Stereospecific synthesis of S-(?)-trans-verbenol and its antipode by inversion of sterically hindered alcohols

Article abstract of DOI:10.1080/10286020.2020.1839433

S-(?)-trans-Verbenol (1) and its antipode, R-(+)-trans-verbenol (1′) have been confirmed as the critical pheromone components of bark beetles. Synthesis of these two active secondary alcohols (1 and 1′) from commercially available starting materials S-α-pinene and R-α-pinene was reported. The key steps were mainly depended on the effective S_N2 stereo-inversion of the hydroxy group of sterically hindered alcohols (3 and 3′), using Mitsunobu reaction or hydrolysis of mesylate ester, alternatively. Our results provide a new and stereo-selectivity way to obtain optically active insect pheromones.

Full text of DOI:10.1080/10286020.2020.1839433

Journal of Asian Natural Products Research
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/ganp20
Stereospecific synthesis of S-(−)-trans-verbenol
and its antipode by inversion of sterically hindered
alcohols
Fu Liu, Jia-Xing Fang, Xiang-Bo Kong, Su-Fang Zhang & Zhen Zhang
To cite this article: Fu Liu, Jia-Xing Fang, Xiang-Bo Kong, Su-Fang Zhang & Zhen Zhang (2020):
Stereospecific synthesis of S-(−)-trans-verbenol and its antipode by inversion of sterically hindered
alcohols, Journal of Asian Natural Products Research, DOI: 10.1080/10286020.2020.1839433
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JOURNAL OF ASIAN NATURAL PRODUCTS RESEARCH
https://doi.org/10.1080/10286020.2020.1839433
Stereospecific synthesis of S-(ꢀ)-trans-verbenol and its
antipode by inversion of sterically hindered alcohols
Fu Liu, Jia-Xing Fang, Xiang-Bo Kong, Su-Fang Zhang and Zhen Zhang
Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Key
Laboratory of Forest Protection of National Forestry and Grassland Administration, Beijing
100091, China
ABSTRACT
ARTICLE HISTORY
S-(ꢀ)-trans-Verbenol (1) and its antipode, R-(þ)-trans-verbenol (10)
have been confirmed as the critical pheromone components of
bark beetles. Synthesis of these two active secondary alcohols (1
and 10) from commercially available starting materials S-a-pinene
and R-a-pinene was reported. The key steps were mainly
depended on the effective SN2 stereo-inversion of the hydroxy
group of sterically hindered alcohols (3 and 30), using Mitsunobu
reaction or hydrolysis of mesylate ester, alternatively. Our results
provide a new and stereo-selectivity way to obtain optically active
insect pheromones.
Received 9 July 2020
Accepted 14 October 2020
KEYWORDS
S-(ꢀ)-trans-verbenol; R-
(þ)-trans-verbenol; sterically
hindered alcohols;
Mitsunobu reaction;
SN2 inversion
1. Introduction
Conifer-feeding bark beetles (Coleoptera: Scolytinae) are among the most economic-
ally important pests of conifers, which cause significant losses to pine forests world-
wide [1]. It is well known that the aggregation of bark beetles is generally mediated
by semichemicals released by the beetles themselves, or volatiles derived from the
host trees [2].
S-(ꢀ)-trans-Verbenol (1, Figure 1(A)) and R-(þ)-trans-verbenol (10, Figure 1(B))
have been confirmed as the critical pheromone components of bark beetles [3,4].
Application of pheromones has become a new trend in the management of this kind
of pest through mass trapping or pheromone avoidance [5]. Commonly, these two
CONTACT Zhen Zhang
zhangzhen@caf.ac.cn
Research Institute of Forest Ecology, Environment and
Protection, Chinese Academy of Forestry, Key Laboratory of Forest Protection of National Forestry and Grassland
Administration, Beijing 100091, China
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
F. LIU ET AL.
Figure 1. The structures of S-(ꢀ)-trans-verbenol (1) and R-(þ)-trans-verbenol (10).
monoterpenols are derived from the host monoterpene a-pinene [6]; however, natural
product extraction cannot provide enough material for economical field trapping.
Presently, allylic oxidation of a-pinene is the main method to synthesize trans-ver-
benol [7]. The oxidization of a-pinene to trans-verbenyl acetate by use of Pb(OAc)4
in benzene, followed by saponification using aqueous KOH in CH3OH, could pro-
duce trans-verbenol directly [8]. However, the oxidation reaction cannot easily be
scaled up, because it involves expensive reagents [Pb(OAc)4], requires the use of ben-
zene as a solvent, and has poor reproducibility and selectivity. Hence, it remains chal-
lenging to develop a versatile and efficient strategy for the total synthesis of
trans-verbenol.
As a part of our project to obtain optically active insect pheromones, two alterna-
tive methods of SN2 inversion were performed to realize the synthesis of S-(ꢀ)-trans-
verbenol (1) and R-(þ)-trans-verbenol (10) in high chemical and optical yields.
2. Results and discussion
The retrosynthetic analysis is shown in Scheme 1 (A and B), which involves the use
of a-pinene (S and R configurations) as the chirons for the construction of the stereo-
genic centers of target compounds (1 and 10). Normally, secondary alcohols can be
prepared via reduction of the corresponding ketone. However, due to steric effects,
there are few stereoselective reductants that could realize the reduction of verbenone
(4 or 40) to obtain the trans products (1 or 10). While, the cis products (3 or 30) could
be obtained easily by the reduction of verbenone (4 or 40). Therefore, some effective
methods should be designed to realize the stereo-inversion of the hydroxy group of
cis-verbenol (3 or 30), so that the optically pure trans-verbenol (1 or 10) can
be prepared.
According to our synthetic plan, the synthesis of S-(ꢀ)-trans-verbenol (1) began
with the preparation of S-(ꢀ)-verbenone (4), which could be produced from S-
a-pinene by allylic oxidation using manganese (III) acetate as the catalyst (Scheme 2)
[9]. The preparation of S-(ꢀ)-cis-verbenol (3) by reducing S-(ꢀ)-verbenone (4) with
(i-Bu)2AlH in CH2Cl2 at ꢀ70ꢁ‘ resulted in higher yields and stereoselectivity [10].
Currently, optically active secondary alcohols can be prepared from the corre-
sponding racemic esters in high chemical and optical yields, mainly by a combination
of enzymatic hydrolysis and chemical transformation [11–13]. In order to realize
stereo-inversion of the hydroxy group of S-(ꢀ)-cis-verbenol (3), Mitsunobu reaction,

JOURNAL OF ASIAN NATURAL PRODUCTS RESEARCH
3
Scheme 1. The retrosynthetic analysis of S-(ꢀ)-trans-verbenol (1) and R-(þ)-trans-verbenol (10).
as well as hydrolysis of the mesyl ester, was carried out, respectively (Scheme 2). S-
(ꢀ)-cis-Verbenol (3) was subjected to Mitsunobu reaction by using 4-nitrobenzoic
acid, PPh3, and diethyl azodicarboxylate, which furnished S-(ꢀ)-trans-verbenyl 4-
nitrobenzoate (2a) with an inverted configuration. LiAlH4 reduction of the benzoate
(2a) provided S-(ꢀ)-trans-verbenol (1) (85.5% yield based on 3). The optical purity
of 1 was determined by GC analysis (96.2% ee). As an alternative to Mitsunobu reac-
tion, the utilization of an SN2 type hydrolysis of a mesylate ester (2b) was also exam-
ined. S-(ꢀ)-trans-verbenol (1) with 93% ee was obtained in 80% yield by mesylation
followed by hydrolysis under neutral condition (85ꢁ‘ in the presence of CaCO3).
R-(þ)-trans-verbenol (10) (Mitsunobu reaction condition, 85.6% yield, 95% ee;
hydrolysis of mesyl ester, 78% yield, 91%ee.) was synthesized, using R-a-pinene as the
starting material as well (Scheme 3).
Both S-(ꢀ)-trans-verbenol (1) and R-(þ)-trans-verbenol (10) can be prepared in
high chemical and optical yields from these two alternative methods of SN2 inversion.
Considering the facts that the chemical and optical yield from Mitsunobu inversion
was better compared with the results obtained via mesylation. The former is prefer-
able to finish the stereo-inversion of sterically hindered alcohols, while there are pos-
sibilities of side reactions such as elimination or SN1 reaction during the hydrolysis
of mesylate.
In conclusion, we have successfully synthesized S-(ꢀ)-trans-verbenol (1) and its
antipode (10) in excellent optical yields, using S-(ꢀ)-a-pinene and R-(þ)-a-pinene as
the starting materials. The stereo-inversion of the hydroxy group was realized effi-
ciently by Mitsunobu inversion, or hydrolysis of mesylate ester. Our results are highly
beneficial for preventing and controlling bark beetles in an economical and environ-
mentally benign way.
3. Experimental
3.1. General experimental procedures
All the commercially available reagents were used without further purification.
Column chromatography was performed on silica gel (200–300 mesh), and TLC

4
F. LIU ET AL.
Scheme 2. The synthesis of S-(ꢀ)-trans-verbenol (1): (a) Mn(OAc)3ꢃ2H2O, TBHP, O2; (b) (i-Bu)2AlH,
CH2Cl2, –70ꢁ‘ in an argon, 2 h; (c) 4-nitrobenzoic acid, PPh3, DEAD, dry THF, 0ꢁ‘ to room tem-
perature (14h), 40 ꢁC (3 h); (d) LiAlH4, dry THF, 0ꢁ‘ to room temperature (4 h); (e) methanesulfonic
anhydride, Et3N, CH2Cl2, –20ꢁ‘ (2 h); (f) CaCO3, H2O, 85ꢁ‘ (4 h).
inspections were performed on silica gel GF-254 plates (Qingdao Haiyang Chemical
Co., Ltd, Qingdao, China). The NMR spectra were recorded on a Bruker NMR spec-
trometer (Bruker, Fallanden, Switzerland) in CDCl3 (1H at 500 MHz and 13C at
€
125 MHz) using tetramethylsilane (TMS) as the internal standard. The purities of the
synthesized compounds were estimated by gas chromatography (GC), which was per-
formed on an Agilent 7890 A (Agilent Technologies, Wilmington, DE, USA). The col-
umn used was CYCLOSIL-B with 30 m ꢂ 0.25 mm i.d., 0.25 lm (Agilent
Technologies). The chemical analysis was carried out using an Agilent GC coupled
with a mass spectrometry system (TRACE GC 2000). The GC system was equipped
with a DB-5 ms column (30 m ꢂ 0.25 mm ꢂ 0.25 lm, Agilent Technologies).
3.2. General procedure for the synthesis of verbenone (4 and 40)
A solution of a-pinene (13.6 g, 100.0 mmol), EtOAc (40 ml), and t-butyl hydroperox-
ide (TBHP) in decane (386 mmol) was stired with 3 Å molecular sieves (25.0 g) for
30 min under nitrogen. Mn(OAc)3ꢃ2H2O (2.67 g, 9.97 mmol) was added. The mixture
was degassed and filled with oxygen (balloon). The reaction was stirred for 48 h,
diluted with 20 ml of diethyl ether, and filtered through a thin pad of Celite. The fil-
trate was concentrated under reduced pressure, and the residue was chromatographed
over SiO2 and eluted with hexane/EtOAc (10:1, v/v) to yield verbenone as a yel-
low oil.
1
S-(2)-Verbenone 4 (9.0 g, 60% yield); H NMR (500 MHz, CDCl3) d: 0.99 (3H, s),
1.47 (3H, s), 1.99 (3H, d, J ¼ 1.5 Hz), 2.05 (1H, d, J ¼ 9.5 Hz), 2.38–2.41 (1H, m),
2.61–2.63 (1H, m), 2.76–2.80 (1H, m), 5.70–5.71 (1H, m). 13C NMR (125 MHz,
CDCl3) d: 21.9, 23.5, 26.5, 40.8, 49.6, 53.9, 57.5, 121.1, 170.1, 203.9.
1
R-(1)-Verbenone 40 (9.3 g, 62% yield); H NMR (500 MHz, CDCl3) d: 0.99 (3H,
s), 1.48 (3H, s), 1.98 (3H, d, J ¼ 1.5 Hz), 2.05 (1H, d, J ¼ 9.5 Hz), 2.39–2.41 (1H, m),
2.62–2.64 (1H, m), 2.77–2.81 (1H, m), 5.70–5.71 (1H, m). 13C NMR (125 MHz,
CDCl3) d: 21.9, 23.5, 26.5, 40.8, 49.6, 54.0, 57.5, 121.1, 170.2, 204.0.
These data were consistent with the literature reports [14].

JOURNAL OF ASIAN NATURAL PRODUCTS RESEARCH
5
Scheme 3. The synthesis of R-(þ)-trans-verbenol (10): (a) Mn(OAc)3ꢃ2H2O, TBHP, O2; (b) (i-Bu)2AlH,
CH2Cl2, –70ꢁ‘, 2 h; (c) 4-nitrobenzoic acid, PPh3, DEAD, dry THF, 0ꢁ‘ to room temperature (14h),
40 ꢁC (3 h); (d) LiAlH4, dry THF, 0ꢁ‘ to room temperature (4 h); (e) methanesulfonic anhydride,
Et3N, CH2Cl2, –20ꢁ‘ (2 h); (f) CaCO3, H2O, 85ꢁ‘ (4 h).
3.3. General procedure for the synthesis of cis-verbenol (3 and 30)
A solution of verbenone (7.5 g, 50.0 mmol) in anhydrous CH2Cl2 (50 ml) was pre-
pared, and 37.0 ml of (i-Bu)2AlH (1.5 M solution in toluene) was added dropwise at
ꢀ70ꢁ‘ under an argon flow. The mixture was stirred for 2 h. At the same tempera-
ture, a mixture of 3.5 ml THF and 3.5 ml H2O was added, and the stirring was con-
tinued for 0.5 h. The separated precipitate was filtered off through celite, and the
organic layer was dried with Na2SO4 and then evaporated. The residue was chroma-
tographed over SiO2 and elution with hexane/EtOAc (8:1, v/v) to yield cis-verbenol as
a white powder.
S-(2)-cis-Verbenol 3 (6.99 g, 92% yield);1H NMR (500 MHz, CDCl3) d: 1.07 (3H,
s), 1.34 (3H, s), 1.67–1.69 (1H, m), 1.72–1.73 (3H, m), 1.95–1.97 (1H, m), 2.27–2.29
(1H, m), 2.42–2.46 (1H, m), 4.44–4.46 (1H, m), 5.35–5.36 (1H, m). 13C NMR
(125 MHz, CDCl3) d: 22.6, 22.6, 26.8, 35.5, 38.9, 47.7, 48.1, 73.5, 119.3, 147.4.
1
R-(1)-cis-Verbenol 30 (6.92 g, 91% yield); H NMR (500 MHz, CDCl3) d: 1.08 (3H,
s), 1.35 (3H, s), 1.70 (1H, m), 1.72–1.74 (3H, m), 1.96–1.98 (1H, m), 2.27–2.31 (1H,
m), 2.43–2.46 (1H, m), 4.45–4.46 (1H, m), 5.36–5.37 (1H, m). 13C NMR (125 MHz,
CDCl3) d: 22.6, 22.6, 26.8, 35.5, 38.9, 47.7, 48.2, 73.5, 119.3, 147.3.
These data were consistent with the literature reports [15].
3.4. General procedure for the synthesis of trans-verbenyl 4-nitrobenzoate (2a
and 2a0)
cis-Verbenol (3.04 g, 20.0 mmol), 4-nitrobenzoic acid (13.4 g, 80.0 mmol), and triphenyl-
phosphine (PPh3) (21.0 g, 80.0 mmol) were dissolved in THF (150 ml), then diethyl azodi-
carboxylate (13.9 g, 80.0 mmol) was added dropwise, and the reaction mixture was
maintained below 10 ꢁC. Upon completion of the addition, the mixture was stirred first
at room temperature overnight (14 h) and then at 40 ꢁC for 3h. The reaction mixture
was cooled to room temperature, diluted with 150 ml of ether and washed twice with
100 ml portions of saturated aqueous NaHCO3 solution. The aqueous layers were com-
bined and back-extracted with 100 ml of ether. The combined organic layers were dried
over Na2SO4. Excess solvent was removed under reduced pressure. The resulting semi-
solid was suspended in 40 ml of ether, and the suspension stand at room temperature

6
F. LIU ET AL.
overnight. The mixture was stirred, while 20 ml of hexanes was slowly added. The result-
ing white solid was filtered, and the filter cake was washed with 200 ml of 50% (v/v)
ether-hexanes. The solvent was removed under reduced pressure. The residue was chro-
matographed over SiO2 and eluted with hexane/EtOAc (8:1, v/v) to yield trans-verbenyl
4-nitrobenzoate as a white powder.
S-(2)-trans-Verbenyl 4-nitrobenzoate 2, [a] 2124 (c ¼ 0.7, CHCl3). (5.42 g, 90%
1
yield); H NMR (500 MHz, CDCl3) d: 0.98 (3H, s), 1.38 (3H, s), 1.59–1.61 (1H, m),
1.78–1.79 (3H, m), 2.11–2.13 (1H, m), 2.37–2.39 (1H, m), 5.44–5.45 (1H, m),
5.61–5.63 (1H, m), 8.17–8.21 (2H, m), 8.25–8.28 (2H, m). 13C NMR (125 MHz,
CDCl3): d: 20.7, 22.8, 26.4, 29.6, 44.4, 46.6, 47.7, 75.7, 114.6, 123.4, 123.4, 130.6,
130.6, 136.4, 150.7, 151.6, 164.5.
R-(1)-trans-Verbenyl 4-nitrobenzoate 20, [a] 1161 (c ¼ 1.0, CHCl3). (5.54 g, 92%
1
yield); H NMR (500 MHz, CDCl3) d: 0.99 (3H, s), 1.39 (3H, s), 1.59–1.62 (1H, m),
1.79–1.80 (3H, m), 2.12–2.14 (1H, m), 2.38–2.40 (1H, m), 5.45–5.46 (1H, m),
5.62–5.64 (1H, m), 8.19–8.21 (2H, m), 8.26–8.28 (2H, m). 13C NMR (125 MHz,
CDCl3): d: 20.7, 22.8, 26.4, 29.6, 44.4, 46.6, 47.7, 75.7, 114.6, 123.4, 123.4, 130.7,
130.7, 136.4, 150.4, 151.6, 164.5.
These data were consistent with the literature reports [7].
3.5. General procedure for the synthesis of cis-verbenyl mesylate (2 b and 2 b0)
A solution of cis-verbenol (3.04 g, 20.0 mmol) in CH2Cl2 (30 ml) was prepared. To
this solution was added triethylamine (2.73 g, 27.0 mmol) and methanesulfonic anhyd-
ride (4.00 g, 23.0 mmol) at ꢀ40ꢁ‘. After stirring at ꢀ40ꢁ‘ for 2 h, the reaction mix-
ture was poured into 30 ml of 1% aq. HCl and extracted with CH2Cl2. The organic
phase was washed with water and concentrated in vacuo to give the crude product.
The crude product was used for the next hydrolysis without purification.
3.6. General procedure for the synthesis of trans-verbenol (1 and 10)
3.6.1. Procedure for the reduction of trans-verbenyl 4-nitrobenzoate
A solution of trans-verbenyl 4-nitrobenzoate (3.01 g, 10.0 mmol) was added dropwise to a
suspension of LiAlH4 (760 mg, 20.0 mmol) in THF (30 ml) under a N2 atmosphere. The
resulting mixture was stirred for 30 min at 0 ꢁC and then warmed to room temperature
for 4 h. Then, the mixture was cooled with an ice bath, and treated by successive drop-
wise addition of 0.68 ml of water and stirred for 15 min, 0.68 ml of 15% NaOH solution
and stirred for 15 min, and then 2.04 ml of water and stirred for 30 min at room tem-
perature. The dry granular precipitate was removed by filtration, the filtrate was dried
over Na2SO4, and the solvent was evaporated. The crude material was purified by flash
chromatography on silica gel using hexane-ethyl acetate (5:1 v/v) as the eluent to provide
trans-verbenol (1, 1.37 g, 90% yield; 10, 1.41 g, 93% yield) as a yellow oil.
3.6.2. Procedure for hydrolysis of cis-verbenyl mesylate
The whole amount of the crude cis-verbenyl mesylate mixture obtained above was
heated in 45 ml of water in the presence of CaCO3 (0.40 g, 4 mmol) at 85ꢁ‘ over 4 h.

JOURNAL OF ASIAN NATURAL PRODUCTS RESEARCH
7
After neutralization with aq. NaHCO3 and extraction with EtOAc, the organic phase
was concentrated in vacuo. The crude product was purified by column chromatog-
raphy on silica gel using hexane-ethyl acetate (5:1 v/v) as the eluent to give trans-ver-
benol (1, 2.43 g, 80% yield based on 3; 10, 2.37 g, 78% yield based on 30) as a
yellow oil.
1
S-(2)-trans-Verbenol 1, [a] 2136 (c 0.80, CHCl3), ee 93%. H NMR (500 MHz,
CDCl3) d: 0.85 (3H, s), 1.32 (3H, s), 1.66–1.67 (1H, m), 1.70–1.71 (3H, m), 1.99–2.02
(1H, m), 2.14–2.17 (1H, m), 2.23–2.26 (1H, m), 4.24–4.26 (1H, m), 5.32–5.34 (1H,
m). 13C NMR (125 MHz, CDCl3): d 20.4, 22.6, 26.6, 28.6, 36.1, 47.0, 48.0, 70.4, 118.8,
148.8. GC-MS (70 eV) m/z: 152.
1
R-(1)-trans-Verbenol 10, [a] 1112 (c 12.0, CHCl3), ee 95%. H NMR (500 MHz,
CDCl3) d: 0.85 (3H, s), 1.32 (3H, s), 1.66–1.68 (1H, m), 1.70–1.71 (3H, m), 1.99–2.02
(1H, m), 2.14–2.17 (1H, m), 2.22–2.26 (1H, m), 4.24–4.26 (1H, m), 5.32–5.34 (1H,
m). 13C NMR (125 MHz, CDCl3): d 20.4, 22.6, 26.6, 28.6, 36.1, 47.0, 48.0, 70.4, 118.8,
148.8. GC-MS (70 eV) m/z: 152.
These data were consistent with the literature reports [7].
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The work was financially supported by the Beijing Natural Science Foundation (6194044), the
National Natural Science Foundation of China (31901320), and the Institute Special Fund for
Basic Research, Institute of Forest Ecology, Environment, and Protection, Chinese Academy of
Forestry (CAFYBB2016SY017).
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