Synthesis of β-Amino Diaryldienones Using the Mannich Reaction

Article abstract of DOI:10.1021/acs.orglett.9b01195

The Mannich reaction has been used for decades to prepare many pharmaceutically important molecules. Here, using a "double-Mannich-β-elimination" synthetic sequence, we report the synthesis and the characterization details of a novel class of β-amino diaryldienones with prominent antiprostate cancer activity. Through these studies, we correct an erroneous structure in the current literature, present a discussion of the stereochemical outcome of a new reaction, and probe the mechanism(s) of byproduct formation through isotopic studies.

Full text of DOI:10.1021/acs.orglett.9b01195

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of β‑Amino Diaryldienones Using the Mannich Reaction
N. G. R. Dayan Elshan,*,† Matthew B. Rettig,‡,§ and Michael E. Jung*,†
†Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
‡Division of Hematology/Oncology, VA Greater Los Angeles Healthcare System West LA, Los Angeles, California 90073, United
States
§Departments of Medicine and Urology, David Geffen School of Medicine, University of California, Los Angeles, California 90095,
United States
S
* Supporting Information
ABSTRACT: The Mannich reaction has been used for decades
to prepare many pharmaceutically important molecules. Here,
using a “double-Mannich−β-elimination” synthetic sequence, we
report the synthesis and the characterization details of a novel
class of β-amino diaryldienones with prominent antiprostate
cancer activity. Through these studies, we correct an erroneous
structure in the current literature, present a discussion of the
stereochemical outcome of a new reaction, and probe the
mechanism(s) of byproduct formation through isotopic studies.
ince its advent over a century ago, the venerable Mannich
failed, because they tended to favor the cis-orientation of the
aryl rings across the double bond (the E-isomer).
1,2
Sreaction has been a staple in synthetic organic chemistry.
Its robust utilization has spread across many different fields of
research, including medicinal chemistry. Mannich bases (β-
aminocarbonyl compounds) represent important pharmaco-
phores in various pharmaceutical compounds.3−6 Our own
prostate cancer drug discovery program encountered a unique
synthetic challenge that was ultimately solved by the utilization
of the Mannich reaction. We found a hit compound (NSC-
89160, Figure 1)7 with potent antiprostate cancer effects in a
high-throughput screening assay.8−10
This led us to an alternative synthetic approach through a
nitrile analogue. The Knoevenagel condensation of benzyl
cyanide with 4-chlorobenzaldehyde yielded the substituted
acrylonitrile Z-1 (Scheme 1, panel A), preferentially in the Z
conformation, which was then used as a key intermediate to
access 89160. Keeping the reaction temperatures at or below
room temperature, devising efficient workup/purification
procedures, and avoiding acidic conditions were essential for
converting 1 to the desired product Z-3.11,12 To our dismay, Z-
3, in neither its hydrochloride nor its neutral form, was able to
replicate the antiprostate cancer effects seen8 with 89160. At
this point we also synthesized the compound in its E
configuration (E-3, Scheme 1, panel B), using a strategic
Mannich reaction performed under mild reaction conditions
between enone 4 and the bis-aminomethyl Mannich reagent13
5. A pre-NMR era preparation of compound E-3 described in
literature14 was unsuccessful in providing pure E-3 in good
yield. Thus, the synthetic methods outlined here are the first
reliable methods reported to obtain stereochemically pure 3 (E
or Z) to the best of our knowledge. Unfortunately, E-3 also
failed to reproduce the biological effectiveness8 of 89160.
At this juncture, we suspected a misassignment of 89160 in
the repository and performed a thorough structural analysis.
Figure 1. Structure of the hit-compound NSC-89160 and a possible
shortest retrosynthetic disconnection.7
1
13C and H NMR spectroscopy revealed that there were only
three sets of sp3-type carbons/protons in 89160. It is
noteworthy here that the α- and the β-carbons (and their
associated hydrogens) in E-3 had accidental overlap in the 13C
and 1H spectra, resulting in also only three aliphatic signals. In
The structure of 89160 given in the PubChem database was
of a β-amino diarylenone derivative, with no known synthetic
methodology available in the literature. The Z-alkene moiety in
its structure provided a significant synthetic challenge in
obtaining a stereochemically pure final compound via an in-
house synthesis. The shortest possible synthetic strategies
through intermediate carbonyl species of type i (Figure 1)
Received: April 8, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01195
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Synthesis of the Z- (Panel A) and the E- (Panel B) β-Amino Diarylenone(s) 3
a
(a) DIBAL-H, toluene, −78 °C, 2 h, 61%; (b) vinylmagnesium bromide, THF, −78 °C, 30 min, 76%; (c) DMP, DCM, 0 °C, 20 min, 56%; (d)
HNBnMe, DCM, 23 °C, 3 h, 84%; (e) Na2CO3 (aq, 10%), DCM, 23 °C, quantitative.
the conventional nonaromatic olefinic region of 5−7 ppm, only
1
one proton signal (6.6 ppm) was visible in the H spectra of
89160. Although integration within the aromatic region
appeared somewhat off, the presence of three aryl rings
overlapping in this region made detailed interpretation
difficult. The most compelling evidence for the determination
of the correct structure came from 2D-NMR. An HSQC
spectrum of 89160 clearly revealed that the olefinic proton
signal at 6.6 ppm correlated to a methylene unit that had a
second sp2-type proton signal for the same carbon at 7.1 ppm.
At this point, we deduced that the true structure of 89160
must have another double bond α to the carbonyl group,
leading to compound 6 (Figure 2). Based on what we knew at
Figure 3. Comparison of previous methodology to the current
synthetic challenge. (a) Synthesis of β-amino enone derivative(s) 7 of
acetophenone. (b) Synthetic approach for β-amino diaryldienone 6.
including the decomposition of the product (initiated by
Nazarov-type processes and by an intramolecular hydride
transfer process), and rapid isomerization of the E double
bond.
A major unexpected byproduct isolated from this reaction
was the dienone Z-8 (stereochemistry further confirmed via an
Figure 2. Correct structure of the hit-compound NSC-89160 (6).
independent synthesis from Z-1).
the time about this type of structure (i.e., NMR chemical shift
of the enone proton in E/Z arrangement) and for steric
reasons, we hypothesized (and later confirmed) that the
stereochemistry across the diaryl substituted double bond
would be E.
Despite its availability in a compound repository, there were
no known synthetic methods to prepare 6. Thus, we adapted
the closest available methodology: the application of the
Mannich reaction on acetophenone type substrates to
synthesize the β-amino enone derivatives of structure 7
(Figure 3a). This type of reaction on acetophenone and the
behavior of resultant β-amino enones 7 are well-known in
literature.15−18 The reaction typically proceeds at high
temperature (120−140 °C), in DMF or acetic acid solvent,
with an excess of paraformaldehyde and the corresponding
amine hydrochloride or the corresponding Eschenmoser salt.
With 6 being ideally set up for Nazarov-type reactions,
heating at high temperatures in acetic acid understandably gave
no discernible product in our system (Figure 3b). While some
nonreproducible minor product formation was observed in
dimethylformamide, there were extensive complications,
In exploring the mechanism of Z-8 formation, we
hypothesized either possible reduction of 6 by dimethylforma-
mide or formaldehyde or the involvement of an intramolecular
hydride transfer process (Figure 4). Use of dimethylacetamide
as the solvent still gave Z-8, eliminating the solvent as a major
contributor for its formation. We thus proceeded to investigate
the intramolecular hydride-transfer hypotheses using isotopi-
cally labeled19 starting amine (Figure 4). Interestingly, 8 was
observed in isolable amounts only for its Z isomeric form.
Thus, we can assume first the formation of Z-6-d2 from 4 as
expected from a double-Mannich−β-elimination sequence
followed by isomerization. Then the protonated species 9
would undergo an intramolecular hydride/deuteride shift
leading to 10.
B
DOI: 10.1021/acs.orglett.9b01195
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the reaction progress by NMR, we realized that the highest
conversion of ketone 4 was achieved at 1 h (Table 1, entry d),
albeit with 32% of the product being isomerized to the Z-
isomer. Observing the depletion of bis-amine adduct
intermediate 5 after 1 h, we increased the amount of
paraformaldehyde in the reaction mixture to 6.0 equiv from
2.2 equiv (Table 1, entries c, e, g, and i). This resulted in
sustained generation of intermediate 5, leading to faster
reaction of starting ketone 4 and the intermediate species 3.
Optimal, reproducible conversion to product was observed at 1
h with the use of 6 equiv of paraformaldehyde (Table 1, entry
e). Through optimized reaction and purification conditions,
yields up to 62% of E-6 had been achieved in excellent purity.
We also noted that there were no discernible levels of Z-3 in
these reaction mixtures, indicating that the isomerization of the
diaryl-substituted double bond under these conditions likely
occurs after the formation of E-6.
Figure 4. Plausible mechanism for the intramolecular hydride-transfer
mediated generation of 8 from 6.
By combining the knowledge gained from our NMR studies
of the reaction intermediates, a plausible reaction mechanism is
shown in Scheme 2. The in situ generated bis-amine adduct 5
The iminium species 11 would then be eliminated to result
in the new methylene group of Z-8-d. Since the deuteride
transfer from the benzylic position would likely be preferred,
the product mixture for Z-8-d has an excess of the deutero
product despite the lower statistical transfer probability (i.e.,
two benzylic deuteriums vs three methyl hydrogens). Switch-
ing to the nonpolar solvent toluene helped to control this side
reaction, boosting the desired product yield. A study done by
heating Z-6 in solvent for 4 h showed the presence of Z-8 to be
10 times lower in toluene compared to reactions done in either
DMF or DMA.
Scheme 2. Plausible Mechanism for the β-Amino
Diaryldienone Synthesis
Thus, we chose toluene as the optimal reaction solvent and
proceeded to optimize the reaction time, with a primary aim to
control the isomerization of the E-double bond (Table 1).
Initial reaction conditions utilized 2.2 equiv of the amine
hydrochloride and 3.3 equiv of paraformaldehyde. In following
Table 1. Optimization of Reaction Conditions for the
a
Synthesis of E-6
facilitates the double-Mannich reaction under near-neutral pH
conditions leading to the compound 12, which then undergoes
β-elimination to yield E-6. The lack of any discernible levels of
isomerized (Z) species prior to E-6 is likely the result of the
rapid kinetics of these mechanistic steps and/or a strong
preference of the starting ketone/intermediates to exist in the
E stereochemistry (e.g., compound 4). In large scale reactions,
isolable levels of Nazarov type cyclized products (13) were
also observed.
The substrate scope for the reaction can be found in the
Supporting Information (p S32). In general, yields of 20−40%
of the E-isomer (analogs of E-6) were achieved for the reaction
across the various substituents evaluated. Given the tendencies
of these substrates to undergo isomerization and/or rearrange-
ment/decomposition during the reaction, such reproducible
yields of the stereopure products are arguably quite reasonable.
Other amines apart from N-benzyl methylamine could be used
a
equivalents in reaction mixture
product 6
time
4
5
E-3
E
Z
a
b
c
d
e
f
g
h
i
15 min
30 min
30 min
1 h
3.20
0.76
0.20
0.30
none
0.14
none
neg.
0.74
0.47
0.75
0.06
0.41
neg.
none
none
none
3.66
3.16
1.14
3.04
neg.
2.30
none
1.60
none
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.08
0.20
0.08
0.48
0.28
0.86
0.70
0.98
0.94
b
b
1 h
2 h
2 h
b
3 h
3 h
b
none
a
b
Equivalents given in comparison to product E-6. 6.0 equiv of
paraformaldehyde. neg. = negligible.
C
DOI: 10.1021/acs.orglett.9b01195
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Organic Letters
Letter
in the reaction, with varying degrees of success. Reaction with
dimethylamine hydrochloride (or with the Eschenmoser salt)
did not produce isolable amounts of the β-amino diary-
ldienone, while the cyclic amines pyrrolidine (8%), piperidine
(30%), and piperazine (34%) gave reasonable yields. The
presence of electron-donating groups on the aryl rings is likely
to be detrimental to this reaction, because it may positively
contribute to isomerization and Nazarov type cyclization
leading to byproducts. In agreement with this fact, when the 4-
Cl substituent of compound 4 is replaced with a 4-methoxy
group, the isomerization reaction predominates, leading only
to the Z-β-amino diaryldienone (Z-SI-17, p S32) and the
corresponding Nazarov cyclized product. If the enone double
bond in compound 4 is not present, the reaction can be
conducted in DMF to reproducibly obtain the resultant β-
amino diarylenone 14 in yields up to 20%.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01195.
■
S
Experimental procedures and characterization for all new
compounds along with 1D and 2D NMR data (PDF)
Accession Codes
CCDC 1896679−1896684 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.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: jung@chem.ucla.edu.
*E-mail: elshan@chem.ucla.edu.
ORCID
NOE NMR data (see Supporting Information) clearly
confirmed the stereochemistry of the E and the Z isomers of 6.
However, neither isomer was amenable for X-ray crystallog-
raphy. Thus, we also studied the spectroscopic characteristics
and X-ray crystal data of several closely related systems and
summarized general trends between the stereochemistry of the
diaryl-substituted enone versus the NMR chemical shift of the
enone proton (see Supporting Information, Table SI-1, p S4).
For compounds of structure 6, a characteristic downfield shift
is observed for this enone proton in the E-isomer than in the
Z-isomer. This may be due to a deshielding magnetic
anisotropic effect by the carbonyl group.
N. G. R. Dayan Elshan: 0000-0001-7830-1477
Michael E. Jung: 0000-0003-1290-8588
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health
(CA164331 and CA12861), the Prostate Cancer Foundation
(Challenge Award and VAlor Award), and the U.S. Depart-
ment of Veterans Affairs.
The β-amino diaryldienone compounds of structure E-6 that
we have synthesized through the chemistry outlined here have
shown excellent in vitro and in vivo antiprostate cancer activity
in androgen receptor (AR) positive tumors.8−10 Functionally,
they have shown potent activity as AR degraders and as
inhibitors of the AR transactivation domain.8 A complete
disclosure of the biological activity of these compounds will be
reported elsewhere. The β-amino enone functionality of E-6
can also undergo further synthetic transformations (Scheme 3)
to yield other compounds of medicinal chemistry interest.20
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D
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