Homogeneous Lewis and Br?nsted acids as catalysts for β-pinene oxide rearrangement to prepare myrtenol and myrtanal

Article abstract of DOI:10.1016/j.mcat.2020.110945

Myrtenol and myrtanal find their utilization as fine chemicals or intermediates for the synthesis of food additive myrtanol. β-Pinene oxide rearrangement was studied using homogeneous Lewis and Br?nsted acids as catalysts for the production of these compounds. The composition of reaction products depended on the type of active sites. Lewis active sites favoured the formation of myrtanal while in presence of Br?nsted active sites undesirable perillyl aldehyde and perillyl alcohol were formed to a high extent. Optimal reaction conditions for myrtenol and myrtanal preparation were found. With 5 mol.% of anhydrous SnCl₂ as Lewis acid catalysts and 1,4-dioxane as the solvent β-pinene oxide achieved total conversion with 95% selectivity to desired products.

Full text of DOI:10.1016/j.mcat.2020.110945

MolecularCatalysis492(2020)110945
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Homogeneous Lewis and Brønsted acids as catalysts for β-pinene oxide
rearrangement to prepare myrtenol and myrtanal
Iva Paterova*, Barbora Fidlerova, Michal Vavra, Eliska Vyskocilova, Libor Cerveny
Department of Organic Technology, UCT Prague, Prague, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords:
β-pinene oxide rearrangement
Lewis acid
Brønsted acids
Myrtenol
Myrtanal
Myrtenol and myrtanal find their utilization as fine chemicals or intermediates for the synthesis of food additive
myrtanol. β-Pinene oxide rearrangement was studied using homogeneous Lewis and Brønsted acids as catalysts
for the production of these compounds. The composition of reaction products depended on the type of active
sites. Lewis active sites favoured the formation of myrtanal while in presence of Brønsted active sites undesirable
perillyl aldehyde and perillyl alcohol were formed to a high extent. Optimal reaction conditions for myrtenol and
myrtanal preparation were found. With 5 mol.% of anhydrous SnCl2 as Lewis acid catalysts and 1,4-dioxane as
the solvent β-pinene oxide achieved total conversion with 95% selectivity to desired products.
1. Introduction
conversion of β-pinene oxide to myrtenol and myrtanal by Brønsted
acids. During testing of the properties of ammonium nitrates for the
β-Pinene oxide (BPO) rearrangement is an important reaction with
respect to the obtained products, such as myrtenol, myrtanal and
perillyl alcohol. The first two products are formed by opening the epoxy
ring of the molecule and perillyl alcohol by both epoxy and carbon ring-
opening (Fig. 1).
catalytic conversion of β-pinene oxide, the influence of Brønsted acid –
nitric acid was obvious. The presence of HNO3 significantly increased
the achieved conversion [12]. The reaction performed only in the
presence of 65% HNO3 (5 wt.% to substrate) in N,N-dimethylforma-
mide as the solvent, gave after 24 h myrtanal (26 rel.%) and myrtenol
(31 rel.%) at total conversion of starting compound.
The use of classic Lewis acids appears to be advantageous. In ni-
tromethane, selectivity to myrtanal of 22% and myrtenol of 2% at total
conversion was achieved by ZnBr2 (8.3% to the substrate) after 3 h of
reaction [7]. Vyskočilová et al. [10] focused mainly on the solvent ef-
fect on product formation in β-pinene oxide rearrangement. Myrtanal
and myrtenol with 64% resp. 2% selectivity were obtained using FeCl3.
The highest selectivities for the preparation of myrtanal and myrtenol
were achieved at 70 °C in acetonitrile and 1,4-dioxane.
Most commonly, the use of heterogeneous catalysts with Lewis ac-
tive sites in the mentioned reaction is discussed in the literature. Using
these the high selectivities towards myrtanal are achieved. The polarity
of the solvents in, which this reaction is carried out, is very variable.
High conversions and selectivities to the desired products are achieved,
for example, in nonpolar toluene, dioxane, but also in polar acetonitrile.
However, these solvents are all aprotic.
Detailed studies dealing with the comparison of Lewis and Brønsted
acids to prepare myrtenol and myrtanal are missing. Therefore, to in-
crease the knowledge about the use of classic Lewis and Brønsted acids
for this reaction we have tested different ones. In this work, we have
These products find their utilization in the chemical industry, per-
fumery, food industry, etc [1,2]. The aim of this work mainly focuses on
products obtained by epoxy ring-opening only, namely myrtenol ((6,6-
dimethylbicyclo[3.1.1]hept-2-en-2-yl)methanol) and myrtanal (6,6-di-
methylbicyclo[3.1.1]heptane-2-carbaldehyde), but since the re-
arrangement is more complicated the selectivity to products obtained
by both epoxy and carbon ring-opening is also discussed. Myrtenol and
myrtanal can be hydrogenated to a food additive myrtanol ((6,6-di-
methylbicyclo[3.1.1]heptan-2-yl)methanol). Opening of the epoxy ring
of β-pinene oxide can be catalysed homogeneously or heterogeneously
by basic or acidic catalysts, such as NaOH modified alumina [3], li-
thium diethylamide [4], aluminosilicates [5,6], modified MCM-41 (Sn,
Ti, Zr, Al, Si) [7], zeolites [8,9], classic Lewis [7,10] and Brønsted acids
[7].
Carr et al. [11] described the use of p-toluenesulfonic acid as
homogeneous acid-catalyst for β-pinene oxide rearrangement to obtain
myrtanal and myrtenol. The reaction was carried out in an 80% aqu-
eous solution of unspecified alcohol at 0 °C and provided myrtanal (17
rel.%) myrtenol (10 rel.%), perillyl aldehyde (9 rel.%) and alcohol (8
rel.%). The authors proposed possible mechanism of the catalytic
⁎ Corresponding author.
E-mail address: iva.paterova@vscht.cz (I. Paterova).
https://doi.org/10.1016/j.mcat.2020.110945
Received 22 January 2020; Received in revised form 30 March 2020; Accepted 2 April 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

I. Paterova, et al.
MolecularCatalysis492(2020)110945
Fig. 1. β-Pinene oxide rearrangement and other identified byproducts of β-pinene oxide rearrangement.
focused on the influence of the concentration of acid, its amount and
possible synergic effect of the catalyst and used solvent on selectivity to
desired product formation.
Brønsted acids in the form of aqueous solution were added into the flask
with an automatic pipette, except for p-toluenesulfonic acid mono-
hydrate, which is solid under standard conditions and was weighed in
the same way as Lewis acids.
Samples taken during the reaction were analysed on a gas chro-
matograph (Agilent 6890 N) equipped with FID and non-polar HP-5
column (30 m, 0.32 mm diameter, 0.25 μm film).
The components of the reaction mixture were identified using the
gas chromatograph (Shimadzu GC-2010 Plus) coupled with a mass
spectrometer (Shimadzu GCMS-QP 2010 Ultra) equipped with DB-1
column (50 m, 0.25 mm diameter, 0.5 μm film).
2. Experimental
β-Pinene oxide, kindly offered by AROMA Praha, a.s., was purified
by vacuum rectification using column with 20 theoretical levels (purity
75–85%). BPO conversion was calculated as the difference between
BPO concentration in the starting material (CBPO0) and the con-
centration of BPO in the sample (CBPOt) divided by BPO concentration
in the starting material (CBPO0).
3. Results and discussion
X = (CBPO0-CBPOt)/CBPO0 * 100
The rearrangement of β-pinene oxide catalysed by either Lewis or
Brønsted acids was monitored. The influence of reaction temperature,
used solvent and amount of catalyst was evaluated.
Selectivity to myrtenol, myrtanal and myrtanol was calculated as
follows:
S
=
((Cmyrtanalt
- Cmyrtanal0)+ (Cmyrtenolt - Cmyrtenol0) +
(Cmyrtanolt - Cmyrtanol0))/ (CBPO0-CBPOt) * 100
3.1. Lewis acids – reaction products
Other used chemicals were purchased from different suppliers and
were used as obtained without any purification. Solvents (acetonitrile,
THF, toluene, and 1,4-dioxane) were stored on molecular sieves (Sigma-
Aldrich, 3 Å, beads, 4–8 mesh).
The composition of the reaction mixture depended on the type of
used acid. The products identified in the reaction mixture in the pre-
sence of Lewis acids are given in Fig. 1.
An appropriate amount of Lewis acid and 4 mL of solvent were in-
serted into 25 mL round-bottomed flask equipped by a magnetic stirrer.
The flask was placed in an ice bath cooled to 0 °C. The mixture was
stirred for about 10 min to dissolve the catalyst. After this, 1 g of β-
pinene oxide was added dropwise to the flask for 1 min. Measurement
of reaction time started after β-pinene oxide dosing.
Besides expected myrtanal and myrtenol, myrtanol was also formed
in the reaction mixture in the presence of Lewis acids. Due to similar
boiling points of myrtenol and myrtanol, their separation by rectifica-
tion is problematic. However, the mixture of myrtanal, myrtenol and
myrtanol may serve as an intermediate for hydrogenation to form
myrtanol.
The arrangement of typical experiments with Brønsted acids was
analogous. The procedure differed in the addition of the catalyst.
In addition to these products, five high-molecular compounds have
been found in the reaction mixture in the total amount of approx.
2

I. Paterova, et al.
MolecularCatalysis492(2020)110945
5–25% Depending on the catalyst used. By mass spectrometry analysis,
it was not possible to identify them. Some of these compounds were
definitely ketals and acetals formed in the reaction mixture from al-
cohols and carbonyl compounds in the presence of Lewis acids. In ad-
dition to these reactions, acid-catalysed aldol condensation may also
take place. The decrease of nopinone (6,6-dimethylbicyclo[3.1.1]
heptan-2-one) concentration (about 5 rel.%), which has been present in
the starting compound, confirmed that ketal or aldol formation is likely
to occur in the reaction mixture in the presence of all Lewis acids. It is
likely that the products of these subsequent reactions also arose from
the desired products, i.e. myrtanal, myrtenol and myrtanol.
During the first experiments, it has been found that the addition of a
whole volume of the substrate to the mixture of catalyst and solvent
was accompanied by a very rapid heat release. Due to an overheating of
the mixture, the product formation could be affected. Thus, the influ-
ence of the substrate addition rate on conversion and selectivity to the
desired products was evaluated. The substrate was added to the reac-
tion mixture from 5 s to 20 min. By comparison of results obtained after
20 min of reaction at room temperature, acetonitrile as the solvent,
using 2.5 wt.% AlCl3, we have found the addition time of the substrate
Table 2
Influence of used solvent on selectivity to the sum of desired products (myr-
tanal, myrtenol, myrtanol) in the presence of Lewis acids.
Solvent
Selectivity [%]
AlCl3
FeCl3
SnCl2
ZrCl4
Acetonitrile
Tetrahydrofurane
Toluene
78
82
89
84
76
70
82
78
87
93
95
95
81
76
79
81
1,4-Dioxane
Solvent: 2 mL; 1 g substrate; catalyst: 5 wt.% to substrate; room temperature;
time: 5 min.
represented by dielectric constant and donor number was observed.
Higher achieved conversion when catalysed by FeCl3 could be caused
by better solubility in ethers such as tetrahydrofurane or dioxane. The
differences in achieved conversions in the other experiments focused on
the solvent effect (Table 1) corresponded to measurement errors.
Table
2 summarizes the selectivity at achieved conversions
(Table 1). The selectivity is given as a sum of selectivities to all desired
products, i.e. myrtanal, myrtenol and myrtanol. However, differences in
selectivity were not very significant using different solvents. The se-
lectivity to desired products was rather influenced by the type of cat-
alyst. Higher selectivities to myrtanal, myrtenol and myrtanol were
achieved when using strong Lewis acids AlCl3 and SnCl2. When cata-
lysed by AlCl3 the selectivity to desired products was lower mainly due
to the formation of high molecular products. The highest selectivity to
perillyl alcohol (6%) was achieved when catalysed by AlCl3.
does not affect the resulting conversion (25
myrtanal (53 1%) and myrtenol (38
1%) and selectivity to
2%). For the following ex-
periments, the substrate addition time of 60 s was chosen, when no
overheating of the mixture was observed. When using 2.5 wt.% AlCl3,
acetonitrile as the solvent, at room temperature and substrate was
dosed for 1 min, the reaction course almost did not change till 20 min of
the reaction. Therefore, the results of all the experiments were eval-
uated after 5 min of the reaction.
Table 3 shows selectivity ratios of myrtanal and myrtenol at the
achieved conversion of β-pinene oxide. When catalysed by FeCl3, SnCl2
and ZrCl4 slight effect of solvent polarity to the formation of myrtanal
can be seen. Using the least polar solvent, 1,4-dioxane, more myrtanal
was formed in the reaction mixture. The interaction between solvent
and catalyst may exist. As the most suitable solvent for the preparation
of myrtanal and myrtenol 1,4-dioxane was chosen.
3.2. Lewis acids – Effect of the solvent
The type of solvent influences the course of α− or β-pinene oxide
rearrangement [7,8,10–16] using other types of acid catalysts than
tested ones in this study. The solvent can interact with the catalyst and
enable easier access of the catalyst to the substrate, provide better
desorption of the product from the active sites of the catalyst, or sta-
bilize catalysts complexes with the reaction intermediates. As discussed
in literature [10], when Lewis acid is used as a catalyst the formation of
organic complexes with solvent can occur. The different nature of these
complexes may influence both selectivity and reaction rate. Therefore,
several solvents of different polarity and basicity, which may be mea-
sured by e.g. donor number described by Gutmann [17], such as acet-
onitrile (ACN), tetrahydrofuran (THF), toluene and 1,4-dioxane were
tested for use in combination with different Lewis acids, namely FeCl3,
AlCl3, SnCl2 and ZrCl4 (Table 1, Table 2).
3.3. Lewis acids – Effect of the reaction temperature
Vyskocilova et al. [10] reported the total selectivity of myrtanal and
myrtenol of 63% achieved in β-pinene oxide rearrangement catalysed
by FeCl3 in 1,4-dioxane. The reaction was carried out at 70 °C using
10 wt.% of catalysts. In our study under the same conditions but at
room temperature, the selectivity to myrtanal and myrtenol of 59% was
achieved.
The results show that the achieved conversion of β-pinene oxide at
5th min was lower when catalysed by ZrCl4 in all tested solvents and by
FeCl3 in acetonitrile and toluene. From the point of view of a central
atom according to Pearson's theory [20] of hard-soft Lewis acids, all the
tested Lewis acids are classified as the so-called hard ones. Especially
AlCl3 and SnCl2 are very strong Lewis acids. In general, the achieved
conversion was dependent on the type of catalyst and was rather in-
dependent on the choice of solvent. No obvious trend between achieved
conversion and different polarity and basicity of the solvents
When cooling the reaction mixture to 0 °C in the presence of AlCl3,
similar results of the conversion and the selectivity to the desired
products were obtained as in the case of rearrangement at the room
temperature.
It can be concluded that the reaction temperature in the presence of
tested Lewis acids did not affect β-pinene oxide conversion and se-
lectivity to the desired products.
Table 1
Influence of used solvent on β-pinene oxide conversion in presence of Lewis acids.
Solvent
Dielectric constant [18][-]
Donor number [19][kcal. mol−1
]
β-Pinene oxide conversion [%]
AlCl3
FeCl3
SnCl2
ZrCl4
Acetonitrile
Tetrahydrofurane
Toluene
37.50
7.58
2.38
2.25
14.1
20.0
0.1
95
99
99
99
67
91
71
94
96
99
99
99
79
69
79
80
1,4-Dioxane
14.8
Solvent: 2 mL; 1 g substrate; catalyst: 5 wt.% to substrate; room temperature; time: 5 min.
3

I. Paterova, et al.
MolecularCatalysis492(2020)110945
Table 3
amount of the catalyst. It can be seen using AlCl3 hydrate significantly
lower conversion of β-pinene oxide was achieved than when the an-
hydrous form was used. It could be due to the lower solubility of
AlCl3.6H2O in the solvent compared to AlCl3 as a higher amount of 1,4-
dioxane had to be used. The second reason for low activity of alumi-
nium chloride hexahydrate could be due to its unique structure. The
hexahydrate consists of octahedral [Al(H2O)6]3+ centres and chloride
counterions. Hydrogen bonds link the cation and anions [21]. The hy-
drated form of aluminium chloride has an octahedral molecular geo-
metry, with the central aluminium ion surrounded by six water ligand
molecules. This means that the hydrated form cannot act as a Lewis acid
since it cannot accept electron pairs.
Using FeCl3.6H2O, the total conversion was achieved with a 10 mol.
% of catalysts related to the substrate. At total conversion using FeCl3
hydrate, lower selectivity was observed compared to an anhydrous
form as perillyl alcohol and also high-molecular products were formed
in the reaction mixture to a higher extent.
Influence of used solvent on selectivity ratio of myrtanal (b) and myrtenol (a) in
presence of Lewis acids.
Solvent
Selectivity ratio Sb/Sa
AlCl3
FeCl3
SnCl2
ZrCl4
Acetonitrile
Tetrahydrofurane
Toluene
1.3
1.4
1.7
1.4
1.6
2.2
2.2
2.8
2.8
3.9
2.8
4.4
1.3
1.9
1.5
2.2
1,4-Dioxane
Solvent: 2 mL; 1 g substrate; catalyst: 5 wt.% to substrate; room temperature;
time: 5 min.
3.4. Effect of Lewis acids and their amount
To compare the catalytic properties of selected Lewis acids, 1,4-di-
oxane was used as a suitable solvent. The comparison was carried out
using the amount of catalyst corresponding to 5, 7.5 and 10 mol.%
relative to the substrate in order to assess the effect of the catalysts
amount on the achieved conversion of β-pinene oxide and the se-
lectivity to the desired products. Lewis acids such as AlCl3 and FeCl3
were used also in the amount corresponding to 2.5 mol.% relative to the
substrate. The results are given in Table 4.
Several catalysts such as AlCl3 and FeCl3 have been tested at
2.5 mol.% in 1,4-dioxane as the solvent. Using 2.5 mol.% FeCl3 lower
conversion (54%) was achieved, using 2.5 mol.% AlCl3 as the catalyst
comparable results, i.e. 98% conversion and 89% selectivity to desired
products was found. The further comparison of catalysts has been done
using only 5 mol.% of the catalyst. Product selectivity at total conver-
sion varies according to the used Lewis acid. It did not change as dif-
ferent amount of catalyst was used. The highest selectivity to the de-
sired products at total conversion was achieved using 5 mol.% of AlCl3
or SnCl2, namely 84% or 95%.
Inorganic Lewis acids are known to be highly hydrophilic and form
hydrates in a solid state. Hydrates of Lewis acids are far more available
than their anhydrous analogues due to less demanding production. It
has been assessed whether the two Lewis acids have the same effect on
β-pinene oxide rearrangement.
For comparison, FeCl3, AlCl3, and their hexahydrates were chosen.
The amount of the respective hydrates was calculated to be the same as
in the case of anhydrous form use. When using AlCl3.6H2O it was ne-
cessary to use twice the amount of solvent to dissolve the respective
Comparison of the selectivities to the main reaction products is
given in Fig. 2.
In the presence of Lewis active sites myrtanal is formed pre-
dominantly. This finding is in good agreement with the results obtained
by Corma et al. [7]. Using ZrCl4 as the catalyst the high amount of
myrtanol was observed in the reaction mixture compared to the use of
other catalysts. The myrtanol may be produced in the reaction mixture
by e.g. disproportionation reaction. Generally, when β-pinene oxide
rearrangement was catalysed by Lewis acids perillyl alcohol and alde-
hyde were formed with selectivity lower than 6%.
3.5. Brønsted acids – reaction products
Unlike using Lewis acids as the catalysts, in the presence of Brønsted
acids myrtanol and 1,5-methadien-7-ol (d, g, Fig. 1) were not found in
the reaction mixture. In the reaction mixture using Brønsted acids as
catalyst three high-molecular products that could not be identified were
present. When the reaction was catalysed by H2SO4, 3 low-molecular
compounds that were not reliably identified were also found in the
reaction mixture in the total amount of approx. 10%. The selectivity to
these compounds increased with the increasing amount of acid used
and with its increasing concentration. Sulfuric acid has dehydrating
effects and therefore these compounds are likely to be the dehydrated
forms of any of the products. It also explains that the selectivity to these
compounds decreased with the increasing amount of water in the re-
action mixture.
Table 4
3.6. Amount of Brønsted acid used
Effect of Lewis acids types on conversion and selectivity to desired products.
The influence of Brønsted acid (H2SO4) on the achieved conversion
within 5 min of the reaction was determined. 1,4-Dioxane was chosen
as a solvent. Similarly, as in the case of using Lewis acid, the substrate
was dosed for 1 min.
To assess the effect of the presence of water on the β-pinene oxide
conversion and the selectivity to myrtanal and myrtenol, sulphuric acid
was added to the reaction mixture in the form of a 20% aqueous so-
lution and in 93% solution for each corresponding amount.
The selectivity to myrtanal and myrtenol with increasing β-pinene
oxide conversion decreased (Table 5), the formation of high-molecular
products such as dimers, trimers, products formed by the addition of
water on double bonds (diols) and the formation of perillyl alcohol
increased. It has been found the increasing concentration of the acid
used has increased the selectivity to myrtanal and myrtenol. The pre-
sence of a higher amount of water in the reaction mixture using a lower
concentration of acid probably caused the addition of water to reaction
products to a higher extent and the selectivity to myrtenol and myrtanal
decreased. Perillyl aldehyde and perillyl alcohol were formed with the
same selectivity (3% resp. 18%) independently of the catalyst amount.
The total conversion of β-pinene oxide was achieved within 5 min
Catalyst
Amount of
catalyst [mol.
%]
Amount of
solvent [ml]
Selectivity [%] β-Pinene oxide
conversion [%]
AlCl3
2.5
5
7.5
10
5
10
2.5
5
7.5
10
5
10
5
7.5
10
5
2
2
4
4
4
8
2
2
4
4
2
4
2
4
4
2
4
4
89
84
86
84
100
100
90
78
68
59
63
40
95
84
83
81
70
62
98
99
99
99
7
AlCl3.6H2O
FeCl3
18
54
94
99
99
72
99
99
99
99
80
99
99
FeCl3.6H2O
SnCl2
ZrCl4
7.5
10
1,4-Dioxane as the solvent; 1 g substrate; room temperature; time: 5 min.
4

I. Paterova, et al.
MolecularCatalysis492(2020)110945
Fig. 2. Effect of Lewis acid on selectivity to main reaction products.
2 mL 1,4-dioxane; 1 g substrate; catalyst: 5 mol.% to substrate; room temperature; time: 5 min
to the reaction mixture in their concentrated form and the amount of
acid corresponded to 10 mol% relative to the substrate. Table 6 sum-
marizes the results of the comparison of used acids. The acids are listed
in the table by decreasing strength.
Table 5
Effect of H2SO4 concentration and amount on maximal achieved conversion and
selectivity to desired products at given conversion.
Concentration of acid
[%]
Amount of
catalyst [mol.
%]
Selectivity [%]
β-Pinene oxide
The total conversion was achieved only in the presence of polyprotic
acids H2SO4 and H3PO4. With decreasing acid strength, an increase in
selectivity to myrtanal and myrtenol has been observed, except for
acetic acid and formic acid. In their presence, β-pinene oxide re-
arrangement did not almost take place. However, this trend was in-
fluenced also by different amounts of water present in the reaction
mixture, which also affected the selectivity. The amount of water in-
fluenced significantly the achieved conversions. The different beha-
viour of p-toluenesulfonic acid was caused by its use in the solid form of
the monohydrate. A comparison of selectivities to the main reaction
products is given in Fig. 3.
conversion m[%]
20
93
1
2.5
5
10
1
2.5
5
51
23
17
4
41
50
28
17
21
35
72
99
21
64
77
99
10
2 ml 1,4-dioxane; 1 g substrate; room temperature; 5 min.
It is clearly visible that using all Brønsted acids (except for nitric
acid) the main product was perillyl alcohol. Thus, these acids were not
suitable for the preparation of myrtanal and myrtenol.
Table 6
Effect of Brønsted acids on conversion and selectivity to desired products
(myrtenol, myrtanal).
Catalyst
pKa
(pKa2)
[22]
Concentration of
acid [wt.%]
Selectivity [%] β-Pinene oxide
conversion [%]
4. Conclusions
β-Pinene oxide is an important intermediate for the fine chemicals
industry. By its rearrangement, several valuable products such as
myrtenol, myrtanal, perillyl alcohol and aldehyde can be obtained.
Using either Lewis or Brønsted acids as catalysts results in a different
composition of reaction products.
In the presence of Lewis acids myrtanal is formed predominantly
while Brønsted acids are not suitable for the preparation of myrtanal
and myrtenol because of undesired perillyl alcohol and aldehyde for-
mation to a high extent. We found that the achieved β-pinene oxide
conversion was dependent on the type of catalyst and its amount but
was independent of the choice of solvent. The highest selectivities ap-
prox. 85% to myrtenol and myrtanal at total conversion were achieved
when catalysed by anhydrous AlCl3 or SnCl2.
HCl
H2SO4
−7
35
93
12
17
22
99
−4,8
(1,9)
−2,8
−1,4
2,1 (7,2)
3,8
pTsOH
HNO3
H3PO4
HCOOH
91
65
84
85
99
20
38
42
0
39
48
99
5
CH3COOH 4,7
0
1
2 ml 1,4-dioxane; 1 g substrate; catalyst: 10 mol.% to substrate; room tem-
perature; time: 5 min.
when the reaction was catalysed by 10 mol.% of H2SO4 to the substrate.
Since higher selectivity to myrtanal and myrtenol was observed when
more concentrated acid was used, the other Brønsted acids were added
5

I. Paterova, et al.
MolecularCatalysis492(2020)110945
Fig. 3. Effect of Brønsted acid on selectivity to main reaction products.
2 mL 1,4-dioxane; 1 g substrate; catalyst: 10 mol.% to substrate; room temperature; time: 5 min
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review & editing. Barbora Fidlerova: Formal analysis, Investigation.
Michal Vavra: . Eliska Vyskocilova: Supervision, Validation. Libor
Cerveny: Supervision, Validation.
Acknowledgments
This work was realized within the Operational Programme Prague -
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.110945.
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