Ring-enlargement of dimethylaminopropenoyl cyclopropanes: An efficient route to substituted 2,3-dihydrofurans

Article abstract of DOI:10.1021/jo801289p

(Chemical Equation Presented) A convenient and efficient synthesis of substituted dihydrofurans is developed via ring-enlargement of 1-dimethylaminopropenoyl-1-carbamoyl/benzoyl cyclopropanes catalyzed by ammonium acetate in acetic acid with high regio- and stereoselectivity. Some of the newly synthesized substituted dihydrofurans are subjected to further synthetic transformation in the presence of NaOH (aq) in ethanol to afford the corresponding 5-aryl-2,3-dihydrofuro[3,2-c]pyridin-4(5H)-ones in high yields.

Full text of DOI:10.1021/jo801289p

SCHEME 1. Ring-Enlargement of Cyclopropyl Ketones
Ring-Enlargement of Dimethylaminopropenoyl
Cyclopropanes: An Efficient Route to Substituted
2,3-Dihydrofurans
Rui Zhang,† Yongjiu Liang,† Guangyuan Zhou,†
Kewei Wang,‡ and Dewen Dong*,†,‡
of intense research for decades, and continues to be an active
area of research today. Extensive work has generated a variety
of synthetic approaches, including ionic or radical [3 + 2]
annulations of 1,3-dicarbonyl compounds with appropriate
olefins,4,5 formal [3 + 2] annulations of ꢀ-ketosulfides/ꢀ-
ketosulfones with aldehydes,6 and the transition metal-catalyzed
[4 + 1] cycloaddition of enones with diazo compounds.7
On the other hand, cyclopropanes are extremely versatile
synthetic intermediates for their ready accessibility and good
reactivity.8,9 Their well-known “unsaturated” character can lead
to a variety of ring-opening reactions under the influence of
electrophiles, nucleophiles, and radicals, or external physical
forces such as heat and light. Since the pioneering work on the
ring-enlargement of cyclopropylketones was first reported by
Cloke in 1929 (Scheme 1),10 this type of transformation became
a notable approach for the synthesis of 2,3-dihydrofurans and
was widely studied by employing varied catalysts, such as Lewis
acids, metal, or strong oxidizing agents.11 Recently, Zhang and
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun, 130022, China, and Department of
Chemistry, Northeast Normal UniVersity,
Changchun, 130024, China
dwdong@ciac.jl.cn
ReceiVed June 16, 2008
A convenient and efficient synthesis of substituted dihydrofurans
is developed via ring-enlargement of 1-dimethylaminoprope-
noyl-1-carbamoyl/benzoyl cyclopropanes catalyzed by am-
monium acetate in acetic acid with high regio- and stereose-
lectivity. Some of the newly synthesized substituted dihydrofurans
are subjected to further synthetic transformation in the presence
of NaOH (aq) in ethanol to afford the corresponding 5-aryl-
2,3-dihydrofuro[3,2-c]pyridin-4(5H)-ones in high yields.
(4) (a) Aso, M.; Ojida, A.; Yang, G.; Cha, O. J.; Osawa, E.; Kamematsu, K.
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Korp, J. D.; Kurosawa, K. J. Chem. Soc., Perkin Trans. 1 1993, 609–616. (f)
McDonald, F. E.; Connolly, C. B.; Gleason, M. M.; Towne, T. B.; Treiber, K. D. J.
Org. Chem. 1993, 58, 6952–6953. (g) Roy, S. C.; Mandal, P. K Tetrahedron 1996,
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The vast number of bioactive natural products as well as
medicinally important unnatural compounds based on the 2,3-
dihydrofuran ring system, such as aflatoxin B1 and clerodin,
are very important in the area of natural product and pharma-
ceutical chemistry.1,2 In addition, the utility of 2,3-dihydrofuran
derivatives as organic intermediates in the synthesis of an array
of highly functionalized tetrahydrofurans with good stereose-
lectivity is well recognized.3 The development of efficient
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† Chinese Academy of Sciences.
‡ Northeast Normal University.
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Dean, F. M.; Sargent, M. V. In ComprehensiVe Heterocyclic Chemistry; Bird,
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10.1021/jo801289p CCC: $40.75
Published on Web 09/18/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 8089–8092 8089

TABLE 1. Synthesis of Doubly Activated Cyclopropanes 2a
SCHEME 2. Ring-Enlargement of
1-Alkenoyl-1-carbamoylcyclopropane 2a
entry
1
Ar
R1
2
yield (%)b
and a major product was obtained and characterized as N-
tolylacetamide, indicating the cleavage of 1a under the employed
acidic conditions. When 2a was subjected to acetic acid under
reflux, the reaction furnished a white solid, which was charac-
terized as 2-(3-(p-tolylcarbamoyl)-4,5-dihydrofuran-2-yl)vinyl
acetate 3a (73% yield) on the basis of its spectral and analytical
data (Scheme 2).
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
4-MeC6H4
C6H5
H
H
H
H
H
H
H
H
2a
2b
2c
2d
2e
2f
2g
2h
2i
88
81
85
83
89
91
78
86
83
2-MeC6H4
2,4-Me2C6H3
4-MeOC6H4
2-MeOC6H4
2-ClC6H4
2-MeO-5-ClC6H3
4-MeC6H4
C6H5
SCHEME 3. Ring-Enlargement of
1-Acyl-1-carbamoylcyclopropanes
a Reagents and conditions: 1 (5.0 mmol), DMFDMA (6.0 mmol),
DMF (15 mL), 85 °C, 5.0-6.0 h. b Isolated yields.
co-workers achieved the synthesis of furoquinolines through
SnCl4-mediated tandem ring-opening/recyclization reaction of
1-acyl-1-carbamoylcyclopropanes.12 From the same precursors,
we developed a facile and efficient one-pot synthesis of highly
substituted pyridin-2(1H)-ones under Vilsmeier conditions.13
Furthermore, we achieved divergent synthesis of fully substituted
1H-pyrazoles and isoxazoles from the oximes of 1-acyl-1-
carbamoylcyclopropanes in the presence of POCl3/DMF (Vils-
meier reagent) and POCl3/CH2Cl2, respectively.14 In connection
with these studies and our continuing interest in the utility of
ꢀ-oxo amide derivatives in the synthesis of carbo- and hetero-
cycles,15 we prepared a series of 1-dimethylaminopropenoyl-
1-carbamoyl/benzoylcyclopropanes 2 and explored their syn-
thetic potential. As a result, we wish to describe herein a strategy
for performing metal-free, mild rearrangements of the doubly
activated cyclopropanes 2 to substituted dihydrofurans 3, and
their further synthetic transformation to substituted 2,3-dihy-
drofuro[3,2-c]pyridin-4(5H)-ones 4.
The substrates, 1-acyl-1-carbamoylcyclopropanes 1a-i, were
synthesized from commercially available ꢀ-oxo amides, 1,2-
dibromoethane/1-phenyl-1,2-dibromoethane in very high yields
following the reported procedure.12,13,16 Upon treatment of
compounds 1 with N,N-dimethylformamide dimethylacetal
(DMFDMA) in DMF at 85 °C for 5.0-6.0 h, a series of
condensation adducts 2a-i was synthesized in high yields (Table
1).
In Zhang’s recent work on the ring-opening/recyclization of
cyclopropanes 1 in the presence of SnCl4 ·H2O, 4-acyl-5-
arylamino-2,3-dihydrofurans were obtained as the key interme-
diates (Scheme 3).12 Their studies and our results suggest that
both the nature of substituents on cyclopropanes and the reaction
conditions employed have a great effect on the selectivity of
the ring-enlargement on the different carbonyl groups of the
doubly activated cyclopropanes. Moreover, it is worth noting
that the ring-enlargement reaction of 2a proceeded to afford 3a
along with an unexpected reversal of the configuration of its
1
CdC bond as determined by H NMR spectra. Two doublet
peaks with resonances at δ 4.70 and 7.72 in the 1H NMR spectra
of 2a are assigned to the two neighboring protons of the CdC
bond. Their coupling constant (J ) 12.5 Hz) reveals that 2a is
an E-isomer. In contrast, 3a displays two doublet peaks at δ
1
6.06 and 7.39, respectively, in its H NMR spectra, and their
coupling constant (J ) 8.0 Hz) indicates that 3a is formed in
the Z-configuration. We could not isolate the E-isomer, which
might be present in trace amounts in the reaction mixture.
The optimization of the reaction conditions, including reaction
temperature and solvents, was then investigated. It was noted
that variation of the reaction temperature in the range of 80-120
°C had no significant influence on the reaction, but the reaction
required prolonged reaction time along with low conversion
when performed at temperatures below 80 °C. Interestingly, the
addition of a small amount of ammonium salt such as NH4OAc
could accelerate the reaction. By employing DMF as the solvent
instead of acetic acid, the reaction resulted in a complex mixture.
Subjecting 2a to acetic anhydride at 100 °C for 24 h, no reaction
was observed as monitored by TLC. With the addition of several
drops of water, the reaction proceeded and produced a mixture
containing 3a. The results suggest that the existence of acetate
anion in the reaction system might play an important role in
the ring-enlargement process. After a series of experiments, the
optimal results were achieved when the reaction of 2a was
performed with NH4OAc (0.1 equiv) in acetic acid at 90 °C for
4.0 h, whereby the yield of 3a reached 85% (Table 2, entry 1).
It is worth mentioning that 3a can be obtained in pure form by
the nonchromatography purification method, namely washing
the crude product with water and cold acetone.
With the doubly activated cyclopropanes 1 and 2 in hand,
we selected 1a and 2a as model compounds to examine their
reaction behaviors. In contrast with the studies on the reactions
of 1 with Lewis acid or Vilsmeier reagent,12,13 in the present
work we envisage the reactions of 1 in the presence of organic
acid. Thus, 1a was treated in acetic acid at room temperature
for 10.0 h, but no reaction was observed as indicated by TLC.
After the mixture was heated to reflux, the reaction proceeded,
(12) Zhang, Z.; Zhang, Q.; Sun, S.; Xiong, T.; Liu, Q. Angew. Chem., Int.
Ed. 2007, 46, 1726–1729.
(13) Pan, W.; Dong, D.; Wang, K.; Zhang, J.; Wu, R.; Xiang, D.; Liu, Q.
Org. Lett. 2007, 9, 2421–2423.
(14) Wang, K.; Xiang, D.; Liu, J.; Pan, W.; Dong, D. Org. Lett. 2008, 10,
1691–1694.
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Soc. 2005, 127, 4578–4579. (b) Dong, D.; Bi, X.; Liu, Q.; Cong, F. Chem.
Commun. 2005, 3580–3582. (c) Wang, Y.; Dong, D.; Yang, Y.; Huang, J.;
Ouyang, Y.; Liu, Q. Tetrahedron 2007, 63, 2724–2728. (d) Xiang, D.; Yang,
Y.; Zhang, R.; Liang, Y.; Pan, W.; Huang, J.; Dong, D. J. Org. Chem. 2007, 72,
8593–8596.
Having established the optimal conditions for the ring-
enlargement reaction, we aimed to determine its scope with
(16) White, D. A. Synth. Commun. 1977, 7, 559–568.
8090 J. Org. Chem. Vol. 73, No. 20, 2008

TABLE 2. Synthesis of Substituted Dihydrofurans 3 from Doubly
Activated Cyclopropanes 2a
A makes it liable to undergo tandem oxa-Michael addition to
the unsaturated ꢀ-carbon and elimination reaction of dimethy-
lamine to generate intermediate C in the Z-configuration via a
transition state B.19 Under the acidic conditions, the carbonyl
group bearing the R2 group of C is protonated to afford oxonium
ion D, followed by a regioselective ring expansion to give
oxonium ion E,20 which is finally converted into the substituted
dihydrofuran of type 3.
entry
2
Ar
R1
3
yieldb
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
2g
2h
2i
4-MeC6H4
C6H5
H
H
H
H
H
H
H
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
85
82
80
83
88
87
81
85
87
It should be noted that the vinyl acetate moiety of 3 can be
regarded as a potential formyl group, which may lead to further
synthetic transformations under appropriate conditions. Inspired
by this, we selected 3a as a model compound to examine its
behavior under basic conditions. Thus, 3a was subjected to
NaOH (aq, 1.5 equiv) in ethanol at room temperature for 2.0 h.
The reaction furnished a white solid, which was characterized
as 5-phenyl-2,3-dihydrofuro[3,2-c]pyridin-4(5H)-one 4a (84%
yield) on the basis of its spectral and analytical data. With the
increase of the amount of NaOH to 2.0 equiv, the reaction
completed within 30 min and the yield of 4a reached 92% (Table
3, entry 1). Under the identical conditions, some of the newly
synthesized 2-(3-(carbamoyl)-4,5-dihydrofuran-2-yl)vinyl ac-
etates 3 were converted into the corresponding substituted 2,3-
dihydrofuro[3,2-c]pyridin-4(5H)-ones 4 in excellent yields
(Table 3, entries 2-7). It is assumed that the vinyl acetate of 3
undergoes hydrolysis reaction mediated by NaOH (aq) in ethanol
to give the corresponding aldehyde and which upon intramo-
lecular cyclization with amide group followed by dehydration
furnishes the 2,3-dihydrofuro[3,2-c]pyridin-4(5H)-one of type
4.
2-MeC6H4
2,4-Me2C6H3
4-MeOC6H4
2-MeOC6H4
2-ClC6H4
2-MeO,5-ClC6H3
4-MeC6H4
C6H5
a Reagents and conditions: 2 (2.0 mmol), NH4OAc (0.2 mmol), AcOH
(15 mL), 90 °C, 2.5-5.0 h. b Isolated yields.
respect to the amide moiety. Thus, a series of reactions of
1-alkenoyl-1-carbamoyl cyclopropanes 2 were carried out under
the optimal conditions. As shown in Table 2, the ring-
enlargement reaction proved to be suitable for 2b-h bearing
varied amide groups, affording the corresponding 2-[3-(aryl-
carbamoyl)-4,5-dihydrofuran-2-yl]vinyl acetates 3b-h in high
yields (Table 2, entries 2-8). In the case of cyclopropane 2i
bearing a 2-substituted phenyl group, the reaction proceeds
smoothly to furnish the corresponding dihydrofuran 3i as a single
regioisomer in high yield (Table 2, entry 9); such regioselective
ring opening is in accordance with the results achieved in similar
ring-enlargement reactions.11a-d,12 It is significant to note that,
in all cases of 3a-i, only Z-isomeric products are isolated,
revealing a high stereoselectivity.
TABLE 3. Intramolecular Annulation Reactions of Dihydrofurans 3a
SCHEME 4. Ring-Enlargement of
1-Alkenoyl-1-benzoylcyclopropane 2j
entry
3
Ar
R1
4
yieldb
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3h
3i
4-MeC6H4
C6H5
H
H
H
H
H
H
C6H5
4a
4b
4c
4d
4e
4h
4i
92
91
93
90
91
94
87
2-MeC6H4
2,4-Me2C6H3
4-MeOC6H4
2-MeO-5-ClC6H3
4-MeC6H4
The versatility of this dihydrofuran synthesis was also
evaluated by using 1-alkenoyl-1-benzoylcyclopropane 2j under
the identical conditions. The reaction proceeded smoothly to
give the corresponding dihydrofuran 3j in 72% yield (Scheme
a Reagents and conditions: 3 (2.0 mmol), NaOH (aq, 4.0 mmol),
EtOH, rt, 0.5-1.0 h. b Isolated yields.
1
4). From the H NMR spectra of 3j, it is noteworthy that the
coupling constant of the two neighboring protons of the CdC
bond is 6.0 Hz, indicating the formation of a Z-isomeric product,
too.
Dihydrofuro[3,2-c]pyridin-4(5H)-one alkaloids are widely
distributed in the Rutaceae family of plants along with useful
bioactivities.21 To date, some synthetic approaches for the
synthesis of this kind of heterocycles have been reported, such
as the oxidative cycloaddition or photoinduced cycloaddition
On the basis of the obtained results, a plausible mechanism
for the ring-enlargement of cyclopropanes 2 is presented in
Scheme 5. The transformation might commence from a nucleo-
philic vinylic substitution (SNV) reaction of 2 in acetic acid.17
The N-protonation of 2 and the hydrogen bonding interaction
between 2 and ammonium acetate (or acetic acid) leads to the
formation of A.18 The strong electron-attracting effect of
dimethylaminium and carbonyl groups on the vinyl moiety of
(19) (a) Basavaiah, D.; Pandiaraju, S.; Krishnamacharyulu, M. Synlett 1996,
747–748. (b) Basavaiah, D.; Muthukumaran, K.; Sreenivasulu, B. Synthesis 2000,
545–548. (c) Ollevier, T.; Mwene-Mbeja, T. M. Tetrahedron 2008, 64, 5150–
5155.
(20) Pittman, C. U., Jr.; MacManus, S. P. J. Am. Chem. Soc. 1969, 91, 5915–
5918.
(17) (a) Bernasconi, C. F.; Schuck, D. F.; Ketner, R. J.; Eventova, I.;
Rappoport, Z. J. Am. Chem. Soc. 1995, 117, 2719–2725. (b) Bernasconi, C. F.
Tetrahedron 1989, 45, 4017–4090. (c) Bernasconi, C. F.; Ketner, R. J.; Chen,
X.; Rappoport, Z. J. Am. Chem. Soc. 1998, 120, 7461–7468.
(18) (a) Shirakawa, S.; Shimizu, S. Synlett 2007, 3160–3164. (b) Yang, Y.;
Xiang, D.; Zhao, X.; Liang, Y.; Huang, J.; Dong, D. Tetrahedron 2008, 64,
4959–4966.
(21) (a) Scheuer, P. J. In Chemistry of Alkaloids; Pelletier, S. W., Ed.; van
Nostrand Reinhold: New York, 1970; p 355. (b) Grundon, M. F. Nat. Prod.
Rep. 1990, 7, 131–138. (c) Michael, J. P. Nat. Prod. Rep. 1997, 14, 605–618.
(d) Basco, L. K.; Mitaku, S.; Skaltsounis, A. L.; Ravelomanantsoa, N.; Tillequin,
F.; Koch, M.; Le Bras, J. Antimicrob. Agents Chemother. 1994, 38, 1169–1171.
(e) Michael, J. P. Nat. Prod. Rep. 2004, 21, 650–668.
J. Org. Chem. Vol. 73, No. 20, 2008 8091

SCHEME 5. Plausible Mechanism of Ring-Enlargement of 2
of 4-hydroxypyridin-2(1H)-one derivatives with olefins.22 Our
present work provided an alternative and novel route to the
synthesis of substituted dihydrofuro[3,2-c]pyridin-4(5H)-ones
from commercially available ꢀ-oxo amides.
In summary, a facile and efficient synthesis of substituted
2,3-dihydrofurans 3 is developed from cyclopropanes 2 cata-
lyzed by NH4OAc in acetic acid involving stereoselective SNV,
regioselective ring-enlargement reactions. Some of the newly
synthesized substituted dihydrofurans 3 bearing an amide group
are subjected to further transformation in the presence of NaOH
(aq) in ethanol to afford the corresponding 5-aryl-2,3-dihydro-
furo[3,2-c]pyridin-4(5H)-ones 4 in high yields. This protocol
is associated with readily available starting materials, mild
conditions, high yields, and potential utility of the products.
Typical Procedure for the Preparation of Substituted
Dihydrofurans 3 (3a as an example). To a 50 mL round-bottomed
flask was added 2a (2.0 mmol), NH4OAc (0.2 mmol), and acetic
acid (15 mL). The mixture was heated and stirred at 90 °C for
4.0 h, then cooled to room temperature. The solvent was removed
under reduced pressure. The residue was neutralized with saturated
aqueous NaHCO3. The precipitated solid was washed with water
(3 × 30 mL) and cold acetone (2 × 20 mL), and dried in vacuum
1
to give 85% yield of 3a: white solid; mp 196-198 °C; H NMR
(400 MHz, DMSO) δ 1.97 (s, 3H), 2.35 (s, 3H), 2.72 (t, J ) 7.5
Hz, 2H), 4.04 (t, J ) 7.5 Hz, 2H), 6.06 (d, J ) 8.0 Hz, 1H), 7.21
(d, J ) 7.5 Hz, 2H), 7.27 (d, J ) 7.5 Hz, 2H), 7.39 (d, J ) 8.0 Hz,
1H), 10.55 (s, 1H); 13C NMR (100 MHz, DMSO) δ 21.3, 21.5,
23.7, 62.5, 100.3, 106.9, 127.3, 129.9, 137.6, 137.8, 139.3, 163.3,
164.0, 171.0. Anal. Calcd for C16H17NO4: C, 66.89; H, 5.96; N,
4.88. Found: C, 66.75; H, 6.01; N, 4.85.
Typical Procedure for the Synthesis of Substituted
Dihydrofuro[3,2-c]pyridin-4(5H)-ones 4 (4a as an example). To
a 50 mL round-bottomed flask was added 3a (2.0 mmol), aqueous
NaOH (30%, 4.0 mmol), and EtOH (10 mL). The mixture was
stirred at room temperature for 30 min, then poured into saturated
aqueous NaCl (50 mL) and neutralized with aqueous HCl (10%).
The resulting mixture was extracted with ethyl acetate (4 × 30
mL). The combined organic phase was washed with water (3 × 20
mL), dried over MgSO4, filtered, and concentrated in vacuum to
give 92% yield of 4a: white solid; mp 208-210 °C; 1H NMR (500
MHz, DMSO) δ 2.32 (s, 3H), 2.60 (t, J ) 7.5 Hz, 2H), 3.41 (t, J
) 7.5 Hz, 2H), 6.08 (d, J ) 8.0 Hz, 1H), 7.17 (d, J ) 8.0 Hz, 2H),
7.24 (d, J ) 8.0 Hz, 2H), 7.33 (d, J ) 7.5 Hz, 1H); 13C NMR (125
MHz, DMSO) δ 21.3, 28.1, 60.3, 100.7, 108.5, 127.3, 129.9, 137.0,
137.7, 139.5, 163.7, 163.8. Anal. Calcd for C14H13NO2: C, 73.99;
H, 5.77; N, 6.16. Found: C, 74.07; H, 5.72; N, 6.21.
Experimental Section
Typical Procedure for the Preparation of Substituted
Cyclopropanes 2 (2a as an example). To a 50 mL round-bottomed
flask was added DMFDMA (6.0 mmol), 1-acetyl-N-phenylcyclo-
propanecarboxamide 1a (5.0 mmol), and DMF (15 mL). Then the
mixture was heated and stirred at 85 °C for 4.0 h, then cooled to
room temperature. The resulting mixture was slowly poured into
saturated aqueous NaCl (100 mL), and extracted with dichlo-
romethane (3 × 20 mL). The combined organic phase was washed
with water and dried over anhydrous Na2SO4. The solvent was
removed under reduced pressure, and the crude product was purified
by flash chromatography (silica gel, petroleum ether:ethyl acetate
1
2:1) to give 88% yield of 2a: white solid; mp 181-182 °C; H
NMR (500 MHz, CDCl3) δ 1.41 (q, J ) 3.5 Hz, 2H), 1.81 (q, J )
3.5 Hz, 2H), 2.28 (s, 3H), 2.79 (s, 3H), 3.09 (s, 3H), 4.71 (d, J )
12.5 Hz, 1H), 7.08 (d, J ) 8.5 Hz, 2H), 7.49 (d, J ) 8.0 Hz, 2H),
7.72 (d, J ) 12.5 Hz, 1H), 11.88 (s, 1H); 13C NMR (125 MHz,
CDCl3) δ 19.7, 20.8, 31.6, 37.2, 45.1, 88.5, 120.0, 129.2, 132.9,
136.2, 154.5, 169.0, 196.6. Anal. Calcd for C16H20N2O2: C, 70.56;
H, 7.40; N, 10.29. Found: C, 70.31; H, 7.47; N, 10.13.
Acknowledgment. Financial support of this research by the
National Natural Science Foundation of China (20572013 and
20711130229), the Ministry of Education of China (105061),
and the Department of Science and Technology of Jilin Province
(20050309) is greatly acknowledged.
(22) (a) Lee, Y. R.; Kim, B. S.; Kweon, H. I. Tetrahedron 2000, 56, 3867–
3874. (b) Barr, S. A.; Boyd, D. R. J. Chem. Soc., Chem. Commun. 1994, 153–
154. (c) Pirrung, M. C.; Blume, F. J. Org. Chem. 1999, 64, 3642–3649. (d)
Senboku, H.; Takashima, M.; Suzuki, M.; Kobayashi, K.; Suginome, H.
Tetrahedron 1996, 52, 6125–6138. (e) Suginome, H.; Kobayashi, K.; Itoh, M.;
Seko, S.; Furusaki, A. J. Org. Chem. 1990, 55, 4933–4943. (f) Suginome, H.;
Seko, S.; Konishi, A.; Kobayashi, K. Tetrahedron Lett. 1991, 38, 5115–5118.
Supporting Information Available: Analytical data and
NMR spectra for 2-4. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO801289P
8092 J. Org. Chem. Vol. 73, No. 20, 2008