Abstract
The powdery scent of orris root oil adds elegance and depth to any fragrance. However, orris root oil is one of the most expensive essential oils due to a lengthy six-year production process. If compounds with the powdery scent could be supplied by chemical synthesis at a lower cost than natural products, there would be a large demand. In the course of our research into the synthesis and olfactory evaluation of irone isomers, we found that (±)-β-irone, a minor component of orris root oil, has an excellent powdery aroma. In this study, we report a new synthetic method for (±)-β-irone and the results of its olfactory evaluation. The synthesis of (±)-β-irone was accomplished using commercially available 3-methylcyclohexanone as the starting material. The olfactory evaluation of synthesized (±)-β-irone was performed. (±)-β-Irone has been found to have a strong, transparent, fruity green top note, a rich violet floral heart note, and a clear, powerful, and long-lasting powdery last note with a woody odor. We considered that some part of the powdery fragrance of orris root oil originates from β-irone, and that β-irone is an excellent flavoring and fragrance material that can be used as a substitute for natural orris root oil.
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Introduction
Irones are the primary aroma component of orris root oil (orris oil, iris oil), an essential oil derived from the rhizomes of iris1,2. The ground rhizomes of some iris, Iris pallida L., Iris florentina L., or Iris germanica L., are steam distilled to provide an orris root oil. The steam distillate, with a violet-like odor, is a light yellow to brown-yellow solid mass (orris butter or orris concrete), as it contains high concentrations of myristic and other fatty acids. Orris absolute is obtained from orris butter after the removal of the fatty acids with alkali. The main components of orris absolute are cis-γ-irone (typically 30–40%) and cis-α-irone (typically 20–30%), which are considered responsible for the typical orris odor. The fresh orris rhizomes are almost odorless. Orris rhizomes take three years to grow in the field, and prior to distillation, the rhizomes are stored for an additional three years. Irones are formed through an oxidative degradation process from iridales, higher molecular precursors, while the orris rhizomes are stored.
The scent of orris root oil containing irones is generally expressed as “a rich and powerful floral note with a powdery character, reminiscent of the scent of violet”, and a small amount of orris root oil is used in high-end perfumes to add powdery and violet-like scent. Its powdery scent adds a touch of elegance and depth to any fragrance. However, orris root oil is known as one of the most expensive natural essential oils3.
Irones are structurally similar to ionones present in sweet violet (Viola odorata L.). There are α-, β-, and γ-isomers depending on the position of the double bond on the cyclohexane ring (Fig. 1), and because they have one or two chiral carbons on the cyclohexane ring, α- and γ-irone have four stereoisomers respectively, and β-irone has two stereoisomers4,5,6. The olfactory evaluations of each isomer have been performed and reported to have different odor properties and strength4,5,6.
Irone isomers (* chiral carbon).
Irones have been reported to be detected also in various plants and fungi7,8,9,10,11,12,13,14,15,16,17,18,19. In all reported cases, only one isomer of irones was detected in low quantities, except for one case where both α- and γ-irone were detected15. For example, β-irone was detected in mangoes, grapes of Shine Muscat, and peaches16,17,18,19, γ-irone as the main constituent in the seeds of Myoporum insulare R. Br.12, and α-irone in the needles of Pinus pinaster10. As the only example, two isomers, α-irone and γ-irone, were detected in Nigella sativa15.
If these fragrant compounds could be supplied by chemical synthesis at a lower cost than natural products, there would be a large demand, and many studies have been conducted for the purpose of synthesizing them. Orris root oil is believed to contain four types of isomers: cis-γ-irone, cis-α-irone, trans-α-irone, and β-irone. Of these, the main constituents of orris root oil are cis-γ-irone and cis-α-irone, and the contents of trans-α-irone and β-irone are small. A synthetic irone (IRONE-ALPHA, with a higher trans isomer ratio than natural orris root oil) containing α-irone as the main component is commercially available; however, its scent seems to be quite different from that of natural orris root oil.
In the course of our research into the synthesis and olfactory evaluation of irone isomers, we found that (±)-β-irone has an excellent powdery aroma. Since the proportion of β-irone in natural orris oil is small, approximately 2% or less of all irone isomers, β-irone is not considered an important component of orris fragrance and has not received much attention so far. Therefore, although there are many documents on the synthesis of γ-irone20,21,22,23,24,25,26,27,28,29,30,34,35,36,37,38,39 and α-irone31,32,33,34,35,36,37,39, there are only a few documents on the synthesis of β-irone38,39,40,41. In this study, we developed a new gram-scale synthetic procedure of (±)-β-irone and performed olfactory evaluation of the synthesized (±)-β-irone. As a result, we have found that (±)-β-irone has an excellent aroma and plays an important role in the aromatic nature of orris root oil.
Results and discussion
Synthesis of (±)-β-Irone
Since none of the methods reported in the literature for synthesizing β-irone38,39,40,41 were easy for our laboratory, which has limited experimental facilities, we decided to investigate a new method for synthesizing β-irone. We have devised a new synthetic method for β-irone, as described below, based on the reported synthesis of γ-irone23.
3-Methylcyclohexanone 4 is used as the starting material because it is commercially available at relatively low cost. As shown in Fig. 2, 3-methylcyclohexanone 4 is reacted with ethyl formate in the presence of a base to give 6-hydroxymethylene-3-methylcyclohexanone 5, which is then reacted with N-methylaniline to produce 6-methylanilinomethylene-3-methylcyclohexanone 642.
Synthesis of 6-hydroxymethylene-2,2,3-trimethylcyclohexanone 8.
Compound 6 is a viscous liquid immediately after work-up, but gradually solidifies when left in a refrigerator (5–10 °C) for several days. The purity of compound 6 can be improved by washing the solid with hexane.
After converting 3-methylcyclohexanone 4 to 6-methylanilinomethylene-3-methylcyclohexanone 6, two methyl groups are introduced into the carbon adjacent to the carbonyl of 6 in one step, followed by acid hydrolysis to remove the protecting group, yielding 6-hydroxymethylene-2,2,3-trimethylcyclohexanone 8, one of the key intermediates for the synthesis of (±)-β-irone.
6-Hydroxymethylene-2,2,3-trimethylcyclohexanone 8 can be converted to γ-methylcyclocitral 9 in six steps by the method described in the literature, as shown below (Fig. 3)23.
Synthesis of γ-methylcyclocitral 9 from 6-hydroxymethylene-2,2,3-trimethylcyclohexanone 8.
A base-promoted isomerization of α-methylcyclocitral to β-methylcyclocitral has been reported39,41. By applying the method to γ-methylcyclocitral 9, β-methylcyclocitral 10 was obtained from γ-methylcyclocitral 9 as indicated in Fig. 4a. β-Methylcyclocitral 10 is unstable under air at room temperature and should be stored under a nitrogen atmosphere in a freezer (below –20 °C).
(a) Synthesis of β-methylcyclocitral 10 from γ-methylcyclocitral 9. (b) Reaction of β-methylcyclocitral 10 with Wittig reagent. (c) Aldol condensation of β-methylcyclocitral 10 with acetone. (d) Preferential epoxidation of α-irone 1 in the mixture of β-irone 2 and α-irone 1 with Oxone (KHSO5·0.5KHSO4·0.5K2SO4) and acetone.
In our synthetic method, β-methylcyclocitral 10 is another key intermediate. Several methods for synthesizing its precursor, γ-methylcyclocitral 921,23,27,29,30,37,39 or α-methylcyclocitral30,37,39,41, have been reported. However, all of these methods require complex multi-step procedures, and no convenient method for the large-scale synthesis has been reported.
Wittig or Horner–Wadsworth–Emmons (HWE) reaction is generally used for the synthesis of α- and γ-irones from α- and γ-methylcyclocitrals23,27,29,37,38,39. However, the reaction of β-methylcyclocitral 10 with Wittig or HWE reagent was very slow, and the reaction did not proceed well even after heating or prolonged reaction time (Fig. 4b)41,43.
Therefore, the application of aldol condensation reactions was examined38. Reaction of β-methylcyclocitral 10 with acetone under basic conditions produced β-irone 2 along with a small amount of α-irone 1. Here, sodium methoxide, sodium hydride, and sodium hexamethyldisilazane (NaHMDS) could be used as the base. A relatively high yield of β-irone 2 was obtained using NaHMDS (Fig. 4c).
When synthesizing β-irone 2, about 2–10% of α-irone 1 relative to the amount of β-irone 2 is produced simultaneously. Although the two can be separated by gas chromatographic analysis, they are difficult to separate by distillation or column chromatography due to their similar physical properties.
As a result of further research into the separation method of β-irone 2 and α-irone 1, it was found that by preferentially converting α-irone 1 into an epoxide by oxidation reaction, as shown in Fig. 4d, it becomes easy to separate epoxidized α-irone by distillation or column chromatography, and β-irone 2 can be isolated in high purity.
We have found that the method using dimethyldioxirane generated in situ from acetone and Oxone® (potassium peroxymonosulfate, KHSO5·0.5KHSO4·0.5K2SO4) under basic conditions44 and the method using methyltrioxorhenium (MTO) as a catalyst and hydrogen peroxide as an oxidizing agent45,46 are effective for the preferential epoxidation of α-irone 1. Among these, the method using Oxone as an oxidizing agent, which can be performed at a lower cost, is preferable. Epoxidation of a mixture of β-irone 2 and α-irone 1 using these methods can reduce the ratio of α-irone 1 to β-irone 2 to less than 1%. At the same time, the β-irone 2 is also epoxidized a little, and the yield of β-irone 2 is reduced, albeit slightly.
The crude β-irone 2 thus obtained, which contained less than 1% α-irone 1 and other impurities, was first purified to a purity of more than 80% by flash chromatography. In the field of organic synthesis, it has become routine to isolate and purify target compounds from reaction mixtures using flash chromatography systems with prepacked columns47,48. Finally, β-irone 2 was purified to a purity of 96% or more by distillation.
Though the overall yield of crude β-irone 2 (purity before flash chromatography was 53%) from 3-methylcyclohexanone 4 was 19%, purification of β-irone 2 to over 96% purity resulted in significant losses. From 28.7 g of crude β-irone 2, 7.3 g of β-irone 2 with a purity of 96% or higher was obtained.
Composition of Irone Samples
Figure 5 shows the GC chromatograms (FID) of the three irone samples subjected to the olfactory evaluation. Retention times of irones are as follows: trans-α-irone (15.6 min), cis-α-irone (15.9 min), cis-γ-irone (16.0 min), and β-irone (16.4 min). Orris absolute consists of trans-α-irone (2.5% of total irones), cis-α-irone (46.4%), cis-γ-irone (50.2%), and β-irone (0.8%) (Fig. 5a). IRONE ALPHA, a commercial synthetic α-irone, is a mixture containing trans-α-irone (52.6% of total irones), cis-α-irone (45.6%), and β-irone (1.8%) (Fig. 5b). The purity of the synthesized (±)-β-irone is over 96% (Fig. 5c).
GC-FID chromatograms of (a) orris absolute (IFF), (b) IRONE ALPHA (Givaudan), and (c) synthesized (±)-β-irone.
Olfactory evaluation
So far, only three olfactory evaluations of β-irone have been reported4,38,49. The first olfactory evaluation of β-irone states, "β-irone has relatively little 'absolute’ character: it smells 'flowery-woody’, somehow 'ordinary’, less elegant and slightly reminiscent of the smell of the β-ionone"49. The second evaluation was by GC-sniffing technique using orris absolute4, and the remaining one was a direct evaluation of synthetic samples38. The GC-sniffing evaluation of (±)-β-irone described the odor as "intensive, iris family, sweet, ionone, anis like carvi oil, green, and liquorice". Further, the stereoisomers are also evaluated. ( +)-β-Irone was reported to have "iris family, anis, liquorice, green note". (–)-β-Irone is reported to have "poor, weakly iris"3. The evaluation of the synthetic sample of (+)-β-irone (91% chemical purity, contaminated by 9% (–)-trans-α-irone) was reported to "possess a β-ionone-type odor of warm floral-woody tonality with green and anisic aspects. The odor is linear, and the tenacity of the note is good. It can be considered a dry-down note. This compound is the strongest of the series"38. The evaluation of the synthetic sample of (–)-β-irone (93% chemical purity, contaminated by 7% (+)-trans-α-irone) was reported to "have a woody odor with a distinct honey note, that is quite sweet. Besides, it shows floral ionone-type facets, and a fruity tonality, but also an unpleasant smoky character. It belongs to the ionone-type family, without being very close to β-ionone"38. These three olfactory evaluations have some similarities but also differences. This shows the difficulty of olfactory evaluation.
We re-evaluated the odor of (±)-β-irone using our synthetic sample and found that (±)-β-irone has an excellent odor comparable to that of orris absolute, especially in its violet and powdery aroma. Therefore, we compared its aroma with that of orris absolute and IRONE ALPHA. We also compared its aroma with that of β-ionone, which has a similar molecular structure and is one of the aroma components of sweet violet. Table 1 indicates the comparison of the odor character of orris absolute (IFF), IRONE ALPHA (Givaudan), the synthesized sample of (±)-β-irone in this study, and β-ionone (Tokyo Chemical Industry).
(±)-β-Irone has a strong, transparent, fruity, green top note, and the heart note is a rich violet floral fragrance, and the last note is a clear, powerful, and long-lasting powdery fragrance with a woody odor. Though the woody odor is strong enough in the last note, the powdery odor is stronger than the woody odor. The reported odor evaluations of β-irone and ours have some similarities and some significant differences. The floral, green, and woody notes are consistent with our odor evaluation; however, no literature has described a powdery odor4,38,49. The fruity, violet, and powdery aroma of (±)-β-irone is quite strong even at concentrations of 0.02% or 0.008%. Orris absolute has a fatty scent in addition to the scents of (±)-β-irone, and lacks the fruity and green aroma of (±)-β-irone. On the other hand, IRONE ALPHA has a weak, green, and fruity top note, with the heart note being floral. The violet floral scent is weak rather than (±)-β-irone. Woody and earthy scents are strong and last long. The powdery character of orris absolute and (±)-β-irone is weak in IRONE ALPHA. β-Ionone had a fruity and green scent that was stronger than orris absolute and IRONE ALPHA, but weaker than (±)-β-irone. The violet and woody scents were weaker than the other three, and the powdery scent was barely perceptible.
The contents of β-irone in 1% orris absolute and IRONE ALPHA are ca. 0.008% and 0.018%. The diluted samples of (±)-β-irone of 0.008% and 0.02% correspond to the contents of β-irone in 1% orris absolute and IRONE ALPHA. The powdery aroma of 1% orris absolute is stronger than that of 0.008% (±)-β-irone and comparable to that of 1% (±)-β-irone. These results suggest that cis-γ-irone and/or cis-α-irone have a strong powdery aroma (comparable to that of (±)-β-irone). On the other hand, the powdery aroma of 1% IRONE ALPHA is weaker than that of 0.02% (±)-β-irone. Why is the powdery aroma of 1% IRONE ALPHA weak, even though IRONE ALPHA contains 1.8% β-irone? We considered that this is because cis-α-irone and trans-α-irone have a very weak or no powdery scent, and the scent of compounds other than β-irone masks the powdery scent. Considering these results, it can be concluded that cis-γ-irone has a strong powdery scent, and that cis-α-irone and trans-α-irone have a very weak or no powdery scent.
To confirm the consideration that the contaminating compounds mask the powdery scent of β-irone, we evaluated the odor of (±)-β-irone of various purities. Table 2 indicates the comparison of the odor character of (±)-β-irone at various purities. The concentration of (±)-β-irone of 27% purity sample is 0.27%, and this is higher than 0.2% (±)-β-irone concentration sample in Table 1. (±)-β-Irone of 0.2% concentration sample has strong powdery aroma, while (±)-β-irone of 27% purity sample has almost no powdery aroma. This may be because the odor of impurities masks the powdery aroma.
Despite much effort4,5,6, it remains unclear which isomers of irone are responsible for the typical odor of orris absolute. From the odor evaluation of the synthesized (±)-β-irone, we considered that β-irone contributes to some part of the powdery fragrance of orris absolute, albeit in a small proportion (< 2%) of total irone isomers.
Table 3 indicates the time course of the intensity of the scent of orris absolute and (±)-β-irone. The scent of both orris absolute and (±)-β-irone lasted over 120 h.
From a smelling blotter containing 0.0001 g (±)-β-irone, the scent of violets will be released. In addition, from a smelling blotter containing 0.001 g (±)-β-irone, the raspberry aroma will be released. In this manner, (±)-β-irone could be used as a substitute for orris absolute as a flavor or fragrance.
Conclusion
We developed a new gram-scale synthetic procedure of (±)-β-irone and performed olfactory evaluation of the synthesized (±)-β-irone. As a result, we have found that (±)-β-irone has a strong, transparent, fruity, green top note, a rich violet floral heart note, and a clear, powerful, and long-lasting powdery last note with a woody odor. It is noteworthy that (±)-β-irone has a powerful and transparent powdery aroma, equal to or better than that of natural orris root oil, and plays an important role in the aromatic nature of orris oil. At present, it is unclear which stereoisomer, ( +)-β-irone or (–)-β-irone, contributes to the powdery aroma of orris oil. It is necessary to synthesize ( +)-β-irone and (–)-β-irone separately, or obtain each isomer by optical resolution to confirm their aromas, which is an issue for the future. In either case, this study confirmed that β-irone has a powdery aroma, even in its racemic form.
We conclude that some part of the violet floral and powdery aroma of orris root oil originates from β-irone. We considered that (±)-β-irone is an excellent flavoring and fragrance material that can be used as a substitute for natural orris oil.
Methods
General information
Unless otherwise stated, commercial solvents and reagents were used as received. Solvents and reagents were purchased from TCI, FUJIFILM Wako Chemicals, and Sigma-Aldrich. Methyltrioxorhenium was prepared according to the reported procedure50. Orris absolute was purchased from IFF, and IRONE ALPHA was purchased from Givaudan. Silica gel column chromatography was performed with Wakogel® 60N (63–212 μm). Flash chromatography was performed on Büchi Pure C-810 with FlashPure EcoFlex Silica column. 1H and 13C NMR spectra were recorded at ambient temperature on JEOL ECS 400 NMR spectrometer. CDCl3 was used as the solvent with TMS as the internal standard. The chemical shifts are referenced to signals at 0.00 ppm (TMS, 1H NMR) and 77.16 ppm (CDCl3, 13C NMR). Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, ddd = double double doublet, t = triplet, m = multiplet), coupling constant (Hz), and integration. Mass spectra were obtained using ThermoScientific TraceGC/ISQ GC–MS instrument (EI, 70 eV) with GL Science InertCap1MS column (0.25 mm × 0.25 μm × 30 m). GC-FID was performed on Shimadzu GC-2010 or Nexis GC-2030 with GL Science InertCap1 column (0.25 mm × 0.25 μm × 30 m). GC–MS and GC-FID conditions are as follows. Column temperature: maintain 50 °C for 2 min, then increase to 270 °C at 10 °C/min, and then hold at 270 °C for 6 min. Injection temperature: 280 °C. Split injection: split ratio 50:1. Carrier gas: Helium (99.9999%, GC–MS and GC-FID). All GC and GC–MS samples were diluted with EtOH.
6-hydroxymethylene-3-methylcyclohexanone 542
To a magnetically stirred solution of 3-methylcyclohexanone 4 (30 g, 0.267 mol) in dry toluene (300 ml) was added NaOMe powder (28.9 g, 0.535 mol, 2.0 equiv) all at once at room temperature, and subsequently, HCOOEt (43.1 ml, 0.535 mol, 2.0 equiv) was added. The color of the reaction mixture turned yellow, and the flask got slightly warm. The reaction mixture solidified within a few minutes and was left at room temperature overnight. Water was added to the reaction mixture, which was stirred magnetically to dissolve the solid. The mixture was transferred to a separation funnel, and the aqueous and toluene layers were separated. The toluene layer was extracted with 5% NaOH aqueous solution (150 mL). The aqueous layer and the alkaline extract were combined and washed once with dichloromethane (CH2Cl2). The aqueous solution was acidified with 36% HCl and then extracted with hexane. The organic extracts were dried over anhydrous Na2SO4 and concentrated to an orange oil. Crude 6-hydroxymethylene-3-methylcyclohexanone (34.1–36.8 g, 91–98% yield, purity of 6-hydroxymethylene-3-methylcyclohexanone 82–87% by GC-FID, the ratio of 6-hydroxymethylene-3-methylcyclohexanone 5 and 2-hydroxymethylene-3-methylcyclohexanone 5’ (90:10–92:8)) obtained was used in the next step without purification.
1H-NMR (CDCl3, 400 MHz)42,51 δ 1.02 (d, J = 6.5 Hz, 3H), 1.18–1.31 (m, 1H), 1.74–1.86 (m, 2H), 1.94–2.05 (m, 1H), 2.27–2.49 (m, 3H), 8.68 (d, J = 2.0 Hz, 1H), 14.38 (d, J = 3.1 Hz, 1H).
13C-NMR (CDCl3, 100 MHz)42,51 δ 21.4, 22.6, 27.8, 30.8, 39.5, 108.4, 184.5, 187.9.
MS (m/z)51 55, 70, 79, 97, 111, 125, 140 (100, M+).
6-Methylanilinomethylene-3-methylcyclohexanone 642
The crude 6-hydroxymethylene-3-methylcyclohexanone 5 (34.8 g, 0.249 mol) was dissolved in methanol (80 mL), and N-methylaniline (80.7 mL, 0.746 mol, 3.0 equiv) was added. The resulting yellow solution was stirred at room temperature for 1 h, then allowed to stand at room temperature for 24 h. The solvent of the reaction mixture was removed by evaporation, and then the remaining N-methylaniline was distilled out under vacuum to give a dark red, viscous oil. The oil solidified gradually on standing in a refrigerator (5–10 °C), typically within a week. The solid was crushed and then washed with hexane, and the resulting powder was collected by filtration as yellow powder, 41.6 g (73%, purity 97% by GC-FID). The yellow powder consisted mainly of 6-methylanilinomethylene-3-methylcyclohexanone 6 along with a small amount of 2-methylanilinomethylene-3-methylcyclohexanone 6’ (98:2–99:1).
1H-NMR (CDCl3, 400 MHz)42 δ 0.97 (d, J = 6.4 Hz, 3H), 1.11–1.23 (m, 1H), 1.66–1.74 (m, 1H), 1.79–1.92 (m, 1H), 1.93–2.17 (m, 3H), 2.51 (ddd, J = 17.4, 4.6, 2.3 Hz, 1H), 3.43 (s, 3H), 7.01–7.14 (m, 3H), 7.28–7.36 (m, 2H), 7.58 (s, 1H).
13C-NMR (CDCl3, 100 MHz)42 δ 21.9, 26.6, 29.6, 31.8, 42.6, 47.4, 111.2, 121.5, 124.2, 129.0, 145.3, 146.1, 199.6.
MS (m/z) 77, 91, 104, 106, 130, 144 (100), 146, 158, 172, 186, 212, 214, 229 (M+).
6-Hydroxymethylene-2,2,3-trimethylcyclohexanone 8
A solution of 6-methylanilinomethylene-3-methylcyclohexanone 6 (25.0 g, 109 mmol) in dry tert-butyl methyl ether (MTBE, 35 mL) was added dropwise to a stirring suspension of tert-BuOK (29.4 g, 262 mmol, 2.4 equiv) in dry MTBE (320 mL) over 70 min at room temperature. After stirring for 30 min, CH3I (16.3 mL, 262 mmol, 2.4 equiv) was added dropwise over 90 min at room temperature. The mixture was stirred at room temperature overnight. Water (350 mL) was added to the reaction mixture, which was stirred to dissolve precipitates, and then the organic layer was separated. The remaining aqueous layer was extracted once with MTBE. The combined organic layer was dried over anhydrous Na2SO4, and the solvent was distilled off using an evaporator to yield a dark red oil containing 6-methylanilinomethylene-2,2,3-trimethylcyclohexanone 7. Water (220 mL) and 36% HCl (65 mL) were added to the oil, and the mixture was stirred vigorously for 2 h at room temperature. The acidic mixture was extracted with CH2Cl2. The CH2Cl2 layer was extracted with aqueous 4% NaOH solution (300 mL). The alkaline extract was washed with hexane. The aqueous alkaline layer was acidified with 36% HCl (35 mL). The acidic aqueous layer was extracted with CH2Cl2. The CH2Cl2 extract was dried over anhydrous Na2SO4, and the solvent was removed by evaporation. The dark red oil obtained was purified by silica gel column chromatography to give 14.6 g (79%, purity 96% by GC-FID) of 6-hydroxymethylene-2,2,3-trimethylcyclohexanone 8 as orange oil.
1H-NMR (CDCl3, 400 MHz)23 δ 0.94 (d, J = 6.8 Hz, 3H), 1.07 (s, 3H), 1.21 (s, 3H), 1.37–1.50 (m, 1H), 1.54–1.70 (m, 2H), 2.27–2.41 (m, 2H), 8.74 (d, J = 2.7 Hz, 1H), 14.76 (d, J = 2.8 Hz, 1H).
13C-NMR (CDCl3, 100 MHz) δ 15.9, 20.9, 22.7, 25.0, 27.0, 38.0, 40.9, 106.8, 189.6, 190.0
MS (m/z) 55, 69, 70, 83, 97, 107, 111, 121, 125 (100), 135, 140, 150, 168 (M+).
β-Methylcyclocitral 1041
To the crude γ-methylcyclocitral 9 (29.1 g, 175 mmol), obtained from 6-hydroxymethylene-2,2,3-trimethylcyclohexanone 8 according to the reported procedure23, was added methanolic KOH (20 g KOH in 250 mL methanol) at room temperature. The mixture was stirred at room temperature for 1.5 h, then poured into water. The mixture was extracted with hexane. The organic layer was washed with brine and then dried over anhydrous Na2SO4. The solvent was distilled out under reduced pressure. The crude β-methylcyclocitral 10, obtained as yellow oil (26.6 g, 91% yield from 2-(dimethylamino)-2-(2,2,3-trimethyl-6-methylenecyclohexyl)acetonitrile, purity 76% by GC-FID), was used in the next step without purification.
Spectral data of the purified sample.
1H-NMR (CDCl3, 400 MHz)39,41 δ 0.91 (d, J = 6.4 Hz, 3H), 1.06 (s, 3H), 1.22 (s, 3H), 1.35–1.49 (m, 2H), 1.53–1.60 (m, 1H), 2.09 (s, 3H), 2.17–2.30 (m, 2H), 10.13 (s, 1H).
13C-NMR (CDCl3, 100 MHz)39 δ 15.8, 19.5, 20.9, 26.3, 26.3, 34.9, 36.2, 40.0, 140.9, 155.6, 192.9.
MS (m/z)39 67, 77, 79, 81 (100), 91, 95, 109, 123, 133, 137, 148, 151, 166 (M+).
Aldol condensation of β-methylcyclocitral 10 with acetone39
A solution of crude β-methylcyclocitral 10 (26.3 g, 76% purity by GC-FID, 158 mmol as 100% purity) in dry MTBE (190 mL) was added dropwise to a solution of sodium hexamethyldisilazane (44.1 mL, 40% in THF, 0.53 equiv to β-methylcyclocitral) during 60 min at 10 °C. Then dry acetone (dried over Molecular Sieves 3A) was added dropwise over 40 min at 10 °C, and the resulting mixture was stirred at 10 °C. After 90 min of stirring, the solvent was distilled off under reduced pressure, and water was added to the dark red residue. Organic materials were then extracted with hexane. The hexane extract was washed twice with 1% HCl and then dried over anhydrous Na2SO4. Distillation of hexane yielded 30.0 g of an orange-colored oil containing β-irone 2 and α-irone 1 in a ratio of 92:8 (GC-FID, purity of β-irone 59%), which was used in the next step without purification.
Preferential epoxidation of α-irone in the mixture of β-irone 2 and α-irone 1 by OXONE44
An aqueous solution of OXONE (26.8 g, 43.5 mmol, 0.30 equiv to irone mixture, 180 mL water) was added dropwise to a stirred mixture of β-irone 2 and α-irone 1 (92:8) (30.0 g, 145 mmol), NaHCO3 (9.15 g, 109 mmol, 2.5 equiv to OXONE), and acetone (240 mL) during 30 min at room temperature. The resulting mixture was stirred at room temperature for 1 h, and then water was added. The mixture was then extracted with hexane. The organic layer was washed with brine, dried over anhydrous Na2SO4, and the solvent was distilled off. Orange color oil (28.7 g) was obtained, containing β-irone and α-irone in the ratio of > 99.5: < 0.5, along with epoxy α-irone and epoxy β-irone (β-irone purity, 53%, GC-FID). Purified β-irone 2 was obtained as follows.
Preferential epoxidation of α-irone in the mixture of β-irone 2 and α-irone 1 by H2O2/methyltrioxorhenium (MTO)45,46
To a stirring solution of the mixture of β-irone 2 and α-irone 1 (90:10) (1.32 g, 6.39 mmol) in CH2Cl2 (10 mL) was added 3-methylpyrazole (51 μL, 0.639 mmol, 10 mol%) and 35% H2O2 (265 μL, 3.2 mmol, 0.50 equiv). MTO (16 mg, 0.064 mmol, 1.0 mol%) was added to the mixture, and the resulting mixture was stirred at room temperature. After stirring for 60 min, additional MTO (16 mg, 0.064 mmol, 1.0 mol%) was added. After stirring additional 60 min, water and Na2S2O3 (1 g) were added to the reaction mixture. The mixture was extracted with CH2Cl2, and the CH2Cl2 layer was washed with brine. The CH2Cl2 layer was dried over anhydrous Na2SO4, and the solvent was distilled off. Orange color oil (1.34 g) was obtained, containing β-irone 2 and α-irone 1 in the ratio of 98.7:1.3.
Purification of β-irone 2
The crude β-irone 2 (7.0 g, purity 53% by GC-FID) obtained as described above was dissolved in hexane and subjected to flash chromatography (Büchi Pure C-810, FlashPure EcoFlex Silica column (80 g)). A flash chromatographic separation was performed, starting with 100% hexane and gradually increasing the gradient to 70% with CH2Cl2 as the second solvent over 20 min. Two UV wavelengths at 265 and 286 nm were used for fraction collection. The eluent was sent to the fraction collector if the signal from the detector matched the selected criteria.
The β-irone 2, obtained with over 80% purity by flash chromatography, was distilled under reduced pressure (4 Pa, 93–94 °C) to yield β-irone 2 with over 96% purity. From the crude β-irone (28.7 g), 7.3 g of β-irone 2, with a purity of over 96%, was obtained.
(±)-β-Irone 2
1H-NMR (400 MHz, CDCl3)38,39,52 δ 0.91 (s, 3H) (11), 0.91 (d, J = 6.5 Hz, 3H) (13), 1.06 (s, 3H) (12), 1.37 − 1.52 (m, 2H) (3), 1.55 − 1.62 (m, 1H) (2), 1.73 (s, 3H) (14), 1.99 − 2.16 (m, 2H) (4), 2.30 (s, 3H) (10), 6.07 (d, J = 16.0 Hz, 1H) (8), 7.26 (d, J = 16.0 Hz, 1H) (7).
13C-NMR (100 MHz, CDCl3)39,53 δ 16.3 (13), 22.0 (14), 22.3 (12), 26.7 (3), 27.3 (10), 27.7 (11), 32.4 (4), 37.3 (1), 39.1 (2), 132.6 (8), 134.3 (5), 136.3 (6), 144.3 (7), 198.9 (9).
The numbers in parentheses after the chemical shifts correspond to the carbon numbers of the β-irone (Fig. 6) in 13C-NMR and to the carbon numbers to which the protons are bonded in 1H-NMR.
Carotenoid numbering.
MS (m/z)39 55, 77, 91, 105, 121, 135, 149, 163, 173, 191 (100), 206 (M+).
Olfactory evaluations
Orris absolute (ORRIS ABSOLUTE ITALY, obtained from rhizomes of Iris pallida Lam.) was purchased from IFF. IRONE ALPHA (product code 6065003) was purchased from Givaudan. β-Ionone (product code I0077) was purchased from Tokyo Chemical Industry (TCI).
The olfactory evaluations were performed by three perfumers (one of the authors, M.M., and two in-house perfumers) using the appropriate concentration of the fragrance substances in ethanol on smelling blotters.
Samples for olfactory evaluations were prepared as follows.
Orris absolute, IRON ALPHA, synthesized (±)-β-irone and β-ionone, were diluted in ethanol (99.5%) at 1% concentration. (±)-β-Irone was diluted further to 0.2%, 0.04%, 0.02%, and 0.008% concentration (Table 1).
Purified (±)-β-Irone (97% purity), and (±)-β-irone of various purities obtained during purification (chromatography and distillation). Due to this background, the impurities in the low-purity (±)-β-irone samples differ. These (±)-β-irone samples of varying purities were diluted in ethanol (99.5%) to a concentration of 1% (Table 2).
The olfactory evaluations were performed by a senior perfumer (one of the authors, M.M.). The evaluations were performed using the fragrance substances without dilution on smelling blotters (Table 3).
Olfactory evaluation of 0.0001 g (±)-β-irone on smelling blotter: Dissolve 0.1 g of (±)-β-irone in 100 mL ethanol, soak 0.1 mL of the solution into a smelling blotter, and evaporate the ethanol. Then the scent was evaluated.
Olfactory evaluation of 0.001 g (±)-β-irone on smelling blotter: Dissolve 0.1 g of (±)-β-irone in 10 mL of ethanol, soak 0.1 mL of the solution into a smelling blotter, and evaporate the ethanol. Then the scent was evaluated.
Data availability
The data that support the findings of this study are available in this Article and its Supplementary Information.
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S.Y. and M.M. conceived and designed this project. S.Y. conducted the synthesis of (±)-β-irone, and M.M. evaluated the scent of (±)-β-irone, orris absolute, IRONE ALPHA, and β-ionone. All authors discussed the results and drafted the manuscript.
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Yamazaki, S., Miyazaki, M. Synthesis and olfactory evaluation of (±)-β-irone. Sci Rep 15, 23477 (2025). https://doi.org/10.1038/s41598-025-08925-z
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DOI: https://doi.org/10.1038/s41598-025-08925-z
