5-Endo-dig electrophilic cyclization of 1,4-disubstituted but-3-yn-1-ones: Regiocontrolled synthesis of 2,5-disubstituted 3-bromo- and 3-iodofurans

Article abstract of DOI:10.1021/ol050372i

(Chemical Equation Presented) 5-Endo-dig electrophilic cyclization of 1,4-diaryl but-3-yn-1-ones with N-bromosuccinimide or N-iodosuccinimide/acetone and iodine monochloride/ CH₂Cl₂, at room temperature, in the absence of base, provi

Full text of DOI:10.1021/ol050372i

ORGANIC
LETTERS
2005
Vol. 7, No. 9
1769-1772
5-Endo-Dig Electrophilic Cyclization of
1,4-Disubstituted But-3-yn-1-ones:
Regiocontrolled Synthesis of
2,5-Disubstituted 3-Bromo- and
3-Iodofurans
Adam Sniady,† Kraig A. Wheeler,‡ and Roman Dembinski*,†
Department of Chemistry, Oakland UniVersity, 2200 North Squirrel Road,
Rochester, Michigan 48309-4477, and Department of Chemistry,
Delaware State UniVersity, DoVer, Delaware 19901
dembinsk@oakland.edu
Received February 21, 2005
ABSTRACT
5-Endo-dig electrophilic cyclization of 1,4-diaryl but-3-yn-1-ones with N-bromosuccinimide or N-iodosuccinimide/acetone and iodine monochloride/
CH2Cl2, at room temperature, in the absence of base, provides 3-halo-2,5-diarylfurans with excellent regiocontrol and high yields (81 94%).
−
The furan unit is found in a number of natural products and
synthetic materials, including industrial intermediates and
pharmaceuticals. The development of synthetic routes that
allow the facile assembly of substituted furans under mild
conditions from simple, readily available starting materials
still remains an important objective.1 In general, substituted
furans are accessed via ring derivatization or cyclization of
acyclic precursors.2 Numerous heteroannulation reactions,
including transition-metal-catalyzed, leading to substituted
furans have been reported.1,3,4 Among the variety of oxygen-
containing compounds that can be subjected to cyclization,
unsaturated alcohols or ketones are substrates of major
interest.3a,5,6
and bromofurans are useful as substrates in a variety of C-C,
C-N, or C-S bond-forming reactions,7-11 and they also
serve as building blocks for combinatorial chemistry.12 Since
(2) (a) Ko¨nig, B. Furan. In Hetarenes and Related Ring Systems; Science
of Synthesis; Maas, G., Ed.; George Thieme Verlag: Stuttgart, 2001;
Category 2, Vol. 9, pp 183-285. (b) Dell, C. P. Benzo[b]furans. In
Hetarenes and Related Ring Systems; Science of Synthesis; Thomas, E. J.,
Ed.; George Thieme Verlag: Stuttgart, 2000; Category 2, Vol. 10, pp 11-
86. (c) Hou, X. L.; Cheung, H. Y.; Hon, T. Y.; Kwan, P. L.; Lo, T. H.;
Tong, S. Y.; Wong, H. N. C. Tetrahedron 1998, 54, 1955-2020. (d)
Eberbach, W. Furan. In Houben-Weyl; George Thieme Verlag: Stuttgart,
1994; E6a, Teil I, pp 16-185d. (e) Hou, X. L.; Yang, Z.; Wong, H. N. C.
Furans and Benzofurans. In Progress in Heterocyclic Chemistry; Gribble,
G. W., Gilchrist, T. L., Eds.; Pergamon Press: Oxford, 2003; Vol. 15, pp
167-205. (f) Friedrichsen, W. Furans and Benzo Derivatives: Synthesis.
In ComprehensiVe Heterocyclic Chemistry II; Katritzky, A. R., Rees, C.
W., Scriven, E. F. V., Eds.; Pergamon Press: Oxford, 1996; Vol. 2, pp
351-394.
(3) Recent reviews: (a) Zeni, G.; Larock, R. C. Chem. ReV. 2004, 104,
2285-2309. (b) Kirsch, G.; Hesse, S.; Comel, A. Curr. Org. Synth. 2004,
1, 47-63. (c) Balme, G.; Bouyssi, D.; Lomberget, T.; Monteiro, N. Synthesis
2003, 2115-2134. (d) Jeevanandam, A.; Ghule, A.; Ling, Y.-C. Curr. Org.
Chem. 2002, 6, 841-864. (e) Cacchi, S.; Fabrizi, G.; Goggiomani, A.
Heterocycles 2002, 56, 613-632.
Halofurans are important derivatives that provide an
opportunity for further functionalization. In particular, iodo-
† Oakland University.
‡ Delaware State University.
(1) Brown, R. C. D. Angew. Chem., Int. Ed. 2005, 44, 850-852.
10.1021/ol050372i CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/02/2005

the HOMO coefficient is greater for the R-C-atoms than for
the â-C-atoms, electrophilic reactions at the 3- or 4-position
are generally more difficult than substitution at the 2- or
5-position.13
Scheme 1. Cyclization/Elimination of
gem-Dihalocyclopropanes
We have designed a homologous family of 2,5-unsym-
metrically substituted 3-halofurans. The preparation of this
class of compounds is a considerable synthetic challenge.
Direct halogenation of 2,5-unsymmetrically (but electroni-
cally similar) substituted furans generally leads to mixtures
of regioisomers.14 Multistep sequences such as ring cleavage
of gem-dihalocyclopropyl ketones proceed in moderate yields
(Scheme 1).7,15 Treatment of but-2-yn-4-ol-1-ones with HX
(X ) Br, Cl, I), typically at 50 °C, yields 3-halo-2,5-
disubstituted furans.16,17 3-Fluoro-2,5-disubstituted furans can
also be obtained via base-promoted cyclization/elimination
of substituted 2,2-difluorobut-3-yn-1-ols.18
Electrophilic cyclization of unsaturated compounds has
proven to be an efficient method for the one-step construction
and functionalization of furan units.19-23 Reactions that
involve the presence of a core aromatic ring such as
electrophilic cyclization of o-alkynyl phenols19 and acetoxy-
or benzyloxypyridines20 have been reported to yield haloben-
zofurans or related aromatic compounds. Additionally, io-
docyclization of alk-3-yn-1,2-diols followed by dehydration
in the presence of base yields 3-iodofurans derivatives.21 We
recently reported the electrophilic heteroannulation of 5-
alkynyl-2′-deoxyuridines to furanopyrimidine nucleosides.24
In this case, amido-iminol tautomerization and the presence
of an sp2 carbon in the core may facilitate the cyclization.
Presently, in pursuit of a regiocontrolled synthesis of 2,5-
unsymmetrically substituted 3-halofurans, we have isolated
the furan acyclic precursor core (but-3-yn-1-one) with various
aromatic endgroups. Phenyl, p-alkylphenyls, and p-halo-
phenyls were selected as aryl substituents. We have focused
on easily handled N-halosuccinimides, which have literature
precedents in electrophilic halocycloisomerizations.22a,24,25
(4) Representative recent examples: (a) Casey, C. P.; Strotman, N. A.
J. Org. Chem. 2005, 70, 2576-2581. (b) Yao, T.; Zhang, X.; Larock, R.
C. J. Am. Chem. Soc. 2004, 126, 11164-11165. (c) Aurrecoechea, J. M.;
Pe´rez, E. Tetrahedron 2004, 60, 4139-4149. (d) Jung, C.-K.; Wang, J.-C.;
Krische, M. J. J. Am. Chem. Soc. 2004, 126, 4118-4119. (e) Sromek, A.
W.; Kel’in, A. V.; Gevorgyan, V. Angew. Chem., Int. Ed. 2004, 43, 2280-
2282. (f) Hashmi, A. S. K.; Sinha, P. AdV. Synth. Catal. 2004, 346, 432-
438. (g) Padwa, A.; Rashatasakhon, P.; Rose, M. J. Org. Chem. 2003, 68,
5139-5146. (h) Ma, S.; Zhang, J.; Lu, L. Chem. Eur. J. 2003, 9, 2447-
2456. (i) Bates, C. G.; Saejueng, P.; Murphy, J. M.; Venkataraman, D. Org.
Lett. 2002, 4, 4727-4729. (j) Flynn, B. L.; Hamel, E.; Jung, M. K. J. Med.
Chem. 2002, 45, 2670-2673. (k) Lo, C.-Y.; Guo, H.; Lian, J.-J.; Shen,
F.-M.; Liu, R.-S. J. Org. Chem. 2002, 67, 3930-3932. (l) Barma, D. K.;
Kundu, A.; Baati, R.; Mioskowski, C.; Falck, J. R. Org. Lett. 2002, 4, 1387-
1389.
(5) Alkynones: (a) Pd-catalyzed: Sheng, H.; Lin, S.; Huang, Y. Z.
Tetrahedron Lett. 1986, 27, 4893-4894. (b) Sheng, H.; Lin, S.; Huang, Y.
Synthesis 1987, 1022-1023. (c) Fukuda, Y.; Shiragami, H.; Utimoto, K.;
Nozaki, H. J. Org. Chem. 1991, 56, 5816-5819. (d) Arcadi, A.; Rossi, E.
Tetrahedron 1998, 54, 15253-15272. (e) Jeevanandam, A.; Narkunan, K.;
Ling, Y.-C. J. Org. Chem. 2001, 66, 6014-6020. (f) Bossharth, E.;
Desbordes, P.; Monteiro, N.; Balme, G. Org. Lett. 2003, 5, 2441-2444.
CuI-catalyzed: Kel’in, A. V.; Gevorgyan, V. J. Org. Chem. 2002, 67, 95-
98. (g) AgNO3-catalyzed: Aucagne, V.; Amblard, F.; Agrofoglio, L. A.
Synlett 2004, 2406-2408. (h) Acid-catalyzed: Brown, C. D.; Chong J. M.;
Shen, L. Tetrahedron 1999, 55, 14233-14242. (i) Base-catalyzed: Vieser,
R.; Eberbach, W. Tetrahedron Lett. 1995, 36, 4405-4408. (j) Arcadi, A.;
Marinelli, F.; Pini, E.; Rossi, E. Tetrahedron Lett. 1996, 37, 3387-3390.
(6) Alkynols: (a) Pd-catalyzed: Wakabayashi, Y.; Fukuda, Y.; Shirag-
ami, H.; Utimoto, K.; Nozaki, H. Tetrahedron 1985, 41, 3655-3661. (b)
Gabriele, B.; Salerno, G.; Lauria, E. J. Org. Chem. 1999, 64, 7687-7692.
(c) Base-catalyzed: Holand, S.; Mercier, F.; Le Goff, N.; Epsztein, R. Bull.
Soc. Chim. Fr. 1972, 4357-4364. (d) Marshall, J. A.; DuBay, W. J. J.
Org. Chem. 1993, 58, 3435-3443.
Our new route to 2,5-substituted 3-halofurans started from
commercially available styrene oxide or its derivatives (1a-
c), as presented in Scheme 2. Terminal alkynes (2a,b) (1.5
equiv) were deprotonated with LDA (1.5 equiv) in DMSO,
as previously described;26 THF and DMF were inefficient
solvents for this protocol. Ring opening of the epoxides
proceeded with full regioselectivity as confirmed by 1H NMR
to yield, after workup, 2,5-diarylbut-3-yn-1-ols (3a-e).
(7) Tanabe, Y.; Wakimura, K.; Nishii, Y.; Muroya, Y. Synthesis 1996,
388-392.
(8) Chinchilla, R.; Najera, C.; Yus, M. Chem. ReV. 2004, 104, 2667-
2722.
Reaction of the alkynyl alcohol 3b with electrophiles such
as N-iodosuccinimide (NIS) or N-bromosuccinimide (NBS)
(9) C-C bond examples: (a) Lin, S.-Y.; Chen, C.-L.; Lee, Y.-J. J. Org.
Chem. 2003, 68, 2968-2971. (b) Bach, T.; Kru¨ger, L. Eur. J. Org. Chem.
1999, 2045-2057. (c) Alvarez-Ibarra, C.; Quiroga, M. L.; Toledano, E.
Tetrahedron 1996, 52, 4065-4078.
(10) C-N bond examples: (a) Hooper, M. W.; Utsunomiya, M.; Hartwig,
J. F. J. Org. Chem. 2003, 68, 2861-2873. (b) Padwa, A.; Crawford, K. R.;
Rashatasakhon, P.; Rose, M. J. Org. Chem. 2003, 68, 2609-2617. (c)
Crawford, K. R.; Padwa, A. Tetrahedron Lett. 2002, 43, 7365-7368.
(11) C-S bond examples: Arroyo, Y.; Rodriguez, J. F.; Sanz-Tejedor,
M. A.; Santos, M. Tetrahedron Lett. 2002, 43, 9129-9132; see also ref
9b.
(18) Sham, H. L.; Betebenner, D. A. J. Chem. Soc., Chem. Commun.
1991, 1134-1135.
(19) Arcadi, A.; Cacchi, S.; Fabrizi, G.; Marinelli, F.; Moro, L. Synlett
1999, 1432-1434.
(20) Arcadi, A.; Cacchi, S.; Di Giuseppe, S.; Fabrizi, G.; Marinelli, F.
Org. Lett. 2002, 4, 2409-2412.
(21) (a) El-Taeb, G. M. M.; Evans, A. B.; Jones, S.; Knight, D. W.
Tetrahedron Lett. 2001, 42, 5945-5948. (b) Bew, S. P.; Knight, D. W.
Chem. Commun. 1996, 1007-1008.
(12) Sauers, R. R.; Van Arnum, S. D. J. Comb. Chem. 2004, 6, 350-
355.
(22) (a) Schultz-Fademrecht, C.; Zimmermann, M.; Fro¨hlich, R.; Hoppe,
D. Synlett 2003, 1969-1972. (b) Nichols, C. J.; Parks, J. J. J. Undergrad.
Chem. Res. 2002, 1, 27-32.
(13) Eicher, T.; Hauptmann, S.; Speicher, A. The Chemistry of Hetero-
cycles, 2nd ed.; Wiley-VCH: Weinheim, 2003; pp 52-62.
(14) NBS bromination of 2-(4-methoxyphenyl)-5-(4-nitrophenyl)furan
(benzene, reflux) yields of 3-bromo-2-(4-methoxyphenyl)-5-(4-nitrophenyl)-
furan in 44% yield: Rees, C. W.; Yue, T.-Y. J. Chem. Soc., Perkin Trans.
1 1997, 2247-2252.
(15) Xu, W.; Chen, Q.-Y. Org. Biomol. Chem. 2003, 1, 1151-1156.
(16) Obrecht, D. HelV. Chim. Acta 1989, 72, 447-456; see also ref 12.
(17) Anionic process (n-BuLi/I2) leads also to iodofuran derivatives:
Buckle, D. R.; Rockell, C. J. M. J. Chem. Soc., Perkin Trans. 1 1985, 2443-
2446.
(23) Knight, D. W. Electrophile-Induced 5-Endo Cyclizations. In Progress
in Heterocyclic Chemistry; Gribble, G. W., Gilchrist, T. L., Eds.; Pergamon
Press: Oxford, 2002; Vol. 14, pp 19-51.
(24) Rao, M. S.; Esho, N.; Sergeant, C.; Dembinski, R. J. Org. Chem.
2003, 68, 6788-6790.
(25) (a) Yue, D.; Della Ca`, N.; Larock, R. C. Org. Lett. 2004, 6, 1581-
1584. (b) Yue, D.; Larock, R. C. J. Org. Chem. 2002, 67, 1905-1909.
(26) Brandsma, L. PreparatiVe Acetylenic Chemistry, 2nd ed.; Studies
in Organic Chemistry 34; Elsevier: Amsterdam, 1988; p 67.
1770
Org. Lett., Vol. 7, No. 9, 2005

Scheme 2. Synthesis of Halofurans 5 and 6
Table 1. Preparation of Halofurans 5 and 6 via
Cycloisomerization of 4
reagent (E-A), halo- yield
solventa
furan (%)
butynone
R
R′
4a
C6H5
p-CH3C6H4 NBS, acetone
NIS, acetone
5a
6a
6a
5b
6b
5c
6c
5d
6d
5e
6e
89
84
82
89
86
83
94
87
81
86
88
ICl, CH2Cl2
4b
4c
4d
4e
p-BrC6H4 p-CH3C6H4 NBS, acetone
NIS, acetone
p-ClC6H4 p-CH3C6H4 NBS, acetone
NIS, acetone
p-BrC6H4 p-t-BuC6H4 NBS, acetone
NIS, acetone
p-ClC6H4 p-t-BuC6H4 NBS, acetone
NIS, acetone
a Reactions were carried out on ca. 0.6-1.4 mmol scale with 1.05 equiv
of electrophilic reagent (E-A) at room temperature. For a representative
procedure see ref 31.
reaction conditions included CH2Cl2 and 1.05 equiv of ICl.
Similarly, the derived product 6a was formed at room
temperature.
GC/MS examination of postreaction mixtures of butynone
4a with ICl, butynone 4b with NBS or NIS, and 4d with
NIS indicated conversion to the halofurans 6a/5b/6b/6d with
97/96/99+/99+% yields, respectively. In all reported cases
in Table 1, quantitative conversion of substrate was con-
(2 equiv, acetone), in the absence of base, showed no
conversion after 48 h at room temperature. It has been
reported that relevant reaction of but-3-yn-1-ols with
elemental iodine in the presence of a base leads to the 1,2-
iodine addition product.27 Thus, the isolated butynols 3a-e
were treated with Dess-Martin reagent28 (1.5 equiv) in CH2-
Cl2 at room temperature (1 h) to yield but-3-yn-1-ones (4a-
e, Scheme 2). Alternative oxidation with MnO2 (8 equiv, 42
h, CH2Cl2) did not show satisfactory progress of the reaction.
Subsequently, the isolated alkynyl ketones 4a-e were
subjected to the reaction with various halogenating electro-
philes.
1
firmed by H NMR. After reaction completion, the solid
products were separated from the major part of spent and
unreacted N-succinimides by extraction with ether.30 Sub-
sequent silica gel filtration (to remove traces of the remaining
N-succinimides) gave halofurans 5a-e and 6a-e with 81-
94% yield.31 The results are summarized in Table 1.
1
The substituted halofurans were characterized by H and
13C NMR, IR, MS, and UV-vis spectroscopy. The charac-
teristic NMR32 (CDCl3) features for 5a-e and 6a-e include
the 1H H-4 signal (6.77-6.79 and 6.83-6.85 ppm) and 13
C
of C-halogen (C-3, 97.5-97.6 and 62.3-62.4 ppm) for
bromo and iodo derivatives, respectively. Mass spectra for
5a-e and 6a-e exhibited intense molecular ions with
appropriate isotope patterns characteristic for halogens, when
applicable. The UV spectra were similar to those reported
for relevant 2,5-diarylfurans.33 Almost all iodo- and bromo-
furans gave highly accurate ((0.1%) elemental analyses.
1-Phenyl-4-(p-tolyl)butynone 4a was treated with NBS
(1.05 equiv) in acetone at room temperature, and complete
conversion into 5a was observed after 2 h. Since it is known
that substituted furans undergo oxidative ring opening with
NBS in acetone in the presence of pyridine,29 and to maintain
mild reaction conditions, all reactions were carried out in
the absence of base. 3-Bromofuran 5a was isolated, after
workup, in 89% yield (Table 1). Alkynone 4a was also
treated with NIS in a similar manner to access the iodo
derivative 6a and to compare the reactivity of 4a toward
analogous halosuccinimide. Practically no difference was
observed in terms of rate of the reaction of NBS vs NIS, as
opposed to different reactivity toward 5-alkynyl uridines.24
The utility of a stronger electrophile, iodine monochloride,
was also examined for the cycloisomerization of 4a. The
(30) Anhydrous (commercial) ether was used. Extraction with reagent
grade ether leads to the “leaching“ of the succinimide likely due to water
and ethanol contents. This step is obviously not necessary for reaction with
ICl.
(31) A round-bottom flask was charged with 4a (0.200 g, 0.854 mmol)
and acetone (9 mL). NBS (0.160 g, 0.900 mmol) in acetone (9 mL) was
added dropwise by a syringe. The solution was stirred under nitrogen
atmosphere at room temperature for 2 h. TLC showed complete conversion
of substrate. Solvent was removed by rotary evaporation and the residue
was extracted with ether. The solid was filtered off. The solvent was
removed by rotary evaporation. Short path silica gel column chromatography
(2.5 × 15 cm; CHCl3) gave a colorless fraction. Solvent was removed by
rotary evaporation and the residue was dried by oil vacuum pump to give
5a as a white solid (0.238 g, 0.760 mmol, 89%). The compound can be
recrystallized from methanol.
(27) Knight, D. W.; Redfern, A. L.; Gilmore, J. J. Chem. Soc., Perkin
Trans. 1 2002, 622-628.
(28) (a) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155-4156.
(b) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277-7287.
(c) Ireland, R. E.; Liu, L. J. Org. Chem. 1993, 58, 2899.
(29) Kobayashi, Y.; Nakano, M.; Kumar, G. B.; Kishihara, K. J. Org.
Chem. 1998, 63, 7505-7515.
(32) Alvarez-Ibarra, C.; Quiroga-Feijo´o, M. L.; Toledano, E. J. Chem.
Soc., Perkin Trans. 2 1998, 679-689.
(33) King, S. M.; Bauer, C. R.; Lutz, R. E. J. Am. Chem. Soc. 1951, 73,
2253-2256.
Org. Lett., Vol. 7, No. 9, 2005
1771

Relevant electrophilic cyclizations are believed to proceed
through an intramolecular, stepwise mechanism involving a
cationic intermediate.20,34 However, we presume that R-hy-
drogen abstraction may also be a mechanistic option.35
A molecular structure of a representative halofuran was
determined by X-ray crystallography. Crystallization of
compound 5e from ether/methanol (7:3) gave single crystals
suitable for X-ray analysis.36 Inspection of Figure 1 reveals
the molecular structure of the expected 3-bromofuran.
Excluding the tert-butyl fragment, the entire molecule of 5e
is planar within 0.173 Å. The maximum atom deviations
from the average plane are 0.39(1) and 0.36(1) Å for C10
and C11, respectively. Interestingly, the CdC bond of 5e
with the bromine atom attached [C2-C3, 1.377(11) Å] is
significantly elongated compared to the hydrogen-substituted
C4-C5 bond [1.344(10) Å].
In summary, we have demonstrated that electrophilic
cyclization of but-3-yn-1-ones with various electrophilic
halogen sources yields halofurans under exceptionally mild
conditions: in the absence of base and in one of the most
environmentally friendly organic solvents, acetone. Com-
bining C-O and C-electrophile bond formation maximizes
reaction efficiency with good atom economy; a cycloisomer-
ization catalyst is at the same time a halogen donor. Our
Figure 1. ORTEP view of 5e with an atom-labeling scheme.
Thermal ellipsoids at the 50% probability level. Selected interatomic
distances (Å): Br1-C3 1.849(9); O1-C2 1.391(8); O1-C5 1.374-
(9); C2-C3 1.377(11); C2-C6 1.437(11); C3-C4 1.419(10); C4-
C5 1.344(10); C5-C13 1.428(11). Key angles (deg): C2-O1-
C5 109.5(7); O1-C2-C3 106.9(7); O1-C2-C6 115.1(7); C3-
C2-C6 138.0(8); C2-C3-C4 106.9(7); Br1-C3-C2 126.4(7);
Br1-C3-C4 126.7(6); C3-C4-C5 109.1(7); O1-C5-C4
107.6(7); O1-C5-C13 117.8(7); C4-C5-C13 134.6(8).
simple isolation protocol facilitates high yields. Our approach
allows for excellent regiocontrol in the preparation of
unsymmetrically 2,5-disubstituted 3-halofurans.
(34) Ren, X.-F.; Turos, E. Tetrahedron Lett. 1993, 34, 1575-1578.
(35) A mechanistic pathway may include cyclization followed by a
deprotonation (right structure) or R-hydrogen abstraction (left structure).
Acknowledgment. We thank the Oakland University
Research Excellence Program in Biotechnology for support
of this research.
Supporting Information Available: Synthetic proce-
1
dures, analytical and spectral characterization data, H and
(36) Crystallographic data (excluding structural factors) for the structure
reported in this paper were also deposited with Cambridge Crystallographic
Data Centre as supplementary publication no. CCDC-265867. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.;
fax: +44(0)1223-336033.
13C NMR spectra for all halofurans (5a-e, 6a-e), and X-ray
tables for 5e and a separate CIF file. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL050372I
1772
Org. Lett., Vol. 7, No. 9, 2005