Synthesis of natural (?)-antrocin and its enantiomer via stereoselective aldol reaction

Article abstract of DOI:10.3390/molecules25040831

The total synthesis of (?)-antrocin and its enantiomer are presented. Antrocin (?)-1 is an important natural product which acts as an antiproliferative agent in a metastatic breast cancer cell line (IC50: 0.6 μM). The key features of this synthesis are: (a) selective anti-addition of trimethylsilyl cyanide (TMSCN) to α,β-unsaturated ketone; (b) resolution of (±)-7 using chiral auxiliary L-dimethyl tartrate through formation of cyclic ketal diastereomers followed by simple column chromatography separation and acid hydrolysis; (c) substrate-controlled stereoselective aldol condensation of (+)-12 with monomeric formaldehyde and pyridinium chlorochromate (PCC) oxidation for synthesis of essential lactone core in (?)-14; and (d) non-basic Lombardo olefination of the carbonyl at the final step to yield (?)-antrocin. In addition, (+)-9 cyclic ketal diastereomer was converted to (+)-antrocin with similar reaction sequences.

Full text of DOI:10.3390/molecules25040831

molecules
Article
Synthesis of Natural (−)-Antrocin and Its Enantiomer
via Stereoselective Aldol Reaction
Venkatachalam Angamuthu 1 and Dar-Fu Tai 2,
*
1
Department of Chemistry, Pondicherry University, Puducherry 605014, India;
venkatachalam_83@yahoo.co.in
Department of Chemistry, National Dong Hwa University, Shoufeng Township, Hualien County 974, Taiwan
Correspondence: dftai@mail.ndhu.edu.tw
2
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 15 January 2020; Accepted: 13 February 2020; Published: 14 February 2020
Abstract: The total synthesis of (
important natural product which acts as an antiproliferative agent in a metastatic breast cancer cell
line (IC50: 0.6 M). The key features of this synthesis are: (a) selective anti-addition of trimethylsilyl
cyanide (TMSCN) to -unsaturated ketone; (b) resolution of ( -7 using chiral auxiliary L-dimethyl
−
)-antrocin and its enantiomer are presented. Antrocin (
−
)-1 is an
µ
α
,
β
±
)
tartrate through formation of cyclic ketal diastereomers followed by simple column chromatography
separation and acid hydrolysis; (c) substrate-controlled stereoselective aldol condensation of (+)-12
with monomeric formaldehyde and pyridinium chlorochromate (PCC) oxidation for synthesis of
essential lactone core in (
step to yield ( )-antrocin. In addition, (+)-9 cyclic ketal diastereomer was converted to (+)-antrocin
with similar reaction sequences.
−
)-14; and (d) non-basic Lombardo olefination of the carbonyl at the final
−
Keywords:
non-basic Lombardo olefination
( )-antrocin; diastereoselective aldol reaction; chiral resolution; natural product synthesis;
−
1. Introduction
Sesquiterpene lactones belong to an important class of naturally occurring terpenoids that represent
a diverse and unique class of natural products. It is mainly constructed by head-to-tail condensation of
three isoprene units, bearing subsequent cis- or trans-fused cyclic lactone rings (Figure 1) [
predominant feature of sesquiterpene lactone is the presence of an -methylene- -lactone ring attached
directly to a six or eight-membered carbon ring [ ]. Antrocin ( )- is a cyclic sesquiterpene containing a
five-membered -lactone ring with an exocyclic double bond. It was isolated from Antrodia camphorata,
1–3]. The most
α
γ
1
−
1
γ
a valuable medicinal fungus grown in the inner wall of a heartwood of Cinnamomum kanehirai—an
indigenous tree in Taiwan. The significant medicinal application of this fungus is well-known to the
native Taiwanese for several decades, even long before it was biologically categorized under the class
of Antrodia camphorata in 1990 [
of this fungus as a remedy for food poisoning, vomiting, and many other purposes [
to that, it is also widely used to treat liver-associated diseases and tumorigenic diseases [
In 1995, Chiang and co-workers first reported the isolation of ( )- from Antrodia camphorata, which is
mainly responsible for the aforesaid medicinal uses [14]. Moreover, the recent studies on cancer
cell lines stimulate enormous interest about ( )- 15 19]. It was demonstrated that antrocin shows
4
–7
]. Traditional Chinese medicine practitioners prescribed the extract
]. In addition
13].
8
9
–
−
1
−
1 [ –
significantly higher activity against proliferation of metastatic breast cancer cell lines (MDA-MB-231)
as compared to the drugs generally used in present days for cancer treatments (e.g., doxorubicin
and cisplatin) [15]. (
lines [15 19], which suggests that it could be used as a superior alternative to the available cancer drugs.
The difficulties for purification of (–)- from natural fungal resources triggers us to find an alternative
−
)-1 also inhibits colony formation and has proved to be nontoxic to normal cell
,
1
Molecules 2020, 25, 831; doi:10.3390/molecules25040831
www.mdpi.com/journal/molecules

Molecules 2020, 25, 831
2 of 15
in vitro synthetic route for it. Moreover, an extensive study on (
necessary in order to get a better understanding on its biological activities.
−
)-
1
and its derivatives is urgently
Figure 1. Basic skeleton of sesquiterpene lactones.
Yang et al. [20] reported the synthesis of (
as a key step. The same group also reported [21] the synthesis of (
acid as a chiral pool. Although carnosic acid possesses the appropriate stereocenters for the synthesis,
however, their higher manufacturing cost and poor availability limit further development. Meanwhile,
isolating the key intermediates from ozonolyses and low temperature reactions require sophisticated
instrumental facilities, along with skilled synthetic knowledge.
±
)-antrocin by using Au(III)-catalyzed tandem reaction
)- from naturally occurring carnosic
−
1
Another synthetic method for (+)-antrocin was reported [22] by Sheng-Han Huang et al.
Their tricyclic lactone core ring was constructed by an intramolecular Diels
of the camphanate-containing triene intermediate. However, this approach is associated with a long
process which did not provide the required natural product ( )- . Therefore, the synthesis of ( )-1
−
Alder (IMDA) reaction
−
1
−
via a simple strategy using inexpensive, readily available starting material 6-methoxy-2-tetralone and
typical operations was patented [23]. In this paper, we reported the detailed synthesis of natural
product ( )-1 (overall yield 7%) and its enantiomer (+)-1 using straightforward methods and simple
−
chemicals starting from 3-methyl-2-cyclohexenone.
2. Results and Discussion
Retrosynthetic analysis of the antrocin ((
bicyclic ketals ( )- and (+)- are the key intermediates. Further manipulations to the cyano group
and -position to the carbonyl group after acetal deprotection could lead either to the target antrocin
)- or (+)- . The final step is the formation of the exocyclic double bond of . This could be obtained
via one carbon homologation of the Lombardo olefination. The segment of tricyclic 13 lactol could
be installed appropriately from bicyclic ketal ( )- via deprotection, followed by substrate-controlled
aldol addition. The key intermediate that contains quaternary stereogenic bicyclic ketal ( )- and
(+)-9 can be easily accessed from (±)-7 by diastereotopic separation. Access to bicyclic core (±)-7 was
contemplated from enone through diastereoselctive anti-trimethylsilyl cyanide (TMSCN) addition.
Enone could be obtained from 3-methyl cyclohex-2-enone (2) by simple alkylation and intramolecular
aldol condensation.
−
)-1 and (+)-1) is shown in Scheme 1, indicating that the
−
8
9
α
(−
1
1
1
−
8
−
8
6
6

Molecules 2020, 25, 831
3 of 15
Scheme 1. Retrosynthetic analysis. PCC: pyridinium chlorochromate.
The synthesis of (
3-methyl-2-cyclohexenone (
by CuI/MeLi provided 3,3-dimethylcyclohexanone
±
)-7 is shown in Scheme 2. Alkylation of commercially available
2) with iodo compound
3
[
24,
25] gave enone
4
[
26]. Michael addition of
29]. Compound was
4
5
in excellent yield [27
–
5
then subjected to in situ deprotection of acetal, followed by intramolecular aldol condensation, to give
α,β-unsaturated bicyclic ketone 6 in 89% yield (two steps).
After having
6 in hand, selective trans-HCN addition with KCN/NH4Cl [30] carried out in DMF
under reflux gave excellent yield (88%), but the ratio of cis/trans-isomer (53:47) was not satisfactory.
This unfavorable stereochemical outcome turned our attention to find different methods for anti-HCN
addition. After complete examination through the study with different reagents, we found diethyl
aluminum cyanide is a suitable reagent for anti-addition. This was generated in situ by mixing five
equivalents of triethyl aluminum in toluene and 2.3 equivalents of TMSCN in the presence of THF
at 0 ◦C. The reaction mixture was immediately treated with
)- as a major product (17:83, cis:trans) [30 33]. When the hexane/THF solvent mixture was used,
the selectivity toward the trans-HCN addition diminished. Compound ( )- was confirmed by X-ray
6 to produce trans-cyano byclic ketone
(±
7
–
±
7
crystallography method, and all other analytical data complied with those published [34].
Scheme 2. Synthesis of (±)-7. TMSCN: trimethylsilyl cyanide.
Resolution of ( )-7 thus synthesized was achieved by utilization of a chiral auxiliary (+)-dimethyl
±
tartrate through ketal formation (Scheme 3). Two commercially available chiral auxiliaries A
((2S,3S)-1,4-bis(benzyloxyobutane-2,3-diol) [35] and B ((2R,3R)-dimethyl-2,3-dihydroxysuccinate)

Molecules 2020, 25, 831
4 of 15
were chosen for the resolution of the compound (
the corresponding ketal as a diastereomeric mixture. Unfortunately, these diastereomers were not
±
)-
7
(Figure 2). Initially, ketone (
±
)-
7
with A gave
separated easily by the column chromatographic method with different solvent mixtures. Secondary,
(
±
)-
by column chromatography on silica gel (long column containing 230–400 mesh silica gel, eluted with
only DCM as an eluant). The diastereomeric purity of the ketal ( )-
was verified by observing the 1H
7
treated with B gave corresponding ketal (
−
)-8 and (+)-9 as a 1:1 diastereomeric mixture separated
−
8
NMR peaks at the methines of the ketal rings of the two diastereomers. The structure of (
further confirmed by the X-ray crystallographic method (Figure 3).
−
)-8 was
Figure 2. Optically active 1,2-diol as a chiral auxiliary.
Scheme 3. Synthesis of ketal diastereomers (−)-8 and (+)-9.
Figure 3. Oak ridge thermal ellipsoid plot (ORTEP) of compound (−)-8.
The complete synthesis of (
−
)-
1
is shown in Scheme 4. Ketal (
−
)-8 was selected for further reaction.
In this event, ketal ( )- was treated with an excess of ethylene glycol, p-toluenesulfonic acid under
−
8
reflux in toluene to give (–)-10 in good yield (89%). Deprotection and protection proceeded in a
25
single operation through the intermediate (
The reduction of ( )-10 using DIBAL-H [36
glacial acetic acid/water mixture (4:1), provided keto aldehyde (+)-12 directly in 87% [35]. Regioselective
-hydroxymethylation of (+)-12 was achieved by reacting with its lithium enolate generated under
low temperature at
78 ◦C with an ethereal saturated solution of gaseous formaldehyde in THF [39].
−
)-7a (optical rotation [
α
]
=
−
50.1 (c = 1, CHCl3)).
D
−
–
38] at 0 ◦C in toluene, followed by acid hydrolysis with
α
−
Single lactol (+)-13 was afforded exclusively in moderate yield (48%), and starting material (+)-12 was
recovered (26%) [40]. The stereochemistry at the C9 position in compound (+)-13 has been confirmed
to be R configuration. Nuclear overhauser effect spectroscopy (NOESY) spectral data strongly supports
in favor of R configuration, and a clear nuclear overhauser effect (NOE) was observed between the
C9 proton and C5 methyl proton (Figure 4) in compound (+)-13
[41] (see Supplementary materials).
Oxidation of (+)-13 with silica-supported pyridinium chlorochromate (PCC) gave lactone (
−
)-14 in

Molecules 2020, 25, 831
5 of 15
good yield (80%) [20]. In this stage, the structure of compound (
crystallographic analysis.
−
)-14 was confirmed by an X-ray
Scheme 4. Complete synthesis of (−)-antrocin 1. DCM: dichloromethane.
Standard and modified Wittig olefination (Ph3PCH2 and THF, 0 ◦C and Ph3PCH2, toluene, and
t-BuOH, rt) [42
,
43] of compound (
reagent (Zn, TiCl4, and CH2Br2) led to smooth olefination of ketone (
(98%) [44]. The spectral data for ( )- , including the [ ] (specific rotation), were identical to the natural
−
)-14 gave poor conversions (30–38%). However, non-basic Lombardo
−
)-14 to ( )- in excellent yield
−
1
−
1
α
product isolated from the fungus. Utilizating of Scheme 4 synthetic procedure, (+)-antrocin
synthesized from ketal diastereomer (+)-9 (See Scheme 5, [α]20 = +118 (c 1.00, CHCl3) (syn).
1 was
D
The mechanism of kinetic-controlled
The result obtained from the isolated yield of (+)-13
α
-hydroxymethylation of (+)-12 is proposed in Figure 5.
13, 48% and 12, 26%) indicates a substrate-controlled
(
addition. Generally, the addition of formaldehyde on to the enolate is reversible. In the point of steric
hindrance at low temperature, comparatively, the transition state 13c is more favorable than 13a and
13b. In this case, the enolate reacted with formaldehyde at C1
The reason, presumably, is the C-C bond formation will proceed via a bicyclic, chelating transition state
with lithium metal (Figure 6) bridged between formaldehyde and -substituted aldehyde. Therefore,
β-position rather than C3 α-position.
β
the equilibrium shifts more favored to transition state 13a, which provides compound (+)-13 lactol via
five-membered cyclic transition state 13d. The transition state 13b is unfavorable due to the opposite
stereocenter in which the formation of the cyclic transition state is difficult. Moreover, the regioselectivity
is determined by the increased stability of alkoxide formed by reaction at C1 position due to the
possibility of forming a lactol. The formation of lactol or a cyclic chelating transition state is not possible
in the case of either aldol addition to the cyano substituent or C3 position. However, this hypothesis
was further checked by a controlled experiment; the reaction of
(Scheme 6) gave regioselective products 15 and 16 at a 1:2.5 ratio with starting compound
Supplementary Materials). As already discussed, in the case of (+)-12, no such mixture of regioisomers
was observed. The regioselectivity is indeed higher with an aldehyde group at -position than the
7
under similar reaction conditions
7
(see
β

Molecules 2020, 25, 831
6 of 15
cyano group. The compound 15 was identified by converting to a compound identical to lactone
14 after DIBAL-H reduction and PCC oxidation (confirmed by comparing with 1H and 13C NMR).
The stereoselectivity at α-position was confirmed by X-ray crystallography studied after oxidation
of 13.
Figure 4. Key nuclear Overhauser effect (NOE) relationship observed for lactol (13): 5CH3 → 9H syn,
4H→1H syn, 9OH→5CH3 anti, and 4H→9OH anti.
Scheme 5. Synthesis of (+)-antrocin 1.
Figure 5. Proposed kinetic-controlled aldol addition of (+)-12 with monomeric formaldehyde.

Molecules 2020, 25, 831
7 of 15
Figure 6. Chelating transition structure.
Scheme 6. Regioselectivity of compound 7 (a controlled experiment).
3. Conclusions
The total synthesis of natural product (
achieved by a simple method (overall yield, 7%) from the commercially available starting material.
The crucial tricyclic core (+)-13 was generated by substrate-controlled -hydroxymethylation leading
−
)-antrocin 1 and its enantiomer (+)-antrocin have been
α
to the target molecule. The chelating transition structure with lithium metal plays an important role
to change the equilibrium in the transition state. The kinetic-controlled mechanisms to compound
(+)-13 have been proposed (Figure 5). The mechanistic path prefers C1 β-position over C3 α-position
at low temperature is realized. We are working on computational studies toward the activation energy
of regio- and stereoselective products and starting materials. That will be completed in due course.
The lactol (+)-13 and lactone ( )-14 have structural similarities like antrocin; therefore, more derivatives
−
and further biological investigations could be performed through this synthetic process.
4. Experimental
General Conditions
All reactions involving air- or moisture-sensitive reagents or intermediates were performed under
an inert atmosphere of nitrogen in oven-dried glassware. All reagents and solvents were obtained from
commercial suppliers and used without further purification, if not mentioned otherwise. THF and
diethyl ether were dried over sodium, monitored with benzophenone ketyl radicals, and distilled
prior to use. Dichloromethane (DCM) and toluene were dried over CaH2 and distilled prior to use.
Thin layer chromatography (TLC) was performed using precoated silica gel 60 F254 plates containing
a fluorescent indicator; purification by chromatography was done using silica gel (230–400 mesh).
1H and 13C NMR chemical shifts are reported in parts per million and referenced to the residual solvent.
All spectra were obtained at 25 ◦C. High-resolution mass spectra were recorded on a Bruker MicroTOF
spectrometer (Bremen, Germany) by electron spray ionization in positive-ion mode (M+H or M+),
as indicated. Optical rotations were recorded on a digital polarimeter (Rudolph Research, Flanders, NJ,
USA) at 589 nm and reported as follows: [α]20D (concentration in g/100 mL and solvent).

Molecules 2020, 25, 831
8 of 15
3-methyl-2-(2-(2-methyl-1,3-dioxolan-2-yl)ethyl)-cyclohex-2-enone (4). A suspension of NaH (60%
suspension in mineral oil, 1.83 g, 49.9 mmol) in DMSO (25 mL) was stirred at room temperature for
15 min. To this suspension was added a solution of 3-methyl-2-cyclohexenone
DMSO (25 mL) at 16 ◦C. The reaction mixture was stirred at room temperature for 30 min, and then,
2-(2-iodoethyl)-2-methyl-1,3-dioxolane ( , 12.05 g, 49.9 mmol) was added. The mixture was stirred
for 17 h and quenched with saturated (sat.) NH4Cl(aq). The aqueous layer was extracted by EtOAc
2 (5 g, 45.3 mmol) in
3
(
3 × 70 mL). The combined organic layer was dried over Na2SO4, filtered, and concentrated. The crude
was further purified by column chromatography on silica gel using EtOAc/hexane (15:85, V:V) as
eluants (Rf 0.46), to obtained
1
4
as yellow oil (7.11 g, 70%). H NMR (400 MHz, CDCl3)
δ
3.85 (m, 4H,
H-ethylene acetal); 2.27–2.22 (m, 6H); 1.86 (s, 3H, H-7); 1.83 (t, J = 7.04 Hz, 2H, H-9); 1.53–1.49 (m,
2H, H-5); 1.26 (s, 3H, H-10); 13C NMR (100 MHz, CDCl3)
δ 198.4, 155.3, 135.2, 109.7, 64.5, 37.8, 37.7,
32.7, 23.5, 22.2, 20.9, and 19.9; and HRMS (ESI) m/z calculated (calcd.) for C13H20O3 (M+) 224.1412
found 224.1417.
3,3-dimethyl-2-(2-(2-methyl-1,3-dioxolan-2-yl)ethyl)cyclohexanone (5). MeLi (36.7 mL, 0.08 mol,
(2.21 M in diethyl ether)) was added over a period of 15 min to slurry of CuI (7.74 g, 0.04 mol)
in dry diethyl ether at
78 ◦C. Then ketone
stirred for 30 min at
−
78 ◦C. The mixture was stirred for 1 h at 10 ◦C and then chilled again to
(4.56 g, 0.02 mol) in dry diethyl ether (20 mL) was added. The mixture was
−
−
4
−
40 ◦C and quenched with sat. NH4Cl(aq) (40 mL). The resulting mixture was
partitioned between water and ethyl acetate. The organic layers were combined, dried over Na2SO4,
filtered, and concentrated under reduced pressure. The resultant crude was further purified by column
chromatography on silica gel using EtOAc/hexane (9:91, V:V) as eluant (Rf 0.7) to obtain 5 as clear oil
1
(4.43 g, 92%). H NMR (400 MHz, CDCl3)
δ
3.90–3.89 (m, 4H, H-ethylene acetal); 2.33–2.18 (m, 2H);
2.8 (d, J = 10.68 Hz, 1H); 1.92–1.73 (m, 2H); 1.72–1.60 (m, 2H); 1.58 (t, J = 7.12, 2H); 1.44–1.33 (m, 2H);
1.29 (s, 3H); 1.01 (s, 3H); 0.75 (s, 3H); 13C NMR (100 MHz, CDCl3)
δ 213.5, 110.0, 64.6, 64.5, 60.9, 41.04,
39.7, 38.8, 37.6, 29.4, 23.6, 23.1, 22.4, and 18.7; and HRMS (ESI) m/z calcd. for C14H24O3 (M+) 240.1725
found 240.1734.
4,4a,5,6,7,8-Hexahydro-5,5-dimethyl-2(3H)-naphthalenone (6). 3 N HCl (54.5 mL) was added to a
solution of
reflux for 19 h, then cooled to room temperature, and methanol was removed under reduced pressure.
Resultant reaction mixture was extracted with ethyl acetate (3 100 mL). The combined organic layers
5 (7.0 g, 0.029 mol) in methanol (150 mL) at room temperature. The mixture was heated under
×
were washed with NaHCO3(aq) (20 mL). Organic layer was dried over MgSO4, filtered, and concentrated
under reduced pressure. The crude product was further purified by column chromatography on silica
gel using EtOAc/hexane (6:94, V:V) as eluants (Rf 0.84) to obtain
1H NMR (400 MHz, CDCl3)
5.89 (t, J = 2.04 Hz, 1H); 2.47–2.38 (m, 2H); 2.29–2.23 (m, 1H); 2.21–2.13
(m, 2H); 2.11–2.03 (m, 1H); 1.83–1.74 (m, 1H); 1.70–1.60 (m, 2H); 1.59–1.43 (m, 2H); 1.05 (s, 3H); 0.81 (s,
6 (4.91 g, 89%) as a yellowish brown oil.
δ
3H); 13C NMR (100 MHz, CDCl3)
δ 200.0, 165.9, 125.9, 53.5, 47.2, 41.5, 36.8, 36.1, 35.78, 29.7, 21.8, 21.8,
and 21.3; and HRMS (ESI) m/z calcd. for C12H18O (M+) 178.1358 found 178.1361.
trans-Octahydro-8a-cyano-5,5-dimethy1-2(1H)-naphthalenone ((±)-7). TMSCN (0.203 mL, 2.05 mmol)
in THF (10 mL) was added to a solution of Et3Al (1M solution in toluene, 4.9 mL, 4.66 mmol) in THF
(5.5 mL) under N2 atm at 0 ◦C. After 20 min of stirring, a solution of
6 (0.166 g, 0.932 mmol) in THF was
added at 0 ◦C over a period of 30 min. The mixture was allowed to warm to room temperature and
refluxed for 20 h, then quenched with sat. NaHCO3(aq) (40 mL) and extracted with EtOAc (3 × 20 mL).
The combined organic layers were dried over MgSO4, filtered, and concentrated. The resultant residue
stirred with 10% HClO4(aq) (12 mL) and THF (12 mL) at room temperature for 40 min. Then, the reaction
mixture was diluted with water and extracted with EtOAc (3
were washed with NaHCO3(aq) (25 mL), dried over MgSO4, filtered, and concentrated. The residue
×
20 mL). The combined organic layers
was purified by column chromatography on silica gel using EtOAc/hexane (10:90, V:V) as eluants (Rf
0.28) to obtain trans-cyano decalone (
±
)
-7 (0.18 g, 89%) as a white crystalline solid, mp 80–82 ◦C. 1H
NMR (400 MHz, CDC13) 2.65–2.58 (m, 2H); 2.36–2.28 (m, 2H); 2.20–2.14 (m, 1H); 2.07 (d, J = 13.4
δ

Molecules 2020, 25, 831
9 of 15
Hz, 1H); 1.95–1.78 (m, 2H); 1.69–1.58 (m, 2H); 1.48–1.25 (m, 3H); 1.07 (s, 3H); 1.02 (s, 3H); 13C NMR
(100 MHz, CDC13) δ 205.7, 121.9, 53.3, 51.1, 41.3, 41.2, 40.8, 38.6, 33.5, 31.84, 24.2, 20.0, and 19.6; and
HRMS (ESI) m/z calcd. for Cl3Hl9NO (M+) 205.1467 found 205.1475.
cis-Octahydro-8a-cyano-5,5-dimethy1-2(1H)-naphthalenone (cis-(±)-7). This compound was purified
by column chromatography on silica gel eluting with 10% EtOAc-hexane (Rf 0.44), Mp 52–54 ◦C. 1H
NMR (400 MHz, CDCl3)
2H); 2.18–2.11 (m, 1H); 2.04 (dd, J = 4.4 Hz, J = 11.9 Hz, 1H); 1.88–1.69 (m, 3H); 1.63 (dt, J = 14.28 Hz,
J = 3.64 Hz, 1H); 1.48 (td, J = 3.8 Hz, J = 13.3 Hz, 1H); 1.36 (s, 3H); 1.33–1.24 (m, 2H); 0.95 (s, 3H); 13
NMR (100 MHz, CDCl3) δ 206.3, 125.0, 51.1, 45.9, 39.9, 39.9, 34.3, 33.5, 31.0, 30.9, 28.9, 24.1, and 19.9.
δ 2.8 (d, J = 14.16 Hz, 1H); 2.46 (dd, J = 2.2 Hz, J = 14.1 Hz); 2.4–2.3 (m,
C
Resolution of ((−)-8). Compound (
±
)- (1 g, 4.8 mmol) was heated to reflux together with dimethyl
7
L-tartrate (4.5 g, 24 mmol) and 92 mg (0.48 mmol) of p-toluenesulfonic acid in dry toluene (30 mL)
under nitrogen atmosphere for 48 h in a round-bottom flask equipped with a Dean-Stark trap. After
48 h, reaction mixture was allowed to cool to room temperature and then quenched with saturated
sodium bicarbonate solution (~30 mL) and extracted with ethyl acetate (3 x 40 mL). The organic layers
were combined, dried over MgSO4, and concentrated under reduced pressure. Then, the resultant
crude mixture (1.7 g) was further purified by chromatography on silica gels using only DCM (100%) as
eluant (a long column loaded with 230–400 mesh size). There were obtained 810 mg (45%) of (
−
)-
8
as a
25
white crystalline solid. Rf 0.68 on TLC (DCM); mp 95 ◦C; [
α
]
−
44.7 (c 1.0, CHCl3) and 806 mg of
D
(+)-9; Rf 0.62 on TLC (DCM); mp 177–180 ◦C; and [α]25D +20.6 (c 1.00, CHCl3).
(4aS,8aS)-3,4,4a,5,6,7,8,8a-Octahydro-8a-carbonitrile-5,5-dimethyl-trans-naphthalene-2(1H)-one-2
[((1S,2S)-di(methoxy-carbonyl))ethylene acetal] ((−)-8). 1H NMR (400 MHz, CDC13)
δ 4.89 (d, J = 4.9
Hz, 1H); 4.77 (d, J = 4.96 Hz, 1H); 3.80 (s, 6H); 2.19 (dd, J = 2.8 Hz, J = 13.9 Hz, 1H); 2.11 (dq, J = 2.9 Hz,
13.0 Hz, 1H); 1.99–1.96 (m, 1H); 1.92–1.81 (m, 2H); 1.78–1.71 (m, 1H); 1.67–1.55 (m, 3H); 1.51 (dd, J =
1.52 Hz, J = 13.44 Hz, 1H); 1.29–1.16 (m, 2H); 1.07 (s, 3H); 0.97 (dd, J = 2.7, J = 11.7 Hz, 1H); 0.92 (s, 3H);
13C NMR (100 MHz, CDC13)
δ 170.2, 169.9, 123.2, 112.7, 52.9, 52.8, 52.3, 47.2, 41.3, 39.4, 38.0, 36.8, 33.3,
32.1, 21.9, 19.9, and 19.4; and HRMS (ESI) m/z calcd. for Cl9H28NO6 [M + H]+ 366.1917 found 366.1906.
(4aR,8aR)-3,4,4a,5,6,7,8,8a-Octahydro-8a-carbonitrile-5,5-dimethyl-trans-naphthalene-2(1H)-one-2
[((1S,2S)-di(methoxy-carbonyl))ethylene acetal] ((+)-9). 1H NMR (400 MHz, CDC13)
δ 4.89 (d,
J = 5.0 Hz, 1H); 4.84 (d, J = 4.92 Hz, 1H); 3.83 (s, 3H); 3.80 (s, 3H); 2.30 (dd, J = 2.7 Hz, J = 13.7 Hz, 1H);
2.04–1.97 (m, 2H); 1.9 (dt, J = 3.2 Hz, J = 13.6 Hz, 1H); 1.85–1.82 (m, 1H); 1.72–1.64 (m, 3H); 1.62–1.50
(m, 2H); 1.29–1.18 (m, 2H); 1.07 (s, 3H); 0.99–0.96 (dd, J = 2.24 Hz, J = 11.44 Hz, 1H); 0.93 (s, 3H); 13
C
NMR (100 MHz, CDC13)
21.9, 20.0, and 19.3.
δ 169.9, 169.8, 122.8, 112.5, 52.9, 52.9, 52.3, 46.8, 41.3, 39.3, 38.0, 37.1, 33.3, 32.1,
trans-Octahydro-8a-cyano-5,5-dimethy1-2(1H)-naphthalenone ((−)-7) and ((+)-7). A mixture of (
−
)-8
(1 g, 2.73 mmol); 12 N HCl (8 mL); and methanol (36 mL) was heated to reflux for 8 h in a round-bottom
flask. The reaction mixture was allowed to cool to room temperature, and the organic solvent was
evaporated under reduced pressure. The resultant aqueous layer was extracted with ethyl acetate
(
3 × 30 mL). The combined organic layers were washed with NaHCO3(aq) (30 mL) solution, dried over
MgSO4, filtered, and concentrated. The crude product was further purified by column chromatography
using EtOAc/hexane (10:90, V:V) as eluants to obtained (
88–89 ◦C and [α]22D −50.1 (c 1.0, CHCl3).
−
)-7 (532 mg, 95%) as a white solid; mp
(+)-7 was prepared from (+)-9 using the same procedure to obtained a white solid (534 mg, 95.2%);
mp 87–90 ◦C; and [α]22D +49.8 (c 1.0, CHCl3).
(4aS,8aS)-3,4,4a,5,6,7,8,8a-Octahydro-8a-carbonitril-5,5-dimethyl-trans-naphthalene-2(1H)-one-2-
[ethylene acetal] (10). Compound ( )-8 (2.1 g, 5.36 mmol) was refluxed with a mixture of p-TSA
−
(20 mg) and ethylene glycol (3.6 mL, 53 mmol) in dry toluene (30 mL) under a nitrogen atmosphere for
4 h in a round-bottom flask equipped with a Dean-Stark trap. After this reaction time, the mixture

Molecules 2020, 25, 831
10 of 15
was cooled to room temperature and then quenched with sat. NaHCO3(aq) (~30 mL). The aqueous
layer was extracted with EtOAc (3 40 mL). The combined organic layers were dried over MgSO4,
×
filtered, and concentrated under reduced pressure. The resultant crude product was further purified
by column chromatography on silica gels using EtOAc/Hexane (8:92, V:V) as eluants (Rf 0.56) to obtain
25
1
pure (
−
)-10 (1.25 g, 89%); mp 56–58 ◦C and [
α
]
−
44.9 (c 1.00, CHCl3). H NMR (400 MHz, CDCl3) δ
D
4.09–4.00 (m, 2H); 3.91 (t, J = 6.56 Hz, 2H); 2.06 (d, J = 13.68 Hz, 1H); 2.00–1.81 (m, 4H); 1.75–1.65 (m,
1H); 1.61–1.48 (m, 4H); 1.29–1.18 (m, 2H); 1.08 (s, 3H); 0.98 (d, J = 12.0 Hz, 1H); 0.93 (s, 3H); 13C NMR
(100 MHz, CDCl3) δ 123.6, 107.0, 64.8, 64.3, 52.4, 46.4, 41.4, 39.4, 38.0, 36.2, 33.3, 32.1, 22.1, 19.9, and 19.4;
and HRMS (ESI) m/z calcd. for C15H23NO2 (M+) 249.1729 found 249.1733.
(4aS,8aS)-3,4,4a,5,6,7,8,8a-Octahydro-5,5-dimethyl-2-oxo-naphthalene-8a-carbaldehyde
((+)-12).
DIBALH (1 M solution in hexane, 2.2 mL, and 2.12 mmol) was added to a solution of (
−
)-10 (443 mg,
1.77 mmol) in toluene (4.8 mL) under N2 atm at room temperature. The mixture was stirred at rt for
2 h. Then, 1 N HCl (20 mL) was added to the reaction mixture at 0 ◦C, diluted with ethyl acetate, and
separated for the organic layer. The organic layer was dried over MgSO4, filtered, and concentrated
under reduced pressure. The resultant crude mixture was stirred with glacial acetic acid/water (4:1)
at 60 ◦C for 20 min and then cooled to 0 ◦C. Then, quenched with NaHCO3(aq) (30 mL) under slow
addition and then extracted with ethyl acetate (10
over MgSO4, filtered, and concentrated. The resultant crude product was further purified by column
×
30 mL). The combined organic layers were dried
chromatography on silica gels using EtOAc/hexane (11:89, V:V) as eluants (Rf 0.31) to obtain 12 as a
23
1
semi-solid (321.9 mg, 87%); [
α
]
+5.9 (c 1.00, CHCl3). H NMR (400 MHz, CDCl3) δ 9.91 (s, 1H); 2.56
D
(dp, J = 2.5 Hz, J = 15.5 Hz, 1H); 2.35–2.26 (m, 1H); 2.21 (dd, J = 2.1 Hz, J = 14.7 Hz, 1H); 2.06–1.98 (m,
3H); 1.86 (td, J = 4.9 Hz, J = 13.04 Hz, 1H); 1.81–1.60 (m, 3H); 1.57–1.53 (m, 1H); 1.40–1.23 (m, 2H); 1.00
(s, 3H); 0.737 (s, 3H); 13C NMR (100 MHz, CDCl3)
δ 208.3, 205.7, 53.5, 51.8, 51.5, 41.5, 41.0, 35.4, 33.6,
31.4, 22.0, 21.9, and 19.0; and HRMS (ESI) m/z calcd. for C13H21O2 [M+H]+ 209.1542 found 209.1531.
(1S,4aS,8aS)-decahydro-5,5-dimethyl-2-oxo-naphthalene[1,8a-c]furan-1S(3H)-ol ((+)-13). Ethereal
solution of formaldehyde: To a 100 mL single-neck round-bottom flask was added paraformaldehyde
◦
(7 g) and dried under vacuum at 80 C for 1 h to remove moisture completely. Then, the round
◦
bottom-flask was fitted with septum and flushed with N2 gas, heated at 140–160 C to generate
monomeric formaldehyde gas, which was passed, through cannula by means of a stream of N2 gas
to one of the single-neck round-bottom flask connected with a transfer arm to another flask and
contained (40 mL) dried THF with constant stirring at −78 ◦C for 40 min. The resultant THF solution
was used immediately.
Compound (+)-12 (0.330 g, 1.584 mmol) in THF (3 mL) was added to a stirred solution of LHMDS
(3.48 mL, 3.48 mmol) in THF (4 mL) at
reaction mixture was transferred the ethereal solution of formaldehyde (40 mL) poured through the
connected transfer arm, also at
78 ◦C. After the addition was completed, the reaction mixture was
−
78 ◦C under N2 atm and then stirred for 5 h. To the above
−
allowed to stir for 15 min at the same◦temperature. Then, the reaction was quenched by drop-wise
addition of aqueous NH4C1 at
40 min. The reaction mass was extracted with ether (3
successively with water and brine and dried over MgSO4. Filtration and evaporation of the filtrate
−
78 C. The reaction mass was stirred at room temperature for
×
30 mL). The combined extracts were washed
afforded a clear oil, which was chromatographed on silica gel using EtOAc/CH2Cl2 (10:90, V:V) as
eluants to obtain (+)-13 (181 mg, 48%) as a white semi-solid. (+)-12 (98 mg, 26%) was recovered from
23
the column. [
α
]
+71.1 (c 1.00, CHCl3). 1H NMR (400 MHz, CDCl3)
δ 5.43 (d, J = 1.48 Hz, 1H);
D
4.66 (dd, J = 1.5 Hz, J = 8.52 Hz, 1H); 3.74 (dd, J = 7.04 Hz, J = 8.4 Hz, 1H); 2.61–2.57 (m, 1H); 2.41 (d,
J = 1.36 Hz, 1H); 2.38 (m, 2H); 2.18 (d, J = 6.72 Hz); 1.93–1.79 (m, 3H); 1.67–1.50 (m, 3H); 1.35–1.34 (m,
1H); 1.23–1.20 (m, 1H); 1.00 (s, 3H); 0.94 (s, 3H); 13C NMR (100 MHz, CDCl3)
δ 209.2, 100.3, 62.3, 57.0,
52.6, 50.6, 42.3, 41.3, 37.3, 34.2, 32.2, 23.0, 21.5, and 19.9; and HRMS (ESI) m/z calcd. for C14H22O3 (M+)
238.1569 found 238.1575.

Molecules 2020, 25, 831
11 of 15
(1S,4aS,8aR)-decahydro-5,5-dimethyl-2-oxo-naphthol[1,8a-c]furan-1(3H)-one((−)-14)PCC(290.8mg,
1.35 mmol) was added at once to a stirred solution of 13 (160 mg, 0.67 mmol) in dried CH2Cl2 (20 mL)
at room temperature under N2 atm. The mixture was stirred for 6 h at room temperature, diluted
with 60 mL of diethyl ether, and filtered through a celite bed. The filtrate was concentrated, and the
resultant crude product was purified by column chromatographed on silica gel using CH2Cl2 (Rf
23
0.84) to obtain (
−
)-14 (151 mg, 91%) as a white solid. mp 153–154 ◦C and [
α
]
−
63.4 (c 1.00, CHCl3).
D
1H NMR (400 MHz, CDCl3)
δ
4.77 (d, J = 9.2 Hz, 1H); 4.30 (dd, J = 5.68 Hz, J = 9.1 Hz, 1H); 2.71–2.66
(m, 1H); 2.49 (d, J = 5.6 Hz, 1H); 2.41–2.32 (m, 1H); 2.20–2.21 (m, 1H); 2.07–2.02 (m, 1H); 1.80–1.70 (m,
1H); 1.67–1.52 (m, 4H); 1.46 (td, J = 3.16 Hz, J = 13.4 Hz, 1H); 1.28 (td, J = 3.28 Hz, J = 13.8 Hz, 1H); 1.20
(s, 3H); 1.04 (s, 3H); 13C NMR (100 MHz, CDCl3)
δ 207.9, 176.0, 65.2, 59.3, 49.6, 49.1, 42.1, 41.4, 35.7,
33.4, 33.3, 22.1, 22.1, and 18.6; and HRMS (ESI) m/z calcd. for C14H20O3 (M+) 236.1412 found 236.1415.
(−)-Antrocin; (1S,4aS,8aR)-decahydro-5,5-dimethyl-2-methylenenaphthol[1,8a-c] furan-l(3H)-one
((−)-1). Zinc dust (571 mg, 8.73 mmol) was briefly flame-dried under high vacuum in a 20 mL
vial fitted with a septum cap. Upon cooling, the vial was flushed with N2, and anhydrous THF was
added (7.1 mL), followed by 1,2-dibromoethane (82 mg) and TMSCl (9.5 mg, 0.087 mmol). After stirring
10 min, dibromoethane (494 mg, 2.84 mmol) was added. The mixture was cooled to
40 ◦C, and TiCl4
−
(385 mg, 2.03 mol) was added slowly drop-wise over 5–10 min. The thick, dark gray mixture was
warmed to 0 ◦C over a period of 45 min and then stirred at that temperature for 2 days. After 20 h,
a further 1 mL of anhydrous THF was added to assist with stirring. An aliquot of the Lombardo
reagent thus-obtained (400
0.0423 mmol, dried 13 h with high vacuum) in CH2Cl2 (600
µ
L, 0.161 mmol, 8 eq) was added to a mixture of lactone (
−
)-14 (10 mg,
µ
L) at 0 ◦C. The mixture was warmed
to room temperature and stirred for 30 min, then diluted with Et2O (2 mL) and quenched with sat.
NaHCO3(aq) (2 mL). The resulting biphasic mixture was stirred vigorously for 5 min, and then the
layers were separated and the aqueous layer extracted with Et2O (2
layers were washed with brine (2 mL), dried over Na2SO4, filtered, and concentrated under reduced
pressure. The resultant crude product was further purified by column chromatography on silica gel
×
2 mL). Combined organic
using EtOAc/hexane (10:90) as an eluant (Rf 0.65) to obtain (
−
)-
1
(9.7 mg, 98%) as a white crystalline
4.81 (d, J = 13.2 Hz,
20
1
solid. mp 97–98 ◦C and [
α
]
−
118 (c 1.00, CHCl3). H NMR (400 MHz, CDCl3) δ
D
1H, methylene-H1); 4.47 (dd, J = 6.8, 9.44 Hz, 1H, methylene-H2); 4.14 (dd, J = 1.32, 9.44 Hz, 1H, H-10);
2.66 (d, J = 6.68 Hz, 1H, H-10); 2.38–2.31 (m, 1H, H-1); 2.27–2.20 (m, 1H, H-3); 2.14 (dq, J = 1.72, 10.32
Hz, 1H, H-4a); 1.85–1.73 (m, 2H, H-8); 1.57–1.50 (m, 2H, H-7); 1.49–1.43 (m, 2H, H-6 and H-7); 1.25–1.20
(m, 1H, H-6); 1.17 (s, 3H); 0.93 (s, 3H); 13C NMR (100 MHz, CDCl3)
δ 178.3, 146.6, 111.1, 69.3, 54.1,
48.4, 46.5, 41.9, 36.7, 33.2, 33.0, 30.3, 22.3, 22.1, and 18.7; and HRMS (ESI) m/z calcd. for C15H22O2(M+)
234.1620 found 234.1624.
(4aR,5S,8aS)-5-(hydroxymethyl)-1,1-dimethyl-6-oxooctahydronaphthalene-4a(2H)-carbonitrile (15).
Ethereal solution of formaldehyde: To a 100 mL sin◦gle-neck round-bottom flask was added
paraformaldehyde (7 g) and dried under vacuum at 80 C for 1 h to remove moisture completely.
Then, the round-bottom flask was fitted with septum and flushed with N2 gas, heated at 140–160 ◦C to
generate monomeric formaldehyde gas, which was passed, through cannula by mean of a stream of
N2 gas to one of the single-neck round-bottom flasks connected with a transfer arm to another flask
containing (40 mL) dried THF with constant stirring at
was used immediately.
−
78 ◦C for 40 min. The resultant THF solution
Compound (
(3.48 mL, 3.48 mmol) in THF (4 mL) at
reaction mixture was transferred the ethereal solution of formaldehyde (40 mL) poured through the
connected transfer arm, also at
78 ◦C. After the addition was completed, the reaction mixture was
allowed to stir for 15 min at the same temperature. Then, the reaction was quenched by drop-wise
addition of aqueous NH4C1 at
78 ◦C, and the reaction mass was stirred at room temperature for
40 min. Then, the reaction mass was extracted with ether (3 30 mL). The combined extracts were
±
)-
7
(0.4 g, 1.58 mmol) in THF (4 mL) was added to a stirred solution of LHMDS
−
78 ◦C under N2 atm and then stirred for 5 h. To the above
−
−
×

Molecules 2020, 25, 831
12 of 15
washed successively with water and brine and dried over MgSO4. Filtration and evaporation of the
filtrate afforded a clear oil as a mixture of 15 and 16, which was separated by column chromatography
on silica gel using EtOAc/CH2Cl2 (10:90, V:V) as eluants to obtain 15 (181 mg, 48%) as a white semi-solid.
98 mg of 7 (26%) was recovered from the column.
Compound 15:1H NMR (400 MHz, CDCl3)
δ 4.12-4.08 (m, 1H); 3.81-3.75 (m, 1H); 2.69 (dd, J = 8.0 Hz,
J = 12.0 Hz, 1H); 2.65–2.59 (m, 1H); 2.46 (dd, J = 4 Hz, J = 12 Hz, 1H); 2.42-2.33 (m, 1H); 2.28-2.18 (m,
1H); 1.98-1.83 (m, 1H); 1.81–1.67 (m, 2H); 1.58–1.55 (m, 2H); 1.41 (dt, J = 4.0 Hz, J = 16.0 Hz, 1H); 1.28
(dt, J = 4.0 Hz, J = 16 Hz, 1H); 1.06 (s, 3H); 1.02 (s, 3H); 13C NMR (100 MHz, CDCl3)
δ 209.3, 120.3, 60.2,
58.2, 51.8, 43.0, 41.5, 40.7, 35.8, 33.9, 32.1, 24.7, 20.2, and 19.4.
1
Compound 16: H NMR (400 MHz, CDCl3)
δ
3.86-3.81 (m, 1H); 3.70-3.67 (m, 1H); 2.64 (d, J = 14 Hz,
1H); 2.50–2.47 (m, 1H); 2.36 (d, J = 14 Hz, 1H); 2.15-2.06 (m, 2H); 1.83-1.76 (m, 4H); 1.44-1.29 (m, 4H);
1.03 (s, 3H); and 0.99 (s, 3H).
Supplementary Materials: The following are available online, Figure S1 1H NMR for the compound 4; Figure S2
13 C NMR for the compound 4; Figure S3 HRMS (ESI) for the compound 4; Figure S4 1H NMR for the compound
5; Figure S5 13C NMR for the compound 5; Figure S6 HRMS (ESI) for the compound 5; Figure S7 1H NMR for the
compound 6; Figure S8 13C NMR for the compound 6; Figure S9 HRMS (ESI) for the compound 6; Figure S10 1H
NMR for the compound trans(
the compound trans( )-7; Figure S13 1H NMR for the compound cis(
cis( )-7; Figure S15 1H NMR for the compound ( )-8; Figure S16 13C NMR for the compound (
HRMS (ESI) for the compound ( )-8; Figure S18 1H NMR for the compound (+)-9; Figure S19 13C NMR for the
compound (+)-9; Figure S20 HRMS (ESI) for the compound (+)-9; Figure S21 1H NMR for the compound (−)-10;
Figure S22 13C NMR for the compound ( )-10; Figure S23 HRMS (ESI) for the compound ( )-10; Figure S24 1H
±
)-7; Figure S11 13C NMR for the compound trans(
)-7; Figure S14 13C NMR for the compound
)-8; Figure S17
±
)-7; Figure S12 HRMS (ESI) for
±
±
±
−
−
−
−
−
NMR for the compound (+)-12; Figure S25 13C NMR for the compound (+)-12; Figure S26 HRMS (ESI) for the
compound (+)-12; Figure S27 1H NMR for the compound (+)-13; Figure S28 13C NMR for the compound (+)-13;
Figure S29 2D NOESY for the compound (+)-13; Figure S30 HRMS (ESI) for the compound (+)-13; Figure S31
1H NMR for the compound (-)-14; Figure S32 13C NMR for the compound (
the compound ( )-14; Figure S34 1H NMR for the compound ( )-1a; Figure S35 13C NMR for the compound
)-1a; Figure S36 DEPT 135, 90 and 45 combined spectra for the compound ( )-1; Figure S37 HRMS (ESI) for the
compound ( )-1; Figure S38 1H NMR for the compound 15 and 16 mixture; Figure S39 1H NMR for the compound
15; Figure S40 13C NMR for the compound 15; Figure S41 1H NMR for the compound 16; Table S1: Crystal data
and structure refinement for trans-cyanobicyclicketone, ( )7; Table S2: Atomic coordinates ( 10 ) and equivalent
isotropic displacement parameters (Å 10 ) for trans-cyanobicycli ketone ( )-7. U(eq) is defined as one third of
the trace of the orthogonalized Uij tensor; Table S3: Bond lengths [Å] and angles [◦] for trans-cyanobicyclic ketone
)-7; Table S4: Anisotropic displacement parameters (Å2 103) for trans bicyclicketone ( )-7. The anisotropic
displacement factor exponent takes the form: 2[ h2 a*2U11 + . . . + 2 h k a* b* U12 ]; Table S5: Crystal data and
structure refinement for ketal diastreomer ( )-8; Table S6: Atomic coordinates ( 104) and equivalent isotropic
displacement parameters (Å2 103) for ketol diastreomer ( )-8. U(eq) is defined as one third of the trace of
the orthogonalized Uij tensor; Table S7: Bond lengths [Å] and angles [◦] for ketol diastreomer (
)-; Table S8:
Anisotropic displacement parameters (Å2 103) for ketol diastreomer, ( )-8. The anisotropic displacement factor
exponent takes the form: 2[ h2 a*2U11 + . . . + 2 h k a* b* U12]; Table S9: Crystal data and structure refinement
for compound ( )-14; Table S10: Atomic coordinates (x 104) and equivalent isotropic displacement parameters
(Å2 103) for lactone ( )-14. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor; Table S11:
Bond lengths [Å] and angles [◦] for lactone (
)-14; Table S12: Anisotropic displacement parameters (Å2 103) for
−
)-14; Figure S33 HRMS (ESI) for
−
−
(
−
−
−
−
×
×
−
(
−
×
−
−
2π
−
×
×
−
−
×
−
−
2π
−
×
−
−
×
(−)-14. The anisotropic displacement factor exponent takes the form: 2[ h2 a*2U11 + . . . + 2 h k a* b* U12].
Author Contributions: D.-F.T. designed research; V.A. performed research; V.A. and D.-F.T. analyzed data and
wrote the paper. Project administration was managed by D.-F.T. All authors have read and agreed to the published
version of the manuscript.
Funding: This research was funded by Taiwan Ministry of Science and Technology (NSC100-2622-M-259-001-CC2)
and Deyew Biotech Corporation.
Acknowledgments: We are grateful to thank Professor Yew-Min Tzeng of National Taitung University (Chaoyang
University of Technology) for helpful discussion.
Conflicts of Interest: The authors declare there are no conflict of interest.

Molecules 2020, 25, 831
13 of 15
References
1.
2.
3.
4.
Neerman, M.F. Sesquiterpene lactones: A diverse class of compounds found in essential oils possessing
antibacterial and antifungal properties. Int. J. Aromatherapy 2003, 13, 114–120. [CrossRef]
Seaman, F.C.; Funk, V.A. Cladistic analysis of complex natural products: Developing transformation series
from sesquiterpene lactone data. Taxon 1983, 32, 1–27. [CrossRef]
Jansen, B.J.M.; de Groot, A. The occurrence and biological activity of drimane sesquiterpenoids. Nat. Prod.
Rep. 1991, 8, 309–318. [CrossRef]
Zhao, S.S.; Leung, K.S.-Y. Quality evaluation of mycelial Antrodia camphorata using high-performance
liquid chromatography (HPLC) coupled with diode array detector and mass spectrometry (DAD-MS). Chin.
Med. 2010, 5, 4. [CrossRef] [PubMed]
5.
Chen, C.-C.; Liu, Y.-W.; Ker, Y.-B.; Wu, Y.-Y.; Lai, E.Y.; Chyau, C.-C.; Hseu, T.-H.; Peng, R.Y. Chemical
Characterization and Anti-inflammatory Effect of Polysaccharides Fractionated from Submerge-Cultured
Antrodia camphorata Mycelia. J. Agric. Food Chem. 2007, 55, 5007–5012. [CrossRef] [PubMed]
Geethangili, M.; Tzeng, Y.-M. Review of Pharmacological Effects of Antrodia camphorata and Its Bioactive
Compounds. Evid Based Complement Alternat. Med. 2011, 2011, 212641. [CrossRef] [PubMed]
Chen, J.C. King of Ganoderma: Antrodia camphorata in Taiwan, 2nd ed.; Yuen Chi Jai Book Publishing: Taipei,
Taiwan, 2008.
Wu, D.-P.; Chiang, H.-C. Constituents of Antrodia Cinnamomea. J. Chin. Chem. Soc. 1995, 42, 797–800.
[CrossRef]
Lee, I.-H.; Huang, R.-L.; Chen, C.-T.; Chen, H.-C.; Hsu, W.-C.; Lu, M.-K. Antrodia camphorata polysaccharides
exhibit anti-hepatitis B virus effects. Microbiol. Lett. 2002, 209, 63–67. [CrossRef]
6.
7.
8.
9.
10. Hseu, Y.-C.; Chen, S.-C.; Chen, H.-C.; Liao, J.-W.; Yang, H.-L. Antrodia camphorata inhibits proliferation of
human breast cancer cells in vitro and in vivo. Food Chem. Toxicol. 2008, 46, 2680–2688. [CrossRef]
11. Lee, Y.-P.; Tsai, W.-C.; Ko, C.-J.; Rao, Y.K.; Yang, C.-R.; Chen, D.-R.; Yang, M.-H.; Yang, C.-C.; Tzeng, Y.-M.
Anticancer Effects of Eleven Triterpenoids Derived from Antrodia camphorata. Anticancer Res. 2012, 32,
2727–2734.
12. Ao, Z.-H.; Xu, Z.-H.; Lu, Z.-M.; Xu, H.-Y.; Zhang, X.-M.; Dou, W.-F. Niuchangchih (Antrodia camphorata)
and its potential in treating liver diseases. J. Ethnopharmacol. 2009, 121, 194–212. [CrossRef] [PubMed]
13. Chen, J.-H.; Wu, A.T.H.; Tzeng, D.T.W.; Huang, C.-C.; Tzeng, Y.-M.; Chao, T.-Y. Antrocin, a bioactive
component from Antrodia cinnamomea, suppresses breast carcinogenesis and stemness via downregulation
of β-catenin/Notch1/Akt signaling. Phytomedicine 2019, 52, 70–78. [CrossRef] [PubMed]
14. Hung-Chen, C.; De-Peng, W.; Cherng, I.W.; Chuen-Her, U. A sesquiterpene lactone, phenyl and biphenyl
compounds from Antrodia cinnamomea. Phytochemistry 1995, 39, 613–616. [CrossRef]
15. Rao, Y.K.; Wu, A.T.H.; Geethangili, M.; Huang, M.-T.; Chao, W.-J.; Wu, C.-H.; Deng, W.-P.; Yeh, C.-T.;
Tzeng, Y.-M. Identification of Antrocin from Antrodia camphorata as a Selective and Novel Class of Small
Molecule Inhibitor of Akt/mTOR Signaling in Metastatic Breast Cancer MDA-MB-231 Cells. Chem. Res.
Toxicol. 2011, 24, 238–245. [CrossRef]
16. Chen, C.-C.; Shiao, Y.-J.; Lin, R.-D.; Shao, Y.-Y.; Lai, M.-N.; Lin, C.-C.; Ng, L.-T.; Kuo, Y.-H. Neuroprotective
Diterpenes from the Fruiting Body of Antrodia camphorata. J. Nat. Prod. 2006, 69, 689–691. [CrossRef]
[PubMed]
17. Han, B.H.; Yang, H.O.; Kang, Y.-H.; Suh, D.-Y.; Go, H.J.; Song, W.-J.; Kim, Y.C.; Park, M.K.
In Vitro Platelet-Activating Factor Receptor Binding Inhibitory Activity of Pinusolide Derivatives:
A Structure−Activity Study. J. Med. Chem. 1998, 41, 2626–2630. [CrossRef]
18. Chiang, Y.-M.; Liu, H.-K.; Lo, J.-M.; Chien, S.-C.; Chan, Y.-F.; Lee, T.-H.; Su, J.-K.; Kuo, Y.-H. Cytotoxic
Constituents of the Leaves of Calocedrus Formosana. J. Chin. Chem. Soc. 2003, 50, 161–166. [CrossRef]
19. Smith, L.; Watson, M.B.; O’Kane, S.L.; Drew, P.J.; Lind, M.J.; Cawkwell, L. The analysis of doxorubicin
resistance in human breast cancer cells using antibody microarrays. Mol. Cancer. Ther. 2006, 5, 2115–2120.
[CrossRef] [PubMed]
20. Shi, H.; Fang, L.; Tan, C.; Shi, L.; Zhang, W.; Li, C.-C.; Luo, T.; Yang, Z. Total Syntheses of Drimane-Type
Sesquiterpenoids Enabled by a Gold-Catalyzed Tandem Reaction. J. Am. Chem. Soc. 2011, 133, 14944–14947.
[CrossRef]

Molecules 2020, 25, 831
14 of 15
21. Li, F.-Z.; Li, S.; Zhang, P.-P.; Huang, Z.-H.; Zhang, W.-B.; Gong, J.; Yang, Z. A chiral pool approach for
asymmetric syntheses of (
12426–12429. [CrossRef]
−
)-antrocin, (+)-asperolide C, and ( )-trans-ozic acid. Chem. Comm. 2016, 52,
−
22. Huang, S.-H.; Liang, K.-H.; Lu, J.-S.; Chang, W.-S.; Su, M.-D.; Yang, T.-F. Total Synthesis of (+)-Antrocin and
Its Diastereomer and Clarification of the Absolute Stereochemistry of (
9576–9584. [CrossRef] [PubMed]
−
)-Antrocin. J. Org. Chem. 2017, 82,
23. Tai, D.-F.; Angamuthu, V.; Tzeng, Y.-M. Process for preparing optically active antrocin. U.S. Patent US0096324
A1, 18 April 2013.
24. Hosmer, C.A.; Comber, R.N.; Brouillette, W.J. Syntheses of alpha.-, beta.-, and gamma.-substituted carnitines
via beta.-keto esters. J. Org. Chem. 1985, 50, 3627–3631. [CrossRef]
25. Feringa, B.L. Trimethylsilyl-Halide-Mediated Conjugated Addition to Allylic Acetals. Syn. Comm. 1985, 15,
87–89. [CrossRef]
26. Lee, H.M.; Nieto-Oberhuber, C.; Shair, M.D. Enantioselective Synthesis of (+)-Cortistatin A, a Potent and
Selective Inhibitor of Endothelial Cell Proliferation. J. Am. Chem. Soc. 2008, 130, 16864–16866. [CrossRef]
[PubMed]
27. House, H.O.; Wilkins, J.M. Reactions involving electron transfer. 12. Effects of solvent and substituents upon
the ability of lithium diorganocuprates to add to enones. J. Org. Chem. 1978, 43, 2443–2454. [CrossRef]
28. Spessard, S.J.; Stoltz, B.M. Progress toward the Synthesis of Garsubellin A and Related Phloroglucins:
The Direct Diastereoselective Synthesis of the Bicyclo[3.3.1]nonane Core. Org. Lett. 2002, 4, 1943–1946.
[CrossRef]
29. Taber, D.F.; Sikkander, M.I.; Storck, P.H. Enantioselective Synthesis of (+)-Majusculone. J. Org. Chem. 2007
,
72, 4098–4101. [CrossRef]
30. Nagata, W.; Yoshioka, M.; Hirai, S. Hydrocyanation. IV. New hydrocyanation methods using hydrogen
cyanide and an alkylaluminum, and an alkylaluminum cyanide. J. Am. Chem. Soc. 1972, 94, 4635–4643.
[CrossRef]
31. Ihara, M.; Katsumata, A.; Egashira, M.; Suzuki, S.; Tokunaga, Y.; Fukumoto, K. Stereoselective Construction
of the Diterpene Part of Indole Alkaloids, Radarins, by Way of Intramolecular Diels-Alder Reaction. J. Org.
Chem. 1995, 60, 5560–5566. [CrossRef]
32. Suryawanshi, S.N.; Fuchs, P.L. Bruceantin support studies. 10. Use of an axial alpha.-face control element in
intramolecular conjugate additions: Synthesis of an ABCD tetracyclic bruceantin precursor. J. Org. Chem.
1986, 51, 902–921. [CrossRef]
33. Utimoto, K.; Wakabayashi, Y.; Horiie, T.; Inoue, M.; Shishiyama, Y.; Obayashi, M.; Nozaki, H.
Cyanotrimethylsilane as a versatile reagent for introducing cyanide functionality. Tetrahedron 1983, 39,
967–973. [CrossRef]
34. Dailey, O.D.; Fuchs, P.L. Synthesis of a model for the BCE ring system of bruceantin. A caveat on the
cyclohexene fwdarw. trans diaxial diol conversion. J. Org. Chem. 1980, 45, 216–236. [CrossRef]
35. Furuichi, N.; Hata, T.; Soetjipto, H.; Kato, M.; Katsumura, S. Common synthetic strategy for optically
active cyclic terpenoids having a 1,1,5-trimethyl-trans-decalin nucleus: Syntheses of (+)-acuminolide,
(−)-spongianolide A, and (+)-scalarenedial. Tetrahedron 2001, 57, 8425–8442. [CrossRef]
36. Cha, J.S.; Kim, E.J.; Kwon, O.O.; Kim, J.M. Chemoselective Reduction of Carbonyl Compounds with
Diisopinocampheylchloroborane. Synlett 1995, 1995, 331–332. [CrossRef]
37. Hori, K.; Hikage, N.; Inagaki, A.; Mori, S.; Nomura, K.; Yoshii, E. Total synthesis of tetronomycin. J. Org.
Chem. 1992, 57, 2888–2902. [CrossRef]
38. Zoretic, P.A.; Zhang, Y.; Fang, H.; Ribeiro, A.A.; Dubay, G. Advanced Tetracycles in a Stereoselective
Approach to d,l-Spongiatriol and Related Metabolites: The Use of Radicals in the Synthesis of Angular
Electrophores. J. Org. Chem. 1998, 63, 1162–1167. [CrossRef]
39. Schlosser, M.; Jenny, T.; Guggisberg, Y. Monomeric Formaldehyde in Ethereal Solution. Synlett 1990, 1990, 704.
[CrossRef]
40. Kato, M.; Matsumura, Y.; Heima, K.; Fukamiya, N.; Kabuto, C.; Yoshikoshi, A. Total synthesis of dl-siccanin
and dl-siccanochromene E. Tetrahedron 1987, 43, 711–722. [CrossRef]
41. NOE spectroscopic date given in Supplementary Materials.
42.
D
í
az, S.; Cuesta, J.; Gonz
á
lez, A.; Bonjoch, J. Synthesis of ( )-Nakamurol A and Assignment of Absolute
−
Configuration of Diterpenoid (+)-Nakamurol, A. J. Org. Chem. 2003, 68, 7400–7406. [CrossRef]

Molecules 2020, 25, 831
15 of 15
43. Toshima, H.; Oikawa, H.; Toyomasu, T.; Sassa, T. Total Synthesis of (+)-Albicanol and (+)-Albicanyl Acetate.
Biosci. Biotechnol. Biochem. 2001, 65, 1244–1247. [CrossRef] [PubMed]
44. Skepper, C.K.; Quach, T.; Molinski, T.F. Total Synthesis of Enigmazole A from Cinachyrella enigmatica.
Bidirectional Bond Constructions with an Ambident 2,4-Disubstituted Oxazole Synthon. J. Am. Chem. Soc.
2010, 132, 10286–10292. [CrossRef] [PubMed]
Sample Availability: Electronic Supplementary Information (ESI) available: Spectral data (1H, 13C NMR, and
HRMS) of all compounds; CIF for (
from the authors.
−
)-7
,
(−)-8, and (−)-14 compounds crystallographic data material are available
©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).