Stereoselective synthesis of (-)-6-acetoxyhexadecanolide: a mosquito oviposition attractant pheromone

Article abstract of DOI:10.1016/j.tetasy.2007.10.006

The stereoselective synthesis of (-)-6-acetoxyhexadecanolide was achieved from the readily available chiral pool compound, l-(+)-tartaric acid. The synthetic sequence includes the elaboration of an α-benzyloxy aldehyde derived from tartaric acid with ring

Full text of DOI:10.1016/j.tetasy.2007.10.006

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Tetrahedron: Asymmetry 18 (2007) 2479–2483
Stereoselective synthesis of (À)-6-acetoxyhexadecanolide:
a mosquito oviposition attractant pheromone
Kavirayani R. Prasad* and Pazhamalai Anbarasan
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India
Received 3 September 2007; accepted 3 October 2007
Abstract—The stereoselective synthesis of (À)-6-acetoxyhexadecanolide was achieved from the readily available chiral pool compound,
L-(+)-tartaric acid. The synthetic sequence includes the elaboration of an a-benzyloxy aldehyde derived from tartaric acid with ring
closing metathesis as the key step.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
lic alcohol 3 by ring closing metathesis. The stereoselective
allylation of a-benzyloxy aldehyde 4 was envisioned for the
formation of 3. The synthesis of 4 from the bis-Weinreb
amide of tartaric acid is a procedure optimized by us in
our laboratory.6
(À)-6-Acetoxyhexadecanolide 1 is an oviposition attractant
pheromone of the female mosquito, Culex pipiens fatigans.1
The mosquito Culex pipiens fatigans (=quinquefasciatus) is
distributed worldwide and in hot climate can be a vector
for filarial disease, malaria as well as having the ability to
transmit the West Nile virus.2 Egg laying is influenced by
a pheromone,3 which, if identified, could lead to an effec-
tive method of controlling the disease. Laurence and Pick-
ett in 1982 identified (À)-6-acetoxyhexadecanolide 1 as the
major component of the attractant pheromone of the mos-
quito.1 Although, several approaches for the enantioselec-
tive synthesis of 1 have been disclosed in the literature,4 a
versatile strategy for the synthesis of the pheromone is of
continuing interest. Our interest in the synthesis of natural
products from the chiral pool tartaric acid resulted in the
synthesis of various bio-active lactones.5 The pivotal strat-
egy in our approach is the synthesis and elaboration of an
a-benzyloxy aldehyde derived from chiral pool tartaric
acid.
Accordingly, by utilizing a procedure described by us, bis-
Weinreb amide 5 was converted to benzyloxy aldehyde 4,
which on allylation under Keck allylation conditions fur-
nished threo alcohol 3 in good yields.7 Reaction of homo-
allylic alcohol 3 with acryloyl chloride and triethylamine
in dichloromethane at 0 ꢁC furnished acryloyl ester 6 in
73% yield. Treatment of 6 with a Grubbs second generation
catalyst in refluxing toluene furnished the expected product
2 albeit in 27% yield (Scheme 2).8
Due to the low yield of lactone 2 in the ring closing metath-
esis reaction, the synthetic sequence was modified as de-
scribed below. Accordingly, the reaction of homoallylic
alcohol 3 with acrolein diethyl acetal in the presence of a
catalytic amount of pyridinium p-toluenesulfonate in
refluxing benzene afforded acetal 7 as an inseparable mix-
ture of diastereomers. Since the acetal formed will be con-
verted to lactone at a later stage, chirality at the acetal
center is of no consequence and the diastereomeric mixture
was used as such in the subsequent transformations. Thus,
the ring closing metathesis reaction of 7 with Grubbs first
generation catalyst in toluene at 60 ꢁC smoothly furnished
the RCM adduct 8 in 86% yield. Catalytic hydrogenation
of 8 with 10% Pd/C in methanol under a hydrogen atmo-
sphere resulted in alcohol 9 in excellent yield. The second-
ary hydroxyl group in 9 was epimerized under Mitsunobu
2. Results and discussion
As shown in Scheme 1, the synthesis of (À)-6-acetoxyhex-
adecanolide 1 was anticipated from the unsaturated lactone
2, the synthesis of which was envisaged from the homoally-
*
Corresponding author. Tel.: +91 80 22932578; fax: +91 80 23600529;
e-mail: prasad@orgchem.iisc.ernet.in
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.10.006

2480
K. R. Prasad, P. Anbarasan / Tetrahedron: Asymmetry 18 (2007) 2479–2483
OH
C10H21
C10H21
OAc
C10H21
OBn
O
O
O
O
OBn
1
2
3
O
O
O
Me
N
C10H21
OBn
MeO
H
N
OMe
Me
O
O
4
5
Scheme 1. Retrosynthesis for the synthesis of (À)-6-acetoxyhexadecanolide 1.
O
O
O
Me
N
SnBu3
Ref.7
C
10H21
C10H21
OBn
MeO
H
HO
N
OMe
OBn
Me
O
O
3
4
5
H2C=CHCOCl
Et3N, DMAP
CH2Cl2, 0 °C
2.5 h, 73%
Grubbs 2nd gen. cat.
toluene, reflux, 22%
C10H21
OBn
C10H21
O
O
O
O
OBn
2
6
Scheme 2. Ring closing metathesis of acryloyl ester 6.
OEt
Grubbs 1st gen. cat. (5 mol%)
toluene, 60 °C, 8 h, 86%
OEt
C10H21
HO
C10H21
O
EtO
PPTS, benzene
reflux, 3 h, 62%
OBn
OBn
7
3
i) DIAD, PNBA, PPh3, THF
0 °C-rt, 6 h
ii) K2CO3, MeOH, rt, 2 h
86% for two steps
H2, 10% Pd/C
MeOH, 3 h, 99%
C10H21
O
C10H21
OBn
EtO
EtO
O
OH
9
8
i) 3M HCl:THF, rt, 3.5 h
ii) PCC, NaOAc, Celite
CH2Cl2, rt, 2.5 h
Ac2O, Et3N
DMAP, CH2Cl2
rt, 2.5 h, 94%
C10H21
OH
C10H21
OAc
1
EtO
O
EtO
O
72% for two steps
11
10
Scheme 3. Stereoselective synthesis of 6-acetoxyhexadecanolide 1.
conditions, furnishing alcohol 10 in 86% yield. Reaction of
10 with acetic anhydride and triethylamine in dichloro-
methane at room temperature yielded acetate 11 in 94%
yield. Treatment of 11 with 3 M HCl in THF at room tem-
perature furnished the corresponding lactol, which was
subjected to oxidation with PCC in dichloromethane at
room temperature to afford (À)-6-acetoxyhexadecanolide
1 in 72% yield over two steps (Scheme 3). The spectroscopic
and physical data [a]D = À37.5 (c 0.4, CHCl3); lit.4b
[a]D = À36.8 (c 1.55, CHCl3) are in good agreement with
that reported in the literature.
step. The synthetic sequence depicted en route to the lac-
tone is highly diastereoselective with good overall yield.
4. Experimental
4.1. General procedures
Column chromatography was performed on silica gel,
Acme grade 100–200 mesh. TLC plates were visualized
either with UV, in an iodine chamber, or with phosphomo-
lybdic acid spray, unless noted otherwise. Unless stated
otherwise, all reagents were purchased from commercial
sources and used without additional purification. THF
was freshly distilled over Na-benzophenone ketyl. Melting
points are uncorrected. Unless stated otherwise, all reac-
tions were performed under an inert atmosphere. Optical
rotations were measured on a JASCO DIP-370 digital
3. Conclusion
In conclusion, a facile stereoselective synthesis of 6-acet-
oxyhexadecanolide was accomplished from L-(+)-tartaric
acid, involving ring closing metathesis reaction as the key

K. R. Prasad, P. Anbarasan / Tetrahedron: Asymmetry 18 (2007) 2479–2483
2481
polarimeter measured at 25 ꢁC. Homoallylic alcohol 3 was
prepared according to the procedure described by us
earlier.7
PPTS (5 mg, 0.02 mmol) followed by acroleindiethyl acetal
(0.12 mL, 0.75 mmol) was then added at room temperature
and further refluxed for 3 h. After the reaction was com-
plete, it was cooled to room temperature and diluted with
20 mL of diethyl ether. The ether layer was washed with
satd NaHCO3 (2 · 6 mL), brine (8 mL) and dried over
Na2SO4. The residue obtained after evaporation of the sol-
vent was purified by column chromatography using petro-
leum ether/ethyl acetate (96:4) as eluent to yield the
diastereomeric mixture of acetal 7 in 62% (0.78 g, 88%
based on 30% of starting material recovery) yield as a col-
orless oil. IR (neat) 2923, 2856, 1645, 1453, 1020 cmÀ1; 1H
NMR (300 MHz, CDCl3): d 7.38–7.23 (m, 5H), 5.98–5.72
(m, 2H), 5.43–5.21 (m, 2H), 5.19–4.88 (m, 3H), 4.70–4.47
(m, 2H), 3.90–3.37 (m, 4H), 2.55–2.38 (m, 1H), 2.32–2.16
(m, 1H), 1.71–1.13 (m, 21H), 0.88 (t, J = 6.6 Hz, 3H);
13C NMR (75 MHz, CDCl3): d 138.9, 135.8(9), 135.8(1),
135.6, 128.3, 127.9, 127.5, 118.1, 118.0, 116.7, 116.5,
102.3, 102.2, 100.5, 80.4, 80.2, 76.7, 76.6, 72.5, 72.4, 60.8,
60.4, 35.1, 34.4, 31.9, 29.6, 29.5, 29.4, 26.2, 25.9, 22.7,
15.2, 15.1, 14.1; HRMS calcd for C27H44O3+Na,
439.3188; found, 439.3184.
4.2. Preparation of (4R,5R)-4-acryloxy-5-benzyloxypenta-
dec-1-ene 6
To a stirred solution of homoallylic alcohol 3 (0.1 g,
0.3 mmol), triethylamine (0.1 mL, 0.6 mmol), and DMAP
(7 mg, 0.06 mmol) in 2.5 mL of dichloromethane at 0 ꢁC
was added acryloyl chloride (0.04 mL, 0.45 mmol) drop-
wise. The reaction mixture was stirred for 2.5 h at the same
temperature. After the reaction was complete (indicated by
TLC), it was poured into water (7 mL) and extracted with
diethyl ether (3 · 6 mL). The combined ethereal extracts
were washed with brine (10 mL) and dried over Na2SO4.
The residue obtained after evaporation of the solvent was
purified by column chromatography using petroleum
ether/ethyl acetate (95:5) as eluent to yield 6 as a colorless
oil in 73% yield (0.085 g). [a]D = +13.0 (c 1.0, CHCl3); IR
1
(neat): 2925, 2854, 1726, 1404, 1269, 1192 cmÀ1; H NMR
(400 MHz, CDCl3): d 7.45–7.28 (m, 5H), 6.46 (dd,
J = 17.3, 1.5 Hz, 1H), 6.19 (dd, J = 17.3, 10.4 Hz, 1H),
5.87 (dd, J = 10.4, 1.5 Hz, 1H), 5.79 (ddt, J = 17.5, 10.2,
6.7 Hz, 1H), 5.28 (quin, J = 4.4 Hz, 1H), 5.18–5.04 (m,
2H), 4.65 (s, 2H), 3.55 (dt, J = 7.4, 4.4 Hz, 1H), 2.58–
2.49 (m, 1H), 2.47–2.36 (m, 1H), 1.69–1.24 (m, 19H),
0.93 (t, J = 7.0 Hz, 3H); 13C NMR (75 MHz, CDCl3): d
165.7, 138.4, 133.9, 130.7, 128.5, 128.3, 127.9, 127.6,
117.6, 78.8, 73.6, 72.4, 34.3, 31.8, 29.8, 29.6, 29.5, 29.4,
29.3, 25.5, 22.6, 14.1; HRMS calcd for C25H38O3+Na,
409.2719; found, 409.2718.
4.5. Preparation of (2R)-2-((R)-1-(benzyloxy)undecyl)-6-
ethoxy-3,6-dihydro-2H-pyran 8
To a solution of 7 (0.08 g, 0.2 mmol) in 5 mL of toluene,
Grubbs 1st generation catalyst (8 mg, 0.009 mmol,
5 mol %) was added and heated to 60 ꢁC, under an argon
atmosphere. After the reaction was complete (ꢀ8 h), it
was cooled to room temperature. Evaporation of the sol-
vent followed by column chromatography of the resulting
residue using petroleum ether/ethyl acetate (95:5) as eluent
afforded 8 as a colorless oil in 86% yield (64 mg, 95% based
on 10% recovery of the starting material). IR (neat) 2924,
4.3. Preparation of (R)-6-((R)-1-(benzyloxy)undecyl)-5,6-
dihydropyran-2-one 2
1
2857, 1643, 1456, 1094, 1021 cmÀ1; H NMR (300 MHz,
Diene 6 (0.08 g, 0.21 mmol) and Grubbs 2nd generation
catalyst (17 mg, 0.02 mmol, 10 mol %) in 10 mL of toluene
were heated at reflux, under an argon atmosphere. After
10 h, it was cooled to room temperature. Evaporation of
the solvent followed by column chromatography of the
resulting residue using petroleum ether/ethyl acetate
(85:15) as eluent afforded 2 as a colorless oil in 22% yield
(18 mg), along with 54% (48 mg) of recovered starting
material. [a]D = +61.9 (c 2.1, CHCl3); IR (neat): 2924,
CDCl3): d 7.43–7.24 (m, 5H), 6.07–5.92 (m, 1H), 5.78–
5.63 (m, 1H), 5.17 and 5.03 (br s, 1H), 4.85–4.59 (m,
2H), 4.09–3.78 (m, 2H), 3.66–3.32 (m, 2H), 2.34–2.08 (m,
1H), 1.96–1.74 (m, 1H), 1.71–1.13 (m, 21H), 0.88 (t,
J = 6.6 Hz, 3H); 13C NMR (75 MHz, CDCl3): d 138.8,
129.1, 128.8, 128.3, 128.1, 127.8, 127.5, 127.4, 125.4, 97.9,
94.5, 81.1, 80.4, 75.4, 73.7, 72.7, 67.7, 63.9, 63.1, 31.9,
30.2, 29.9, 29.8, 29.6, 29.5, 29.4, 26.8, 26.4, 25.6, 25.4,
22.7, 15.3, 15.2, 14.1; HRMS calcd for C25H40O3+Na,
411.2875; found, 411.2879.
1
2854, 1727, 1455, 1380, 1248 cmÀ1; H NMR (300 MHz,
CDCl3): d 7.40–7.23 (m, 5H), 6.89 (ddd, J = 9.3, 6.0,
2.1 Hz, 1H), 5.99 (dd, J = 9.6, 1.8 Hz, 1H), 4.64 and 4.59
(AB q, J = 11.4 Hz, 2H), 4.52 (dt, J = 12.6, 4.2 Hz, 1H),
3.55 (quin, J = 4.5 Hz, 1H), 2.57–2.42 (m, 1H), 2.32–2.20
(m, 1H), 1.75–1.14 (m, 19H), 0.86 (t, J = 7.2 Hz, 3H);
13C NMR (75 MHz, CDCl3): d 164.0, 145.5, 138.1, 128.4,
127.9, 127.8, 121.2, 79.1, 78.3, 72.9, 31.8, 29.6, 29.5(4),
29.5(1), 29.3, 25.7, 24.8, 22.6, 14.1; HRMS calcd for
C23H34O3+Na, 381.2406; found, 381.2405.
4.6. Preparation of (2R)-2-((R)-1-hydroxyundecyl)-6-ethoxy-
tetrahydropyran 9
To a solution of 8 (60 mg, 0.15 mmol) in 2 mL of methanol
at room temperature was added activated 10% Pd/C
(30 mg). The reaction mixture was stirred for 3 h under a
hydrogen atmosphere (balloon) at the same temperature.
After the reaction was complete (TLC), it was filtered
through a short pad of Celite and the Celite pad was
washed with diethyl ether (15 mL). The residue obtained
after evaporation of the solvent was purified by column
chromatography using petroleum ether/ethyl acetate (9:1)
as eluent to yield 9 as a colorless oil in 99% (46 mg) yield.
IR (neat): 3438, 2923, 2853, 1458, 1127, 1031, 980 cmÀ1; 1H
NMR (300 MHz, CDCl3): d 4.87 and 4.5–4.38 (m, 1H),
4.4. Preparation of (4R,5R)-5-benzyloxy-4-(1-ethoxyallyl-
oxy)pentadec-1-ene 7
Single neck round-bottomed flask (10 mL) fitted with
Dean-Stark apparatus and a condenser was charged with
homoallylic alcohol 3 (0.1 g, 0.3 mmol) in 5 mL of benzene.

2482
K. R. Prasad, P. Anbarasan / Tetrahedron: Asymmetry 18 (2007) 2479–2483
4.05–3.19 (m, 4H), 2.53 and 2.07 (d, J = 4.2 Hz, 1H,
exchangeable with D2O), 1.97–1.15 (m, 27H), 0.86 (t,
J = 7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3): d 96.9,
74.3, 71.5, 62.2, 32.8, 31.9, 29.6, 29.5, 29.3, 27.1, 25.5,
22.6, 17.6, 15.0, 14.0; HRMS calcd for C18H36O3+Na,
323.2562; found, 323.2560.
0.88 (t, J = 6.9 Hz, 3H); 13C NMR (75 MHz, CDCl3): d
170.7, 96.9, 75.9, 69.5, 62.1, 31.9, 29.8, 29.7, 29.5(6),
29.5(3), 29.3, 26.6, 25.4, 22.7, 21.1, 17.7, 15.1, 14.1; HRMS
calcd for C20H38O4+Na, 365.2668; found, 365.2666.
4.9. Preparation of (À)-(5R,6S)-6-acetoxyhexadecanolide 1
4.7. Preparation of (2R)-2-((S)-1-hydroxyundecyl)-6-ethoxy-
tetrahydropyran 10
To a stirred THF (1 mL) solution of 11 (30 mg, 0.09 mmol)
was added 1 mL of 3 M HCl at room temperature and stir-
red for 3.5 h. After the reaction was complete (indicated by
TLC), it was poured into water (5 mL) and extracted with
diethyl ether (3 · 5 mL). The combined ether layer was
washed with brine (5 mL) and dried over Na2SO4. Evapo-
ration of the solvent yielded the lactol, which was used in
the next step without further purification.
In a 10 mL single neck round-bottomed flask was placed 9
(40 mg,
0.13 mmol),
triphenylphosphine
(87 mg,
0.33 mmol), and p-nitrobenzoic acid (56 mg, 0.33 mmol)
dissolved in 2 mL of THF, under an argon atmosphere.
It was cooled to 0 ꢁC and DIAD (0.07 mL, 0.33 mmol)
was added dropwise over a period of 7 min. It was then
warmed up to room temperature and stirred for 6 h at
the same temperature. After the reaction was complete
(indicated by TLC), it was poured into water and extracted
with diethyl ether (3 · 6 mL). The combined ether layers
were washed with brine (10 mL) and dried over Na2SO4.
Column chromatography of the residue obtained after
evaporation of the solvent using petroleum ether/ethyl ace-
tate (93:7) as an eluent yielded the p-nitrobenzoate along
with uncharacterized impurity. This was subjected to the
next reaction without further purification.
To a solution of the lactol (obtained above) in 1 mL of
dichloromethane was added Celite (10 mg) and sodium
acetate (22 mg, 0.27 mmol) at room temperature and stir-
red for 5 min. PCC (58 mg, 0.27 mmol) was then intro-
duced into the reaction mixture at the same temperature.
It was stirred at room temperature for 2.5 h. After the reac-
tion was complete (TLC), it was filtered through a short
pad of Celite and the Celite pad was washed with diethyl
ether (15 mL). The ether layers were combined and the res-
idue obtained after evaporation of the solvent was purified
by column chromatography using petroleum ether/ethyl
acetate (8:2) as eluent to yield 1 as a colorless oil in 72%
(20 mg). [a]D = À37.5 (c 0.4, CHCl3); lit.4b [a]D = À36.8
The p-nitrobenzoate obtained above was dissolved in 2 mL
of methanol and K2CO3 was added at room temperature
and stirred vigorously at the same temperature for 2 h.
After the reaction was complete (TLC), it was poured into
water and extracted with diethyl ether (3 · 5 mL). The
combined ether extracts were washed with brine (7 mL)
and dried over Na2SO4. The residue obtained after evapo-
ration of the solvent was purified by column chromatogra-
phy using petroleum ether/ethyl acetate (9:1) as eluent to
furnish 10 in 69% (28 mg, 86% yield based on 19% recovery
of the starting material) as a colorless oil. IR (neat): 3415,
1
(c 1.55, CHCl3); H NMR (300 MHz, CDCl3): d 4.98 (dt,
J = 7.2, 5.1 Hz, 1H), 4.35 (ddd, J = 11.1, 4.8, 3.6 Hz,
1H), 2.69–2.38 (m, 2H), 2.08 (s, 3H), 2.02–1.76 (m, 2H),
1.74–1.53 (m, 4H), 1.48–1.16 (m, 16H), 0.88 (t,
J = 6.6 Hz, 3H); 13C NMR (75 MHz, CDCl3): d 170.8,
170.5, 80.5, 74.3, 31.9, 29.6, 29.5, 29.4(4), 29.4, 29.3, 25.2,
23.5, 22.7, 21.0, 18.2, 14.1. HRMS calcd for C18H32O+Na,
335.2198; found, 335.2198.
1
2923, 2854, 1645, 1549, 1020 cmÀ1; H NMR (300 MHz,
CDCl3): d 4.86 (br s, 1H), 3.78–3.29 (m, 4H), 1.98 (d,
J = 3.0 Hz, 1H, exchangeable with D2O), 1.94–1.17 (m,
27H), 0.88 (t, J = 6.9 Hz, 3H); 13C NMR (75 MHz,
CDCl3): d 97.2, 73.6, 71.2, 62.2, 31.9, 31.8, 29.9, 29.7,
29.6, 29.3, 26.0, 23.9, 22.7, 17.5, 15.1, 14.1; HRMS calcd
for C18H36O3+Na, 323.2562; found, 323.2558.
Acknowledgments
We thank the Department of Science and Technology
(DST), New Delhi for funding of this project. P.A. thanks
the Council of Scientific and Industrial Research (CSIR),
New Delhi for a fellowship.
4.8. Preparation of (2R)-2-((S)-1-acetoxyundecyl)-6-ethoxy-
tetrahydropyran 11
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To the stirred solution of 10 (30 mg, 0.1 mmol), DMAP
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followed by column chromatography of the resultant resi-
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