Bioinspired total synthesis of montanine-type amaryllidaceae alkaloids

Article abstract of DOI:10.1002/anie.201307324

Finding a common path: A synthetic strategy inspired by the proposed biogenesis of the montanine-type alkaloids enabled the concise asymmetric synthesis of these compounds in a divergent fashion on the basis of an unprecedented tandem oxidative dearomatization/intramolecular aza-Michael addition as the key step. This bioinspired approach revealed a chemical connection between the cherylline- and montanine-type alkaloids (see scheme). Copyright

Full text of DOI:10.1002/anie.201307324

Angewandte
Chemie
DOI: 10.1002/anie.201307324
Asymmetric Synthesis
Hot Paper
Bioinspired Total Synthesis of Montanine-Type Amaryllidaceae
Alkaloids**
Xu Bao, Ye-Xing Cao, Wen-Dao Chu, Hu Qu, Ji-Yuan Du, Xian-He Zhao, Xiao-Yan Ma,
Cheng-Tao Wang, and Chun-An Fan*
Inspiration from natural products biosynthesis for chemical
synthesis has been playing an increasingly crucial role in the
discovery and invention of new synthetic strategies and
methods:[1] one of the central issues in the field of natural
products synthesis. Significantly, the tracking or mimicking of
a possible biogenetic pathway to natural molecules could
provide a fascinating and meaningful perspective to explore
the frontiers of modern organic synthesis with respect to high
molecular complexity, structural diversity, and synthetic
efficiency. With research interests in the development of
biologically inspired approaches to bioactive natural prod-
ucts, we recently selected some montanine-type Amaryllida-
ceae alkaloids[2] (Scheme 1) as the targets for probing such
synthesis design in a bioinspired fashion.
The montanine-type alkaloids (Scheme 1) were first
isolated by Wildman and co-workers in 1955 from plants of
the Amaryllidaceae family,[3a] and other congeners were found
subsequently in the following decades.[3b–j] Biologically, these
alkaloids and derivatives preliminarily exhibited some anti-
microbial and psychopharmacological activities.[2e,3h,4] Struc-
Scheme 1. Representative montanine-type Amaryllidaceae alkaloids.
turally, naturally occurring montanine-type alkaloids are
characterized by a common bridged pentacyclic 5,11-meth-
anomorphanthridine ring system, which constitutes one of
twelve main skeleton types of Amaryllidaceae alkaloids,[2i]
and are distinguished primarily by positional and stereo-
chemical diversity of oxygen-containing substituents in
ring E. Clearly, the architectural assembly of the unique
5,11-methanomorphanthridine core in combination with the
selective installation of the requisite oxygen-containing func-
tional groups is the most challenging aspect in the enantio-
selective synthesis of montanine-type alkaloids.
Regarding the chemical synthesis of montanine-type
alkaloids, great effort has been made over the past two
decades since the pioneering synthetic studies by Overman
and Shim[5a] and Hoshino and co-workers[5b] in 1991. In
various approaches to the assembly of the 5,11-methanomor-
phanthridine skeleton, the five types of advanced synthons I–
V (Scheme 2) have been strategically involved,[6–10] and the
closure of ring C, D, C/D, or E has constituted the final step in
the construction of the polycyclic system. Significantly, the
synthetic elaboration of these synthons I–V has involved the
elegant development and application of many novel method-
[*] X. Bao, Y.-X. Cao, W.-D. Chu, H. Qu, J.-Y. Du, X.-H. Zhao, X.-Y. Ma,
C.-T. Wang, Prof. Dr. C.-A. Fan
State Key Laboratory of Applied Organic Chemistry
College of Chemistry and Chemical Engineering, Lanzhou University
222 Tianshui Nanlu, Lanzhou 730000 (China)
E-mail: fanchunan@lzu.edu.cn
Prof. Dr. C.-A. Fan
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences, Peking University
38 Xueyuan Lu, Beijing 100191 (China)
[**] This research was supported by the 973 Program of MOST
(2010CB833200), the NSFC (21322201, 21290180, 21072082), the
Fok Ying Tung Education Foundation (121015), NCET (NCET-08-
0254), PCSIRT (IRT1138), FRFCU (lzujbky-2012-k39 and lzujbky-
2013-ct02), and the 111 Project of MOE (111-2-17). We thank Prof.
Jian Liao (CIB, CAS) and Prof. Jian-Hua Xie (Nankai University) for
generous gifts of chiral phosphine ligands. We also thank one of the
referees for valuable comments on the stereoselectivity of the
tandem reaction.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307324.
Scheme 2. Known non-bioinspired strategies for assembling the 5,11-
methanomorphanthridine skeleton.
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ologies (e.g., an aza-Cope rearrangement/Mannich cycliza-
tion,[5a,6a] an allenylsilane imino ene reaction,[9b,c]
[3+2] cycloaddition,[6b,g,10] and
chemoenzymatic ap-
a
a
proach[6i,k,l]). Despite the substantial progress made in the
racemic[5,6c–e,o,7,8,9a,10] and asymmetric[6a,b,f–n,9b,c] synthesis of the
montanine-type alkaloids, a general strategy for the effective
bioinspired total synthesis of these architecturally unique
molecules has not previously been explored.
From a biosynthetic point of view, the intriguing biogen-
esis of the montanine-type Amaryllidaceae alkaloids, which
can be traced back to norbelladine or its derivatives, has
illuminated two distinct biosynthetic proposals for the
biochemical conversion of A1 (norbelladine-type alkloids)
into A2 (montanine-type alkaloids; Scheme 3).[2d,11] One
Scheme 4. Retrosynthetic analysis.
substituents at C2 and C3 in the target alkaloids could be
derived by prefunctionalization of the 1,2-diketone mono-
ketal moiety in the common synthon B. Strategically, the
À
logical disconnection of the C4a N bond in ring D of the key
montanine-type synthon B through a retro-aza-Michael
addition would deliver a quinone monoketal intermediate,
which could be generated in situ by oxidative phenol
dearomatization of the cherylline-type synthon C. A selective
Pictet–Spengler cyclization of synthon D and the asymmetric
conjugate addition of organoboron synthon E with alkene
synthon F could be conceived for the formation of the
isoquinoline ring C and the crucial C11 diaryl methine
stereocenter in the synthon C, respectively.
To expediently access the cherylline-type synthon C
(Scheme 4) for the proposed bioinspired transformation, we
began our study with a rhodium-catalyzed asymmetric con-
jugate addition of aryl boronic acid 1 to nitroalkene 2 with the
chiral sulfinylphosphine ligand L discovered by Liao and co-
workers[13] (Scheme 5). The desired adduct 3 was obtained in
90% yield with 95% ee. Reduction of the diaryl nitroethane 3
with Zn powder, followed by fluoride-mediated desilylation
and carbamation/esterification, gave the diaryl carbamate 4
(96% ee) in 81% yield over two steps. We recrystallized 4 to
further enhance its enantiomeric purity. Gratifyingly, the
enantiomeric purity of the mother liquor was readily enriched
through heterochiral crystallization[14] to afford 4 in 75%
overall yield with 99% ee after separation of the solid
racemate by filtration. Next, following the highly site-
selective assembly of a methylene unit on ring B of 4 by
a regioselective Pictet–Spengler cyclization, cleavage of the
carbamate and removal of the ester protecting group on the
phenol by hydrazinolysis readily gave the cherylline-type
precursor 5 in 91% overall yield. For the selective Pictet–
Spengler transformation, the presence of an alkoxycarbonyl
group (R = CO2Et) in 4 was essential for the observed site
selectivity,[15] as it decreased the nucleophilicity of ring E by
its electron-withdrawing effect.
Scheme 3. Proposed biosynthetic pathways.
pathway (right-hand side) proposed by Wildman and co-
workers[11a,c,d] involves a plausible 11-hydroxyvittatine-type
precursor and was supported by the serendipitous discovery
of the chemical conversion of haemanthamine-type alkaloids
into montanine-type derivatives through a stereospecific
rearrangement.[3c,4c] Alternatively,
a tentative biogenetic
route (left-hand side) was deduced by Jin,[2d] who proposed
the intermediacy of p-quinone methide and dienone species.
This state-of-the-art knowledge of the biogenetic origin of the
montanine-type alkaloids led us to consider that chemically
stable cherylline-type precursors A3 (Scheme 3) might be
involved in the multistep transformation of norbelladine-type
intermediates A1 into montanine-type alkaloids A2, and that
such intermediates could offer an opportunity to explore
a new bioinspired strategy for an expeditious approach to the
unique 5,11-methanomorphanthridine ring system.
Retrosynthetic analysis inspired by these considerations
(Scheme 4) pointed to the use of an unprecedented tandem
oxidative dearomatization/intramolecular aza-Michael addi-
tion as a key step[12] in our divergent asymmetric synthesis of
five montanine-type alkaloids. Chemically, the diverse oxygen
With the chiral cherylline-type building block 5 in hand,
we chemically explored the feasibility of the proposed
bioinspired approach to the montanine-type pentacyclic
skeleton (Scheme 6). On the basis of recent progress in
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hypervalent-iodine-mediated phenol dearomatization,[16] we
subjected the substituted isoquinoline 5 bearing a free
secondary-amine moiety to the preliminarily optimized con-
ditions with PhI(OAc)2 as an oxidant and MeOH as the
solvent in the presence of CF3CO2H.[17] Significantly, our
proposed key tandem oxidative dearomatization/intramolec-
ular aza-Michael addition proceeded smoothly with high
diastereoselectivity via an ortho-quinone monoketal inter-
mediate.[18,19] This tandem reaction led exclusively to the
multifunctionalized chiral intermediate 6 (99% ee) with the
5,11-methanomorphanthridine core in 61% yield. The abso-
lute configuration of 6 was established unambiguously by X-
ray crystallographic analysis.[20,21]
To account for the observed stereoselectivity of this
bioinspired transformation, we propose a plausible model
based on the Curtin–Hammett principle (Scheme 6).[22] On
the basis of kinetic accessibility in the intramolecular aza-1,4-
conjugate addition of the ortho-quinone monoketal inter-
mediate formed in situ, two main conformers G2 and G3
would be adopted in the current addition reaction. Owing to
the presence of energetically unfavorable 1,3-allylic strain in
G2,[23] the excellent diastereoselectivity of this tandem
reaction might be appropriately rationalized by a preferential
Si-face attack of the enone moiety via the kinetically
accessible conformer G3.
Scheme 5. Asymmetric synthesis of the cherylline-type precursor.
Having established the crucial 5,11-methanomorphanthri-
dine pentacyclic framework with the requisite configuration
and functionality in 6, we then pursued the divergent
asymmetric synthesis of montanine-type alkaloids
À
(Scheme 7). To access the stereochemically defined C3 OH
group in ring E of the target alkaloids (Scheme 4), we
examined the diastereoselective reduction of the diketone
monoketal 6.[24] The resulting diastereomers 7a (equatorial
[9c]
À
À
C3 OH) and 7b (axial C3 OH)
could be separated
chromatographically. Interestingly, the Luche reduction of
ketone 6 (NaBH4/CeCl3) gave 7a as the main product in 69%
yield, whereas the iridium-catalyzed hydrogenation of 6
([{Ir(cod)Cl)}2]/H2)[25] gave 7b as the main product in 58%
yield.[24] The absolute configuration of 7a was unequivocally
assigned by X-ray crystallographic analysis.[20,21]
After the above reduction of the C3 carbonyl group in 6,
the further reduction of the C2 ketal moiety of less polar 7b
[9c]
with (iBu2AlH)2 directly furnished readily separable (À)-
montanine[3a,c,9c] and (À)-coccinine[3a,c,9b,c] in 51 and 30%
yield, respectively (Scheme 7). Analogously, when more polar
7a was subjected to reduction with (iBu2AlH)2, (À)-manthi-
dine originally assigned by Wildman[3a,c] was obtained readily
for the first time in 53% yield, together with separable (À)-2-
epi-manthidine in 36% yield. The configuration of our
synthetic sample as that previously proposed for (À)-man-
thidine[3c,26] was definitively confirmed by X-ray crystallo-
graphic analysis.[20,21]
To further demonstrate the synthetic diversity of our
current strategy (Scheme 7), we subjected 7a and 7b to acidic
ketal hydrolysis in parallel and obtained ketone 8a (equato-
[27]
[6a,27]
À
À
rial C3 OH) and 8b (axial C3 OH)
in 99 and 98%
yield, respectively. Upon the treatment of 8a with (iBu2AlH)2
at À788C, readily separable (À)-brunsvigine and (À)-2-epi-
brunsvigine were obtained in 76 and 15% yield, respectively.
Scheme 6. Bioinspired tandem oxidative dearomatization/intramolecu-
lar aza-Michael addition.
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Scheme 7. Divergent asymmetric synthesis of (À)-montanine, (À)-coccinine, (À)-brunsvigine, (À)-pancracine, and previously proposed (À)-
manthidine: a) (iBu2AlH)2, CH2Cl2; b) oxalic acid, THF/H2O; c) NaBH(OAc)3, CH3CN/AcOH. cod=1,5-cyclooctadiene.
The configuration of (À)-brunsvigine[3b,e,f,6f,j] was also eluci-
dated by X-ray crystallographic analysis.[20,21] Meanwhile, the
mild reduction of 8b by NaBH(OAc)3 exclusively gave (À)-
pancracine[3d,6a] in 70% yield.
Keywords: alkaloids · bioinspired synthesis · dearomatization ·
domino reactions · total synthesis
.
In conclusion, a new strategy featuring a bioinspired
tandem oxidative dearomatization/intramolecular aza-
Michael addition was developed for a diversity-oriented,
expeditious asymmetric total synthesis of five montanine-type
Amaryllidaceae alkaloids. Significantly, an unusual conver-
sion of the cherylline-type isoquinoline skeleton into the
unique 5,11-methanomorphanthridine core of montanine-
type alkaloids was proposed and chemically explored. In
comparison with previous synthetic studies, it is particularly
noteworthy that our bioinspired strategy positively enables
a step-economic, effective, divergent, and enantioselective
route to (À)-montanine (8 steps, 11% yield), (À)-coccinine
(8 steps, 6.5% yield), (À)-brunsvigine (9 steps, 19% yield),
and (À)-pancracine (9 steps, 15% yield), as well as previously
proposed (À)-manthidine (8 steps, 14% yield). The current
investigation not only synthetically provides a bridge between
the cherylline-type and montanine-type alkaloids, but also
manifests the potential of bioinspired design in the chemical
synthesis of natural products.
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Received: August 20, 2013
Published online: November 8, 2013
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[17] The absence of CF3CO2H resulted in a complex reaction
mixture. For details on the optimization of the reaction
conditions, see the Supporting Information.
[18] For pioneering examples of the formation of ortho-quinone
monoketals by oxidation with hypervalent iodine reagents, see:
a) J.-T. Hwang, C.-C. Liao, Tetrahedron Lett. 1991, 32, 6583;
b) A. S. Mitchell, R. A. Russell, Tetrahedron Lett. 1993, 34, 545;
c) A. Pelter, R. S. Ward, Q. Li, J. Nat. Prod. 1993, 56, 2204.
[19] For reviews on the chemistry of ortho-quinone monoketals, see:
a) S. Quideau, L. Pouysꢃgu, Org. Prep. Proced. Int. 1999, 31, 617;
b) S. Quideau in Modern Arene Chemistry (Ed.: D. Astruc),
Wiley-VCH, Weinheim, 2002, p. 539; c) C.-C. Liao, R. K.
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Meek, T. R. R. Pettus, Chem. Rev. 2004, 104, 1383; e) C.-C. Liao,
Pure Appl. Chem. 2005, 77, 1221.
[11] a) W. C. Wildman in The Alkaloids, Vol. 11 (Ed.: R. H. F.
Manske), Academic Press, New York, 1968, p. 307; b) C.
Fuganti, D. Ghiringhelli, P. Grasselli, J. Chem. Soc. Chem.
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Chem. Commun. 1976, 551; d) A. I. Feinstein, W. C. Wildman, J.
Org. Chem. 1976, 41, 2447; e) J. Bastida, R. Lavilla, F. Viladomat
in The Alkaloids, Vol. 63 (Ed.: G. A. Cordell), Academic Press,
New York, 2006, p. 87.
[20] The absolute configuration was determined by X-ray crystallo-
graphic analysis. The intensity data were collected on an Agilent
SuperNova diffractometer by using graphite-monochromated
CuKa radiation. CCDC 955421 (6), 955422 (7a), 955423 {((À)-
Angew. Chem. Int. Ed. 2013, 52, 14167 –14172
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 14171

.
Angewandte
Communications
À
brunsvigine)2·(H2O)3}, and 955424 (synthetic (À)-manthidine)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
equatorial C3 OH group was kinetically much faster than that
À
of 8b with an axial C3 OH group. This process provides an
alternative to the individual preparation of 8a and 8b. See the
Supporting Information for details.
[21] For an ORTEP drawing from a different perspective, see the
Supporting Information.
[22] J. I. Seeman, Chem. Rev. 1983, 83, 83.
[23] For reviews on 1,3-allylic strain, see: a) F. Johnson, Chem. Rev.
1968, 68, 375; b) R. W. Hoffmann, Chem. Rev. 1989, 89, 1841.
[24] See the Supporting Information for details.
[25] S. Özkar, R. G. Finke, J. Am. Chem. Soc. 2005, 127, 4800.
[26] Since no NMR spectroscopic data were reported for natural
(À)-manthidine (Refs. [3a,c]), its structure originally assigned by
Wildman remains to be verified by further spectroscopic analysis
of the natural sample. For detailed comments, see the Supporting
Information.
[27] A diastereoselective kinetic resolution of a mixture of 8a and 8b
under silylation conditions (iPr3SiOSO2CF3, iPr2NEt, À788C)
was discovered: the silylation of a-hydroxyketone 8a with an
14172 www.angewandte.org
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Angew. Chem. Int. Ed. 2013, 52, 14167 –14172