Synthesis of Polycyclic Imidazolidinones via Amine Redox-Annulation

Article abstract of DOI:10.1021/acs.orglett.7b03309

α-Ketoamides undergo redox-annulations with cyclic secondary amines, such as 1,2,3,4-tetrahydroisoquinoline, pyrrolidine, piperidine, and morpholine. Catalytic amounts of benzoic acid significantly accelerate these transformations. This approach provides

Full text of DOI:10.1021/acs.orglett.7b03309

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Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Synthesis of Polycyclic Imidazolidinones via Amine Redox-
Annulation
Zhengbo Zhu, Xin Lv,†,‡,∥ Jason E. Anesini, and Daniel Seidel*
†
,∥
†
,†,§
†Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854,
United States
‡
Department of Chemistry, College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, People’s Republic of
China
*
S Supporting Information
ABSTRACT: α-Ketoamides undergo redox-annulations with cyclic
secondary amines, such as 1,2,3,4-tetrahydroisoquinoline, pyrroli-
dine, piperidine, and morpholine. Catalytic amounts of benzoic acid
significantly accelerate these transformations. This approach provides
polycyclic imidazolidinone derivatives in typically good yields.
midazolidinones are frequently encountered as substructures
of natural products and synthetic, biologically active
compounds (Figure 1). Among the most common methods
Scheme 1. Selected Routes to Polycyclic 4-Imidazolidinones
I
1
−3
Figure 1. Selected 4-imidazolidinones.
as catalysts or promoters, and result in the formation of a new
11
six-membered ring. Although there are examples of redox-
neutral amine α-C−H bond functionalizations of secondary
amines that give rise to the formation of new five-membered
rings, typically via (3 + 2) cycloaddition of azomethine ylide
used to build the imidazolidinone motif are condensations of α-
aminoacetamide derivatives with aldehydes or ketones, various
cycloadditions, ring expansions, and others. Methods have also
1
12
intermediates or 1,5-electrocyclic ring-closure of conjugated
emerged that are particularly suitable for the preparation of
ring-fused imidazolidinones (Scheme 1). One such approach
involves an oxidative intramolecular coupling of α-amino-
1
3,14
azomethine ylides,
this chemistry remains underdeveloped
13c,i
and has rarely been applied to C−N bond formation.
We
4
reasoned that such an annulation could be applied to the
synthesis of bi- or polycyclic imidazolidinones via the
condensation of cyclic amines with α-ketoamides (Scheme 1,
acetamide derivatives (eq 1). A decarboxylative strategy
involving the condensation of proline with α-ketoamides to
build bicyclic imidazolidinones containing a pyrrolidine ring has
15
5
,6
eq 3).
,2,3,4-Tetrahydroisoquinoline (THIQ) and 2-oxo-N,2-
also been established (eq 2). Here we report a redox-neutral
annulation approach to polycyclic imidazolidinones (eq 3).
1
7
8
diphenylacetamide (1a) were selected as model substrates in
order to evaluate the proposed annulation process (Table 1). A
We and others have developed a range of redox-neutral
annulation reactions that proceed via the condensation of a
secondary amine with an aldehyde/ketone possessing a
pendent (pro)nucleophile. These annulations feature con-
current N-alkylation and the functionalization of an amine α-
2
:1 mixture of THIQ and 1a, upon heating under reflux in
toluene for 2 days, resulted in an incomplete reaction and the
9
,10
C−H bond.
The majority of these transformations proceed
Received: October 23, 2017
through azomethine ylide intermediates, utilize carboxylic acids
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03309
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Evaluation of Reaction Conditions
Scheme 2. Ketoamide Scope
entry
THIQ (equiv)
catalyst
time (h)
yield (%)
1
2
3
4
2
48
7
50
95
92
91
56
93
88
95
2
PhCOOH
AcOH
2
23
21
44
12
12
15
2
2-EHA
b
5
2
PhCOOH
PhCOOH
PhCOOH
PhCOOH
6
7
1.5
1.2
1.5
c
8
a
Reactions were performed on a 0.2 mmol scale. All yields correspond
b
to isolated yields. dr >25:1 in all cases. Reaction was run at 50 °C and
remained incomplete. Without 4 Å MS.
c
isolation of desired product 2a as a single diastereomer in 50%
yield (entry 1). Utilization of catalytic amounts of benzoic acid
(
20 mol %) resulted in a significant improvement (entry 2).
Complete consumption of 1a was observed within 7 h, and 2a
was obtained in 95% yield. Replacement of benzoic acid with
either acetic acid or 2-ethylhexanoic acid (2-EHA) facilitated
the formation of 2a in similar yields but required prolonged
reaction times (entries 3 and 4). A reaction that was performed
at 50 °C remained incomplete after 44 h and led to product in
moderate yield (entry 5). A reduction of the amount of THIQ
to 1.5 equiv was well tolerated (entry 6), whereas further
reduction to 1.2 equiv led to a slight drop in yield (entry 7).
Notably, the reaction performed equally well in the absence of
molecular sieves (entry 8).
a
Reactions were performed on a 0.5 mmol scale. All yields correspond
b
to isolated yields. Transamidation product (1-(3,4-dihydroisoquino-
lin-2(1H)-yl)-2-phenylethane-1,2-dione) was obtained in 38% yield.
The scope of the redox-annulation was explored under the
optimized conditions of Table 1 (entry 8). A range of α-
ketoamides with different substitution patterns were inves-
tigated (Scheme 2). The corresponding 4-imidazolidinone
products 2 were isolated in good to excellent yields. Both
aromatic and aliphatic substituents on the amide nitrogen were
tolerated. Likewise, nonenolizable and enolizable α-ketoamides
participated in the annulation reaction. In the case of the
primary amide-derived product 2n, which was obtained in 53%
yield, a competing pathway was identified. Specifically, the
corresponding transamidation product was obtained in 38%
a
Scheme 3. Amine Scope
1
6
yield. An enantiomerically pure α-ketoamide, derived from
S)-1-phenylethan-1-amine, provided product 2o in 86% yield
(
as a 1.3:1 mixture of diastereomers.
The scope of the amine component is summarized in
Scheme 3. Benzylic amines other than THIQ, including the
sterically hindered 1-phenyl-THIQ, readily formed annulation
products upon reaction with α-ketoamide 1a. Amines with
attenuated reactivities, such as pyrrolidine and azepane,
provided 4-imidazolidinone products in good yields. Partic-
ularly challenging substrates such as piperidine, morpholine,
and thiomorpholine underwent the title reaction at elevated
temperatures.
As shown in Schemes 2 and 3, reactions involving THIQ,
related benzylic amines, and pyrrolidine underwent redox-
annulations with α-ketoamides in highly diastereoselective
fashion. In contrast, reactions with azepane, piperidine,
morpholine, and thiomorpholine were poorly diastereoselec-
tive. We suspected that the aminal stereogenic center might be
configurationally unstable under the reaction conditions. Thus,
a
Reactions were performed on a 0.5 mmol scale. All yields correspond
b
to isolated yields. Reaction was performed in PhMe (0.25 M) under
microwave irradiation for 30 min at 220 °C. Reaction was performed
in PhMe (0.25 M) under microwave irradiation for 1 h at 220 °C.
c
product ratios may reflect the different thermodynamic
stabilities of the two diastereomers. To test this hypothesis,
B
DOI: 10.1021/acs.orglett.7b03309
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the readily available pure diastereomers of product 3g were
exposed to the reaction conditions (eqs 4 and 5). Upon
Experimental procedures and characterization data,
including X-ray crystal structures of products 2a and
3d (PDF)
X-ray data for compound 2a (CIF)
X-ray data for compound 3d (CIF)
AUTHOR INFORMATION
■
*
Corresponding Author
E-mail: seidel@chem.ufl.edu.
ORCID
Daniel Seidel: 0000-0001-6725-111X
Present Address
§(D.S.) Department of Chemistry, University of Florida,
Gainesville, FL 32611.
extended heating, both mixtures converged to a final 2.1:1 ratio
of diastereomers. These experiments establish that product
isomerization can indeed occur under the reaction conditions.
Two plausible mechanistic scenarios are shown in Scheme 4,
depicting pyrrolidine and α-ketoamide 1a as prototypical
Author Contributions
∥Z.Z. and X.L. contributed equally.
Notes
Scheme 4. Mechanistic Considerations
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the NIH−NIGMS (R01GM101389) is
gratefully acknowledged. We thank Dr. Tom Emge (Rutgers
University) for crystallographic analysis and Dr. Wazo Myint
(
Rutgers University) for assistance with NMR assignments.
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ASSOCIATED CONTENT
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■
*
S
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03309.
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D
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