Cannabidiol as the Substrate in Acid-Catalyzed Intramolecular Cyclization

Article abstract of DOI:10.1021/acs.jnatprod.0c00436

The chemical reactivity of cannabidiol is based on its ability to undergo intramolecular cyclization driven by the addition of a phenolic group to one of its two double bonds. The main products of this cyclization are Δ9-THC (trans-Δ-9-tetrahydrocannabinol) and Δ8-THC (trans-Δ-8-tetrahydrocannabinol). These two cannabinoids are isomers, and the first one is a frequently investigated psychoactive compound and pharmaceutical agent. The isomers Δ8-iso-THC (trans-Δ-8-iso-tetrahydrocannabinol) and Δ4(8)-iso-THC (trans-Δ-4,8-iso-tetrahydrocannabinol) have been identified as additional products of intramolecular cyclization. The use of Lewis and protic acids in different solvents has been studied to investigate the possible modulation of the reactivity of CBD (cannabidiol). The complete NMR spectroscopic characterizations of the four isomers are reported. High-performance liquid chromatography analysis and 1H NMR spectra of the reaction mixture were used to assess the percentage ratio of the compounds formed.

Full text of DOI:10.1021/acs.jnatprod.0c00436

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Article
Cannabidiol as the Substrate in Acid-Catalyzed Intramolecular
Cyclization
Paola Marzullo,† Francesca Foschi,† Davide Andrea Coppini, Fabiola Fanchini, Lucia Magnani,
Selina Rusconi, Marcello Luzzani, and Daniele Passarella*
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ABSTRACT: The chemical reactivity of cannabidiol is based on its ability
to undergo intramolecular cyclization driven by the addition of a phenolic
group to one of its two double bonds. The main products of this cyclization
are Δ9-THC (trans-Δ-9-tetrahydrocannabinol) and Δ8-THC (trans-Δ-8-
tetrahydrocannabinol). These two cannabinoids are isomers, and the first
one is a frequently investigated psychoactive compound and pharmaceutical
agent. The isomers Δ8-iso-THC (trans-Δ-8-iso-tetrahydrocannabinol) and
Δ
4(8)-iso-THC (trans-Δ-4,8-iso-tetrahydrocannabinol) have been identified
as additional products of intramolecular cyclization. The use of Lewis and
protic acids in different solvents has been studied to investigate the possible
modulation of the reactivity of CBD (cannabidiol). The complete NMR
1
spectroscopic characterizations of the four isomers are reported. High-performance liquid chromatography analysis and H NMR
spectra of the reaction mixture were used to assess the percentage ratio of the compounds formed.
ecent years have seen a dramatically increasing interest in
for Alzheimer’s disease, Parkinson’s disease, cancer, and
infertility.8
R
phytocannabinoids. Isolated from Cannabis in 1940,1,2
Δ9-THC is the key compound of Cannabis sativa with major
psychoactive effects.5 From a pharmacological perspective, Δ9-
THC is a partial agonist at both cannabinoid receptors: CB1, a
modulator of psychoactive effects, and CB2, a modulator of
immunological and anti-inflammatory effects.5 The psycho-
active effects of Δ9-THC include anxiety, paranoia, perceptual
alterations, and cognitive deficits. All these CB1-mediated
effects are caused by the perturbation of GABA (γ-amino-
butyric acid)/glutamatergic neurotransmission and dopamine
release, and above all, they are generally acute, transient, and
self-limited.5 Moreover, a low Δ9-THC acute toxicity in
murine models has also been observed. Lastly, after Δ9-THC
administration, hypolocomotion, hypothermia, catalepsy, an-
algesia, and increased food intake have been reported.5
The possibility of inducing intramolecular cyclization of
CBD to create the THC skeleton is well-known. Because of the
remarkable difference in terms of the activity between CBD,
Δ9-THC, and its isomers, we decided to study (a) the
feasibility of this reaction, (b) its selectivity, and (c) the
cannabidiol (CBD) is one of the most abundant phytocanna-
binoids in the species of Cannabis for textile uses.3,4 Despite
the structural similarity between CBD and Δ9-THC (trans-Δ-
9-tetrahydrocannabinol) (Figure 1), CBD has a low agonistic
Figure 1. Structures of cannabidiol (CBD) and Δ-9-tetrahydrocanna-
binol (Δ9-THC).
effect for cannabinoid receptors; in particular, it is considered
an allosteric negative modulator of CB1 and CB2 receptors
(cannabinoid receptor types 1 and 2).5,6 Current evidence
shows that CBD exerts pharmacological effects via specific
molecular targets such as adenosine, glycine, opioid, serotonin,
nonendocannabinoid G protein-coupled, nicotinic acetylcho-
line, and proliferator-activated receptors.7 Moreover, CBD
shows anticonvulsant, antispasmodic, anxiolytic, antinausea,
antirheumatoid arthritis, and neuroprotective properties.5
Recently, it has been demonstrated that CBD is an inverse
agonist for G protein-coupled orphan receptors, such as GPR3,
GPR6, and GPR12, suggesting new therapeutic uses of CBD
Received: April 20, 2020
© XXXX American Chemical Society and
American Society of Pharmacognosy
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A

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Scheme 1. CBD Acid-Promoted Cyclization
Scheme 2. CBD Conversions with Acids and the Structures of the Products
a
Table 1. Reaction Conditions Screening of Acid-Catalyzed Cyclization of CBD Using Lewis Acids
b
reaction mixture composition (%)
entry
acid
solvent
T (°C)
−10
time (h)
Δ9-THC
Δ8-THC
Δ8-iso-THC
Δ
4(8)-iso-THC
1
2
3
4
5
6
7
8
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
TMSOTf
TMSOTf
In(OTf)3
In(OTf)3
In(OTf)3
In(OTf)3
ZnBr2
CH2Cl2
CH2Cl2
CH2Cl2
Tol
Tol
THF
MeCN
CH2Cl2
Tol
CH2Cl2
CH2Cl2
Tol
Tol
CH2Cl2
CH2Cl2
4
6
48
3
6
6
6
6
6
6
44
2
10
41
1
52
11
2
36
NR
5
93
75
6
72
NR
98
NR
9
3
0
−78 to −30
−10
0
5
29
26
NR
5
−10
−10
−10
−10
−10
0 to RT
−10
0
NR
NR
30
9
12
52
10
11
12
13
14
15
4
NR
NR
48
4
24
96
NR
NR
NR
RT
−10
NR
34
TiCl4
6
a
b
1
RT, room temperature; NR, no reaction. Determined via HPLC and H NMR analysis.
availability of an efficient and quick method for monitoring this
conversion.
Thus, CBD was treated with Lewis and protic acids, and the
composition of the resulting mixture was evaluated using high-
performance liquid chromatography (HPLC) or direct NMR
spectra analysis.
formed by internal ether formation of one of the phenolic
groups with one of the double bonds. The two double bonds in
the CBD structure are responsible for the formation of two
different compounds (Scheme 1). If the activation occurs on
the Δ8 double bond, the products show the THC scaffold (Δ9-
THC, path b); otherwise, the Δ1 double bond activation leads
to the formation of the iso-THC scaffold (Δ8-iso-THC, path a).
The latter cyclization is much less frequent. However, acidic
conditions are responsible for further isomerization toward the
corresponding thermodynamically more stable compounds,
Δ8-THC and Δ4(8)-iso-THC, respectively.11
RESULTS AND DISCUSSION
■
According to the literature, the cyclization reaction of CBD
seems to occur following an acid-catalyzed activation of a
specific double bond.9,10 A dihydrobenzopyran ring moiety is
B
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a
Table 2. Reaction Conditions Screening of Acid-Catalyzed Cyclization of CBD Using Protic Acids
b
reaction mixture composition (%)
entry
acid
solvent
T (°C)
RT
time (h)
Δ9-THC
Δ8-THC
Δ8-iso-THC
Δ
4(8)-iso-THC
1
2
3
4
5
6
7
8
HCl
H2O
72
36
36
18
48
96
96
72
96
24
96
96
24
96
57
94
66
pTSA
pTSA
pTSA
pTSA
pTSA cat
CSA
H2SO4
H2SO4
ascorbic acid
ascorbic acid
citric acid
HOAc
CH2Cl2
n-Hex
DMSO
Tol
Tol
Tol
CH2Cl2
Tol
CH2Cl2
Tol
EtOH
CH2Cl2
Tol
RT
RT
RT
RT
RT
RT
0
RT
0
RT
RT
0
13
NR
82
9
13
11
89
c
61
5
NR
NR
NR
NR
NR
NR
NR
4
NR
NR
NR
NR
NR
NR
NR
11
NR
NR
NR
NR
NR
NR
NR
9
NR
NR
NR
NR
NR
NR
NR
10
11
12
13
14
15
HOAc
H3PO4
RT
−10 to 50
Tol
48
1
a
b
c
RT, room temperature; NR, no reaction. Determined via HPLC and H NMR analysis. pTSA 10% catalytic amount.
Although Δ9-THC and its derivatives have been widely
cyclization reaction even at room temperature (Table 1, entry
14), while TiCl4 showed a trend similar to that of BF3 (Table
1, entry 15). The activity of AlCl3, AgOTf, and Ti(OiPr)4 was
also investigated, without any noteworthy results. Considering
these outcomes, a unique preferential formation path for any of
the possible isomers cannot be determined based on the
characteristics of the Lewis acid used to induce the cyclization.
Subsequently, protic acid screening was performed (Table
2). The best results for CBD conversion were obtained with
HCl, pTSA (p-toluenesulfonic acid), and CSA (camphorsul-
fonic acid). As reported,9 pTSA in CH2Cl2 led directly to the
formation of Δ8-THC as the sole product (Table 2, entry 2).
The nature of the solvent clearly affected the reaction outcome.
The reaction in n-hexane afforded a mixture of Δ9-THC, Δ8-
THC, and Δ8-iso-THC in a ratio of 1:5:1 (Table 2, entry 3),
while the reaction in toluene gave a higher selectivity. pTSA
gave different isomers in different percentages depending on
the solvent and reaction time (Table 2, entries 2−5). The best
selectivity of Δ9-THC and Δ8-THC formation was obtained
with toluene and CH2Cl2, respectively. On the contrary, the
use of 10% mmol catalytic amounts of acid in toluene resulted
in almost complete isomerization of the double bond because
of the increased reaction time that shifted the outcome to the
thermodynamic isomer (Table 2, entry 6). Interestingly, CSA
promoted the cyclization of CBD to Δ9-THC with complete
selectivity and satisfactory yields regardless of the reaction time
(Table 2, entry 7). Other protic acids gave worse results for the
CBD conversion (Table 2, entries 8−15).
explored and recognized as the major psychoactive Cannabis
constituents, the iso-THC isomers have received little
attention. For this reason, we wish to fill the literature gap,
especially regarding the provision of full NMR data.
To investigate the susceptibility and selectivity of CBD
cyclization, different reactions, including the use of Lewis and
protic acids in different solvents and varying the temperature
and reaction time, were performed (Scheme 2).
The Lewis acids were evaluated first, starting with the
recorded use of BF3·OEt2.12,13 The data suggest that
performing the reaction with BF3·OEt2 in CH2Cl2 at a low
temperature affords Δ9-THC as the main product, but
increasing the temperature and reaction time results in
preferential formation of the more stable Δ8-THC. The results
support this assertion (Table 1, entries 1 and 2). Lowering the
temperature also lowers the yields (Table 1, entry 3). Using
other solvents gave different degrees of selectivity. In
particular, toluene gave results similar to those in CH2Cl2,
but iso-THCs always accompanied the Δ8- and Δ9-THCs
(Table 1, entries 4 and 5). A reaction conducted in MeCN at
−10 °C for 6 h yielded Δ8-iso-THC as the main product
accompanied by trace amounts of Δ4(8)-iso-THC (Table 1,
entry 7).
To enhance the yields and the selectivity of the process, a
series of tests with different acids were conducted, following
the hypothesis that other Lewis acids could actively induce
cyclization. Starting from the positive literature results
regarding the use of TMSI (trimethylsilyl iodide), which
showed a high yield of Δ9-THC formation without isomer-
ization,14 TMSOTf (trimethylsilyl triflate) was tested as an
acidic reagent. Contrary to the expectations, it displayed a high
affinity for the formation of Δ8-THC when CH2Cl2 or toluene
were used as solvents, even at a low temperature (Table 1,
entries 8 and 9).
Some Δ4(8)-iso-THC formation was detected in three cases
(Table 1, entries 5 and 7; Table 2, entry 8), and this
compound was isolated and characterized.
The experimental results indicate that toluene is the most
suitable solvent for the conversion of CBD into THC isomers.
This solvent particularly affects the selectivity of the isomers
according to the other experimental conditions (the reaction
temperature and nature of the acid cyclization promoter).
Increasing the temperature reduces the selectivity of the
activation of the double bond and favors the formation of the
corresponding most stable isomers. The Lewis acids BF3·OEt2,
In(OTf)3, and TMSOTf have a proven effect and affected the
major formation of Δ8-THC and the product mixtures. As for
protic acids, pTSA promotes the reaction to selectively afford
In(OTf)3 in CH2Cl2 converted CBD into Δ9-THC at a low
temperature in a better yield than that of BF3·OEt2 (Table 1,
entry 10). As in previous tests, higher temperatures caused the
production of the thermodynamically more stable isomer in
higher yields (Table 1, entry 11). Using toluene, the selectivity
shifted to the formation of Δ8-THC in excellent yields (Table
1, entry 13). The use of ZnBr2 in CH2Cl2 did not promote the
C
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a
Table 3. Reaction Conditions of the Acid-Catalyzed Cyclization of CBD Using CSA
b
reaction mixture composition (%)
entry
solvent
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
T (°C)
time (h)
Δ9-THC
Δ8-THC
Δ8-iso-THC
1
2
3
4
5
6
7
8
RT
RT
RT
30
40
40
40
40
50
50
RT
RT
30
30
30
30
48
96
120
96
24
48
72
96
3
61
20
53
48
45
28
13
37
62
33
64
48
31
54
NR
28
20
19
52
72
87
10
19
5
36
52
41
26
NR
9
Tol
Tol
10
11
12
13
14
15
16
4
CH2Cl2
CH2Cl2
CH2Cl2
n-Hex
MTBE
cyclohexane
24
48
24
96
96
9
NR
96
a
b
1
RT, room temperature; NR, no reaction. Determined via HPLC and H NMR analysis.
Δ9- and Δ8-THCs, depending on the reaction time. CSA
emerges as an interesting cyclization inducer, giving Δ9-THC
in good yields through readily accessible reaction conditions.
With these encouraging screening results, the influence of
CSA was more thoroughly investigated; therefore, the solvent,
temperature, and time were considered variables. Using
toluene as the solvent at room temperature gave a 61% yield
of Δ9-THC accompanied by unreacted CBD (Table 2, entry
7). A longer reaction time led to isomerization of Δ9-THC and
to decomposition of compounds (Table 3, entry 3). Increasing
the temperature led to the formation of a mixture that was
enriched with the Δ8 isomer over time (Table 3, entries 4−8).
Increasing the temperature further drastically reduced the
CBD conversion time and isomerization, obtaining Δ9-THC as
a kinetic reaction product (Table 3, entries 9 and 10). In
CH2Cl2, the reaction was faster and less selective toward Δ9-
THC formation (Table 3, entries 11−13). n-Hexane and
MTBE (t-butyl methyl ether) induced a high degree of CBD
conversion without a marked preferential selectivity for the
formation of a THC isomer even at a short reaction time
(Table 3, entries 14 and 15). In all experiments, iso-THC
isomers were not detected except when the reaction was
performed in MTBE (Table 3, entry 15).
Table 4. NMR Spectroscopy Data (400 MHz, Methanol-d4)
of CBD
Toluene showed the best selectivity; however, the long
reaction time seemed to be a drawback. CH2Cl2 appeared to
be promising in this respect, but continuous monitoring of the
reaction was required to avoid the prevalence of isomerization.
Δ9-THC, Δ8-THC, Δ8-iso-THC, and Δ4(8)-iso-THC were
1
fully characterized using NMR data, and the complete H and
13C NMR assignments (Table 4−6) have been determined on
the basis of 1D and 2D NMR spectra (1H and 13C NMR,
correlation spectroscopy (COSY), heteronuclear single quan-
tum coherence (HSQC), and heteronuclear multiple bond
correlation (HMBC)). The data were compared with those
available in the literature.15,16
a
Chemical shifts (in ppm) were determined with reference to TMS.
Spectra recorded at 101 MHz. Spectra recorded at 400 MHz.
b
c
d−
e
Chemical shifts bearing the same symbol overlap.
Diagnostic and distinguishable NMR peaks permit identi-
fication of compounds derived from intramolecular cyclization
within the crude reaction mixture and also allow the
composition percentage to be determined from integration
ratios (Figure 2). In CDCl3, Δ9-THC is characterized by the
presence of signals at 6.34 ppm (H-10), 3.23 ppm (H-10a),
1
2.22−2.16 ppm (H-8), and 4.88 ppm (OH). The H NMR
data of Δ8-THC show two signals for H-10 (3.21 and 2.19−
2.15 ppm), while the signal due to H-10a is present at 2.71
ppm. The olefinic (H-8) and the hydroxy proton appear at
D
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1
Figure 2. H NMR spectrum in CDCl3 of a mixture of CBD, Δ9-THC, Δ8-THC, and Δ8-iso-THC.
5.45 and 4.63 ppm, respectively. For Δ8-iso-THC, the
corresponding characteristic signals are the doublet at 4.98
ppm (H-9) and the two signals at 3.49 and 2.37 ppm that
match the protons H-3 and H-4, respectively. The spectrum of
Δ
4(8)-iso-THC shows a characteristic signal at 4.29 ppm (H-3).
On the basis of the literature data,17 an HPLC method was
used to follow the cyclization of CBD. The reactions
monitored via HPLC provided a composition of the reaction
1
mixture comparable with that of the H NMR data analysis.
The analysis was performed on an ASCENTIS RP-C18
column (5 μm × 4.6 × 150 mm). The pressure was set at
101 bar, and the temperature was maintained at 40 °C with a
constant flow rate of 0.95 mL/min. UV spectra were recorded
at 228.8 nm using a gradient elution method. The mobile
phase consisted of a mixture of A (0.1% v/v HCOOH in H2O)
and B (0.1% v/v HCOOH in MeCN). The gradient elution
program was adapted to a 30 min duration to obtain RRT 1.00
for CBD and RRT 1.28 for Δ9-THC. After 30 min, the column
was purged with 100% B in 7 min; subsequently, the system
was washed under these conditions for 3 min and restored to
the initial conditions. The retention times were CBD, 23.63
min; Δ4(8)-iso-THC, 29.62 min; Δ9-THC, 29.92 min; Δ8-
THC, 30.77 min; and Δ8-iso-THC, 30.77 min (Figure 3).
The method allowed an excellent separation of CBD from
the THC isomers; in particular, it was possible to recognize
Δ9-THC from Δ8-THC and Δ8-iso-THC; however, the peaks
were quite close. The drawback was that it remained difficult to
obtain a better resolution between the peaks of Δ9-THC/
Figure 3. Representative chromatogram of the standard cannabinoid
mixture.
Δ
4(8)-iso-THC, and Δ8-THC/Δ8-iso-THC, which have similar
°C for 96 h selectively afforded Δ8-THC in high yields. The
use of BF3·OEt2 in toluene led to the formation of the iso-THC
isomer, depending on the reaction temperature. At −10 °C, a
separable mixture of Δ9-THC and Δ8-iso-THC was obtained,
whereas a temperature increase to 0 °C shifted the result
toward the corresponding most stable isomers, Δ8-THC and
retention times. For this reason, the HPLC results were always
1
compared with those obtained from the H NMR data.
In conclusion, all THC isomers were fully characterized via
1H and 13C NMR spectroscopy. An analytical method was
optimized to monitor the course of the reactions. In particular,
it was found that CSA in toluene at room temperature (RT)
for 96 h and pTSA in toluene for 48 h at RT were the best
conditions for the selective formation of Δ9-THC. TMSOTf in
CH2Cl2 at −10 °C for 6 h, In(OTf)3 in toluene at 0 °C for 24
h, pTSA in CH2Cl2 at RT for 36 h, and CSA in toluene at 40
Δ
4(8)-iso-THC. CBD is a challenging substrate that permits the
chemical reactivity of natural alkenes and phenols to be
addressed and exploited.
E
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Table 5. NMR Spectroscopy Data (400 MHz, CDCl3) of Δ9-THC and (300 MHz, CDCl3) of Δ8-THC
a
b
c
d
Chemical shifts (in ppm) were determined with reference to TMS. Spectra recorded at 101 MHz. Spectra recorded at 400 MHz. Spectra
e−i
recorded at 300 MHz.
Chemical shifts bearing the same symbol overlap.
eter, and HR-ESI mass spectra were recorded on a FT-ICR APEXII
instrument (Bruker Daltonics). EI mass spectra were recorded at an
ionizing voltage of 6 kEv on a VG 70−70 EQ. Optical rotation values
were measured on a Jasco P-1030 polarimeter at 20 °C, using a
sodium D line wavelength λ = 589 nm.
EXPERIMENTAL SECTION
■
General Experimental Procedures. Unless otherwise stated,
reagents and solvents were purchased from Sigma-Aldrich (Milan,
Italy), Fluorochem (Hadfield, United Kingdom), or TCI (Zwijn-
drecht, Belgium) and used without further purification. All reactions
were carried out in oven-dried glassware and dry solvents under a
nitrogen atmosphere and were monitored by TLC on silica gel
(Merck precoated 60F254 plates), with detection by UV light (254
nm) or by cerium molybdate stain (Hanessian’s stain). Analytical
HPLC was performed on an ASCENTIS RP-C18 column (5 μm × 4.6
× 150 mm). The pressure was set at about 101 bar, and the
temperature was maintained at 40 °C, with a constant flow rate of
0.95 mL/min. UV spectra were recorded at 228 nm using a gradient
elution method. The mobile phase consisted of a mixture of A (0.1%
v/v HCOOH in H2O) and B (0.1% v/v HCOOH in MeOH). The
gradient was programmed linearly from 60% B to 90% B in 30 min.
Flash column chromatography (FCC) was performed using silica gel
(240−400 mesh, Merck) as a stationary phase. 1H NMR spectra were
recorded on a Bruker Avance Spectrometer 300 or 400 MHz, and
chemical shifts are reported relative to residual CDCl3, methanol-d4,
or acetone-d6. 13C NMR spectra were recorded on the same
instrument (101 MHz), and chemical shifts are reported relative to
residual CDCl3, methanol-d4, or acetone-d6. All 1D and 2D NMR
spectra were acquired using the standard pulse sequences available
with Bruker Topspin 1.3. Chemical shifts (δ) for proton and carbon
resonances are quoted in parts per million (ppm) relative to TMS,
used as an internal standard. Data for 1H NMR are reported as
follows: chemical shift (δ/ppm), multiplicity, and coupling constants
(Hz). Multiplicities are reported as follows: s = singlet, d = doublet, t
= triplet, m = multiplet, and br s= broad singlet. Data for 13C NMR
are reported in terms of chemical shifts (δ/ppm). MS spectra were
recorded using the Electrospray Ionization (ESI) technique on a
Waters Micromass quadrupole time-of-flight micro-mass spectrom-
General Procedure Using Lewis or Protic Acids. All the
reactions were performed under a nitrogen atmosphere in different
anhydrous solvents and at different temperatures. To a CBD stirred
solution at the specified temperature (more details follow below) was
slowly added the corresponding Lewis or protic acids, and the mixture
was stirred. The reaction was quenched with a saturated aqueous
NaHCO3 solution, stirred for 30 min, and washed with a saturated
aqueous NaHCO3 solution and with brine. The organic phase was
dried over Na2SO4, filtered, and evaporated under reduced pressure.
All the reactions were monitored by TLC (CH2Cl2/n-hexane 1:3)
developed by cerium molybdate stain, and the crudes were analyzed
by 1H NMR spectroscopy in CDCl3 and HPLC to determine
composition. All the residues were purified by FCC on silica gel
(CH2Cl2/n-hexane 1:3), providing four possible THC isomers.
CBD. [α]2D0 −113 (c 1, EtOH); [HPLC ASCENTIS C18; RT CBD
= 23.63 min]; 1H and 13C NMR data see Table 4; HRMS (ESI) m/z
[M + Na]+ 337.2137 (calcd. for C21H30O2Na, 337.213).
Δ9-THC. [α]2D0 −159 (c 1, CHCl3); [HPLC ASCENTIS C18; RT
1
Δ9-THC = 29.92 min]; H and 13C NMR data see Table 5; HRMS
(ESI) m/z [M + Na]+ 337.2132 (calcd for C21H30O2Na, 337.2138).
Δ8-THC. [α]2D0 −238 (c 1, CHCl3); [HPLC ASCENTIS C18; RT
1
Δ8-THC = 30.77 min]; H and 13C NMR data see Table 5; HRMS
(ESI) m/z [M + Na]+ 337.2136 (calcd. for C21H30O2Na, 337.2138).
Δ8-iso-THC. [α]D20 −249 (c 1, CHCl3); [HPLC ASCENTIS C18;
1
RT Δ8-iso-THC = 30.77 min]; H and 13C NMR data see Table 6;
HRMS (ESI) m/z [M + Na]+ 337.2141 (calcd. for C21H30O2Na,
337.2138).
Δ
4(8)-iso-THC. [α]D20 −236 (c 1, CHCl3); [HPLC ASCENTIS C18;
RT Δ4(8)-iso-THC = 29.62 min]; 1H and 13C NMR data see Table 6;
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Table 6. NMR Spectroscopy Data (300 MHz, Acetone-d6) of Δ8-iso-THC and (400 MHz, CDCl3) of Δ4(8)-iso-THC
a
b
c
d
Chemical shifts (in ppm) were determined with reference to TMS. Spectra recorded at 101 MHz. Spectra recorded at 300 MHz. Spectra
e−k
Chemical shifts bearing the same symbol overlap.
recorded at 400 MHz.
HRMS (ESI) m/z [M + Na]+ 337.2133 (calcd. for C21H30O2Na,
337.2138).
Conditions (Table 1, entry 9): CBD (80 mg, 0.25 mmol); solvent,
anhydrous toluene (1.25 mL); T = −10 °C; TMSOTf (91 μL, 0.5
mmol); reaction time, 6 h. Yields: Δ9-THC, 10 mg (12%); Δ8-THC,
61 mg (75%).
In(OTf)3-Catalyzed Reactions (Table 1). Reactions were
performed as specified in the general procedure for Lewis acids.
Conditions (Table 1, entry 10): CBD (317 mg, 1 mmol); solvent,
anhydrous CH2Cl2 (5 mL); T = −10 °C; In(OTf)3 (675 mg, 1.2
mmol); reaction time, 6 h. Yields: Δ9-THC, 165 mg (52%); Δ8-THC,
18 mg (6%); Δ8-iso-THC, 12 mg (4%).
Conditions (Table 1, entry 11): CBD (317 mg, 1 mmol); solvent,
anhydrous CH2Cl2 (5 mL); T = 0 °C to RT; In(OTf)3 (58 mg, 0.1
mmol); reaction time, 48 h. Yields: Δ8-THC, 228 mg (72%).
Conditions (Table 1, entry 13): CBD (156 mg, 0.5 mmol); solvent,
anhydrous toluene (2.5 mL); T = 0 °C; In(OTf)3 (563 mg, 1 mmol);
reaction time, 24 h. Yields: Δ8-THC, 153 mg (98%).
TiCl4-Catalyzed Reaction (Table 1). The reaction was
performed as specified in the general procedure for Lewis acids.
Conditions (Table 1, entry 15): CBD (315 mg, 1 mmol); solvent,
anhydrous CH2Cl2 (5 mL); T = −10 °C; TiCl4 (167 μL, 1.2 mmol);
reaction time, 6 h. Yields: CBD, 38 mg (12%); Δ9-THC, 108 mg
(34%); Δ8-THC, 27 mg (9%).
HCl-Catalyzed Reaction (Table 2). Reaction was performed as
specified in the general procedure for protic acids.
Conditions (Table 2, entry 1): CBD (156 mg, 0.5 mmol); solvent,
H2O (1.6 mL); T = RT; HCl 37% (1.6 mL); reaction time, 72 h.
Yields: Δ8-THC, 89 mg (57%).
BF3·OEt2-Catalyzed Reactions (Table 1). Reactions were
performed as specified in the general procedure for Lewis acids.
Conditions (Table 1, entry 1): CBD (315 mg, 1 mmol); solvent,
anhydrous CH2Cl2 (5 mL); T = −10 °C; BF3·OEt2 (151 μL, 1.2
mmol); reaction time, 4 h. Yields: Δ9-THC, 138 mg (44%); Δ8-THC,
4 mg (1%); Δ8-iso-THC, 11 mg (3%).
Conditions (Table 1, entry 2): CBD (315 mg, 1 mmol); solvent,
anhydrous CH2Cl2 (5 mL); T = 0 °C; BF3·OEt2 (151 μL, 1.2 mmol);
reaction time, 6 h. Yields: Δ9-THC, 5 mg (2%); Δ8-THC, 164 mg
(52%).
Conditions (Table 1, entry 3): CBD (315 mg, 1 mmol); solvent,
anhydrous CH2Cl2 (5 mL); T = −78 to −30 °C; BF3·OEt2 (151 μL,
1.2 mmol); reaction time, 48 h. Yields: Δ9-THC, 32 mg (10%); Δ8-
THC, 35 mg (11%); Δ8-iso-THC, 16 mg (5%).
Conditions (Table 1, entry 4): CBD (156 mg, 0.5 mmol); solvent,
anhydrous toluene (2.5 mL); T = −10 °C; BF3·OEt2 (76 μL, 0.6
mmol); reaction time, 3 h. Yields: Δ9-THC, 64 mg (41%); Δ8-THC,
3 mg (2%); Δ8-iso-THC, 45 mg (29%).
Conditions (Table 1, entry 5): CBD (316 mg, 1 mmol); solvent,
anhydrous toluene (5 mL); T = 0 °C; BF3·OEt2 (151 μL, 1.2 mmol);
reaction time, 6 h. Yields: Δ8-THC, 115 mg (36%); Δ4(8)-iso-THC, 83
mg (26%).
Conditions (Table 1, entry 7): CBD (315 mg, 1 mmol); solvent,
anhydrous MeCN (5 mL); T = −10 °C; BF3·OEt2 (151 μL, 1.2
mmol); reaction time, 6 h. Yields: Δ8-THC, 16 mg (5%); Δ8-iso-
THC, 95 mg (30%); Δ4(8)-iso-THC, 17 mg (5%).
TMSOTf-Catalyzed Reactions (Table 1). Reactions were
performed as specified in the general procedure for Lewis acids.
Conditions (Table 1, entry 8): CBD (315 mg, 1 mmol); solvent,
anhydrous CH2Cl2 (5 mL); T = −10 °C; TMSOTf (217 μL, 1.2
mmol); reaction time, 6 h. Yields: Δ8-THC, 293 mg (93%).
pTSA·H2O-Catalyzed Reactions (Table 2). Reactions were
performed as specified in the general procedure for protic acids.
Conditions (Table 2, entry 2): CBD (154 mg, 0.5 mmol); solvent,
anhydrous CH2Cl2 (2.5 mL); T = RT; pTSA·H2O (189 mg, 1 mmol);
reaction time, 36 h. Yields: Δ8-THC, 145 mg (94%).
Conditions (Table 2, entry 3): CBD (155 mg, 0.5 mmol); solvent,
n-hexane (2.5 mL); T = RT; pTSA·H2O (190 mg, 1 mmol); reaction
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time, 36 h. Yields: Δ9-THC, 20 mg (13%); Δ8-THC, 102 mg (66%);
Author Contributions
Δ8-iso-THC, 20 mg (13%).
†These authors contributed equally. The manuscript was
written through contributions of all authors. All authors have
given approval to the final version of the manuscript.
Conditions (Table 2, entry 5): CBD (318 mg, 1 mmol); solvent,
anhydrous toluene (5 mL); T = RT; pTSA·H2O (386 mg, 2 mmol);
reaction time, 48 h. Yields: Δ9-THC, 262 mg (82%); Δ8-THC, 34 mg
(11%).
Conditions (Table 2, entry 6): CBD (79 mg, 0.25 mmol); solvent,
anhydrous toluene (1.25 mL); T = RT; pTSA·H2O (6 mg, 0.025
mmol); reaction time, 96 h. Yields: Δ9-THC, 7 mg (9%); Δ8-THC,
70 mg (89%).
CSA-Catalyzed Reaction (Table 2). The reaction was performed
as specified in the general procedure for protic acids.
Conditions (Table 2, entry 7): CBD (79 mg, 0.25 mmol); solvent,
anhydrous toluene (1.25 mL); T = RT; CSA (117 mg, 0.5 mmol);
reaction time, 96 h. Yields: CBD, 28 mg (36%); Δ9-THC, 48 mg
(61%).
Notes
The authors declare no competing financial interest.
REFERENCES
■
(1) Adams, R.; Baker, B. R.; Wearn, R. B. J. Am. Chem. Soc. 1940, 62
(8), 2204−2207.
(2) Jacob, A.; Todd, A. R. Nature 1940, 145 (3670), 350−350.
(3) Hanus, L. O.; Meyer, S. M.; Munoz, E.; Taglialatela-Scafati, O.;
Appendino, G. Nat. Prod. Rep. 2016, 33, 1357−1392.
(4) Izzo, A. A.; Borrelli, F.; Capasso, R.; Di Marzo, V.; Mechoulam,
R. Trends Pharmacol. Sci. 2009, 30, 515−527.
(5) Pertwee, R. G. Br. J. Pharmacol. 2008, 153 (2), 199−215.
(6) Casajuana Koguel, C.; Lopez-Pelayo, H.; Balcells-Olivero, M.
M.; Colom, J.; Gual, A. Adicciones 2018, 30 (2), 140−151.
(7) Ibeas Bih, C.; Chen, T.; Nunn, A. V. W.; Bazelot, M.; Dallas, M.;
Whalley, B. J. Neurotherapeutics 2015, 12 (4), 699−730.
(8) Laun, A. S.; Shrader, S. H.; Brown, K. J.; Song, Z.-H. Acta
Pharmacol. Sin. 2019, 40 (3), 300−308.
H2SO4-Catalyzed Reactions (Table 2). Reactions were
performed as specified in the general procedure for protic acids.
Conditions (Table 2, entry 8): CBD (315 mg, 1 mmol); solvent,
anhydrous CH2Cl2 (5 mL); T = 0 °C; H2SO4, 98% (54 μL, 1 mmol);
reaction time, 72 h. Yields: Δ8-THC, 16 mg (5%); Δ8-iso-THC, 13
mg (4%); Δ4(8)-iso-THC, 37 mg (11%).
General Procedure for CSA Screening Reactions (Table 3).
The procedure is the same as described in the general procedure for
Lewis acids, but the reactions were quenched by diluting with EtOAc
and were monitored by TLC (n-hexane/EtOAc 7:3, eluted 2 times)
(9) Gaoni, Y.; Mechoulam, R. Tetrahedron 1966, 22 (4), 1481−
1488.
(10) Mechoulam, R.; Hanus, L.ır Chem. Phys. Lipids 2002, 121 (1−
2), 35−43.
1
developed by cerium molybdate stain. Crudes were analyzed by H
(11) Gaoni, Y.; Mechoulam, R. J. Am. Chem. Soc. 1966, 88 (23),
5673−5675.
NMR spectroscopy in CDCl3 and HPLC to determine the
composition. Conditions: CBD (1 equiv); CSA (2 equiv); solvent,
toluene (0.2 M) or as specified in Table 3; T as specified in Table 3.
(12) Gaoni, Y.; Mechoulam, R. J. Am. Chem. Soc. 1971, 93 (1), 217−
224.
(13) Nikas, S. P.; Grzybowska, J.; Papahatjis, D. P.; Charalambous,
A.; Banijamali, A. R.; Chari, R.; Fan, P.; Kourouli, T.; Lin, S.;
Nitowski, A. J.; Marciniak, G.; Guo, Y.; Li, X.; Wang, C. L. J.;
Makriyannis, A. AAPS J. 2004, 6 (4), 23.
(14) Denis, P.; Mortreux, A.; Petit, F.; Buono, G.; Peiffer, G. J. Org.
Chem. 1984, 49 (26), 5274−5276.
(15) Choi, Y. H.; Hazekamp, A.; Peltenburg-Looman, A. M. G.;
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jnatprod.0c00436.
NMR spectra of compounds (PDF)
́
́
Frederich, M.; Erkelens, C.; Lefeber, A. W. M.; Verpoorte, R.
Phytochem. Anal. 2004, 15 (6), 345−354.
(16) de A. Leite, J.; de Oliveira, M. V.L.; Conti, R.; de S. Borges, W.;
Rosa, T. R.; Filgueiras, P. R.; Lacerda, V.; Romao, W.; Neto, A. C. Sci.
Justice 2018, 58 (5), 355−365.
AUTHOR INFORMATION
■
Corresponding Author
Daniele Passarella − Dipartimento di Chimica, Universita degli
(17) Gul, W.; Gul, S. W.; Radwan, M. M.; Wanas, A. S.; Mehmedic,
Z.; Khan, I. I.; Sharaf, M. H. M.; ElSohly, M. A. J. AOAC Int. 2015, 98
(6), 1523−1528.
̀
Studi di Milano, Milan 20133, Italy; orcid.org/0000-0001-
6180-9581; Phone: +39 02 5031 4081;
Email: daniele.passarella@unimi.it; https://users.unimi.it/
passalab/
Authors
̀
Paola Marzullo − Dipartimento di Chimica, Universita degli
Studi di Milano, Milan 20133, Italy
̀
Francesca Foschi − Dipartimento di Chimica, Universita degli
Studi di Milano, Milan 20133, Italy
̀
Davide Andrea Coppini − Dipartimento di Chimica, Universita
degli Studi di Milano, Milan 20133, Italy
̀
Fabiola Fanchini − Dipartimento di Chimica, Universita degli
Studi di Milano, Milan 20133, Italy
̀
Lucia Magnani − Dipartimento di Chimica, Universita degli
Studi di Milano, Milan 20133, Italy
̀
Selina Rusconi − Dipartimento di Chimica, Universita degli
Studi di Milano, Milan 20133, Italy
̀
Marcello Luzzani − Dipartimento di Chimica, Universita degli
Studi di Milano, Milan 20133, Italy
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jnatprod.0c00436
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