Ultrasound assisted synthesis of 5,9-dimethylpentadecane and 5,9-dimethylhexadecane - The sex pheromones of Leucoptera coffeella

Article abstract of DOI:10.3390/12082080

Racemic 5,9-dimethylpentadecane and 5,9-dimethylhexadecane, the major and minor constituents, respectively, of the sex pheromone of Leucoptera coffeella, have been synthesized from citronellol in 56-58% overall yield through six steps. Ultrasound irradiation efficiently supported tosylation of alcohols (two steps) as well as the subsequent cross coupling reactions with the pertinent Grignard reagents (also two steps).

Full text of DOI:10.3390/12082080

Molecules 2007, 12, 2080- 2088
molecules
ISSN 1420-3049
© 2007 by MDPI
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Full Paper
Ultrasound Assisted Synthesis of 5,9-Dimethylpentadecane and
5,9-Dimethylhexadecane – the Sex Pheromones of Leucoptera
coffeella
Nhuan Ngoc Doan 1,§, Thach Ngoc Le 2, Hao Cong Nguyen 3, Poul Erik Hansen 1 and
Fritz Duus 1,*
1 Department of Science, Systems and Models, Roskilde University, P. O. Box 260, DK-4000
Roskilde, Denmark; E-mails: dnnhuan@hcmuns.edu.vn, poulerik@ruc.dk
2 Department of Organic Chemistry, University of Natural Sciences, National University of
Ho Chi Minh City, 227 Nguyen Van Cu, Ho Chi Minh City, Vietnam; E-mail: lenthach@hcm.vnn.vn
3 Institute of Natural Products Chemistry of Ho Chi Minh City, 01 Mac Dinh Chi, Ho Chi Minh City,
Vietnam; E-mail: nguyenconghao@yahoo.com
§
On leave from the University of Natural Sciences, National University of Ho Chi Minh City,
Vietnam.
* Author to whom correspondence should be addressed; E-mail: fd@ruc.dk.
Received: 8 August 2007; in revised form: 23 August 2007 / Accepted: 23 August 2007 / Published: 28
August 2007
Abstract: Racemic 5,9-dimethylpentadecane and 5,9-dimethylhexadecane, the major and
minor constituents, respectively, of the sex pheromone of Leucoptera coffeella, have been
synthesized from citronellol in 56-58% overall yield through six steps. Ultrasound
irradiation efficiently supported tosylation of alcohols (two steps) as well as the subsequent
cross coupling reactions with the pertinent Grignard reagents (also two steps).
Keywords: 5,9-Dimethylpentadecane; 5,9-Dimethylhexadecane; Ultrasound irradiation;
Pheromones; Leucoptera coffeella.

Molecules 2007, 12
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Introduction
Recent research has shown that several hydrocarbons with a 1,5-dimethyl skeleton are insect
pheromones [1]. 5,9-Dimethylpentadecane (1) and 5,9-dimethylhexadecane (2) are known as the major
and minor constituents, respectively, of the sex pheromone of Leucoptera coffeella, a pest of coffee
trees [2]. Several ways of synthesizing 1 have been described. Stereoisomers of 1 were synthesized
starting from (-)-isopulegol by Moreira and Correa [3] and from methyl (R)- and (S)-3-hydroxy-2-
methylpropanoate by Kuwahara and co-workers [4]. Zarbin et al. carried out an unsymmetrical double
Wittig olefination as the key reaction to obtain also a stereoisomeric mixture of 1 in a yield of 54% [5].
Using citronellol as the starting material, Liang et al. synthesized stereoisomeric mixtures of 1 as well
as of 2 in yields of about 25% [6]. Using (R)-citronellol as the starting material, Poppe et al.
synthesized 1 as a mixture of the (5S,9S) and (5R,9S) stereoisomeric molecules in a related way in an
overall yield of 22% [7]. However, the stereochemistry of the natural pheromone is still undetermined
[5]. Zarbin et al. [5] have tested the biological activity of a synthetic stereoisomeric mixture of 1. Their
field experiments showed, that it was highly attractive to L. coffeella males [5].
Ultrasonic irradiation has been found useful as support for quite a few organic reactions [8]. In this
paper, we describe a new six-step synthetic route to 1 and 2 (as a mixture of stereoisomers) from
citronellol (3) (see Scheme 1) in which ultrasound irradiation was used to support the tosylation and
cross coupling reactions, which were used repeatedly (four out of six steps, see Scheme 1).
Scheme 1: Synthesis of 5,9-dimethylpentadecane (1) and 5,9-dimethylhexadecane (2).
[O]
(iii)
RMgBr
(ii)
TsCl
(i)
CH2OH
CH2OTs
R
R
O
3
4
5a, 5b
6a, 6b
Me
CHMgBr
(vi)
NaBH4
(iv)
TsCl
(v)
Bu
R
R
R
OH
7a, 7b
OTs
8a, 8b
1, 2
a: R = n-C4H9; b: R = n-C5H11; 1: R = n-C4H9; 2: R = n-C5H11.
(i): Tosyl chloride, pyridine, CHCl3, ultrasound; (ii): n-C4H9MgBr or n-C5H11MgBr, Li2CuCl4,
o
THF, -78 C, ultrasound; (iii): perphthalic acid, then HIO4, THF; (iv): NaBH4, MeOH; (v): the
same as (i); (vi): BuMeCHMgBr, Li2CuCl4, THF, -78 oC, ultrasound.

Molecules 2007, 12
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Results and Discussion
Citronellol (3), an easily obtainable starting material [9], was tosylated to give the tosylate 4,
which subsequently was coupled with either n-butylmagnesium bromide or n-pentylmagnesium
bromide to give the alkenes 5a and 5b, respectively. Upon oxidation, 5a and 5b gave the aldehydes 6a
and 6b, respectively, which in turn were reduced into the corresponding alcohols 7a and 7b by NaBH4.
Repeated tosylations and cross coupling reactions converted the alcohols 7a and 7b into the
hydrocarbons 1 and 2 via the tosylates 8a and 8b (Scheme 1).
The preparation of the alkyl tosylates 4, 8a and 8b were carried out following a Varma-like
procedure [10]. According to Kabalka et al. [10], alkyl tosylates can be prepared from alcohols and
p-toluenesulfonyl chloride, in the presence of pyridine, by stirring the reactants together in chloroform
o
solution for 2.5-9.0 hours at 0 C. In our procedure, the reactions were supported by ultrasound
irradiation from an ultrasonic processor (steps i and v). The results of a series of experiments are
shown in Tables 1 and 2.
Table 1: Preparation of the tosylate 4 under ultrasound irradiation.
3 : TsCl : pyridine
Entry
Time (minutes)
Yield of 4 (%)
(molar ratios)
1:1:1
1
2
30
60
30
60
60
30
20
40
30
20
30
82
83
86
84
80
91
84
92
92
86
92
1:1:1
3
1:1:2
4
1:1:2
5
1:1.5:1
1:1.5:2
1:1.5:2
1:1.5:2
1:1.5:3
1:2:2
6
7
8
9
10
11
1:2:2
Table 2: Preparation of the tosylates 8a and 8b under ultrasound irradiation.
Entry
Tosylate
8a
Time (minutes)
Yield (%)a
1
2
3
4
5
6
30
45
60
30
45
60
74
92
93
78
91
91
8a
8a
8b
8b
8b
a 7a, 7b : TsCl : pyridine = 1.0 : 1.5 : 2.0 (molar ratio).

Molecules 2007, 12
2083
p-Toluenesulfonyl chloride was added to the reaction solutions within a few minutes, i.e. not over
an extended period, as described by Kabalka’s group [10], and with the assistance of ultrasound
irradiation, the reactions occurred quickly (within 30-45 minutes) to give the tosylates in good yields
(Tables 1 and 2).
In 1974 Fouquet and Schlosser [11] reported the formation of a C-C single bonds by a cross
coupling reaction of alkyl tosylates with pertinent Grignard reagents in the presence of lithium
o
tetrachlorocuprate at -78 C after overnight stirring. Adopting the reaction conditions of Fouquet and
Schlosser, but using ultrasound to promote the reactivity, we were able to perform the cross coupling
reactions between the tosylates and the Grignard reagents (steps ii and vi) within 30-45 minutes, while
still obtaining the products in high yields (Table 3).
Table 3: Ultrasound assisted cross coupling reactions between tosylates and Grignard reagents.
Entry
Product
Time (minutes)
Yield (%)a
1
2
3
4
5
6
5a
5a
5a
5b
1
20
30
45
30
45
45
80
95
95
94
93
91
2
The aldehydes 6a and 6b can be prepared from the alkenes 5a and 5b by ozonolysis [12]. In this
work, the alkenes 5a and 5b were oxidized by perphthalic acid, then periodic acid [13], to give 6a and
6b, in of 81-82% yields, respectively, The alcohols 7a and 7b were easily prepared from the aldehydes
6a and 6b in yields of 96-97% by reduction of the latter with sodium borohydride in methanol for 15
minutes.
Conclusions
5,9-Dimethylpentadecane was prepared from citronellol in the excellent overall yield of 58%
through six steps. By the same procedure, 5,9-dimethylhexadecane was prepared in the overall yield of
56%. The use of ultrasound irradiation combined with the use of optimized repetitive synthetic steps
(Scheme 1) offer a convenient method for the synthesis of a variety of related branched hydrocarbons.
It should be noticed that also the alkyl groups of the Grignard reagent in steps (ii) and (vi) can be
varied.
Acknowledgement
We thank DANIDA for financial support via the ENRECA programme.

Molecules 2007, 12
2084
Experimental
General
13
1H- and C-NMR spectra were recorded in CDCl3 with TMS as internal reference standard at
frequencies 300 MHz and 75 MHz, respectively, on a Varian Mercury 300 NMR spectrometer, or
alternatively, at frequencies 600 MHz and 150 MHz, respectively, on a Varian Inova 600 NMR
spectrometer. GC-MS analyses were performed on a Hewlett Packard 5890 Series II apparatus with a
Hewlett Packard 5791A MS detector (column: RTX-5MS, 30 m, 0.25 mm, 0.25 µm). A Sonics GE130
ultrasonic processor was used both in the tosylation and the cross coupling reactions. All reagents were
purchased from Aldrich, while solvents were from Labscan. The chloroform used in the tosylation
reactions was filtered through a plug of alumina to remove ethanol (present as stabilizer) [10].
General procedure for the tosylation of alcohols: 3,7-dimethyl-6-octenyl 4-methylbenzenesulfonate (4)
A mixture of citronellol (6.24 g, 40 mmol), pyridine (6.32 g, 80 mmol), and chloroform (20 mL)
was placed in a 3-neck round flask and cooled to 0 oC in an ice bath. p-Toluenesulfonyl chloride (11.4
g, 60 mmol) was added to the mixture within a few minutes under irradiation from an ultrasonic
processor. The reaction was finished within 30 minutes. After adding water (10 mL), the resulting
mixture was extracted with diethyl ether (70 mL). The ethereal layer was washed with 2 M aqueous
HCl, aqueous NaHCO3, and brine. The solvent was removed by evaporation at reduced pressure, and
the residue was chromatographed on a column (24 x 400 mm, 60 g of silica gel, eluent: 2% diethyl
ether in hexane) to give 11.46 g (91%) of the tosylate 4. MS (m/e): 310 (M+); 227; 185; 173; 155; 138;
1
123; 91 (100%); 81; 69; 55; 41; H-NMR (300 MHz) δ: 0.82 (d, J = 6.3 Hz, 3H), 1.10-1.65 (m, 5H),
1.57 (s, 3H), 1.67 (d, J = 1.5 Hz, 3H), 1.82-1.94 (m, 2H), 2.45 (s, 3H), 4.06-4.07 (m, 2H), 5.00-5.06
(m, 1H), 7.34 (d, J = 8.3 Hz, 2H), 7.79 (d, J = 8.3 Hz, 2H); 13C-NMR (75 MHz) δ: 17.63, 19.05, 21.63,
25.27, 25.70, 28.88, 35.66, 36.71, 69.06, 124.33, 127.89 (2C), 129.82 (2C), 131.48, 133.26, 144.66.
General procedure for the cross coupling reactions: 2,6-dimethyl-2-dodecene (5a)
n-Butylmagnesium bromide was prepared from 1-bromobutane (12.33 g, 90 mmol) and
magnesium turnings (2.16 g, 90 mmol) in dry diethyl ether (30 mL) during 30 minutes with assistance
of an ultrasonic bath (Branson 1210) at room temperature. The slurry formed was added dropwise to a
solution of the tosylate 4 (9.3 g, 30 mmol) in THF (25 mL) at -78 oC. A solution of 0.1 M Li2CuCl4 in
o
THF (3.0 mL) was added to the mixture under ultrasonic irradiation. The mixture was kept at -78 C
for 10 minutes, after which the temperature was raised slowly to room temperature. The reaction was
completed after ultrasound irradiation for a total of 30-45 minutes after the addition of the Li2CuCl4
solution. The obtained mixture was poured into a saturated aqueous solution of NH4Cl containing
crushed ice, which was subsequently extracted with diethyl ether (3x50 mL). The combined organic
layers were washed with saturated aqueous NaHCO3, with brine, and then dried (anhydrous Na2SO4).
After removal of the solvent by evaporation at reduced pressure, the crude product was purified by
column chromatography (24 x 400 mm, 30 g of silica gel, eluent: n-hexane) to give 5.58 g of 5a

Molecules 2007, 12
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(95%). MS (m/e): 196 (M+, 14), 126, 111, 98, 83, 69 (100%), 56, 41; 1H-NMR (300 MHz) δ: 0.84-0.88
(m, 6H), 1.06-1.16 (m, 2H), 1.22-1.44 (m, 11H), 1.60 (s, 3H), 1.68 (d, J = 1.2 Hz, 3H), 1.93-1.99 (m,
2H), 5.08-5.13 (m, 1H); 13C-NMR (75 MHz) δ: 14.34, 17.83, 19.84, 22.94, 25.83, 25.93, 27.24, 29.93,
32.20, 32.66, 37.23, 37.40, 125.36, 131.13.
2,6-Dimethyl-2-tridecene (5b): By the same procedure as described for the cross coupling reaction
above, n-pentylmagnesium bromide, prepared from 1-bromopentane (9.06 g, 20 mmol) and
magnesium turnings (1.44 g, 60 mmol) in 30 mL of dry diethyl ether, was reacted with the tosylate 4
(6.20 g, 20 mmol) to yield 3.95 g (94%) of 5b. MS (m/e): 210 (M+, 36), 182, 140, 125, 111, 98, 83, 69
1
(100%), 56, 41; H-NMR (300 MHz) δ: 0.85-0.91 (m, 6H), 1.05-1.16 (m, 2H), 1.26-1.42 (m, 13H),
13
1.60 (s, 3H), 1.68 (d, J = 1.2 Hz, 3H), 1.88-2.04 (m, 2H), 5.04-5.14 (m, 1H); C-NMR (75 MHz) δ:
14.14, 17.63, 19.64, 22.73, 25.63, 25.74, 27.09, 29.44, 32.03, 31.98, 32.47, 37.03, 37.19, 125.15,
130.92.
General procedure for oxidation: 4-methyldecanal (6a)
A 0.6 M solution of perphthalic acid (70 mL, 42 mmol) in diethyl ether was added dropwise to a
o
solution of 5a (5.5 g, 28 mmol) in dry diethyl ether (10 mL) at 0 C, after which the mixture was
placed in a refrigerator overnight. The reaction mixture was washed with aqueous NaHCO3, and dried
(anhydrous Na2SO4). Then the solvent was removed by evaporation at reduced pressure to leave crude
intermediate 2,6-dimethyl-2-dodecene epoxide, which was dissolved in diethyl ether (20 mL) and
added dropwise to a stirred solution of HIO4 (6.84 g, 30 mmol) in THF (30 mL) during 1.5 hours.
Removal of the solvents at reduced pressure gave crude 6a, which was purified by column
chromatography (24 x 400 mm, 30 g of silica gel, eluent: 5% diethyl ether in hexane), to yield 3.85 g
1
(81%) of pure 6a as a light yellow oil. MS (m/e): 155, 142, 126, 111, 95, 85, 69, 56 (100%), 41; H-
NMR: (300 MHz) δ: 0.84-0.90 (m, 6H), 1.20-1.32 (m, 10H), 1.40-1.47 (m, 2H), 1.60-1.66 (m, 1H),
2.39-2.46 (m, 2H), 9.77 (t, J = 1.8 Hz, 1H); 13C-NMR (75 MHz) δ: 14.30, 19.56, 22.87, 27.11, 29.11,
29.77, 32.09, 32.59, 36.89, 41.93, 203.30.
4-Methylundecanal (6b): In the same manner as described above for the preparation of 6a, we
obtained 2.80 g (82%) of 6b from 3.90 g of the alkene 5b. MS (m/e): 169, 156, 140, 125, 112, 95, 85,
69, 56 (100%), 41; 1H-NMR: (300 MHz) δ: 0.86-0.90 (m, 6H), 1.20-1.34 (m, 12H), 1.37-1.50 (m, 2H),
13
1.60-1.70 (m, 1H), 2.39-2.46 (m, 2H), 9.77 (t, J = 1.8 Hz, 1H); C-NMR (75 MHz) δ: 14.31, 19.57,
22.89, 27.16, 29.11, 29.55, 30.08, 32.10, 32.60, 36.90, 41.93, 203.28.
General procedure for reduction: 4-methyl-1-decanol (7a)
A solution of sodium borohydride (0.26 g, 6.8 mmol) in methanol (7 mL) was added dropwise to
3.80 g (22 mmol) of 6a under stirring during 15 minutes at room temperature. The reaction mixture
was quenched with 1.0 M aqueous HCl (4 mL), and the resulting mixture was extracted with diethyl
ether (3 x 15 mL). The combined organic layers were washed successively with aqueous NaHCO3 and
brine, and then dried (anhydrous Na2SO4). After removal of the solvent by evaporation at low pressure,

Molecules 2007, 12
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the residue was purified by column chromatography (24 x 400 mm, 30 g of silica gel, eluent: 10%
diethyl ether in hexane) to give 3.69 g (96%) of 7a. MS (m/e): 126, 111, 98, 84, 69 (100%), 56, 41; 1H-
NMR (300 MHz) δ: 0.86-0.90 (m, 6H), 1.09-1.42 (m, 13H), 1.52-1.61 (m, 2H), 1.80 (s, br, 1H), 3.63
13
(t, J = 6.6 Hz, 2H); C-NMR (75 MHz) δ: 14.19, 19.72, 22.82, 27.15, 29.81, 30.40, 32.08, 32.79,
33.15, 37.16, 63.20.
4-Methyl-1-undecanol (7b): In the same manner as described above for the preparation of 7a, we
obtained 2.74 g (97%) of 7b from 2.80 g of the aldehyde 6b. MS (m/e): 140, 125, 112, 97, 84, 69
1
(100%), 56, 41; H-NMR (300 MHz) δ: 0.86-0.90 (m, 6H), 1.11-1.40 (m, 15H), 1.52-1.63 (m, 2H),
13
1.78 (s, br, 1H), 3.62 (t, J = 6.6 Hz, 2H); C-NMR (75 MHz) δ: 14.12, 19.65, 22.71, 27.07, 29.40,
29.99, 30.36, 31.94, 32.65, 32.97, 37.01, 63.45.
4-Methyl-1-decanyl 4-methylbenzenesulfonate (8a) and 4-methyl-1-undecanyl 4-methylbenzene-
sulfonate (8b): The tosylates 8a and 8b were prepared following the general procedure described
above for the preparation of the tosylate 4. We obtained 6.29 g (92%) of 8a from 3.61 g of 7a. Data of
8a: MS (m/e): 207 (10), 173 (74), 154 (39), 126 (49), 111 (34), 97 (36), 91 (100), 84 (46), 69 (91), 56
(65), 41 (61);1H-NMR: (300 MHz) δ: 0.80 (d, J = 6.6 Hz, 3H), 0.86-0.90 (m, 3H), 1.01-1.35 (m, 13H),
1.56-1.72 (m, 2H), 2.45 (s, 3H), 4.01 (t, J = 6.6 Hz, 2H), 7.36 (d, J = 8.4 Hz, 2H), 7.79 (d, J = 8.4 Hz,
13
2H); C-NMR: (75 MHz) δ: 14.12, 19.41, 21.63, 22.68, 26.48, 26.90, 29.60, 31.91, 32.28, 32.49,
36.78, 71.12, 127.90 (2C), 129.80 (2C), 133.33, 144.62. Analogously, 4.49 g (91%) of 8b was
obtained from 2.70 g of 7b. Data of 8b: MS (m/e): 207 (3), 173 (100), 155, 140, 125, 111, 91, 83, 69,
1
56, 43, 41; H-NMR: (300 MHz) δ: 0.80 (d, J = 6.3 Hz, 3H), 0.88 (t, J = 6.6 Hz, 3H), 1.07-1.32 (m,
15H), 1.56-1.72 (m, 2H), 2.45 (s, 3H), 4.01 (t, J = 6.6 Hz, 2H), 7.34 (d, J = 8.3 Hz, 2H), 7.79 (d, J =
13
8.3 Hz, 2H); C-NMR: (75 MHz) δ: 14.06, 19.35, 21.58, 22.69, 26.48, 26.95, 29.35, 29.90, 31.91,
32.27, 32.49, 36.77, 71.11, 127.88 (2C), 129.79 (2C), 133.31, 144.60.
5,9-Dimethylpentadecane (1): 2-Hexylmagnesium bromide was prepared by reaction of 4.80 g (29
mmol) of 2-bromohexane [14] with magnesium turnings (0.77 g, 32 mmol) in dried diethyl ether (30
mL) for 2 hours. In the same manner as described for the preparation of the alkene 5a (see general
procedure above), we obtained 1, albeit together with a very small amount of 5,6-dimethyldecane.
After removal of 5,6-dimethyldecane by vacuum distillation (65-68 oC/6 mbar), 2.15 g (93%) of pure 1
could be isolated (from 3.10 g (9.5 mmol) of (8a). MS (m/e): 240 (M+, 5), 225 (4), 211 (2), 197 (1),
1
183 (21), 155 (32), 127 (10), 112 (35), 99 (22), 85 (77), 71 (93), 57 (100), 43 (58); H-NMR: (600
MHz) δ: 0.84 (t, J = 6.6 Hz, 3H), 0.84 (t, J = 6.6 Hz, 3H) [15], 0.87-0.90 (m, 6H), 1.02-1.11 (m, 4H),
13
1.16-1.32 (m, 18H), 1.33-1.39 (m, 2H); C-NMR: (150 MHz) δ: 14.14, 14.19, 19.72, 19.78, 22.73,
23.09, 24.50, 27.09 (27.10) [16], 29.38 (29.40) [16], 29.74, 32.01, 32.77 (32.79) [16], 32.80 (32.82)
[16], 36.79 (36.89) [16], 37.12 (37.21) [16], 37.43, 37.48.
5,9-Dimethylhexadecane (2): In the same manner as described above for the preparation of 1, 1.43
g (91%) of 2 was obtained from 2.10 g (6.5 mmol) of 8b. MS (m/e): 254 (M+, 5), 239 (5), 211 (2), 197
(31), 169 (10), 155 (35), 126 (37), 99 (22), 85 (97), 71 (90), 57 (100), 43 (76); 1H-NMR: (300 MHz) δ:
13
0.84 (d, J = 6.6 Hz, 6H), 0.87-0.91 (m, 6H), 1.02-1.11 (m, 4H), 1.16-1.39 (m, 22H); C-NMR: (75

Molecules 2007, 12
2087
MHz) δ: 14.14, 14.19, 19.72, 19.78, 22.73, 23.08, 24.51, 27.13 (27.14) [16], 29.38 (29.39) [16], 29.44,
30.03, 31.98, 32.77 (32.79) [16], 32.80 (32.82) [16], 36.79 (36.89) [16], 37.11 (37.20) [16], 37.42,
37.48.
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13. Furniss, B. S.; Tatchell, A. R.; Hannaford, A. J.; Smith, P. W. G. Vogel's Textbook of Practical
Organic Chemistry, 5th ed.; Longman: London, 1989; pp. 454-457.
14. Sufficiently pure 2-bromohexane could not be procured commercially, minor contents of the
isomeric 3-bromohexane being an apparently unavoidable impurity. Therefore, 2-bromohexane
o
was prepared with the purity of 98% (GC) from 2-hexanol and PBr3 at a temperature below 5 C
([13]; pp. 559-564).
15. The signals are separated from each other by δ = 0.002 ppm.
16. Resolved resonances for the same carbon atom of two diastereomerically different forms.
Sample Availability: Available from the authors.
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