Enantioselective Synthesis of All-Carbon Quaternary Centers Structurally Related to Amaryllidaceae Alkaloids

Article abstract of DOI:10.1002/chem.201802493

Enantioselective synthesis of all-carbon quaternary centers remains a considerable challenge for synthetic organic chemists. Here, we report a two-step protocol to synthesize such centers including tandem cyclization/Suzuki cross-coupling followed by halocarbocyclization. During this process, two rings, three new C?C bonds and a stereochemically defined all-carbon quaternary center are formed. The absolute configuration of this center is controlled by the stereochemistry of the adjacent stereocenter, which derives from an appropriate enantioenriched starting material. Using this method, we synthesized polycyclic compounds structurally similar to Amaryllidaceae alkaloids in high enantiomeric excesses. Because these products resemble naturally occurring compounds, our protocol can be used to synthesize various potentially bioactive compounds.

Full text of DOI:10.1002/chem.201802493

A Journal of
Accepted Article
Title: Enantioselective Synthesis of All-Carbon Quaternary Centers
Structurally Related to Amaryllidaceae Alkaloids
Authors: Jiří Mikušek, Petr Jansa, Pratap R. Jagtap, Tomáš Vašíček,
Ivana Císařová, and Eliška Matoušová
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To be cited as: Chem. Eur. J. 10.1002/chem.201802493
Link to VoR: http://dx.doi.org/10.1002/chem.201802493
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Enantioselective Synthesis of All-Carbon Quaternary Centers
Structurally Related to Amaryllidaceae Alkaloids
Jiří Mikušek, Petr Jansa, Pratap R. Jagtap, Tomáš Vašíček, Ivana Císařová, and Eliška Matoušová*
Abstract: Enantioselective synthesis of all-carbon quaternary
centers remains a considerable challenge for synthetic organic
chemists. Here, we report a two-step protocol to synthesize such
centers including tandem cyclization/Suzuki cross-coupling followed
by halocarbocyclization. During this process, two rings, three new C-
C bonds and a stereochemically defined all-carbon quaternary
center are formed. The absolute configuration of this center is
Scheme 1. Retrosynthetic analysis of crinane-type Amaryllidaceae alkaloids.
controlled by the stereochemistry of the adjacent stereocenter, which
derives from an appropriate enantioenriched starting material. Using
this method, we synthesized polycyclic compounds structurally
Hence, our attention turned to the enantioselective
similar to Amaryllidaceae alkaloids in high enantiomeric excesses.
synthesis of C-like compounds. We decided to start with the five-
Because these products resemble naturally occurring compounds,
membered analogues of our target alkaloids because we
our protocol can be used to synthesize various potentially bioactive
expected a preferential cis-fusion between the two rings, which
compounds.
would increase the stereoselectivity. The formation of
a
tetracyclic skeleton similar to that of compound C has been
previously described in only three reports.13–15 However, in
contrast to our targets, the reported products were carbocyclic
(angular triquinanes), their syntheses were based on acid-
promoted rearrangements and lacked enantiocontrol.
Alkaloids with
a crinane skeleton are an important
subclass of Amaryllidaceae alkaloids containing all-carbon
quaternary centers. These alkaloids have valuable biological
effects, including cytotoxic,1 antimalarial2 or antiinflammatory3
activities. Crinamine and haemanthamine, for example, are
potent and selective cytotoxic agents (Scheme 1).4,5 However,
the synthesis of such natural products requires the
enantioselective formation of an all-carbon quaternary center.
Although many research groups have developed methods for
the enantioselective synthesis of all-carbon quaternary
centers,6–12 this remains a challenging task requiring further
investigation.
Considering our interest in the synthesis of biologically
active alkaloids and their analogues, we sought to develop an
approach that would allow us to form all-carbon quaternary
centers with good stereocontrol. Our proposed retrosynthetic
analysis of the target alkaloids (Scheme 1) shows that they
could be derived from
A by intramolecular nucleophilic
substitution forming the methylene bridge. In turn, compound A
could be made from dicarbonyl B, which would be the product of
oxidative cleavage of a double bond in tetracycle C. The
stereochemistry of the all-carbon quaternary center in C would
remain unaffected by this process.
Scheme 2. Tandem reaction – comparison between reported procedures and
our approach.
[*]
Dr. J. Mikušek, P. Jansa, Dr. P. R. Jagtap, T. Vašíček, Dr. E.
Matoušová
Department of Organic Chemistry
Faculty of Science, Charles University
Praha, 128 43 (Czech Republic)
E-mail: eliska.matousova@natur.cuni.cz
Dr. I. Císařová
Department of Inorganic Chemistry
Faculty of Science, Charles University
Praha, 128 43 (Czech Republic)
In order to develop an efficient method, we explored the
possibility of including tandem reactions in our sequence. We
envisioned that palladium-catalyzed tandem cyclization/Suzuki
cross-coupling followed by halocarbocyclization could be a
convenient approach to the synthesis of all-carbon quaternary
centers. The tandem reaction consisting of cyclic
carbopalladation16 and subsequent Suzuki cross-coupling with
Supporting information for this article is given via a link at the end of
the document.
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aryl halides as starting materials17–21 has been previously
applied to the synthesis of natural products and bioactive
compounds.22–25 However, to our knowledge, only two methods
have been reported using alkenyl halides,26 and alkenyl
dihalides27 as starting compounds. Moreover, the reported
procedures prevented further cyclization of the products to form
an all-carbon quaternary center (Scheme 2, refs. 17-25) or they
lacked any form of stereocontrol (refs. 26 and 27).
In this study, we selected alkenyl iodides 1-3 as starting
materials to transform the products of the tandem reaction 4-6
into the polycyclic compounds 7-9, respectively, by
halocarbocyclization28 (Scheme 2). In the second step, the all-
carbon quaternary center of the defined configuration is formed.
In fact, cis-fusion between the two five-membered rings should
be the only possible stereochemical outcome of cyclization
because the energy difference between cis- and trans-fused
bicyclo[3.3.0]octanes is sufficiently high for any trans-fused
compound to equilibrate to the cis-isomer immediately.29 Thus,
we should be able to control the absolute configuration of the
quaternary center when the corresponding enantioenriched
Table 1. Scope of the tandem cyclization/Suzuki cross-coupling reaction.
Time
[h]
Yield[c] Ratio
Entry 1-3
R
Conditions[b]
Product
[%]
98
98
89
98
74
67
94
96
98
90
98
94
83
Z / E
1
2
1
1
1
1
1
1
2
2
2
2
2
2
3
3,4-OCH2O
4-OCH3
4-CH3
A
A
4
4
4a
4b
4c
4d
4e
4f
73 / 27
75 / 25
87 / 13
74 / 26
77 / 23
91 / 9
3
A
3
4
H
A
3
5
4-CF3
B[d]
B[d,e]
B
25
19
23
45
3
6
2,4-diF
3,4-OCH2O
4-OCH3
4-CH3
7
5a
5b
5c
5d
5e
5f
93 / 7
8
B
92 / 8
9
A
88 / 12
90 / 10
90 / 10
93 / 7
10
11
12
13
H
A
3.5
19
17.5
16
4-CF3
B
starting material 1,
2
or
3
is employed. Notably,
2,4-diF
4-OCH3
B
halocarbocyclization has been used before to prepare
spirocycles,30–32 and also in total synthesis33–35 but not to
synthesize quaternary centers.
B
6b
1 / 99
[a] MBS = 4-methoxybenzenesulfonyl. [b] A: Pd(PPh3)4 (4 mol%), K2CO3 (4
eq), and boronic acid (1.6 eq) at 80 °C in toluene/H2O (4:1). B: Pd(PPh3)4 (5
mol%), Cs2CO3 (2 eq) and boronic acid (1.6 eq) in THF/H2O (10:1). [c] Isolated
yield. [d] 2.6 eq of Cs2CO3 was used. [e] 1.3 eq of boronic acid was used.
To test our method, we subjected compound 1 to a
reaction with methylenedioxyphenylboronic acid under Suzuki
cross-coupling conditions (Pd(PPh3)4 (4 mol%), Na2CO3,
refluxing benzene). Fortunately, we were able to isolate the
desired product 4a with an 80% yield in this initial experiment.
The yield was further improved, up to 98%, by changing the
solvent and the base. We then used these modified conditions to
explore the scope of the tandem reaction (Table 1). Starting
compounds 1 and 2 were converted into products 4 and 5,
respectively, with high yields and good Z/E ratios with all tested
arylboronic acids, regardless of their electronic properties.
Generally, the Z/E ratios were better for nitrogen-containing
products 5 (entries 7–12) than for their oxygen congeners
(entries 1–6) and always yielding the desired Z isomer as the
main product, which is the only product that undergoes
subsequent halocyclization (see Table S1 in the Supporting
Information for additional experiments). The minor E isomer was
presumably formed by isomerization of intermediate 10 (Scheme
2). Such isomerization was previously described for 1-silyl-1-
metalloalkenes.36 In addition to the aforementioned heteroatom-
containing substrates, we performed an experiment with
carbocyclic substrate 3, generating product 6b with a good yield
and with an excellent E/Z ratio (Entry 13).
The presence of triethylsilyl group in our starting
compounds was necessary to ensure the high yield of the
reaction. We also tested different capping groups. However, in
these cases, the following side reactions were observed,
depending on the capping group: isomerization of the product,
when using the methyl group; partial desilylation in the following
step, when using trimethylsilyl; low yield and isomerization of the
product, when using uncapped substrate. Moreover, in the
subsequent halocarbocyclization the triethylsilyl group hindered
the attack of electrophile on the adjacent double bond.
Halocarbocyclization of compounds 4-6 was first attempted
using N-bromosuccinimide (NBS), albeit with unsatisfactory
results. Further reagent screening showed that N-
iodosuccinimide (NIS) provided the best yields in the cyclizations
of 4a and 5a (Table 2, entries 1, 8), whereas Snyder’s BDSB37
was more suitable for all other starting compounds (compare
entries 5 vs. 6, and 10 vs. 11). The reaction proceeded well
when using substrates containing electron-donating groups on
the aromatic ring (entries 3, 10, 11, 12). Conversely, no
compound with electron-poor aromatics was involved in
halocarbocyclization; accordingly, we observed only traces or no
product formation in such instances (entries 7, 14, 15). Nitrogen-
containing products 8 were formed with higher yields (entries 8–
13), especially after changing the solvent to DCM, than oxygen
derivatives 7 (entries 1–6). This result may be attributed to the
lower stability of compounds 4, both during storage and under
the reaction conditions tested. Carbocyclic compound 6b was
also subjected to halocyclization with BDSB, and the
corresponding product 9bb was generated with a 68% yield
(Entry 16).
To gather solid evidence on the stereochemistry of this
reaction, we prepared a crystal of product 7ai suitable for X-ray
analysis. The resulting data38 confirmed the expected relative
configuration of 7ai and that the two saturated five-membered
rings are cis-fused, as predicted. This result prompted us to
explore an enantioselective version of our synthesis.
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Table 2. Scope of halocarbocyclization.
Encouraged by these results, we proceeded with the
enantioselective synthesis of the nitrogen-containing compound
8ai. Our original reaction sequence was based on the Mitsunobu
reaction of the enantioenriched alcohol (S)-12 with a protected
propargylamine (Section 5.2 in the Supporting Information).
Unfortunately, the anticipated inversion of configuration did not
occur in this step, and the ee value decreased from 99% to 72%.
This result was attributed to the competing SN2' mechanism,
which has been previously reported for allylic alcohols.40
Importantly, the enantiomeric excess did not decrease in the
following reactions, and the final product (S,R,R)-8ai was
isolated with a 76% ee.
Time
[h]
Entry 4-6
R
Reagent[b]
Solvent Product Yield[c]
1
2
4a
4a
4b
4c
4d
4d
4e
5a
5a
5b
5b
5c
5d
5e
5f
3,4-OCH2O
3,4-OCH2O
4-OCH3
4-CH3
NIS
BDSB[d]
BDSB
BDSB
NIS
1
MeCN
MeCN
DCM
7ai
7ab
7bb
7cb
7di
82
26
0.5
1
3
50
4
0.3
15
MeCN
MeCN
MeCN
MeCN
MeCN
DCM
32
5
H
0
6
H
BDSB
BDSB
NIS
0.6
0.6
1
7db
7eb
8ai
37
7
4-CF3
traces
98
8
3,4-OCH2O
3,4-OCH2O
4-OCH3
4-OCH3
4-CH3
9
BDSB
NIS
0.5
1.3
0.6
0.5
1.3
0.6
1.5
0.2
8ab
8bi
59
10
11
12
13
14
15
16
MeCN
MeCN
DCM
60
BDSB
BDSB
BDSB
BDSB
BDSB
BDSB
8bb
8cb
8db
8eb
8fb
77
80
H
DCM
74
4-CF3
DCM
traces
0
2,4-diF
4-OCH3
DCM
6b
MeCN
9bb
68
[a] MBS = 4-methoxybenzenesulfonyl. [b] Reactions were performed with 1.2
eq. of NIS or 1.05 eq. of BDSB. [c] Isolated yield based on the Z isomer of the
starting material. [d] Bromodiethylsulfonium bromopentachloroantimonate (see
ref. 34).
To prepare enantioenriched compounds 1 and 2, we used
asymmetric Corey-Bakshi-Shibata reduction of ketone 11,39
yielding alcohol (S)-12 with high enantiomeric excess (up to 99%
ee after recrystallization). The proof-of-concept synthesis of
oxygen-containing compound (R,S,S)-7ai was performed using
alcohol (S)-12 with 93% ee (Scheme 3). The alcohol was
alkylated, and the resulting terminal alkyne (S)-13 was protected
with triethylsilyl group to give (S)-1. Then, (S)-1 was subjected to
the tandem reaction with methylenedioxyphenylboronic acid
followed by halocarbocyclization under standard conditions, and
the final pentacyclic product, (R,S,S)-7ai, was formed with an
87% ee. Notably, the overall loss of optical purity was only 6% in
the entire reaction sequence.
Scheme 4. Chemoenzymatic synthesis of the nitrogen-containing (R,S,S)-20.
These results showed that our method is suitable for
enantioselective formation of all-carbon quaternary centers but
only if the substrate can be prepared with higher
enantioselectivity. To this end, we decided to use
a
commercially available Candida antarctica lipase B (CAL-B),
which has been shown to be an effective catalyst to prepare
chiral amines and amides,41 for enzymatic kinetic resolution. The
enzymatic reaction was performed with the racemic primary
amine 14, prepared from alcohol 12 by Gabriel synthesis, and
generated a mixture of amine (S)-14 and amide (R)-15 in high
enantiomeric excesses (Scheme 4). Due to difficulties in
isolating the amine, the mixture was directly treated with Boc
anhydride to yield carbamate (S)-16 and amide (R)-15, easily
separable from each other by column chromatography. The
isolated carbamate (S)-16 was then alkylated with propargyl
bromide, providing alkyne (S)-17 in 94% yield, albeit only when
replacing the solvent by DMF. Subsequent capping with Et3SiCl
yielded compound (S)-18, which was then subjected to the
tandem reaction/halocarbocyclization sequence under standard
conditions. We observed no significant decrease in enantiomeric
excess throughout the reaction sequence, and the final product
Scheme 3. Enantioselective synthesis of the oxygen-containing (R,S,S)-7ai.
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Entry for the Table of Contents
COMMUNICATION
Jiří Mikušek, Petr Jansa, Pratap R.
Jagtap, Tomáš Vašíček, Ivana Císařová,
and Eliška Matoušová*
Page No. – Page No.
Enantioselective Synthesis of All-
Carbon Quaternary Centers
Structurally Related to
A two-step protocol for an enantioselective synthesis of all-carbon quaternary
centers is reported. Using this method, we synthesized polycyclic compounds
structurally similar to Amaryllidaceae alkaloids in high yield and enantioselectivity.
Amaryllidaceae Alkaloids
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