(S)-2-pentyl (R)-3-hydroxyhexanoate, a banana volatile and its olfactory recognition by the common fruit fly, Drosophila melanogaster

Article abstract of DOI:10.1021/np800597k

The volatile organic compounds emitted from ripening bananas that elicit an antennal response from the common fruit fly, Drosophila melanogaster, were analyzed by a combination of gas chromatographic-electroantennographic detection, mass spectrometry, and ¹H NMR spectroscopy. These analyses revealed that the headspace of ripening bananas contains a number of EAD-active components including the new ester (S)-2-pentyl (R)-3-hydroxyhexanoate, the structural assignment of which was confirmed by chemical synthesis.

Full text of DOI:10.1021/np800597k

772
J. Nat. Prod. 2009, 72, 772–776
(S)-2-Pentyl (R)-3-Hydroxyhexanoate, a Banana Volatile and Its Olfactory Recognition by the
Common Fruit Fly, Drosophila melanogaster
Jeffrey Mowat,† Regine Gries,‡ Grigori Khaskin,‡ Gerhard Gries,‡ and Robert Britton*,†
Departments of Chemistry and Biological Sciences, Simon Fraser UniVersity, 8888 UniVersity DriVe, Burnaby, British Columbia,
V5A 1S6, Canada
ReceiVed September 23, 2008
The volatile organic compounds emitted from ripening bananas that elicit an antennal response from the common fruit
fly, Drosophila melanogaster, were analyzed by a combination of gas chromatographic-electroantennographic detection,
mass spectrometry, and 1H NMR spectroscopy. These analyses revealed that the headspace of ripening bananas contains
a number of EAD-active components including the new ester (S)-2-pentyl (R)-3-hydroxyhexanoate, the structural
assignment of which was confirmed by chemical synthesis.
Fruit flies in the family Drosophilidae are a nuisance in homes,
restaurants, fruit markets, and canneries, and wherever fruit and
vegetable matter is left exposed.1 Early research directed at
identifying the natural volatiles that lure fruit flies to rotting organic
material, with the intention of exploiting these compounds in
commercial traps, led to the discovery of various attractants
including EtOH, HOAc, EtOAc, and acetaldehyde.2 More recently,
it was revealed that overripe mango fruit produces the fruit fly
attractive volatiles EtOH, HOAc, amyl acetate, 2-phenylethanol,
and phenylethyl acetate.3 It has also been demonstrated that
Drosophila melanogaster is attracted by volatile compositions that
include a short-chain carboxylic acid, a short-chain alcohol, a
volatile aryl-substituted alcohol, a nitrogen compound, a sugar, a
terpene, EtOAc, 2-phenylethyl acetate, and H2O.4 Despite these
discoveries, the development of effective means to remove Droso-
phila from areas of food preparation and storage remains an
important challenge. Given that banana mash fermented by bakers’
yeast has been used as a bait to attract fruit flies since the 1930s,5
as a prelude to the development of an efficient semiochemical lure
for Drosophila, we initiated an investigation of antenna-stimulating
volatiles produced by ripening bananas.6 The results of this study
as well as the isolation, structural elucidation, and synthesis of (S)-
2-pentyl (R)-3-hydroxyhexanoate, a new natural product, are
reported herein.
Approximately 50 ripening bananas were aerated individually
for three days, and the volatile organic compounds were collected
on Porapak Q (ethylvinylbenzene-divinylbenzene polymer) and
subsequently extracted into pentane. Gas chromatographic-electro-
antennographic detection (GC-EAD) analyses7 of the volatiles
revealed that many components elicited a response from D.
melanogaster antennae (Figure 1). With the exception of one
compound, the structures of all antennal stimulatory volatile
constituents were assigned on the basis of their characteristic
retention indices and MS fragmentation patterns,8 and assignments
were confirmed by comparison with authentic standards. The
behavioral activity of each candidate semiochemical is typically
determined by preparing a synthetic blend that mimics in concentra-
tion and composition the antennal-stimulatory volatiles and by
bioassaying this synthetic blend as well as blends from which
specific compounds (e.g., alcohols, esters, or hydrocarbons) are
eliminated.9 While the structurally uncharacterized compound (tR
) 10.69 min, Figure 1) elicited a relatively weak antennal response
(approximately 0.2 mV), this was not necessarily indicative of the
compound’s behavioral significance.10 Consequently, a confident
structural assignment and synthetic source for this new candidate
semiochemical were required. In its mass spectrum (Figure 2) the
fragmentation ion m/z 89 was suggestive of a butyrate, which is
commonly found in banana volatiles6 and the molecular ion m/z
203 (M + 1) was consistent with the chemical formula C11H22O3,
indicating an additional alcohol function. Moreover, the retention
indices for this compound on DB-23 (1838), DB-5 (1347), DB-
210 (1657), and DB-17 (1480) were consistent with those of a polar
ester, such as a butyrate with an OH group in the alcohol or acid
portion. While the fragmentation ion m/z 133 (loss of 69 from the
molecular ion m/z 203) suggested an ester derived from pentanol,
and m/z 45 further suggested a 2-pentyl ester,11 the molecular mass
of the candidate structure (2-pentyl hydroxybutyrate) deduced from
the MS data was not a match with that of the unknown. With this
in mind, the gas chromatographic retention indices for 2-heptyl
3-hydroxybutyrate, which has a molecular weight consistent with
that of the unknown, were measured on DB-23 (1861), DB-5
(1345), DB-210 (1672), and DB-17 (1482) columns. These data
closely resembled those of the unknown, but the mass spectrum of
this substance lacked a strong m/z 89 ion. Considering that the MS
information derived from the unknown appeared insufficient to
assign a molecular structure and that many hydroxy esters seemed
plausible, we decided to isolate this substance and to elucidate its
molecular structure by 1H NMR spectroscopy. Normal-phase flash
chromatographic separation (Et2O/pentane) of the crude mixture
of volatiles followed by reversed-phase HPLC afforded a small
amount (300 ng as determined by GC analysis with an internal
standard) of this unidentified volatile. 1H NMR spectroscopy with
extended acquisition time (11 000 scans, 600 MHz TCI Cryoprobe)
permitted the detection of a number of signals related to the natural
product. However, the 300 ng sample also contained relatively large
amounts of residual solvents and their associated impurities that
masked key resonances. The presence of an ester function was
suggested by a multiplet that resonated at δ 4.95 (acyloxy methine)
and two doublets of doublets with resonances at δ 2.46 and 2.36
(R-methylene). Importantly, the multiplicity and splitting pattern
of the latter two resonances were consistent with those of a
ꢀ-hydroxy ester, and both signals showed a 1D TOCSY correlation
to a multiplet that resonated at δ 3.96. On the basis of an additional
1D TOCSY correlation between the acyloxy methine proton and a
methyl resonance at δ 1.20 (doublet), we were able to construct
the partial structure depicted in Figure 3. Further observation of
1D TOCSY correlations from the acyloxy methine and carbinol
protons to at least two methyl resonances centered around δ 0.90
reduced to 22 the number of potential structural isomers for the
ester, but due to the limited amount of material, a confident
* To whom correspondence should be addressed. Tel: (778)-782-4889.
Fax: (778)-782-3765. E-mail: rbritton@sfu.ca.
† Department of Chemistry.
‡ Department of Biological Sciences.
10.1021/np800597k CCC: $40.75
2009 American Chemical Society and American Society of Pharmacognosy
Published on Web 03/11/2009

Notes
Journal of Natural Products, 2009, Vol. 72, No. 4 773
Figure 1. Representative recording of flame ionization detector (FID) and electroantennographic detector (EAD: D. melanogaster antenna)
responses to volatile organic compounds collected from ripening bananas. Unidentified substance is indicated with arrow (V). Chromatography:
DB-5 column; splitless injection; temperature of injection port and FID, 240 °C; temperature program, 1 min at 50 °C, then 10 °C/min to
280 °C.
Scheme 1. Synthesis of (S)-2-Pentyl (R)-3-Hydroxyhexanoate
Figure 2. Saturn 2000 ion trap mass spectrum of (S)-2-pentyl (R)-
3-hydroxyhexanoate (5).
1
respectively, as diastereomeric mixtures. While H NMR spectra
of esters 2-4 indicated that none of these substances possessed
the connectivity of the natural product, similarities in the chemical
shifts of the alkoxy fragment in 4 with those of the natural product
suggested the later substance was derived from 2-pentanol, which
was consistent with the MS data (Vide supra). On the basis of this
observation, the isomeric ꢀ-hydroxy ester 5 was synthesized, and
the planar structure for the natural product was unambiguously
assigned as 2-pentyl 3-hydroxyhexanoate. This is the first example
of this compound as a synthetic or natural product.12 As the four
stereoisomers represented by 5 proved to be inseparable by HPLC,
assignment of both the relative and absolute configuration of the
natural product would require the stereoselective syntheses of these
compounds.
Repetition of the synthesis of 5 starting with (S)-2-pentanol
Figure 3. Synthesis and structures of candidate ꢀ-hydroxy esters
2-5.
structural assignment would ultimately rely on comparison with
synthetic standards.
provided an inseparable mixture of diastereomeric hydroxy esters
that, when compared to natural 5 by chiral GC,13 confirmed the
configuration of the acyloxy methine stereocenter as S. On the basis
of this result, syntheses of both the R- and S-configured ꢀ-hydroxy
esters were carried out. While the R,ꢀ-epoxy acid 8 required to
access 5 is available via Sharpless asymmetric epoxidation of
2-hexen-1-ol followed by oxidation of the alcohol function,14,15
an alternative route to this substance (Scheme 1) exploits methodol-
ogy developed in one of our laboratories for the synthesis of trans-
epoxides.16 Thus, addition of the lithium anion derived from
trimethylsilyl acetylene to (S)-2-chloropentanal (6) provided the
In order to gain further insight into the structure of this new
volatile ester, the synthesis and spectroscopic analysis of a small
family of synthetic ꢀ-hydroxy esters (MW ) 202) was carried out.
As detailed in Figure 3, acetylation of 2-butanol provided an acetate
(not shown), the subsequent treatment of which with LDA followed
by 2-methylbutanal afforded the ꢀ-hydroxy ester 2 as a mixture of
diastereomers. Alternatively, acetylation of 3-methyl-2-pentanol or
2-pentanol followed by sequential treatment with LDA and propanal
or isobutyraldehdye afforded the ꢀ-hydroxy esters 3 and 4,

774 Journal of Natural Products, 2009, Vol. 72, No. 4
Notes
air (Praxair Canada Inc., Mississauga, Ontario, Canada), which delivered
the GC column eluent through a custom-built interface kept a 250 °C.
Antennal receptor potentials (measured in mV by a custom-built
amplifier) elicited by specific compounds were recorded by a HP 3392A
chart recorder. Identical retention times of compounds detected by the
flame ionization detector of the GC and by the insect antenna allowed
assignment of antennal responses to specific compounds in the eluent.
Volatiles that elicited responses from antennae were analyzed by GC-
MS, employing a Varian Saturn 2000 ion trap GC-MS fitted with a
DB-5 column. As (S)-2-pentyl (R)-3-hydroxyhexanoate occurred in
greatest relative abundance between 24 and 48 h, in 50 additional
aerations of single bananas, volatiles were Porapak Q-collected for 3
days.
chlorohydrin 7 in excellent yield (dr > 20:1). Following epoxide
formation (Cs2CO3, EtOH), the alkyne function was oxidatively
cleaved17 to provide the desired R,ꢀ-epoxy acid 8 in good overall
yield. Completion of the synthesis of the (R)-ꢀ-hydroxy ester
involved esterification with (S)-2-pentanol and a subsequent regi-
oselective reduction18 of the epoxide. Following a similar sequence
of reactions that started with (R)-2-chloropentanal, the (S)-ꢀ-
hydroxy ester (3S,2S)-5 (see inset in Scheme 1) was also con-
structed. Although the 1H and 13C NMR spectra derived from
(3R,2S)-5 and (3S,2S)-5 were indistinguishable, chiral GC analysis13
allowed confident configurational assignment of the natural product
as (S)-2-pentyl (R)-3-hydroxyhexanoate.19
Isolation of (S)-2-Pentyl (R)-3-Hydroxyhexanoate. Single or
combined Porapak Q volatile extracts were evaporated to dryness under
a stream of N2 (5 min). The samples were reconstituted in 100 µL of
pentane and fractionated on silica gel (50 g) in a glass column (14 ×
0.5 cm i.d.). After prerinsing the silica with pentane, the extract was
applied and then successively eluted with 2 mL of pentane, 1 mL of
pentane/Et2O (9:1), and 4 mL of pentane/Et2O (3:1). The pentane/Et2O
(3:1) fraction was further purified by reversed-phase HPLC (Nova Pak
C18, 3.9 × 300 mm column) using a H2O/MeCN (2:3) mobile phase.
(S)-2-Pentyl (R)-3-hydroxyhexanoate eluted between 6.25 and 6.5 min.
For characterization data see below.
In summary, the volatile organic compounds emitted from
ripening bananas that elicit an antennal response from the common
fruit fly, D. melanogaster, have been identified by a combination
of GC-EAD analysis, MS, NMR spectroscopy, and synthesis. These
efforts resulted in the isolation of submicrogram quantities of the
new antenna-stimulating ester (S)-2-pentyl (R)-3-hydroxyhexanoate,
the structural assignment of which was confirmed by synthesis.
Notably, the stereoselective synthesis of this compound involved
the development of a new method to access R,ꢀ-epoxy acids.
Bioassaying the behavioral response of D. melanogaster to
individual components and/or blends of the antenna-stimulating
banana volatiles9 identified in this study may lead to the develop-
ment of effective lures for the common fruit fly.
Representative Procedure for the Synthesis of the ꢀ-Hydroxy
Esters 2-4. Synthesis of 2-Butyl 3-Hydroxy-4-methylhexanoate (2).
To a cold (-78 °C), stirred solution of diisopropylamine (0.66 mL,
4.7 mmol) in THF (40 mL) was added n-butyllithium (2.5 M solution
in hexanes, 1.9 mL, 4.7 mmol). After 30 min at -78 °C, the reaction
mixture was warmed to 0 °C for an additional 15 min. The resulting
slightly yellow solution was cooled to -78 °C, and sec-butyl acetate
(500 mg, 4.31 mmol) was added followed after 15 min by the addition
of 2-methylbutanal (0.55 mL, 5.17 mmol). The resulting mixture was
stirred for an additional 30 min, after which time it was warmed to 0
°C and treated with saturated aqueous NH4Cl (10 mL). The mixture
was then diluted with Et2O (20 mL), and the layers were separated.
The aqueous layer was extracted with Et2O (3 × 10 mL), and the
combined organic phases were washed with brine (15 mL), dried
(MgSO4), and concentrated to provide a crude yellow oil. Purification
of the crude material by flash chromatography (silica gel, 10:1 hexanes/
EtOAc) afforded (2-butyl)-3-hydroxy-4-methylhexanoate (2) as a
Experimental Section
General Experimental Procedures. All reactions were performed
under an atmosphere of dry Ar using oven-dried glassware. THF, Et2O,
and CH2Cl2 were used directly from an MBraun solvent purifier system
(MB-SP Series). Commercial anhydrous EtOH (reagent grade) was used
without further purification. Caledon pentane non-UV was distilled prior
to use. Flash chromatography was carried out with 230-400 mesh silica
gel (E. Merck, silica gel 60). Chiral GC analyses were preformed on a
Hewlett-Packard 5890 gas chromatograph equipped with a flame
ionization detector and a custom-made chiral GC column coated with
a 1:1 mixture of heptakis(2,6-di-O-methyl-3-O-pentyl)-ꢀ-cyclodextrin
and OV-1701 (30 m length, 0.320 mm i.d., 0.25 µm film).13 NMR
spectra were recorded using deuterochloroform (CDCl3) as the solvent
on a Bruker Avance 600 equipped with a QNP or TCI cryoprobe (600
MHz), Varian Inova 500 (500 MHz), or Varian MercuryPlus (400
MHz). Signal positions (δ) are given in parts per million from TMS
and were measured relative to the signal of the solvent (CDCl3: δ 7.26,
1H NMR; δ 77.0, 13C NMR). Infrared (IR) spectra were recorded on a
MB-series Bomem/Hartman & Braun Fourier transform spectropho-
tometer with internal calibration as films between sodium chloride
plates. Selected, characteristic absorption data are provided for each
compound. Chemical ionization (CI) mass spectra were recorded on a
Varian 4000 mass spectrometer at 70 eV. High-resolution fast atom
bombardment (HR-FABMS) mass spectra were recorded on a JEOL
JMS-AX505HA mass spectrometer at 3 kV. Optical rotations were
measured on a Perkin-Elmer polarimeter 341.
Collection of Volatiles from Bananas. A single yellow-ripe banana
was placed in a Pyrex glass aeration chamber (15.5 cm inner diameter
(i.d.) × 20 cm). For 12 days, a water-driven aspirator drew purified air
at 1 L/min through the chamber and a downstream Pyrex glass column
(140 × 5 mm i.d.) filled with Porapak Q (50-80 mesh, Waters, Milford,
MA). Every 24 h, volatiles were eluted from Porapak Q with 2 mL of
freshly distilled pentane.
Analyses of Volatiles. The volatile extracts eluted from the Porapak
Q with pentane were concentrated to a volume of 1 mL and analyzed
by coupled GC-EAD7 employing a Hewlett-Packard (HP) 5890 gas
chromatograph fitted with a GC column (30 m × 0.32 mm i.d.) coated
with DB-5 (J&W Scientific, Folsom, CA). For GC-EAD recordings a
fruit fly’s head with both antennae intact was severed from the body
and inserted into the opening of a glass capillary electrode (0.58 mm
i.d. × 65 mm length) (A-M Systems, Inc., Calsborg, WA) filled with
saline solution.20 The tip of one antenna was removed by spring
microscissors (Fine Science Tools Inc., North Vancouver, British
Columbia, Canada) and then placed into the opening of the recording
electrode mounted on a portable micromanipulator and positioned in
front of a constant stream (250 mL/min) of warm (20 °C) nonhumidified
1
mixture of four diastereomers. H NMR (500 MHz) δ: 4.90 (m, 4H),
3.94 (m, 2H), 3.85 (m, 2H), 3.00 (m, 2H), 2.83 (m, 2H), 2.49-2.35
(m, 8H), 1.66-1.43 (m, 16H), 1.22 (d, 12H, J ) 6.2 Hz), 1.17 (m,
4H), 0.93-0.88 (m, 36H). 13C NMR (100 MHz) (all resolved carbon
resonances reported) δ: 173.2, 173.2, 173.1, 173.1, 72.6, 72.6, 71.6,
71.5, 70.9, 70.9, 39.7, 39.7, 39.7, 39.0, 39.0, 38.1, 28.7, 25.4, 24.9,
19.3, 19.3, 14.3, 14.3, 13.7, 13.7, 11.6, 11.6, 11.4, 9.6. IR (neat): 3494,
2967, 2878, 1714, 1463, 1380, 1278, 1183, 1095, 1061, 1028 cm-1
.
Exact mass calcd for C11H23O3: 203.1647 (M + H); found 203.1651.
3-Methyl-2-pentyl 3-Hydroxypentanoate (3) (mixture of four
diastereomers). 1H NMR (500 MHz) δ: 4.87-4.78 (m, 4H), 3.85 (m,
4H), 3.16 (m, 4H), 2.42 (dd, 4H, J ) 3.3, 16.2 Hz), 2.33 (ddd, 4H, J
) 1.0, 8.9, 16.2 Hz), 1.57-1.34 (m, 16H), 1.13-1.01 (m, 16H), 0.90
(t, 12H, J ) 7.5 Hz), 0.83 (m, 24H). 13C NMR (150 MHz) (all resolved
carbon resonances reported) δ: 173.4, 173.4, 173.3, 173.3, 72.8, 72.7,
71.6, 71.6, 71.0, 71.0, 39.7, 39.7, 39.7, 39.0, 39.0, 38.1, 28.8, 25.5,
25.0, 19.5, 19.5, 14.4, 14.4, 13.9, 13.8, 11.8, 11.5, 9.7. IR (neat): 3444,
2965, 2879, 1730, 1463, 1380, 1179, 1026, 983 cm-1. Exact mass calcd
for C11H23O3: 203.1647 (M + H); found 203.1654.
2-Pentyl 3-Hydroxy-4-methylpentanoate (4) (mixture of two
1
diastereomers). H NMR (600 MHz) δ: 4.96 (apparent sextet, 2H, J
) 6.1 Hz), 3.76 (m, 2H), 3.00 (m, 2H), 2.47 (dd, 2H, J ) 2.4, 16.2
Hz), 2.37 (dd, 2H, J ) 9.7, 16.2 Hz), 1.70 (m, 2H), 1.58 (m, 2H), 1.46
(m, 2H), 1.38-1.27 (m, 4H), 1.21 (dd, 6H, J ) 0.7, 6.1 Hz), 0.95-0.89
(m, 18H). 13C NMR (150 MHz) (all resolved carbon resonances
reported) δ: 173.2, 173.2, 72.7, 72.7, 71.3, 38.6, 38.0, 33.1, 20.0, 19.9,
18.6, 18.6, 18.3, 17.8, 17.8, 13.9. IR (neat): 3466, 2961, 2875, 1731,
1468, 1382, 1327, 1282, 1186, 1121, 1106, 1056 cm-1. Exact mass
calcd for C11H23O3: 203.1647 (M + H); found 203.1653.
(S)-2-Chloropentanal (6). To a cold (0 °C), stirred solution of
pentanal (430 mg, 5.0 mmol) in CH2Cl2 (20 mL) were added
D-prolinamide (57.5 mg, 0.5 mmol) and NCS (870 mg, 6.5 mmol).
The reaction mixture was stirred for 1 h and then slowly warmed to

Notes
Journal of Natural Products, 2009, Vol. 72, No. 4 775
25
room temperature over the course of 3 h, at which temperature it was
stirred until complete consumption of pentanal (as determined by H
mass calcd for C11H21O3 (M + H): 201.1491; found 201.1487. [R]D
+14.3 (c 10.0, CHCl3).
1
Preparation of (S)-2-Pentyl (R)-3-hydroxyhexanoate (5). To a
solution of diphenyldiselenide (47 mg, 0.15 mmol) in EtOH (2 mL)
was added NaBH4 (13 mg, 0.34 mmol). After gas evolution had
subsided the reaction was cooled to 0 °C and HOAc (12 µL, 0.4 mmol)
was added. The reaction mixture was stirred for 5 min, a solution of
epoxy ester 9 (20 mg, 0.1 mmol) in a 1:0.5 mixture of THF/EtOH (1.5
mL) was added, and the resulting mixture was stirred at 50 °C for
24 h. The reaction mixture was then cooled, purged with air for 5 min,
and concentrated. Purification of the crude product by flash chroma-
tography (silica gel, 15:1 hexanes/EtOAc) afforded (S)-2-pentyl (R)-
NMR spectroscopy). The mixture was diluted with pentane (20 mL),
cooled (-78 °C), filtered through a fritted funnel, and concentrated on
a rotary evaporator in an ice water bath. The resulting oil was dissolved
in pentane (20 mL), cooled (-78 °C), filtered through a fritted funnel,
and concentrated on a rotary evaporator in an ice-water bath to give
(S)-2-chloropentanal (6) (600 mg, >97% yield, 85% enantiomeric
excess21) as a clear oil. H NMR (500 MHz) δ: 9.44 (d, 1H, J ) 2.5
1
Hz), 4.14 (ddd, 1H, J ) 2.5, 5.0, 7.5 Hz), 1.90 (m, 1H), 1.77 (m, 1H),
1.47 (m, 2H), 0.91 (t, 3H, J ) 7.0 Hz). 13C NMR (125 MHz, CDCl3)
δ: 195.2, 63.6, 33.8, 18.7, 13.2. IR (neat): 2963, 2936, 2876, 2849,
1735, 1466, 1434, 1382, 1262, 1206, 1055 cm-1. Exact mass calcd for
1
3-hydroxyhexanoate (5) (18 mg, 90%) as a clear oil. H NMR (600
25
MHz, CDCl3) δ: 4.97 (apparent sextet, 1H, J ) 6.3 Hz), 4.00 (m, 1H),
3.01 (d, 1H, J ) 3.8 Hz), 2.48 (dd, 1H, J ) 2.9, 16.4 Hz), 2.38 (dd,
1H, J ) 9.2, 16.4 Hz), 1.61-1.27 (m, 8H), 1.22 (d, 3H, J ) 6.3 Hz),
0.93 (t, 3H, J ) 7.2 Hz), 0.91 (t, 3H, 7.1 Hz). 13C NMR (150 MHz,
CDCl3) δ: 173.1, 71.5, 67.9, 41.7, 38.8, 38.2, 20.2, 18.9, 18.8, 14.2,
14.1. IR (neat): 3450, 2932, 2874, 1731, 1466, 1379, 1176, 1120, 1019,
996 cm-1. Exact mass calcd for C11H23O3: 203.1647 (M + H); found
C5H10ClO: 121.5868; found 121.5870. [R]D -11.4 (c 0.6, CHCl3).
(3R,4S)-4-Chloro-3-hydroxy-1-(trimethylsilyl)-1-heptyne (7). To
a cold (-78 °C), stirred solution of trimethylsilylacetylene (98 mg, 1
mmol) and dry THF (10 mL) was added n-butyllithium (0.54 mL, 1.84
M in hexanes, 1.0 mmol). After 10 min, a solution of (2S)-2-
chloropentanal (6) (132 mg, 1.1 mmol) in THF (1.0 mL) was added,
and the resulting mixture was stirred for 10 min. The solution was
then treated with saturated aqueous NH4Cl (10 mL) and diluted with
Et2O (10 mL), and the phases were separated. The aqueous phase was
extracted with Et2O (3 × 10 mL), and the combined organic phases
were washed with brine (15 mL), dried (MgSO4), and concentrated to
give the crude chlorohydrin (>20:1 mixture of diastereomers as
determined by 1H NMR spectroscopy). Purification of the crude product
by flash chromatography (silica gel, 15:1 hexanes/EtOAc) afforded
(3R,4S)-4-chloro-3-hydroxy-1-trimethylsilyl-1-heptyne (7) (180 mg,
83%) as a colorless oil. 1H NMR (500 MHz) δ: 4.51 (dd, 1H, J ) 3.5,
8.3 Hz), 4.03 (m, 1H), 2.43 (t, 1H, J ) 8.3 Hz), 1.81 (m, 2H), 1.62
(m, 1H), 1.45 (m, 1H), 0.96 (t, 3H, J ) 7.4 Hz), 0.19 (s, 9H). 13C
NMR (150 MHz) δ: 101.7, 92.0, 66.5, 66.4, 35.4, 19.7, 13.5, -0.3. IR
(neat): 3367, 2961, 2875, 2177, 1466, 1382, 1251, 1123 cm-1. Exact
25
203.1630. [R]D +3.4 (c 10.0, CHCl3).
Acknowledgment. Financial support was provided by the Natural
Sciences and Engineering Research Council of Canada (NSERC)
(Discovery grant to R.B.) and by a NSERC-Industrial Research Chair
to G.G. with Pherotech Int., S.C. Johnson Canada, and Global Forest
Science (GF-18-2007-226; GF-18-2007-227) as industrial sponsors. J.M.
was supported, in part, by a Michael Smith Foundation for Health
Research Trainee Award.
Supporting Information Available: 1H NMR and 1D TOCSY
1
spectra recorded on the natural product and H and 13C NMR spectra
for all new synthetic compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
25
mass calcd for C10H18ClSi: 201.0861 (M - OH); found 201.0867. [R]D
-11.1 (c 8.7, CHCl3).
(2S,3R)-2,3-Epoxyhexanoic Acid (8). To a solution of (3R,4S)-4-
chloro-3-hydroxy-1-trimethylsilyl-1-heptyne (7) (90 mg, 0.4 mmol) in
ethanol (4 mL) was added Cs2CO3 (202 mg, 0.6 mmol), and the reaction
mixture was stirred for 16 h at room temperature. The mixture was
treated with H2O (5 mL) and diluted with pentane (10 mL), and the
phases were separated. The aqueous phase was extracted with pentane
(3 × 10 mL), and the combined organic phases were washed with brine
(3 × 15 mL), dried (MgSO4), and concentrated to give a crude epoxide,
which was used without further purification.
References and Notes
(1) Mallis, A. Handbook of Pest Control, 5th ed.; MacNair-Dorland Co.:
New York, 1969; p 158.
(2) (a) Barrows, W. M. J. Exp. Zool. 1907, 4, 515. (b) Hunter, S. H.;
Kaplan, H. M.; Enzmann, E. V. Am. Nat 1937, 71, 575. (c) West,
A. S. J. Econ. Entomol. 1961, 54, 677.
(3) Zhu, J.; Park, K.-C.; Baker, T. C. J. Chem. Ecol. 2003, 29, 899.
(4) Baker, T. C.; Zhu, J.; Park, K.-C. United States Patent No. 6,543,181,
April 8, 2003.
(5) (a) Reed, M. R. Physiol. Zool. 1938, 11, 317. (b) Spencer, W. P. Am.
Midland Nat. 1950, 43, 79. (c) Phaff, H. J.; Miller, M. W.; Recca,
J. A.; Shifrine, M.; Mrak, E. M. Ecology 1956, 37, 533.
To a cold (0 °C), stirred solution of the crude epoxide in a 1.5:1:0.3
mixture of MeCN/H2O/EtOAc (11.5 mL) were added Oxone (834 mg,
1.4 mmol) and NaHCO3 (342 mg, 4.1 mmol). After 5 min, RuCl3 ·3H2O
(3 mg, 0.01 mmol) was added and the reaction mixture was stirred at
0 °C for 1 h. The mixture was then treated with 10% aqueous NaHSO3
(10 mL), acidified with 2 N aqueous HCl to pH < 2, and diluted with
EtOAc (15 mL), and the phases were separated. The aqueous phase
was extracted with EtOAc (3 × 10 mL), and the combined organic
phases were washed with brine (20 mL), dried (MgSO4), and
concentrated. Purification of the crude product by flash chromatography
(Iatrobeads, 2.5% MeOH in CH2Cl2) afforded (2R,3R)-2,3-epoxyhex-
(6) For the analysis of volatiles produced by various banana cultivars see:
(a) Nogueira, J. M. F.; Fernandes, P. J. P.; Nascimento, A. M. D.
Phytochem. Anal. 2003, 14, 87. (b) Jordan, M. J.; Tandon, K.; Shaw,
P. E.; Goodner, K. L. J. Agric. Food. Chem. 2001, 49, 4813. (c) Pe´rez,
A. G.; Cert, A.; R´ıos, J. J.; Ol´ıas, J. M. J. Agric. Food. Chem. 1997,
´
45, 4393. (d) Cano, M. P.; Ancos, B. d.; Matallana, M. C.; Camara,
M.; Reglero, G.; Tabera, J. Food. Chem 1997, 59, 411. (e) Salmon,
B.; Martin, G. J.; Remaud, G.; Fourel, F. FlaVour Fragr. J. 1996, 11,
353. (f) Shiota, H. J. Agric. Food Chem. 1993, 41, 2056. (g) Macku,
C.; Jennings, W. G. J. Agric. Food Chem. 1987, 35, 845.
(7) (a) Arn, H.; Sta¨dler, E.; Rauscher, S. Z. Naturforsch. C 1975, 30, 1137.
(b) Gries, R.; Khaskin, G.; Gries, G.; Bennett, R. G.; King, G. G. S.;
Morewood, P.; Slessor, K. N.; Morewood, W. D. J. Chem. Ecol. 2002,
28, 2283.
(8) The retention times, retention indices, amounts (ng/µL), and percent
composition of all antennal stimulatory compounds can be found in
the Supporting Information.
(9) Byers, J. A. J. Chem. Ecol. 1992, 18, 1603.
(10) For example see: Schwarz, J.; Gries, R.; Hillier, K.; Vickers, N.; Gries,
1
anoic acid (8) (35 mg, 65% over two steps) as a clear oil. H NMR
(600 MHz) δ: 8.11 (br s, 1H), 3.26 (s, 1H), 3.20 (t, 1H, J ) 4.8 Hz),
1.68-1.46 (m, 4H), 0.98 (t, 3H, 7.4 Hz). 13C NMR (150 MHz) δ: 174.8,
58.9, 52.5, 33.3, 19.0, 13.7. IR (neat): 2963, 2876, 1730, 1462, 1383,
1198, 899 cm-1. Exact mass calcd for C6H11O3: 131.0708 (M + H);
25
found 131.0703. [R]D +11.6 (c 8.0, CHCl3).
(S)-2-Pentyl (2R,3R)-2,3-expoxyhexanoate (9). To a solution of
epoxy acid 8 (100 mg, 0.77 mmol) in CH2Cl2 (10 mL) were added
(2S)-2-pentanol (92 µL, 0.85 mmol), DCC (238 mg, 1.2 mmol), and
DMAP (9 mg, 0.08 mmol). The resulting mixture was stirred at room
temperature for 48 h, filtered through a plug of Celite, and concentrated.
Purification of the crude product by flash chromatography (silica gel,
CH2Cl2) afforded (S)-2-pentyl (2R,3R)-2,3-expoxyhexanoate (9) (92 mg,
60%) as a clear oil. 1H NMR (500 MHz) δ: 5.00 (apparent sextet, 1H,
J ) 6.3 Hz), 3.18 (d, 1H, J ) 1.9 Hz), 3.12 (dt, 1H, J ) 1.9, 6.2 Hz),
1.67-1.45 (m, 6H), 1.33 (m, 2H), 1.23 (d, 3H, J ) 6.3 Hz), 0.97 (t,
3H, J ) 7.2 Hz), 0.91 (t, 3H, J ) 7.3 Hz). 13C NMR (100 MHz) δ:
169.0, 72.3, 58.2, 53.2, 37.9, 33.4, 19.8, 19.1, 18.6, 13.8, 13.7. IR (neat):
2961, 2875, 1749, 1465, 1381, 1290, 1244, 1200, 1121 cm-1. Exact
G. J. Chem. Ecol. 2009, 35, 58.
(11) For the detection of 2-pentanol and esters derived from 2-pentanol in
collections of banana volatiles see ref 6.
(12) The related ester ethyl 3-hydroxyhexanoate has been reported as a
volatile component in bananas. However, its structure was only
tentatively assigned using the NIST library (see ref 6b).
(13) GC analysis of 5 was carried out on a HP5890 GC equipped with a
custom-made chiral GC column coated with a 1:1 mixture of
heptakis(2,6-di-O-methyl-3-O-pentyl)-ꢀ-cyclodextrin and OV-1701 as
described in: Gries, R.; Gries, G.; Khaskin, G.; King, S.; Olfert, O.;
Kaminski, L.-A.; Lamb, R.; Bennett, R. Naturwissenschaften 2000,

776 Journal of Natural Products, 2009, Vol. 72, No. 4
Notes
87, 450 Temperature program: 100 °C isothermal. (3R,2S)-5 tR ) 26.7
min; (3S,2S)-5 tR ) 27.1 min; (3R,2R)-5 and (3S,2R)-5 tR ) 29.9 and
30.7 min.
(19) Treatment of synthetic (3R,2S)-5 with LiAlH4 afforded (3R)-1,3-
hexandiol; its specific rotation ([R]D25-9.2, c 2.4 EtOH) was consistent
with that reported ([R]D25-10.5, c 1.45 EtOH) in: Crump, D. Aust.
J. Chem. 1982, 35, 1945.
(20) Staddon, B. W.; Everton, I. J. Comp. Biochem. Physiol. A Comp.
Physiol. 1980, 65, 371.
(21) The optical purity of 6 was determined by chiral GC analysis following
reduction (NaBH4, MeOH) to (2S)-2-chloropentanol. Temperature
program: 70 °C held for 5 min then increased by 20 °C per minute
until 150 °C and run for 30 min. tR ) 21.0 min ((R)-enantiomer);
tR ) 21.8 min ((S)-enantiomer).
(14) For example see: Apfel, C.; Banner, D. W.; Bur, D.; Dietz, M.; Hirata,
T.; Hubschwerlen, C.; Locher, H.; Page, M. G. P.; Pirson, W.; Rosse´,
G.; Specklin, J.-L. J. Med. Chem. 2000, 43, 2324.
(15) Optically pure ꢀ-hydroxy esters are also available via reduction of
the corresponding ꢀ-keto ester, see: Ager, D. J.; Laneman, S. A.
Tetrahedron: Asymmetry 1997, 8, 3327.
(16) Kang, B.; Britton, R. Org. Lett. 2007, 9, 5083.
(17) Yang, D.; Chen, F.; Dong, Z.-M.; Zhang, D.-W. J. Org. Chem. 2004,
69, 2221.
(18) Miyashita, M.; Suzuki, T.; Hoshino, M.; Yoshikoshi, A. Tetrahedron
1997, 53, 12469.
NP800597K