Exploring sesquiterpene alkaloid-like scaffolds via Beckmann-transannular remodelling of beta-caryophyllene

Article abstract of DOI:10.1039/c7ra08196k

High-throughput screening (HTS) is the dominant approach to identify lead compounds in drug development. However, most compound-screening collections provide little structural or stereochemical complexity, which do not offer enough diversity to merit the modulation of many drug targets. Here we describe a facile strategy for the creation of diverse compounds with high structural and stereochemical complexity using readily available natural products, β-caryophyllene, as a synthetic starting point. Our findings demonstrate that a cascaded Beckmann-transannular protocol transforms macrocyclic natural products into poly-heterocyclic unnatural skeletal types. These compounds are significantly more complex and diverse than those in standard screening collections.

Full text of DOI:10.1039/c7ra08196k

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PAPER
Exploring sesquiterpene alkaloid-like scaffolds
via Beckmann-transannular remodelling of
beta-caryophyllene†
Cite this: RSC Adv., 2017, 7, 40510
a
a
c
a
Shuang-Jiang Ma,a Jie Yu,
Hua-Fang Fan, Zi-Han Li, An-Ling Zhang
*
ab
*
and Qiang Zhang
High-throughput screening (HTS) is the dominant approach to identify lead compounds in drug
development. However, most compound-screening collections provide little structural or
stereochemical complexity, which do not offer enough diversity to merit the modulation of many drug
targets. Here we describe a facile strategy for the creation of diverse compounds with high structural
and stereochemical complexity using readily available natural products, b-caryophyllene, as a synthetic
starting point. Our findings demonstrate that a cascaded Beckmann-transannular protocol transforms
macrocyclic natural products into poly-heterocyclic unnatural skeletal types. These compounds are
significantly more complex and diverse than those in standard screening collections.
Received 25th July 2017
Accepted 13th August 2017
DOI: 10.1039/c7ra08196k
rsc.li/rsc-advances
(acetylcholinesterase inhibition IC50 2.1 mM),12 sessilistemon-
amine E (acetylcholinesterase inhibition IC50 9.1 mM),13 and
cryptopleurin (NF kB, IC50 1.4 nM)14 (Fig. 1). Compounds of this
type have been reported to show a wide range of potential
applications in antitumor, anti-inammatory and Alzheimer's
disease therapy.
Introduction
Natural products (NPs) have played an essential role in devel-
oping novel drugs due to their structural diversity.1–4 Never-
theless, pharmaceutical research into NPs has recently declined
owing to factors such as the difficulty of collecting novel
compounds containing privileged structures.2 Thus, new strat-
egies to augmenting chemical diversity are crucial for retaining
the utility of natural products and their derivatives. Recently,
creating complex small molecules directly from NPs has
appeared as a convenience approach in diversity-oriented
synthesis (DOS) area.5–9 NPs are being considered as excep-
tional starting points due to the high abundance and structural
variation. Additionally, NPs provide an excellent framework for
site selective and stereoselective transformations.
The propensity of macrocyclic precursors to undergo trans-
annular cyclization has been dely harnessed by nature to build
The construction of heterocyclic molecules with potentially
interesting biological activities represents an important part of
the drug discovery process. Especially there has been a growing
interest in the synthesis of medium-sized rings because of their
many applications.10 Among them, poly-N-heterocyclic NPs
displayed excellent bioactivities, such as homoharringtonine
(topoisomerase inhibition, IC50 9.0 nM),11 stenine
B
aShaanxi Key Laboratory of Natural Products
& Chemical Biology, College of
Chemistry & Pharmacy, Northwest A&F University, Yangling 712100, PR China.
E-mail: zhangq@nwsuaf.edu.cn; anlingzh@nwsuaf.edu.cn
bState Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin
300071, PR China
cCollege of Life Sciences, Northwest A&F University, Yangling 712100, PR China
† Electronic supplementary information (ESI) available: Experimental,
characterization data, X-ray structures of compounds 6 and 9 and NMR spectra.
CCDC 1552864 and 1519447. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c7ra08196k
Fig. 1 Bioactive natural products with N-containing poly-heterocyclic
skeleton.
40510 | RSC Adv., 2017, 7, 40510–40516
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The structures of compounds 4–8 were unambiguously
elucidated using 1D and 2D NMR and MS spectra. As a repre-
sentative example, the structural analysis of 6 was elucidated in
detail herein. Its molecular formula (MF) was assigned as
C
16H26N2O2 by HRESIMS, indicating 5 degrees of unsaturation.
Its 13C and DEPT NMR spectroscopic data revealed the presence
of two imine groups (dC 161.1, 162.4) and other 15 aliphatic
carbons, including three methyls, ve methenes, three
methines and two quaternary carbon atoms. Apart from those
two degrees of unsaturated C]N bonds, the remaining implied
that 6 possesses three rings in the structure.
Extensive analysis of the HSQC, 1H–1H COSY, and HMBC
correlations (Table 1) established the whole planar structure.
1H–1H COSY correlations disclosed two isolated spin systems.
HMBC correlations from H3-15 to C-1, C-10, C-11 and C-14
indicated a moiety of cyclobutane with a gem-dimethyl group.
The HMBC correlations from H2-10 and H2-7 to C-8 indicated
that the C-7 and C-9 are linked up at C-8. The HMBC correla-
tions from H3-12 to C-3, C-4 and C-5 disclosed a 9 membered
ring. The dihydrooxazole ring with a methyl was identied by
the HMBC correlations of H-5/C-16 and H3-17/C-16. The relative
conguration were analysed on the basis of NOESY correlations
(Table 1). The NOE correlation of H-1/H-5 indicated these H
atoms are in b-orientation. The NOE correlations of H-9/Ha-7/
H3-12 indicated the H-9 and H3C-12 are in a-orientation. The
relative conguration were also conrmed by Cu Ka X-ray
diffraction analysis (Table 1). However, high values of Flack
parameters (0.03) indicated that the X-ray data did not provide
unambiguous information about absolute conguration.
Alternatively, based on the unchanged (1R) and (9S) congura-
tions, the absolute conguration was deduced as therein on the
basis of NOESY correlations related to H-1 and H-9.
Scheme 1 Beckmann-transannular strategy to NP-like library.
thousands of polycyclic terpenoids and alkaloids with medium-
sized rings.9,15–17 Our previous diversity-oriented transformation
demonstrated the convenience protocol to build natural-like
libraries containing medium-sized rings from macrocyclic car-
yophyllene.18,19 In this paper, we consider applying Beckmann
rearrangement associated with transannular cyclization to
explore chemical space covering poly-N-heterocyclic skeletons.
The natural sesquiterpenes, such as b-caryophyllene and its
epoxide (1), are readily abundant to be ideal starting points to
build diverse natural-like scaffolds along NP-driven DOS
strategy. Due to the tensional cyclobutane, macrocyclic moiety
and exocyclic olen, compound 1 plays as a key precursor in
nature to form diverse tricyclic sesquiterpenes by transannular
rearrangements.20 In this paper, we reported a concise proce-
dure to build natural-like skeletons with N-containing poly-
heterocyclic skeletons via Beckmann-transannular rearrange-
ment on the macrocycle of caryophyllene (Scheme 1). To start
Beckmann rearrangements on the macrocycle, compound 1 was
introduced oxime group at C-8 as starting site (Scheme 1).
Compound 6 possesses a 2-oxazoline moiety which was
generated from epoxyethane moiety in 3 and the solvent
acetonitrile. The 2-oxazoline ring was reported to be synthesized
21
in good yields catalyst by BF3–OEt2 or SiF4.22 In the current
Results and discussion
reaction, LiBF4 was considered to decompose into BF3 and LiF.
Compound 3 was transformed into carbonium ion A catalysed
by BF3 (Scheme 2). The intermediate A was attacked by Fꢀ or
MeCN to yield uoride 4 and 2-oxazoline derivate 6. Simulta-
neously, elimination reactions of A afforded products 5, 7 and 8.
The end olen at C-8 in 1 was cleaved by RuCl3/NaIO4 oxidation
to yield a 13-nor product (2) with carbonyl function group. The
key intermediate oxime derivative (3) was prepared via
condensation with hydroxylamine. In the next step, Lewis acid
LiBF4/MeCN inspired 3 into a DOS library containing ve
compounds 4–8 (Scheme 2).
Scheme 2 Diversity-oriented transformation to oxime. Reaction condition: (i) NaIO4, RuCl3, MeCN–EtOAc–H2O; (ii) NH2OH$HCl, NaOH; (iii)
LiBF4, MeCN, 45 ꢁC.
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Table 1 Structural analysis of compounds 6 and 9a
a
See Table S1 in ESI for the 2D NMR correlations of compounds 4–15.
These ndings showed that the LiBF4 did not provide access NMR. All the detailed correlations in 2D NMR were summarized
to the targeted remodeling skeletons. Pleasingly, we success- in the Table S1 in ESI material.† For example, the MF of
fully formed the 2-oxazoline moiety at milder condition than compound 9, C14H23NO2, was determined by HRESIMS. Two
that previously reported.21,22 To fully exploit build-in reactivity isolated spin systems were recognized by the COSY correlations
and increase rigid polycyclic molecules, Beckmann rearrange- as shown in Table 1. The HMBC correlations of H-5 and H2-6 to
ments of compound 3 were investigated with catalyst p-TsCl C-8 revealed cascaded Beckman-transannular remodeling on
(Scheme 3). Compound 3 gave entirely different cyclization the caryophyllene's macrocycle. Furthermore, the X-ray analysis
products 9–11, with polycyclic lactam skeletons. To acquire using Cu Ka diffraction disclosed the N-containing 4/7/5 scaf-
similar rearranged derivatives, the epoxyethane moiety in the fold 9. The conguration was determined by NOESY correla-
starting material 3 was reduced into an endocyclic olen. A tions. The H-1(S) was selected as the reference group whose
following Beckmann rearrangement afforded a macrolactam 14 absolute conguration kept unchanged in the reaction process.
and two other polycyclic products 10 and 15.
The NOESY correlations among H-1, H3-15 and H-5 indicated
To verify the skeleton remodeling, the structures of products that the newly formed chiral centre C-5 is S-conguration. The
9–11, 14 and 15 were investigated by HRMS, IR, 1D and 2D NOESY correlations from H-9 to H3-14 and H3-12 indicated that
Scheme 3 Beckmann-transannular remodelling. (i) NH2OH$HCl, NaOH; (ii) p-TsCl, Et3N, DMAP (cat.), DCM, 3 h, reflux; (iii) Zn powder; (iv)
NH2OH$HCl, NaOH.
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naturally occurring bioactive blocker, will yield highly diverse
products with NP-like scaffolds.
Experimental procedures
General procedures
UV spectra were obtained using an Evolution 300 UV-vis Spec-
trophotometer (Thermo Fisher Scientic Inc., USA). IR were
recorded on a Bruker Tensor 27 spectrophotometer with KBr
disks (Bruker Corp., German). Optical rotations were measured
on an Autopol III automatic polarimeter (Rudolph Research
Analytical, USA). ECD spectra were obtained on a Chirascan CD
Spectrometer (Applied Photophysics Ltd., United Kingdom).
ESI-MS were performed on an LTQ Fleet instrument (Thermo
Fisher Scientic Inc., USA). HR ESIMS data were obtained by an
AB Sciex Triple TOF 4600 system (AB Sciex Pte. Ltd., USA). 1D
and 2D NMR spectra were recorded on an AVANCE III (500
MHz) instrument (Bruker Corp., Germany). Chemical shis
were reported using solvent residual as the internal standard.
Crystallographic data collection was carried out on a Bruker
Smart Apex CCD area detector diffractometer with graphite-
Scheme
rearrangements to 9–11.
4 Proposed mechanism for the Beckmann-transannular
the chiral C-4 is R-conguration. The whole conguration was
also conrmed by the X-ray diffraction as shown in Table 1.
Based on the observed stereochemistry, a proposed mecha-
nism for the formation of 9–11 is shown in Scheme 4. In the
Beckmann process, the departure of the leaving group, TsO–,
was accompanied by the [1,2]-shi of the C-7 or C-8 to afford the
lactam tautomers B or C. In the next step, transannular rear-
rangements occurred between the imines in B or C with the epo
C-4 or C-5 to form the polycyclic skeletons of 9–11. Their
structures indicated a cascaded Beckmann-transannular rear-
rangement in the process.
To evaluate the biological relevancy of product library,
butyrylcholinesterase (BChE) was selected as bioassay target.
Previous relationship observed between cholinergic dysfunc-
tion and Alzheimer's disease (AD) severity provides a rationale
for the therapeutic use of cholinesterase inhibitors (ChEIs).23
Administration of ChEIs to increase the amount of cholines
available for neural transmission has proven successful in
relieving some cognitive as well as behavioral symptoms of
AD.24,25 BChE inhibition was found to elevate brain acetylcho-
line, augment learning and reduce Alzheimer b-amyloid
˚
monochromated Cu Ka radiation (l ¼ 1.54184 A) at 293(2) K
using u-scan technique. The diffraction data were integrated by
using the SAINT program, which was also used for the intensity
corrections for the Lorentz and polarization effects. Semi-
empirical absorption corrections were applied using SADABS
program. Analytic and semi-preparative HPLC were performed
on a Waters 1525 instrument (Waters Corp., USA). Column
chromatography was performed on silica gel (90–150 mm;
Qingdao Marine Chemical Inc., China), Sephadex LH-20 (40–70
mm; Amersham Pharmacia Biotech AB, Sweden), and Chroma-
torex C18 gel (40–75 mm; Fuji Silysia Chemical LTD., JPN). GF254
plates (Qingdao Marine Chemical Inc., China) were used for
thin-layer chromatography (TLC).
Synthesis of oxime 3
(ꢀ)-b-Caryophyllene epoxide (5.0 g, 22.7 mmol), NaIO4 (36.4 g,
170.25 mmol, 7.5 eq) was added to a mixed solvent (25 ml
MeCN, 25 ml EtOAc and 37.5 H2O) and RuCl3 (2.2% mol) in
a 250 ml round-bottom ask equipped with a stir bar. Aer
being stirred for 6 h, the reaction mixture was ltered. The
ltrate was extracted with EtOAc (100 ml, 3 times). The
combined organic layers were dried over Na2SO4 and concen-
trated under reduced pressure. The residue was puried on
peptide.26 The bioassay indicated
compound 15 (IC50 134.6 mM), with positive control, galanth-
amine (IC50 82.6 mM).
a weak inhibition of
Conclusions
In conclusion, highly functionalized epoxy-macrocyclic sesqui- column chromatography (silica gel, petroleum ether : EtOAc ¼
terpenes, such as b-caryophyllene epoxide, endowed with 10 : 1, v/v) to give the product 2 (3.1 g, 61% yield).
a potential to generate rigid polycyclic structures not easily
A solution of 2 (1.11 g, 5 mmol in 15 ml EtOH) was added to
accessed by other methods. Our ndings demonstrated that a stirred solution of NH2OH HCl (2.08 g, 30 mmol) in 15 ml 2 N
a Beckmann-transannular rearrangement on such a macrocycle aqueous NaOH. Aer being stirred for 30 min at room
is a concise approach to generate NP-like library with a lactam temperature, 60 ml H2O was added. Then the reaction mixture
unit. In addition, b-caryophyllene and its epoxide are readily was extracted with DCM (100 ml, 3 times). The combined
plentiful in natural source. Thus they can be served as ideal organic layers were dried over Na2SO4 and concentrated under
staring points to construct complex cyclic scaffolds covering reduced pressure to afford the crude residue. A further puri-
unknown chemical space or bioactive space. On the basis of the cation on column chromatography (silica gel, petroleum–EtOAc
present ndings, combination of macrocyles with other 20 : 1, v/v) to yield the product 3 (1.04 g, 87% yield).
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(C-12), 21.9 (C-14), 30.0 (C-15); ESIMS m/z 238 [M + H]+; HR
ESIMS m/z 238.1807 [M + H]+ (calcd for C14H24NO2 238.1807).
Compound 6. Colorless needle, mp 191.2–191.8 ꢁC;
[a]3D0 ꢀ5.1 (c 0.09, CH3OH); IR (KBr) nmax 3436, 2929, 1644, 1457,
1392 cmꢀ1; 1H NMR (500 MHz, CDCl3) d ppm 1.80 (dd, J ¼ 9.6,
3.2 Hz, H-1), 1.67 (m, H-2a), 1.47 (m, H-2b), 1.96 (m, H-3a), 1.61
(d, J ¼ 7.9 Hz, H-3b), 4.166 (dd, J ¼ 10.2, 3.6 Hz, H-5), 2.39 (m, H-
6a), 1.83 (dd, J ¼ 8.4, 2.0 Hz, H-6b), 3.12 (dt, J ¼ 13.6, 5.7 Hz, H-
7a), 1.87 (m, H-7b), 2.56 (td, J ¼ 9.6, 7.9 Hz, H-9), 2.03 (t, J ¼
10.6 Hz, H-10a), 1.58 (d, J ¼ 7.9 Hz, H-10b), 1.19 (s, H3-12), 0.99
(s, H3-14, H3-15), 1.91 (s, H3-17); 13C NMR (125 MHz, CDCl3)
d ppm 54.3 (C-1), 24.6 (C-2), 42.5 (C-3), 70.8 (C-4), 85.2 (C-5), 25.4
(C-6), 26.7 (C-7), 161.1 (C-8), 44.3 (C-9), 34.4 (C-10), 34.9 (C-11),
20.1 (C-12), 21.5 (C-14), 29.7 (C-15), 162.4 (C-16), 14.0 (C-17);
ESIMS m/z 279 [M + H]+; HR ESIMS m/z 279.2055 [M + H]+
(calcd for C16H27N2O2 279.2072).
Compound 3. Colorless oil; [a]3D0 ꢀ1.4 (c 0.23, CH3OH); IR
(KBr) nmax 3315, 2938, 2864, 1452, 946, 861 cmꢀ1 1H NMR
;
(CDCl3, 500 MHz): d (ppm) 1.74 (dd, J ¼ 9.3, 2.4 Hz, H-1, 2a),
1.65 (m, H-2b, 10b), 2.09 (dt, J ¼ 12.8, 3.5 Hz, H-3a), 0.98 (m, H-
3b); 2.62 (ddd, J ¼ 12.2, 7.5, 4.3 Hz, H-5), 2.33 (m, H-6a), 1.45 (m,
H-6b), 2.81 (q, J ¼ 9.8 Hz, H-7a), 2.42 (m, H-7b), 2.86 (dd, J ¼
11.0, 4.1 Hz, H-9), 1.95 (t, J ¼ 9.9 Hz, H-10a), 1.15 (s, H3-12), 1.02
(s, H3-14), 1.03 (s, H3-15); 13C NMR (CDCl3, 125 MHz): d (ppm)
49.4 (C-1), 23.1 (C-2), 37.7 (C-3), 59.8 (C-4), 64.2 (C-5), 26.6 (C-6),
38.4 (C-7), 162.3 (C-8), 45.9 (C-9), 23.7 (C-10), 35.2 (C-11), 16.0 (C-
12), 21.2 (C-14), 29.8 (C-15); ESIMS m/z 238 [M + H]+; HR ESIMS
m/z 238.1807 [M + H]+ (calcd for C14H24NO2 238.1807).
DOS procedure catalysed by LiBF4
LiBF4 (164.3 mg, 1.75 mmol) was added slowly to a stirred
Compound 7. Colorless oil; [a]3D0 +52.0 (c 0.15, CH3OH); IR
solution of 3 (1.04 g, 4.38 mmol) in 10 ml MeCN. The reaction (KBr) nmax 3386, 2932, 1697, 1450, 943 cmꢀ1; 1H NMR (500 MHz,
mixture was stirred at 45 ꢁC for 20 h. The reaction was quenched CDCl3) d ppm 1.81 (m, H-1), 1.42 (m, H2-2), 1.73 (m, H-3a), 1.45
with water (30 ml). And the aqueous phase was extracted with (m, H-3b), 2.55 (m, H-4), 2.63 (m, H-6a), 2.51 (m, H-6b), 2.84
DCM for 3 times. The combined organic layers were dried over (ddd, J ¼ 18.3, 8.2, 1.8 Hz, H-7a), 2.63 (d, J ¼ 9.5 Hz, H-7b), 2.48
Na2SO4, and concentrated under reduced pressure to give (m, H-9), 1.75 (m, H-10a), 1.69 (m, H-10b), 1.06 (d, J ¼ 6.9 Hz,
a crude product (0.717 g).
H3-12), 0.96 (s, H3-14), 1.00 (s, H3-15); 13C NMR (125 MHz,
The crude was chromatographed on a silica gel column CDCl3) d ppm 50.1 (C-1), 26.8 (C-2), 30.3 (C-3), 47.6 (C-4), 217.7
being eluted with CHCl3–MeOH (from 100 : 1 to 1 : 1) to give (C-5), 35.9 (C-6), 25.2 (C-7), 163.6 (C-8), 41.9 (C-9), 37.1 (C-10),
three fractions (Fr.1–Fr.3). Fr.1 (110 mg) was separated on an 34.7 (C-11), 18.0 (C-12), 21.4 (C-14), 29.6 (C-15); ESIMS m/z
HPLC C18 column (Thermo BDS Hypersil 250 ꢂ 10 mm) with 260.23 [M + Na]+; HR ESIMS m/z 238.1807 [M + H]+ (C14H23NO2,
80% methanol to yield compounds 7 (15 mg) and 8 (14 mg). Fr.2 calcd for [M + H]+ 238.1807).
(330 mg) was separated by a silica gel column chromatography
(CHCl3–MOH 100 : 1) to afford compounds 4 (200 mg). Fr.3 (260 (KBr) nmax 3373, 2935, 2865, 1699, 1451, 945 cmꢀ1
mg) was isolated on an HPLC C18 column with elution of 60% (500 MHz, CDCl3) d ppm 2.01 (m, H-1), 1.46 (m, H-2a), 1.40 (dt,
Compound 8. Colorless oil; [a]3D0 43.4 (c 0.14, CH3OH); IR
1H NMR
;
MeOH to yield 5 (160 mg) and 6 (40 mg).
J ¼ 10.0, 4.5 Hz, H-2b), 1.57 (m, H-3a), 1.00 (m, overlapped, H-
Compound 4. Colorless oil; [a]3D0 ꢀ85.1 (c 0.18, CH3OH); IR 3b), 2.40 (m, H-4), 2.71 (m, H-6a), 2.57 (dd, J ¼ 12.1, 10.6 Hz, H-
(KBr) nmax 3391, 2942, 1641, 1435, 1378, 1059 cmꢀ1; H NMR 6b), 2.70 (m, H-7a), 2.30 (m, H-7b), 3.26 (q, J ¼ 9.5 Hz, H-9),
1
(CDCl3, 500 MHz): d (ppm) 2.10 (td, J ¼ 10.9, 5.4 Hz, H-1), 1.79 1.94 (dd, J ¼ 10.7, 9.5 Hz, H2-10), 1.06 (d, J ¼ 6.9 Hz, H3-12),
(m, H-2a), 1.41 (m, H-2b), 2.01 (d, J ¼ 6.0 Hz, H-3a), 1.91 (m, H- 0.99 (s, H3-14), 1.00 (s, H3-15); 13C NMR (125 MHz, CDCl3)
3b), 3.98 (dd, J ¼ 7.3, 3.5 Hz, H-5), 3.95 (dd, J ¼ 7.1, 3.0 Hz, H-5), d ppm 43.9 (C-1), 28.3 (C-2), 29.3 (C-3), 50.7 (C-4), 217.6 (C-5),
2.21 (m, H-6a), 1.63 (m, H-6b), 3.05 (dt, J ¼ 13.7, 5.4 Hz, H-7a), 29.3 (C-6), 35.4 (C-7), 164.5 (C-8), 32.6 (C-9), 37.2 (C-10), 35.5
1.98 (m, H-7b), 2.54 (td, J ¼ 9.9, 7.9 Hz, H-9), 2.05 (m, H-10a), (C-11), 18.0 (C-12), 22.1 (C-14), 30.0 (C-15); ESIMS m/z 260 [M +
1.60 (d, J ¼ 7.6 Hz, H-10b), 1.32 (s, H3-12), 1.37 (s, H3-12), 1.00 Na]+; HR ESIMS m/z 238.1809 [M + H]+ (calcd for C14H24NO2
(s, H3-14), 1.06 (s, H3-15); 13C NMR (CDCl3, 125 MHz): d (ppm) 238.1807).
52.1 (C-1), 21.6 (C-2), 21.5 (C-2), 36.0 (C-3), 35.8 (C-3), 100.0 (C-
4), 101.3 (C-4), 73.2 (C-5), 73.1 (C-5), 28.1 (C-6), 28.0 (C-6), 26.4
(C-7), 162.2 (C-8), 40.5 (C-9), 33.9 (C-10), 34.7 (C-11), 20.0 (C-12),
Beckmann remodeling of compound 3
19.8 (C-12) 22.2 (C-14) 30.1 (C-15); ESIMS m/z 258 [M + H]+; 280 To a solution of compound 3 (1.00 g, 4.2 mmol) in 10 ml DCM,
[M + Na]+; HR ESIMS m/z 258.1867 [M + H]+ (calcd for a solution of paratoluenesulfonyl chloride (p-TsCl, 2.00 g, 10.54
C
14H25FNO2 258.1869).
mmol), Et3N (1.5 ml, 10.54 mmol) and DMAP (10.3 mg, 2 mol%)
Compound 5. Colorless needle, mp 260.8–261.1; in DCM (20 ml) was added dropwise. Aer being stirred for 3 h
[a]3D0 ꢀ105.26 (c 0.13, CH3OH); IR (KBr) nmax 3378, 2946, 1640, at room temperature, the reaction mixture was diluted with
1453, 1021 cmꢀ1; 1H NMR (500 MHz, CDCl3) d ppm 2.32 (m, H- 20 ml DCM and washed with water (2 ꢂ 100 ml) and brine (2 ꢂ
1, 2a), 1.93 (m, H-2b), 5.46 (dd, J ¼ 4.9, 3.9 Hz, H-3), 4.57 (dd, J ¼ 100 ml). Aer being concentrated in vacuum and dried over
11.5, 3.9 Hz, H-5), 2.14 (m, H-6a), 2.01 (m, H-6b, 7b), 2.70 (ddd, J Na2SO4, the crude product (820 mg) was yielded.
¼ 11.7, 6.9, 4.1 Hz, H-7a), 2.449 (m, H-9), 1.81 (t, J ¼ 10.4 Hz, H-
The crude product (820 mg) was run on a silica gel column
10a), 1.61 (dd, J ¼ 10.4, 8.2 Hz, H-10b), 1.67 (s, H3-12), 1.00 (s, (gradient petroleum–EtOAc from 20 : 1 to 1 : 2) and a semi-
H3-14), 1.04 (s, H3-15); 13C NMR (125 MHz, CDCl3) d ppm 51.2 preparative HPLC C18 column (Thermo BDS Hypersil 250 ꢂ 10
(C-1), 25.6 (C-2), 125.2 (C-3), 137.8 (C-4), 70.1 (C-5), 29.9 (C-6), mm, 60% methanol) to yield compounds 9 (80 mg), 10 (30 mg)
25.3 (C-7), 163.0 (C-8), 41.0 (C-9), 35.1 (C-10), 35.1 (C-11), 16.5 and 11 (8 mg).
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Compound 9. Colorless needle, mp 222.8–223.6 ꢁC;
RSC Advances
reaction mixture was extracted with DCM (100 ml) for 3 times.
The combined organic layers were dried over Na2SO4 and
concentrated under reduced pressure to yield the crude
product. The crude product was run on a silica gel column
chromatography (petroleum–EtOAc 20 : 1) to afford compound
13 (960 mg, 90% yield).
[a]3D0 +112.9 (c 0.19, CH3OH); IR (KBr) nmax 3440, 2978, 2935,
2866, 1657, 1447, 1291, 1252 cmꢀ1; 1H NMR (500 MHz, CDCl3)
d ppm 1.74 (dt, J ¼ 12.3, 1.8 Hz, H-1), 1.63 (m, H-2a), 1.31 (dt, J ¼
12.3, 4.4 Hz, H-2b), 1.90 (m, H-3a), 1.55 (m, H-3b), 3.90 (t, J ¼
7.6 Hz, H-5), 2.05 (m, H2-6), 2.38 (ddd, J ¼ 17.0, 10.1, 0.9 Hz, H-
7a), 2.29 (m, H-7b), 3.42 (ddd, J ¼ 16.7, 6.9, 1.8 Hz, H-9), 2.33 (m,
H-10a), 1.59 (m, H-10b), 1.08 (s, H3-12), 1.01 (s, H3-14), 1.05 (s,
H3-15); 13C NMR (125 MHz, CDCl3) d ppm 48.7 (C-1), 24.4 (C-2),
45.0 (C-3), 75.1 (C-4), 66.4 (C-5), 20.4 (C-6), 30.7 (C-7), 174.8 (C-
8), 51.7 (C-9), 40.9 (C-10), 34.2 (C-11), 20.3 (C-12), 20.9 (C-14),
30.2 (C-15); ESIMS m/z 238 [M + H]+; 260 [M + Na]+; HR ESIMS
m/z 238.1812 [M + H]+ (calcd for C14H24NO2 238.1807).
Compound 13. Colorless oil; [a]3D0 +91.9 (c 0.13, CH3OH); IR
(KBr) nmax 3445, 2939, 1640, 1451 cmꢀ1 1H NMR (500 MHz,
;
CDCl3) d ppm 1.89 (m, H-1), 1.56 (m, H-2a), 1.49 (m, H-2b), 2.10
(dt, J ¼ 11.7, 3.4 Hz, H-3a), 1.93 (m, H-3b), 5.39 (dd, J ¼ 10.1,
6.1 Hz, H-5), 2.58 (m, H-6a), 2.21 (td, J ¼ 12.6, 6.1 Hz, H-6b), 2.44
(m, H-7a), 2.37 (m, H-7b), 2.53 (m, H-9), 1.82 (t, J ¼ 10.6 Hz, H-
10a), 1.68 (dd, J ¼ 10.6, 8.5 Hz, H-10b), 1.61 (d, J ¼ 0.9 Hz, H3-12),
1.00 (s, H3-14), 1.04 (s, H3-15); 13C NMR (125 MHz, CDCl3) d ppm
52.2 (C-1), 28.9 (C-2), 39.6 (C-3), 136.3 (C-4), 125.1 (C-5), 22.4 (C-
6), 27.9 (C-7), 164.7 (C-8), 46.4 (C-9), 37.9 (C-10), 34.1 (C-11), 15.8
(C-12), 22.1 (C-14), 29.9 (C-15); ESIMS m/z 222 [M + H]+; HR
ESIMS m/z 222.1875 [M + H]+ (calcd for C14H24NO 222.1858).
Compound 10. Colorless oil; [a]3D0 +51.8 (c 0.15, CH3OH); IR
1
(KBr) nmax 3459, 2080, 1638 cmꢀ1; H NMR (500 MHz, CDCl3)
d ppm 1.41 (dd, J ¼ 11.3, 3.2 Hz, H-1), 1.53 (dd, J ¼ 11.0, 3.2 Hz,
H-2a), 1.59 (m, H-2b), 1.44 (dd, J ¼ 9.9, 3.6 Hz, H-3a), 2.05 (dt, J
¼ 11.0, 3.2 Hz, H-3b), 3.73 (t, J ¼ 8.1 Hz, H-5), 1.89 (m, H2-6),
2.40 (t, J ¼ 4.3 Hz, H-7a), 2.43 (ddd, J ¼ 18.0, 10.1, 7.6 Hz, H-7b),
3.14 (td, J ¼ 10.7, 7.3 Hz, H-9), 2.09 (t, J ¼ 10.4, 6.9 Hz, H-10a),
2.32 (dd, J ¼ 10.4, 6.9 Hz, H-10b), 1.27 (s, H3-12), 1.10 (s, H3-14,
15); 13C NMR (125 MHz, CDCl3) d ppm 52.9 (C-1), 22.3 (C-2), 37.2
(C-3), 61.4 (C-4), 74.5 (C-5), 25.7 (C-6), 30.3 (C-7), 169.0 (C-8),
49.4 (C-9), 43.3 (C-10), 37.1 (C-11), 17.2 (C-12), 20.6 (C-14),
30.3 (C-15); ESIMS m/z 238 [M + H]+, 260 [M + Na]+; HR ESIMS
m/z 238.1813 [M + H]+ (calcd for C14H24NO2, 238.1807).
Beckmann remodeling of 13
To a 10 ml DCM solution of compound 13 (0.95 g, 4.3 mmol),
a 10 ml DCM solution of p-TsCl (2.10 g, 10.75 mmol), Et3N
(1.5 ml, 10.75 mmol) and DMAP (10 mg) was added dropwise.
Aer being stirred for 3 h at room temperature, the mixture was
diluted with 20 ml DCM and washed with water (2 ꢂ 100 ml)
and brine (2 ꢂ 100 ml). The organic layer was dried over Na2SO4
and concentrated in vacuum to afford the crude product (820
mg). Remaining reactants were removed on a silica gel column
(petroleum–EtOAc 20 : 1–1 : 2). A further HPLC C18 chroma-
tography (Thermo BDS Hypersil 250 ꢂ 10 mm, 60% MeOH)
afforded compounds 10 (8 mg), 14 (90 mg) and 15 (16 mg).
Compound 14. Colorless oil; [a]3D1 ꢀ11.4 (c 0.14, CH3OH); IR
(KBr) nmax 3308, 2939, 2860, 1655, 1548, 1454, 1336, 1287, 1186
cmꢀ1; 1H NMR (500 MHz, CDCl3) d ppm 1.44 (m, H-1), 1.51 (m, H-
2a), 1.58 (dd, J ¼ 3.0, 1.9 Hz, H-2b), 1.88 (td, J ¼ 12.7, 3.0 Hz, H-
3a), 2.24 (d, J ¼ 12.0 Hz, H-3b), 5.12 (dt, J ¼ 11.7, 1.3 Hz, H-5), 2.15
(m, H-6a), 2.77 (qd, J ¼ 11.7, 5.4 Hz, H-6b), 1.94 (ddd, J ¼ 12.0,
11.7, 5.8 Hz, H-7a), 2.36 (ddd, J ¼ 12.0, 5.4, 2.0 Hz, H-7b), 3.97 (dt,
J ¼ 18.0, 8.9 Hz, H-9), 1.47 (m, H-10a), 2.04 (dd, J ¼ 10.7, 7.9 Hz,
H-10b), 1.60 (s, H3-12), 0.96 (s, H3-14), 1.02 (s, H3-15), 4.96 (d, J ¼
8.9 Hz, HN); 13C NMR (125 MHz, CDCl3) d ppm 58.1 (C-1), 26.4 (C-
2), 41.0 (C-3), 138.9 (C-4), 122.7 (C-5), 26.6 (C-6), 38.2 (C-7), 174.3
Compound 11. Colorless needle, mp 222.1–222.5 ꢁC;
[a]3D0 ꢀ47.4 (c 0.14, CH3OH); IR (KBr) nmax 3453, 2950, 2080,
1
1638, 1436, 1017 cmꢀ1; H NMR (500 MHz, CDCl3) d ppm 1.74
(m, H-1), 1.62 (m, H2-2), 2.12 (dt, J ¼ 14.2, 3.5 Hz, H-3a), 1.32 (m,
H-3b), 3.85 (dd, J ¼ 11.0, 6.3 Hz, H-5), 2.07 (m, H-6a), 1.78 (td, J
¼ 9.3, 1.9 Hz, H-6b), 3.69 (dd, J ¼ 9.3, 2.5 Hz, H-7a), 3.20 (td, J ¼
11.7, 7.3 Hz, H-7b), 2.82 (td, J ¼ 8.2, 2.2 Hz, H-9), 1.93 (t, J ¼
10.4 Hz, H-10a), 1.70 (m, H-10b), 1.22 (s, H3-12), 1.08 (s, H3-14),
1.04 (s, H3-15); 13C NMR (125 MHz, CDCl3) d ppm 49.6 (C-1),
23.9 (C-2), 38.5 (C-3), 63.8 (C-4), 80.5 (C-5), 28.2 (C-6), 42.3 (C-
7), 175.7 (C-8), 39.8 (C-9), 35.0 (C-10), 35.1 (C-11), 16.7 (C-12),
21.4 (C-14), 29.5 (C-15); ESIMS m/z 238 [M + H]+; 260 [M +
Na]+; HR ESIMS m/z 238.1806 [M + H]+ (calcd for C14H24NO2
238.1807).
Synthesis of compound 13
To a solution of compound 2 (2.00 g, 9 mmol) in anhydrous (C-8), 47.9 (C-9), 42.1 (C-10), 31.3 (C-11), 15.3 (C-12), 21.9 (C-14),
EtOH (90 ml), activated zinc powder (100 g) was added. The 29.9 (C-15); ESIMS m/z 222 [M + H]+; 244 [M + Na]+; HR ESIMS
reaction mixture was reuxed for 48 h. Then the reaction m/z 222.1853 [M + H]+ (calcd for C14H24NO 222.1858).
mixture was cooled to room temperature and ltered. The
Compound 15. Colorless oil; [a]2D2 +65.9 (c 0.18, MeOH); IR
residue was washed with water, and then extracted with EtOAc (KBr) nmax 2943, 2871, 1641, 1451, 1369, 1154 cmꢀ1; H NMR
(3 ꢂ 120 ml). The combined extraction were dried over Na2SO4 (500 MHz, CDCl3) d ppm 1.43 (m, H-1), 1.53 (m, H2-2), 1.67 (m,
and concentrated under reduced pressure to afford crude H2-3), 1.54 (m, H-6a), 1.66 (m, H-6b), 1.72 (m, H-7a), 1.83 (m, H-
product. The crude product was puried on a silica gel column 7b), 2.30 (m, H2-8), 3.10 (dt, J ¼ 14.2, 3.5 Hz, H-9), 2.16 (t, J ¼
(petroleum–EtOAc 60 : 1) to yield the compound 12 (1.00 g, 54% 10.1 Hz, H-10a), 2.29 (m, H-10b), 1.29 (s, H3-12), 1.08 (s, H3-14,
1
yield).
15); 13C NMR (125 MHz, CDCl3) d ppm 53.5 (C-1), 22.5 (C-2), 38.5
To a stirred solution of NH2OH HCl (2.00 g, 28.8 mmol) in (C-3), 58.3 (C-4), 40.5 (C-5), 17.0 (C-6), 32.2 (C-7), 169.4 (C-8),
14.5 ml aqueous NaOH (2 M), a solution of compound 12 49.3 (C-9), 43.3 (C-10), 36.8 (C-11), 23.5 (C-12), 20.5 (C-14),
(1.00 g, 4.8 mmol) in ethanol (15 ml). Aer being stirred for 30.2 (C-15); ESIMS m/z 222 [M + H]+; HR ESIMS m/z 222.1860
30 min at room temperature, 60 ml water was added. The [M + H]+ (calcd for C14H24NO, 222.1858).
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Paper
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Bioassay
Butyrylcholine esterase inhibitory activity was measured by
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ylcholinesterase solution, 10 ml of test compound solution,
ꢁ
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¨
11 K. Burger, B. Muhl, T. Harasim, M. Rohrmoser,
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¨
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This work is nancially supported by State Key Laboratory of
Medicinal Chemical Biology, Nankai University (No. 201502003)
and the Natural Science and Basic Research Plan of Shaanxi
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40516 | RSC Adv., 2017, 7, 40510–40516
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