Designing and Accurately Developing a [6 + 2] Dipolar Cycloaddition for the Synthesis of Benzodiazocines

Article abstract of DOI:10.1021/acs.orglett.1c01770

1,6-Dipolar cycloadditions represent a valuable strategy for the rapid construction of medium-sized rings. Herein, we describe the concept for the design of 1,6-dipoles that bypasses the regioselectivity. Through the introduction of an amino group into Morita-Baylis-Hillman (MBH) carbonates, unprecedented [6 + 2] dipolar cycloadditions were accurately developed with Cs2CO3, efficiently delivering a series of benzodiazocines in mild conditions. Computational studies bring a deeper understanding of this reaction.

Full text of DOI:10.1021/acs.orglett.1c01770

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Letter
Designing and Accurately Developing a [6 + 2] Dipolar
Cycloaddition for the Synthesis of Benzodiazocines
Wei Cai, Yiming Zhou, Yanlin He, Kaihong Chen, Cui Yu, and You Huang*
Cite This: Org. Lett. 2021, 23, 5430−5434
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ABSTRACT: 1,6-Dipolar cycloadditions represent a valuable
strategy for the rapid construction of medium-sized rings. Herein,
we describe the concept for the design of 1,6-dipoles that bypasses
the regioselectivity. Through the introduction of an amino group
into Morita−Baylis−Hillman (MBH) carbonates, unprecedented
[6 + 2] dipolar cycloadditions were accurately developed with
Cs2CO3, efficiently delivering a series of benzodiazocines in mild
conditions. Computational studies bring a deeper understanding of
this reaction.
regulation of substituents, I participated in [4 + 2] or [6 + n]
onstruction of medium-sized rings, which are widely
Cpresent in bioactive molecules and natural products, is (n = 4−6) cycloaddition reactions as a 1,4-4 or 1,6-dipole,
1
respectively (Scheme 1A, panel I).5 γ-Methylidene-δ-valer-
olactones could be catalyzed to yield intermediate II, which
usually serves as a 1,3-, 1,4-, or 1,6-dipole (Scheme 1A, panel
II).6 These three dipoles were selectively obtained by
regulating solvents6e and ligands.6b,d,f The only 1,6-dipole
was obtained in a [6 + 3] cycloaddition with the carboxyl
group, which remained instead of leaving as CO2.6d Also by
means of the strategy of retaining a carboxyl group, Lu and
Xiao developed the 1,8-dipolar cycloaddition of vinyl
carbamates via intermediate III (Scheme 1A, panel III).7
When CO2 was released by regulating the protective group,
there was no selectivity between the 1,4-dipole8 and the 1,6-
dipole (1:1, 32% yield). During these investigations, 1,6-
dipoles could not be precisely regulated, and products were
unpredictable. To make 1,6-dipolar cycloadditions more
powerful and general in the synthesis of medium-sized rings,
we hope to develop an accurate process to construct 1,6-
dipoles.
an enduring issue.2 Among the ongoing efforts, 1,6-dipolar
cycloadditions are a promising method to achieve this goal as
long as an intermolecular reaction occurs. To date, three types
of 1,6-dipoles have been established, all of which are based on
Tsuji−Trost chemistry (Scheme 1A).3 Palladium catalysts
catalyze vinyl oxetanes to generate intermediate I. Through the
a
Scheme 1. Regulation of 1,6-Dipoles
Driven by our continuous interest in MBH carbonates,9 we
imagine that the introduction of a nucleophilic site into the
adjacent position of such an electrophile would make it turn
into a 1,6-dipole under Brønsted base catalysis. With this
concept, 1,6-dipoles can be accurate synthesized by bypassing
regioselectivity and thus [6 + n] dipolar cycloadditions can be
realized (Scheme 1B). Different from the previous Lewis base-
Received: May 27, 2021
Published: July 1, 2021
a
E, electron withdrawing group; PG, protective group.
https://doi.org/10.1021/acs.orglett.1c01770
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5430−5434
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a
catalyzed [3 + n] or [1 + n] dipolar cycloadditions of MBH
carbonates,10 this newly designed building block will bring
more possibilities to the synthesis of medium-sized rings.
To fill the vacancy of [6 + 2] dipolar cycloaddition, herein
we designed o-amino-acylation aryl MBH carbonates and
applied dipolarophile isocyanates as two-atom synthons.
However, although it is reasonable in principle, there are two
inherent issues that make the proposed pathway challenging:
(1) [6 + 2] dipolar cycloadditions are limited by entropic
effects, enthalpic effects, and transannular steric interactions11
and (2) competition between intermolecular cycloaddition and
intramolecular cycloaddition under Brønsted base catalysis,12
which always requires the use of a high dilution (typically at
the micromolar level) or the slow addition of substrates.
To verify the above proposal, we initiated our investigation
using MBH carbonate 1a and isocyanate 2a as the model
substrates. As shown in Table 1, no reaction occurred without
Scheme 2. Scope of [6 + 2] Dipolar Cycloadditions
a
Table 1. Optimization of the Reaction Conditions
b
entry
1a/2a
solvent
catalyst
yield of 3aa (%)
1
2
3
4
5
6
7
8
1:1
1:1
1:1.2
1:1.5
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
MeCN
MeCN
MeCN
MeCN
MeCN
toluene
CHCl3
EtOAc
THF
MeCN
MeCN
MeCN
MeCN
MeCN
no reaction
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Na2CO3
K2CO3
62
73
79
91
no reaction
no reaction
no reaction
no reaction
67
9
10
11
12
13
14
86
90
43
75
Ru2CO3
NaOAc
K3PO4
a
Reactions were performed on a 0.1 mmol scale at room temperature
b
for 1 h as noted. Isolated yields. Structures were confirmed by X-ray
analysis (Supplementary Information Section 7). ND, not detected.
a
Reactions were carried out on a 0.1 mmol scale with 10 mol %
b
catalyst in 1 mL of the solvent at rt. Isolated yield. rt, room
temperature.
isocyanate 2h was used. Substrates on the meta-position and
the ortho-position showed similar electronic effects, and the
products 3ai−3aq and 3as were produced in moderate to
excellent yields. In addition, we successfully produced the
single crystal of 3an to prove the feasibility of the reaction.
Polyhalogen-substituted phenyl isocyanate influenced the
outcome, and product 3ar was obtained in a lower yield.
Benzyl isocyanate could not make the reaction proceed
smoothly, but n-butyl isocyanatoacetate 2u produced 3au in
a moderate yield. The steric hindrance of the ester group
−CO2R2 of 1 had little impact on the reaction, and
cycloadducts 3ba−3fa were produced with outstanding data.
In sharp contrast, the steric hindrance of the acyl group
−NHCOR3 of 1 showed a slight influence on the yield (3ga−
3ka). The influence of substituents on the phenyl group of
MBH carbonates showed that substituents next to the eight-
membered ring had a great influence on the outcome,
providing 3la and 3ua−3va in relatively lower yields. The
electron-donating substituents at other positions were also
compatible with this protocol, giving the corresponding
cycloadducts 3ma and 3pa−3qa with satisfactory results.
The substitution of electron-withdrawing groups was also
a catalyst (Table 1, entry 1). To our gratification, the target
product benzodiazocine 3aa was produced with Cs2CO3 as a
catalyst in a 62% yield in MeCN at room temperature (Table
1, entry 2). By increasing the amount of isocyanate, the yield
can be increased to 91% (Table 1, entries 3−5). A series of
solvents were screened, and none of them made the reaction
occur (Table 1, entries 6−9). Other carbonates showed slightly
lower reactivities than Cs2CO3 (Table 1, entries 10−12).
When NaOAc or K3PO4·3H2O was used, the yield was also not
improved (Table 1, entry 13 or 14, respectively).
With the optimal conditions in hand, we started to explore
the substrate scope and limitations of Cs2CO3-catalyzed [6 +
2] dipolar cycloadditions. The electronic effects of different
aryl isocyanates were first examined (Scheme 2). The
introduction of an electron-donating group, p-OMe, to the
aryl isocyanates led to the corresponding product 3ab in a
good result. The substitutions of electron-withdrawing groups
at the para-position provided 3ac−3ag in modest to good
yields. Notably, no reaction occurred when p-NO2 phenyl
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Letter
compatible, producing products 3na−3oa and 3ra−3ta with
fair yields.
Scheme 4. Synthetic Applications
To elucidate the detailed mechanism, the density functional
theory (DFT) method M06-2X was employed to study the
mechanism.13 As shown in Scheme 3, which corresponds to
Scheme 3. DFT Calculations
process caused by the intramolecular nucleophilic attack of an
oxygen anion to the alkene under basic conditions (56% yield).
Moreover, p-methoxyphenyl in 3ab could be removed by CAN
to produce 4d in a 70% yield.
In conclusion, we have developed efficient, accurate, and
unprecedented [6 + 2] dipolar cycloadditions of o-amino-
acylation aryl MBH carbonates with isocyanates that are
catalyzed by Cs2CO3. For the first time, 1,6-dipoles were
obtained by bypassing regioselectivity. Benzodiazocines could
be expeditiously achieved in good to excellent yields under
mild conditions through these newly designed building blocks,
which will bring more possibilities to the synthesis of medium-
sized benzo-N-heterocycles. Conceptually, the strategy of
bypassing regioselectivity to precisely construct remote dipoles
provides ideas to design new reagents and reactions. Further
investigations to expand this new concept and the reaction
mode of o-amino-acylation aryl MBH carbonates are ongoing
in our laboratory.
the [6 + 2] dipolar cycloaddition, 1b was activated by Cs2CO3
to generate the 1,6-dipole INT1. The nucleophilic addition of
INT1 to Ph−NCO formed INT2 via TS2. In the following
rate-determining step of this reaction, the newly formed
nitrogen anion was more negative than the oxygen anion,
which attacked the terminal olefin through 1,4-addition to
generate INT3 via TS3 (9.8 kcal/mol). Sequentially, the enol
carbanion caused an SN2′ process, which decomposed OBoc
t
into CO2 and BuO−; benzodiazocine 3 was obtained at the
t
same time. Besides, BuO− as the base can promote the above
−
reaction or remove the hydrogen in HCO3 to complete the
ASSOCIATED CONTENT
* Supporting Information
■
catalytic cycle. These calculated results confirmed our
speculation on the mechanisms.
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01770.
To display the potential applicability of these strategies, we
performed these reactions on gram scales. To our delight, the
yield of 3aa could be maintained even when the loading
amount of catalyst was reduced to 1 mol % (Scheme 4A).
Benzodiazocines could be easily transformed into a series of
other skeletons (Scheme 4B), revealing the potential to
synthesize multitudinous benzo-N-heterocycles. 3aa could
undergo a [3 + 2] dipolar cycloaddition with N-(methox-
ymethyl)-N-(trimethylsilylmethyl)benzyl-amine to get the
corresponding polycyclic compound 4a in an 89% yield. The
treatment of 3aa with TMS-I in DCM hydrolyzed 3aa into 4b
in a 63% yield. 4b was rearranged into 4c through an SN2′
Experimental procedures for all reactions and character-
1
ization data for all products including HRMS, H and
13C NMR spectra, and crystal data (PDF)
HRMS spectra (ZIP)
Accession Codes
CCDC 2023952 contains the supplementary crystallographic
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 Cam-
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AUTHOR INFORMATION
Corresponding Author
■
You Huang − State Key Laboratory and Institute of Elemento-
Organic Chemistry, College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China; orcid.org/
0000-0002-9430-4034; Email: hyou@nankai.edu.cn
Authors
Wei Cai − State Key Laboratory and Institute of Elemento-
Organic Chemistry, College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
Yiming Zhou − State Key Laboratory and Institute of
Elemento-Organic Chemistry, College of Chemistry, Nankai
University, Tianjin 300071, People’s Republic of China
Yanlin He − State Key Laboratory and Institute of Elemento-
Organic Chemistry, College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
Kaihong Chen − State Key Laboratory and Institute of
Elemento-Organic Chemistry, College of Chemistry, Nankai
University, Tianjin 300071, People’s Republic of China
Cui Yu − State Key Laboratory and Institute of Elemento-
Organic Chemistry, College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01770
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the National Natural Science Foundation of
China (21871148 and 21672109) for financial support.
■
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Tunge, J. A. Decarboxylative cyclizations and cycloadditions of
palladium-polarized aza-ortho-xylylenes. Tetrahedron 2009, 65, 5102−
5109. (c) Li, T.-R.; Tan, F.; Lu, L.-Q.; Wei, Y.; Wang, Y.-N.; Liu, Y.-
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palladium-catalysed decarboxylation-cycloaddition sequence. Nat.
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DEDICATION
■
Dedicated to the 100th anniversary of Chemistry at Nankai
University.
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