Synthesis of β-Methyl Alcohols: Influence of Alkyl Chain Length on Diastereoselectivity and New Attractants of Rhynchophorus ferrugineus

Article abstract of DOI:10.1021/acs.jafc.1c01875

The diastereoselectivity of adducts in the addition reaction via the Felkin-Anh model is affected significantly by the steric effect of bulky groups. However, the influence of steric alkyl chain length has not been studied for the diastereoselectivity. In this work, we present a new strategy for the racemic synthesis of β-methyl alcohols to obtain various diastereomer ratios using the Felkin-Anh model. The addition of alkyl Grignard reagents to α-methyl aldehydes afforded diastereomer ratios of threo/erythro ≈ 2:1, while the reduction in structurally related ketones using LiAlH4 afforded ratios of threo/erythro ≈ 1:1. The experimental data showed no effect of alkyl chain length on either side on the stereoselectivity of adducts. All synthesized analogues were evaluated for attractiveness to Rhynchophorus ferrugineus weevils in the field. Five novel derivatives, including two alcohols and three ketones, were found to attract weevils in the field trials. Among them, 3-methyldecan-4-one (5b) and 4-methyldecan-5-ol (11a) were found to be the most attractive to the insects.

Full text of DOI:10.1021/acs.jafc.1c01875

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Article
Synthesis of β‑Methyl Alcohols: Influence of Alkyl Chain Length on
Diastereoselectivity and New Attractants of Rhynchophorus
ferrugineus
Van-Dung Le, Chi-Hien Dang,* Cong-Hao Nguyen, Hong-Ung Nguyen, and Thanh-Danh Nguyen*
Cite This: J. Agric. Food Chem. 2021, 69, 5882−5886
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ABSTRACT: The diastereoselectivity of adducts in the addition reaction via the Felkin−Anh model is affected significantly by the
steric effect of bulky groups. However, the influence of steric alkyl chain length has not been studied for the diastereoselectivity. In
this work, we present a new strategy for the racemic synthesis of β-methyl alcohols to obtain various diastereomer ratios using the
Felkin−Anh model. The addition of alkyl Grignard reagents to α-methyl aldehydes afforded diastereomer ratios of threo/erythro ≈
2:1, while the reduction in structurally related ketones using LiAlH4 afforded ratios of threo/erythro ≈ 1:1. The experimental data
showed no effect of alkyl chain length on either side on the stereoselectivity of adducts. All synthesized analogues were evaluated for
attractiveness to Rhynchophorus ferrugineus weevils in the field. Five novel derivatives, including two alcohols and three ketones, were
found to attract weevils in the field trials. Among them, 3-methyldecan-4-one (5b) and 4-methyldecan-5-ol (11a) were found to be
the most attractive to the insects.
KEYWORDS: Grignard coupling, Felkin−Anh model, pheromone, microwave irradiation, diastereoselectivity
application of EAG-GC is screening of components released by
insects for semiochemical activity. The third method is field
screening, a simple method that uses plant volatile compounds,
such as essential oils and extracts, to attract a particular insect.6
The disadvantage of this method is understanding which major
components are attractive to the insect. At present, trials of
synthetic analogues of insect pheromones to find new
attractants are rarely performed.
Many insect pheromones possess chiral centres with a
methyl branch.7 Among them, β-methyl alcohols and ketones
bearing chiral centres have been found in the pheromones of
many species, especially in the genus Rhynchophorus (Figure
1). Similar activity between the absolute configuration and
racemic mixture of pheromones has been documented in insect
attraction. For instance, 4-methyl-5-nonanol (10c) and 4-
methyl-5-nonanone (10b) are major components of an
aggregation pheromone of two Asian red palm weevils, R.
ferrugineus and R. vulneratus. The absolute configuration of R.
ferrugineus pheromone is (4S,5S)-10. However, the attractive-
ness of both (4S,5S)-10 and racemic 10 was found to be
similar in trial programmes to control the red palm weevil.8−10
Moreover, a racemic mixture of 9a has also been used to
manage populations of R. bilineatus.11 Similarly, a combination
of a racemic mixture of 4-methyl-3-heptanol (7a) and other
INTRODUCTION
■
Semiochemicals are chemical compounds that are released by
insects for the purpose of communication. Pheromones are a
family of semiochemicals that significantly influence the
survival and social organization of insects.1 The use of insect
pheromones as an alternative to conventional pesticides, which
have negative impacts on the environment and other
organisms, is of particular relevance to integrated pest
management (IPM) strategies.2−4 Several methods can be
used to identify the components of new attractants. The first
method is isolation and extraction of pheromones by solvent
extraction or aeration (volatile collection).5 Volatile collection
depends on the insect species and the number of insects
available. Organic solvent extraction is the most common way
of rapidly obtaining semiochemicals from the pheromone
production glands of insects. However, the samples obtained
can be contaminated with the many organic compounds
present in insect organs. In the aeration model, volatile
compounds that insects release are adsorbed onto polymers
and subsequently desorbed using organic solvents. Airflow is
used to drag the volatile compounds from the insect to the
aeration system. An advantage of this technology is that it
produces samples with low contaminant levels. The semi-
ochemicals are then identified using gas chromatography
coupled with mass spectrometry. The second method is
electroantennography (EAG),5 which uses a bioassay to record
electrophysiologically the olfactory response of insects via their
antennal receptor. When the insect receives a chemical
stimulus, the electrical potential between two electrodes fixed
to the insect’s head or antenna fluctuates. A mixture of active
compounds can be tested for insect response using EAG
coupled with gas chromatography (EAG-GC). An important
Received: April 3, 2021
Revised: May 12, 2021
Accepted: May 12, 2021
Published: May 24, 2021
https://doi.org/10.1021/acs.jafc.1c01875
J. Agric. Food Chem. 2021, 69, 5882−5886
© 2021 American Chemical Society
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Figure 1. Examples for pheromones of β-methyl alcohols and ketones.
Figure 2. Felkin−Anh model for the synthesis of β-methyl-branched alcohols.
components is highly attractive to the smaller European elm
RESULTS AND DISCUSSION
■
bark beetle, Scolytus multistriatus.12 Given the volumes of
To create low-cost pheromones, we prepared an R. ferrugineus
pheromone, 4-methyl-5-nonanol (10), with dominant ratios of
either threo or erythro by application of the Felkin−Anh model,
which requires a nucleophilic attack on the α-methyl carbonyl
group (Figure 2). Attack by different nucleophiles (e.g., a
Grignard reagent or hydride) can generate different diaster-
eomer mixtures of the same alcohol structures. Our previous
report described the different racemic mixtures of compound
(10) produced using Grignard coupling between 2-methyl-
pentanal and n-butylmagnesium bromide to obtain a dominant
amount of threo isomer (threo/erythro = 1.67:1.0) and a
mixture of (threo/erythro = 1.0:1.0) obtained from n-pentanal
and pent-2-ylmagnesium bromide.22 A dominant erythro
isomer is required in the synthesis of this pheromone, and
reduction of α-methyl ketone (10b) using LiAlH4 as a
nucleophilic resource was expected to achieve the appropriate
mixture. Unfortunately, it was somewhat surprising that
comparatively low diastereoselectivity (10c, threo/erythro =
1.0:0.98) was found for this addition. The synthesis of the
other substrate, 4-methyl-3-heptanol (7c), yielded a ratio of
threo/erythro = 1:1, as reported in the literature.12,23 Therefore,
we decided to study the influence of steric alkyl chain length
with respect to the diastereoselectivity of the Felkin−Anh
addition. In particular, the analogues of the β-methyl
derivatives synthesized were used to investigate attractiveness
to R. ferrugineus in field trials.
pheromones needed for large-scale trapping of insects, racemic
synthesis should be considered as a cost-effective solution.
In the diastereoselective synthesis of β-methyl alcohols,
additions of an organometallic reagent or a hydride source to a
ketone or an aldehyde with an existing α-chiral centre to its
carbonyl group reflect Cram’s rule or the preferred Felkin−
Anh (FA) model. In this model, both syn- and anti-
diastereomeric adducts can be generated, and the predominant
form can be predicted, as illustrated in Figure 2. It is well
known that the addition of an organometallic reagent into the
carbonyl group of α-methyl aldehydes forms the syn-isomer
(FA) as a major product, while the attack of the carbonyl
group of α-methyl ketone by hydride ions generates a favored
anti-isomer (AF).13 Diastereoselective synthesis of β-methyl
alcohols via the Felkin−Anh model is controllable by one of
two factors.14,15 The first factor is the bulky effect of the large
group (L) and the organometallic reagent (or the R group).
Stereoselectivity generally increases in the order of alkyl group
size, i.e., Me < Et < i-Pr < t-Bu. The second factor is the polar
effect that stabilizes a transition state with maximum separation
between the nucleophile (Nu−) and a large group (L).
Although the stereoselective synthesis in the Felkin−Anh
model has been explored extensively,16−21 to the best of our
knowledge, the influence of steric alkyl chain length has not
been studied for diastereoselectivity. Here, we investigate the
effect of alkyl chain lengths on the diastereoselective synthesis
of β-methyl alcohols via the Felkin−Anh model. Oxidation of
the secondary alcohols using 2-iodoxybenzoic acid (IBX)
under microwave-assisted conditions was applied to the α-
methyl ketones. All synthetic alcohol and ketone compounds
were screened to find new attractants for R. ferrugineus in field
trials.
In the first pathway described, 1-bromoalkanes were utilized
to prepare the Grignard reagents as nucleophile sources in the
addition reaction with racemic α-methyl aldehydes to form the
secondary alcohols 1a−15a, which were expected to afford
diastereomers with a dominant quantity of threo isomers
according to the Felkin−Anh model. The ratios of diaster-
eomers were determined by nuclear magnetic resonance
analysis. The peak relating to the proton at C−OH of the
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threo isomer appears as a multiplet at δ ∼ 3.50 (CDCl3), while
a multiplet peak corresponding to the erythro isomer is at δ ∼
3.45 (CDCl3).10,11,22 The results show that the β-methyl
alcohols generated from the addition of a nucleophile to the α-
methyl aldehydes possess a ratio of threo/erythro ≈ 2:1 (Table
1). However, it is noteworthy that an increase in the alkyl
reagent has no effect on the diastereoselectivity of the Felkin−
Anh adduct. It is also surprising that a slight increase in the
steric bulk of the group of aldehyde substrates (n = 1 and 2;
entries 1−14) also induces poor selectivity of diastereomers. A
similar observation for the synthesis of other compounds has
been found in previous studies.24,25 To lead to a clear
conclusion, we set out to prepare a product with long alkyl
chains by coupling 2-methylheptanal (n = 4), which was
synthesized from commercial 2-methylheptanoic acid, in three
steps (Supplementary Information), and n-octylmagnesium
bromide (m = 7) to produce 15a. Also, 1H-NMR data showed
that this compound possesses a ratio of threo/erythro =
1.79:1.0.
Table 1. Percentage of Diastereomers and Yields for the
Synthesis of β-Methyl Alcohols
For the preparation of α-methyl ketones, oxidation of
alcohols using IBX as an oxidant on heating with or without
solvents revealed a facile means of preparing the required
aldehydes or ketones. To enhance the oxidation yields and
reduce the reaction time, the α-methyl ketones needed for the
present work were produced by microwave-assisted oxidation
using IBX. In the search for a suitable procedure for oxidation,
10a was submitted to a range of conditions with several
different solvents. Oxidation in each solvent was carried out by
heating, coupled with stirring and microwave irradiation
(Table S1). At the first attempt, the oxidation of 10a was
explored in the presence of β-CD in water/acetone (86:14) or
nonsolvent;26,27 however, unfortunately, the oxidation was
unsuccessful (entries 1 and 2 in Table S1). For synthesis in
common solvents, including acetone, dimethylformamide,
acetonitrile, dimethyl sulfoxide, tetrahydrofuran, and chloro-
form, microwave irradiation showed higher yields compared to
heating. Among the selected solvents, the reaction in
acetonitrile with 1.5 molar equivalents of IBX exhibited the
highest isolated yield of 70% under microwave irradiation for
20 min.
threo/
yield
b
a
entry product
n
m
erythro
(%)
boiling point
1
1a
1
1
1
1
1
1
1
2
2
2
2
2
2
2
4
1
2
3
4
5
6
7
1
2
3
4
5
6
7
7
1.71:1.0
1.80:1.0
1.61:1.0
1.61:1.0
1.69:1.0
1.89:1.0
2.01:1.0
1.74:1.0
2.14:1.0
1.74:1.0
1.96:1.0
1.87:1.0
1.74:1.0
1.82:1.0
1.79:1.0
84.2
85.6
83.7
86.6
88.8
95.4
90.9
87.9
86.5
94.0
93.4
90.6
93.3
92.4
95.8
75 °C/60 mmHg
85 °C/60 mmHg
91 °C/60 mmHg
62 °C/6 mmHg
82 °C/5 mmHg
102 °C/5 mmHg
128 °C/5 mmHg
87 °C/60 mmHg
90 °C/60 mmHg
99 °C/30 mmHg
155 °C/80 mmHg
110 °C/5 mmHg
130 °C/5 mmHg
140 °C/2 mmHg
2
2a
3
3a
4
4a
5
5a
6
6a
7
7a
8
8a
9
9a
c
10
11
12
13
14
15
10a
11a
12a
13a
14a
15a
209 °C/7 mmHg
a
b
Percent of diastereomers estimated by 1H-NMR data. Isolated
yields of pure products. Reported in the literature.21
c
Rapid microwave-assisted oxidation was explored for
substrates of the secondary alcohols 1a−15a (Table 2)
under the same reaction conditions. Good yields of the
respective ketones 1b−15b were obtained in all cases.
chain length of the nucleophile reagents leads to an
insignificant change in the ratio of the diastereomers obtained,
showing that the alkyl chain length of the alkyl nucleophile
Table 2. Microwave-Assisted Synthesis of α-Methyl Ketones
a
no.
com.
n
m
products
yield (%)
boiling point
1
2
3
4
5
6
7
8
1a
2a
3a
4a
5a
6a
7a
8a
1
1
1
1
1
1
1
2
2
2
2
2
2
2
4
1
2
3
4
5
6
7
1
2
3
4
5
6
7
7
1b
2b
3b
4b
5b
6b
7b
8b
56
54
60
65
60
70
73
64
63
75
73
74
80
82
85
60 °C/30 mmHg
75 °C/30 mmHg
80 °C/30 mmHg
92 °C/30 mmHg
75 °C/5 mmHg
90 °C/5 mmHg
110 °C/5 mmHg
74 °C/40 mmHg
80 °C/40 mmHg
85 °C/13 mmHg
85 °C/5 mmHg
90 °C/5 mmHg
112 °C/5 mmHg
120 °C/2 mmHg
132 °C/2 mmHg
9
9a
9b
10
11
12
13
14
15
10a
11a
12a
13a
14a
15a
10b
11b
12b
13b
14b
15b
a
Isolated yield of pure products.
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Substrates with a low length of alkyl chains (1b, 2b, 8b, and
9b) showed slightly lower isolated yields compared to those
with a long chain length; this could be due to the high
evaporation of ketones with a low molecular weight.
The effectiveness of the synthesized compounds is shown in
Figure 3A. In traps baited with a pheromone, which was
In the next step, reduction of the corresponding α-methyl
1
ketones (1b−15b) using LiAlH4 was carried out. The H
NMR analysis showed that all the alcohols obtained (1c−15c)
possessed a mixture of threo/erythro ≈ 1:1 even with the
reduction of the longest ketone (15b) (Table 3); this shows
Table 3. Percentage of Diastereomers and Yields for the
Synthesis of β-Methyl-Branched Alcohols
Figure 3. Means ( SE) of R. ferrugineus weevils caught in traps baited
with (A) five synthesized compounds and (B) components of
compound 11. Different letters indicate significant differences at p <
0.05. The asterisk indicates significant differences at p < 0.01.
threo/
yield
b
a
entry product
n
m
erythro
(%)
boiling point
blended between components 10c and 10b in the ratio of 10:1
1
2
3
4
5
6
7
8
1c
2c
3c
4c
5c
6c
7c
8c
1
1
1
1
1
1
1
2
2
2
2
2
2
2
4
1
2
3
4
5
6
7
1
2
3
4
5
6
7
7
1.0:1.06
1.0:0.94
1.0:1.06
1.0:1.00
1.0:1.03
1.0:1.02
1.0:0.95
1.0:0.97
1.0:1.03
1.0:0.98
1.0:1.05
1.0:1.09
1.0:1.03
1.0:1.05
1.0:1.07
95.6
96.4
98.2
96.7
95.7
97.4
97.0
95.4
96.8
95.5
98.2
97.3
98.2
97.6
98.4
80 °C/72 mmHg
78 °C/50 mmHg
85 °C/48 mmHg
70 °C/10 mmHg
90 °C/7 mmHg
96 °C/4 mmHg
120 °C/4 mmHg
82 °C/50 mmHg
86 °C/50 mmHg
110 °C/40 mmHg
135 °C/40 mmHg
105 °C/4 mmHg
133 °C/5 mmHg
148 °C/3 mmHg
(v/v), 2.2
0.44 weevils were captured per three traps per
week. The attractiveness of the pheromone was observed in the
same way as in a previous report.8 Compounds 4b, 4c, and 11b
elicited weak responses by the weevils, with the number of
weevils captured lower than that for both pheromone
components and the blended pheromone mixture. Alcohol
11c exhibited slightly higher attractiveness than its pheromone
components, although the number of weevils captured was
lower than that for the blended pheromone. It is noteworthy
that the number of weevils captured by ketone 5b (3.6 0.54
weevils per three traps per week) was significantly greater than
that captured by the others, including its pheromone. As a
result, this derivative, 3-methyldecan-4-one (5b), is deter-
mined to be a new attractant for RFW.
Interestingly, the structures of analogues 11b and 11c were
very similar to their pheromone components (10b and 10c).
Thus, we conducted field trials for diastereomer 11a (threo/
erythro ≈ 2:1) and blending mixtures of (11b and 11a) and
(11b and 11c). The results are illustrated in Figure 3B.
Statistically significant effectiveness in weevil responses was
observed in traps using 11a (threo/erythro ≈ 2:1) compared to
the others. The number of weevils captured with 11a was 3.8
0.45 weevils per three traps per week, which is much more
than the pheromone, while the blending of ketone 11b into the
relative alcohols induced inhibition of the bioactivity of these
alcohols. The high attractiveness of 11a indicates the
importance of various β-methyl alcohol diastereomers in
enhancing the attraction of insects.
9
9c
10
11
12
13
14
15
10c
11c
12c
13c
14c
15c
195 °C/4 mmHg
a
b
Percent of diastereomers estimated by 1H-NMR data. Isolated
yields of pure products.
the independence of alkyl chain length in both sides of the
substrates for the stereoselectivity of the Felkin−Anh adduct.
Even though no conclusion could be drawn on the effect of
alkyl chain length on the diastereoselectivity, similar results
have been observed for short alkyl chains (e.g., methyl and
ethyl) in the other substrates,14,28 suggesting that the bulk
effects of the adjacent carbons of two stereocenters are a
prerequisite, but alkyl chain length has no effect in
diastereoselective addition in the Felkin−Anh model.
In conclusion, a straightforward, effective, and low-cost
synthetic strategy has been uncovered for pheromones based
on α-methyl alcohols, which have a particular value in large-
scale application of pheromones in IPM. The β-methyl
alcohols with two different diastereomer ratios (threo/erythro
= 2:1 and 1:1) were prepared via the addition of nucleophiles
into an α-methyl carbonyl group. No effect of alkyl chain
length on the stereoselectivity of the Felkin−Anh adduct was
demonstrated in the synthesis of β-methyl alcohols. The α-
methyl ketones were synthesized simply by the oxidation of the
respective alcohols using IBX under microwave-assisted
conditions. Five compounds were found to attract R.
ferrugineus weevils in screening in field trials. Among them,
compounds 5b and 11a possessed a higher bioactivity than
their pheromones. This finding can enable the development of
a novel method of finding new attractants of the insect.
Field Trials of R. ferrugineus Attractiveness. To find
new attractants for R. ferrugineus weevils (RFW), all the
compounds synthesized, including alcohols (1c−15c) and
ketones (1b−15b), were used to evaluate attractiveness in the
field. Pitfall traps have been shown to be effective in capturing
the weevils in field trials in previous reports29,30 and were
utilized in this work. A block of three traps with 10 mg of the
corresponding compound baited in each one was put in a
coconut field and left for 1 week. The number of captured
weevils was noted weekly, and the experiments were replicated
five times. The results showed that weevils responded to seven
chemicals, including two pheromone components (10c and
10b) with 1.4 0.45 captured weevils per three traps per week
and five novel attractants (Table S2). These potential
attractants consisted of two alcohols (4c and 11c) and three
ketones (4b, 5b, and 11b).
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jafc.1c01875.
■
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AUTHOR INFORMATION
Corresponding Authors
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Chi-Hien Dang − Graduate University of Science and
Technology, Vietnam Academy of Science and Technology,
Cau Giay, Hanoi 100000, Vietnam; Institute of Chemical
Technology, Vietnam Academy of Science and Technology,
Ho Chi Minh City 70000, Vietnam; Email: dangchihien@
gmail.com
Thanh-Danh Nguyen − Graduate University of Science and
Technology, Vietnam Academy of Science and Technology,
Cau Giay, Hanoi 100000, Vietnam; Institute of Chemical
Technology, Vietnam Academy of Science and Technology,
Ho Chi Minh City 70000, Vietnam; orcid.org/0000-
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Van-Dung Le − Graduate University of Science and
Technology, Vietnam Academy of Science and Technology,
Cau Giay, Hanoi 100000, Vietnam; Institute of Chemical
Technology, Vietnam Academy of Science and Technology,
Ho Chi Minh City 70000, Vietnam
Cong-Hao Nguyen − Graduate University of Science and
Technology, Vietnam Academy of Science and Technology,
Cau Giay, Hanoi 100000, Vietnam; Institute of Chemical
Technology, Vietnam Academy of Science and Technology,
Ho Chi Minh City 70000, Vietnam
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Hong-Ung Nguyen − Department of Agriculture and
Aquaculture, Tra Vinh University, Tra Vinh City 87000,
Vietnam
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jafc.1c01875
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Notes
The authors declare no competing financial interest.
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