Imine allylation using 2-alkoxycarbonyl allylboronates as an expedient three-component reaction to polysubstituted α-exo-methylene-γ-lactams

Article abstract of DOI:10.1016/j.tetlet.2008.09.112

α-exo-Methylene-γ-lactams are key structural units in a wide variety of biologically active natural products. A concise route to the formation of polysubstituted α-exo-methylene-γ-lactams is described. In this three-component reaction, an imine is formed

Full text of DOI:10.1016/j.tetlet.2008.09.112

Tetrahedron Letters 49 (2008) 6995–6998
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Imine allylation using 2-alkoxycarbonyl allylboronates as an expedient
three-component reaction to polysubstituted
a-exo-methylene-c-lactams
*
Tim G. Elford, Dennis G. Hall
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2008
Revised 18 September 2008
Accepted 18 September 2008
Available online 23 September 2008
a
-exo-Methylene-
c
-lactams are key structural units in a wide variety of biologically active natural prod-
-exo-methylene- -lactams is described. In this
ucts. A concise route to the formation of polysubstituted
three-component reaction, an imine is formed from an aldehyde and ammonia in situ, and is subse-
quently allylated through the use of a 2-alkoxycarbonyl allylboronate. Due to the ester functionality,
the addition intermediate subsequently undergoes in situ cyclization to form the observed
ylene- -lactam products. This route allows access to highly substituted -exo-methylene-
moderate to excellent yields.
a
c
a-exo-meth-
c
a
c-lactams in
Keywords:
Imine allylboration
Ó 2008 Elsevier Ltd. All rights reserved.
c
-Lactam
Multi-component reaction
The
a-exo-methylene c-butyrolactone and c-lactam ring is a
H
key structural motif in many natural products,1–6 most notably
the sesquiterpene lactones (Fig. 1). These natural products have
proven to be quite useful as DNA polymerase inhibitors, nuclear
vitamin D receptor inhibitors, cellular steroidal inhibitors, blockers
O
H
O
O
O
O
OH
O
of tumour necrosis factor-
a production, as well as many other
uses.7 The wide inhibitory action of these natural products makes
them potential drug candidates due to their cytotoxic, anti-inflam-
matory, phytotoxic and antimicrobial properties.8 However, many
arglabin1
helenalin2
O
O
H
OH
a
-methylene-
when tested in vitro and are thus not suitable for pharmaceutical
purposes.8,9 One closely related class of molecules to the
-lac-
tones is the corresponding -exo-methylene- -lactam scaffold.
Even though there are significantly fewer reports compared to
the -lactones, there are still a considerable number of natural
products that contain a -lactam moiety as part of their structures
(Fig. 1). Furthermore, since the -exo-methylene- -lactones are of-
ten too toxic to be used for pharmaceutical purposes, it has been
suggested that the use of substituted -exo-methylene- -lactams
in place of the -lactone moiety might help to mitigate their pro-
miscuous biological toxicity.9 Synthetic routes to access
-lactams
have been the focus of several reports in the literature;8,10,11 how-
ever, many of the reported routes are tedious in accessing the
lactam core structure. To satisfy the demands of parallel synthesis,
c-lactone natural products display toxic side effects
N
O
N
N
H
MeO
c
O
a
c
OH
pukeleimide E4
c
(+)-clausenamide3
O
c
O
O
a
c
HO
NH
Cl
O
NH
a
c
O
c
C9H19
OH
HO
OH
O
c
(_)-pramanicin5
salinosporamide A6
-substituted- -lactones or a-
c
-
Figure 1. Selected natural products containing
substituted- -lactam rings.
a
c
c
more efficient routes to N-unsubstituted
a-methylene-c-lactams
are greatly needed.11a
As a solution to this synthetic problem, it was envisioned that
one could simply add 2-alkoxycarbonyl allylboronates to imines
and thus form
a-exo-methylene-c-lactams in a single step via a
* Corresponding author. Tel.: +1 780 492 3141; fax: +1 780 492 8231.
E-mail address: dennis.hall@ualberta.ca (D. G. Hall).
tandem allylation/lactamization reaction. Villiéras and co-workers
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.112

6996
T. G. Elford, D. G. Hall / Tetrahedron Letters 49 (2008) 6995–6998
Table 1
repeated, but with a higher reaction temperature of 70 °C. Gratify-
ingly, this time both method A16 and method B17 provided the de-
sired lactam product 3a. Since the yields of 3a were similar for
these two different sets of reaction conditions, the modified Kobay-
Imine allylation using 2-alkoxycarbonyl allylboronate 1a
O
A: NH4OH,
EtOH
CHO
ashi conditions13 were chosen for further investigation of
a
-exo-
HN
MeO2C
O
B
or
+
methylene-
c-lactam formation. This choice was based on the fact
B: NH4OAc,
MeOH, 4 Å MS
O
that method A is experimentally easier to perform, the reaction
time is shorter and the purification steps are more straightforward.
Various aldehydes were selected and then subjected to the
Br
1a
Br
3a
2a
imine allylation conditions, forming a wide variety of
c-substituted
Entry
Method
Temperature (°C)
Time (h)
Yielda (%)
a
-exo-methylene- -lactams (Table 2). Not only allylboronate 1a,
c
1
2
3
4
A
B
A
B
20
20
70
70
4
16
4
0
0
48
36
but also the corresponding Z- and E-crotylboronates 1b and 1c18
were used in the investigation of these imine allylation conditions
(vide infra). Initially, unsubstituted allylboronate 1a was used with
a number of electron-poor aromatic aldehydes (Table 2, entries 1–
3), an electron-rich aromatic aldehyde (entry 4) and even a number
of aliphatic aldehydes (entries 7 and 8). The yields were typically
good to excellent; however, a few entries displayed less satisfac-
tory results. The lower yield observed for entry 4 can most likely
be attributed to poor imine formation. p-Anisaldehyde is less reac-
tive towards imine formation, and under the standard reaction
conditions would be slow in forming the required imine interme-
diate. Longer reaction times may help to increase the yield for sim-
ilarly less reactive aldehydes. Lower yields were also observed for
the aldehydes containing an enolizable proton (entries 7 and 8).
However, this result could be predicted due to the strongly basic
conditions under which the reactions are performed. Enamine for-
mation is expected to be a competing side reaction, and would re-
sult in a lower observed yield. Kobayashi and co-workers have
shown that modifying the order of reagent addition is advanta-
geous for obtaining the desired products with aliphatic alde-
hydes.13 In the typical reaction procedure, the aldehyde is
dissolved in a mixture of NH4OH and ethanol, and the allylboro-
nate is added to this reaction solution and subsequently heated.
However, the order of addition was switched for aldehydes con-
taining an enolizable proton (entries 7 and 8), whereby allylboro-
nate 1a was added first. The aldehyde was then added to the
reaction mixture and this was subsequently heated to provide
16
a
Isolated yield after flash chromatography.
have demonstrated that this chemistry is indeed possible.12 Their
work described the use of highly activated, preformed imines
reacting with 2-alkoxycarbonyl allylboronates to form
through the same allylation/cyclization process described with
aldehydes. Despite the fact that this route provided facile access
c-lactams
to the c-lactam core structure, the imines utilized in this protocol
are severely limiting, not to mention the need to first pre-form the
imines before carrying out the allylboration reactions. Further-
more, the reaction times are long (14 days at room temperature).
In this regard, the Kobayashi13 and Morken14 groups have recently
shown that imines formed in situ from aldehydes can be allylated
to form homoallylic amines. However, both these reports make use
of significantly more reactive allylboronates as compared to the
less reactive 2-alkoxycarbonyl allylboronates that are required to
access c-lactams. We wondered whether either of these reported
conditions would also be suitable with less reactive allylboronates,
such as 2-alkoxycarbonyl allylboronate 1a.15 Initially, both sets of
conditions were attempted on a typical aromatic aldehyde, 2a (Ta-
ble 1).
Not surprisingly, under both Kobayashi’s conditions (Table 1,
entry 1) and Morken’s conditions (Table 1, entry 2), none of the de-
the desired
a-exo-methylene-c-lactams 3g and 3h (see Supple-
sired
c
-lactam product 3a was obtained. The reactions were then
mentary data for a more detailed description).
Table 2
Substrate scope using allylboronate 1 and aldehydes 2 to form
a
-methylene-c-lactams 3
O
RCHO
MeO2C
R1
O
B
NH4OH, EtOH
HN
R
+
RCHO
O
R1
70 oC, 4 h
R2
2
R2
1
3
Entry
Allylboronate
R1
R2
Aldehyde 2 (RCHO)
Product
Yielda (%)
1
2
3
4
5
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1c
1c
1c
1c
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
4-BrC6H4CHO
4-NO2C6H4CHO
4-FC6H4CHO
4-MeOC6H4CHO
4-MeC6H4CHO
PhCHO
PhCH2CH2CHO
n-C10H21CHO
4-BrC6H4CHO
4-NO2C6H4CHO
4-CH3C6H4CHO
2-BrC6H4CHO
4-BrC6H4CHO
4-NO2C6H4CHO
4-CH3C6H4CHO
2-BrC6H4CHO
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
48
76
62
39
90
53
39
35
63
52
56
98
73
66
95
74
6
7b
8b
9
CH3
10
11
12
13
14
15
16
CH3
CH3
CH3
H
H
H
H
a
Isolated yields after flash chromatography and are the result of only one run for each reaction.
Reverse order of addition used in reaction procedure (see Refs. 16 and 17, and Supplementary data).
b

T. G. Elford, D. G. Hall / Tetrahedron Letters 49 (2008) 6995–6998
6997
ratio for the c-lactam products in these four examples was 19:1,
which was identical to the diastereomeric ratio of 1c. The complete
transfer of stereochemistry from starting material to products fur-
ther demonstrated that the imine allylboration reaction is indeed
diastereospecific, even under these harsh, basic reaction condi-
tions. The reaction described thus proceeds in a manner that can
be considered to be a three-component reaction. Imines are formed
in situ from ammonia and aldehydes, and are subsequently allylat-
ed highly diastereoselectively using 2-alkoxycarbonyl allylboro-
nates. After the allylboration step, the intermediate homoallylic
amine undergoes in situ lactamization to form the observed
tam products (Fig. 3).
c-lac-
In conclusion, we have optimized a new procedure for accessing
highly substituted -exo-methylene- -lactams through the use of
a
c
a diastereospecific, three-component reaction. The reaction is quite
general in that a wide variety of aromatic aldehydes and aliphatic
aldehydes are suitable substrates, providing the corresponding
exo-methylene- -lactams in moderate to excellent yields. One
should note that this protocol furnishes polysubstituted -methy-
lene- -lactams that lack any activating or protecting groups on the
nitrogen, which allows for immediate functionalization of the
a-
c
a
c
c-
lactam nitrogen without an extra deprotection step. Very few
examples are found in the literature where this is the case.11a,24
Further studies are currently being carried out to expand this work
to include other types of aldehydes, as well as investigations into
Figure 2. ORTEP diagram of crystal structure for 3i.
the functionalization of these substituted
tams. As mentioned earlier, the lactam sub-unit is a key compo-
nent in a wide variety of bioactive natural products and synthetic
a-exo-methylene-c-lac-
We were also interested in the diastereoselectivity of these
imine allylboration reactions. According to the rules set out by
Hoffman,19 allylborations with E- and Z-crotylboronates and alde-
hydes provide distinct diastereomers depending on the initial
geometry of the crotylboronate. 2-Alkoxycarbonyl 3-methylallyl-
boronates have also been shown to react with the same diastereo-
specificity, either under thermal conditions,20 with Lewis acid
catalysts21 and Bronsted acid catalysts.18,22 Villiéras and co-work-
ers have also shown that imine allylboration reactions carried
out at room temperature also proceed in a diastereospecific man-
ner.12 Although we suspected that the diastereospecific nature of
the allylboration reaction would still manifest itself under these
new imine allylation conditions, it needed to be verified. Thus, both
Z-crotylboronate (1b) and E-crotylboronate (1c) were utilized un-
der the same imine allylboration conditions to test the diastereose-
lectivity of the reaction under these strongly basic conditions. Z-
Crotylboronate 1b was reacted with four different aromatic alde-
hydes and, in each case, the desired trans lactam product was ob-
tained as the sole product in moderate to excellent yields after
flash chromatography purification (entries 9–12). The X-ray crys-
tallographic structure of 3i was obtained and allowed us to conclu-
sively assign the relative stereochemistry for this product as being
trans with respect to the aldehyde substituent and the methyl
drugs. Additional modifications of the
a-exo-methylene-c-lactam
sub-unit would allow for analogues to be made in a convergent
fashion. Studies towards this end are ongoing in our laboratories.
Acknowledgements
This work was funded by the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the University of Alberta.
T.G.E. thanks NSERC for a PGS-D and Alberta Ingenuity for a grad-
uate doctoral scholarship.
Supplementary data
Supplementary data (experimental procedures, NMR spectra)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2008.09.112.
References and notes
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c
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c-lactam products as the major diastereomers in
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O
HN
O
B
MeO2C
CHO
NH4OH
EtOH
H
CO2Me
pinBHN
HN
70 ºC
4 h
CO2Me
Bpin
O
N
H
+
30 min.
25 oC
1b
Br
Br
Br
Br
Br
Figure 3. Proposed mechanism for the three-component imine allylation/lactamization reaction.
3i
2a

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hexanes) to provide the desired
a-exo-methylene-c-lactam 3a. 5-(4-Bromo-
phenyl)-3-methylene-pyrrolidin-2-one (3a): Obtained as a white solid in a yield
of 48% (method A) or 36% (method B). 1H NMR (400 MHz, CDCl3): d 7.51–7.46
(m, 2H), 7.17–7.12 (m, 2H), 6.09–6.04 (m, 1H), 6.00–5.92 (br s, 1H), 5.43–5.38
(m, 1H), 4.73 (dd, 1H, J = 8.4, 4.7 Hz), 3.31 (app. ddt, 1H, J = 17.2, 8.2, 2.3 Hz),
2.63 (ddd, 1H, J = 17.2, 4.5, 2.5 Hz). 13C NMR (100 MHz, CDCl3): d 170.7, 141.9,
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3177, 1697. HRMS (EI, m/z) calcd for C11H10ON81Br: 252.99252, found:
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stirred for 2 h at room temperature. Allylboronate 1 (0.22 mmol) was dissolved
in 0.5 mL MeOH and added to the reaction, which was subsequently heated to
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a-exo-
c
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