Direct and highly diastereoselective synthesis of azaspirocycles by a dysprosium(III) triflate catalyzed aza-piancatelli rearrangement

Article abstract of DOI:10.1002/anie.201102102

Ring the changes: The first example of an intramolecular aza-Piancatelli rearrangement is reported. Dysprosium(III) trifluoromethanesulfonate efficiently catalyzes the synthesis of functionalized azaspirocycles in a trans-selective manner by a 4π electroc

Full text of DOI:10.1002/anie.201102102

DOI: 10.1002/anie.201102102
Domino Reactions
Direct and Highly Diastereoselective Synthesis of Azaspirocycles by a
Dysprosium(III) Triflate Catalyzed Aza-Piancatelli Rearrangement**
Leoni I. Palmer and Javier Read de Alaniz*
The azaspirocycle structural motif of type 1 (Figure 1) is
embedded in numerous remarkable natural products, such as
cephalotaxine (2),[1] stemonamine (3),[2] halichlorine (4),[3]
Recently, our group developed a new cascade strategy for
the efficient synthesis of trans-4,5-disubstituted cyclopenten-
ones based on the aza-Piancatelli rearrangement.[9] The
overall transformation is highly diastereoselective and
believed to proceed through a cascade sequence that termi-
nates with a 4p-electrocyclic ring closure to give a pentadienyl
cation; a step that is analogous to the Nazarov cyclization.[10]
Given the synthetic importance of the azaspirocycle structural
motif and the stereospecificity of the 4p electrocyclizations,
we hypothesized that 2-furylcarbinols bearing an aminoalkyl
side chain at the 5-position of the furan ring would be
effective for the general synthesis of functionalized azaspiro-
cycles.
histrionicotoxin (5),[4] nankakurine A (6),[5] and others.[6]
A
Piancatelli et al. demonstrated that 2-furylcarbinols with
substituents at the 5-position undergo an intermolecular
rearrangement to give the corresponding cyclopentenone
containing a tertiary alcohol; often a decrease in both yield
and reaction rate was observed.[11] More recently, Wu et al.
described the reactivity of 2-furylcarbinols containing a
hydroalkyl side chain at the 5-position of the furan ring. In
their case the rearrangement led exclusively to the more
stable oxa-bicyclic cyclopentenone and they believe the
reaction does not proceed through an intramolecular Pianca-
telli rearrangement.[12]
Figure 1. Alkaloids that contain 1-azaspirocycle motifs.
number of elegant approaches to construct azaspirocycles
have been developed.[6,7] The methods differ in the strategy
used to address the two main synthetic challenges: construc-
tion of the tertiary carbon center bearing the nitrogen atom,
and the formation of the spirocyclic ring system. Generally,
the synthetic routes rely on a two-step process where the
tertiary carbon center and the spirocycle are formed in
separate, discrete synthetic steps. The most practical, but also
the most difficult and rare approach to construct this
challenging azaspirocyclic framework is to combine the
construction of the tertiary stereocenter and the formation
of the spirocyclic ring system within a single operation.[6,8]
We envisioned a mechanistic scenario that would proceed
in an intramolecular fashion, analogous to the proposed aza-
Piancatelli rearrangement (Scheme 1). By employing the
proposed cascade rearrangement, we sought to overcome the
challenges associated with the generation of an azaspirocyclic
framework by relying on the electrocyclization to construct
the sterically congested azaspirocycle (D–E). Herein, we
report an efficient and highly diastereoselective synthesis of
functionalized azaspirocycles, based on the first example of an
intramolecular aza-Piancatelli rearrangement.
Furylcarbinol 9 was selected as a model substrate to probe
the feasibility of the new intramolecular rearrangement and
dysprosium(III) triflate (Dy(OTf)3) was chosen as the catalyst
[*] L. I. Palmer, Prof. J. Read de Alaniz
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: http://www.chem.ucsb.edu/~read
[**] This work was supported by UCSB. Additional support was kindly
provided by Eli Lilly (New Faculty Award to J.R.A.). 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.201102102.
Scheme 1. The proposed mechanism of the intramolecular aza-Pianca-
telli rearrangement. conr.=conrotatory, LA=Lewis acid.
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because of our our long-term interest in this intriguing Lewis
acid.[9,13] We were delighted to discover that 9 undergoes
quantitative rearrangement to the azaspirocycle 10 in
15 minutes upon exposure to 5 mol% of Dy(OTf)3 in
acetonitrile heated to reflux [Eq. (1)].
Investigation of the scope of the intramolecular aza-
Piancatelli rearrangement revealed that this method provides
access to a range of substituted azaspirocycles in excellent
yield and as a single diastereomer (Scheme 2). Single-crystal
X-ray analysis of products 10, 19, and 39 confirmed that the
rearrangement had taken place with the expected trans
selectivity, consistent with a 4p electrocyclization.[14]
As illustrated in Scheme 2 the rearrangement is compat-
ible with 2-aryl furylcarbinols possessing common functional
groups on the aromatic ring; these groups range from
electron-withdrawing nitro (14)[15] and bromo groups (15) to
electron-rich methyl ether (17) and methylenedioxy groups
(18–19). Sterically bulky aryl substituents also successfully
gave the rearranged products (16, 18, and 19) in good yield.
Furylcarbinols substituted at the 2-position with an alkyl
group participated in the rearrangement to afford the desired
product in excellent yield (23–25), but in the sterically
demanding case of 22 a modest 53% yield was observed.
Importantly, substrates possessing the removable para-
methoxyphenyl (PMP) group on the nitrogen atom were also
tolerated, although we did notice a decrease in both yield and
reaction rate when compared to the analogous substrates with
a phenyl group on the nitrogen atom (10 vs. 13 and 20 vs.
21).[9,16] In contrast to our previous intermolecular aza-
Piancatelli rearrangement, simple unsubstituted furylcarbi-
nols 20 and 21 underwent the reaction smoothly. This
rearrangement presumably benefits from the unimolecular
nature of the reaction.
The 6-azaspiro[4.5]decane framework has received sig-
nificant attention in recent years because of its interesting
biological activities and challenging architecture.[3,6] Conse-
quently, we turned our attention to this important structural
motif and found it can also be constructed by the intra-
molecular aza-Piancatelli rearrangement in good yield as a
single diastereomer (Table 1).
Scheme 2. The scope of the intramolecular aza-Piancatelli rearrange-
ment. [a] Isolated as a 5:1 mixture of products, see the Supporting
Information for details. [b] Reaction conducted at RT. [c] 10 mol% of
catalyst was used.
A more pronounced effect on the rate of the rearrange-
ment was observed while investigating the formation of 6-
azaspirocycles. Perhaps the most striking example is shown in
entry 3 (Table 1). In this case, the rearrangement product 30,
which has no substituent adjacent to the alcohol and possesses
an electron-rich para-methoxy aniline tether in the furylcar-
binol starting material, was obtained in only 37% yield, and
20 mol% of Dy(OTf)3 catalyst was necessary to achieve
moderate conversion. Within the series, longer reaction times
were required as the N-aryl group became more electron rich
(entries 1–8). In addition, substrates that lack substitution are
more difficult to activate.
We have observed that the efficiency of the intramolec-
ular cascade rearrangement benefits from a polarized sub-
strate bearing an electron-donating group adjacent to the
alcohol and an electron-withdrawing group on the nitrogen
nucleophile. This result could occur because the more
electron-rich anilines are competing with the hydroxy group
on the 2-furylcarbinol for Lewis acid coordination. The high
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Table 1: Scope of the 6-azaspirocycle.
slightly with the para-chloro (40), para-bromo (41), and para-
trifluoromethyl (42) anilines, suggesting a possible change in
mechanism or change in the rate-determining event (Table 2;
entries 5–7).[17]
In conclusion, we have developed a general and practical
intramolecular aza-Piancatelli rearrangement that constructs
a fully substituted carbon center bearing a nitrogen atom and
a spirocyclic ring system in a single operation. The rearrange-
ment is catalyzed by commercially available dysprosium (III)
triflate under operationally simple reaction conditions. The
azaspirocycles are formed exclusively as the trans diaster-
eomer, which is consistent with a 4p electrocyclization.
Further investigation of this rearrangement and its applica-
tion toward complex azaspirocyclic natural products will be
forthcoming.
Entry
Product
R
t [h]
Yield
[%][a]
1
28
29
30
p-IC6H4
Ph
p-OMeC6H4
8
48
72
90
70
37
2
3[b]
4
5
6
31
32
33
p-IC6H4
Ph
p-OMeC6H4
5 min
1
75
69
75
74
Experimental Section
7
8
34
35
p-IC6H4
Ph
2
15
65
54
General procedure for the intramolecular aza-Piancatelli rearrange-
ment: To a stirred solution of 5-substituted furylcarbinol (11;
0.16 mmol) in MeCN (< 6 mL) was added Dy(OTf)3 (5 mol%) at
238C. The reaction flask was immediately fitted with a reflux
condenser and placed in an oil bath preheated to 808C. Upon
completion (as evident by TLC), the reaction mixture was cooled to
RT then quenched with saturated aqueous NaHCO3 and extracted
with ethyl acetate. The combined organic layers were dried over
MgSO4, filtered and then concentrated in vacuo. The residue was
purified by flash column chromatography on silica gel (eluent:
hexanes/ethyl acetate 9:1) to afford the desired azaspirocycle (12).
[a] Yield of the isolated product. [b] 20 mol% of catalyst was used.
trans diastereoselectivity and the increased aptitude of the
polarized substrates to undergo the rearrangement are
consistent with a 4p conrotatory electrocyclization. These
results are similar to the Nazarov reaction, a process well
known to proceed through a 4p conrotatory electrocyclization
and benefit from polarization of the substrates.[10]
Received: March 24, 2011
Revised: May 20, 2011
Intrigued by the rate acceleration observed during the
formation of the 6-azaspirocycles, we prepared a series of 2-
furylcarbinols bearing an aminoalkyl side chain containing
different para-substituted anilines. Our preliminary experi-
ments were designed to probe the net electronic effect of the
aniline substituent on the intramolecular aza-Piancatelli
rearrangement. Consistent with our previous observation in
the formation of the 6-azaspirocycles, the electronic nature of
the aniline group affected reaction rate (Table 2). In general,
more electron-rich anilines were slower to rearrange relative
to more electron-poor anilines (Table 2; entries 1–7). How-
ever, it is interesting to note that the reaction time increased
Published online: June 17, 2011
Keywords: aza-Piancatelli · azaspirocycles · domino reactions ·
.
lanthanides · rearrangement
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Entry
R
Product
t [min]
Yield [%][a]
1
2
3
4
5
6
7
OMe
Me
H
I
Cl
13
38
10
39
40
41
42
150
30
15
5
10
10
15
74
79
96
84
96
78
82
Br
CF3
[a] Yield of the isolated product.
Angew. Chem. Int. Ed. 2011, 50, 7167 –7170
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7169

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7170
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