Versatile method for the synthesis of 4-aminocyclopentenones: Dysprosium(III) triflate catalyzed aza-piancatelli rearrangement

Article abstract of DOI:10.1002/anie.201005131

A sweet dysprosition: A dysprosium(III) trifluoromethanesulfonate catalyzed rearrangement of furylcarbinols into 4-aminocyclopentenones by a 4π electrocyclization has been developed. The aza-Piancatelli rearrangement affords a single trans diastereomer fr

Full text of DOI:10.1002/anie.201005131

Communications
DOI: 10.1002/anie.201005131
Rearrangement Reactions
Versatile Method for the Synthesis of 4-Aminocyclopentenones:
Dysprosium(III) Triflate Catalyzed Aza-Piancatelli Rearrangement**
Gesine K. Veits, Donald R. Wenz, and Javier Read deAlaniz*
In memory of Marianna Rovis
Well-represented in natural products and biologically active
molecules, the cyclopentenone scaffold has long been an
inspiration for the development of new methodologies.[1] In
1976, Piancatelli and co-workers reported a new method for
the synthesis of 4-hydroxycyclopentenone derivatives by an
acid-catalyzed rearrangement of suitable 2-furylcarbinols
(Scheme 1).[2] The overall transformation is believed to
proceed through a cascade sequence that terminates with a
4p electrocyclic ring closure of a pentadienyl cation (D),
analogous to the Nazarov cyclization.[3]
Denisov and et al. also required stoichiometric amounts of
acid (BF3·OEt2 or p-TsOH) and was limited to 2-furylcarbi-
nols that were activated with a cobalt/alkyne complex.[5]
The Piancatelli rearrangement caught our attention
because both the cascade rearrangement and access to
trans-4,5-disubstituted cyclopentenones appear ideally
suited for various applications in synthesis. We reasoned
that an efficient catalytic aza-Piancatelli rearrangement
would be a powerful synthetic reaction for the preparation
of trans-substituted 4-amino-5-alkylcyclopentenones, a func-
tional scaffold that is rich in potential for the synthesis of
biological and medicinal compounds. Few processes are
available for the synthesis of 4-aminocyclopentenones,[6] and
all of the previously reported methods require multiple steps
and typically lack substitution at the 5-position. Herein, we
report a mild catalytic single-step procedure for the con-
version of readily available 2-furylcarbinols into their corre-
sponding trans-substituted 4-amino-5-alkylcyclopentenones.
Our investigation began by identifying a catalyst capable
of activating 2-furylcarbinols in the presence of potentially
problematic Lewis basic amines. We were encouraged by a
report by Li and Batey that rare-earth Lewis acids mediate
the rearrangement of furfural-derived iminium cations in the
presence of excess Lewis basic amines.[7] Therefore, we
hypothesized that such acids would allow us to extend the
range of possible nucleophiles beyond electron-deficient
para-substituted anilines.[8]
Initial studies were conducted by examining the addition
of commercially available para-iodoaniline 5 to furylcarbinol
4 in the presence of 5 mol% of either scandium or dysprosium
trifluoromethanesulfonate (Table 1). We were pleased to find
that both Lewis acids catalyzed the desired transformation,
affording 4-aminocyclopentenone 6 in excellent yield as a
single diastereomer (Table 1, entries 1 and 2). The rearrange-
ment was found to be most effective at 808C (Table 1,
entry 3). Although 5 mol% of triflic acid can serve as an
active catalyst for this rearrangement (Table 1, entry 4),
control experiments demonstrated that a trace quantity of
triflic acid was not solely responsible for the catalysis when a
metal triflate was employed (Table 1, entries 5 and 6).[9] We
chose to develop the reaction with Dy(OTf)3 because of its
lower cost compared to Sc(OTf)3 and because it is exper-
imentally easier to handle than triflic acid.[10] Lewis acid
reactions mediated by Dy(OTf)3 have not attracted tremen-
dous interest from the synthetic community, despite the fact
that it exhibits similar reactivity and shares the advantageous
properties of other lanthanide salts: low toxicity and cost,
ease of handling, and stability toward moisture.[11]
Scheme 1. Proposed mechanism of the Piancatelli reaction. LA=Lewis
acid, conr.=conrotatory
Investigations by Piancatelli and co-workers focused
exclusively on accessing 4-hydroxycyclopentenones, presum-
ably because reaction development was largely driven by
application of this methodology to the synthesis of prosta-
glandins.[4] The synthetic utility of the Piancatelli rearrange-
ment has been limited because the reaction often requires
stoichiometric amounts of acid, dilute reaction conditions
(<0.005m), and excess water. Furthermore, there has been
only one subsequent investigation that probes this interesting
cascade rearrangement to access compounds besides substi-
tuted 4-hydroxycyclopentenones. This seminal study by
[*] G. K. Veits, D. R. Wenz, Prof. J. Read deAlaniz
Department of Chemistry and Biochemistry
University of California-Santa Barbara
Santa Barbara, CA 93106-9510 (USA)
Fax: (+1)805-893-4120
E-mail: javier@chem.ucsb.edu
Homepage: Homepage: http://www.chem.ucsb.edu/~read
[**] This research was supported by the UCSB. We thank Professors
Lipshutz, Pettus, Zakarian, and Zhang for helpful discussions and
access to chemicals and equipment. We also thank Dr. Guang Wu
(UCSB) for X-ray analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005131.
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Table 1: Optimization of the rearrangement conditions.
Under the optimized reaction conditions (5 mol% Dy-
(OTf)3, MeCN, 808C), we investigated the scope of this
transformation. Scheme 2 summarizes results obtained with
ortho-, meta-, and para-substituted aniline derivatives. The
reaction of anilines substituted at the para- or meta-positions
generated the best results. Sterically hindered 2,4,6-trimethyl-
aniline also successfully participated in the reaction (14).
Secondary acyclic and cyclic anilines produced the desired
product in 74%, 88%, and 67% yield, respectively (17, 18,
and 19).
Several additional observations merit note. Typically, it is
difficult to prevent product isomerization (Scheme 1, 2!3) in
the Piancatelli rearrangement; however, presumably because
of the mild nature of the Dy(OTf)3 catalyst, product isomer-
ization was not observed in the aza-Piancatelli rearrange-
ment. One significant side-reaction did occur when 2,6-
dimethylaniline was employed. In this case, 21 was formed in
only 33% yield (Scheme 2), with the rest of the remaining
starting material consumed by Friedel–Crafts alkylation to
give 22 and 23 (Scheme 3).[12]
Entry[a]
Catalyst (mol%)
T [8C]
t [h]
Yield [%]
1
2
3
Sc(OTf)3 (5)
Dy(OTf)3 (5)
Dy(OTf)3 (5)
HOTf (5)
40
40
80
80
80
3
2
0.5
0.5
4
83
79
92
62
0
4
5[b]
HOTf (5)
K2CO3 (100)
Dy(OTf)3 (5)
K2CO3 (100)
6
80
1
93
[a] Reaction conducted in MeCN. [b] The starting material was recovered
from the reaction.
Scheme 3. Products resulting from Friedel–Crafts alkylation.
Subsequently, we performed a series of experiments to
explore the initial scope of the furylcarbinol component 24
using 25 in this rearrangement to give 26. As shown in Table 2,
the rearrangement is compatible with 2-aryl furylcarbinols
possessing electron-donating or electron-withdrawing groups
on the aromatic ring (Table 2, entries 1–6). It is noteworthy
that 2-alkyl-substituted furylcarbinols can be accommodated
without a significant loss in reactivity (Table 2, entries 7–12).
Increasing the steric bulk of the alkyl group on the 2-
substituted furylcarbinol from methyl to isopropyl groups
appreciably decreased the formation of the competing
Friedel–Crafts alkylation with meta-chloroaniline (Table 2,
entries 8 and 11).
We believe that the high trans diastereoselectivity is a
result of a 4p conrotatory electrocyclization.[13] It seems likely
that the initial step in the cascade rearrangement involves loss
of the alcohol and the formation of a stabilized carbocation.
At this point, the stabilized carbocation can react with the
aniline through two predominant pathways: Friedel–Crafts
alkylation at the benzylic position or addition at the 5-
position of the furylcarbinol, the latter triggering the product-
forming cascade reaction.
To highlight the synthetic utility of this methodology, we
began to explore the application of the cascade rearrange-
ment for the efficient synthesis of biological and medicinal
molecules. A recent structure–activity relationship (SAR)
study by Merck found 1,2-trans-2,3-trans-cyclopentane-based
scaffolds of type 39 to be comparable to the current clinical
Scheme 2. Scope of the rearrangement with substituted anilines.
[a] 20 mol% Dy(OTf)3. [b] 3 equiv of the corresponding aniline.
[c] Formed as a 1:1 ratio of diastereomers at the stereocenter marked
with an *. Yields reported are those of the isolated products.
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Table 2: Scope of the rearrangement with various 2-furylcarbinols.
Entry
Product
R
Yield [%]
1[a]
2[a]
3[a]
p-IC6H4 (27)
m-ClC6H4 (28)
2,4,6-Me3C6H2 (29)
68
82
89
4
p-IC6H4 (30)
m-ClC6H4 (31)
2,4,6-Me3C6H2 (32)
83
87
78
Scheme 5. Direct synthesis of the cyclopentane scaffold: a) 5 mol%
Dy(OTf)3, para-anisidine, MeCN, 808C, 60% (1 gram scale) b) NaBH4,
CeCl3·7H2O, MeOH, 98% (2:1 trans/cis); c) 10 mol% Pd/C, H2,
MeOH, 500 psi, 76% (2:1 trans/cis; 41=49%); d) NaH, 3,5-bis(tri-
fluoromethyl)benzyl bromide, THF, 75%; e) H5IO6, H2SO4, MeCN/
H2O (1:1), RT, 58%. THF=tetrahydrofuran.
5
6[b]
7
p-IC6H4 (33)
m-ClC6H4 (34)
2,4,6-Me3C6H2 (35)
68
10
74
8[c]
9
10
11
12
p-IC6H4 (36)
m-ClC6H4 (37)
2,4,6-Me3C6H2 (38)
73
52
89
In conclusion, we have developed an efficient aza-
ꢀ
Piancatelli rearrangement that constructs a carbon carbon
ꢀ
bond plus a carbon nitrogen bond and two stereocenters in a
[a] Reaction conducted at RT. [b] 10 mol% Dy(OTf)3. [c] Products arising
from Friedel–Crafts alkylation were mainly observed. Yields reported are
those of the isolated products.
single operation. This strategy offers a practical solution for
the synthesis of 4-aminocyclopentenones, a versatile building
block for the synthesis of structurally diverse biologically
active molecules. Reactions are performed in reagent grade
acetonitrile, open to air, with commercially available
Dy(OTf)3. Further investigation of this rearrangement and
its application toward complex synthetic targets will be
forthcoming.
Received: August 16, 2010
Published online: November 4, 2010
Keywords: cyclization · domino reactions · dysprosium ·
.
lanthanides · rearrangement
Scheme 4. Representative cyclopentane-based hNK1 antagonist.
[1] For recent reviews of the Nazarov and Pauson–Khand reactions,
see: a) M. A. Tius, Eur. J. Org. Chem. 2005, 2193; b) H. Pellissier,
Tetrahedron 2005, 61, 6479; c) A. J. Frontier, C. Collison,
Tetrahedron 2005, 61, 7577; d) T. N. Grant, C. J. Rieder, F. G.
West, Chem. Commun. 2009, 5676; e) J. Blanco-Urgoiti, L.
Anorbe, L. Perez-Serrano, G. Dominguez, J. Perez-Castells,
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[2] G. Piancatelli, A. Scettri, S. Barbadoro, Tetrahedron Lett. 1976,
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[3] A. N. Faza, C. S. Lopez, R. Alvarez, I. R. de Lera, Chem. Eur. J.
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[4] G. Piancatelli, M. Dauria, F. Donofrio, Synthesis 1994, 867.
[5] V. R. Denisov, S. E. Shustitskaya, M. G Karpov, Zh. Org. Khim.
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[6] For select examples, see: a) F. A. Davis, Y. Z. Wu, Org. Lett.
2004, 6, 1269; b) J. Dauvergne, A. M. Happe, V. Jadhav, D.
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compound 40 in assays of hNK1 inhibition (Scheme 4).[14]
Aprepitant (40) is an hNK1 antagonist and FDA approved for
use as an antiemetic for chemotherapy-induced nausea and
vomiting.
The synthesis of a cyclopentane scaffold containing an
oxygen functionality at the C1 position, the required aryl
group at the C2 position, and a functionalizable amine group
at the C3 position began with the rearrangement of 4 with
para-anisidine on a gram scale (Scheme 5). Luche reduction
of cyclopentenone 10 gave the 1,2-trans-2,3-trans-cyclopen-
tene-based scaffold in two steps from commercially available
reagents. Subsequent alkene reduction and hydroxyl alkyla-
tion with 3,5-bis(trifluoromethyl)benzyl bromide and sodium
hydride gave the racemic ether 42. Oxidative dearylation of
the para-methoxyphenyl group with periodic acid (H5IO6)
provided the primary amine.[15] The hNK1 binding affinity for
racemic cyclopentane 43 was moderate (IC50 = 5.7 nm); how-
ever, enhanced affinity (IC50 < 0.1 nm) and aqueous solubility
were achieved by simple derivatization of 43.[14]
[7] S. W. Li, R. A. Batey, Chem. Commun. 2007, 3759.
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[8] The rearrangement reported by Denisov et al. was limited to
three para-substituted anilines; see Ref. [5].
[9] D. C. Rosenfeld, S. Shekhar, A. Takemiya, M. Utsunomiya, J. F.
Hartwig, Org. Lett. 2006, 8, 4179.
[10] Price from Strem Chemicals: Dy(OTf)3 = $36.00/5 g, Sc(OTf)3 =
$172.00/5 g.
[11] For select examples of dysprosium(III)-catalyzed reactions, see:
a) S. Kobayashi, I. Hachiya, J. Org. Chem. 1994, 59, 3590;
b) R. A. Batey, D. A. Powell, A. Acton, A. J. Lough, Tetrahedron
Lett. 2001, 42, 7935; c) W. Li, Y. Ye, J. Zhang, R. Fan,
Tetrahedron Lett. 2009, 50, 5536.
I. Matsuo, Adv. Synth. Catal. 2001, 343, 71; c) L. L. Anderson, J.
Arnold, R. G. Bergman, J. Am. Chem. Soc. 2005, 127, 14542.
[13] Alternative mechanisms cannot be ruled out at this time.
[14] a) P. E. Finke, L. C. Meurer, D. A. Levorse, S. G. Mills, M.
MacCoss, S. Sadowski, M. A. Cascieri, K.-L. Tsao, G. G. Chicchi,
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R. G. Ball, S. G. Mills, M. MacCoss, S. Sadowski, M. A. Cascieri,
K.-L. Tsao, G. G. Chicchi, L. A. Egger, S. Luell, J. M. Metzger,
D. E. MacIntyre, N. M. J. Rupniak, A. R. Williams, R. J. Har-
greaves, Bioorg. Med. Chem. Lett. 2006, 16, 4504.
[12] For select related examples of Friedel–Crafts acylation and
alkylation of aniline derivatives, see: a) M. Beller, O. R. Thiel,
H. Trauthwein, Synlett 1999, 243; b) S. Kobayashi, I. Komoto, J.-
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P. L. Alsters, F. L. van Delft, F. P. J. T. Rutjes, Tetrahedron Lett.
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