Asymmetric Total Synthesis of (?)-Stemonamine and Its Stereochemical Stability

Article abstract of DOI:10.1002/chem.201706057

The first asymmetric total synthesis of (?)-stemonamine is described. The key reactions included intramolecular acylation to construct the seven-membered ring and a tandem [2+2] cycloaddition-Dieckmann condensation reaction using an ynolate to form the fully substituted cyclopentenone moiety. Racemization and epimerization of the natural product were first experimentally demonstrated.

Full text of DOI:10.1002/chem.201706057

A Journal of
Accepted Article
Title: Asymmetric Total Synthesis of (-)-Stemonamine and its
Stereochemical Stability
Authors: Satoshi Fujita, Keisuke Nishikawa, Takayuki Iwata, Taishi
Tomiyama, Hiroshi Ikenaga, Kenji Matsumoto, and Mitsuru
Shindo
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201706057
Link to VoR: http://dx.doi.org/10.1002/chem.201706057
Supported by

10.1002/chem.201706057
Chemistry - A European Journal
Asymmetric Total Synthesis of (-)-Stemonamine and its Stereochemical
Stability
Satoshi Fujita,[a] Keisuke Nishikawa,[b] Takayuki Iwata,[b] Taishi Tomiyama,[a] Hiroshi
Ikenaga,[a] Kenji Matsumoto[b] and Mitsuru Shindo*[b]
aInterdisciplinary Graduate School of Engineering Sciences, Kyushu University,
Kasuga-koen, Kasuga 816-8580, Japan
bInstitute for Materials Chemistry and Engineering, Kyushu University, Kasuga-koen,
Kasuga 816-8580, Japan
Key Words: Alkaloids; Asymmetric synthesis; Total synthesis; Ynolate; Dieckmann
condensation
Abstract: The first asymmetric total synthesis of (-)-stemonamine is described. The key
reactions included intramolecular acylation to construct the seven-membered ring and a
tandem [2+2] cycloaddition-Dieckmann condensation reaction using an ynolate to form
the fully substituted cyclopentenone moiety. Racemization and epimerization of the
natural product were first experimentally demonstrated.
Natural products are usually produced in an optically pure form of only one
enantiomer, because the stereochemistry of natural products is strictly controlled during
biosynthesis processes.[1] It is also well known that both enantiomers of natural products
can be produced in different organisms.[2] However, the production of a racemic or a
scalemic mixture from a single organism only occurs in rare cases.[3] For example, some
natural products have optically unstable structures. Thus, racemization can occur both in
nature and during isolation of compounds even if the compounds are produced in their
optically pure forms.[4]
Optically unstable natural products have attracted much interest, in particular,
stemonamine (1),[5] which has interesting structural features such as sterically hindered
consecutive spiro-quaternary centers and a fused ring system containing a fully
substituted cyclopentanone (A ring), a pyrrolidine (B ring), an azepane (C ring), and a
butenolide (D ring). Stemonamine is one of the Stemona alkaloids,[6] which can be
isolated from plants belonging to the Stemonaceae family. These alkaloids consist of
more than 140 compounds that can be classified into eight structural groups based on
the connection and the number of rings and side chains.[6b] Stemonamine group, one of
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
these groups, includes stemonamine (1), isostemonamine (2),[5] stemonamide (3),[5b]
maistemonine (4),[5b,7] and other related compounds (Figure 1).[8] Intriguingly, although
alkaloids 1–4 were isolated from the root of Stemona japonica Miq., which has been
used as traditional Chinese medicine for curing respiratory diseases and infections with
worms, only stemonamine and its diastereomer, isostemonamine have been isolated as
racemates.[5] Therefore, it was anticipated that these two alkaloids could have both a
racemic and an epimeric pathway from one to the other via a retro-Mannich/Mannich
type intermediate 5.[5a] Although the total synthesis of racemic stemonamine has been
reported by Tu/Zhang et al. and Ishibashi et al.,[ 9] no subsequent work has been carried
out to explore the stereochemical stability of stemonamine. Ishibashi et al. have reported
that both stemonamine and isostemonamine were obtained from a single synthetic
intermediate, which has an identical stereochemistry to isostemonamine.[9d] This
supports the fact that stemonamine can be epimerized to isostemonamine, but there are
still questions regarding its stereochemical stability, including the kinetics of the
interconversion process. Moreover, since nonracemic stemonamine has not yet been
obtained, the racemization of stemonamine is yet to be demonstrated experimentally.
Thus, the complex structure and unexplored stereochemical stability of
stemonamine prompted an investigation into its asymmetric synthesis. This was
anticipated to be challenging because of the possibility of the racemization of synthetic
intermediates and the product itself, and also because this would provide information
about the fundamental physical properties of stemonamine such as specific optical
rotation, which is otherwise difficult to obtain. Herein, we report the first total synthesis
of (-)-stemonamine using an intermolecular acylation and an ynolate-initiated tandem
reaction. Furthermore, the study provides a better understanding of the stereochemical
stability of stemonamine and the kinetics of its interconversion between enantiomeric
and diastereomeric forms.
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
Figure 1. Stemona alkaloids and the proposed interconversion mechanism between
stemonamine (1) and isostemonamine (2).
The retrosynthetic pathway to stemonamine (1) is shown in Scheme 1. The D-ring
was constructed using a modified version of the procedure reported by Tu and Zhang.[10]
To obtain the key intermediate 6, we utilized two unique reactions developed by our
group. The cyclopentenone moiety (A ring) could be formed by an ynolate (8)-initiated
tandem [2+2] cycloaddition-Dieckmann condensation reaction[11] of ketoester 7. This
reaction is regarded as a formal [4+1]-type annulation, which is generally suitable for
the synthesis of sterically crowded, multisubstituted cycloalkenones. Since seven
membered rings are not always synthesized in high yield,[12] we decided to use an
intramolecular acylation of iodide 9 for construction of the seven membered ring of 7.
This approach was derived from an analogous reaction used to construct the seven
membered ring in our total synthesis of sundiversifolide.[13] Iodide 9 could be prepared
from proline derivative 10 and hydroxylactone 11, both of which are readily available as
an optically active form.
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
Scheme 1. Retrosynthetic analysis of (-)-stemonamine (1)
The first step of the synthesis is a condensation reaction of proline derivative 10
with hydroxylactone 11, both of which are easily prepared from L-proline[14] and
L-malic acid,[15] respectively (Scheme 2). The sequential removal of the Boc group of
ester 12 and the subsequent lactamization afforded lactam 13 in high yield. After several
attempts to directly transform alcohol 13 into the corresponding iodide 9, the method
developed by Khazdooz (using P2O5 and KI)[16] was found to be suitable as it could be
scaled up easily, affording good yield and easy purification.
Next, the first key reaction was carried out. The intramolecular acylation of iodide 9
via lithiated intermediate 14 (formed using tBuLi at -78 °C), successfully provided
seven-membered hemiacetal 15. Optimization of the reaction conditions afforded the
product in 91% yield, even on over 10 mmol scales.
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
Scheme 2. Synthesis of hemiacetal intermediate 15.
The next step involved the reductive removal of the -oxygen atom adjacent to the
carbonyl group of lactam 15 (Scheme 3). Interestingly, the equilibrium between the
hemiacetal and the ketol forms of 15 emphatically favored the hemiacetal form,
resulting in 15, which shows a low propensity for acetal-opening reactions. After
numerous attempts, the Appel reaction[17] followed by the reductive removal of the iodo
group provided compound 17 in high yield (see Scheme 3). The terminal olefin 17 was
subjected to oxidative cleavage, followed by methylation using TMSCHN2 to form
ketoester 7.
In an effort to construct the cyclopentenone moiety (A ring), an ynolate-initiated
tandem reaction was investigated as the second key reaction (Scheme 3). Ketoester 7
was subjected to a tandem reaction initiated by ynolate 8, generated from
-dibromoester 18[18] using the procedure developed by our group.[19] Without
purification, the resulting -lactone 19 was further treated with silica gel (pH = 6.0 ±
1.0) to facilitate decarboxylation, which afforded tricyclic intermediate 6 in 88% yield
over two steps. It is noteworthy that the ynolate was able to react with the sterically
hindered ketone moiety (adjacent to a quaternary carbon) even at temperatures as low as
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
-60 °C, demonstrating the high reactivity of ynolates.
I2 (3.0 equiv.)
PPh3 (3.0 equiv.)
Imidazole
Zn
(3.0 equiv.)
HO
N
O
(6.0 equiv.)
O
N
O
O
C
AcOH
rt, 15 min
CH2Cl2
rt, 2 days
H
I
15
95%
99%
16
1) RuCl3•nH2O (0.1 equiv.)
NaIO4 (5.0 equiv.)
CH3CN, AcOEt, H2O
rt, 15 min
MeO2C
O
O
N
N
O
O
2) TMSCH2N2 (1.2 equiv.)
benzene, MeOH
0 C, 1.5 h
17
7
93% (2 steps)
Me
Br Br
Me CO2Ph
tBuLi
O
OLi
(5.0 equiv.)
8
Me
N
O
18
O
O
THF
60 °C, 2 days
19
O
B
N
SiO2
A
O
C
benzene
reflux, 10 h
6
88% (2 steps)
Scheme 3. Synthesis of key intermediate 6.
With key intermediate 6 in hand, construction of the butenolide moiety (D ring) was
attempted, based on the work carried out by Tu and Zhang[10] (Scheme 4). An aldol
reaction of 6 with propanal using LiHMDS diastereoselectively formed alcohol 20. A
Dess-Martin oxidation afforded 1,3-diketone 21, which was treated with cerium
trichloride under an oxygen atmosphere[20] to form -oxidized products 22 and 23 in 4:1
d.r., along with unidentified chlorinated products 24. Compound 22, with the desired
stereochemistry, was then converted into carbonate 25 using ethyl chloroformate and
DMAP.[21] A Dieckmann condensation reaction of 25 using KHMDS followed by
methylation afforded the tetracyclic product 26 in 62% yield over two steps; however,
an enantiomeric excess of 61% was obtained for 26. The optical purity of 25 could not
be verified;[22] it is possible that the racemization process may initiate over the course of
the synthesis, and/or that the basic conditions used for the Dieckmann condensation
reaction may cause racemization through the ring-opening process (see the
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
Supplementary Information). Thiolactamization of 26 using Lawesson’s reagent
followed by recrystallization provided thioamide 27 in 80% ee. As the final step,
reduction of the thiocarbonyl group using Raney Ni afforded stemonamine (1). However,
work-up under room temperature conditions and purification with silica gel column
chromatography gave a completely racemic mixture of stemonamine, along with
significant amounts of isostemonamine (combined yield: 89%). HPLC analysis of the
products using a chiral column (CHIRALPAK AD) revealed that stemonamine easily
racemizes and epimerizes during the chromatography process; this is likely due to the
acidity of silica gel, and furthermore the compound has a tendency to racemize
gradually in solution at room temperature (vide infra). Therefore, following the
reduction using Raney Ni, a rapid work-up was carried out under low-temperature
conditions, which afforded optically active (-)-stemonamine (1) with a significant
specific optical rotation ([]23 = -47) and optical purity (58% ee), although with the
D
co-occurrence of (-)-isostemonamine (2, 2%,[23] 55% ee). The HPLC separation of the
product using a chiral column (CHIRALPAK AD-H) provided more optically active
(-)-stemonamine, with 98% ee and []23D = -75.
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
Scheme 4. Synthetic route to (-)-stemonamine.
The “apparent” rate constants and half-lives of the racemization and epimerization
reactions[24] were determined under several conditions (Tables 1). It was found that
(-)-stemonamine racemizes faster in protic solvents than in aprotic solvents (entries 1-7),
and the process in 20% isopropanol/hexane speeds up higher temperature (entries 1, 3, 4
and 5). Moreover, the epimerization to (±)-isostemonamine was found to proceed much
slower than the racemization (entries 1, 3 and 4). This is reasonable because the
epimerization via intermediate 5 requires rotation of bond A, which has more
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
double-bond character, while racemization requires rotation around bond B, which has
more single-bond character (Scheme 5). In addition, the steric effect of the B ring is
expected to affect the rotation of bond B more than that of bond A. The equilibrium ratio
of stemonamine and isostemonamine (80 °C, in isopropanol) was also revealed to be 2:1
(equilibrium constant: Kc = 0.50), which indicates that stemonamine is more
thermodynamically stable than isostemonamine.
Table 1. Rate constants and half-lives of racemization (ee) of (-)-1 and epimerization of
(±)-1 into (±)-2.
racemization
temp. k (10-4 s-1) half-life
epimerization
entry
1
solvent
k (10-4 s-1)
half-life
74 h
20% isopropanol 60 °C
in hexane
0.74
2.6 h
0.026
[a]
[a]
2
3
isopropanol
25 °C
0.54
0.22
3.6 h
9.0 h
-
-
20% isopropanol 45 °C
in hexane
0.0071
270 h
4
5
20% isopropanol 25 °C
in hexane
0.035
55 h
<0.0015
>1000 h
[a]
[a]
20% isopropanol 10 °C
in hexane
0.0097
200 h
-
-
[a]
[a]
6
7
toluene
CHCl3
25 °C
25 °C
0.0078
0.0072
250 h
270 h
-
-
[a]
[a]
-
-
[a] not determined.
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
Scheme 5. Proposed mechanisms for racemization and epimerization.
In conclusion, the first total synthesis of nonracemic (-)-stemonamine is achieved in
17 steps from known compounds. Key reactions included the intramolecular acylation
reactionand the ynolate-initiated tandem [2+2] cycloaddition-Dieckmann condensation
reaction,. Although the obtained natural product stemonamine 1 was not enantiopure
because it is optically unstable, racemization and epimerization of stemonamine were
verified experimentally, and the half-lives of the constituent processes were successfully
determined. The optical rotation of the enantio-enriched natural product was also
demonstrated, and thus the absolute configuration was assigned. The investigation of
the biological activity of stemonamine is under progress and the results will be reported
in due course.
Acknowledgements
This work was partially supported by JSPS KAKENHI Grant Number (No. JP22390002,
JP24106731, JP16H01157, JP26293004, JP17K14449), Science and Technology
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
Research Promotion Program for Agriculture, Forestry, Fisheries and Food industry
(M.S.) and the MEXT Project of “Integrated Research Consortium on Chemical
Sciences” (T.I.). This work was performed under the Cooperative Research Program of
“Network Joint Research Center for Materials and Devices.” We thank Mr. T.
Matsumoto at Kyushu University for the X-ray crystal structure analyses. We are
grateful to Prof. K. Tomooka, and Dr. K. Igawa at Kyushu University for instruction in
the chiral HPLC analysis and determination of half-lives.
References
[1] S. M. Colegate, R. J. Molyneux, Bioactive Natural Products:Detection, Isolation,
and Structural Determination, 2nd ed.,Taylor and Francis, Boca Raton, 2008.
[2] J. M. Finefield, D. H. Sherman, M. Kreitman, R. M. Williams, Angew. Chem. Int. Ed.
2012, 51, 4802.; Angew. Chem. 2012, 124, 4886.
[3] Recent examples for the natural product isolated as a racemic mixture; a) X.-L.
Wang, M. Dou, Q. Luo, L.-Z. Cheng, Y.-M. Yan, R.-T. Li, Y.-X. Cheng, Fitoterapia
2017, 116, 93; b) Q. Luo, X.-H. Yang, Z.-L. Yang, Z.-C. Tu, Y.-X. Cheng, Tetrahedron
2016, 72, 4564; c) H.-X. Liu, K. Chen, Y. Liu, C. Li, J.-W. Wu, Z.-F. Xu, H.-B. Tan,
S.-X. Qiu, Fitoterapia 2016, 115, 142; d) Y. Zhu, Y. Wang, B.-B. Gu, F. Yang, W.-H.
Jiao, G.-H. Hu, H.-B. Yu, B.-N. Han, W. Zhang, Y. Shen, H.-W. Lin, Tetrahedron 2016,
72, 2964; e) Q. Luo, L. Di, X.-H. Yang, Y.-X. Cheng, RSC Adv., 2016, 6, 45963; f) K.
Nisa, T. Ito, T. Kodama, M. Tanaka, Y. Okamoto, Y. Asakawa, H. Imagawa, H. Morita,
Fitoterapia 2016, 109, 236.
[4] Selected examples of the natural products which is racemized: a) G. Volpin, N. A.
Veprˇek, A. B. Bellan, D. Trauner, Angew. Chem. Int. Ed. 2017, 56, 897; b) L. C.
Konkol, F. Guo, A. A. Sarjeant, R. J. Thomson, Angew. Chem. Int. Ed. 2011, 50, 9931;
c) J. K. Harper, A. M. Arif, E. J. Ford, G. A. Strobel, J. A. Porco, Jr., D. P. Tomer, K. L.
Oneill, E. M. Heidere, D. M. Grant, Tetrahedron 2003, 59, 2471.
[5] a) H. Iizuka, H. Irie, N. Masaki, K. Osaki, S. Uyeo, J. Chem. Soc. Chem. Comm.
1973, 4, 125; b) Y. Ye, G.-W. Qin, R.-S. Xu, J. Nat. Prod. 1994, 57, 665.
[6] a) R. A. Pilli, M. d. C. F. d. Oliveira, Nat. Prod. Rep. 2000, 17, 117; b) R. A. Pilli, G.
B. Rosso, M. d. C. F. d. Oliveira, Nat. Prod. Rep. 2010, 27, 1908; c) F-P. Wang, Q-H.
Chen, Nat. Prod. Commun. 2014, 9, 1809. d) X-Y. Liu, F-P. Wang, Nat. Prod. Commun.
2015, 10, 1093.
[7] W. Lin, Y. Ye, R.-S. Xu, Youji Huaxue 1991, 11, 500.
[8] a) Isostemonamide: see ref. 5b. b) Oxymaistemonine: see ref. 7. c) Isomaistemonine
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
and isooxymaistemonine: A. Guo, L. Jin, Z. Deng, S. Cai, S. Guo, W. Lin, Chem.
Biodiversity 2008, 5, 598. d) Sessilistemonamines A−C: P. Wang, A.-L. Liu, Z. An, Z.-H.
Li, G.-H. Du, H.-L. Qin, Chem. Biodiversity 2007, 4, 523. e) 3β-n-Butylstemonamine,
8-oxo-3β-n-butylstemonamine, and 8-oxo-oxymaistemonine: S.-Z. Huang, F.-D. Kong,
Q.-Y. Ma, Z.-K. Guo, L.-M. Zhou, Q. Wang, H.-F. Dai, Y.-X. Zhao, J. Nat. Prod. 2016,
79, 2599.
[9] a) Y.-M. Zhao, P. Gu; Y.-Q. Tu, C.-A. Fan, Q. Zhang, Org. Lett. 2008, 10, 1763; b)
Y.-M. Zhao, P. Gu, H.-J. Zhang, Q.-W. Zhang, C.-A. Fan, Y.-Q. Tu, F.-M. Zhang, J. Org.
Chem. 2009, 74, 3211; c) T. Taniguchi, G. Tanabe, O. Muraoka, H. Ishibashi, Org. Lett.
2008, 10, 197; d) T. Taniguchi, H. Ishibashi, Tetrahedron 2008, 64, 8773. Formal
synthesis: e) C. Kim, S. Kang, Y. H. Rhee, J. Org. Chem. 2014, 79, 11119.
[10] a) Z.-H. Chen, Y.-Q. Zhang, Z.-M. Chen, Y.-Q. Tu, F.-M. Zhang, Chem. Commun.
2011, 47, 1836; b) Z.-H. Chen, Z.-M. Chen, Y.-Q. Zhang, Y.-Q. Tu, F.-M. Zhang, J. Org.
Chem. 2011, 76, 10173.
[11] a) M. Shindo, Y. Sato, K. Shishido, J. Am. Chem. Soc. 1999, 121, 6507; b) M.
Shindo, Y. Sato, K. Shishido, J. Org. Chem. 2001, 66, 7818.
[12] a) Modern Physical Organic Chemistry (Eds.: E. V. Anslyn, D. A. Dougherty),
Higher Education Press, Beijing, 2009; b) L. Yet, Chem. Rev. 2000, 100, 2963.
[13] a) K. Ohtsuki, K. Matsuo, T. Yoshikawa, C. Moriya, K. Tomita-Yokotani, K.
Shishido, M. Shindo, Org. Lett. 2008, 10, 1247; b) K. Matsuo, K. Ohtsuki, T.
Yoshikawa, K. Shishido, K. Yokotani-Tomita, M. Shindo, Tetrahedron 2010, 66, 8407.
[14] G. D. Artman III, R. J. Rafferty, R. M. Williams. Org. Synth. 2009, 86, 262.
[15] S. E. Denmark, S.-M. Yang, J. Am. Chem. Soc. 2004, 126, 12432.
[16] L. Khazdooz, A. Zarei, H. Aghaei, G. Azizi, M. M. Gheisari, Tetrahedron Lett.
2016, 57, 168.
[17] E. Ideue, J. Shimokawa, T. Fukuyama, Org. Lett. 2015, 17, 4964.
[18] M. Shindo, Y. Sato, R. Koretsune, T. Yoshikawa, K. Matsumoto, K. Itoh, K.
Shishido, Chem. Pharm. Bull., 2003, 51, 477.
[19] a) M. Shindo, K. Matsumoto, K. Shishido, Org. Synth. 2007, 84, 11; b) M. Shindo,
Y. Sato, K. Shishido, Tetrahedron 1998, 54, 2411.
[20] a) J. Christoffers, T. Werner, Synlett 2002, 119; b) J. Christoffers, T. Werner, S.
Unger, W. Frey, Eur. J. Org. Chem. 2003, 425.
[21] This reaction was accelerated when solvent amount of ethyl chloroformate and
trimethylamine were used.
[22] Chiral HPLC analysis of 20, 21, 22, 23, and 25 was attempted to determine their
optical purity, but only a single peak was detected under all conditions.
This article is protected by copyright. All rights reserved.

10.1002/chem.201706057
Chemistry - A European Journal
[23] The yield was estimated by chiral HPLC analysis.
[24] The rate constants and the half-lives shown in this report are not real values
because the racemization and the epimerization occur at the same time.
This article is protected by copyright. All rights reserved.