance, after 15 min, the reaction
mixture was composed of 35% remaining starting material (1a),
40% diiodoalkene 3a and 25% iodofuran 2a. In light of this
observation, it is reasonable to assume that 3a exhibited a poor
thermal stability and eliminated iodine upon heating to regenerate
the original alkyne, thereby enabling the iodocyclization process
to take place unhampered.10 To confirm this assumption, a sample
of 3a was heated in refluxing DCE for several hours. Progress was
monitored by crude 1H NMR analysis, which showed the expected
appearance and disappearance of the alkynyl coumarin (1a) as an
intermediate in the conversion of 3a into 2a, thereby confirming
that iodine was being eliminated and subsequently re-used in the
next (cyclization) step.11 Notably, if the reaction was terminated
after 4 h,12 the alkynyl coumarin 1a was isolated in 40% yield. If the
reaction was allowed to continue overnight, essentially all of the
starting diiodoalkene was then converted into the iodocyclization
product (2a), which was then obtained in an estimated 58% yield
(5 h, 85% yield), mp 175–177 ◦C (toluene/cyclohexane). FTIR
1
(neat): nmax/cm-1 1660 (C O). H NMR (300 MHz, CDCl3): d
7.38–7.50 (m, 4H), 7.54 (dd, J = 8.5 and 0.7 Hz, 1H), 7.71 (ddd,
J = 8.5, 7.1 and 1.7 Hz, 1H), 8.04 (dd, J = 8.4 and 1.5 Hz, 2H),
8.34 (dd, J = 7.9 and 1.7 Hz, 1H). 13C NMR (100 MHz, CDCl3): d
56.2, 104.6, 117.6, 123.6, 125.8, 126.7, 126.8, 128.6, 128.7, 129.3,
133.7, 146.0, 152.7, 161.8, 173.3. HRMS (EI): M+, 387.9596; calc.
for C17H9I03: 387.9596. The synthesis of 2a was also performed on
a 2.1 mmol preparative scale (85% yield).
3-Iodo-2-(4-fluoro)phenyl-4H-furo[2,3-b]benzopyran-4-one (2b)
(6 h, 89% yield), white solid, mp 189–192 ◦C. FTIR (neat):
1
n
max/cm-1 1657 (C O). H NMR (300 MHz, CDCl3): d 7.16 (t,
J = 8.6 Hz, 2H), 7.50 (m, 1H), 7.56 (br. d, J = 7.6 Hz, 1H), 7.73
(ddd, J = 8.5, 7.0 and 1.6 Hz, 1H), 8.00–8.07 (dd, J = 5.2 and
8.8, 2H), 8.36 (dd, J = 7.8 and 1.6 Hz, 1H). 13C NMR (100 MHz,
CDCl3): d 56.2, 104.6, 116.0 (d, J = 21.9 Hz), 117.9, 123.7, 124.9
(d, J = 3.6 Hz), 125.9, 126.9, 128.9 (d, J = 8.0 Hz), 133.8, 145.4,
152.7, 161.9, 163.2 (d, J = 249.4 Hz), 173.3. HRMS (ESI): MH+,
406.9566; calc. for C17H9FIO3: 406.9575.
1
based on its crude weight and H NMR analysis. Overall, these
results demonstrate that—at least in some particular cases—the
propensity of vicinal diiodoalkenes to undergo iodine elimination
might be successfully exploited to hamper the diiodination of
alkyne-containing derivatives, often reported as a predominant
competing pathway in attempted iodocyclization processes.
3-Iodo-2-(4-methoxycarboxy)phenyl-4H-furo[2,3-b]benzo-pyran-
4-one (2c)
After completion of the reaction (96 h), the precipitated
furocoumarin was collected by filtration and recrystallized
(AcOEt/cyclohexane). 70% yield, white solid, mp > 200 ◦C. FTIR
Conclusions
1
(neat): nmax/cm-1 1716 and 1660 (C O). H NMR (100 MHz,
In summary, we have developed an easy, straightforward access
to 3-iodofurochromones based on iodine-promoted electrophilic
cyclization/O-demethylation of 3-alkynyl-4-methoxycoumarins.
An interesting feature of this process pertains the possibility
of ‘reversing’ the competitive 1,2-diiodination of the coumarin
acetylene moiety by simply increasing the reaction temperature.
CDCl3): d 3.95 (s, 3H), 7.48–7.51 (m, 1H), 7.58 (br. d, J = 8.0 Hz,
1H), 7.71–7.77 (m, 1H), 8.12–8.20 (m, 4H), 8.36 (dd, J = 8.0
and 1.4 Hz, 1H), 13C NMR (75 MHz, CDCl3): d 52.5, 58.7,
105.01, 117.9, 123.6, 126.0, 126.3, 127.0, 130.0, 132.8, 134.0, 144.9,
152.8, 162.1, 166.6, 173.3. HRMS (ESI): MH+, 446.9713; calc. for
C19H12IO5: 446.9724.
Experimental section
Commercially available reagents and solvents were used as pur-
chased without further drying. Acetylenic substrates 1a–i and 4
were prepared from readily available 4-methoxy(benzo)pyrones
and terminal acetylenes by following procedures previously devel-
oped in our laboratory.13
3-Iodo-2-(4-methoxy)phenyl-4H-furo[2,3-b]benzopyran-4-one (2d)
(0.5 h, 92% yield), white solid, mp > 200 ◦C. FTIR (neat):
1
n
max/cm-1 1651 (C O). H NMR (300 MHz, CDCl3): d 3.87 (s,
3H), 7.00 (d, J = 9.0 Hz, 2H), 7.49 (m, 1H), 7.56 (br, d, J =
7.8 Hz, 1H), 7.72 (ddd, J = 8.6, 7.2 and 1.6 Hz, 1H), 7.98 (d,
J = 9.0 Hz, 2H), 8.36 (dd, J = 7.9 and 1.6 Hz, 1H). 13C NMR
(100 MHz, CDCl3): d 54.5, 55.5, 104.6, 114.2, 117.8, 121.2, 123.7,
125.7, 126.9, 128.5, 133.6, 146.3, 152.7, 160.4, 161.7, 173.3. HRMS
(ESI): MNa+, 440.9587; calc. for C18H11INaO4: 440.9594.
General procedure
A solution of the selected alkyne (0.1 mmol) in dichloroethane
(1 mL) was treated with iodine (0.2 mmol) and left to stir under
1476 | Org. Biomol. Chem., 2011, 9, 1474–1478
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
3-Iodo-2-(3,4,5-trimethoxy)phenyl-4H-furo[2,3-b]benzo-pyran-4-
one (2e)
3H), 6.13 (s, 1H), 7.38–7.49 (m, 3H), 8.00 (dd, J = 8.5 and 1.7 Hz).
13C NMR (100 MHz, CDCl3): d 19.3, 55.6, 107.1, 113.9, 126.8,
128.5, 128.7, 129.3, 145.9, 159.7, 161.9, 175.4. HRMS (EI): M+,
351.9597; calc. for C14H9IO3: 351.9596.
(0.5 h, 98% yield), white solid, mp 192–195 ◦C (dec.). FTIR (neat):
1
n
max/cm-1 1656 (C O). H NMR (300 MHz, CDCl3): d 3.92 (s,
3H), 3.96 (br s, 6H), 7.32 (s, 2H), 7.50 (m, 1H), 7.56 (br. d, J =
8.3 Hz, 1H), 7.73 (ddd, J = 8.7, 7.3 and 1.7 Hz, 1H), 8.36 (dd, J =
8.0 and 1.7 Hz, 1H). 13C (100 MHz, CDCl3): d 55.7, 56.4, 61.1,
104.1, 104.7, 117.8, 123.6, 123.8, 125.8, 126.7, 133.7, 139.0, 145.7,
152.6, 153.3, 161.6, 173.2. HRMS (EI): M+, 477.9913; calc. for
C20H15IO6: 477.9913.
Acknowledgements
This research was assisted financially by a grant to G. R. from
the French Ministry of Higher Education and Research (MESR).
We thank Dr E. Jeanneau (Centre de Diffractome´trie Henri
Longchambon, Universite´ Lyon 1) for the X-ray crystallographic
analyses. I. Aillaud (Universite´ Lyon 1) is acknowledged for
preliminary experiments.
6-Chloro-3-iodo-2-phenyl-4H-furo[2,3-b]benzopyran-4-one (2f)
(72 h, 88% yield), white solid, mp 198–200 ◦C. FTIR (neat):
1
n
max/cm-1 1660 (C O). H NMR (300 MHz, CDCl3): d 7.41–
Notes and references
7.54 (m, 4H), 7.66 (dd, J = 9.0 and 2.5 Hz, 1H), 8.04 (dd, J = 8.2
and 1.5 Hz, 2H), 8.29 (d, J = 2.5 Hz, 1H). 13C NMR (100 MHz,
CDCl3): d 56.0, 104.8, 119.5, 124.7, 126.3, 126.8, 128.4, 129.5,
131.9, 133.8, 146.4, 161.9, 171.9. HRMS (EI): M+, 421.9208; calc.
for C17H8ClIO3: 421.9207.
1 For reviews, see: (a) Y. Yamamoto, I. D. Gridnev, N. T. Patil and T. Jin,
Chem. Commun., 2009, 5075; (b) M. J. Mphahlele, Molecules, 2009, 14,
4814; (c) H. Togo and S. Iida, Synlett, 2006, 2159.
2 For recent examples, see: (a) A. Arcadi, S. Cacchi, S. Di Giuseppe, G.
Fabrizi and F. Marinelli, Org. Lett., 2002, 4, 2409; (b) Y. Liu and S.
Zhou, Org. Lett., 2005, 7, 4609; (c) T. Yao, X. Zhang and R. C. Larock,
J. Org. Chem., 2005, 70, 7679; (d) T. Yao, D. Yue and R. C. Larock,
J. Org. Chem., 2005, 70, 9985; (e) D. Yue, T. Yao and R. C. Larock,
J. Org. Chem., 2005, 70, 10292; (f) S. P. Bew, G. M. M. El-Taeb, S. Jones,
D. W. Knight and W.-F. Tan, Eur. J. Org. Chem., 2007, 5759; (g) C.-H.
Cho, B. Neuenswander, G. H. Lushington and R. C. Larock, J. Comb.
Chem., 2008, 10, 941; (h) Y.-X. Xie, X.-Y. Liu, L.-Y. Wu, Y. Han, L.-B.
Zhao, M.-J. Fan and Y.-M. Liang, Eur. J. Org. Chem., 2008, 1013; (i) S.
Arimitsu, J. M. Jacobsen and G. B. Hammond, J. Org. Chem., 2008,
73, 2886; (j) T. Okitsu, D. Nakazawa, R. Taniguchi and A. Wada, Org.
Lett., 2008, 10, 4967.
3 For leading references, see: (a) R. A. Hollins and M. P. A. Campos,
J. Org. Chem., 1979, 44, 3931; (b) V. L. Heasley, D. F. Shellhamer, L. E.
Heasley, D. B Yaeger and G. E. Heasley, J. Org. Chem., 1980, 45, 4649;
(c) J. Barluenga, M. A. Rodriguez and P. J. Campos, J. Org. Chem.,
1990, 55, 3104; (d) J. Duan, W. R. Dolbier Jr. and Q.-Y. Chen, J. Org.
Chem., 1998, 63, 9486.
3-Iodo-6-methyl-2-phenyl-4H-furo[2,3-b]benzopyran-4-one (2g)
(6 h, 89% yield), white solid, mp 200–201 ◦C. FTIR (neat):
n
max/cm-1 1648 (C O). 1H NMR (300 MHz, CDCl3): d 2.47
(s, 3H), 7.38–7.52 (m, 5H), 8.04 (dd, J = 8.6 and 1.6 Hz, 2H),
8.1 (d, J = 0.8 Hz, 1H). 13C NMR (100 MHz, CDCl3): d 21.0,
56.3, 104.5, 117.5, 123.2, 126.2, 126.7, 128.6, 129.2, 134.7, 135.7,
145.7, 150.9, 161.8, 173.3. HRMS (EI): M+, 401.9753; calc. for
C18H11IO3: 401.9753.
3-Iodo-7-methoxy-2-phenyl-4H-furo[2,3-b]benzopyran-4-one (2h)
After completion of the reaction (6 h), the precipitated
furocoumarin was collected by filtration and recrystallized
(AcOEt/cyclohexane). 70% yield, white solid, mp > 200 ◦C. FTIR
4 G. Raffa, M. Rusch, G. Balme and N. Monteiro, Org. Lett., 2009, 11,
5254.
5 (a) Y. R. Lee and J. Y. Suk, Tetrahedron, 2002, 58, 2359; (b) S.
Tollari, G. Palmisano, S. Cenini, G. Cravotto, G. B. Giovenzana and
A. Penoni, Synthesis, 2001, 735; (c) K. Kobayashi, K. Sakashita,
H. Akamatsu, K. Tanaka, M. Uchida, T. Uneda, T. Kitamura, O.
Morikawa and H. Konishi, Heterocycles, 1999, 51, 2881; (d) Y. R. Lee,
B. S. Kim and H. C. Wang, Tetrahedron, 1998, 54, 12215; (e) Y. R.
Lee, M. W. Byun and B. S. Kim, Bull. Korean Chem. Soc., 1998, 19,
1080.
6 Benzo-fused furochromones possessing interesting biological proper-
ties have been recently isolated from plants used in Chinese folklore:
(a) Q. Y. Shou, Q. Tan and Z. W. Shen, Bioorg. Med. Chem. Lett.,
2009, 19, 3389; A. Ahmed-Belkacem, S. Macalou, F. Borrelli, R.
Capasso, E. Fattorusso, O. Taglialatela-Scafati and A. Di Pietro,
J. Med. Chem., 2007, 50, 1933; Some naturally occurring dihydrofuro-
chromone sesquiterpenes have been reported recently; for examples,
see: (b) M. Iranshahi, F. Shaki, A. Mashlab, A. Porzel and L. A.
Wessjohann, J. Nat. Prod., 2007, 70, 1240; (c) T. Motai and S. Kitanaka,
J. Nat. Prod., 2005, 68, 1732; (d) B.-N. By Su, Y. Takaishi, G. Honda,
M. Itoh, Y. Takeda, O. K. Kodzhimatov and O. Ashurmetov, J. Nat.
Prod., 2000, 63, 520.
1
(neat): nmax/cm-1 1639 (C O). H NMR (300 MHz, CDCl3): d
3.94 (s, 3H), 6.97 (d, J = 2.3 Hz, 1H), 7.04 (dd, J = 8.8 and 2.3 Hz,
1H), 7.42–7.51 (m, 3H), 8.05 (dd, J = 8.3 and 1.3 Hz, 2H), 8.26
(d, J = 8.8 Hz, 1H). 13C NMR (100 MHz, CDCl3): d 56.1, 56.5,
101.1, 104.4, 114.1, 117.3, 126.8, 128.1, 128.7, 128.8, 129.3, 145.8,
154.4, 161.8, 164.1, 173.1. HRMS (EI): M+: 417.9702; calc. for
C18H11IO4: 417.9702.
2-Butyl-3-iodo-4H-furo[2,3-b]benzopyran-4-one (2i)
(72 h, 60% yield), white solid, mp 96–99 ◦C. FTIR (neat): nmax/cm-1
1656 (C O). 1H NMR (300 MHz, CDCl3): d 0.96 (t, J =
7.4 Hz, 3H), 1.40 (sext., J = 7.4 Hz, 2H), 1.68 (quint., J =
7.4, 2H), 2.77 (t, J = 2.4 Hz, 2H), 7.47 (t, J = 7.9 Hz, 1H),
7.52 (d, J = 7.9 Hz, 1H), 7.69 (m, 1H), 8.33 (dd, J = 8.0 and
1.4 Hz, 1H). 13C NMR (100 MHz, CDCl3): d 13.9, 22.1, 26.8,
29.9, 58.1, 103.3, 117.8, 123.8, 125.6, 126.8, 133.5, 141.4, 152.7,
161.7, 173.1. HRMS (EI): M+, 367.9907; calc. for C15H13IO3:
367.9909.
7 I. Aillaud, E. Bossharth, D. Conreaux, P. Desbordes, N. Monteiro and
G. Balme, Org. Lett., 2006, 8, 1113.
8 Electrophilic activation of the triple bond occurs presumably via
coordination to iodine to generate a cyclic iodonium ion. For a leading
reference, see ref. 1a. See also: T. Okazaki and K. K. Laali, J. Org.
Chem., 2005, 70, 9139and references therein.
3-Iodo-6-methyl-2-phenyl-4H-furo[2,3-b]pyran-4-one (5)
9 Aliquots were submitted to 1H NMR after work-up by treatment with
aqueous bisulfite to remove any free iodine.
(6 h, 73% yield), orange solid, mp 162–164 ◦C. FTIR (neat):
10 Vicinal diiodo compounds, including 1,2-diiodoalkenes but to a much
1
n
max/cm-1 1649 (C O). H NMR (300 MHz, CDCl3): d 2.38 (s,
lesser extent than 1,2-diiodoalkanes, have been reported as relatively
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 1474–1478 | 1477
©

View Article Online
unstable compounds that may easily eliminate iodine: (a) M. Zanger
and J. L. Rabinowitz, J. Org. Chem., 1975, 40, 248; (b) R. M. Pagni,
G. W. Kabalka, R. Boothe, K. Gaetano, L. J. Stewart, R. Conaway, C.
Dial, D. Gray, S. Larson and T. Luidhardt, J. Org. Chem., 1988, 53,
4477 See also ref. 3b.
on the iodine elimination reaction of 1,2-diiodoethylene by iodide ion
catalysis have been published: S. I. Miller and R. M. Noyes, J. Am.
Chem. Soc., 1952, 74, 3403.
12 At this point, the reaction mixture was composed of 42% remaining
3a, 42% 1a and 16% 2a, as determined by 1H NMR analysis.
13 D. Conreaux, S. Belot, P. Desbordes, N. Monteiro and G. Balme, J. Org.
Chem., 2008, 73, 8619 and ref. 4.
11 It is likely that the elimination process is triggered by traces of iodine
present as a contaminant in the starting diiodo compound. Studies
1478 | Org. Biomol. Chem., 2011, 9, 1474–1478
This journal is
The Royal Society of Chemistry 2011
©