Filbertone: A Review - Journal of Agricultural and Food Chemistry

Review Cite This: J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Filbertone: A Review Eva Puchl’ová† and Peter Szolcsányi*,‡ †

Axxence Slovakia Ltd., Mickiewiczova 9, SK-811 07 Bratislava, Slovakia Department of Organic Chemistry, Slovak University of Technology, Radlinského 9, SK-812 37 Bratislava, Slovakia

J. Agric. Food Chem. Downloaded from pubs.acs.org by WESTERN UNIV on 10/21/18. For personal use only.

‡

ABSTRACT: This comprehensive review of filbertone, a principal flavor compound of hazelnut, evaluates the current state of the art of all relevant aspects of the title molecule: its occurrence and properties, laboratory preparation and bulk synthesis, analytical issues regarding stereochemistry and purity, sensory evaluation, and practical uses. Comparisons are made between different synthetic approaches, and a critical assessment of various applications is presented. KEYWORDS: filbertone, hazelnut, flavor, synthesis, applications

1. OCCURRENCE AND PROPERTIES OF FILBERTONE Filbertone, (2E)-5-methylhept-2-en-4-one, 1 (C8H14O, MW 126.22; CAS: 102322-83-8; FEMA (U.S.): 3761; FLAVIS (EU): 07.139), is a key flavor compound in fruits of hazel trees (Corylus maxima and Corylus avellana); its presence was identified1 in hazelnuts in 1989. The trivial name, filbertone, is derived from filberts, a name synonymous with hazelnuts and related to St. Philibert, a seventh-century monk in France whose feast day of August 20th overlaps with the nutting season. Filbertone exists in two enantiomeric forms, (S)-1A and (R)-1B (Figure 1), and rather surprisingly, naturally

SPME varies broadly in unroasted hazelnut oils (44−98% ee), in most cases,5 typical values between 70 and 90% ee were observed. Likewise, the enantiomeric composition of filbertone 1A in unroasted oils analyzed by techniques other than SPME ranged from 73 to 85% ee (with one exceptional case reporting only 43% ee).2 As in the case of the fruits, significantly lower enantiomeric-excess values of filbertone 1A (14−43% ee) were observed when roasted nuts were used to obtain the hazelnut oil.4 The results obtained from both roasted hazelnut fruits and oils suggest that thermal processing remarkably changes the enantiomeric composition of filbertone, a process which initially looked like heat-induced partial racemization with a simultaneous loss of water. In this context, the racemization of filbertone under various conditions was evaluated. No racemization was observed; however, notable thermal decomposition of (S)-1A was observed in the gas phase at 120 °C after 21 days.1 Also, when filbertone 1A and distilled water were heated in an autoclave at 100 °C for up to 96 h, the initial enantiomeric excess remained unchanged.6 Both experiments with synthetic (S)-1A clearly revealed that this enantiomer is not heat sensitive. Even a distillation−extraction procedure at pH 6 conducted with the (S)-enantiomer, 1A, did not cause the slightest isomerization to the (R)-enantiomer, 1B.4 Moreover, in the presence of silica, no racemization occurred at room temperature for over 21 h; however, the enantiomeric excess dropped from 92 to 74% in the presence of basic alumina under same conditions.1 These results suggest that the relative decrease of (S)-filbertone 1A content during hazelnut roasting must thus be explained in a different way. Regarding the olfactory properties, Jauch et al.1 originally claimed that the sensory evaluation of (S)-1A and (R)-1B filbertone revealed a significant olfactory difference between the enantiomers in respect to odor intensity and quality. Authors also stated that the sensory evaluation of synthetic racemic filbertone and enantiomerically pure (S)-1A revealed striking olfactory differences. However, no further details were

Figure 1. Stereoisomers of (E/Z)-filbertone, 1 and 2.

occurring filbertone exhibits low-to-medium enantiomeric excess, with (S)-enantiomer 1A being the prevalent isomer in ratios varying according to the origin of the hazelnuts, thermal treatment, analytical sample collection, and technological processing (vide infra). The enantiomeric content of natural filbertone in hazelnuts of different geographic origins ranges from 54 to 73% ee. For example,1 hazelnuts from Turkey contain (S)-1A with 54−56% ee, whereas those from Italy exhibit 62−63% ee. The highest enantiomeric excess of natural filbertone was reported in hazelnuts from California (70% ee).2 Moreover, the enantiomeric purity of isolated filbertone is usually higher under mild extraction conditions. There is also a remarkable difference between raw and roasted hazelnuts;3 the enantiomeric excess of filbertone 1A drops from values of around 67% ee in raw hazelnuts to about 45% ee in roasted hazelnuts.4 Analogous results were observed for hazelnut oil too. Although the enantiomeric purity of filbertone 1A in samples obtained by © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

August 11, 2018 October 5, 2018 October 10, 2018 October 10, 2018 DOI: 10.1021/acs.jafc.8b04332 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry

2.1.1. Grignard Addition. The first employment10 of Grignard addition in the synthesis of filbertone 1 features the ring-opening of oxazoline, 6, which, in turn was initially prepared by thermal condensation of 2-methylbutanoic acid, 4, with amino alcohol, 5. Thus, low-temperature addition of an excess of allylmagnesium chloride to oxazoline, 6, followed by acid hydrolysis of the intermediary imine with the final alkene isomerization furnished racemic filbertone 1 in 37% overall yield in two steps (Scheme 2). Later, Grignard additions to other electrophiles, such as aldehydes and lactones, were also successfully employed. A short, two-step route11 to filbertone 1 used the initial addition of crotonaldehyde, 8, to a cold solution of sec-butylmagnesium bromide, 7, to obtain the intermediary allyl alcohol, 9, which was finally oxidized to the title compound in 19% overall yield. However, despite the straightforward strategy and minimal number of steps, there are two major disadvantages besides the modest yield of this approach: First, the starting α,βunsaturated aldehyde, 8, may generate an undesired side product via concurrent 1,4-addition in the first step. Second, the oxidation of alkenol 9 is carried out in sulfuric acid, although the compound is prone to allylic rearrangement under acidic conditions. These factors complicate the product composition and significantly reduce the final content of filbertone 1. Moreover, the sequence generates a nonnegligible amount of (toxic) waste, making this process rather unsuitable for scaling up or industrial production (Scheme 3). With the aim of partially overcoming the inherent problems of the above-mentioned approach and thus increasing the yield and purity of filbertone 1, Huang et al.12 utilized 2methylbutanal, 10 (instead of crotonaldehyde, 8), and allylic Grignard reagents to obtain homoallyl alcohol 11. This was subsequently oxidized to ketone 12, which was finally isomerized to filbertone 1. Although the best total yield (39%) via such a three-step sequence is notably higher, this approach still uses a toxic stoichiometric oxidant, which largely limits its scaling potential (Scheme 4). Eventually, Wang et al.13 disclosed a preparation method of filbertone 1 using γ-butyrolactone, 13, as a raw material, which initially underwent a Grignard addition with 7 to provide cyclic hemiacetal 14. The latter was dehydrated to 2,3-dihydrofuran, 15, which underwent ring-opening by acid-catalyzed hydration, furnishing γ-hydroxyketone, 16. Finally, sequential dehydration−isomerization of 16 yielded filbertone 1 in 37% overall yield over four steps (Scheme 5). However, because of the number of steps and use of phosphine reagent, this approach is also unlikely to be directly adaptable for industrial scale-up. 2.1.2. Fragmentation. Another synthetic approach leading to filbertone 1 features the base-promoted anionic cleavage of tertiary bis-homoallylic alcohols leading to the corresponding enones. For this purpose, stoichiometric or overstoichiometric amounts of various strong organic or inorganic bases, such as alkali metal hydrides and alkoxides, were used.14 In a typical example,15 the key dienol, 18, readily prepared from ester 17 by double addition of an allylic nucleophile, was deprotonated

disclosed regarding this issue.1 On the other hand, Güntert et al.4 performed a full assessment and olfactory comparison of all four possible stereoisomers of filbertone (Figure 1). Thus, sniffing the effluent of a chiral GC analysis using a βcyclodextrin column yielded a sensory impression of the (E,S)1A, (E,R)-1B, (Z,S)-2A, and (Z,R)-2B stereoisomers. Notably, the odors of all four compounds were typical of hazelnuts. However, the odor threshold of 1A showed about a 10-fold decrease over that of 1B, and about a 3-fold decrease over that of the 2A stereoisomer. Interestingly, at higher concentrations, all four isomers tended to smell metallic. The tastes of the 1A and 1B enantiomers isolated by preparative HPLC from racemic (synthetic) filbertone were also evaluated. Descriptions were as follows (both enantiomers, 1A and 1B, were evaluated at concentration of 25 ppb in water): (E,S)filbertone 1A had the flavors of ‘hazelnut’, ‘metallic’, ‘fatty’, and ‘pyridine’ and a stronger impact, whereas (E,R)-filbertone 1B had ‘hazelnut’, ‘soft’, ‘butter’, ‘chocolate’, and ‘metallic’ and a weaker impact.4 The odor threshold of filbertone 1 is reported with value 0.05 μg/L (in water at 25 °C).7 The flavorperception threshold is about 5 ppt, and the recognition threshold is about 30 ppt (in a 3% sucrose solution).8

2. SYNTHESES OF FILBERTONE Considering the obvious commercial potential of filbertone, there has been naturally a strong interest in synthetically efficient or cost-effective preparations of the title compound. To date, several synthetic approaches have been described in the literature, and accordingly, most of the published preparations are disclosed in the patent literature. In general, these can be divided into two main categories: racemic and enantioselective syntheses. An assessment and comparison of the respective strategies follows. 2.1. Racemic Syntheses. The very first mention of 5methylhept-2-en-4-one synthesis (although it was not named filbertone yet) dates from 1939. The authors describe9 an acidcatalyzed dehydration of β-hydroxyketone, 3 (Scheme 1), itself Scheme 1. First Preparation of Filbertone, 1

made through the use of acetoacetic ester. Although the exact yield of isolated filbertone was not disclosed (only the range of 70−90%), its first physicochemical and analytical data were reported. Since then, various synthetic strategies to produce filbertone 1 have been published. All known racemic syntheses of filbertone 1 can be grouped into three main categories according to the key step of the respective approach: Grignard addition, fragmentation, and aldol−Knoevenagel condensation, respectively.

Scheme 2. First Use of Grignard Addition in the Preparation of Filbertone 1

B

DOI: 10.1021/acs.jafc.8b04332 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry Scheme 3. Grignard Addition with Subsequent Oxidation in the Synthesis of Filbertone 1

Scheme 4. Grignard Addition with Oxidation−Isomerisation in the Synthesis of Filbertone 1

Scheme 5. Grignard Addition with Repeated Dehydration−Hydration in the Synthesis of Filbertone 1

Scheme 6. Base-Promoted Fragmentation in the Synthesis of Filbertone 1

Scheme 7. Double Use of Aldol Condensation in the Synthesis of Filbertone 1

Scheme 8. Use of Knoevenagel Condensation in the Synthesis of Filbertone 1

inherent unfavorable atom economy and low chemoselectivity, resulting in poor efficiency of filbertone 1 production. 2.1.3. Aldol and Knoevenagel Condensation. With the aim of making the synthesis of filbertone 1 more suitable for the prospective industrialization (i.e., to avoid highly reactive reagents, e.g., organometallics and hydrides, and to reduce environmental pollution, e.g., chromium waste and HMPT),

by NaH, whereupon the in situ generated alkoxide spontaneously fragmented to gaseous propene and a mixture of major ketone 12 and minor filbertone 1, providing only 16% overall yield for the latter (Scheme 6). Clearly, this approach is severely limited not only by the use of carcinogenic hexamethylphosphoric triamide (HMPT) but also by the C


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Journal of Agricultural and Food Chemistry Scheme 9. Optimized Enantioselective Synthesis of (S)-Filbertone, 1A

Scheme 10. Alternative Enantioselective Synthesis of (S)-Filbertone, 1A

cially available starting material. In the first asymmetric1 synthesis of 1A, the initial Jones oxidation of alcohol 26 led to the corresponding aldehyde, 27, which subsequently underwent nucleophilic attack at the carbonyl function by propynyl lithium, 28, forming a diastereomeric mixture of alcohols 29. The stereospecific alkyne reduction afforded a mixture of diastereomeric (E)-allylic alcohols, 30, which upon oxidation eventually furnished (S)-filbertone 1A (with 92% ee) in an unoptimized 20% overall yield. This protocol was later modified and used by others20 for the synthesis of racemic [2H2]-5-methylhept-2-en-4-one, a dideuterated analogue of filbertone 1. In their own modification of the original route, Jauch et al.21 later used innocuous TEMPO-based oxidation instead of toxic chromium salts in the first step, whereas the rest of the sequence remained synthetically the same. Particular yields were also improved by optimized workup procedures (i.e., careful hydrolyses and extractions of crude reaction mixtures as well as careful distillations of intermediates). Thus, the overall yield of (S)-filbertone 1A eventually reached 40% over four steps (Scheme 9). An alternative but similar approach22 exploited allylation of known aldehyde 27 with an organozinc nucleophile to provide a diastereomeric mixture of homoallylic alcohols 11A. This was oxidized to afford ketone 12A, which was finally isomerized to the desired target. Although this synthetic sequence furnished (S)-filbertone 1A in an overall yield (42%) very similar to that of the previous approach, a drop in the target molecule’s enantiomeric purity was observed (down to 75% ee). This was probably due to partial racemization during the final conjugation step (Scheme 10). It is noteworthy that the synthesis of enantiomerically enriched or pure (E,R)-filbertone, 1B, has not been reported yet, neither independently nor via the application of the strategy used in the synthesis of (E,S)-filbertone, 1A. This

Cheng16 devised a straightforward three-step sequence that comprised an initial acid-catalyzed aldol condensation of acetaldehyde, 19, with butanone, 20, followed by catalytic hydrogenation of enone 21 to ketone 22. This was followed by aldol condensation of 22 with acetaldehyde, 19, again to provide the racemic (E)-filbertone 1 in 36% overall yield, with the last reaction being the lowest-yield step (Scheme 7). Notably, by using [2H4]-acetaldehyde instead of unlabeled ethanal 19, this protocol was later adapted by others17 for the synthesis of racemic [2H4]-5-methylhept-2-en-4-one, a tetradeuterated analogue of filbertone 1. Another aldol-based approach18 uses the Knoevenagel condensation of racemic β-ketoester 23 with acetaldehyde, 19, to generate an intermediary ketol, 3, which is then dehydrated to racemic filbertone 1. This three-step process was optimized to a one-pot protocol without isolation of either the acid or the ketol. Thus, the initial basic saponification of 23 released a potassium salt of the acid, which was subsequently condensed with ethanal 19 to generate intermediary βhydroxyketone 3. Ketone 3 was in situ dehydrated under acidic conditions to the final target in 64% overall yield (Scheme 8). To date, this synthetic sequence represents the highest-yielding preparation of racemic filbertone 1. However, the necessary preliminary access to the starting ketoester, 23, must be accounted for when considering the overall efficiency of filbertone preparation from readily available commercial substrates. As an example, ketoester 23 was prepared19 from ethyl acetoacetate, 24, by reaction with acyl chloride 25. The ketoester, 23, was then condensed with acetaldehyde, 19, in an analogous manner as above to finally provide racemic (E)filbertone 1 (Scheme 8). 2.2. Enantioselective Syntheses. To date, three enantioselective syntheses of (E,S)-filbertone 1A are known, and all are based on the chiral-pool approach, employing enantiomerically pure (S)-2-methylbutanol, 26, as commerD


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Journal of Agricultural and Food Chemistry

4. USES OF FILBERTONE 4.1. Perfumery. As already mentioned, the odor of racemic filbertone 1 is described as ‘fruity’, ‘hazelnut’, ‘green’, and ‘dried fruit’. For perfumery uses, it is used as a top-note booster and highly diffusive material. The compound gives a natural impression of exotic fruit and citrus notes; moreover, it combines well with rose ketones. Filbertone exhibits very good stability for use in personal-care products (body lotions, shampoos and shower gels, soaps, and roll-ons) and cleaning agents (detergent powder, cleaner-liquid citric acid, and cleaner APC liquid); on the other hand, its stability in bleach is rather poor. For such practical purposes, filbertone 1 is usually applied as a 1% solution (TEC 736664) at the use level of 0.1−2%. 4.2. Flavor and Food Industry. It is reported30 that racemic filbertone 1 has a flavor-perception threshold as low as about 5 ppt, and a recognition threshold of about 30 ppt (as a 3% sucrose solution). The flavor30 of filbertone 1 is described as ‘hazelnut’, ‘nutty’, ‘nougat’, and ‘roasted’. Its application is useful in nuts; cocoa; coffee; mints; meat; and citrus, tropical, or fruity foods. For these purposes, filbertone 1 is used as 5% sugar solution at dosage of 0.1 ppm.

might be due to the fact that the respective substrate, R-(+)-2methylbutanol, is rather expensive or must in turn be obtained via preliminary synthesis itself. All in all, when pure (E,R)filbertone, 1B, is needed for the study of its properties, the semipreparative enantioselective separation of filbertone is warranted.23

3. APPLICATIONS OF FILBERTONE Recently, filbertone has been considered a suitable marker for analysis of hazelnut contents in various products. The methods based on the determination of filbertone can be classified according to the purposes of such analyses: (a) methods for determination hazelnut-oil addition when possessing olive oil and similar fatty acids and (b) methods for the detection of allergens and authenticity evaluation of hazelnut products. 3.1. Adulteration of Olive Oils. The adulteration of olive oil with the commercially much less valuable hazelnut oil is relatively frequent. In general, the detection of such malpractice with previous methods has been rather difficult, and standard analyses of fatty acid and sterol contents24 fail because of their similar profiles in both oils. However, filbertone, which is naturally occurring in hazelnut oil, is not present in olive oil. Therefore, it was proposed that the presence or absence of this analyte can be used as an adequate biomarker to distinguish between hazelnut oil and olive oil. The described analytical methods25 for the detection of olive oil adulteration with hazelnut oil usually involve preconcentration steps, and (multidimensional) chromatographic techniques are used on GC systems equipped with programmable temperature vaporizers or MS detectors with selected ion monitoring to improve sensitivity. In some cases, identification of filbertone using 1H NMR is also possible.26 The detection threshold ranges from 0.3 to 50 μg of filbertone per liter. It allows the recognition from 5 to 25% of hazelnut oil in olive oil, depending on its variety, place of origin, degree of refining, and methodology applied. In this context, consideration of refined hazelnut oil is further complicated by the harsh conditions typically used in the process, which may result in partial or even total elimination of volatile compounds such as filbertone. 3.2. Authenticity Evaluation of Hazelnut-Based Products. The possible use of filbertone as a key marker for quality-sorting of hazelnut-based products has attracted certain interest. The methodology uses hazelnut-paste contents as the principal qualitative criterion.27 Filbertone is naturally occurring in raw hazelnuts, but its original content increases considerably by roasting.28 In a model experiment, its content increased up to 1000-times after 15 min of heating at 180 °C. In fact, 315 μg/kg were found in commercially produced oil from roasted fruits, whereas oil from unroasted hazelnuts contained less than 10 μg/kg filbertone.29 However, because of the high variability of filbertone contents in hazelnuts and hazelnut-derived products, the compound itself cannot be used for the precise quantification of hazelnut contents. On the other hand, analysis of five authentic hazelnut pastes from Italy and Turkey (304−584 μg/kg filbertone)27 allowed the practical grading of commercially available products according to filbertone content into three quality groups: samples with less than 1% hazelnuts (45 μg/kg filbertone).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +421 2 59325745. E-mail: [email protected]. ORCID

Eva Puchl’ová: 0000-0001-6380-8048 Peter Szolcsányi: 0000-0003-1957-914X Funding

This work was supported by the Slovak Research and Development Agency under contract No. APVV-15-0355. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Peter Zálupský for proof-reading and language-editing as well as to Dr. Kvetka Vranková for valuable suggestions.

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REFERENCES

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Filbertone: A Review - Journal of Agricultural and Food Chemistry

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