C-H functionalization of cyclic amines: Redox-annulations with α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1039/c5cc03390j

Cyclic amines such as pyrrolidine and 1,2,3,4-tetrahydroisoquinoline undergo redox-annulations with α,β-unsaturated aldehydes and ketones. Carboxylic acid promoted generation of a conjugated azomethine ylide is followed by 6π-electrocylization, and, in so

Full text of DOI:10.1039/c5cc03390j

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DOI: 10.1039/C5CC03390J
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C–H Functionalization of Cyclic Amines: Redox-Annulations with
α,β-Unsaturated Carbonyl Compounds
YoungKu Kang,a,† Matthew T. Richers,a,† Conrad H. Sawickia and Daniel Seidela,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cyclic
amines
such
as
pyrrolidine
and
1,2,3,4- electron-withdrawing groups (e.g., eq 2).6
Oxidative C–H
tetrahydroisoquinoline undergo redox-annulations with α,β- functionalization methods for the generation of conjugated
unsaturated aldehydes and ketones. Carboxylic acid promoted azomethine ylides have also emerged.7–9 Here we report a
generation of a conjugated azomethine ylide is followed by 6π- carboxylic acid facilitated method for the in situ generation of
electrocylization, and, in some cases, tautomerization. The conjugated azomethine ylides and their subsequent 1,5-
resulting ring-fused pyrrolines are readily oxidized to the electrocyclizations. Simple cyclic amines such as pyrrolidine or
corresponding pyrroles or reduced to pyrrolidines.
1,2,3,4-tetrahydroisoquinoline
(THIQ) and
α,β-unsaturated
aldehydes/ketones serve as the starting materials in these cascade
reactions (eq 3).
Electrocyclic ring-closures of conjugated azomethine ylides enable
efficient access to 5- and 7-membered azacycles.1,2 A number of
mechanistically distinct methods for the generation of the required
dipolar intermediates have been developed, the most common of
which involve decarboxylation or the deprotonation of a preformed
iminium salt.1,2 In contrast, the direct generation of conjugated
Table 1. Evaluation of Reaction Conditions.
azomethine
ylides
via
redox-neutral
amine
α-C–H
functionalization3,4 as an avenue for 1,5- and 1,7-electrocyclizations
has been explored to only a limited extent.1 Previous examples
include the reaction of enamines or related compounds with
dimethyl acetylenedicarboxylate (DMAD) (e.g., eq 1)5 or
intramolecular rearrangements of dienamines bearing multiple
additive
(equiv)
time
[h]
yield
(%)
entry
X
solvent (M)
1
2
3a
5
5
5
5
5
5
5
5
5
5
5
3
2
PhMe (0.25)
PhMe (0.25)
PhMe (0.25)
PhMe (0.1)
PhMe (0.1)
PhMe (0.1)
PhMe (0.1)
PhMe (0.1)
PhMe (0.1)
n-BuOH (0.1)
1,2-DCE (0.1)
PhMe (0.1)
PhMe (0.1)
-
15
5
trace
65
BzOH (0.5)
BzOH (0.5)
BzOH (0.5)
AcOH (0.5)
2-EHA (0.5)
HCO2H (0.5)
BzOH (0.2)
BzOH (1.0)
BzOH (1.0)
BzOH (1.0)
BzOH (1.0)
BzOH (1.0)
5
58
4
5
74
5
5
65
6
5
69
7
5
trace
68
8
6
9
3
77
10
11
12
13
5
23
12
3
trace
62
15
64
a
Without molecular sieves. 2-EHA = 2-ethylhexanoic acid.
Our group has recently advanced a general amine α-C–H bond
functionalization concept to access reactive azomethine ylide
intermediates via the condensation of a secondary amine with an
aldehyde or a ketone.3u, 10 This enabled the development of a range
a.Department of Chemistry and Chemical Biology, Rutgers, The State University of
New Jersey, Piscataway, New Jersey, 08854, USA.
† These authors contributed equally.
Electronic Supplementary Information (ESI) available: Experimental procedures
and characterization data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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of new reactions in which azomethine ylides are transformed in Scheme 1. Scope of the reaction with pyrrolidine.
Journal Name
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DOI: 10.1039/C5CC03390J
non-pericyclic ways.11,12 Carboxylic acids were found to be essential
additives in many of these processes as they substantially lower the
barriers for azomethine ylide formation. In addition, carboxylic
acids serve to readily protonate azomethine ylides to form iminium
ions or related N,O-acetal intermediates that undergo further
transformations. Thus, the presence of a carboxylic acid, while
required to access azomethine ylides, appears to be incompatible
with well-established pericyclic azomethine ylide chemistry.
However, we could recently show that azomethine ylides, accessed
via a benzoic acid catalyzed process, readily undergo intramolecular
[3+2]-cycloadditions.13 To test whether this strategy is compatible
with other types of pericyclic reactions, we decided to explore 1,5-
electrocyclizations using pyrrolidine and chalcone as model
substrates.
A range of diversely substituted chalcones readily underwent
reactions with pyrrolidine under the optimized conditions,
providing the corresponding pyrrolizidine-type annulation products
in moderate to good yields (Scheme 1).15 Substitution of either of
chalcone’s phenyl groups for
a methyl substituent was not
tolerated; no 1,5-electrocyclization products could be isolated.
Similarly, reactions of pyrrolidine with cinnamaldehyde or α-
methyl-cinnamaldehyde led to complex reaction mixtures and no
appreciable formation of the desired annulation products. The
scope of the reaction was easily extended to THIQ’s (eq 4).
However, the expected products 3 were not observed. Rather,
tautomeric products 2 were obtained, indicating that isomerization
to the thermodynamically more stable enamines is rapid under the
reaction conditions.
As summarized in Table 1, reactions between pyrrolidine and
chalcone proceeded under a range of conditions. No formation of
1a was observed in the absence of a carboxylic acid additive (entry
1). Instead, analysis of the crude reaction mixture by 1H-NMR
indicated the presence of the conjugate addition product (not
shown) in addition to unmodified chalcone.14 Addition of benzoic
acid (0.5 equiv) under otherwise identical conditions led to the
formation of 1a as a single diastereomer in 65% yield (entry 2). The
presence of molecular sieves was not essential, but slightly
diminished yields were obtained in their absence (entry 3).
Improved results were obtained upon lowering the reaction
molarity from 0.25 M to 0.1 M (entry 4). Carboxylic acid additives
other than benzoic acid were less effective (entries 5–7). While a
reduction in benzoic acid loading was tolerated (entry 8), the best
results were obtained with one equivalent of this additive (entry 9).
Solvents other than toluene were explored briefly but provided
inferior results (entries 10, 11). Finally, while the amount of
pyrrolidine could be reduced, this led to longer reaction times and
slightly diminished yields (entries 12, 13).
Scheme 2. Scope of the reaction with cinnamaldehydes.
In contrast to pyrrolidine, THIQ’s and tryptoline readily underwent
annulation reactions with cinnamaldehydes (Scheme 2). Parent
cinnamaldehyde itself was a poor substrate, resulting in complex
product mixtures out of which product 4a could be isolated in only
3–6% yield. However, α-substituted cinnamaldehydes gave rise to
fast reactions and moderate to good yields of annulation products.
This is consistent with what would be expected based on a simple
conformational analysis. The presence of an α-substituent should
2 | J. Name., 2012, 00, 1-3
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2
Other selected reviews on azomethine ylides: (a) A. Padwa,
result in an increased amount of the requisite conformer for the 6π-
electrocyclization, as this avoids an unfavorable allylic 1,3-
interaction present in the non-productive conformer (Scheme 2).16
The amine annulation products could be reduced to the
corresponding pyrrolidine ring-systems (Scheme 3). Reduction of
1d with sodium borohydride led to the formation of 5 as a nearly
1:1 mixture of readily separable diastereomers. The BH3 complex of
the major diastereomer, which was found to be stable to
View Article Online
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chromatographic
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was
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X-ray
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crystallography. A moderately selective reduction was achieved
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Scheme 3. Product transformation.
In summary, we have developed a simple method for the redox-
neutral C–H annulation of amines with α,β-unsaturated aldehydes
and ketones, providing easy access to alkaloid-like structures from
simple starting materials. This study further establishes that the
generation of azomethine ylides via a carboxylic acid promoted
process is compatible with the subsequent pericyclic transformation
of these dipolar species. This concept, which was applied here for
the first time to 6π-electrocyclizations, is expected to find
widespread use in related transformations.
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Amine C–H functionalization in the context of 1,7- 15 For selected reviews on pyrrolizidine natural products, see:
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4
12 Other recent examples of redox-neutral amine α-C–H
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