Stereoselective synthesis of bicyclic nitrocyclopropanes by a radical-anion domino reaction

Article abstract of DOI:10.1002/chem.200901920

A novel one-step radical-ionic cyclopropanation was carried out to prepare aza- or oxabicyclo[3.1.0]hexane from β-nitroamides or β-nitroethers in a stereoselective manner (see scheme). The present procedure involves a higher-order domino process that incl

Full text of DOI:10.1002/chem.200901920

DOI: 10.1002/chem.200901920
Stereoselective Synthesis of Bicyclic Nitrocyclopropanes by a Radical–Anion
Domino Reaction
Akio Kamimura,*[a] Ayako Kadowaki,[b] Takayuki Yoshida,[a] Ryota Takeuchi,[a] and
Hidemitsu Uno[c]
Cyclopropanation is regarded as an important transforma-
tion in organic synthesis.[1] Synthesis of 3-azabicyclo-
ACHTUNGTRENNUNG[3.1.0]hexane structures has been of interest because com-
Preparation of the cyclization precursors 1, 2 and 3 was
carried out by the conjugate addition of formamides, alkox-
ides, and thiolate to nitroalkenes.[12] Treatment of 1a with
Ag2O and iodine in the presence of DBU resulted in a
smooth consumption of 1a. After the usual workup, bicyclic
adduct 4a was isolated in 72% yield as a mixture of two dia-
stereomers. GC analysis revealed that the diastereomeric
ratio was 88:12 (Scheme 1, Table 1, entry 1).
Cyclopropanation from b-nitroamide 1 occurred smoothly
to yield 4 in moderate to good yields. For example, treat-
ment of 1b with DBU, Ag2O and I2 resulted in the forma-
tion of 4b in 69% yield (entry 2). The product ratio was
found to be 83:17. When R1 was a primary alkyl group, the
diastereoselectivity was slightly lowered (entries 3 and 4).
pounds with these structures, for example, indolizomycin,[2]
trovafloxacin,[3] duocarmycin, and CC-1065[4] show signifi-
cant biological activity. Preparation of these structures in-
volves low-valent titanium-mediated cyclization (the Kulin-
kovich reaction), which has recently been explored exten-
sively by de Meijere and others.[5,6] Another method using
cyclopropyllithium reagents has also been reported.[7]
Aliphatic nitro compounds are recognized as useful syn-
thetic building blocks because the nitro group has a strong
electron-withdrawing property.[8] The a-anions of the nitro
compounds are easily oxidized to the corresponding carbon
radicals by treatment with ceric ammonium nitrate, silver(I)
salts, K3Fe(CN)6, and MnACHTNUTGRNEUNG
(OAc)3.[9] These radicals are fre-
Table 1. Preparation of bicyclic nitrocyclopropanes 4 and 5 by domino
cyclopropanation.
[10]
À
quently used to form new carbon carbon bonds through
radical cyclization.[11] However, to the best of our knowl-
edge, there are currently no successful studies reported to
show a direct cyclopropanation process using primary nitro
compounds. In this paper, we report a novel intramolecular
cyclopropanation from b-nitroamides or b-nitroethers
through a higher-order domino reaction to give bicyclic ni-
trocyclopropanes in a stereoselective manner.
Entry
X
R1
R2
Product
Yield [%][a]
dr[b]
1
2
3
4
5
6
7
8
NCHO
NCHO
NCHO
NCHO
NCHO
NCHO
O
O
O
O
O
iPr
cC6H11
Pr
C5H11
Ph
iPr
iPr
cC6H11
Pr
Ph
1-naphthyl
Ph
H
H
H
H
H
Me
H
H
H
H
H
H
4a
4b
4c
4d
4e
4 f
5a
5b
5c
5d
5e
6a
72
69
74
57
55
45
54
64
72
58
56
0
88:12
83:17
65:35
66:34
90:10
99:1
93:7
92:8
96:4
97:3
99:1
–
9
[a] Prof. Dr. A. Kamimura, T. Yoshida, R. Takeuchi
Department Applied Molecular Bioscience
Graduate School of Medicine, Yamaguchi University
Ube 755-8611 (Japan)
10
11
12
S
Fax : (+81)836-85-9231
E-mail: ak10@yamaguchi-u.ac.jp
[a] Isolated yield. [b] Determined by GC analyses.
[b] Dr. A. Kadowaki
Department of Applied Chemistry
Graduate School of Science and Engineering
Yamaguchi University, Ube 755-8611 (Japan)
[c] Prof. Dr. H. Uno
Department of Chemistry
Graduate School of Science of Engineering, Ehime University
Matsuyama 790-8577 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901920.
Scheme 1. Dominocyclopropanation of 1, 2 and 3.
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COMMUNICATION
For example, the same treatment of 1c resulted in the for-
mation of 4c in 74% yield, but the product contained two
diastereomers in 65:35 ratio (entry 3). The stereoselectivity
was improved to 71:29 when the reaction was carried out at
08C, although the yield of 4c decreased to 45%. Thus, the
stereoselectivity was higher with a secondary alkyl group as
R1 than a primary alkyl group as R1. Placing a phenyl group
at the R1 position offered good stereoselectivity (entry 5).
The prenyl derivative stereoselectively formed 4 f, which
was isolated as a single diastereomer in 45% yield (entry 6).
The same treatment for b-nitroether 2 also formed corre-
chemical course of the present cyclopropanation of the
ether derivatives was the same as that of the amide deriva-
tives. Compound 9b was liquid and did not give good crys-
tals for X-ray analysis. We thus attempted to remove the
formyl group under acidic conditions (Scheme 3), which was
Scheme 3. Conversion to N-tosyl derivative 12. i) HCl, MeOH, reflux,
12 h, 92%; ii) TsCl, Et3N, DMAP, CH2Cl2, 76%.
sponding oxabicycloACHTUNRGTNE[NUG 3.1.0]hexane (5) in good yields (en-
tries 7–11). Cyclopropanation again occurred in a highly ste-
reoselective manner. For example, b-nitroether 2a was con-
verted into oxabicyclopropane 5a in 54% yield with 93:7
stereoselectivity (entry 7). This selectivity was higher than
that of the corresponding formation of aza-derivative 4a.
The present high selectivity was also observed in the prepa-
ration of 5c which has a primary alkyl group at the R1 posi-
tion (entry 9). A major isomer of 5c was obtained with 96:4
selectivity, slightly higher compared with the corresponding
aza-derivative 4c (65:35). The highest selectivity was ach-
ieved in the formation of 5e, where mostly a single isomer
was formed (entry 11). Treatment of b-nitrosulfide 3a under
the same conditions did not produce the desired 6a and the
corresponding nitroalkene was observed in the reaction mix-
ture (entry 12).
successfully achieved; N-H bicyclopiperidine 11 was ob-
tained in 76% yield as a single isomer.[15] No epimerization
occurred during the deprotection. Compound 11 was then
converted to the corresponding tosylate 12 by treatment
with TsCl. Recrystallization of tosylamide 12 afforded good
crystals for the X-ray analysis.[16] It should be mentioned
that the structure obtained for 12 showed a cis-configuration
between the cyclohexyl group and the cyclopropane ring,
which was opposite to the configuration of 4a. Fortunately,
10b gave good crystals for the X-ray analysis that clearly
showed a cis-configuration.[17] Again the stereochemical
courses of homoallylic amides and ethers derivatives are the
same. Thus, the present domino cyclopropanation occurred
in a trans-selective manner in the formation of aza- and
Application of the present reaction to homoallyl amide or
ether derivatives also successfully yielded the corresponding
bicyclic [4.1.0]heptanes 9 and 10 (Scheme 2). For example,
oxabicyclo
ACHTUNGTRENNUNG
formation of aza- and oxabicycloAHCNUTGTRENNUNG
chemistry of the reaction depended on the length of the
tether. We attempted to prepared seven-membered ring by
use of the one-carbon extended precursor of 7, but no
bicycloACHTNUTRGNE[NUG 5.1.0]octane derivatives were observed in the reac-
tion mixture.
To clarify the reaction mechanism, we carried out the re-
action using 1a under various conditions (Scheme 4). First,
we attempted the reaction with AgI instead of Ag2O and I2,
but no reaction occurred. Then the reaction in the absence
of Ag2O was examined. Compound 1a was consumed slowly
and remained after 2 h under refluxing conditions. The reac-
tion became sluggish and the yield of 4a dropped to 21%. It
should be mentioned that after isolation 4a contained the
two isomers in an almost 1:1 ratio along with recovery of
the starting material. Thus, the stereoselectivity was totally
lost, when the reaction was performed without Ag2O.
Scheme 2. Dominocyclopropanation of 7 and 8.
treatment of a homoallylic amide with Ag2O/I2/DBU result-
ed in the smooth formation of 9a in 80% yield. Compound
9a consisted mostly of a single isomer, indicating that the
stereoselectivity of the reaction was very high. Thus, the
present method is applicable to homoallylic amides and
ethers that yielded a one-carbon extension analogue of the
bicyclopropanes.
The stereochemistry was determined by X-ray analyses.
For example, CH2Cl2 solution of rotameric mixture of 4a
gave a crystal of single rotamer, which was used for the X-
ray crystallographic analysis.[13] The X-ray data revealed the
configuration of 4a, in which the R1 group and the cyclopro-
pane ring were located in a trans relationship. Compound
5b also gave nice crystals, X-ray analysis of which unambig-
uously indicated trans configuration.[14] Thus, the stereo-
Scheme 4. Reactions of 1a under various conditions.
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A. Kamimura et al.
The reaction without iodine also afforded an interesting
result (Scheme 5). Treatment of 1a with Ag2O in the ab-
sence of I2 did not give 4a; instead, substituted pyrrolidine
X-ray crystallographic analysis for major-15 unambiguously
indicated 2,4-trans configuration.[18] It should be noted that
the trans relationship in major-15 was the same as in 4a.
Based on these results, we assumed the following reaction
mechanism (Scheme 6): The starting compound 1 or 2 is de-
protonated by DBU to give a nitronate anion A, which im-
mediately undergoes single-electron oxidation by Ag2O to
generate the a-nitro radical B.[9f] If b-nitrosulfide 3 is used
in the reaction, the generated radical B readily loses a sulfe-
nyl radical through b-elimination to afford nitroalkene so
that no cyclization takes place. The allylic or homoallylic
double bond is located in a good position for radical cycliza-
tion by the a-nitro radical generated from 1 and 2 that
forms cyclized radical D. During the cyclization, the two
conformations, B and C, are proposed. Conformer B is pre-
ferred to conformer C because of the steric bias of the R
group. Radical D is likely to be trapped by iodine to give io-
domethylpyrrolidine E. If the reaction is carried out without
iodine, the intermediate radical D abstracts hydrogen from
the solvent to give monocyclic pyrrolidine 13. This is strong-
ly supported by the observation that both compounds 13
and 4a have the same 2,4-trans configuration and the forma-
tion of these compounds occurs with almost the same level
of diastereoselectivity. Iodine in intermediate E, being a
good leaving group, is displaced through intramolecular SN2
Scheme 5. Reactions of 1a in the absence of I2.
attack by the a-nitroanion to form bicycloACTHNUGRTENUNG[3.1.0]hexanes 4
13 was isolated in 73% yield as a mixture of diastereomers.
Due to overlapping signals, the GC analysis of 13 showed
only two peaks. To confirm the diastereomeric ratio, the
nitro group was reduced to the amine under hydrogenation
conditions. The corresponding amine 16 was prepared in
89% yield. The present diaste-
and 5 in a trans-selective manner. Thus, the diastereoselec-
tivity of the present domino cyclopropanation is determined
in the radical cyclization step. The opposite cis-selectivity
during the formation of bicyclo
ACHTUNGERTN[NUNG 4.1.0]heptanes 9 and 10
from the homoalylic derivatives should also be explained by
reomeric mixture of 16 gave
well-resolved GC peaks, which
allowed determination of the
diastereomeric ratio of 16 as
65:25:5:5. To reduce the
number of diastereomers, we
then removed the nitro group
by treatment with Bu3SnH.
The removal took place
smoothly and 14 was isolated
in 50% yield. As expected, it
contained only two diastereo-
mers in the ratio 86:14. Re-
moval of the formyl group
under acidic conditions fol-
lowed by N-tosylation pro-
duced 15 in 65% yield. Com-
pound 15 contained two diaste-
reomers in the ratio 82:18,
which was slightly different
from the ratio found for 14.
The two isomers were separat-
ed by careful HPLC treatment
and the major isomer of 15
(major-15) was recrystallized.
Scheme 6. Plausible reaction mechanism.
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Bicyclic Nitrocyclopropanes
COMMUNICATION
the assumption that the radical cyclization takes place
through an analogous conformation B’.
tion of heterocyclic bicyclo
AHCTUNGTREG[NNUN 4.1.0]heptanes which are of interest for their unique bioac-
ACHTUNGTNER[NUNG 3.1.0]hexanes and bicyclo-
If Ag2O is absent in the reaction mixture, bicyclic propane
can be formed through an alternative route. Thus, iodine at-
tacks the allylic double bond to form the epi-iodonium ion
G. In this route, the rate of the reaction can become very
slow. No stereoselectivity should be expected because there
are no substituents close to the double bond to provide a
proper steric bias. As a result, two diastereomeric epi-iodo-
nium ions G and G’ should be generated in a 1:1 ratio. The
iodomethylpyrrolidine E intermediate should be generated
by an intramolecular nucleophilic attack by the nitronate
anion to give intermediate E, but this should be formed as a
1:1 diastereomeric mixture through the process. This as-
sumption is in good agreement with the observation that the
reaction without Ag2O occurred very slowly, and a 1:1 dia-
stereomeric mixture of 4a was obtained in 21% yield along
with recovery of the starting material 1a. No reaction takes
place in the absence of Ag2O and I2. For example, AgI does
not oxidize the nitronate anion A so no reaction occurred.
Hence, we conclude that the present reaction is a novel
higher-order domino process in which oxidation of the nitro-
nate, radical cyclization and intramolecular SN2 reaction
takes place in a one-pot process in a highly stereoselective
manner. The combination use of Ag2O and iodine is a key
factor for the smooth formation of bicyclopropanes 4, 5, 9
and 10.
tivity. Further investigation of this reaction is now underway
in our laboratory.
Experimental Section
See Supporting Information for additional details.
Preparation of N-formyl-2-isopropyl-1-nitro-3-azabicycloACTHNUTRGNEUGN[3.1.0]hexane
(4a): Ag2O (0.5562 g, 2.40 mmol) and I2 (0.6091 g, 2.40 mmol) were
added to a solution of 1a (0.2403 g, 1.20 mmol) and DBU (0.21 mL,
1.44 mmol) in anhydrous THF (5 mL) at 08C, and the mixture was al-
lowed to stir for 3 h. The reaction mixture was filtered and the filtrate
was concentrated in vacuo. The obtained crude product was purified by
flash chromatography (silica gel, hexane/ethyl acetate 2:1 then 1:1) to
give 4a as oil (0.1700 g, 72%). GC analysis revealed that the diastereo-
meric ratio of 4a was 88:12. Pure trans-4a contained two rotational iso-
mers at room temperature (ratio ca. 2:1), and the ratio was about 2:1.
1H NMR (270 MHz, CDCl3, TMS): d =0.86 (d, 3J=6.9 Hz, 3H, major
rotamer), 0.87 (d, 3J=6.9 Hz, 3H, minor rotamer), 1.04 (d, 3J=6.9 Hz,
3H, major rotamer), 1.05 (d, 3J=6.9 Hz, 3H, minor rotamer), 1.19 (t,
3J=5.9 Hz, 1H, minor rotamer), 1.24 (t, 3J=5.9 Hz, 1H, major rotamer),
1.82 (m, 1H, minor rotamer), 1.86 (dd, 3J=5.9, 8.9 Hz, 1H, major rota-
mer), 2.58–2.74 (m, 1H), 2.74–2.90 (m, 1H), 3.45 (dd, 3J=14.0, 12.4 Hz,
1H, major rotamer), 3.52 (d, 3J=5.9 Hz, 1H, minor rotamer), 3.82 (dd,
3J=3.0, 9.9 Hz, 1H, minor rotamer), 3.97 (d, 3J=11.9 Hz, 1H, major
rotamer), 4.24 (d, 3J=2.0 Hz, 1H, major rotamer), 4.84 (d, 3J=3.0 Hz,
1H, minor rotamer), 8.13 ppm (s, 1H); 13C NMR (67.5 MHz, CDCl3) d=
16.8, 17.2, 20.1, 20.7, 20.8, 21.6, 24.5, 24.8, 29.0, 29.6, 45.2, 48.8, 59.8, 63.7,
162.5, 163.2 ppm; IR (CHCl3): n˜ =1373, 1530, 1672 cmÀ1; elemental analy-
sis calcd (%) for C9H14N2O3; C 54.54, H 7.12, N 14.13; found: C 54.49, H
7.03, N 14.10.
Finally, we examined a much higher-order domino reac-
tion starting from nitroalkene and formamide. Treatment of
a mixture of formamide and nitroalkene in the presence of
a base smoothly formed the nitronate anion of 1b, which
underwent the present domino cyclization process by addi-
tion of Ag2O/I2 to give 4b in 53% yield (Scheme 7). Addi-
Preparation of N-formyl-2-isopropyl-4-methyl-3-nitropyrrolidine (13):
Ag2O (0.463 g, 2.0 mmol) was added to
a solution of 1a (0.200 g,
1.0 mmol) and DBU (0.18 mL, 1.2 mmol) in anhydrous THF (15 mL) at
08C, and the mixture was allowed to stir for 2 h. The reaction mixture
was filtered and the filtrate was concentrated in vacuo. The obtained
crude product was purified by flash chromatography (silica gel, hexane/
ethyl acetate 1:1) to give 13 as pale yellow oil (0.1456 g, 73%). GC analy-
sis showed two peaks: ratio 67:33. 1H NMR (270 MHz, CDCl3): d=0.71–
1.20 (m, 9H), 1.65–2.20 (m, 1H), 2.53–2.83 (m, 1H), 2.90–3.45 (m, 1H),
3.50–4.38 (m, 2H), 4.38–4.56 (m, 0.5H), 4.80 (t, 3J=5.7 Hz, 1H), 8.07 (s,
0.2H), 8.17 (s, 0.6H), 8.24 ppm (s, 0.2H); 13C NMR (67.5 MHz, CDCl3):
d=12.1, 12.3, 14.9, 15.8, 16.8, 17.4, 17.5, 18.2, 18.4, 18.8, 18.9, 28.6,29.5,
30.3, 32.0, 32.2, 35.2, 35.6, 40.1, 40.6, 48.7, 48.9, 51.2, 52.5, 64.8, 65.7, 67.4,
67.9, 89.5, 90.1, 92.7, 92.9, 160.7, 161.1, 162.0, 162.1 ppm; IR (CHCl3):
n˜ = 1375, 1551, 1674, 2880, 2940, 2967 cmÀ1; HRMS (EI+): m/z: calcd
for C9H16N2O3: 200.1161; found: 200.1164 [M]+.
Scheme 7. One-pot conversion to 4b.
À
tion of DBU improved the yield of 4b. Thus, a carbon ni-
Keywords: cyclopropanes · domino reactions · oxidation ·
radical cyclization · stereoselective reaction
À
trogen bond and two carbon carbon bonds were formed in
a one-pot process through five domino reactions: a conju-
gate addition, oxidation, radical cyclization, trapping by
iodine, and an intramolecular SN2 reaction. The reaction
showed almost the same level of stereoselectivity as the re-
action using 1b.
In conclusion, we found a new reaction to prepare bicyclic
cyclopropanes in one step from the conjugate adducts of for-
mamides or alcohols with nitroalkenes. The present method
involves a higher-order domino process in which three reac-
tions occur sequentially in a one-pot process. With this reac-
tion, primary nitroalkanes are regarded as an equivalent to
a-nitrocarbene. This method will be useful for the prepara-
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Chem. Eur. J. 2009, 15, 10330 – 10334