Gold-catalyzed tandem 1,3-migration/[2 + 2] cycloaddition of 1,7-enyne benzoates to azabicyclo[4.2.0]oct-5-enes

Article abstract of DOI:10.1021/ja2052304

A synthetic method that relies on gold(I)-catalyzed tandem 1,3-migration/[2 + 2] cycloaddition of 1,7-enyne benzoates to prepare azabicyclo[4.2.0]oct-5- enes is described.

Full text of DOI:10.1021/ja2052304

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pubs.acs.org/JACS
Gold-Catalyzed Tandem 1,3-Migration/[2 + 2] Cycloaddition of
1,7-Enyne Benzoates to Azabicyclo[4.2.0]oct-5-enes
Weidong Rao, Dewi Susanti, and Philip Wai Hong Chan*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S Supporting Information
b
Scheme 1. Gold-Catalyzed Reactivities of 1,n-Enyne
Carboxylates
ABSTRACT: A synthetic method that relies on gold(I)-
catalyzed tandem 1,3-migration/[2 + 2] cycloaddition of
1,7-enyne benzoates to prepare azabicyclo[4.2.0]oct-5-enes
is described.
old-catalyzed 1,n-enyne cycloisomerizations offer a highly
G
efficient and atom-economical synthetic strategy for increas-
ing molecular complexity and diversity in a single step.1À5 The
field has seen rapid growth in recent years, with a number of
impressive methods for the conversion of 1,n-enyne derivatives
to various synthetically valuable products being developed. This
has hitherto included approaches that make use of easily acces-
sible 1,n-enynes bearing a carboxylic ester at the propargylic
position.3À5 As shown in Scheme 1, the reaction relies on the
propensity of the acyloxy moiety to undergo 1,2- or 1,3-migra-
tion to give the corresponding gold carbene and allene inter-
mediates I and II that are poised for further functionalization by a
remaining pendant group. In the case of 3,3-rearrangements,
which DFT calculations have recently shown could also possibly
occur via two sequential 1,2-migrations,5d allene activation is
typically required before further functional group transforma-
tions can take place. On the other hand, a tandem process
involving 1,3-O-ester migration followed by selective coordina-
tion of the gold catalyst to an appropriately placed unactivated
alkene rather than the allenic moiety is not known (paths b and c
in Scheme 1).6 We anticipated that the gold(I)-coordinated
species III generated from this change in selectivity by the metal
catalyst might then be more prone to undergo a stepwise [2 + 2]
process and provide synthetically useful bicyclic derivatives fused
with a cyclobutane ring. To our knowledge, this mode of reactivity
is not observed in 1,n-enyne cycloisomerizations because other
more generally favored skeletal rearrangements occur.1À5 Added
to this is a recent report showing acyloxy-substituted 1,6-enynes
derived from dipropargylic amides to be resistant to the gold-
catalyzed cycloisomerization process.7 As part of an ongoing
program exploring the scope of gold catalysis in heterocyclic
synthesis,8 we disclose herein the detailsof this chemistryinvolving
Au(I)-mediated tandem 1,3-migration/[2 + 2] cycloaddition of
1,7-enyne benzoates (Scheme 1). This process delivers a regiose-
lective and stereoconvergent route to azabicyclo[4.2.0]oct-5-enes,
a key class of compounds in synthetic and natural product
chemistry,9 that gives the products in good to excellent yields
as single regio- and diastereomers. The bicyclic ring adducts could
also be obtained as single enantiomers with up to four stereogenic
centers, demonstrating the reaction to proceed with efficient
transfer of chirality from the enantiopure substrate to the product.
To test the feasibility of our hypothesis, the enantiopure syn-
1,7-enyne benzoate 1a, prepared from L-leucine following litera-
ture procedures,10 was chosen as the model substrate to establish
the reaction conditions (Table 1). The (3S,4S) absolute config-
uration of the starting ester was determined by X-ray crystal
structure analysis of a precursor of the substrate.11 This study
revealed that treating 1a with 5 mol % Au(I) catalyst A and 4 Å
molecular sieves (MS) in (CH2Cl)2 at 80 °C for 15 h gave the
best result, furnishing 2a in 82% yield as a single regio-, diastereo-,
and enantiomer (entry 1). The (1R,4S,7R) absolute configura-
tion of the cyclobutane-fused piperidine product was determined
by X-ray crystallography.11 Lower product yields were obtained
when the reaction was repeated at room temperature, with the
more sterically crowded Au(I) complex B or C in place of A as
the catalyst, or in the absence of 4 Å MS (entries 2, 3, 8, and 9).10
The reaction at room temperature also afforded allenene 3a in
48% yield (entry 3).12 In marked contrast, only the allenene was
obtained in 77À95% yield when the solvent was changed from
(CH2Cl)2 to either MeCN, MeNO2, 1,4-dioxane, or toluene
(entries 4À7). A survey of other Au(I) and Au(III) complexes as
Received: June 7, 2011
Published: August 29, 2011
r
2011 American Chemical Society
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|

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COMMUNICATION
Table 1. Optimization of the Reaction Conditionsa
Table 2. Tandem 1,3-Migration/[2 + 2] Cycloaddition of
1,7-Enyne Benzoates 1bÀx Catalyzed by Aa
yield (%)b
3a
entry
catalyst
solvent
2a
1
2c
3d
4
A
A
A
A
A
A
A
B
C
D
E
F
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
MeCN
82
53
44
À
À
À
À
69
68
À
À
À
À
À
À
À
À
À
À
48
95
88
80
77
À
5
1,4-dioxane
MeNO2
6
7
PhMe
8
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
9
À
10
11
12
13
14
15
16
17
29e
93
86
83
88
95
63
96
a All reactions were performed on a 0.15 mmol scale with an A:1
ratio of 1:20 and 4 Å MS (150 mg) at 80 °C for 15À24 h. Values in
parentheses denote isolated product yields. b The reaction was con-
ducted with (3R,4S)-1a and 10 mol % A. c The reaction was conducted
with an inseparable 1:1 mixture of diastereomers of the substrate.
d The reaction was conducted with an inseparable 4:1 mixture of
diastereomers of the substrate. e The reaction was conducted with an
inseparable 3.3:1 mixture of diastereomers of the substrate.
G
Ph3PAuNTf2
H
AuBr3
PtCl2
a All reactions were performed on a 0.15 mmol scale with a catalyst:1a ratio
of 1:20 and 4 Å MS (150 mg) at 80 °C for 15 h. b Isolated yields. c The
reaction was carried out in the absence of 4 Å MS. d The reaction was carried
out at room temperature for 30 h. e Pyrrole 4a was obtained in 20% yield.
1p, respectively), showing that such functional groups were well-
tolerated under the reaction conditions. Similarly, reactions of
substrates containing an activated alkene moiety (1qÀx) were
found to proceed well and provide the corresponding bicyclic
adducts in excellent yields. More notably, these intramolecular
cyclizations also demonstrated that the ring-forming process occurs
in a highly selective manner. We found the [2 + 2] cycloaddition
step to occur regioselectively at the distal 2π component of the
in situ-formed allene moiety, affording the cyclobutane-fused piper-
idine as the sole product in all of the reactions shown in Table 2. The
pyrrolidine regioisomer resulting from [2 + 2] cycloaddition at the
proximal 2π component of the in situ-formed allene moiety was not
observed using NMR analysis of the crude reaction mixtures.13 The
reaction was additionally shown to give the corresponding products
as single diastereomers irrespective of whether it started from a
single isomer or a diastereomeric mixture of the substrate. In the
case of reactions with 1bÀd, 1gÀk, and 1pÀx, the corresponding
adducts were also furnished as single enantiomers with up to four
stereogenic centers, illustrating efficient transfer of chirality from the
enantiopure substrate to the bicyclic product. Moreover, the forma-
tion of cyclobutane-fused piperidines as single diastereomers from
well as PtCl2 as the catalyst gave similar outcomes (entries
10À17). In all but one of these latter experiments, the allenene
adduct was again obtained in 63À96% yield. In our hands, the
analogous reaction of 1a with gold(I) carbene complex D as the
catalyst was the only instance that afforded the pyrrole byproduct
4a along with 3a, in 20 and 29% yield, respectively (entry 10).10
The structure of the aromatic N-heterocycle was structurally
confirmed by X-ray crystallographic analysis.11
With the optimal conditions in hand, we next sought to evaluate
their generality for a series of 1,7-enyne benzoates prepared from the
corresponding L-α-amino acids, and the results are summarized in
Table 2. These experiments showed that with the Au(I) complex A
as the catalyst, the conditions proved to be broad, and a variety of
highly substituted azabicyclo[4.2.0]oct-5-enes could be obtained in
41À90% yield from the corresponding substrates 1bÀx. This
hitherto included starting 1,7-enynes containing a thiophene,
cyclopropane, OTBS, or MeSO2 moiety (1d and 1m, 1f, 1n, and
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COMMUNICATION
Scheme 2. Control Experiments with 2a and 3a Catalyzed
by A
Scheme 3. Tenative Mechanism for Tandem
1,3-Migration/[2 + 2] Cycloaddition of 1 Catalyzed by A
stereoisomeric starting 1,7-enynes that presumably were epimeric at
C3 (1l, 1n, 1o, and 1w) showed that the transformation is stereo-
convergent. This was further exemplified by the Au(I)-catalyzed
reaction of (3R,4S)-1a, which also provided (1R,4S,7R)-2a in 48%
yield. On the other hand, the stereochemistry of the alkene moiety was
found to be retained in the product. Substrates containing a trans-
alkene group (1qÀw) were found to give the corresponding bicyclo-
[4.2.0] adducts having the bridgehead proton and R4 group syn with
respect to each other. In contrast, the reaction of 1x with a pendant cis-
alkene moiety gave 2x, in which the stereochemical relationship
between these groups in the product was anti and epimeric at C8.
The absolute configurations (1R,4S,7R) for 2b and 2i, (1R,4S,7R,8S)
for 2r, and (1R,4R,7R,8R) for 2x were determined on the basis of
X-ray crystal structure measurements.11 Similarly, the relative syn
diastereochemistry of 2m was determined by X-ray crystallography.11
The mechanistic premise presented in Scheme 1 predicts that the
Au(I)-catalyzed cycloisomerization proceeds through a tandem 3,3-
rearrangement/[2 + 2] cycloaddition pathway involving an allenene
intermediate. While the isolation of allenene 3a produced by
reactions of 1a under various conditions as described in entries
3À7 and 10À17 in Table 1 was fortuitous, the result argues in favor
of this being the actual intermediate in the Au(I)-catalyzed trans-
formation. This argument is further corroborated by the fact that 2a
was furnished in 88% yield when a (CH2Cl)2 solution containing 3a
was treated with 5 mol % Au(I) complex A under the conditions
depicted in eq 1 in Scheme 2. The role of the gold(I) catalyst in
triggering the stepwise [2 + 2] cycloaddition by preferentially
coordinating to the unactivated alkene moiety was also shown by
first repeating the reaction in the absence of the metal catalyst. This
test led to recovery of only the starting allenene in nearly quantitative
yield, mirroring the lack of reactivity observed in thermal [2 + 2]
cycloadditions of allenenes containing an unactivated alkene
moiety.14 Moreover, conducting the reaction again for a second
time with the gold(I) catalyst in the presence of D2O provided cis-
piperidine d1-5a in 30% yield with 90% deuterium incorporation
(eq 2 in Scheme 2). On the other hand, resubjecting 2a to these
latter conditions with the gold(I) catalyst and D2O was found to
give the debenzoylated product 6a in 31% yield (eq 1 in Scheme 2).
A tentative mechanism for the present Au(I)-catalyzed reaction to
form cyclobutane-fused piperidines is outlined in Scheme 3. This could
involve activation of the alkyne moiety of the 1,7-enyne substrate by
the gold(I) catalyst, resulting in syn 1,3-migration of the carboxylic
ester group and formation of allenene 3 via IV and V. To avoid
unfavorable steric interactions between the gold complex and the
substituents on the allene moiety, it is possible that the catalyst then
selectively coordinates to the alkene bond of this newly formed
allenene intermediate to give gold-activated adduct III. This is the
active species that undergoes the stepwise [2 + 2] cycloaddition
process involving anti addition of the allenic group to the gold-
coordinated alkene moiety to give piperidine adduct VI.15 Subsequent
nucleophilic addition of the AuÀC(sp3) bond to the carbonyl carbon
center of the benzoyl cationic moiety generated from this initial
intramolecular cyclization step would then deliver 2 with release of
the gold(I) catalyst.16 The pyrrole byproduct 4a could originate from
coordination of the gold catalyst to the alkyne unit in 1 or the allene
moiety in 3followed by nucleophilic attack of the tethered sulfonamide
moiety and a deauration step involving 1,3-migration of the Ts
group.17 We surmise that the obtained product regioselectivities result
from the greater electron-donating ability of the OBz moiety relative to
that of the R2 group in 3. This would consequently make the distal 2π
component of the allene side chain more nucleophilic in charac-
ter and therefore more likely to take part in the first step of the
[2 + 2] cycloaddition. The observed stereoconvergence leading to a
single diastereomer regardless of the stereochemistry at C3 in the
substrate could be due to the oxonium species adopting the con-
formation shown in Scheme 3. This would provide optimal orbital
alignment and thus overlap between the HOMO of the AuÀC(sp3)
bond and the LUMO of the carbonyl carbon center, enabling the
second CÀC bond-forming step of the [2 + 2] cycloaddition to
proceed efficiently. It would also explain the lower reactivities observed
in reactions with the anti isomer of 1a and substrates containing this
diastereomer (1l, 1n, 1o, and 1w), since the close proximity of the
bulky alkyl gold side chain could significantly restrict the rotational
freedom of the benzoyl CÀC bond (eq 1 in Scheme 4). A similar
rationale could be applied to account for the preferential nucleophilic
attack of the AuÀC(sp3) bond from below the plane of the benzoyl
moiety and the moderate product yield obtained for the reaction of 1x
containing a cis-alkene unit (eq 2 in Scheme 4). The formation of the
product as a single enantiomer from an enantiopure substrate
additionally suggests that neither the starting material nor any of the
putative intermediates are prone to racemization. This consequently
allows efficient transfer of the retained chirality at the α-carbon center
to the amino group, giving the enantioselectivities observed at the
newly formed stereogenic centers.
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COMMUNICATION
Scheme 4. Possible Conformational States for Tandem
1,3-Migration/[2 + 2] Cycloaddition of 1 Catalyzed by A
(3) Reviews of cycloisomerizations of 1,n-enyne carboxylic esters:
(a) Marco-Contelles, J.; Soriano, E. Chem.—Eur. J. 2007, 13, 1350.
(b) Marion, N.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46, 275.
(c) Ma, S.; Yu, S.; Gu, Z. Angew. Chem., Int. Ed. 2006, 45, 200.
(4) Selected examples: (a) Garayalde, D.; Kr€uger, K.; Nevado, C.
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In summary, we have demonstrated gold(I)-catalyzed tandem 1,3-
migration/[2 + 2] cycloaddition of 1,7-enyne benzoates to be a
regioselective and stereoconvergent strategy for the construction of
highly functionalized azabicyclo[4.2.0]oct-5-enes. The reaction has
been shown to tolerate a diverse set of 1,7-enyne substrates and
furnish stereochemically well-defined cyclobutane-fused piperidines
for applications in natural product synthesis and as versatile privileged
scaffolds in medicinal chemistry. Our studies suggest that when allene
activation is not possible in 1,n-enyne cycloisomerizations involving a
1,3-migration step, the gold catalyst preferentially coordinates to the
alkene moiety. This results in bicyclic ring formation via a [2 + 2]
cycloaddition pathway, which is not typically favored in 1,n-enyne
cycloisomerizations. Efforts to study this mechanistic dichotomy
further and explore the scope and synthetic applications of the present
reaction are in progress and will be reported in due course.
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(10) For the synthesis of 1 and gold complexes AÀH, see the
Supporting Information (SI) for details.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, char-
b
(11) See Figures S104ÀS113 in the SI for ORTEP drawings of the
acterization data, crystal structure data (CIF), and complete ref 5c
and 9a. This material is available free of charge via the Internet at
http://pubs.acs.org.
crystal structures reported in this work.
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’ AUTHOR INFORMATION
Corresponding Author
waihong@ntu.edu.sg
’ ACKNOWLEDGMENT
This work was supported by a University Research Committee
Grant (RG55/06) from Nanyang Technological University and a
Science and Engineering Research Council Grant (092 101 0053)
from A*STAR, Singapore.
(15) (a) Xiao, Y.-P.; Liu, X.-Y.; Che, C.-M. Angew. Chem., Int. Ed. 2011,
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references therein.
(16) The involvement of an in situ-formed organogold species
containing a AuÀC(sp3) bond that participates in a nucleophilic
addition reaction has also been recently proposed in the Au(I)-catalyzed
dimerization of propiolic acid. See: Luo, T.; Dai, M.; Zheng, S.-L.;
Schreiber, S. L. Org. Lett. 2011, 13, 2834.
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