Rh(II)-Catalyzed Monocyclopropanation of Pyrroles and Its Application to the Synthesis Pharmaceutically Relevant Compounds

Article abstract of DOI:10.1021/acs.orglett.9b02250

Here we report Rh(II)-catalyzed monocyclopropanation reactions on pyrroles in the presence of aryldiazoacetates, providing the corresponding dearomatized products with high levels of enantioselectivity (up to >99% ee). Under the catalysis of Rh₂

Full text of DOI:10.1021/acs.orglett.9b02250

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rh(II)-Catalyzed Monocyclopropanation of Pyrroles and Its
Application to the Synthesis Pharmaceutically Relevant Compounds
Jiantao Fu,†,§ Nikolai Wurzer,†,‡,§ Verena Lehner,†,‡ Oliver Reiser,*,‡ and Huw M. L. Davies*,†
†Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United States
‡
̈
Institute of Organic Chemistry, University of Regensburg, Universitatsstraße 31, 93053 Regensburg, Germany
S
* Supporting Information
ABSTRACT: Here we report Rh(II)-catalyzed monocyclo-
propanation reactions on pyrroles in the presence of
aryldiazoacetates, providing the corresponding dearomatized
products with high levels of enantioselectivity (up to >99%
ee). Under the catalysis of Rh2(R-p-PhTPCP)4, a broad range
of pyrrole substrates and aryldiazoacetates are shown to be
compatible. Utilizing these valuable chiral building blocks, we
further demonstrate the application of this transformation by
synthesizing a homo-β-proline analog and a β-aminocarboxylic acid (β-ACC) derivative from the monocyclopropanated
product.
Scheme 1. Rh(II)-Catalyzed Reactions of Donor/Acceptor
Diazo Compounds with Furans and N-Protected Pyrroles
etal-catalyzed reactions of carbenes with electron-rich
Mheterocycles such as furans and pyrroles are syntheti-
cally useful transformations.1 When protected pyrroles are used
as substrates, the outcomes of the reaction usually depend on
the electronic nature of the heterocycle. With N-H or N-alkyl
derivatives the common transformation is alkylation at either
the C2 or C3 position via a zwitterionic pathway.2 However,
N-acylation causes the pyrrole to be less electron-rich and
cyclopropane intermediates are typically generated.2c,3 Re-
cently, the Reiser group described the asymmetric cyclo-
propanation of furans and pyrroles catalyzed by Cu(I)-
bis(oxazoline) with acceptor-only carbenes derived from alkyl
diazoacetates,4 and Davies and Reiser reported the asymmetric
monocyclopropanation of furans with donor/acceptor car-
benes (Scheme 1A).5 In addition, the Davies group has shown
that cyclopropanes generated from rhodium-catalyzed reac-
tions of donor/acceptor carbenes with N-Boc-pyrroles tend to
react further under the reaction conditions. Two established
reactions involving such reactive intermediates are the tandem
cyclopropanation/Cope rearrangement to form tropane
derivatives (Scheme 1B),6 and double cyclopropanation of
protected pyrroles to generate products with six new
stereogenic centers (Scheme 1C).7 Even though the
monocyclopropane intermediates derived from donor/accept-
or carbenes are implicated in these reactions, their isolation
was only possible when the substrate was used as the solvent
(in large excess) and still double cyclopropanation was a
competing reaction.7 In this manuscript, we describe
conditions for the monocyclopropanation of pyrroles and
demonstrate that the products can be applied to the syntheses
of a homo-β-proline analog as well as a β-aminocarboxylic acid
derivative, highlighting the potential medicinal relevance of our
strategy (Scheme 1D).
Recently we reported that Rh2(S-TCPTTL)4 is an excep-
tional catalyst for the asymmetric monocyclopropanation of
furans.5 However, applying this catalyst to N-protected
Received: June 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02250
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
pyrroles was not effective. When N-Boc pyrrole was used as the
substrate, the reaction yielded the undesired double cyclo-
propanated product in only trace amount, whereas the reaction
of N-tosyl pyrrole (1) and methyl p-bromophenyldiazoacetate
(2) gave some of the monocyclopropanated product 3, but
also a considerable amount of the double cyclopropanated
product 4 (Table 1, entry 1). In order to obtain a higher yield
With the optimized conditions in hand, the reaction of a
series of aryldiazoacetates with N-tosyl pyrrole was examined.
As shown in Scheme 2, many aryldiazoacetates are compatible
Scheme 2. Scope of Monocyclopropanation of N-
Tosylpyrrole
Table 1. Catalyst Optimization of N-Tosyl Pyrrole
a
Monocyclopropanation
b
c
d
entry
catalyst
3:4
3, yield (%)
3, ee (%)
1
2
3
4
5
6
7
Rh2(S-TCPTTL)4
Rh2(R-TCPTAD)4
Rh2(R-PTAD)4
Rh2(R-PTTL)4
Rh2(S-NTTL)4
Rh2(R-p-BrTPCP)4
Rh2(R-p-PhTPCP)4
Rh2(R-p-PhTPCP)4
1.4:1
3.1:1
6.7:1
22.7:1
>30:1
>30:1
>30:1
>30:1
33
39
54
59
60
59
56
64
66
−70
−97
−95
85
82
91
88
e
8
a
Reaction conditions: 2 (0.3 mmol) in 2.5 mL of DCM was added
over 90 min to a solution of the pyrrole substrate (0.6 mmol, 2.0
equiv) and catalyst (0.5 mol %) in 2.5 mL of DCM at 0 °C. The
b
reaction was allowed to stir for an additional 30 min. Ratios were
c
determined by reaction crude 1H NMR spectra. All yields are isolated
d
yields. The enantioselectivity was determined by chiral HPLC
e
analysis of the isolated product. The reaction was conducted at reflux
(40 °C).
and more selective monocyclopropanation reaction, a system-
atic catalyst screening was conducted using a wide variety of
chiral dirhodium catalysts that have been developed for the
reactions of donor/acceptor carbenes. A series of related
phthalimido catalysts were examined (entries 1−4, Figure 1).
with this chemistry, and the desired pyrrolocyclopropanes 3,
5−11 were obtained in good yields and with moderate to high
levels of enantioselectivity (75−96% ee). Modifications of the
electronic factors and substitution patterns on the aryl ring do
not have a major influence on the outcome of the reaction.
However, when the aryldiazoacetate contains a pyridine
functionality, the pyrrolocyclopropane 12 was formed with a
marked drop in enantioselectivity (67% ee).
Extension of the optimized conditions to functionalized
pyrroles did not lead to good results. For example, reaction
with the 2-methoxycarbonyl derivative 13 with the methyl
aryldiazoacetate 2 gave a low yield of the pyrrolocyclopropane
15 (34%) (Table 2, entry 1), even though the level of
enantioselectivity remained high (96% ee). We reasoned that
this may be due to inefficient trapping of the carbene by the
substrate under these conditions based on the crude 1H NMR.
Previously, we had found in our C−H functionalization studies
that, in reactions of carbenes with unactivated C−H bonds, the
yields tended to be higher when a trichloroethyl ester of the
carbene was used instead of the methyl ester.8 Hence, the
reaction was reoptimized for 13 using trichloroethyl p-
bromophenyldiazoacetate 14. The initial study was conducted
at 0 °C, which gave an even lower yield of the desired product
16 (entry 2), but when the reaction was conducted at reflux,
the yield was much higher (87%, entry 3) and the
Figure 1. Structures of rhodium catalysts used in the study.
Rh2(S-TCPTAD)4 gave a cleaner monocyclopropanation
reaction, but low enantioselectivity. Rh2(R-PTAD)4 and
Rh2(R-PTTL)4 resulted in high enantioselectivity, but still
the undesired side product 4 was formed. The naphthalimido
catalyst Rh2(S-NTTL)4 gave clean monocyclopropanation, and
3 was formed in 85% ee (entry 5). The triarylcyclopropane
carboxylate catalyst Rh2(R-p-BrTPCP)4 also gave cleanly the
monocyclopropanated product (entry 6), but Rh2(R-p-
PhTPCP)4 produced 3 in higher ee (91% ee, entry 7). In an
attempt to enhance yield, the reaction was performed at 40 °C
(reflux), but a lower enantioselectivity was observed with
marginal improvement in yield (entry 8).
B
DOI: 10.1021/acs.orglett.9b02250
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Organic Letters
Letter
Table 2. Optimization of Reaction with Methyl N-
Tosylpyrrole-2-carboxylate
Scheme 3. Scope of Monocyclopropanation of Pyrroles with
2,2,2-Trichloroethyl Aryldiazoacetates
a
b
c
Entry
R
temp (°C)
yield (%)
ee (%)
1
2
3
Me
0
0
40
40
34
15
87
69
96
98
97
97
CH2CCl3
CH2CCl3
CH2CCl3
d
4
a
Reaction conditions: 2 (0.3 mmol) in 2.5 mL of DCM was added
over 90 min to a solution of the pyrrole substrate (0.6 mmol, 2.0
equiv) and catalyst (0.5 mol %) in 2.5 mL of DCM. The reaction was
b
allowed to stir for an additional 30 min. All yields are isolated yields.
c
The enantioselectivity was determined by chiral HPLC analysis of
d
the isolated product. Reaction conducted with 0.1 mol % of catalyst.
The absolute stereochemistry of 16 was assigned by X-ray
crystallography study and used for analogy for other compounds.
enantioselectivity remained high (97% ee). Due to the
presence of the electron-withdrawing group on the pyrrole,
no double cyclopropanation was observed. In addition, the
reaction could be conducted with a 0.1 mol % catalyst loading,
and the same level of enantioselectivity could be obtained, but
the yield was slightly decreased (entry 4).
The reaction of N-tosylpyrroles with a range of trichlor-
oethyl aryldiazoacetates was examined, and the results are
summarized in Scheme 3. The reaction with 13 gave very high
enantioselectivity for pyrrolocyclopropanes 17−24 with
virtually every aryldiazoacetate (94% to >99% ee) including
the pyridine and thiophene derivatives 18 and 23. Notably, the
scalability of the reaction was demonstrated with 16 at 3 mmol
(10-fold increase), where the product was produced in 72%
yield and 96% ee at gram scale, even at a lowered catalyst
loading of 0.25 mol %. The only product that did not give
exceptionally high enantioselectivity was the o-fluorophenyl
derivative 25 (74% ee). The reaction also gave high
enantioselectivity with other 2-substituted pyrroles to form
26 and 27 in 96% and 86% ee, respectively. The low yield of
the trifluoroacetyl derivative 27 (24% yield) is presumably
because the pyrrole is now too deactivated for effective
trapping of the carbene. Good yields and high levels of
enantioselectivity of the monocyclopropanated product were
obtained with the unsubstituted pyrrole to form 28 and with
the N-Boc protected 2-methoxycarbonyl pyrrole to form 29.
However, compound 28 was prone to decomposition even at
room temperature within a few hours. The methyl ester
version, 3, nonetheless, was stable. While 2-substituted pyrroles
are well tolerated, placing the carboxylate group at the 3-
position resulted in no product formation. Instead, substantial
dimerization of the diazo compound was observed. This result
contrasts with the reactions with furans as 3-methoxycarbo-
nylfuran is an exceptional substrate for cyclopropanation.5
The asymmetric cyclopropanation of pyrrole displayed some
distinctive trends. Unlike the case with furan, the unsubstituted
pyrrole system is prone to double cyclopropanation when
donor/acceptor carbenes are used. However, this can be
controlled by using an N-tosyl group with an appropriate,
bulky catalyst. 2-Substituted pyrroles are effective, yet less
reactive substrates, but the reaction fails with 3-substituted
pyrroles. These trends are consistent with the general
mechanisms for the rhodium-catalyzed reactions of donor/
acceptor carbenes with electron-rich heterocycles as illustrated
in Figure 2.2c It is well established that even when the
approach of the heterocycle occurs toward a single face of the
carbene, either enantiomer of the product can be generated
depending on which position of the heterocycle is preferen-
tially attacked. In the case of furans, the concerted
asynchronous cyclopropanation initiates at C2, to ensure
greater delocalization of the positive charge build up, as shown
C
DOI: 10.1021/acs.orglett.9b02250
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Organic Letters
Letter
Scheme 4. Synthetic Applications of Monocyclopropanated
Pyrroles
Figure 2. Model for Rh(II)-catalyzed cyclopropanations with
substituted furan and pyrrole substrates.
in transition state (TS) 30. However, in the case of an N-
protected pyrrole, attack is preferred at C3 to avoid a steric
clash between the protecting group and the catalyst.2c
Consequently, a C3 substituent is easily accommodated for
the cyclopropanation of a furan but it is sterically unfavored for
an N-protected pyrrole, as illustrated for TS renditions 30 and
31 (Figure 2). The greater tendency of the pyrrole to undergo
double cyclopropanation can also be explained by the initial
trajectory of the attack during the cyclopropanation. Assuming
the attack at the carbene preferentially occurs at the Re face as
illustrated in structure 32, the formed enantiomer 33 is the
“matched” enantiomer for the second cyclopropanation, since
the cyclopropane moiety in 33 is now pointing away from the
carbene as illustrated TS 34. The tendency of the N-tosyl
group to favor the monocyclopropanation compared to the N-
Boc group can now be explained because the N-sulfonyl
nitrogen is closer to be sp3 hybridized, whereas the N-Boc is sp2
hybridized.9 Consequently, the tosyl group will reside out of
the plane of the dihydropyrrole ring (bond angle approx-
imately 158°; see Supporting Information) and be on the
opposite face to the cyclopropane. This arrangement can be
seen in the X-ray structure of 16 (Scheme 3; CCDC 1936964).
The tosyl group would then be partially shielding the
pyrrolocyclopropane from undergoing the second cyclo-
propanation.
Monocyclopropanated pyrroles have been shown to be
readily transformed to enantiomerically pure building blocks
such as β-aminocyclopropane carboxylic acids,10 pyrroli-
dones,11 or piperidines,12 being of great value in medicinal
chemistry. The utility of the cyclopropane product was
demonstrated with the synthesis of a chiral homo-β-proline
derivative, a cyclic, strained analog of GABA-16 and several
structurally related derivatives that display useful pharmaco-
logical activity.13 Starting from the enantioenriched mono-
cyclopropane 9, its homo-β-proline analog can be synthesized
in a two-step sequence,4f featuring a hydrogenation followed
by an acid-catalyzed ring opening, which gave rise to
pyrrolidine 37 as the major diastereomer in 72% yield and
>99% ee (Scheme 4). The absolute configuration of 37 was
unambiguously established by X-ray crystallography (CCDC
1936965). Lastly, the global deprotection was achieved by
treatment with HBr in acetic acid in the presence of phenol to
reveal the free amino acid 38.14 Furthermore, a β-ACC analog
was synthesized by subjecting 9 to ozonolysis conditions giving
rise to highly substituted cyclopropane 39 in 90% yield with
retention of all stereogenic centers, which can be further
transformed by known literature procedures.15 Such amino
acids are of great interest due to their intriguing structural
properties in peptides.10
In conclusion, we have developed a highly enantioselective
monocyclopropanation procedure for pyrrole substrates using
donor/acceptor carbenes. The ability to isolate the mono-
cyclopropane intermediate also represents an advancement in
the field of dirhodium catalyst development, as previously this
nonisolable intermediate would immediately undergo a second
cyclopropanation. With the choice of an appropriate catalyst,
monocyclopropanated pyrroles can now be isolated in good
yields and high enantioselectivity. The utility of this trans-
formation is also showcased through the synthesis of
biologically relevant compounds.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02250.
Complete experimental procedures and compound
characterizations (PDF)
Accession Codes
CCDC 1936964−1936965 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
D
DOI: 10.1021/acs.orglett.9b02250
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(7) Davies, H. M. L.; Bruzinski, P. R.; Lake, D. H.; Kong, N.; Fall, M.
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Model. 2008, 48, 1.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: hmdavie@emory.edu.
*E-mail: oliver.reiser@chemie.uni-regensburg.de.
ORCID
(10) For applications of β-ACCs, see: (a) Koglin, N.; Zorn, C.;
Beumer, R.; Cabrele, C.; Bubert, C.; Sewald, N.; Reiser, O.;
BeckSickinger, A. G. Angew. Chem., Int. Ed. 2003, 42, 202.
(b) Lang, M.; Bufe, B.; De Pol, S.; Reiser, O.; Meyerhof, W.; Beck-
Sickinger, A. G. J. Pept. Sci. 2006, 12, 258. (c) Urman, S.; Gaus, K.;
Yang, Y.; Strijowski, U.; Sewald, N.; De Pol, S.; Reiser, O. Angew.
Chem., Int. Ed. 2007, 46, 3976. (d) De Pol, S.; Zorn, C.; Klein, C. D.;
Zerbe, O.; Reiser, O. Angew. Chem., Int. Ed. 2004, 43, 511. (e) D’Elia,
V.; Zwicknagl, H.; Reiser, O. J. Org. Chem. 2008, 73, 3262. (f) Schmid,
M. B.; Fleischmann, M.; D’Elia, V.; Reiser, O.; Gronwald, W.;
Gschwind, R. M. ChemBioChem 2009, 10, 440.
Jiantao Fu: 0000-0002-1067-6915
Oliver Reiser: 0000-0003-1430-573X
Huw M. L. Davies: 0000-0001-6254-9398
Author Contributions
§J.F. and N.W. contributed equally to this work.
Notes
The authors declare the following competing financial
interest(s): H.M.L.D. is a named inventor on a patent entitled,
Dirhodium Catalyst Compositions and Synthetic Processes
Related Thereto (US 8,974,428, issued March 10, 2015). The
other authors have no competing financial interests.
(11) (a) Gheorghe, A.; Schulte, M.; Reiser, O. J. Org. Chem. 2006,
71, 2173. (b) Roy, S.; Reiser, O. Angew. Chem., Int. Ed. 2012, 51,
4722.
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ACKNOWLEDGMENTS
■
We thank Dr. John Bacsa (Emory University) and Michael
Bodensteiner (University of Regensburg) for determination of
the X-ray structure and Stephan Reichl (University of
Regensburg) for its bond angles. Financial support
(H.M.L.D.) was provided by the National Science Foundation
(CHE 1465189). The collaboration between the groups was
supported by the Alexander v. Humboldt Foundation
(Humboldt Senior Award for H.M.L.D.). Financial support
(O.R.) was provided by Fonds der Chemischen Industrie
(fellowship to N.W.) and an NSF international exchange grant
(OISE-1827201) (J.F.). Funds to purchase the NMR and X-
ray spectrometers used in these studies were supported by the
NSF (CHE 1531620 and CHE 1626172).
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DEDICATION
■
This paper is dedicated to our mentor (O.R.) and good friend
Armin de Meijere on the occasion of his 80th birthday.
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