Trisubstituted 2-trifluoromethyl pyrrolidines via catalytic asymmetric michael addition/reductive cyclization

Article abstract of DOI:10.1021/ol500679w

The stereoselective synthesis of trisubstituted 2-trifluoromethyl pyrrolidines by asymmetric Michael addition/hydrogenative cyclization is described. The direct organocatalytic addition of 1,1,1-trifluoromethylketones to nitroolefins proceeds under mild reaction conditions and low catalyst loadings to provide Michael adducts in high yield with excellent diastereo- and enantioselectivity. Catalytic hydrogenation of the Michael adducts stereoselectively generates 2-trifluoromethylated pyrrolidines bearing three contiguous stereocenters. A stereospecific route to epimeric 2-trifluoromethyl pyrrolidines from a common intermediate is described.

Full text of DOI:10.1021/ol500679w

Letter
pubs.acs.org/OrgLett
Trisubstituted 2‑Trifluoromethyl Pyrrolidines via Catalytic
Asymmetric Michael Addition/Reductive Cyclization
Michael T. Corbett, Qihai Xu, and Jeffrey S. Johnson*
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
S
* Supporting Information
ABSTRACT: The stereoselective synthesis of trisubstituted 2-trifluoromethyl
pyrrolidines by asymmetric Michael addition/hydrogenative cyclization is
described. The direct organocatalytic addition of 1,1,1-trifluoromethylketones
to nitroolefins proceeds under mild reaction conditions and low catalyst
loadings to provide Michael adducts in high yield with excellent diastereo- and
enantioselectivity. Catalytic hydrogenation of the Michael adducts stereo-
selectively generates 2-trifluoromethylated pyrrolidines bearing three con-
tiguous stereocenters. A stereospecific route to epimeric 2-trifluoromethyl
pyrrolidines from a common intermediate is described.
his letter describes a simple reaction platform for the
Scheme 1. Strategy for the Synthesis of Highly Substituted 2-
T
enantio- and diastereoselective preparation of 2-trifluor-
Trifluoromethylated Pyrrolidines
omethyl pyrrolidines. Pyrrolidines are highly desirable building
blocks due to their prevalence in the pharmaceutical and
agrochemical industries.1 Given the unique physicochemical
properties engendered by the trifluoromethyl (CF3) group,2
considerable effort has been directed toward new method-
ologies for the preparation of CF3-containing compounds.3 A
productive merger of these functionalities has led to interest in
the development of methodologies for the construction of N-
containing organofluorine compounds.4 Consequently, meth-
ods toward the generation of optically active 2- and 3-
trifluoromethylated pyrrolidines have garnered increasing
attention in the literature. A common strategy for the synthesis
of 3-trifluoromethylated proline derivatives utilizes the
asymmetric 1,3-dipolar cycloaddition of azomethine ylides
and CF3-substituted alkenes.5 A majority of the methods for the
preparation of optically active 2-trifluoromethylated pyrroli-
dines rely on the use of chiral starting materials or auxiliaries.6
A more attractive method to generate these 2-trifluoro-
methylated pyrrolidines would utilize de novo pyrrolidine
syntheses to rapidly generate molecular complexity from simple
starting materials via asymmetric catalysis.7 Here, we report a
formal (3 + 2)-annulation strategy for the preparation of
trisubstituted 2-trifluoromethyl pyrrolidines via organocatalytic
asymmetric Michael addition of 1,1,1-trifluoromethylketones to
nitroolefins followed by diastereoselective reductive cyclization.
A number of highly functionalized pyrrolidines have
previously been accessed via asymmetric Michael addition/
reductive cyclization protocols employing aldehydes,8 aryl
ketones,9 α-keto ester/amides,10 and β-keto esters11 as two-
carbon donor synthons with nitroolefins for the de novo
synthesis of substituted pyrrolidines. We postulated that an
analogous (3 + 2)-annulation approach could be utilized in the
preparation of trisubstituted 2-trifluoromethyl pyrrolidines 4
(Scheme 1a). The realization of this approach would require
the invention of a heretofore unknown catalytic, enantiose-
lective Michael addition of 1,1,1-trifluoromethylketones 1 to
nitroolefins 2 that would provide the requisite γ-nitro carbonyl
intermediate 3. An attractive feature of the projected
experimental plan was the high level of atom efficiency
associated with the catalytic addition/hydrogenative cyclization
sequence (Scheme 1b).
Despite significant interest in the synthetic utility of 1,1,1-
trifluoromethylketones as electrophiles, their application as
nucleophiles has gone relatively unexplored.12 Yan et al.
recently reported the formal (4 + 2)-cycloaddition of 4,4,4-
trifluoroacetoacetates to β,γ-unsaturated-α-keto esters;13 how-
ever, the direct catalytic asymmetric α-functionalization of
simple 1,1,1-trifluoromethylketones is to the best of our
knowledge unknown. While the electron-withdrawing nature
of the CF3 group engenders enhanced C−H acidity relative to
Received: March 4, 2014
Published: April 18, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
normal ketones, by corollary it also stabilizes the resultant
enolate, rendering trifluoromethylketone enolates poorly
nucleophilic.12 We anticipated that application of a bifunctional
catalyst bearing thiourea and tertiary amine moieties would
provide a pseudointramolecular pathway to overcome this
inherent barrier to reactivity.14 We sought to harness the
Brønsted acid/base ambifunctionality of these catalyst systems
to develop the Michael addition of 1,1,1-trifluoromethylketones
to nitroolefins.15
We began our studies by evaluating the feasibility of the
Michael addition of 1,1,1-trifluoromethylketone 1a to nitro-
olefin 2a. Our experiments led to the use of bifunctional
catalyst QD-TU,16 affording the desired γ-nitro carbonyl 3aa in
quantitative yield with >20:1 dr and 95:5 er (Table 1, entry 1;
see the Supporting Information (SI) for full optimization
studies). A variety of reaction partners were next examined in
the asymmetric Michael addition of 1,1,1-trifluoromethyl-
ketones 1 to nitroolefins 2 (Table 1). Electron-releasing and
-withdrawing ortho-substituted aromatic nitroolefins were well
tolerated, providing Michael adducts 3ab−3ae as single
diastereomers with excellent levels of enantioselectivity (entries
2−5). Less sterically encumbering substitution at the meta-
position of the nitroolefin resulted in a slight drop in diastereo-
and enantiocontrol providing 3af in 98% yield with 19:1 dr and
93.5:6.5 er (entry 6). Examination of electron-releasing and
-withdrawing para-substituents on the aromatic nitroolefin
revealed significant electronic effects with respect to the
diastereo- and enantioselectivity of the reaction (entries 7−
11). Whereas electron-withdrawing groups (Br, NO2, CN)
provided Michael adducts 3ag−3ai with high levels of
selectivity, electron-releasing groups (Me and OMe) provided
3aj and 3ak, respectively, in diminished diastereo- and
enantioselectivity. This electronic effect was also observed in
the reaction of electron-rich heteroaromatic 2-thienyl- and 3-N-
Ts-indoyl-substituted nitroolefins 2l and 2m, which afforded
their respective Michael adducts 3al and 3am in diminished
enantioselectivity (entries 12 and 13). Despite noticeably
reduced reactivity, alkenyl and aliphatic nitroolefins were also
found to be competent reaction partners providing 3an and
3ao in moderate conversion, but with high diastereo- and
enantiocontrol (entries 14 and 15).
Table 1. Scope of Michael Addition of 1,1,1-
a
Trifluoromethylketones 1 to Nitroolefins 2
We next turned our attention to examining electronic and
structural variations of the 1,1,1-trifluoromethylketone donor 1
by placing methoxy groups at the ortho-, meta-, and para-
positions of the aromatic ring. Although sterically demanding
ortho-substituted 1b required longer reaction times (3 h) to
achieve full conversion, 3bg was obtained in 97% yield with
>20:1 dr and 96:4 er (entry 16). Less sterically encumbered
electron-releasing nucleophiles 1c and 1d proceeded efficiently
under the standard reaction conditions to provide 3cg and 3dg,
respectively, in excellent yield and diastereoselectivity, but with
reduced enantioselectivity (entries 17 and 18). This electronic
effect was confirmed by utilizing electron-withdrawing para-
substituted 1e as the nucleophile, which provided 3eg in 95%
yield with 17:1 dr and 93:7 er (entry 19). It is worth noting that
subjecting 3eg, which possesses enhanced C−H acidity, to silica
gel chromatography resulted in erosion of diastereoselectivity
from 17:1 to 2:1 dr presumably due to facile epimerization via
enol formation. Lastly, we employed aliphatic 1,1,1-trifluoro-
methylketone 1f in the Michael addition to nitroolefin 2a to
provide 3ea in 73% yield with >20:1 dr and 97:3 er, although
the reaction required a higher catalyst loading and longer
reaction times to achieve acceptable conversion (entry 20). The
majority of adducts are crystalline solids. A single recrystalliza-
tion of 3ai provided a useful upgrade in the enantiomeric
composition (to 99.5:0.5 er).
b
c
d
entry
1
2
3
yield (%)
dr
er
1
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
C6H5
3aa
3ab
3ac
3ad
3ae
3af
98
95
94
96
98
98
99
99
96
96
91
97
92
>20:1
>20:1
>20:1
>20:1
>20:1
19:1
95:5
96.5:3.5
97.5:2.5
96.5:3.5
94:6
2
2-BrC6H4
3
2-NO2C6H4
2-CF3C6H4
2-OMeC6H4
3-ClC6H4
e
4
5
6
93.5:6.5
95.5:4.5
95.5:4.5
96:4
7
4-BrC6H4
3ag
3ah
3ai
>20:1
7:1
8
4-NO2C6H4
4-CNC6H4
4-MeC6H4
4-OMeC6H4
2-thienyl
9
>20:1
8:1
10
11
12
3aj
93:7
3ak
3al
6:1
86.5:13.5
91.5:8.5
87.5:12.5
87:13
94:6
16:1
f
13
3-N-Ts-indoyl
CHCHC6H5
cyclohexyl
4-BrC6H4
3am
3an
3ao
3bg
3cg
3dg
3eg
3fa
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
g
14
15
16
17
18
19
62 (65)
42 (43)
97
g
96:4
4-BrC6H4
96
88:12
87:13
93:7
4-BrC6H4
97
4-BrC6H4
95
h
g
20
C6H5
73 (82)
97:3
Having developed a highly diastereo- and enantioselective
Michael addition to access γ-nitro trifluoromethyl ketones 3
(Table 1), we sought to exploit this functional array toward the
synthesis of 3,4-diaryl-2-trifluoromethyl pyrrolidines 4 (Scheme
2). Subjecting the crude products obtained via Michael addition
to Raney-Ni hydrogenation conditions resulted in the clean
formation of the desired pyrrolidines with excellent levels of
diastereocontrol (>20:1 in all cases) bearing an all cis-
relationship as determined by NOESY analysis. Attempts
employing a one-pot protocol were promising; however, the
molecular sieves from step 1 were found to be detrimental to
the hydrogenation step, resulting in only moderate conversions.
a
Reactions were performed with 1 (0.21 mmol) and 2 (0.20 mmol)
b
and proceeded to full conversion as adjudged by TLC. Isolated yield.
The diastereomers were not separable, and this represents the
c
combined yield. The diastereomeric ratio was determined by 19F
d
NMR spectroscopic analysis of the crude product. The enantiomeric
ratio was determined by HPLC or SFC analysis on a chiral stationary
phase. The enantiomeric ratio was determined following reduction of
3ad with NaBH4 (see the SI). The reaction was performed at 0 °C for
e
f
g
3 h. Number in parentheses is conversion of nitroolefin as determined
h
by 1H NMR spectroscopic analysis of the crude product. The
reaction was performed employing QD-TU (10 mol %) at 0 °C for 12
h.
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Organic Letters
Letter
the Michael addition of 1a to 2g by addition of NaBH4/MeOH
mixture results in the highly diastereoselective reduction of the
carbonyl to provide 5ag in 87% yield. An X-ray diffraction study
of alcohol 5 was performed to assign the relative and absolute
stereochemistries as (2S,3S,4R).17 By analogy, the Michael
adducts were assigned as (3S,4R)-3 and the pyrrolidines as
(2S,3S,4R)-4. Next, sequential carbonyl/nitro reduction was
employed to provide the Boc-protected amino alcohol 6
bearing three contiguous stereocenters in 80% overall yield
from 3aa as a single diastereomer.
Scheme 2. Synthesis of Enantioenriched 2-Trifluoromethyl
Pyrrolidines
a
The high syn-selectivity observed in the NaBH4 reduction of
Michael adducts 3aa and 3ag (Scheme 3) led us to pursue a
unified strategy to access both C(2)-epimers of 2-trifluoro-
methylated pyrrolidine 4a. We envisioned stereospecific
intramolecular SN2-displacement of an alcohol derivative by
the terminal amine would provide 2-epi-4a (Scheme 4).
Scheme 4. Access to Diastereomerically Pure Epimeric 2-
Trifluoromethylated Pyrrolidines
a
Reactions were performed as described in Table 1. The yield is for
both diastereomers. The diastereomeric ratio was determined by 19F
NMR spectroscopic analysis of the crude product. The enantiomeric
ratio was determined by HPLC analysis on a chiral stationary phase. b
Number in parentheses is dr following column chromatography. c The
Michael addition was performed at −10 °C for 1 h.
In addition to bis(phenyl) pyrrolidine 4a, ortho- and para-
substituents were tolerated at both the 3- and 4-positions of the
pyrrolidine providing products 4b−4e in high yield and
enantioselectivity. No erosion in diastereo- or enantiomeric
composition was observed during the hydrogenation, indicating
that neither epimerization nor retro-Michael pathways are
operative during the reaction.
In addition to providing expedient access to new classes of
enriched 2-trifluoromethyl pyrrolidines, the obtained Michael
adducts 3 are also amenable to a number of other secondary
transformations (Scheme 3). Employing a modified workup to
Reduction of the alcohol with NaBH4, conversion of the free
alcohol to its derived mesylate, and hydrogenation of the nitro
group provided the unstable primary amine intermediate 7. The
latter was immediately treated with DBU to induce cyclization
to diastereomerically pure 2-epi-4a in 52% yield over the four
steps.
In conclusion, we have developed a formal (3 + 2)-
annulation strategy for the highly selective synthesis of
trisubstituted 2-trifluoromethyl pyrrolidines via asymmetric
Michael addition/reductive cyclization. A direct catalytic
Michael addition of 1,1,1-trifluoromethylketones to nitroolefins
was developed, providing access to 3,4-disubstituted-5-nitro-
1,1,1-trifluoromethylketones with excellent levels of diastereo-
and enantioselectivity. The obtained Michael adducts were then
utilized in a diastereoselective reductive cyclization to afford
functionalized 3,4-disubstituted-2-trifluoromethyl pyrrolidines
bearing three contiguous stereocenters. We also demonstrated
that C(2)-epimeric 2-trifluoromethyl pyrroldines can be
accessed from a common intermediate with excellent
diastereocontrol utilizing an intramolecular SN2-displacement.
Scheme 3. Secondary Transformations of Michael Adducts
and Determination of Relative and Absolute
Stereochemistries
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization of the products, and
CIF data for CCDC 979177. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jsj@unc.edu.
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Organic Letters
Letter
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The project described was supported by Award No. R01
GM103855 from the National Institute of General Medical
Sciences. X-ray crystallography was performed by Dr. Peter S.
White (UNC Chapel Hill).
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