Organic nitrate esters are key products of terpene
oxidation in the atmosphere. We report here the preparation and purification
of nine nitrate esters derived from (+)-3-carene, limonene, α-pinene, β-pinene and perillic alcohol. The availability of these
compounds will enable detailed investigations into the structure–reactivity
relationships of aerosol formation and processing and will allow individual
investigations into aqueous-phase reactions of organic nitrate esters.
Introduction
Biogenic volatile organic compound (BVOC) emissions account for
∼88 % of non-methane VOC emissions. Of the total BVOC
estimated by the Model of Emission of Gases and Aerosols from Nature version
2.1 (MEGAN2.1), isoprene is estimated to comprise half, and methanol,
ethanol, acetaldehyde, acetone, α-pinene, β-pinene, limonene,
ethene and propene together encompass another 30 %. Of the terpenoids,
α-pinene alone is estimated to generate ∼66 Tg yr-1 (Guenther et al., 2012). These monoterpenes can be oxidized by
nitrate radicals that are projected to account for more than half of the
monoterpene-derived secondary organic aerosol (SOA) in the US (Pye et al.,
2010). Nitrate oxidation pathways have been shown to be important
particularly during nighttime. A large portion (30 %–40 %) of monoterpene
emissions occur at night (Pye et al., 2010). These emissions can then react
with NO3 radicals, formed from the oxidation of NO2 emissions by
O3 (Pye et al., 2010).
The full role of organic nitrates (ONs) is complicated with many different
sources and sinks (Perring et al., 2013). Fully deconvoluting the
atmospheric processing of terpene-derived ON is difficult, particularly due
to partitioning into the aerosol phase in which hydrolysis and other
reactivity can occur (Bleier and Elrod, 2013; Rindelaub et al., 2014, 2015;
Romonosky et al., 2015; Thomas et al., 2016). Hydrolysis reactions of
nitrate esters of isoprene have been studied directly (Jacobs et al., 2014)
and the hydrolysis of ON has been studied in bulk (Baker and Easty, 1950).
These and other studies have shown that the hydrolysis of ON is dependent on
structure (Darer et al., 2011). For example, primary and secondary ON are
thought to be relatively stable (Hu et al., 2011). In contrast, tertiary
nitrates have been shown to hydrolyze on the order of hours (Boyd et al.,
2015; Liu et al., 2012) to minutes (Darer et al., 2011). To the best of our
knowledge there is only one study of the hydrolysis of an isolated
terpene-derived hydroxynitrate (2 in Fig. 1) (Rindelaub et al., 2016a).
Two hydroxynitrate esters with available spectral data. Relative
stereochemistry is undefined.
Furthermore, fully understanding the atmospheric processes of organic
molecules is restricted by the ability to identify these species (Nozière
et al., 2015). Part of this challenge is, of course, related to the lack of
available standards. While one certainly cannot synthesize all of the
atmospherically relevant ON, having access to representative compounds from
monoterpenes would enable key studies. With these molecules in hand, the
atmospheric chemistry community could directly study the ON reactivity, such
as hydrolysis, and deconvolute the structure–reactivity relationships.
Additionally, novel method development would be enabled and validated
(Rindelaub et al., 2016b). For example, Nozière and co-authors called
attention to “the lack of NMR spectra libraries for atmospheric markers”
as a barrier to utilizing nuclear magnetic resonance (NMR) spectroscopy in atmospheric science. While
the preparation of α-hydroxynitrates of terpenes has been alluded to
in a handful of reports, we have only been able to identify NMR data for two
species (Fig. 1). Two main methods appear to have been attempted, treating
an epoxide with either fuming nitric acid (Rollins et al., 2010) or bismuth
nitrate (Rindelaub et al., 2016b; Romonosky et al., 2015). The utility of
the former is limited by the extreme hazards involved with mixing fuming
nitric acid with organic materials (Parker, 1995; Univ. of California, Berkeley,
2019) and only provides characterization data for a single compound. On the
surface the latter appears to be a usable method but on closer inspection
lacks spectral data, perhaps due to intractable and inseparable mixtures
(Romonosky et al., 2015), and has been the subject of a retracted study
(Pöschl et al., 2011).
ExperimentalInstrumentation and materials
All starting materials and reagents were purchased from commercial sources
and used without further purification unless otherwise noted. Bismuth
nitrate (98 %) was purchased from Strem Chemicals and ground to a fine
powder prior to use. Isoprene, methyl vinyl ketone (MVK), (+)-3-carene, β-pinene, α-pinene oxide and 1,2-limonene oxide were purchased from Sigma-Aldrich. Silica
gel chromatography was performed on a Teledyne Isco CombiFlash+ Lumen using
25 µm SiliCycle spherical silica gel. 1H and 13C NMR spectra
were recorded on a Bruker AV400 spectrometer with chloroform (7.26 ppm) or
benzene (7.28 ppm) as internal standards. Infrared (IR) spectra were recorded on a
Thermo Nicolet IR100 spectrometer using a Thunderdome attenuated total
reflectance (ATR) sample accessory. Melting points were collected on a
DigiMelt 160 melting point apparatus. Exact masses were collected at the
BioAnalytical Mass Spectrometry Facility at Portland State University using
a ThermoElectron LTQ-Orbitrap Discovery high-resolution mass spectrometer
with a dedicated Accela HPLC system. Low-resolution masses were collected on
a Varian Saturn 2000 gas chromatographer–mass spectrometer (GC–MS) with a Restek Crossbind Rxi-5ms column.
Abbreviations include meta-chloroperoxybenzoic acid (m-CPBA), hexanes (Hex),
ethyl acetate (EtOAc), column volume (CV), thin layer chromatography (TLC),
retention factor (Rf), diastereomeric ratio (d.r.), broad (br), singlet
(s), doublet (d), doublet of doublets (dd), doublet of doublets of doublets
(ddd), doublet of quartet (dq), triplet (t), multiplet (m), broad (b), weak
(w), medium (m), strong (s) and very strong (vs).
Preparation of epoxidestrans-Carene oxide (3)
To a solution of (+)-3-carene (0.86 g, 1 mL, 7.34 mmol) in dichloromethane
(25 mL) at 0 ∘C was added meta-chloroperoxybenzoic acid (m-CPBA)
(1.85 g, 8.1 mmol, 1.1 equiv.). The solution was warmed to 23 ∘C over 1 h. The solution was poured onto saturated aqueous (sat. aq) sodium
bicarbonate (30 mL) and extracted with dichloromethane (3×30 mL). The
combined organics were washed with sat. aq sodium bicarbonate (2×30 mL),
dried (MgSO4) and concentrated to yield a crude clear, colorless oil
(1.17 g). The crude oil was purified by column chromatography (SiO2; 0 %–25 % EtOAc/Hex over 10 column volumes) to yield clear colorless oil
(685 mg, 4.5 mmol; 61 % yield): 1H NMR (400 MHz, chloroform-d) δ 2.82 (s, 1 H), 2.28 (ddd, J=16.4, 9.0, 1.9 Hz, 1 H), 2.13 (dd, J=16.2,
9.1 Hz, 1 H), 1.63 (dt, J=16.4, 2.3 Hz, 1 H), 1.48 (dd, J=16.1, 2.3 Hz,
1 H), 1.24 (s, 3 H), 1.00 (s, 3 H), 0.72 (s, 3 H), 0.52 (td, J=9.1, 2.3 Hz,
1 H), 0.44 (td, J=9.1, 2.3 Hz, 1 H) ppm. Spectral data are consistent with
literature reports (Cabaj et al., 2009).
cis-Carene oxide (6)
According to the methods of Cocker and Grayson (1969), a round bottom flask
was charged with a solution of (+)-3-carene (2.7 g, 20 mmol) in dioxane
(20 mL) and water (10 mL). Calcium carbonate (2 g, 20 mmol) and
N-bromosuccinimide (7 g, 40 mmol) were added to the solution. The internal
temperature rose to 50 ∘C after the initial addition. The
mixture was stirred for 2 h then poured onto water (50 mL), filtered and
washed with diethyl ether. The filtrate was extracted with ether (2×100 mL). The combined extract was washed with water (3×100 mL) and sodium
thiosulfate (5 % aq, 20 mL), dried (Na2SO4), and concentrated to
yield a crude pale yellow oil. Purification by column chromatography
(SiO2; 0 %–50 % EtOAc / Hex) yielded the bromohydrin as a white
crystalline solid (4.6 g, 99 % yield): 1H NMR (400 MHz,
chloroform-d) δ 4.07 (dd, J=11.1, 7.6 Hz, 1 H), 2.50–2.36 (m,
2 H), 2.21 (dd, J=14.6, 10.1 Hz, 1 H), 1.41 (dd, J=4.9, 1.2 Hz, 1 H), 1.38
(s, 3 H), 1.03 (s, 3 H), 1.00 (s, 3 H), 0.91–0.80 (m, 1 H), 0.70 (m, 1 H).
Spectral data are consistent with literature reports (Cocker and Grayson,
1969). Bromohydrin (2.44 g, 10.5 mmol) was dissolved in 100 mL tBuOH (warmed
with a water bath). Potassium tert-butoxide (2.44 g, 21.8 mmol, 2.08 equiv.) was
added, and the solution was stirred at rt for 2 h. The solution was
poured onto water (50 mL), extracted with diethyl ether (3×75 mL), washed
with water (3×50 mL), dried (MgSO4) and concentrated to yield 2.5 g
crude clear oil. Purification by column chromatography (40 g SiO2; 0 %–25 % EtOAc / Hex over 10 CVs) yielded a clear colorless oil (1.31 g, 63 %
yield): 1H NMR (400 MHz, chloroform-d) δ 2.89 (d, J=5.6 Hz,
1 H), 2.30 (ddd, J=16.7, 9.0, 5.6 Hz, 1 H), 2.08 (dd, J=16.4, 8.9 Hz, 1 H),
1.82 (s, 1 H), 1.78 (s, 1 H), 1.32 (s, 3 H), 0.98 (s, 3 H), 0.94 (s, 3 H), 0.63–0.49 (m, 2 H). Spectral data are consistent with literature reports (Cocker
and Grayson, 1969).
8,9-Limonene oxide (9)
Methyl vinyl ketone (10 mL, 90 mmol), isoprene (10 mL, 75 mmol) and dichloromethane (DCM) (90 mL) were added to a round bottom flask with stir bar. The flask was purged
with inert atmosphere and chilled to 0 ∘C with stirring.
AlCl3 (1.2 g, 9 mmol) was added in three portions over 10 min. The
ice bath was removed and the reaction mixture was stirred for 1.5 h. The
crude reaction mixture was filtered through a 1.5 in. (3.8 cm) silica gel pad (350 mL, 9 cm diameter) and washed with 8 % EtOAc : Hex (4×160 mL). The filtrate
was concentrated to yield 1-(4-methyl-3-cyclohexene) ethenone
(8 in Fig. 4) as a clear yellow liquid (9.68 g, 88 % yield): 1H NMR
(400 MHz, chloroform-d) δ 5.44–5.38 (m, 1 H), 2.59–2.49 (m, 1 H),
2.15–2.20 (s, 4 H), 2.06–1.95 (m, 3 H), 1.67 (s, 3 H), 1.61 (m, 2 H).
Spectral data are consistent with literature reports (Buss et al., 1987).
Sodium hydride (1.766 g, 44.16 mmol) was suspended in dry dimethylsulfoxide (DMSO) (25 mL). A
solution of trimethylsulfonium iodide (10.356 g, 47.06 mmol) in DMSO (50 mL)
was added via cannula. The solution was stirred at rt for 30 min and then
heated to 50 ∘C until gas evolution ceased (1 h). To this
solution was added 1-(4-methyl-3-cyclohexen-1-yl) ethenone (8 in Fig. 4)
(5 g, 36 mmol) and heated to 70 ∘C for 2 h. The
reaction mixture was cooled to room temperature and poured onto sat. aq.
NH4Cl (100 mL), extracted with methyl tert-butyl ether (MTBE) (4×100 mL). The combined organics
were washed with water (4×50 mL), dried (MgSO4) and concentrated to
yield 5.1 g crude oil. Purification by column chromatography (80 g SiO2; 0 %–20 % EtOAc / Hex over 10 CVs) yielded 8,9-limonene oxide (1 : 1
d.r.) as a clear, colorless oil (3.76 g, 68 % yield): 1H NMR (400 MHz, chloroform-d) δ 5.43–5.33 (br s, 1 H), 2.67 (t, J=5.3 Hz,
1 H), 2.58 (dd, J=10.9, 4.8 Hz, 1 H), 2.17–1.73 (m, 5 H), 1.66 (s, 3 H),
1.44 (m, 2 H), 1.29 (d, J=4.5 Hz, 3 H). Spectral data are consistent with
literature reports (Almeida and Jones Jr., 2005)
cis-1,2-Limonene oxide (12-cis)
According to the methods of Steiner et al. (2002), a 20 mL scintillation
vial was charged with 1,2-limonene oxide, pyrrolidine and water. The vial
was sealed with a Teflon-lined cap and heated to 88 ∘C for
24 h. The vial was cooled to rt, transferred to a separatory funnel with 100 mL pentane, and washed with sat. aq. ammonium chloride (3×30 mL) and water (1×50 mL). The organics were dried (MgSO4) and concentrated to yield a
light yellow oil. Purification by column chromatography (80 g spherical
SiO2; 0 %–15 % EtOAc / Hex over 10 CVs) yielded cis-1,2-limonene oxide as a
clear, colorless oil. (1.52 g, 64 %): 1H NMR (400 MHz,
chloroform-d) δ 4.77–4.71 (m, 1 H), 4.69 (s, 1 H), 3.07 (s, 1 H),
2.22–2.07 (m, 2 H), 1.90–1.84 (m, 2 H), 1.75–1.64 (m, 5 H), 1.33 (s,
3 H), 1.28–1.15 (m, 1 H) ppm. Spectral data are consistent with literature
reports (Steiner et al., 2002).
trans-1,2-Limonene oxide (12-trans)
According to the methods of Steiner et al. (2002), a 50 mL flask was charged
with 1,2-limonene oxide (4.57 g, 30 mmol), pyrazole (0.34 g, 5 mmol) and
water (16.2 mL). The reaction was heated to reflux for 5 h. The reaction
mixture was cooled to 80 ∘C and transferred to a separatory
funnel. Warm water (80 ∘C; 60 mL) was added. The emulsion
was extracted with pentane (3×50 mL). The combined organic layers were
dried (MgSO4) and concentrated to yield 1.481 g crude oil. Purification
by column chromatography (80 g spherical SiO2; 0 %–20 % EtOAc / Hex
over 13.5 CVs) yielded trans-1,2-limonene oxide (485 mg, 20 % yield): 1H
NMR (400 MHz, chloroform-d) δ 4.68 (s, 2 H), 3.01 (d, J=5.3 Hz, 1 H),
2.11–2.00 (m, 2 H), 1.96–1.82 (m, 1 H), 1.75–1.64 (m, 5 H), 1.45–1.36 (m, 2 H), 1.34 (s, 3 H) ppm. Spectral data are consistent with literature
reports (Steiner et al., 2002).
Perillic alcohol epoxide (15)
Perillic alcohol (5 mL, 31.46 mmol) was combined with dichloromethane (62 mL) in a round bottom flask with stir bar and cooled to 0 ∘C under inert atmosphere. m-CPBA (5.97 g, 34.61 mmol) was added in five
portions over the course of 10 min. After 30 min the reaction was
warmed back up to room temperature and allowed to stir for an hour at
room temperature. The reaction mixture was filtered through celite, washed
with DCM (3×30 mL) and then transferred to a separatory funnel where it was
washed with sodium bicarbonate (50 mL) and brine (50 mL). The organic layer
was dried (Na2SO4) and concentrated to yield a crude oil (5.1 g).
Purification by column chromatography (80 g spherical SiO2; 0 %–100 % EtOAc / Hex over 15 CVs) yielded the epoxide (15) as a clear,
colorless oil (3.363 g, 19.8 % yield): 1H NMR (400 MHz,
chloroform-d) δ 4.77–4.66 (m, 2 H), 3.78–3.59 (m, 2 H), 3.41–3.31 (m, 1 H), 2.26–2.14 (m, 1 H), 2.09 (doublet of triplet of doublet (dtd) J=14.9, 5.6, 1.8 Hz, 1 H),
1.88–1.83 (m, 1 H), 1.80–1.59 (m, 7 H), 1.56–1.38 (m, 1 H), 1.32–1.16 (m, 1 H). Spectral data are consistent with literature reports (Thomas et
al., 2016).
β-Pinene oxide
Potassium peroxide monosulfate (4.750 g, 31.20 mmol) was dissolved in
deionized water (60 mL). Sodium bicarbonate (4.019 g, 47.84 mmol) was placed
into a 125 mL Erlenmeyer flask and acetone (40 mL) was added, followed by
β-pinene (1.59 mL, 9.974 mmol). The solution of potassium peroxide
monosulfate mixture was slowly added into the β-pinene mixture over
the course of 3 min via syringe while the mixture was stirring at
800 rpm. The reaction mixture was stirred for exactly 30 min while
stirring at 800 rpm at room temperature. The reaction mixture was
transferred to a separatory funnel and extracted with dichloromethane (2×40 mL), dried (MgSO4) and concentrated to produce a clear oil (1500 g,
96 % yield). 1H NMR (400 MHz, chloroform-d) δ 2.79 (d, J=4.8 Hz, 1 H), 2.62 (d, J=4.8 Hz, 1 H), 2.30 (dtd, J=10.3, 5.9, 1.5 Hz,
1 H), 2.19 (ddd, J=15.1, 10.7, 8.1 Hz, 1 H), 2.06–1.97 (m, 1 H), 1.97–1.79 (m, 2 H), 1.75–1.65 (m, 2 H), 1.53 (t, J=5.4 Hz, 1 H), 1.27 (s, 3 H),
0.94 (s, 3 H). Spectral data are consistent with literature reports
(Charbonneau et al., 2018).
General nitration method
A round bottom flask was charged with a solution of epoxide (2 mmol) in
toluene, dioxane or dichloromethane (10 mL, 0.2 M). Bismuth nitrate (1.164 g, 2.4 mmol, 1.2 equiv.) was added to the reaction mixture. The reaction
mixture was stirred for 30–60 min. When TLC indicated complete
consumption of starting material, the reaction was filtered through a 1 in. (2.5 cm)
celite pad and washed with DCM (2×15 mL). The filtrate was transferred to
a separatory funnel and washed with sodium bicarbonate (3×15 mL). The
organic layer was dried with sodium sulfate, filtered and concentrated to
yield a crude liquid. Desired products were isolated via column
chromatography.
Nitrate ester (4)
Nitration of trans-3-carene oxide (3 mmol) was carried out according to the
general method with dioxane. Purification of the crude oil by column
chromatography (12 g SiO2, 0 %–30 % EtOAc / Hex) yielded nitrate ester (4) as a white crystalline solid (345 mg, 53 % yield): mp = 52.2–54.0 ∘C; IR (ATR) cm-1 3412 (br w, OH), 2944 (w), 2360
(w), 1616 (vs, NO2), 1291 (m, NO2), 868 (m, NO2); high-resolution mass spectrometry (HRMS) electrospray ionization (ESI) +m/z calculated for [C10H18O4N]+ (216.1230, observed; 216.1236,
expected) 1H NMR (400 MHz, chloroform-d) δ 3.70 (dd, J=9.6,
7.6 Hz, 1 H), 2.86–2.78 (m, 1 H), 2.16 (dd, J=14.8, 7.5 Hz, 1 H), 1.83
(ddd, J=14.8, 9.6, 7.9 Hz, 1 H), 1.65 (s, 3 H), 1.47 (dd, J=14.3, 4.1 Hz, 1 H), 1.03 (s, 3 H), 1.01 (s, 3 H), 0.83–0.73 (m, 2 H); 13C NMR (101 MHz, chloroform-d) δ 95.68, 71.03, 28.37, 28.34, 27.02, 19.95, 18.98,
17.95, 15.70, 14.39 ppm.
cis-4-Caranone (7)
Nitration of cis-3-carene oxide was attempted according to the general method
with dioxane or dichloromethane. Purification of the crude oil by column
chromatography (12 g SiO2, 0 %–40 % EtOAc / Hex) yielded cis-4-caranone as a
clear, colorless oil (53 % yield): Rf 0.51 (5 % EtOAc : Hex;
anisaldehyde); 1H NMR (400 MHz, chloroform-d) δ 2.54 (ddd, J=18.0, 8.4, 1.0 Hz, 1 H), 2.44–2.25 (m, 3 H), 1.33–1.20 (m, 1 H), 1.14–0.99 (m, 5 H), 0.97 (d, J=6.4 Hz, 3 H), 0.86 (s, 3 H); 13C NMR (101 MHz, chloroform-d) δ 216.79, 41.99, 36.84, 29.77, 27.91, 22.83,
20.34, 19.47, 14.89, 14.11 ppm. Spectral data are consistent with literature
reports (Kolehmainen et al., 1993).
Nitration of cis-1,2-limonene oxide was carried out according to the general
method with dioxane or dichloromethane. Purification by column
chromatography (12 g SiO2 gold column, 0 %–50 % EtOAc / Hex over 30 CVs)
yielded nitrate ester (13) as a clear, colorless oil (53 % yield in
dioxane; 62 % yield in DCM): IR (ATR) cm-1 3660 (w,
alcoholic OH), 3340 (bd w, alcoholic OH), 2980 (m), 1618 (s, NO2), 1291
(m, NO2), 860 (vs, NO2); HRMS ESI +m/z calculated for
[C10H17O4NNa]+ (238.1052, observed; 238.1055,
expected); 1H NMR (400 MHz, chloroform-d) δ 4.76 (dd, J=8.1,
1.5 Hz, 2 H), 4.13 (s, 1 H), 2.35 (doublet of doublet of triplet (ddt), J=11.6, 8.4, 4.1 Hz, 1 H), 2.24 (dtd,
J=14.8, 3.7, 1.4 Hz, 1 H), 1.98–1.78 (m, 3 H), 1.76–1.73 (m, 4 H), 1.68–1.58 (m, 4 H), 1.49 (triplet of doublet of doublet (tdd), J=13.3, 11.6, 3.6 Hz, 1 H); 13C NMR (101 MHz, chloroform-d) δ 148.49, 109.48, 91.34, 69.22, 36.84, 33.86,
29.90, 25.77, 20.98, 20.93 ppm.
Nitrate ester (14)
Nitration of trans-1,2-limonene oxide was carried out according to the general
method with dioxane or dichloromethane. Purification by column
chromatography (12 g SiO2, 0 %–50 % EtOAc / Hex over 30 CVs) yielded
nitrate ester (14) as a clear, colorless oil (63 % in dioxane;
54 % in DCM): Rf 0.39 (15 % EtOAc : Hex; anisaldehyde); IR
(ATR) cm-1 3414 (bd w, alcoholic OH), 2940 (w), 1627 (s, NO2),
1279 (m, NO2), 876 (m, NO2), 851 (m, NO2); HRMS ESI +m/z calculated
for [C10H17O4NNa]+ (238.1048, observed; 238.1055,
expected); 1H NMR (400 MHz, chloroform-d) δ 5.01 (s, 1 H),
4.76 (d, J=6.6 Hz, 2 H), 2.19 (tt, J=10.9, 3.1 Hz, 1 H), 2.03 (ddd, J=14.7, 12.1, 2.6 Hz, 1 H), 1.92 (dq, J=14.4, 3.5, 2.7 Hz, 1 H), 1.74 (s, 3 H),
1.69–1.56 (m, 4 H), 1.32 (s, 3 H); 13C NMR (101 MHz, chloroform-d)
δ 148.27, 109.57, 84.43, 69.55, 37.96, 34.78, 29.91, 26.96, 20.89 ppm.
Nitration of perillic alcohol epoxide (15)
Nitration of
4-(1-methylethenyl)-7-oxabicyclo[4.1.0]heptane-1-methanol
(15) was carried out according to the general method with
dichloromethane. Purification by column chromatography (12 g SiO2,
0 %–40 % EtOAc / Hex over 30 CVs) yielded two regioisomers and nitrate esters
(16) and (17) in 23 % and 32 % yield, respectively.
Nitration of α-pinene oxide was carried out according to the general
method. Crude oil from a 10 mmol reaction was purified by column
chromatography (40 g SiO2, 0 %–40 % EtOAc / Hex over 30 CVs) yielded the
following isomerization products and nitrate esters.
α-Campholenic aldehyde (19)
α-Campholenic aldehyde (19) was isolated as a clear, colorless
oil: Rf 0.59 (10 % EtOAc : Hex; anisaldehyde); 1H NMR (400 MHz,
benzene-d6) δ 9.51 (t, J=2.1 Hz, 1 H), 5.23 (s, 1 H), 2.41
(doublet of doublet of doublet of triplet (dddt), J=13.8, 6.0, 2.7, 1.6 Hz, 1 H), 2.25–2.16 (m, 1 H), 2.11 (ddd,
J=15.9, 4.4, 1.8 Hz, 1 H), 2.00 (ddd, J=15.9, 10.3, 2.3 Hz, 1 H), 1.80
(doublet of doublet of pentet (ddp), J=15.8, 9.2, 2.5 Hz, 1 H), 1.58 (dt, J=3.0, 1.6 Hz, 3 H), 0.90 (s,
3 H), 0.68 (s, 3 H). Spectral data are consistent with literature reports
(Thomas et al., 2016).
Nitration of β-pinene oxide (11.1 mmol) was carried out according to
the general procedure in toluene. Purification by column chromatography (80 g SiO2, 0 %–30 % EtOAc / Hex over 20 CVs) yielded a 1 : 1 mix of myrtenol (26) and nitrate (29) (344 mg, 9 % combined yield),
perillic alcohol (28, 600 mg, 36 % yield) and nitrate (27)
(71 mg, 3 % yield). The combined myrtenol and nitrate (29) were
separated by column chromatography (40 g SiO2; 5 %–20 % EtOAc / Hex over
25 CVs).
Myrtenol (26)
Myrtenol was isolated as a clear, colorless oil: Rf 0.25 (15 %
EtOAc : Hex; anisaldehyde); 1H NMR (400 MHz, chloroform-d) δ 5.49
(dt, J=3.0, 1.5 Hz, 1 H), 4.00 (t, J=1.8 Hz, 2 H), 2.42 (dt, J=8.6,
5.6 Hz, 1 H), 2.37–2.20 (m, 2 H), 2.19–2.09 (m, 2 H), 1.61–1.45 (m,
1 H), 1.31 (s, 3 H), 1.19 (d, J=8.6 Hz, 1 H), 0.85 (s, 3 H) ppm. Spectral
data are consistent with literature reports (Motherwell et al., 2004).
Previous reports have described the opening of epoxides and aziridines using
bismuth nitrate in acetonitrile (Das et al., 2007). Others report optimal
conditions in dichloromethane (Rindelaub et al., 2016b), or 1,4-dioxane with
undesired side reactions in acetonitrile (Pinto et al., 2007). Thus, we
began our investigations into the nitration of trans-carene oxide (3)
using bismuth nitrate in various solvents (Fig. 2). In our hands, dioxane
clearly outperformed dichloromethane, yielding 53 % of the desired product
after 45 min (entry 3). A diol was isolated as a major (16 %)
byproduct. Attempts to mitigate hydrolysis by the use of base or molecular
sieves were ineffective. Acetonitrile produced a complex intractable mixture
with a high amount of diol and caronaldehyde (entry 2). Other solvents, such
as tetrahydrofuran (THF) and nitromethane, also produced complex mixtures with no trace of
desired nitrate. Methanol, interestingly, only produced an undesired methyl
ether. It is unclear if this product was generated through methanolysis of
the nitrate ester or direct substitution of the epoxide. The regiochemistry
of the nitration was easily elucidated with gHSQC NMR data. A doublet of
doublets at 3.69 ppm correlated with a carbon shift at 71.1 ppm and was thus
assigned to the alcohol methyne carbon. A tetrasubstituted carbon, with no
correlations in the gHSQC at 95.6 ppm was consistent with the tertiary
nitrate ester. Finally, the IR of compound (4) displayed the expected
strong absorbances at 1616, 1291 and 868 cm-1.
Effect of solvent on nitration of trans-3-carene oxide.
Preparation of cis-3-carene oxide and attempted nitration.
Next, we aimed to investigate the impact of relative stereochemistry on the
nitration. We hypothesized that the secondary nitrate would predominate due
to classical stereoelectronic effects. (+)-3-Carene was treated with
N-bromosuccinimide and calcium carbonate to produce a bromohydrin which
subsequently cyclized under basic conditions to produce cis-carene oxide (Cocker
and Grayson, 1969). Interestingly, we never observed the desired nitrate
ester. Instead a 1,2-hydrogen shift generated cis-4-caranone (7) in
53 % yield.
Preparation of 8.9-limonene oxide and attempted nitration.
The preparation of 8,9-limonene oxide (9) began with a Diels–Alder
reaction to form 1-(4-methyl-3-cyclohexene) ethenone (8) in
88 % yield. Corey–Chaykovsky addition of a sulfur ylide cleanly produced
8,9-limonene oxide (9). Nitration of (9) was attempted in a
variety of solvents (DCM, benzene, dioxane, acetonitrile, nitromethane). In
acetonitrile, oxazoline (10a) (1 : 1 d.r.) was cleanly produced
presumably via nucleophilic addition of acetonitrile and subsequent
cyclization to the oxazoline. Pinto et al. (2007) observed a similar bismuth
nitrate mediated epoxide opening with acetonitrile. In dioxane, aldehyde (11) (1 : 1 d.r.) was observed along with a product consistent with a
tertiary nitrate ester (10) (1 : 1 d.r.) in a 0.25 : 1 mixture. Formation
of 11-enol presumably proceeds via a facile intramolecular
elimination of nitric acid. The IR of the crude mixture showed the expected
strong absorbances at 1613, 1288 and 865 cm-1. A set of signals in the
13C NMR at 97.40 and 97.17 ppm, with no gHSQC correlations, is
consistent with a tertiary nitrate. Attempts to purify this mixture on
silica gel gave small amounts of (11) and unidentifiable
decomposition products. Furthermore, the fast hydrolysis rates of tertiary
nitrate esters hinder the ability to isolate these products (Boyd et al.,
2015; Darer et al., 2011; Liu et al., 2012). No reaction was observed when
non-polar solvents such as DCM and benzene were employed. We hypothesize
that the steric hindrance at the β-carbon severely limits reactivity.
Nitration of cis- and trans-1,2-limonene oxide.
Commercially available 1,2-limonene oxide was resolved into pure samples of
cis- and trans-1,2-limonene oxide by treating with cyclic amine bases (Steiner et
al., 2002). The nitration of the cis isomer was predicted to produce the
tertiary nitrate ester via stereoelectronic effects. The dominant
conformation of cis-1,2-limonene oxide is expected to be a pseudo-half chair
(Fig. 5). Nucleophilic substitution at the less substituted position would
proceed through a lower-energy chair-like transition state, whereas attack
at the more substituted carbon leads to a very unstable twist-boat. The
nitration of cis-1,2-limonene oxide (cis-12)
proceeded smoothly in both dioxane (53 %) and dichloromethane (63 %).
The trans isomer was expected to produce the secondary nitrate
ester. The nitration of trans-1,2-limonene oxide
(trans-12) also proceeded smoothly in both
dioxane (63 %) and dichloromethane (61 %). Contrary to previous reports
(Romonosky et al., 2015), we did not observe the desired nitrate esters in
acetonitrile. gHSQC NMR data were used to verify nitrate ester (13)
was tertiary and (14) was secondary. For (13), a
broad singlet at 4.12 ppm correlated with a carbon shift at 69.2 ppm and was
thus assigned to the methyne adjacent to the alcohol. A tetrasubstituted
carbon at 91.4 ppm was consistent with the tetrasubstituted nitrate ester.
Similarly for (14), a broad triplet at 5.00 ppm correlated with a
carbon shift at 84.45 ppm and was thus assigned to the methyne adjacent to
the nitrate ester. A tetrasubstituted carbon at 69.5 ppm was consistent with
the tertiary alcohol.
Nitration of perillic alcohol oxide.
Under acidic conditions, β-pinene oxide undergoes facile
rearrangement to form perillic alcohol. Accordingly, we prepared the epoxide (15) by treating perillic alcohol with one equivalent of m-CPBA. The
inseparable mixture of diastereomers was treated with bismuth nitrate under
the standard reaction conditions. Nitrate esters (16) and (17)
were easily separated by column chromatography and assigned to the
secondary and tertiary nitrate esters, respectively. These isomers were
formed from the cis- and trans-epoxide diastereomers in an analogous fashion to the
limonene isomers. For (16), a broad singlet at 5.21 ppm correlated
with a carbon shift at 80.1 ppm and was thus assigned to the methyne
adjacent to the nitrate ester. A tetrasubstituted carbon at 70.9 ppm was
consistent with the tertiary alcohol. Similarly for (15), a broad
singlet at 4.22 ppm correlated with a carbon shift at 65.6 ppm and was thus
assigned to the methyne adjacent to the alcohol. A tetrasubstituted carbon
at 92.8 ppm was consistent with the tertiary nitrate ester.
Nitration of α-pinene oxide.
The facile rearrangements of α-pinene with both Brønsted and
Lewis acids (Kaminska et al., 1992) can be thought to proceed via a
nonclassical isobornyl cation (18) (Kong et al., 2010). We
expected the reaction with bismuth nitrate to generate a complex mixture of
products due to the many reactive sites. As shown in
Fig. 7, three rearrangement products (campholenic
aldehyde 19, diene 20 and trans-carveol 21) and three
nitrate esters (22, 23, 24) were observed. The
structure of nitrate ester (23) was identified by a clear singlet at
4.64 ppm (s, 1 H). Two-dimensional-NMR data (gCOSY, gHSQC and gHMBC) along with comparison
to 6-exo-hydroxyfenchol, the analogous diol, confirmed the structural
assignment (Miyazawa and Miyamoto, 2004). Correspondingly, nitrate ester (24) displayed a distinct doublet of doublets of doublets at 5.31 ppm
that correlated to the methyne adjacent to the nitrate ester. Again, 2-D-NMR
data (gCOSY, gHSQC and gHMBC) were used in conjunction with the literature
spectra for platydiol and its trans diastereomer to verify the assignment (Kuo et
al., 1989).
Under all conditions campholenic aldehyde was the major product (20 %–28 %
yield). First generation nitrate ester (25) was not found under all
conditions. All six components were isolated in small amounts when the
reaction was run in dichloromethane (Fig. 7, entry 1). Cooling the
reaction to -78∘C completely shut down all reactivity
(entry 2). Interestingly, adding one equivalent of tetrabutylammonium
nitrate (TBAN) as an external nitrate source in addition to bismuth nitrate
at -78∘C generated mostly nitrate (22) (entry 3).
More polar solvents, like dioxane (entry 4), generated slightly higher
amounts of nitrates at room temperature. Aromatic solvents, such as benzene
and toluene, provided the best yield of (23). Interestingly, adding
TBAN to the nitration in toluene dramatically increased the amounts of diene (20) and trans-carveol (21). Using acetonitrile as a solvent
generated a complex mixture that appeared to be mainly diols. Zirconium
nitrate produced a similar distribution of products (Das et al., 2006), but
other metal nitrate complexes (Y(NO3)3⚫6H2O,
Co(NO3)2⚫6H2O) resulted in no observable reaction.
Nitration of β-pinene oxide.
The nitration of β-pinene was similarly complicated by isomerization
pathways through the corresponding non-classical carbocation (Fig. 8). Nitrate (27) initially co-eluted with myrtenol (26) but was
separable upon a second purification by column chromatography. The methyne
proton of (27) at 5.51 ppm is correlated with a carbon at 85.3 ppm in
the gHSQC NMR. These data are consistent with the secondary nitrate ester.
Similarly, the methyne proton in nitrate (29) is a doublet at 4.91 ppm (J4= 1.6 Hz; W coupling). This signal correlates with a carbon at
89.5 ppm. Interestingly, nitrates (30) and (31) were not
observed.
Spectral data
As shown in Table 1, all nitrate esters displayed the expected strong
nitrate ester absorbances at ∼1600, 1300 and 900 cm-1.
Methyne protons next to the secondary nitrate esters appeared between 4.64
and 5.51 ppm. Carbon chemical shifts were found over a broader range
than anticipated from 80.1 to 95.6 ppm. Finally, we observed masses
consistent with methanolysis products (185.1 m/z M+; 223.1 m/z M + K) when
nitrate esters were analyzed (GC–MS) as solutions in methanol. Further
experimentation is necessary to evaluate the implications of methanolysis.
Collated 1H and 13C NMR data with IR absorbances for
nitrate esters.
We were surprised to find that five of the nitrate esters
(4, 16, 23, 24 and 29) are solids. We have stored these compounds at 0 ∘C for up
to 9 months with no noticeable decline in purity. The remaining nitrate
esters are stable at 0 ∘C for 2–4 weeks. Interestingly,
some of these compounds appear to deviate from the expected stability
patterns (vide supra). For example, tertiary nitrate ester (4) is particularly
stable stored as a solid at 0 ∘C. In contrast, when
secondary nitrate ester (14) was stored as a neat oil at 0 ∘C, it decomposed to a complex mixture within a few weeks.
Freezing the samples as a solution in benzene is recommended for longer-term
(∼6 months) storage.
Conclusions
We have clearly delineated successful methods required to synthesize and
purify nine nitrate esters derived from mono-terpenes. Seven of these
compounds are undescribed in the literature and the remaining two had gaps
in their characterization. Using our methods it is possible to cleanly
isolate 50–100 mg even of the least prevalent isomers from the nitration
reactions of α-pinene and β-pinene. Interestingly, we did not
observe the formation of α-pinene oxide and β-pinene oxide
products that retained their bicyclic ring structures. This is consistent
with the solution-phase synthetic literature (Kaminska et al., 1992) but in
contrast to many atmospheric reports (for example, see Rindelaub et al., 2016b;
Duporte et al., 2016). We believe that the availability of these
compounds will enable further study of the structure–reactivity
relationships. For example, comparing the specific hydrolysis rates for
tertiary versus secondary nitrate esters in (13) and (14) as
well as (17) and (16) could help deconvolute the fates of each
terpene in the atmosphere. We also believe that these compounds will assist
in confirming the identities of organic nitrates that have previously been
limited to detection by MS-based methods (Rindelaub et al., 2016b). A forthcoming
report will describe our investigation into the behavior of these authentic
compounds in mass spectrometers. In particular, the detailed MS–MS data,
impact of various ionization conditions (ESI, chemical ionization (CI), etc.) and sample
preparation will be described. Finally, the availability of these compounds
is important for further studies into the influence of terpene structure on
the fate and roles of organic nitrates in SOA formation.
Data availability
All spectral data for new compounds are available in the Supplement to this article.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-4241-2020-supplement.
Author contributions
RLL designed the experiments and prepared the manuscript with contributions
from all co-authors. All co-authors carried out experiments and contributed
to reviewing and editing the manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This research was supported by generous start-up funding from Reed College.
Elena Ali McKnight is grateful for funding from the Clark Fellowship. We thank Julie Fry
for drawing our attention to this class of compounds. We are particularly
grateful to Drew Gingerich, Nick Till, Stewart Green, Alyssa Harrison and Carlo Berti for preliminary experiments and to the 2014 and 2015 Chem 343 students
for their assistance in this research.
Review statement
This paper was edited by Jason Surratt and reviewed by five anonymous referees.
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