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2501
Formal total syntheses of classic natural product target
molecules via palladium-catalyzed enantioselective alkylation
Yiyang Liu, Marc Liniger, Ryan M. McFadden, Jenny L. Roizen, Jacquie Malette,
Corey M. Reeves, Douglas C. Behenna, Masaki Seto, Jimin Kim, Justin T. Mohr,
Scott C. Virgil and Brian M. Stoltz
*
Full Research Paper
Open Access
Address:
Warren and Katharine Schlinger Laboratory of Chemistry and
Chemical Engineering, Division of Chemistry and Chemical
Engineering, California Institute of Technology, 1200 E. California
Boulevard, Pasadena, CA, USA
Email:
Brian M. Stoltz
*
- stoltz@caltech.edu
* Corresponding author
Keywords:
enantioselective alkylation; natural products; palladium
Beilstein J. Org. Chem.
2014,
10,
2501–2512.
doi:10.3762/bjoc.10.261
Received: 11 August 2014
Accepted: 09 October 2014
Published: 28 October 2014
Associate Editor: S. Bräse
© 2014 Liu et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Pd-catalyzed enantioselective alkylation in conjunction with further synthetic elaboration enables the formal total syntheses of a
number of “classic” natural product target molecules. This publication highlights recent methods for setting quaternary and tetra-
substituted tertiary carbon stereocenters to address the synthetic hurdles encountered over many decades across multiple compound
classes spanning carbohydrate derivatives, terpenes, and alkaloids. These enantioselective methods will impact both academic and
industrial settings, where the synthesis of stereogenic quaternary carbons is a continuing challenge.
2501
Introduction
Catalytic enantioselective allylic alkylation has emerged as a
powerful method for the construction of building blocks bearing
quaternary carbon and fully substituted tertiary centers [1,2]. A
recent addition developed by our laboratory is the allylic alkyl-
ation of nonstabilized enolate precursors to form
α
-quaternary
carbonyl compounds (Scheme 1) [3]. Once the key stereocenter
is set by this chemistry, further elaboration allows access to
many bioactive small molecules. In our lab alone, this palla-
dium-catalyzed alkylation has enabled the enantioselective total
syntheses of dichroanone [4], elatol [5], cyanthiwigins [6-8],
carissone [9], cassiol [10], chamigrenes [11], and liphagal [12].
Other labs have also utilized our method in natural product total
synthesis [13,14]. Often, it is the case that a new technology
that allows the synthesis of building blocks will open up new
avenues to complex structures of long standing interest [15,16].
Herein we detail the application of this asymmetric chemistry in
formal total syntheses of “classic” natural product targets
across a range of compound families by strategic selection of
allylic alkylation substrates and subsequent product transforma-
tions.
Beilstein J. Org. Chem.
2014,
10,
2501–2512.
2502
Scheme 1:
Three classes of Pd-catalyzed enantioselective allylic
alkylations.
Results and Discussion
A) Thujopsene
The Japanese hiba tree,
Thujopsis dolabrata
has been used for
centuries as decoration and within traditional architecture [17].
The plant is a member of the order
Cupressaceae
, and its
fragrant wood oil contains numerous sesquiterpenes including
mayurone (
8
) [18,19], widdrol (
9
) [20], and (
)-thujopsene (
10
)
(Figure 1) [21,22]. The wood oil is a potent dust mite deterrent;
thus, in addition to its ornamental value, the hiba tree also
provides and environmentally benign means of pest control
[23,24].
Figure 1:
Selected natural products from
Thujopsis dolabrata
.
(
)-Thujopsene (
10
) has attractive features to the synthetic
chemist. Its tricyclo[5.4.0.0
1,3
]undecane skeleton contains three
contiguous all-carbon quaternary centers, two of which are
stereogenic. Being a hydrocarbon, (
)-thujopsene (
10
) has few
natural handles for retrosynthetic analysis. Inspired by the
complexity of this relatively small natural product, several total
syntheses of racemic
10
have been reported [25-29] along with
at least two enantioselective routes [30-32].
One enantiospecific total synthesis of (+)-thujopsene (
10
) by
Srikrishna and Anebouselvy began with (
R
)-carvone (
11
)
(Scheme 2) [33]. During the total synthesis, the authors
prepared carboxylic acid (+)-
12
over a 14-step sequence. We
planned to intercept the antipode of (+)-
12
using the palladium-
catalyzed enantioselective alkylation chemistry described
above.
Scheme 2:
Srikrishna and Anebouselvy’s approach to (+)-thujopsene.
We commenced a formal total synthesis of (
)-thujopsene (
10
)
with the goal of improved efficiency compared to the Srikr-
ishna/Anebouselvy route and to use enantioselective palladium
catalysis to install the initial stereocenters (Scheme 3). Treat-
ment of
16
with LiHMDS in THF, followed by allyl chlorofor-
mate, furnished the known carbonate
17
in high yield [34]. This
substrate smoothly undergoes palladium-catalyzed enantiose-
lective decarboxylative allylation in the presence of (
S
)-
t
-Bu-
PHOX (
5
), giving allyl ketone (
)-
18
in 94% yield and 91% ee
[34]. Treatment of the ketone (
)-
18
with MeMgBr at 23 ºC
provided a mixture of two diastereomeric alcohols
19A
and
19B
in 94% yield. Without separation, the diastereomers were
rapidly carried through a three-step sequence of hydroboration/
oxidation, terminal alcohol silylation, and tertiary alcohol dehy-
Beilstein J. Org. Chem.
2014,
10,
2501–2512.
2503
Scheme 3:
Formal total synthesis of (
)-thujopsene.
dration, affording methylene cyclohexane (
)-
20
. Treatment of
this silyl ether with Jones reagent simultaneously cleaved the
silyl group and oxidized the resulting alcohol, furnishing
carboxylic acid (
)-
12
in 65% yield. With this enantioenriched
acid in hand, the formal total synthesis of (
)-thujopsene (
10
) is
completed in only 9 steps from trimethylcyclohexanone
16
.
B) Quinic acid
(
)-Quinic acid (
21
) [35,36] serves as a useful chiral building
block that has been employed in numerous syntheses [37],
including our own syntheses of (+)- and (
)-dragmacidin F [38-
41], and the initial commercial-scale synthesis of Tamiflu [42].
In Renaud’s formal total synthesis of (
)-quinic acid (
21
) [35],
a key carboxylic acid
22
was accessed, intercepting Novak’s
older synthesis of the natural product (Scheme 4) [36]. To
begin, Renaud transformed the chiral glycolic acid ketal
23
(enantioenriched to 80% ee) to the more elaborate diene
24
via
two diastereoselective alkylations. After a sequence of three
reactions including removal of the pinacolone portion of the
auxiliary, carboxylic acid
22
could be accessed. Novak’s syn-
thesis applied a bromolactonization of
22
to build in the requi-
site syn relationship between the carboxylate group and the
3-hydroxy group, ultimately leading to quinic acid.
Unlike the allylic alkylations in Scheme 1, which form all-
carbon stereocenters, we envisioned a unique modification of
the silyl enol ether version to access nonracemic tertiary alco-
hols (Scheme 5) [43]. The planned modification would involve
the use of dioxanone-derived substrates instead of the prototyp-
ical cycloalkanone-derived ones. To demonstrate this new tech-
nology in the context of formal total synthesis, we chose to
intercept the acid
22
in the Renaud and Novak routes to quinic
acid (
21
). Conversion of dioxanone
25
to a cyclohexylimine
Scheme 4:
Renaud’s formal total synthesis of (
)-quinic acid.
enabled alkylation via a metalloenamine. On acidic work-up,
imine hydrolysis furnished an alkylated dioxanone in good
yield. The targeted silyl enol ether
26
was prepared by thermo-
dynamic silylation in 66% yield [43]. Optimal conversions and
enantioselectivities were achieved from triethylsilyl enol ether
26
on exposure to [Pd(dmdba)
2
] (5 mol %), (
S
)-
t
-BuPHOX (
5
,
5.5 mol %), and diallyl carbonate (1.05 equiv) at 25 °C, in
PhMe with an equivalent of Bu
4
NPh
3
SiF
2
(TBAT) [43]. Recog-
nizing that enantioenriched
α
,
ω
-dienes could be transformed
into cycloalkenes with a stereocenter remote to the olefin [44],
chiral diene
27
was submitted to ring closing metathesis to
generate
28
in 90% yield and 92% ee [43]. Cyclohexene
28
readily undergoes acetonide cleavage and periodic acid oxi-
dation to provide carboxylic acid (
S
)-
22
[43], completing the
formal synthesis of (
)-quinic acid (
21
). Additionally, one could
in principle also access the less commercially abundant
antipode (+)-quinic acid (
21
) using the catalyst (
R
)-
t
-Bu-PHOX.
Beilstein J. Org. Chem.
2014,
10,
2501–2512.
2504
Scheme 5:
Formal total synthesis of (
)-quinic acid.
C) Dysidiolide
Dysidiolide (
29
, Scheme 6) was isolated from the marine
sponge
Dysidea etheria
and found to have inhibitory activity
toward protein phosphatase cdc25, with an IC
50
value of
9.4 μM [45]. This enzyme is a member of the protein family re-
sponsible for dephosphorylation of cyclin-dependent kinases
[46]. Thus, inhibitors of cdc25 might allow for targeted cell-
cycle disruption [45]. The relative stereochemistry of
dysidiolide (
29
) was determined via single-crystal X-ray
diffraction analysis, revealing a molecule with six stereocenters,
two of which are quaternary carbons [45]. Several groups have
reported total syntheses of this natural product [47-53], three of
which are enantioselective [54-56].
In Danishefsky’s approach to racemic dysidiolide, the cyclo-
hexene ring of
30
was installed via diastereoselective
Diels–Alder reaction of a transient dioxolenium dienophile and
chiral vinylcyclohexene
31
[48]. Triene
31
was prepared from
α
-quaternary ketone (±)-
32
in racemic form. We anticipated the
interception of (
)-
32
[57] in Danishefsky’s route using
enantioselective palladium-catalyzed allylic alkylation to set the
quaternary stereocenter.
The formal total synthesis of (
)-dysidiolide (
29
) commenced
with known allyl
β
-ketoester
4
(Scheme 7), which was
converted to 2-allyl-2-methylcyclohexanone (
2
) in 85% yield
and 88% ee [58] with a catalytic amount of [Pd
2
(dba)
3
] and (
S
)-
t
-BuPHOX (
5
, Scheme 1). The allyl ketone was enriched to
98% ee via recrystallization of semicarbazone
33
[59]. Using
the Grubbs 2
nd
generation metathesis catalyst, allyl ketone (
)-
2
was crossed with methyl vinyl ketone in 62% yield [34]. Reduc-
tion of enone
34
was achieved in the presence of Pd/C with H
2
Scheme 6:
Danishefsky’s approach to (±)-dysidiolide.
in EtOAc to furnish diketone
35
[34]. Chemoselective Wittig
mono-olefination of
35
provided
ω
-enone (
)-
32
, spectroscopi-
cally identical to the material in Danishefsky’s racemic syn-
thesis. This formal synthesis shows the power of the enantiose-
lective allylic alkylation to access formerly racemic constructs
as single enantiomers; Danishefsky’s synthesis is now rendered
enantioselective.
D) Aspidospermine
The aspidosperma alkaloids have garnered much attention as
beautiful targets for the synthetic chemist. Most of the 250-plus
Beilstein J. Org. Chem.
2014,
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2505
Scheme 7:
Formal total synthesis of (
)-dysidiolide.
compounds in this class share a pentacyclic core, from the clin-
ical anticancer therapeutics vincristine and vinblastine to the
simpler aspidospermidine [60]. To address the challenging syn-
thetic features of the aspidosperma alkaloids, many clever syn-
thetic approaches have been reported [61,62]. One popular
target in this family is aspidospermine (
36
, Scheme 8).
Although its medicinal potency is inferior to other members of
the class, this alkaloid has served as a proving ground for many
synthetic chemists.
In 1989, Meyers reported an enantioselective synthesis of the
(4a
S
,8a
R
,8
S
)-hydrolilolidone core
37
[63,64] present in aspi-
dospermine (
36
), and thus a formal total synthesis of the alka-
loid itself, intercepting Stork’s classic route [61]. One precursor
described in the core synthesis is enone
38
, which bears the
quaternary stereocenter of the natural product. Contrasting
Meyers’ approach, which employed a chiral auxiliary as part of
39
, we thought a catalytic enantioselective alkylation strategy
would be ideal for a formal total synthesis of natural (
)-aspi-
dospermine (
36
) via the antipode of
38
.
The formal synthesis began with 1,3-cyclohexanedione (
40
),
which was converted to isobutyl vinylogous ether
41
under acid
Scheme 8:
Meyers’ approach to unnatural (+)-aspidospermine.
promotion (Scheme 9) [65]. The
β
-ketoester
42
was prepared
using a two-step sequence of acylation and alkylation, then
treated with the (
S
)-
t
-Bu-PHOX catalyst system (with
[Pd(dmdba)
2
]) to generate (+)-
43
in 86% ee. The challenge of
installing the
γ
-stereocenter of the target
38
was addressed as
follows: LiAlH
4
treatment of (+)-
43
gave exclusive 1,2-reduc-
tion. When the crude product was hydrolyzed,
β
-elimination
Beilstein J. Org. Chem.
2014,
10,
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2506
Scheme 9:
Formal total synthesis of (
)-aspidospermine.
gave the desired enone
38
. The overall formal synthesis repre-
sents a rare example of enantioselective Stork–Danheiser chem-
istry [66,67].
E) Rhazinilam
(
)-Rhazinilam (
44
) has been isolated from various plants
including
Rhazya strica decaisne
[68],
Melodinus australis
[69], and
Kopsia singapurensis
[70]. Shortly after the first isola-
tion, its structure was elucidated by single crystal X-ray diffrac-
tion analysis [71]. It features a tetracyclic scaffold with a nine-
membered ring and an all-carbon quaternary stereocenter. This
alkaloid is a microtubule-disrupting agent that displays similar
cellular effects to paclitaxel [72,73]. Because of its biological
activities and potential pharmaceutical use, many groups have
pursued its total synthesis [74-77], including a number of
enantioselective syntheses [78-82].
In 2001, Magnus reported a total synthesis of rhazinilam in
racemic form (Scheme 10) [83]. In their approach, the first
retrosynthetic disconnection of the amide C–N bond in the nine-
membered ring led to tricyclic compound
45
. The pyrrole ring
of
45
was formed by intramolecular condensation of cinnamyl
amide
46
, which is prepared via union of quaternary piperidi-
none
47
and cinnamyl electrophile
48
. We envisioned that our
allylic alkylation of lactam enolates would furnish enantioen-
riched piperidinone
47
, and thus a single enantiomer of
rhazinilam may be prepared.
The formal synthesis of (+)-rhazinilam commenced with palla-
dium-catalyzed decarboxylative allylic alkylation of known
carboxy-lactam
49
to afford benzoyl-protected piperidinone
50
in 97% yield and 99% ee (Scheme 11) [84]. Cleavage of the
benzoyl group under basic conditions furnished piperidinone
(+)-
47
[84], which can be advanced to (+)-rhazinilam via
Scheme 10:
Magnus’ approach to (±)-rhazinilam.
Magnus’ route. This formal synthesis demonstrates the utility of
our recently developed asymmetric lactam alkylation chemistry.
F) Quebrachamine
Quebrachamine (
51
) is an indole alkaloid isolated from the
Aspidosperma quebracho
tree bark [60]. It has been found to
possess adrenergic blocking activities for a variety of urogen-
ital tissues [85]. Structurally, it features a tetracycle including
an indole nucleus, a 9-membered macrocycle, and an all-carbon
quaternary stereocenter. Due to its structural complexity and
biological activities, quebrachamine has received considerable
attention from the chemistry community. A number of total
syntheses have been reported [86-88], with several examples of
asymmetric syntheses [89-91].
In 2007, Amat reported an enantioselective total synthesis of
quebrachamine (Scheme 12) [92]. In their planning, disconnec-
Beilstein J. Org. Chem.
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2507
Scheme 11:
Formal total synthesis of (+)-rhazinilam.
tion at the macrocycle led to amide
52
, which was prepared
from 3,3-disubstituted piperidine
53
. The all-carbon quaternary
stereocenter in
53
was installed by double alkylation of lactam
55
, using an auxiliary to control the stereoselectivity. We envi-
sioned that an alternative way of constructing this motif would
again make use of our recently developed palladium-catalyzed
asymmetric alkylation of lactam enolates.
Scheme 12:
Amat’s approach to (
)-quebrachamine.
The formal synthesis of (+)-quebrachamine commenced with
benzoyl lactam
50
(Scheme 13), which was prepared in excel-
lent yield and ee by alkylation of carboxy-lactam
49
(see
Scheme 11) [84]. Oxidative cleavage of the terminal double
bond and subsequent reduction with LiAlH
4
afforded
N
-benzylpiperidine-alcohol
56
[84]. Hydrogenolysis of the
N
-benzyl group and re-protection with di-
tert
-butyl dicarbonate
Scheme 13:
Formal total synthesis of (+)-quebrachamine.
furnished
N
-boc piperidine-alcohol (+)-
53
[84], thus inter-
cepting an intermediate in Amat’s synthesis of quebrachamine.
G) Vincadifformine
Vincadifformine (
59
) was isolated in both enantioenriched and
racemic forms from the leaves and roots of
Rhazya stricta
in
1963 [93]. Not only is it a representative member of the
Aspi-
dosperma
alkaloid family, but it also holds particular signifi-
cance as a valuable precursor to pharmaceutically important
vincamine, vincamone, and cavinton [94-97]. The molecule has
a fused pentacyclic framework with three contiguous stereocen-
ters, two of which are all-carbon quaternary centers. The medic-
inal relevance and structural complexity of vincadifformine
Beilstein J. Org. Chem.
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2508
Scheme 14:
Pandey’s approach to (+)-vincadifformine.
have led to a large number of total syntheses [98-104],
including several enantioselective examples [105-108].
Recently, Pandey reported a highly efficient synthesis of (+)-
vincadifformine (Scheme 14) [106]. The key step in the syn-
thesis was an iminium ion cascade reaction that formed the
fused ring systems by coupling 3,3-disubstituted tetrahydropyri-
dine
57
with indole derivative
58
. The former coupling partner
was derived from chiral
α
-quaternary lactam
60
, which was
constructed using a chiral auxiliary strategy. We envisioned that
chiral lactam
60
could again be readily accessed by our palla-
dium-catalyzed enantioselective alkylation chemistry.
The formal synthesis of (
)-vincadifformine commenced with
ruthenium-catalyzed isomerization of the terminal olefin moiety
in unprotected piperidinone (+)-
47
(made previously in the
formal synthesis of (+)-rhazinilam shown in Scheme 11) to
produce internal olefin
63
(Scheme 15) [109]. Ozonolysis of the
double bond furnished aldehyde
64
, which was reduced under
Luche conditions to alcohol
65
, a compound identical in struc-
ture and enantiomeric to the intermediate employed by Pandey
in the synthesis of (+)-vincadifformine.
Conclusion
The development of a series of Pd-catalyzed methods for
constructing stereogenic quaternary carbons has provided two
generations of building blocks (Scheme 16). The described
derivatization enabled the formal total syntheses of an array of
classic natural products including sugar derivatives, terpenes,
and alkaloids, adding significantly to the growing list of uses
for this powerful C–C bond construction. An efficient route to
the sesquiterpenoid (
)-thujopsene (
10
) has been delineated,
allowing access to the compound’s natural antipode. Our lab’s
novel approach to (
)-quinic acid (
21
) allowed access to either
Scheme 15:
Formal total synthesis of (
)-vincadifformine.
enantiomer of this important substance. We have also inter-
cepted a key intermediate in Danishefsky’s synthesis of (±)-
dysidiolide (
29
), rendering the former racemic route enantiose-
lective. Additionally, a rapid approach to a compound in
Meyers’ formal synthesis of (+)-aspidospermine (
36
) granted
access to the natural product without the use of a chiral auxil-
Beilstein J. Org. Chem.
2014,
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2509
Scheme 16:
Two generations of building blocks.
iary. Finally, we have demonstrated the application of lactam
alkylation products in the catalytic asymmetric syntheses of (+)-
rhazinilam (
44
), (+)-quebrachamine (
51
), and (
)-vincadif-
formine (
59
). The powerful catalytic enantioselective allylic
alkylation will undoubtedly enable new synthetic endeavors in
the context of both academic and industrial research.
Supporting Information
Supporting information features experimental procedures,
characterization data of synthesized compounds, copies of
1
H and
13
C NMR spectra, and single crystal structure data.
Supporting Information File 1
Experimental data, NMR spectra and X-ray data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-261-S1.pdf]
Acknowledgements
We are grateful to NIH (R01GM080269), Amgen, the Gordon
and Betty Moore Foundation, and Caltech for funding. We also
thank the Caltech Minorities Undergraduate Research Fellow-
ship program, PREM program, Eli Lilly, the Resnick Sustain-
ability Institute at Caltech (fellowship for Y. L.), and the Swiss
National Science Foundation (SNSF, fellowship for M. L.). Dr.
Michael L. Krout and Dr. David E. White are acknowledged for
preliminary experimental work related to their results. Dr.
Michael Takase (Caltech) and Larry Henling (Caltech) are
gratefully acknowledged for X-ray crystallographic structural
determination.
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