A Green and Sustainable Approach: Celebrating the 30th Anniversary of the Asymmetric l-Menthol Process

Article abstract of DOI:10.1002/cbdv.201400063

Takasago has been devoted to producing l-menthol since 1954, and our long history of manufacturing this important aroma chemical is reviewed here. The current asymmetric catalytic process had its 30th anniversary in 2013. Our l-menthol process is consider

Full text of DOI:10.1002/cbdv.201400063

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A Green and Sustainable Approach: Celebrating the 30th Anniversary of the
Asymmetric l-Menthol Process
by Makoto Emura* and Hiroyuki Matsuda
Takasago International Corporation, Corporate Research and Development Division 4–11, 1-Chome,
Nishi-yawata, Hiratsuka City, Kanagawa 254-0073, Japan
(phone: þ81-463-252000; fax: þ81-463-252084; e-mail: makoto_emura@takasago.com)
Dedicated to the memory of Professor Mugio Nishizawa
Takasago has been devoted to producing l-menthol since 1954, and our long history of manufacturing
this important aroma chemical is reviewed here. The current asymmetric catalytic process had its 30th
anniversary in 2013. Our l-menthol process is considered carbon-neutral, and, therefore, ꢀgreenꢁ and
sustainable. It uses renewable myrcene obtained from gum rosin as a starting material. In addition, the
Rh-BINAP (¼2,2’-bis(diphenylphosphino)-1,1’-binaphthyl) catalytic system is highly efficient. This
pathway not only leads l-menthol, but a variety of 100% biobased aroma chemical products as well. By
measuring the 14C levels in a material, one can determine the percentage of carbon that is biobased. This
biobased assay, described as the ratio plant-derived C/fossil-derived C, can clarify how renewable a
product really is. This will be highlighted for several of Takasagoꢁs key aroma chemicals.
Introduction. – l-Menthol ((ꢀ)-1) is one of the most abundant chiral chemicals in
the world. Before World War II, it was prepared mainly in Japan and China by
crystallization of the extracted oil of Mentha arvensis. It is reported that, by 1938, 886 t
of natural l-menthol were produced, and 375 t were exported from Japan to the world.
Japan was the world leader in menthol production. Takasago submitted the first patent
for the synthesis of l-menthol in 1928 [1] and began the synthetic manufacturing of l-
menthol in 1954. At that time, the starting material was d-citronellal ((þ)-2) isolated
from citronella oil, and the total production volume was ca. 300 t. By 1972, with the
advance of the petroleum industry, thymol (3) replaced d-citronellal as the starting
material. However, this only lasted ca. five years due to the oil crisis of the late 1970s,
which suddenly increased the price of petroleum. Thus, we began to prioritize
sustainable or renewable materials over petroleum-based materials when developing
aroma chemicals. Other natural products such as d-limonene ((þ)-4) and d-D3-carene
(5) have also been utilized as starting materials for l-menthol synthesis. In 1983,
Takasago completed the asymmetric synthesis of l-menthol which is still the basis of our
process today (Fig. 1). The current asymmetric catalytic process uses renewable
myrcene (6) as the starting material, and, through the years, the process has
continuously been modified and improved. From this process, a variety of aroma
chemicals are produced. Our three key commitments are i) to use renewable or
sustainable raw materials when available, ii) to apply catalytic processes when possible,
and iii) to target optically pure chemicals.
ꢂ 2014 Verlag Helvetica Chimica Acta AG, Zꢃrich

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Fig. 1. History of the production of l-menthol
Chronological Review. – From 1954 to 1972. The 60-year history of synthetic l-
menthol ((ꢀ)-1) began when d-citronellal ((þ)-2) was isolated in 35% yield from
citronella oil. At that time, citronellal (2) was a lesser-used aroma chemical, while other
components of citronella oil, such as geraniol and its esters, were much more highly
valued. Because the optical purity of (þ)-2 was 80% ee, it was considered as a useful
chiral starting material. The first industrial l-menthol synthesis from (þ)-2 was
performed in Japan [2] (Scheme 1). Purification via the 2-chloroacetate 9 of the
menthol isomers was a key step, from which ultimately 300 t of l-menthol were
produced.
From 1972 to 1977. To manufacture 300 t of l-menthol, 1,300 t of citronella oil were
required to obtain the necessary (þ)-2. This comprised nearly half of the worldwide
natural citronella oil consumption at the time. As a result, thymol (3), which is derived
from the petroleum-based compound m-cresol, was developed as an alternative
starting material for the l-menthol synthesis. Since 3 is achiral, optical resolution was
Scheme 1. d-Citronellal Process from 1954 to 1972

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necessary, which was accomplished by preferential crystallization of the corresponding
diastereoisomer, l-menthyl d-menthoxyacetate (12), obtained from the reaction with
d-menthoxyacetic acid, and was subsequently converted to pure l-menthol by
hydrolysis [3] (Scheme 2). The antipodal d-menthyl d-menthoxyacetate remaining in
the mother liquor was converted to dl-menthol by Ni-catalyzed isomerization under
high pressure and subsequently recycled back into the process. A fine distillation
system, equipped with a successively feeding 80 theoretical plate setup, was installed to
enable separation of the desired dl-n-menthol from iso-, neo-, and neoisomenthol.
From 1977 to 1980. The oil crisis of 1973 caused a drastic price increase in petroleum
feedstocks and led to a shortage of thymol, hence, the thymol-based process needed to
be replaced. The search thus began for a new natural feedstock to support a novel l-
menthol synthesis. d-Limonene ((þ)-4), which occurs in very high concentrations in
orange oil, was selected as an inexpensive raw material [4] (Scheme 3). Although the
optical purity of (þ)-4 is very high, the manufacturing process was too costly to take
advantage of this chirality, and it afforded only dl-menthol. As a consequence, dl-
menthol still needed to be subjected to the same optical resolution techniques as in the
aforementioned thymol process.
From 1980 to 1983. Both the thymol and d-limonene processes mentioned above
required an optical resolution step to produce l-menthol. The equivalent amount of
undesired d-menthol produced from the resolution process was subjected to further
racemization and recycling steps. This in turn yielded higher fuel costs, which became a
serious issue during the second oil crisis of 1979. Takasago decided to adopt a
technology from PFW that used optically active d-D3-carene (5) as the feedstock for the
l-menthol synthesis. This process was improved through isomerization of 18 to a
mixture of 19 and 20 by Takasago in 1982 [5] (Scheme 4). Consequently, (1R)-menth-3-
ene (21) was obtained from 19 and 20 by selective reduction, respectively, without
formation of (1R)-menth-2-ene. This avoided the production of the undesired p-
menthan-2-ol in the reaction mixture, which could not easily be separated from l-
menthol by distillation and could not be recycled.
Scheme 2. Thymol Process from 1972 to 1977

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Scheme 3. Limonene Process from 1977 to 1980
Scheme 4. d-D3-Carene Process from 1980 to 1983

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From 1983 to Present. In 1983, Takasago established one of the largest industrial
asymmetric catalytic manufacturing plants in the world with the collaboration of
professors, Dr. R. Noyori, Dr. H. Takaya. Dr. K. Tani, and Dr. S. Otsuka [6–9]. This
process (Scheme 5), which just celebrated its 30th anniversary in 2013, has not changed
much in more than three decades since its inception, notwithstanding the fluctuations
of the world economy. The current process contains many aspects of past technologies.
For instance, the fine distillation that separates n-menthol from iso-, neo-, and
neoisomenthol, and the industrial use of metal/amine-catalyzed reaction was intro-
duced in the d-citronellal and d-D3-carene processes, respectively. One advantage of
this method is its efficiency. By simply switching the configuration of the homogeneous
organometallic catalyst, the Rh-BINAP (¼2,2’-bis(diphenylphosphino)-1,1’-binaphth-
yl), one can change the chirality of the product. Both optically active isomers can be
produced in the same manufacturing plant, as the chemical purity and enantiomeric
ratios of these products are completely identical. In contrast, a biocatalyst does not
have such flexibility.
Although there has been little change in this process over the last 30 years, we have
implemented a ꢀKAIZENꢁ (continuous improvement) approach. Instead of using a
petroleum-based Et2NH, a bio-ethanol-derived Et2NH is now used to reduce our
reliance on petroleum-based chemicals. Another improvement has been implemented
in the cyclization of (þ)-2 to give (ꢀ)-7, which originally gave 92% selectivity. Here, the
Scheme 5. Myrcene Process from 1983 to Present
Scheme 6. ATPH-Catalyzed Cyclization

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original ZnBr2 catalyst was replaced with aluminum tris(2,6-diphenylphenoxide)
(ATPH; 27), which now provides (ꢀ)-7 with 99.5% selectivity [10] (Scheme 6).
Results and Discussion. – A Green and Sustainable Process. The current process
uses renewable myrcene (6), which can be isolated from gum rosin, wood rosin, toll
rosin, tree sap, wood chips, or the waste fluid of wood pulp. Currently, gum rosin is
being used as our feedstock in the asymmetric l-menthol synthesis. The entire process
involves four different catalytic reactions in an effort to maintain a focus on green
chemistry. The Rh-BINAP catalyst works highly efficiently, as one molecule of Rh-
BINAP can produce more than 200,000 molecules of l-menthol.
In terms of molecular carbon, there is no increase in atmospheric CO2 from the use
of natural l-menthol due to the equilibrium between the CO2 released by biodegra-
dation and the CO2 adsorbed by photosynthesis. In a similar manner, 6 produced from
gum rosin can be considered carbon-neutral. It is well-known that the use of petroleum
feedstock for chemical synthesis poses problems for future sustainability. In addition,
products obtained from these sources can release CO2 stored in fossil fuels and increase
the CO2 levels in the environment which may exacerbate climate change. In this study,
the biobased index was investigated for a variety of products derived from our
sustainable myrcene source.
Biobased Index. When a plant or animal dies, the amount of 14C in its composition
begins to slowly decrease over time at a known rate. The half-life of radioactive 14C is
5,730 years. For fossil material, the quantities of 14C are much lower, since life activity
ceased in the distant past. Using this principle [11], by measuring the radioactive 14C in
a material, one can tell not only what is the age of the material but also the ratio of
petroleum-derived material to plant-derived material within a product. In this biobased
analysis, the 14C from fossil sources is essentially zero. From this, the balance of fossil-
Table. Biobased Analytical Result of Aroma Chemicals
Compounds
Biobased [%]
l-Citronellol ((ꢀ)-30)
100
0
100
100
0
100
0
100
34
100
100
100
100
0
Petroleum-derived citronellol ((ꢁ)-30)
Natural l-menthol ((ꢀ)-1)
Myrcene-process l-menthol ((ꢀ)-1)
Petroleum-derived l-menthol ((ꢀ)-1)
Myrcene-process l-citronellyl nitrile ((ꢀ)-32)
Petroleum-derived citronelly nitrile ((ꢁ)-32)
d-p-Menthane-3,8-diol ((þ)-33)
N,N-Diethyl-3-methylbenzamide (35)
(1R)-8-sulfanylmenthone ((þ)-37)
(1S)-8-sulfanylmenthone ((ꢀ)-37)
8-Sulfanylmenthone as a natural flavor substance ((þ)-37)
l-cis-Rose oxide ((ꢀ)-cis-40)
Petroleum-derived rose oxide ((ꢁ)-40)
Thesaronꢀ ((þ)-trans-43)
100
0
b-Damascone (45)
g-Decalactone from Castor oil (48)
Synthetic g-decalactone ((ꢁ)-48)
100
70

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and plant-derived carbon in the molecule can be calculated. Samples of l-menthol
synthesized from our myrcene process, isolated from a natural mint oil, and syn-
thesized from a fossil feedstock were analyzed by this assay. Both the l-menthol
from the myrcene process and the one isolated from natural mint showed a 100%
biobased index, whereas l-menthol from the fossil material showed a 0% biobased
index (Table).
Scheme 7. Myrcene Process-Derived Aroma Chemicals

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Myrcene-Derived Aroma Chemicals. From the aforementioned myrcene process,
l-citronellal ((ꢀ)-2), l-citronellol (30), and their analogs are synthesized using the
(R)-TolBINAPꢀRh catalyst. Alternatively, d-citronellal ((þ)-2), l-citronellyl nitrile
((ꢀ)-32), l-isopulegol ((ꢀ)-7), and their analogs are prepared by the (S)-TolBI-
NAPꢀRh catalyst (Scheme 7). They are all found to have a 100% biobased index. In
addition, the following specialty chemicals have been investigated.
(1R)-p-Menthane-3,8-diol ((þ)-33). Naturally identical (1R)-p-menthane-3,8-diol
((þ)-33) was synthesized from (þ)-2, derived from myrcene, and has been developed as
a commercial cooling material [12] (Scheme 8). It also acts as a mosquito repellant. It
was found that the (þ)-33 from mycrene has a 100% biobased index, while the popular
artificial insect repellant N,N-diethyl-3-methylbenzamide (DEET; 35) contains only
34% biobased carbon. This result suggest that the 34% biobased carbon in 35 is from
Scheme 8. Synthesis of d-p-Menthane-3.8-diol and the Structure of N,N-Diethyl-3-methylbenzamide
(DEET)
Scheme 9. Synthesis of 8-Sulfanylmenthone

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Et2NH (24), derived from bioethanol, while the residual 66% carbon is contributed by
the petroleum-based 3-methylbenzoic acid (34). Interestingly, this observed biobased
index for 35 is almost identical to that calculated if we assume the four biobased
carbons (33% of total) are derived from bioethanol precursor of 24, and the eight
fossil-based carbons (67% of total) are from 34.
(1S)-8-Sulfanylmenthone ((1S)-37). Enantioselective GC analysis gave a clear
separation of the isomers of 8-sulfanylmenthone. It is known that there is a significant
olfactory difference for the optical isomers of 8-sulfanylmenthone [13][14], and while
the (S)-isomer is strong, the (R)-isomer is dirty and oily. The conventional production
of 8-sulfanylmenthone (37) starts from commercially available d-pulegone ((þ)-36),
which has a configuration opposite to that of the desired natural (1S)-37. Its odor is not
preferable, therefore. Enantioselective GC analysis revealed that 37 from l-pulegone,
derived from the myrcene process (Scheme 9), has the same configuration as (1S)-37
found in natural Buchu oil [15] (Fig. 2). In addition, it was found to possess an intense
odor reminiscent of natural Buchu. Interestingly, 8-sulfanylmenthone, labeled as
natural flavoring substance in accordance with the EU Flavor Regulation 1334/2008/
Fig. 2. GC Analysis of the four isomers of 8-sulfanylmenthone. Upper to lower: mixture of four isomers;
(1R,4R)-8-sulfanylmenthone and (1R,4S)-8-sulfanylmenthone; (1S,4S)-8-sulfanylmenthone and
(1S,4R)-8-sulfanylmenthone; and 8-sulfanylmenthone labeled as a natural flavoring substance.

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EC, is identical to (1R)-37, and is the antipode of (1S)-37, which is identified in natural
Buchu (Table). Both of these 37 have a 100% biobased index, since they are prepared
from natural starting materials.
l-cis-Rose Oxide ((ꢀ)-cis-40). Rose oxide (40) has four optical isomers, and it is
well-known that l-cis-rose oxide ((ꢀ)-cis-40) is the preferred enantiomer in terms of
both odor threshold and olfactory character [16]. The odor thresholds of l-cis-rose
oxide, l-trans-rose oxide, d-cis-rose oxide, and d-trans-rose oxide are 0.005, 0.160, 0.05,
and 0.08 ppm, respectively. Using our myrcene process, (ꢀ)-cis-40 was synthesized
selectively (Scheme 10) from (ꢀ)-30 and was found to have a 100% biobased index.
Whereas, the conventional synthetic rose oxide ((ꢁ)-40) comprises a mixure of those
four isomers and has a 0% biobased index.
Thesaronꢄ ((þ)-trans-43). An important family of chemicals derived from natural
rose oil are the rose ketones, represented by a-damascone (44), b-damascone (45), and
d-damascone (46). It is known that use of rose ketones is restricted in some cases,
because they cause skin sensitization. Takasago developed the aroma chemical
Thesaronꢄ ((þ)-trans-43; Scheme 11), and specifically designed it to reduce this skin
sensitization problem by eliminating the conjugated C¼C bond of 44–46 and replacing
it with an O-atom to form an ester [17]. Another advantage of (þ)-trans-43 is that the
sterically hindered ester function is rather stable and difficult to hydrolyze in a variety
of consumer product applications, thus enhancing its odor stability.
Scheme 10. Synthesis of l-cis-Rose Oxide
Scheme 11. Synthesis of Thesaronꢀ and Structures of Damascones

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Although there are four possible stereoisomers, (þ)-trans-43 was selectively
developed (Scheme 11) for its unique odor intensity and character. Thesaronꢄ ((þ)-
trans-43) has an intense sweet flowery character with an odor threshold of 0.02 ppm,
while (ꢀ)-trans-43 possesses an oily character with an odor threshold of 0.10 ppm. It
was initially found that the biobased index of Thesaronꢄ ((þ)-trans-43) was 80%, since
the cyclic terpene portion is derived from myrcene, with the remaining 20% being
derived from synthetic EtOH, which was replaced with bioethanol to improve the
biobased index to 100%. In contrast, it was found that commercially available b-
damascone (45), which is synthesized from a petroleum feedstock, has a 0% biobased
index.
(R)-g-Decalactone (48). Other green and sustainable approaches include chemical
biotransformations utilizing microorganisms or enzymes from a natural feedstock, as
they require less organic solvent and generate less chemical waste. In addition, products
derived by these methods can be labeled as natural flavors, in accordance with CFR21
Sec. 101.22 and EU Flavor Regulation 1334/2008/EC. (R)-g-Decalactone (48) was
developed from the renewable castor oil via 47 by screening a variety of natural
microorganisms [18] (Scheme 12). Compound 48 was identified to have a (R)-
configuration by enantioselective GC analysis and was preferred olfactively. Interest-
ingly, some natural g-decalactone products on the market were found to be racemic,
although all products have 100% biobased indices. In contrast, synthetic 48 was found
to be 70% biobased.
Scheme 12. Biotransformation of g-Decalactone
Conclusions. – Public interest in environmental concerns has been continuously
increasing, and environmentally friendly products have become an important area for
both chemical producers and consumers. Finding natural and sustainable sources of key
aroma chemicals has been a focus of the flavor and fragrance industry even before the
environmental concerns became main stream. As discussed in this review, the biobased
index can play a major role in identifying the use of plant-derived feedstock, as well as
in helping to identify the synthetic pathway to prepare a chemical. Throughout the
history of the l-menthol process, we have shared our successes with the world. Today,
we continue to retain our core commitments to use renewable or sustainable raw
materials, to apply catalytic processes, and to target optically pure chemicals.
Experimental Part
General. (ꢁ)-Citronellol ((ꢁ)-30), (ꢁ)-rose oxide ((ꢁ)-40), 8-sulfanylmenthone ((þ)-37) as
natural flavoring substances, and b-damascone (45) were purchased from SigmaꢀAldrich Corporation
(St. Louis, Missouri, USA). DEET (35) and synthetic g-decalactone (48) were purchased from Tokyo

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Chemical Industry Co., Ltd. (Tokyo, Japan). Other materials, unless otherwise noted, are produced or
obtained from Takasago International Corporation. Enantioselective gas charomatography (GC): a GC
system (Agilent Technologies Inc., Santa Clara, CA) combined with a flame ionization detector (FID)
equipped with a 25 mꢂ0.25 mm i.d. b-DEX325 fused cap. column coated with 0.25-mm film; carrier gas,
He; 100 kPa; oven temp., started at 1308 up to 1358 at an rate of 0.18/min, detector, FID; in m/z.
Synthesis of (1S)-Sulfanylmenthone ((ꢀ)-37) from l-Pulegone ((ꢀ)-36). A reactor equipped with a
thermometer and a gas-introducing tube was charged with (ꢀ)-36 (3 g, 23 mmol) CH2Cl2 (30 ml) and
anh. AlCl3 (612 mg, 0.2 equiv.) were added, and then H2S gas was blown through the gas-introducing tube
at 608 for 3 h. After completion of the reaction, a portion of the mixture was taken out, and the
conversion ratio (100%) was determined by GC.
After releasing the remaining H2S from the mixture by N2, a crude product was obtained in the usual
way by dil. HCl treatment, washing with H2O, and concentration. By distilling the obtained crude product
under a reduced pressure (958/600 Pa), 2.8 g (65%) of (ꢀ)-37 was obtained with a purity of 96%.
The ratio (4S)/(4R) of this compound was 60 :40. A 1.0-g portion of the obtained (1S)-8-
sulfanylmenthone ((ꢀ)-37) with a (4S)/(4R) ratio of 60 :40 was purified by a recycling method using GC.
(1S,4R)-8-Sulfanylmenthone (cis-form). [a]2D0 ¼ ꢀ43.5 (c¼1.3, MeOH). IR (neat): 1709, 2583.
1H-NMR (CDCl3): 1.02 (d, J¼6.35, 3 H); 1.40 (s, 6 H); 2.30 (s, 1 H). MS (m/z): 153 ([MꢀSH]þ ).
(1S,4S)-8-Sulfanylmenthone (trans-form). [a]2D0 ¼ þ29.6 (c¼1.5, MeOH). IR (neat): 1709, 2583.
1H-NMR (CDCl3): 0.96 (d, J¼7.15, 3 H); 1.40 (s, 3 H); 1.45 (s, 3 H); 2.35 (s, 1 H). MS: 153 ([MꢀSH]þ ).
The authors are grateful to Dr. Jonathan Warr, Dr. Michael Lankin, and Louis Lombard for helpful
discussions.
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Received February 5, 2014