A dual-catalysis approach to the asymmetric steglich rearrangement and catalytic enantioselective addition of O-acylated azlactones to isoquinolines

Article abstract of DOI:10.1021/ja208156z

A dual-catalysis approach, namely the combination of an achiral nucleophilic catalyst and a chiral anion-binding catalyst, was applied to the Steglich rearrangement to provide α,α-disubstituted amino acid derivatives in a highly enantioselective fashion. Replacement of the nucleophilic co-catalyst for isoquinoline resulted in a divergent reaction pathway and an unprecedented transformation of O-acylated azlactones. This strategy provided highly substituted α,β-diamino acid derivatives with excellent levels of stereocontrol.

Full text of DOI:10.1021/ja208156z

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pubs.acs.org/JACS
A Dual-Catalysis Approach to the Asymmetric Steglich Rearrangement
and Catalytic Enantioselective Addition of O-Acylated Azlactones to
Isoquinolines
Chandra Kanta De, Nisha Mittal, and Daniel Seidel*
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854,
United States
S Supporting Information
b
ABSTRACT: A dual-catalysis approach, namely the com-
bination of an achiral nucleophilic catalyst and a chiral
anion-binding catalyst, was applied to the Steglich rearran-
gement to provide α,α-disubstituted amino acid derivatives
in a highly enantioselective fashion. Replacement of the
nucleophilic co-catalyst for isoquinoline resulted in a diver-
gent reaction pathway and an unprecedented transforma-
tion of O-acylated azlactones. This strategy provided highly
substituted α,β-diamino acid derivatives with excellent
Figure 1. New concept for the azlactone rearrangement.
levels of stereocontrol.
rearrangement. Specifically, we envisioned that intermediates
such as 4 (Figure 1), featuring an achiral nucleophilic catalyst and
a chiral anion receptor, should enable the efficient enantioselec-
tive conversion of 1 into 2.
he rearrangement of O-acylated azlactones to the corre-
We initiated our studies by exposing O-acylated azlactone 1
T
sponding C-acylated products (1 f 2, Figure 1), also known
(Ar = 4-MeO-C6H4) to a combination of DMAP (20 mol %) and
various HB catalysts (20 mol %) at À78 °C in toluene (Table 1).
Relatively fast consumption of starting material was observed in
all cases. Catalysts 5a and 5b, compounds that were previously
used successfully in the kinetic resolution and desymmetrization
of amines, gave rise to poor selectivities. Promising results were
obtained with the Takemoto catalyst14 (5c, entry 3), while the
highest level of selectivity was observed with catalyst 5d (79% ee,
entry 4), previously developed by the Jacobsen group.11c A brief
survey of the aryl group in 1 revealed the superiority of 3,5-di-
MeO-C6H3 over 4-MeO-C6H4, allowing for the recovery of
product 2 in 89% ee (entry 10). Interestingly, this change also
significantly decreased the reaction time from 4 h to 1 h, likely the
result of an increase in nucleophilicity of the anionic intermedi-
ate. Notably, azlactone rearrangements are typically conducted at
higher temperatures and to our knowledge have never been
reported to proceed at temperatures as low as À78 °C. Im-
portantly, the use of DMAP as the only catalyst under otherwise
identical conditions led to very sluggish reactions. These com-
bined observations nicely illustrate the unique reactivity profile of
the dual-catalyst system.
as the Steglich reaction,1,2 represents a valuable strategy for the
synthesis of α,α-disubstituted amino acid derivatives.3 Ever since
the seminal work by Ruble and Fu, who described the first
catalytic enantioselective version of this transformation,4 this
reaction has become and continues to be a fertile testing ground
for the evaluation of new chiral nucleophilic catalysts. The latter
currently include chiral variants of 4-(dimethylamino)pyridine
(DMAP), phosphines, isothioureas, imidazoles, carbenes, and
betaines.5À7 These reactions typically proceed through chiral ion
pairs related to 3, with the betaine catalyst developed by Ooi et al.
being a notable exception.5i Here we report a conceptually new
strategy for the catalytic enantioselective Steglich reaction that
takes advantage of a dual-catalysis approach. In addition, we
describe the unprecedented reaction between O-acylated azlac-
tones and isoquinolines, a transformation that is likely outside
the realm of traditional nucleophilic catalysis.
We recently developed a new concept for asymmetric nucleo-
philic catalysis in which a simple nucleophilic catalyst such as
DMAP is used in combination with a chiral hydrogen-bonding
(HB) catalyst.8,9 Anion binding10À12 is thought to be a vital
component of this strategy, which was successfully applied to a
number of asymmetric acyl-transfer reactions.13 These include
the kinetic resolution of benzylic,8a propargylic,8b and allylic
amines8c as well as the desymmetrization of meso-1,2-diamines.8d
Very recently, the Jacobsen group, who pioneered much of the
early work on chiral anion binding catalysis, reported a highly
enantioselective acylation of silyl ketene acetals with acyl fluo-
rides via a closely related dual-catalysis approach.11q
The scope of the azlactone rearrangement was explored under
the optimized conditions (Chart 1). A number of substrates were
tested, and all reactions went to full conversion within short
reaction times. The rearranged products 2 were recovered with
high levels of enantioselectivity.15
In an effort to extend our dual-catalysis strategy to other acyl-
transfer reactions, we decided to apply it to the Steglich
Received: August 29, 2011
Published: September 29, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja208156z J. Am. Chem. Soc. 2011, 133, 16802–16805
|

Journal of the American Chemical Society
COMMUNICATION
Table 1. Evaluation of Reaction Parameters for the Azlactone
Rearrangementa
Figure 2. Potential reaction pathways for isoquinoline-derived ion pairs.
Table 2. Evaluation of Reaction Parameters for the Azlactone
Additiona
entry
R
Ar
cat time (h) yield (%) ee (%)
1
Ph 4-MeO-C6H4
Ph 4-MeO-C6H4
Ph 4-MeO-C6H4
Ph 4-MeO-C6H4
Ph 4-MeO-C6H4
Ph 4-MeO-C6H4
Ph 4-MeO-C6H4
Bn 4-MeO-C6H4
Ph 1-naphthyl
5a
5b
5c
5d
5e
5f
4
4
4
4
4
4
4
8
5
1
57
54
44
72
77
52
58
52
53
70
12
10
entry cat. temp (°C)
solvent
PhMe
time (h) yield (%) dr ee (%)
2
1
2
3
4
5
6
5a
5b
5d
5e
5f
À10
À10
À10
À10
À10
À10
À10
À10
À10
À20
À20
À25
À25
À25
À25
À25
À25
À25
2
90
91
85
91
91
92
85
86
88
88
65
91
90
92
91
92
61
52
70:30 À10
3
À74
79
PhMe
2
69:31
86:14
83:17
82:18
77:23
69:31
84:16
88:12
92:08
92:08
95:05
96:04
96:04
91:09
96:04
96:04
95:05
0
64
50
52
35
13
69
73
77
82
88
91
93
84
93
93
93
4
PhMe
2.5
3
5
71
PhMe
6
55
PhMe
3
7
8b
5g
5d
5d
47
5g
PhMe
3
60
7b 5d
PhMe
2.5
4
9
85
8
9
5d
5d
TBME
mesitylene
mesitylene
pentane
10
Ph 3,5-di-MeO-C6H3 5d
89
a Reactions were performed on a 0.1 mmol scale. The ee’s were
determined by HPLC analysis; see the Supporting Information for
details. b Reaction was performed at À60 °C.
3
10 5d
11c 5d
12 5d
13 5d
14 5d
15 5d
16 5dd
17c 5dd
18c 5dd
10
24
mes/pent (2:1) 14
mes/pent (1:1) 17
mes/pent (1:2) 22
Chart 1. Scope of the Azlactone Rearrangement
mes/hex (1:2)
20
mes/pent (1:2) 27
mes/pent (1:3) 24
mes/pent (1:4) 24
a Reactions were performed on a 0.1 mmol scale. The dr’s and ee’s were
determined by HPLC analysis; see the Supporting Information for details.
b Starting material with Ar = 3,5-di-MeO-C6H3 was used. c Reaction was
incomplete. d 10 mol % of catalyst was used.
In order to explore this proposed transformation, we exposed
1a to 1.2 equiv of isoquinoline in the presence of various HB
catalysts (Table 2). Isoquinoline possesses a significantly lower
nucleophilicity than DMAP, and consequently, the reactions
were conducted at elevated temperatures. Gratifyingly, the
reaction proceeded as anticipated to provide product 7a in
typically excellent yield and with promising levels of enantio-
selectivity. Under the initial screening conditions (toluene,
À10 °C), catalyst 5d again gave rise to the best results, enabling
the isolation of 7a in 85% yield with dr = 86:14 and 64% ee in a
reaction that was completed within 2.5 h (entry 3).
The stereoselectivity could be improved to 73% ee (dr =
88:12, 3 h reaction time) upon exchange of toluene for mesity-
lene (entry 9). Interestingly, under the same conditions but in the
absence of the HB catalyst, the reaction went to completion
within 11.5 h, and 7a was formed in 95% yield (dr = 64:36),
indicating a substantial background rate. Lowering the reaction
temperature to À20 °C increased the enantioselectivity to 77%
In considering alternate reaction pathways, we reasoned that
replacement of the nucleophilic co-catalyst DMAP for a different
nucleophilic species might result in the incorporation of the latter
into the final product. Specifically, we envisioned the reaction of
an O-acylated azlactone with isoquinoline in the presence of a
HB catalyst to give intermediates such as 6 (Figure 2). Ion pair 6
could rearrange in the usual manner, with isoquinoline acting as a
nucleophilic promoter to give rearranged product 2 (pathway a).
Alternatively, the acylisoquinolinium ion could be attacked by
the enolate at the 1-position of the isoquinoline ring to give
rise to the highly functionalized α,β-diamino acid derivative 7
(pathway b). While catalytic enantioselective additions to iso-
quinolines have been limited to relatively few examples,16À18
significant efforts have focused on the catalytic enantioselective
preparation of α,β-diamino acid derivatives.19À21
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COMMUNICATION
Chart 2. Scope of the Azlactone Addition to Isoquinolines
the nucleophilic catalyst for a stoichiometric amount of isoquino-
line enabled an unprecedented reaction of O-acylated azlactones,
a process that provides rapid access to densely functionalized
α,β-diamino acid derivatives.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
characterization data, including X-ray crystallographic data in
CIF format for 7a and 7j. This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
seidel@rutchem.rutgers.edu
’ ACKNOWLEDGMENT
Financial support received from the Charles and Johanna
Busch Memorial Fund at Rutgers, The State University of New
Jersey, is gratefully acknowledged. We thank Dr. Tom Emge for
crystallographic analysis. D.S. is a fellow of the Alfred P. Sloan
Foundation and the recipient of an Amgen Young Investigator
Award.
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obtained at this temperature when pentane was used as the
solvent (entry 11). However, in this case, the reaction did not go
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nucleophilic catalyst. We also demonstrated that replacement of
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Journal of the American Chemical Society
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(23) It is interesting to note that the facial selectivity for the
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authors have previously noted the need for rapid chromatographic
purification. See, for instance, refs 4 and 5a.
(16) For examples, see: (a) Takamura, M.; Funabashi, K.; Kanai, M.;
Shibasaki, M. J. Am. Chem. Soc. 2000, 122, 6327. (b) Takamura, M.;
Funabashi, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2001,
123, 6801. (c) Alexakis, A.; Amiot, F. Tetrahedron: Asymmetry 2002,
13, 2117. (d) Taylor, M. S.; Tokunaga, N.; Jacobsen, E. N. Angew. Chem.,
Int. Ed. 2005, 44, 6700. (e) Yamaoka, Y.; Miyabe, H.; Takemoto, Y.
J. Am. Chem. Soc. 2007, 129, 6686. (f) Black, D. A.; Beveridge, R. E.;
Arndtsen, B. A. J. Org. Chem. 2008, 73, 1906.
(17) For a review on catalytic enantioselective additions to aromatic
heterocycles, see: Ahamed, M.; Todd, M. H. Eur. J. Org. Chem. 2010, 5935.
(18) For selected reviews on catalytic enantioselective Mannich
reactions, see: (a) Friestad, G. K.; Mathies, A. K. Tetrahedron 2007,
16805
dx.doi.org/10.1021/ja208156z |J. Am. Chem. Soc. 2011, 133, 16802–16805