Iron(II) Halide Promoted Cyclization of Cyclic 2-Enynamides: Stereoselective Synthesis of Halogenated Bicyclic γ-Lactams

Article abstract of DOI:10.1021/acs.orglett.6b00916

A simple and mild process was developed for the highly stereoselective synthesis of halogenated bicyclic [4.3.0] and [3.3.0] γ-lactams, possessing four stereocenters, from easily available cyclic 2-enynamides. The reaction required only an inexpensive iron(II) halide under dry air and was tolerant of aryl, heteroaryl, and alkyl groups at the alkyne terminus.

Full text of DOI:10.1021/acs.orglett.6b00916

Letter
pubs.acs.org/OrgLett
Iron(II) Halide Promoted Cyclization of Cyclic 2‑Enynamides:
Stereoselective Synthesis of Halogenated Bicyclic γ‑Lactams
Ming-Chang P. Yeh,* Yuan-Shin Shiue, Hsin-Hui Lin, Tzu-Yu Yu, Ting-Chia Hu, and Jia-Jyun Hong
Department of Chemistry, National Taiwan Normal University, 88 Ding-Jou Road, Sec. 4, Taipei 11677, Taiwan R.O.C.
S
* Supporting Information
ABSTRACT: A simple and mild process was developed for the highly
stereoselective synthesis of halogenated bicyclic [4.3.0] and [3.3.0] γ-lactams,
possessing four stereocenters, from easily available cyclic 2-enynamides. The
reaction required only an inexpensive iron(II) halide under dry air and was
tolerant of aryl, heteroaryl, and alkyl groups at the alkyne terminus.
Scheme 1. Reaction of Lewis Acids with 1a
actams are common core structures that can be found in
various biologically important natural products and
L
pharmaceuticals.1 Recently developed methods for the syn-
thesis of lactams involved the transition-metal-catalyzed
insertion of an external CO into C(sp3)−H of alkylamines,2
the intramolecular Ugi multicomponent reaction of γ-ketoacids
with an amine and an isocyanide,3 the Al(OTf)3-catalyzed
cascade cyclization and ionic hydrogenation reaction of
nitrogen substituted ketoamides,4 the gold-catalyzed tandem
cycloisomerization/oxidation of homopropargyl amides,5 the
Sc(OTf)3-catalyzed Mukaiyama−aldol-type reaction of 2,5-
bis(trimethylsilyloxy)furan with imines,6 the Pd-catalyzed
oxidative intramolecular cyclization of amides with an alkene,7
the cobalt-catalyzed intramolecular reductive coupling reaction
of nitriles and acrylamides,8 the gold(I)-catalyzed cyclization of
N-alkenyl β-ketoamides,9 the Pd-catalyzed oxidation cyclization
of N-allylpropiolamides,10 and the FeCl2-catalyzed intra-
molecular chloroamination of cyclohexene-tethered acyl
azides.11 Here, we report our results on a simple and
unprecedented synthesis of halogenated bicyclic [4.3.0] and
[3.3.0] γ-lactams in stereoselective manners by reaction of six-
and five-membered ring 2-enynamides with inexpensive and
environmentally friendly iron(II) halides and green oxidant
air.12
The readily prepared six-membered ring N-tosyl-2-enyna-
mide 1a was chosen as a model substrate for screening Lewis
acids and optimizing the reaction conditions. Compound 1a
was prepared from cyclohex-2-enol using the known literature
protocols13 via a Mitsunobu reaction of cyclohex-2-enol with
NHTsBoc,14 deprotection of the Boc group, and a copper-
catalyzed amidation15 of phenylethynyl bromide with the amide
to afford 1a in 70% yield over three steps. First, several
chlorine-containing Lewis acids at 0.01 M concentration in
THF were tested for the cyclization of 1a at room temperature
under nitrogen (see the SI for details). While employing CuCl,
ZnCl2, and InCl3 resulted in the quantitative recovery of the
enynamide 1a (see the SI), the use of TiCl4, TMSCl, and AlCl3
led to predominantly the hydrochlorination product 3a and a
trace amount of the hydrolysis product 4a (Scheme 1). Only 3a
was obtained in 30% yield when 1a was treated with CuCl2. To
our delight, when 1a was subjected to FeCl2 or FeCl3 (0.01 M
in THF) at room temperature under nitrogen for 52−60 h, a
major product, identified as the bicyclic γ-lactam 2a, was
obtained as a single diastereomer and in 49 and 46% yield,
respectively. The relative stereochemistry of 2a, derived from
anti-addition of a chloride ion and the ynamide tether across
the pendant double bond, was determined with NOESY
(nuclear Overhauser effect spectroscopy) measurements and
was further confirmed by X-ray crystallography.16
Encouraged by the successful transformation of 1a into 2a
using inexpensive iron chlorides, we next screened the effect of
solvent, concentration, temperature, and atmosphere to
improve the yield of 2a (Table 1). The most relevant results
of evaluating proper solvents revealed that THF was the best
among the solvents (THF, ether, toluene, and DCM) screened
at rt (Table 1, entries 1−4) for the cyclization of 1a with FeCl2
(1.1 equiv). Pleasingly, the yield of 2a could be substantially
raised to 77% when the concentration of 1a was increased to
0.25 M in THF (Table 1, entry 5). The effect of the reaction
temperature was also significant, as when 1a was treated with
1.1 equiv of FeCl2 in THF (0.25 M) under nitrogen at 40 °C
for 40 h, the desired γ-lactam 2a was isolated in 88% yield
(Table 1, entry 6). To our surprise, the reaction time could be
significantly reduced to 6 h when the reaction was conducted in
open air at 40 °C and delivered 2a in 75% yield (Table 1, entry
7), together with a trace amount of the hydrolysis product 4a.
We envisioned that the undesired hydrolysis product 4a could
be avoided by attaching a calcium chloride drying tube to the
reaction flask. Indeed, treatment of 1a with 1.1 equiv of FeCl2
in THF at 0.25 M concentration under dry air gave an 84%
Received: March 31, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00916
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Letter
various oxides (N-oxides, sulfoxides, or epoxides) as oxidant for
the formation of α-oxo metal carbene intermediates, which
were then trapped with an alkene or alkyne to form bicyclic
lactams.17 Furthermore, halogenated bicyclic γ-lactams had
been synthesized via metal-catalyzed radical additions of
cycloalkene-tethered acid derivatives, such as acyl azides or
trichloroacetamides,11,18 and our synthesis of halogenated
bicyclic γ-lactams using the simple cyclic NTs-tethered enynes,
nontoxic and inexpensive iron(II) halides, and dry air.
Table 1. Optimization of Reaction Conditions
a
T
(°C)
time
(h)
yield
entry
FeXn
solvent
[M]
atmos
(%)
b
1
2
3
4
5
6
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl3
FeBr2
FeBr3
FeCl2
THF
ether
toluene
DCM
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
0.01
0.01
0.01
0.01
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
rt
rt
N2
60
49
4
With the optimal reaction conditions in hand, we examined
the substrate scope of the cyclization using various six-
membered ring 2-enynamides (Scheme 2). Good to excellent
b
N2
24
b
rt
N2
24
9
b
rt
N2
24
b
rt
N2
48
77
Scheme 2. Scope of the Cyclization of Cyclohexene-
Tethered 2-Enynamides
b
c
40
40
40
40
40
40
40
40
40
N2
40
88
d
7
air
6.0
9.5
9.5
4.0
1.0
3.5
2.5
6.5
75
c
8
dry air
84
e
9
-
18
69
74
68
69
10
11
12
13
O2
dry air
dry air
dry air
dry air
f
14
a
b
NMR yields. N2 is contaminated with a small amount of O2.
c
d
e
Isolated yield. A small amount of 4a was isolated. The reaction
mixture in a reaction vessel was first evacuated to remove all air and
f
the vessel was then sealed and heated to 40 °C. 1.1 equiv of
hydroquinone was added.
yield of 2a after the reaction mixture was heated at 40 °C for
9.5 h (entry 8). To further understand the effect of atmosphere,
the following experiments were performed. The reaction
mixture (1a, 1.1 equiv of FeCl2, in 0.25 M THF) in a reaction
vessel was evacuated to remove all air, and the vessel was then
sealed. No trace of 2a, 3a, and 4a was detected by TLC analysis
(entry 9), and 1a was recovered in 82% yield after the reaction
mixture was heated at 40 °C for 9.5 h. Therefore, oxidation of
FeCl2 with oxygen in the air or in the oxygen-contaminated
nitrogen gas (entries 1, 5, and 6) was essential for the
cyclization. However, treatment of 1a with FeCl2 (1.1 equiv)
under an atmosphere of oxygen at 40 °C for 4 h led to a small
amount of 2a (18%, entry 10). The result may indicate that a
great portion of 1a was decomposed under oxygen. Although
the more reactive iron(III) chloride (1.1 equiv) reduced the
reaction time to 1 h, it could cause the decomposition of 1a and
afforded 2a in only 69% yield (entry 11). Switching iron
chlorides to iron bromides revealed that FeBr2 was also capable
of transforming 1a to 2a′ (entry 12, X = Br) in 3.5 h and in
74% yield. Due to the slow decomposition of 1a with FeBr3, 1a
was consumed in 2.5 h, and the reaction provided 2a′ in 68%
yield (entry 13, X = Br). Therefore, it was concluded that the
use of 1.1 equiv of FeCl2 (entry 8) and FeBr2 (entry 12) in dry
THF under dry air, to avoid the formation of hydrolysis
product 4a, at 40 °C were the best conditions for the
cyclization. To further understand the reaction mechanism, the
standard cyclization was conducted in the presence of 1.1 equiv
of hydroquinone (a radical scavenger). The reaction produced
bicyclic γ-lactam 2a in 69% yield (entry 14). Since the radical
scavenger did not impede the cyclization, it was suggested that
the reaction may not proceed through a radical process. It must
be mentioned that a few transition metals have been employed
for the synthesis of bicyclic lactams from ynamides tethering an
unsaturated C−C bond; however, these methods required
a
Isolated yields from column chromatography over silica gel.
yields were observed when the cyclic enynamides bearing an
aryl group were at the alkyne terminus. In general, electron-
neutral substituents on the phenyl ring, 1a,c−f, afforded the
corresponding bicyclic γ-lactams in good yields (61−88%),
though the sterically hindered o-tolyl enynamide 1b gave the
chlorinated bicyclic lactam 2b (57%) and brominated bicyclic
lactam 2b′ (48%) in lower yields and required longer reaction
times. Substrates 1g and 1h bearing the electron-rich methoxy
group on the phenyl ring reacted efficiently and provided the
corresponding lactams in high yields (81−90%), whereas
electron-deficient aryl enynamides 1i and 1j led to the desired
products in lower yields (62−83%). Substrates that were
substituted with a chlorine or bromine atom on the phenyl ring,
for example, 1k and 1l, performed with equal levels of efficiency
with FeCl2 and FeBr2, resulting in good yields (64−76%) of the
desired products 2k, 2k′, 2l, and 2l′. Thienyl-substituted
enynamides 1m and 1n were also effective, however, affording
the corresponding lactams in diminished yields (35-61%). Next,
B
DOI: 10.1021/acs.orglett.6b00916
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Letter
substrates with an alkyl group (methyl-, hexyl-, cyclopropyl-, or
2-phenylethyl) at the alkyne terminus were also tolerated,
delivering the target bicyclic γ-lactams 2o−r16 and 2o′−r′ in
the range of 68−82% yields. Furthermore, the replacement of
the N-tosyl group with easily deprotected N-4-methoxybenze-
nesulfonyl (N-Mbs) or N-4-nitrobenzenesulfonyl (N-Ns)
groups, for example, 1s and 1t, required much longer reaction
times. While the electron-rich N-Mbs-protected enynamide 1s
afforded higher yields of lactams 2s (89%) and 2s′ (86%), the
electron-poor N-Ns-protected enynamide 1t gave lactams 2t
and 2t′ in only 40 and 38% yield, respectively. Furthermore, the
tosyl group of 2a could be removed by treatment of 2a with
magnesium powder and ammonium chloride in MeOH under
sonication for 1 h, providing the deprotected bicyclic γ-lactam
quantitatively (see the SI for details).19
Scheme 4. Proposed Mechanism for the Formation of 2a
from 1a
C−C double bond gave intermediate II.20 Anti-addition of a
chloride ion to the activated double bond followed by reductive
elimination afforded intermediate III. Protonolysis of III upon
workup generated the iminium ion IV with the phenyl group at
the more stable convex face. Hydrolysis of the resulting
iminium ion delivered the bicyclic lactam 2a. For the system of
FeCl2/NaI (Scheme 3), anti-addition of an iodide ion onto the
olefin of II would lead to the iodinated bicyclic lactam 9.
Next, five-membered ring starting substrates 10a−r were
subjected to FeCl2 (1.5 equiv, THF, 40 °C, dry air), and results
are summarized in Scheme 5. Although shorter reaction times
To gain further insight into the reaction path, substrate 5
with an additional trans-acetoxy substituent at the 4-position of
the ring was subjected to the optimal reaction conditions
(Figure 1). The transformation provided exclusively the desired
Scheme 5. Scope of the Cyclization of Cyclopentene-
Tethered 2-Enynamides
Figure 1. Compounds 5−8.
bicyclic γ-lactam 6 (7 h, 54% yield). Moreover, the 3,5,5-
trimethyl-substituted cyclohex-2-ene-tethered ynamide 7 was
also reactive, yielding the corresponding bicyclic γ-lactam 8 in
57% yield (Figure 1). Both lactams 6 and 816 were obtained as
a single diastereomer with the same stereochemistry as those of
2. Therefore, the additional acetoxy or methyl groups on the
six-membered ring did not alter the stereo preference (anti-
addition) for the cyclization.
Finally, 1a was subjected to 1.1 equiv of FeCl2 and NaI under
the optimal reaction conditions. The reaction was performed
with the expectation that the more nucleophilic iodide would
lead to an iodinated γ-lactam. Indeed, addition of 1.1 equiv of
NaI to the reaction mixture provided the corresponding
iodinated bicyclic γ-lactam 9 in 72% yield and 2a in 11%
yield under the standard reaction conditions (Scheme 3).
Scheme 3. Synthesis of Iodinated Bicyclic γ-Lactam 9
a
Isolated yields from column chromatography over silica gel.
Since hydroquinone (a radical scavenger) did not impede the
cyclization (Table 1, entry 14) and the formation of lactams 2q
and 2q′ with the cyclopropyl ring intact (Scheme 2), it may
indicate that the cyclization does not proceed through a radical
process. Scheme 4 shows a plausible path for the cyclization of
1a with FeCl2 or FeCl3. First, FeCl2 was oxidized to [O]-FeCl2.
Metalation of the cyclic 2-enynamide 1a with [O]-FeCl2
afforded the keteniminium ion I (X = [O]). In the case of
FeCl3, 1a may react faster with more reactive FeCl3 to afford
intermediate I (X = Cl). Trapping intermediate I by a chloride
ion followed by coordination of the iron center to the pendant
(1.0−6.0 h) were observed, the reactions gave lower yields of
the desired chlorinated bicyclic [3.3.0] γ-lactams 11a16−r (20−
58%). The shorter reaction times and lower yields resulting
from 10a−r may be partly due to the fact that the five-
membered ring substrates were less stable and decomposed
under the reaction conditions. In some cases, a small amount of
the isomers resulting from syn-addition of a chloride ion and
the iron moiety to the olefin (Figure 2, intermediate VI) were
observed (11a′,16 11d′, 11e′, 11h′, 11l′, and 11n′). Compared
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Figure 2. Intermediates V and VI and compound 12.
to intermediate V, the α-face of the five-membered ring
intermediate VI is less sterically congested (Figure 2).
Therefore, a syn-addition of a chloride ion and the iron moiety
to the olefin would lead to syn products in some cases.
Unfortunately, acyclic NTs-tethered enyne 12 (Figure 2) failed
to give lactams under the standard reaction conditions (THF,
dry air, 40 °C, 20 h).
In conclusion, we have developed a simple and reliable
method for the synthesis of halogenated bicyclic [4.3.0] and
[3.3.0] γ-lactams from readily available cyclic NTs-tethered
enynes. The reaction only required inexpensive iron halides and
green oxidant air as promoters, providing direct access to the
bicyclic γ-lactams with four stereocenters in a single step. The
key feature of this reaction is an anti-addition of a halide ion
and the postulated vinyl iron species across the pendant olefin.
Further mechanistic studies on the reaction and applications on
the use of the present method to the synthesis of other
heterocycles are currently underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b00916.
■
(b) Danielec, H.; Klugge, J.; Schlummer, B.; Bach, T. Synthesis 2006, 3,
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Experimental procedures, characterizations, and NMR
spectra of all new compounds (PDF)
X-ray crystallographic data for 2a (CIF)
X-ray crystallographic data for 2a′ (CIF)
X-ray crystallographic data for 2n (CIF)
X-ray crystallographic data for 2o (CIF)
X-ray crystallographic data for 2r (CIF)
X-ray crystallographic data for 8 (CIF)
X-ray crystallographic data for 11a (CIF)
X-ray crystallographic data for 11a′ (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: cheyeh@ntnu.edu.tw. Fax: (+886)-2-2932-4249.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology of the
Republic of China (MOST 104-2113-M-003-003) for financial
support.
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Sutherland, A. Org. Biomol. Chem. 2010, 8, 3418.
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Ragnarsson, U. Chem. Commun. 1997, 1017.
DEDICATION
■
This paper is dedicated to Professor Masahiro Murakami
(Kyoto University) on the occasion of his 60th birthday.
(20) Liu, H.; Yang, Y.; Wang, S.; Wu, J.; Wang, X.-N.; Chang. Org.
Lett. 2015, 17, 4472.
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DOI: 10.1021/acs.orglett.6b00916
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