Formal enantioselective total synthesis of bisdehydroneostemoninine

Article abstract of DOI:10.1080/10286020.2019.1608956

A formal enantioselective total synthesis of bisdehydroneostemoninine employing L-glutamic acid as the chiral pool is described. The key features of the synthesis include regioselective and enantioselective opening the chiral epoxide with dimethylsulfonium methylide and tandem Friedel–Crafts cyclization followed by lactonization to form the 5-7-5 tricyclic core of the target stemona alkaloids. The synthetic route provides opportunities to explore the biological behavior of enantiopure bisdehydroneostemoninine. (Figure presented.).

Full text of DOI:10.1080/10286020.2019.1608956

Journal of Asian Natural Products Research
ISSN: 1028-6020 (Print) 1477-2213 (Online) Journal homepage: https://www.tandfonline.com/loi/ganp20
Formal enantioselective total synthesis of
bisdehydroneostemoninine
Kai-Qing Ma, Hu-Bin Ren, Jian-Bin Chao & Xue-Mei Qin
To cite this article: Kai-Qing Ma, Hu-Bin Ren, Jian-Bin Chao & Xue-Mei Qin (2019): Formal
enantioselective total synthesis of bisdehydroneostemoninine, Journal of Asian Natural Products
Research, DOI: 10.1080/10286020.2019.1608956
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JOURNAL OF ASIAN NATURAL PRODUCTS RESEARCH
https://doi.org/10.1080/10286020.2019.1608956
Formal enantioselective total synthesis
of bisdehydroneostemoninine
Kai-Qing Maa, Hu-Bin Rena,b, Jian-Bin Chaoc and Xue-Mei Qina
aModern Research Center for Traditional Chinese Medicine, Shanxi University, Taiyuan 030006, China;
bCollege of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China;
cScientific Instrument Center, Shanxi University, Taiyuan 030006, China
ABSTRACT
ARTICLE HISTORY
Received 4 January 2019
Accepted 15 April 2019
A formal enantioselective total synthesis of bisdehydroneostemoni-
nine employing L-glutamic acid as the chiral pool is described.
The key features of the synthesis include regioselective and enan-
tioselective opening the chiral epoxide with dimethylsulfonium
methylide and tandem Friedel–Crafts cyclization followed by lacto-
nization to form the 5-7-5 tricyclic core of the target stemona alka-
loids. The synthetic route provides opportunities to explore the
biological behavior of enantiopure bisdehydroneostemoninine.
KEYWORDS
Natural product; stemona
alkaloids; bisdehydroneoste-
moninine; total synthesis
1. Introduction
The Stemona alkaloids constitute a family of natural products featuring a characteris-
tic pyrrolo[1, 2-a]azepine nucleus, which are categorized into eight different groups
[1–3]. Within this family, rich sources of complex natural products have been shown
to possess a variety of biological activity, such as antitumor, antitussive, anthelmintic,
and insecticidal effects [4–7]. Bisdehydroneostemoninine (1) (Figure 1) was isolated
from the roots of Stemona tuberosa in 2006 [8] and is a representative member of the
stemoamide group.
In the previous study, we have completed the first total synthesis of bisdehydroste-
moninine and bisdehydroneostemoninine in racemic forms employing palladium-cat-
alyzed carbonylative spirolactonization as the key step [9]. In addition to the
synthetic challenges posed by the oxaspirolactone moiety, constructing bisdehydro-
neostemoninine in enantiopure form is nontrivial as well and has not been reported
CONTACT Kai-Qing Ma
makaiqing@sxu.edu.cn
Supplemental data for this article is available online at https://doi.org/10.1080/10286020.2019.1608956.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
K.-Q. MA ET AL.
Figure 1. The structure of bisdehydroneostemoninine (1).
so far. We envisioned that the chiral allylic alcohol 5 could serve as a common pre-
cursor for the enantioselective synthesis of bisdehydroneostemoninine and could be
prepared from asymmetric reduction of the corresponding enone 4. The existence of
a highly nucleophilic enone moiety and the terminal alkene in the substrate 4 gener-
ates significant synthetic challenge in control of the stereochemistry. Herein, we
report the formal enantioselective syntheses of bisdehydroneostemoninine employing
L-glutamic acid as chiral pool.
2. Result and discussion
Our initial attempt at the synthesis of the desired allylic alcohol 5 began with the enone
compound 4, which was prepared from 2, 5-dimethoxytetrahydrofuran (2) and ethyl 4-
aminobutanoate hydrogen chloride salts (3) in three steps [9]. We assumed that the
allylic alcohol could be accessed through Corey–Itsuno asymmetric reduction [10].
Unfortunately, the enone 4 treated with the (S)-2-Methyl-CBS-oxazaborolidine and bor-
ane dimethylsulfide under various conditions failed to afford any useful level of enan-
tioselectivity (determined by 1H-NMR analysis of the ester derived from the (R)-
Mosher acid) (Scheme 1). These failures were presumably due to the alkene moiety,
which could not provide enough steric interactions between the oxazaborolidine and
the substrate for discrimination [11]. BH3Et2NPh, instead of borane dimethylsulfide,
was also evaluated but led only to the 1, 4-reduction by-product [12]. We also investi-
gated the possibility of employing (þ)-diisopinocampheyl chloroborane as the reducing
reagent [13]. Again, this method was not fruitful which was probably due to the
instability of the enone 4 under the conditions. We also examined the feasibility of
enzymatic kinetic resolution (EKR) of compound 5 employing Candida antarctica lip-
ase B and vinyl acetate as an acylating agent. As a result, compound 5 was
almost recovered.
When our synthetic endeavors toward compound 5 using the enone 4 failed, we
then turned our attention to the intermediate 6, which could be prepared from enone
4 in two steps (Scheme 2) [9]. We assumed that compound 7 with substitution on the
double bond could potentially be the proper substrate for the Corey–Itsuno asymmetric
reduction to obtain useful level of enantioselectivity. Unfortunately, after extensive
screening of the oxidation conditions, compound 6 was consumed and desired product
7 could not be observed, which are presumably due to the instability of compound 7.
In view of these results, a revised strategy was required for the enantioselective
synthesis. We queried whether we might be able to achieve an enantioselective syn-
thesis of the key intermediate 5 using a single, enantiomerically pure starting

JOURNAL OF ASIAN NATURAL PRODUCTS RESEARCH
3
Table 1. Conversion of enone 4 to the allylic alcohol 5.
Entry
Conditions(equiv)
T(ꢀC)/time(h)
Yield
ee
1
2
3
4
5
(S)-CBS-Me (2 equ), BH3-SMe2 (5 equ), THF
(S)-CBS-Me (0.2 equ), BH3-SMe2 (5 equ), THF
(S)-CBS-Me (0.2 equ), BH3-SMe2 (2 equ), THF
(S)-CBS-Me (0.25 equ), BH3-Et2NPh (1 equ), THF
(þ)-DIP-Chloride (2 equ),THF
–40 ꢀC/1.5 h
–40 ꢀC/7 h
ꢁ40 ꢀC/7 h
0 ꢀC/3 h
64%
62%
60%
NA
76%
32%
65%
NA
rt/1 h
NA
NA
material. Accordingly, a new approach was envisaged using the L-glutamic acid as the
chiral pool (Scheme 3). Treated with HBr, NaNO2, and KBr, the amino group of L-
glutamic acid was converted to the corresponding bromide 10 with retention of con-
figuration via a diazonium salt-mediated double inversion strategy [14]. Subsequently,
two carboxylic acid groups were reduced with borane to provide a diol 11 in a 60%
two-step yield. The diol was then subjected to NaH-promoted epoxidation and in situ
esterification of remaining hydroxyl group with 4-toluenesulfonyl chloride (TsCl) to
afford the epoxide 12. The epoxide ring opening of compound 12 to the allyl second-
ary alcohol proves problematic [15]. Compound 12, treated with dimethylsulfonium
methylide, failed to furnish the desired allylic alcohol, which are presumably due to
the toluenesulfonate moiety in the structure.
We then went back one step and protected the alcohol with tert-butyldimethylsilyl
to afford the epoxide 14. Gratifyingly, the epoxide ring of 14 was treated with the
dimethylsulfonium methylide to give the allyl secondary alcohol 15 in 48% yield for
two steps. The resulting secondary alcohol was protected with triisopropylsilyl chlor-
ide to afford the product 16. The diprotected compound 16 was selectively depro-
tected to afford the primary alcohol, which reacted with TsCl to furnish the
corresponding p-toluenesulfonate 17 in one pot. The pyrrole efficiently reacted with
the toluenesulfonate 17, followed by deprotection to give the key intermediate 5 in
1
enantiopure form (ee > 95%) [16]. The ee value was determined by H-NMR analysis
of the ester derived from the (R)-Mosher acid (see the supporting information). The
a, b-unsaturated ester 8 was afforded in 91% yield via a cross-metathesis reaction
between allylic alcohol 5 and methyl acrylate [17]. The azepine ring as well as the
transfused c-butyrolactone ring of compound 19 was constructed through a boron
trifluoride etherate promoted tandem Friedel–Crafts cyclization and lactonization.
Accordingly, preparation of enantiomerically pure 19 completes the formal enantiose-
lective synthesis of 1.
In summary, we have completed the formal enantioselective total synthesis of the
stemona alkaloids bisdehydroneostemoninine (1), employing the L-glutamic acid as

4
K.-Q. MA ET AL.
the chiral pool. The key intermediate 19 was synthesized through nine steps, which
could serve to access the enantiopure bisdehydroneostemoninine in five steps and
other stemoamide group natural products. Further studies focusing on the biological
evaluation of enantiopure bisdehydroneostemoninine and its derivatives are in pro-
gress and will be reported in due course.
3. Experimental
3.1. General experimental procedures
All reactions were carried out under an argon atmosphere with dry solvents. All
reagents were obtained from commercial sources and used without further purifica-
tion. NMR spectra were recorded on Bruker AV-600 spectrometers (Bruker Co.
Billerica, USA) at ambient temperature, using TMS as an internal standard. High-
resolution mass spectra (HRMS) were recorded at a Mass Spectrometry Laboratory
using a Thermo Scientific Q Exactive (Thermo Fisher Scientific Inc. Waltham, USA).
The value of optical rotation was recorded at AUTOPOL IV (Rudolph Research
Analytical Inc. Hackettstown, USA).
3.2. General procedures for the synthetic compounds
3.2.1. Synthesis of (S)-2-bromopentane-1, 5-diol (11)
(S)-2-bromopentanedioic acid (10 g, 47.6 mmol) was dissolved in distilled tetrahydro-
furan (THF) (125 mL) under argon atmosphere and cooled to –10 ꢀC. Borane dime-
thylsulfide (71 mL, 142.8 mmol) was added dropwise at –10 ꢀC. The mixture was
warmed to rt and stirred for 10 h. The reaction was quenched with dropwise addition
of MeOH (125 mL) at 0 ꢀC and concentrated in vacuo. The crude product was puri-
fied by flash chromatography (dichloromethane(DCM)/acetone ¼ 1:1) to give the title
1
compound (7.2 g, two steps: 60%) as a colorless oil; H NMR (600 MHz, CDCl3) d
4.22 – 4.09 (m, 1H), 3.80 (qd, J¼ 12.2, 5.6 Hz, 2H), 3.69 (t, J¼ 6.2 Hz, 2H), 2.41 (s,
2H), 2.03 (tdd, J¼ 10.1, 5.8, 4.6 Hz, 1H), 1.97 – 1.76 (m, 2H), 1.76 – 1.63 (m, 1H)
13C NMR (151 MHz, CDCl3) d 67.0, 62.0, 58.8, 31.2, 30.2. [a]20 -2.0 (c 0.09,
D
MeOH); HRESIMS: m/z 183.0014 [M þ H]þ (calcd for C5H12BrO2, 183.0015).
3.2.2. (2R)-2-[3-[[(1,1-Dimethylethyl)dimethylsilyl]oxy]propyl]oxir (14)
NaH (60% dispersion in oil, 280 mg, 7 mmol) was dissolved in distilled THF (5 mL)
under argon atmosphere and cooled to 0 ꢀC. Bromodiol (425 mg, 2.33 mmol) in dis-
tilled THF (2 mL) was added dropwise, and the reaction stirred for 30 min at 0 ꢀC
and tert-butyldimethylsilyl chloride (526.8 mg, 3.49 mol) was introduced. The reaction
mixture was stirred for 3 h at room temperature, then quenched with saturated
NH4Cl, and extracted with Et2O. The combined organic layers were dried over
anhydrous Na2SO4, filtered, and concentrated in vacuo. The crude product was puri-
fied by flash chromatography (petroleum ether (PE)/ethyl acetate (EtOAc)¼8:1) to
give the title compound (256 mg, 54%) as a yellow oil.

JOURNAL OF ASIAN NATURAL PRODUCTS RESEARCH
5
Scheme 1. The attempt to synthesize compound 8.
Scheme 2. The synthesis of intermediate (13). Reagents and conditions: (a) HBr, KBr, NaNO2,
ꢁ15 ꢀC to rt, 3 h. (b) BH3 Me2S, THF, MeOH, rt, 10 h, two steps: 60%. (c) NaH, THF, 0 ꢀC, 0.5 h, then
TsCl, py, 1 h, 52%. (d) Me3SI, LHMDS, THF, –10 ꢀC.
Scheme 3. The synthesis of intermediate (19). Reagents and conditions: (a) NaH, THF, 0 ꢀC, 0.5 h,
then TBSCl, 3 h, 54%. (b) Me3SI, LHMDS, THF, –10 ꢀC, room temperature, 2 h, 89%. (c) TIPSOTf,
Et3N, 0 ꢀC, 1 h, 93%. (d) PPTS, THF, rt, 12 h and then TsCl, Et3N, 4-DMAP, THF, rt, 12 h, 45%. (e)
NaH, DMF, 2 h, and then TBAF, overnight, 93%. (f) Grubbs (2nd) (5%), methyl acrylate, DCM, 40 ꢀC,
24 h, 92%. (g) BF3 ether, DCM, 0 ꢀC to rt, overnight, 57%.

6
K.-Q. MA ET AL.
3.2.3. (R)-6-((Tert-butyldimethylsilyl)oxy)hex-1-en-3-ol (15)
To a suspension of trimethylsulfonium iodide (72 mg, 0.35 mmol) in anhydrous THF
(0.7 mL) was added lithium bis(trimethylsilyl)amide (0.7 mL, 0.7 mmol, 1 M in THF)
at –10 ꢀC. After 30 min, epoxide (65 mg, 0.32 mmol) in THF (0.4 mL) was introduced,
producing a milky suspension. The reaction was allowed to warm to 0 ꢀC about
30 min, followed by stirring for 2 h at room temperature. The reaction was quenched
with water at 0 ꢀC, extracted with EtOAc. The combined organic layers were dried
over MgSO4, filtered, and concentrated in vacuo. The crude product was purified by
flash chromatography (PE/EtOAc ¼ 5:1) to give the title compound (65.8 mg, 89%)
as a colorless oil; 1H NMR (600 MHz, CDCl3) d 5.97–5.82 (m, 1H), 5.26 (d,
J¼ 17.2 Hz, 1H), 5.12 (d, J¼ 10.4 Hz, 1H), 4.15 (s, 1H), 3.68 (t, J¼ 5.2 Hz, 2H), 2.69
(s, 1H), 1.77 – 1.55 (m, 4H), 0.92 (s, 9H), 0.09 (s, 6H), 13C NMR (151 MHz, CDCl3)
d 141.2, 114.3, 72.7, 63.4, 34.5, 28.8, 25.9, 18.3, 0.3. [a]20 -7.8 (c 0.03, MeOH);
D
HRESIMS: m/z 231.1774 [M þ H]þ (calcd for C12H27O2Si, 231.1775).
3.2.4. (R)-10,10-Diisopropyl-2, 2, 3, 3, 11-pentamethyl-8-vinyl-4, 9-dioxa-3, 10-disi-
ladodecane (16)
To a solution of alcohol (200 mg, 0.87 mmol) in anhydrous methylene chloride
(5 mL) were added triethylamine (0.6 mL, 4.35 mmol) and TIPSOTf (0.35 mL,
1.30 mmol) at –10 ꢀC. After stirring 1 h at –10 ꢀC, the reaction mixture was quenched
with water. The organic layer was separated, and the aqueous layer was extracted
with methylene chloride and washed with brine. The combined organic extracts were
dried over MgSO4, filtered, and concentrated in vacuo. Purification by flash column
chromatography (PE/EtOAc ¼ 20:1) gave title compound (336 mg, 93%) as a colorless
1
oil. H NMR (600 MHz, CDCl3) d 5.82 (ddd, J¼ 17.0, 10.4, 6.5 Hz, 1H), 5.16 (d,
J¼ 17.2 Hz, 1H), 5.06 (d, J¼ 10.4 Hz, 1H), 4.26 (d, J¼ 5.2 Hz, 1H), 3.62 (dd, J¼ 11.0,
5.2 Hz, 2H), 1.71–1.49 (m, 4H), 1.08 (m, 21H), 0.91 (s, 9H), 0.12 (s, 6H). 13C NMR
(151 MHz, CDCl3) d 141.6, 114.3, 73.7, 72.7, 63.4, 34.5, 28.8, 25.9, 18.3, 1.9, 0.3.
[a]20 -2.0 (c 0.10, MeOH); HRESIMS: m/z 409.2933 [M þ Na]þ (calcd for
D
C21H46O2Si2Na, 409.2929).
3.2.5. (R)-4-((Triisopropylsilyl)oxy)hex-5-en-1-yl-4-methylbenzenesulfonate (17)
To a solution of di-protected compound (380 mg, 0.98 mmol) in anhydrous THF
(5 ml) were added pyridinium p-toluenesulfonate solution (50 mg, 0.20 mmol in 5 mL
of anhydrous methanol) at rt and stirred overnight at room temperature.
Triethylamine (0.4 mL, 2.94 mmol), 4-dimethylaminopyridine (12.2 mg, 0.1 mmol),
TsCl (280 mg, 1.47 mmol) was added at 0 ꢀC and then stirred for overnight at room
temperature. The reaction mixture was quenched with water at 0 ꢀC. The aqueous
layer was extracted with ethyl acetate. The combined organic extracts were washed
with brine, dried over Na2SO4, filtered, and concentrated in vacuo. Purification by
flash column chromatography (PE/EtOAc ¼ 8:1) gave compound (188 mg, 45%) as a
colorless oil. 1H NMR (600 MHz, CDCl3) d 7.80 (d, J¼ 8.3 Hz, 2H), 7.36 (d,
J¼ 8.0 Hz, 2H), 5.73 (ddd, J¼ 16.8, 10.4, 6.3 Hz, 1H), 5.16–5.09 (m, 1H), 5.07–5.03
(m, 1H), 4.25 (q, J¼ 5.9 Hz, 1H), 4.09–4.01 (m, 2H), 2.47 (s, 3H), 1.78–1.64 (m, 2H),
1.60–1.49 (m, 2H), 1.04 (m, 21H). 13C NMR (151 MHz, CDCl3) d 144.6, 140.9, 133.2,

JOURNAL OF ASIAN NATURAL PRODUCTS RESEARCH
7
129.8, 127.9, 114.5, 73.0, 70.9, 33.8, 23.9, 21.6, 18.1, 18.1, 12.3. [a]20 -7.2 (c 0.27,
D
MeOH); HRESIMS: m/z 427.2328 [M þ H]þ (calcd for C22H40O4SSi, 427.2333).
3.2.6. (R)-6-(1H-Pyrrol-1-yl)hex-1-en-3-ol (5)
Pyrrole (0.017 mL, 0.24 mmol) was added to a stirred suspension of NaH (10 mg,
0.24 mmol) in dry N, N-dimethylformamide (1 mL), and the resulting solution was
starred at room temperature for 5 min. Then, a solution of tosylate (51.8 mg,
0.12 mmol) in DMF (0.5 mL) was added, and the flask containing the tosylate was
rinsed with additional DMF (0.2 mL). The resulting mixture was stirred at room tem-
perature for 2 h, and then, a solution of TBAF (0.5 mL, 0.5 mmol, 1 M in THF) was
added and stirred at room temperature overnight. The reaction mixture was
quenched with water, and the aqueous layer was extracted with ethyl acetate. The
combined organic extracts were washed with brine, dried over Na2SO4, filtered, and
concentrated in vacuo. Purification by flash column chromatography (PE/EtOAc ¼
4:1) gave title compound (18.6 mg, 93%) as a colorless oil. 1H NMR (600 MHz,
CDCl3) d 6.66 (s, 2H), 6.14 (s, 2H), 5.84 (ddd, J¼ 17.2, 10.4, 6.2 Hz, 1H), 5.22 (d,
J¼ 17.2 Hz, 1H), 5.12 (d, J¼ 10.4 Hz, 1H), 4.09 (q, J¼ 6.2 Hz, 1H), 3.91 (tt, J¼ 8.5,
4.2 Hz, 2H), 1.99–1.76 (m, 2H), 1.54–1.49 (m, 2H). 13C NMR (151 MHz, CDCl3) d
140.8, 120.5, 115.1, 107.9, 72.8, 49.5, 33.9, 27.4. [a]20 -2.5 (c 0.12, MeOH);
D
HRESIMS: m/z 166.1221 [M þ H]þ (calcd for C10H16NO, 166.1226).
3.2.7. Methyl (R, E)-4-hydroxy-7-(1H-pyrrol-1-yl) hept-2-enoate (8)
To a solution of the alcohol (0.26 g, 1.6 mmol) in DCM (4 mL) were added methyl
acrylate (1.4 g, 16 mmol) and the Grubbs second generation catalyst (66 mg,
0.082 mmol) at room temperature in sealed tube. The deep brown reaction mixture
was raised to 40 ꢀC and stirred for 24 h. The reaction mixture was concentrated to
afford residue, which was purified by column chromatography (hexane: EtOAc ¼
4:1) to afford the desired product (0.32 g) as brown oil (Yield: 92%). 1H NMR
(600 MHz, CDCl3) d 6.92 (dd, J¼ 15.7, 4.9 Hz, 1H), 6.66 (s, 2H), 6.16 (t, J¼ 2.1 Hz,
2H), 6.05 (dd, J¼ 15.7, 1.7 Hz, 1H), 4.34–4.25 (m, 1H), 3.97–3.88 (m, 2H), 3.77 (s,
3H), 1.98–1.81 (m, 2H), 1.54–1.62 (d, 2H). 13C NMR (126 MHz, CDCl3) d 166.9,
150.0, 120.5, 120.1, 108.1, 70.5, 51.7, 49.3, 33.5, 27.2. [a]20 -3.4 (c 0.27, MeOH).
D
3.2.8. (3aR, 10bR)-1, 3a, 4, 5, 6, 10b-Hexahydro-2H-furo[3, 2-c]pyrrolo[1, 2-a]aze-
pin-2-one (19). To a solution of ester (25 mg, 0.11 mmol) in DCM (3.8 mL) was
added the boron trifluoride diethyl etherate (0.017 ml, 0.14 mmol) at 0 ꢀC. The reac-
tion mixture was warmed to room temperature and stirred overnight. Triethylamine
was added to the reaction mixture and stirred for 30 min. The organic layer was
washed with H2O and concentrated to afford residue, which was purified by column
chromatography (PE: EtOAc ¼ 4:1) to afford the desired product 19 (9.6 mg) as
1
white solid (Yield: 57%). H NMR (600 MHz, CDCl3) d 6.70–6.59 (m, 1H), 6.05 (dd,
J¼ 3.4, 2.8 Hz, 1H), 5.99–5.92 (m, 1H), 4.14 (dd, J¼ 14.6, 5.4 Hz, 1H), 4.00 (ddd,
J¼ 11.2, 10.1, 3.5 Hz, 1H), 3.91 (dd, J¼ 14.2, 11.8 Hz, 1H), 3.43 (dd, J¼ 15.4, 6.0 Hz,
1H), 2.98–2.87 (m, 2H), 2.60–2.50 (m, 1H), 2.20–2.10 (m, 1H), 1.88–1.80 (m, 1H),

8
K.-Q. MA ET AL.
1.77–1.69 (m, 1H). 13C NMR (126 MHz, CDCl3) d 175.2, 128.6, 122.8, 106.4, 105.5,
83.7, 49.1, 42.2, 34.0, 33.7, 26.1. [a]20 -4.7 (c 0.38, MeOH).
D
Disclosure statement
The authors declare no conflict of interest.
Funding
This work was financially supported by Key R&D Program of Shanxi Province (No.
201803D421059) and Science and Technology Innovation Project of Shanxi Province
(No. 2016115).
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