Stereoselective Lewis base catalyzed formal 1,3-dipolar cycloaddition of azomethine imines with mixed anhydrides

Article abstract of DOI:10.1039/c4sc02612h

Stereoselective synthesis of pyrazolidinones via dipolar cycloaddition of azomethine imines with active esters under Lewis base catalysis is presented. The active esters are readily generated in situ from the corresponding acids. Products, which are obtai

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Stereoselective Lewis Base Catalyzed Formal 1,3-
Dipolar Cycloaddition of Azomethine Imines with
Mixed Anhydrides
Cite this: DOI: 10.1039/x0xx00000x
Received
Accepted
Lena Hesping, Anup Biswas, Constantin G. Daniliuc, Christian Mück-Lichtenfeld*
and Armido Studer*
DOI: 10.1039/x0xx00000x
www.rsc.org/
catalysis.6 Chen,7 Sibi,8a Maruoka9 and others8b-d disclosed
Stereoselective synthesis of pyrazolidinones via dipolar
cycloaddition of azomethine imines with active esters under
Lewis base catalysis is presented. The active esters are readily
generated in situ from the corresponding acids. Products,
which are obtained with excellent diastereocontrol and high
enantioselectivity, contain along with the pyrazolidinone core
also the tetrahydroisoquinoline structural motif. Theoretical
studies give insight into the mechanism of the formal
cycloaddition reaction.
enantioselective cycloadditions of azomethine imines with electron
poor alkenes and the corresponding enantioselective dipolar
cycloaddition with electron-rich alkenes was reported by Leighton10
and Maruoka.11 Very recently, Wang et al. published amine-catalyzed
enantioselective 1,3-dipolar cycloadditions of aldehydes to C,N-cyclic
azomethine imines, where intermediately generated electron-rich
enamines are the actual dipolarophiles.12
As a continuation of our studies on oxidative carbene catalysis,13 we
decided to investigate the reaction of aliphatic aldehydes with
azomethine imines in the presence of a N-heterocyclic carbene (NHC)
under oxidative conditions as a novel method for the preparation of
Introduction. Many natural products with interesting biological
activity are based on the C1-substituted tetrahydroisoquinoline core 1 1
.
It is obvious that many synthetic methods have been developed for the
stereoselective construction of this important core structure.2
compounds of type 3. Disappointingly, we found that the azomethine
imine 1a reacted with phenylacetaldehyde in the presence of triazole
precatalyst and DBU under oxidative conditions to cycloadduct 414
(Figure 2). The targeted pyrazolidinone 3aa was not identified.
Pyrazolidinones
2 are also interesting compounds which have found
wide applications in different fields. This structural motif can be found
in dyes, in pharmaceutical and agricultural relevant compounds.3
Pyrazolidinones of type
and should therefore be valuable compounds which have so far not
been intensively investigated.4 Herein we present
novel
straightforward method for the stereoselective synthesis of compounds
of type via 1,3-dipolar cycloaddition of azomethine imines with
3 combine the structural features of both 1 and
2
a
3
mixed anhydrides.
Figure 1. Compounds 3 combining structural features of both 1 and 2.
Results and Discussion.
Experimental studies. The intermolecular [2+3] cycloaddition of
azomethine imines with alkynes is known for more than 45 years,5 and
an enantioselective version was developed by Fu et al. using Cu-
Figure 2. Transformation of 1a to either 4 or 3aa under different conditions.
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The reaction of phenylacetaldehyde with the NHC, oxidation to the low in that case (Table 1, No. 5). As compared to
acylazolium ion and subsequent enolization15 to give an enolate of type provided slightly improved selectivities (Table 1, No. 6, 7). From the
is obviously slower than direct reaction of phenylacetaldehyde with experiment run with the acid chloride as a substrate it became obvious
1a under the applied basic conditions. We therefore switched to that the leaving group at the activated acid derivatives may play an
enolates of type as potential dipolarophiles for the [2+3] important role on the selectivity. Therefore, we tested the in situ
cycloaddition with 1a. It is important to note that stereoselective generated mixed anhydride derived from 2,4,6-trimethylbenzoyl
reactions with enolates formally deriving from acylammonium or chloride. Disappointingly, both diastereoselectivity and
D, catalyst E
A
B
acylpyridinium ions have been intensively studied in asymmetric enantioselectivity decreased (Table 1, No. 8). However, the mixed
catalysis.16 However, the application of such enolates as dipolarophiles anhydride formed from benzoyl chloride gave 3aa with excellent ee
in the reaction with azomethine imines is unknown.
(99%) and high diastereoselectivity (94:6) in good yield (Table 1,
Pleasingly, the mixed anhydride in situ generated from phenylacetic No. 9). We found that reaction works far more efficiently at room
acid and isobutyric acid chloride17 in the presence of NEt3 reacted with temperature without diminishing selectivity to a large extent and
1a and DMAP (10 mol %) to pyrazolidinone 3aa which was isolated in pyrazolidinone 3aa was obtained in 95% yield, 94:6 diastereoselectivity
56% yield as a 99:1 mixture of diastereoisomers, as analyzed by with 98% ee (Table 1, No. 10). The absolute and relative configuration
HPLC.18 Encouraged by this result, we continued the studies by testing of 3aa were unambiguously assigned by X-ray analysis (Figure 4).
the chiral commercially available Lewis bases C 19
, , which
D20 and E 20
have successfully been used in asymmetric catalysis (Figure 3).21,22
Figure 3. Chiral Lewis bases
C
,
D
and
E
used in this study.
Table 1. Reaction optimization.
Figure 4. X-ray structure of pyrazolidinone 3aa. Thermals ellipsoids are shown
with 30% probability.
No.
R
Cat.
Base
Temp (°C)/
Time (h)
rt / 16
rt / 14
0 / 23
0 / 23
0 / 24
0 / 43
rt / 43
5 / 24
dr
ee
(%)
6
rac
56c
76c
92c
64
84
32
99
98
Yield
(%)
46
71
90
74
12
64
86
(mol%)
C (10)
C (10)
D (10)
D (20)
D (10)
E (10)
E (10)
E (10)
E (10)
E (10)
1
2
3
4
5
6
7
8
9
i-Pr
i-Pr
i-Pr
i-Pr
Et3Na
95:5
95:5
67:33
91:9
99:1
95:5
94:6
83:17
94:6
94:6
i-Pr2NEta
i-Pr2NEtb
i-Pr2NEta
i-Pr2NEtb
i-Pr2NEta
i-Pr2NEtb
i-Pr2NEtb
i-Pr2NEtb
i-Pr2NEtb
d
-
i-Pr
i-Pr
Mes
Ph
33
68
95
5 / 24
rt / 16
10
Ph
a With 2.1 equiv base. b With 1.1 equiv base. c Other enantiomer formed as
major isomer. d Phenylacetyl chloride used as substrate.
With catalyst C, 3aa was obtained in moderate to good yield with high
diastereoselectivity (95:5) but no or very low enantioselectivity by
using i-Pr2NEt or Et3N as base (Table 1, No. 1, 2). Pleasingly,
tetramisole
D (10 mol%) provided 3aa with significant ee (56%) but
low diastereoselectivity (67:33) which was improved to 91:9 upon
increasing the catalyst loading to 20 mol% (Table 1, No. 3, 4). Likely,
back ground reaction, which is cycloaddition of the free ketene with the
azomethine imine, competes at lower catalyst loading. This assumption
is supported by the fact that the diastereoselectivity at lower catalyst
loading was significantly lower (67:33 versus 91:9) and accordingly
also the ee was lower. Enantioselectivity was determined by HPLC
analysis (see Supporting Information). With phenylacetyl chloride as
substrate, ee and dr were further improved; however, yield was very
Figure 5. Variation of the acetic acid component.
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With optimized conditions in hand, we tested scope and limitation of
the stereoselective cycloaddition by varying the acid component and
also the azomethine imine (Figures 5 and 6). All azomethine imines
used in this study were prepared according to literature procedures (see
Supporting Information). Phenylacetic acids with both electron-
donating and electron-withdrawing groups were tolerated to afford
in high selectivities and good to excellent yields. Moreover, the
sterically hindered 8-methyl-substituted azomethine imine 1c was a
good substrate and cycloadducts 3ca and 3ch were obtained in good to
excellent selectivitites with good yields.
All attempts to conduct the cycloaddition of the chiral ammonium
enolate derived from phenylacetic acid with in situ generated C,N-
cyclic azomethine imines according to the elegant work of Maruoka et
al.9 failed. Moreover, the N,N-cyclic azomethine imine 2-benzylidene-
5-oxopyrazolidin-2-ium-1-ide, often used in dipolar cycloaddtions,6-8
did not react with the in situ generated ammonium enolate under the
tested conditions.
cycloaddition products 3ac-3ag in good to high yields and high enantio-
and diastereoselectivity. Furthermore, the sterically demanding
o-tolylacetic acid could be utilized furnishing 3ab with high ee and dr.
Excellent selectivity and good yield were achieved with
2-naphthylacetic acid as the substrate (product 3ah
, >99% ee,
dr = 98:2, 82% yield). Additionally, the heteroaromatic substrate
thiopheneacetic acid worked well providing cycloadduct 3ai in good
yield and excellent diastereoselectivity, albeit with slightly lower ee.
To show the value of pyrazolidinones as building blocks in synthesis,
we tested
a first follow-up reaction. To this end, N-benzoyl
deprotection in pyrazolidinones 3aa and 3da was achieved with
DBU/LiBr23 in MeOH to give the corresponding NH-pyrazolidinones.
N-N bond cleavage by reduction with Raney-Ni/H2 eventually provided
stereospecifically the tetrahydroisoquinolines 5aa and 5da in good
yields (Figure 7). These
-amino acid derivatives might be valuable for
preparation of

-peptides.24
Figure 7. Reductive cleavage of the N-N bond for preparation of -amino acid
derivatives.
Theoretical studies of the mechanism. In our DFT study of the
mechanism of the cycloaddition reaction, we used TPSS-D3/def2-
TZVP and an implicit solvation model (COSMO), for details, see
Supporting Information. We chose the reaction of intermediate
F
deriving from phenylacetic acid and catalyst with azomethine imine
E
1a as our model system. The free energies, including solvation energies
in CH2Cl2 and thermodynamic corrections at 298 K starting from the
enolate
F and 1a are given in Figure 8.
Figure 6. Variation of the azomethine imine and the acetic acid component.
Next, differently substituted azomethine imines were tested in the
reaction with various acetic acid derivatives (Figure 6). Good to high
yields were obtained by using 7-bromo and 7-methoxy-substituted
azomethine imines 1d and 1e (see 3da-3eh). The electron rich
azomethine imine 1d provided significantly higher ee as compared to
the ee obtained with the Br-derivative 1e. Along these lines, 5-methyl-
substituted azomethine imine 1b afforded pyrazolidinones 3ba and 3bh
Figure 8. Chemical structure of enolate
TZVP+COSMO(CH2Cl2) free energies of the cycloaddition of
PR = 3aa/E
F and DFT calculated (TPSS-D3/def2-
F and 1a. RE = 1a/F,
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We were not able to locate a transition structure of synchronous
formation of both bonds (C-C and C-N) of the product (3aa). The
mechanism proceeds stepwise with C-C bond formation as the first step
Notes and references
a Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster, Germany. Email: studer@uni-
muenster.de; Web: http://www.uni-muenster.de/Chemie.oc/
(
TS-1), leading to an intermediate (IN-1), which subsequently forms
studer/; Fax: +49-251-83-36523; Tel: +49-251-83-33291
the second C-N bond with a very low barrier. Thus, the first step is
determining the rate and the stereochemical outcome of the process.
The preferred orientation of the two reactants in the pre-reactive
complex CP-1 and for TS-1 is exo (see Figure 9), in accordance with
the observed diastereoselectivity of the reaction. Moreover, the absolute
stereochemistry obtained in the calculations agreed with the
stereochemistry observed in the experiment. In the exo reaction, we
could not identify a tetrahedral intermediate IN-2 as for endo, the
†
Electronic Supplementary Information (ESI) available: Detailed
experimental procedures, and spectral data for all compounds, including
scanned images of 1H and 13C NMR spectra. CCDC reference number
1019046 for 3aa. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b000000x/
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4
Acknowledgement
4
We thank the University of Münster and the Deutsche
Forschungsgemeinschaft (DFG) within the SFB 858 (project Z1) for
supporting our research. We also thank Ulrich Schreiber for the
preparation of some cycloadducts.
configuration at the acetal stereocenter could not be unambiguously
assigned. The relative configuration of the other two stereocenters
could be assigned as trans by oxidation of
4 to 3aa. The structure of
3aa was assigned by X-ray analysis, see below.
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1
,
4
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