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The hydration of nootkatone in aqueous acids
Technical Note The Hydration of Nootkatone in Aqueous Acids
ABSTRACT
Nootkatone 1 undergoes facile hydration to the 13-hydroxy derivative 2 in aqueous citric acid (pH 2.4). The non-detection of 2 in grapefruit juice is attributed to the protective effect of the cloud particles. The odour potency of 2 is about 1/60 of 1.
INTRODUCTION
In the course of studies of the acid-catalysed transformations of terpenoids, Baxter et al. (1978) have noted that compounds possessing an isopropenyl group undergo facile hydration in dilute aqueous acids. Nootkatone 1, which has such a function, is allegedly the character-impact component of the aroma of grapefruit (MacLeod & Buigues, 1964; MacLeod, 1966) and as such its stability in dilute citric acid is of intrinsic importance. The characteristic musty-grapefruit odour of a dilute solution of nootkatone in aqueous citric acid (pH 2-4) was found to diminish in intensity over a period of several weeks. Solvent extraction of the solution after four weeks afforded a mixture of nootkatone and a more polar product (2: 3) which was separated by preparative thin layer chromatography. The latter product was an oil, [~]2o + 133 ° (c 0.99, CHC13) , which was assigned the molecular formula C15H240 2 on the basis of elemental analysis and mass spectroscopy. Absorptions at 239 nm (e 17,100 EtOH) in the ultra-violet and at 1665 c m - 1 in the infra-red spectra indicated that the product retained the a, fl-unsaturated ketone structure. An absorption at 3460 c m - 1 in the infra-red spectrum, coupled with the observation of the base peak in the mass spectrum at m/e 59, were indicative of the presence of a hydroxyisopropyl group. Confirmation that this product was the expected 13-hydroxy derivative 2 was 319 Fd. Chem. 0308-8146/79/0004-0319/$02-25 © Applied Science Publishers Ltd, London, 1979
Printed in Great Britain
320
R.L. BAXTER, D. McHALE
obtained by comparison of its ir and nmr spectra with previously reported values (Hikino et al., 1968). Similarly, a solution of nootkatone in 2M H2SO * afforded a 2:3 mixture of starting material and 2 after 30 h. The ready hydration of nootkatone in aqueous acids contrasts with an earlier report of its acid stability under anhydrous conditions (MacLeod & Buigues, 1964).
a 1
2
The compound 2, purified by repeated preparative thin-layer chromatography on silica until no significant odoriferous impurities were apparent by gas-liquid chromatography-odour port analysis, had a burnt floral-woody odour. The odour potency of 2 in aqueous solution was found to be about 1/60 of that of nootkatone. This is significantly lower than the odour potencies reported by Teranishi (1971) for other nootkatone related compounds and reflects the introduction of the hydroxyl group into the molecule. While nootkatone readily affords the hydration product 2 in homogenous dilute acid, analysis (by successive column and thin layer chromatography) of extracts of grapefruit juices which had been stored at various temperatures showed that no significant amounts of 2 had been formed. However, thin layer chromatography analysis of extracts from cloud particles and supernatant liquor established that the nootkatone is mainly associated with the former and as such is not exposed to the action of aqueous citric acid.
ACKNOWLEDGEMENTS We thank Firmenich et Cie for a gift of ( +)-nootkatone, Mr R. Rosen for technical assistance and the Directors of Cadbury Schweppes Ltd for permission to publish this work.
REFERENCES BAXTER,R. L., LAURIE,W. A. & MCHALE,D. (1978). Transformationsof monoterpenoidsin aqueous acids. The reactionsoflinalool, geraniol,nerol and their acetatesin aqueouscitricacid, Tetrahedron,
34, p. 2195. HIKINO,H., SUZUKI,N. & TAKEMOTO,T. (1968). Structureand absoluteconfigurationof kusunol, Chem. Pharm. Bull. (Tokyo), 16, 832.
THE HYDRATION OF NOOTKATONE IN AQUEOUS ACIDS
321
M ACLEOD, W. D. (1966). Nootkatone, grapefruit flavour and the citrus industry, CaliJl Citrograph, 51, 120. MACLEOD, W. D. & BUIGUES,N. M. (1964). Sesquiterpenes. !. Nootkatone, a new grapefruit flavour constituent, J. Food Sci., 29, 565. TERANISHt, R. (1971). Odour and molecular structure. In Gustation and olJaction ((Eds.), Ohloff, G. & Thomas, A. F.) London, Academic Press, 165 77. R. L. BAXTER & D . MCHALE,
( R e c e i v e d : 7 J u l y , 1978)
Group Research Laboratory, Cadbury Schweppes Ltd, Garrick Rd., London N W 9 6 A N , Great Britain
ABSTRACT
Nootkatone 1 undergoes facile hydration to the 13-hydroxy derivative 2 in aqueous citric acid (pH 2.4). The non-detection of 2 in grapefruit juice is attributed to the protective effect of the cloud particles. The odour potency of 2 is about 1/60 of 1.
INTRODUCTION
In the course of studies of the acid-catalysed transformations of terpenoids, Baxter et al. (1978) have noted that compounds possessing an isopropenyl group undergo facile hydration in dilute aqueous acids. Nootkatone 1, which has such a function, is allegedly the character-impact component of the aroma of grapefruit (MacLeod & Buigues, 1964; MacLeod, 1966) and as such its stability in dilute citric acid is of intrinsic importance. The characteristic musty-grapefruit odour of a dilute solution of nootkatone in aqueous citric acid (pH 2-4) was found to diminish in intensity over a period of several weeks. Solvent extraction of the solution after four weeks afforded a mixture of nootkatone and a more polar product (2: 3) which was separated by preparative thin layer chromatography. The latter product was an oil, [~]2o + 133 ° (c 0.99, CHC13) , which was assigned the molecular formula C15H240 2 on the basis of elemental analysis and mass spectroscopy. Absorptions at 239 nm (e 17,100 EtOH) in the ultra-violet and at 1665 c m - 1 in the infra-red spectra indicated that the product retained the a, fl-unsaturated ketone structure. An absorption at 3460 c m - 1 in the infra-red spectrum, coupled with the observation of the base peak in the mass spectrum at m/e 59, were indicative of the presence of a hydroxyisopropyl group. Confirmation that this product was the expected 13-hydroxy derivative 2 was 319 Fd. Chem. 0308-8146/79/0004-0319/$02-25 © Applied Science Publishers Ltd, London, 1979
Printed in Great Britain
320
R.L. BAXTER, D. McHALE
obtained by comparison of its ir and nmr spectra with previously reported values (Hikino et al., 1968). Similarly, a solution of nootkatone in 2M H2SO * afforded a 2:3 mixture of starting material and 2 after 30 h. The ready hydration of nootkatone in aqueous acids contrasts with an earlier report of its acid stability under anhydrous conditions (MacLeod & Buigues, 1964).
a 1
2
The compound 2, purified by repeated preparative thin-layer chromatography on silica until no significant odoriferous impurities were apparent by gas-liquid chromatography-odour port analysis, had a burnt floral-woody odour. The odour potency of 2 in aqueous solution was found to be about 1/60 of that of nootkatone. This is significantly lower than the odour potencies reported by Teranishi (1971) for other nootkatone related compounds and reflects the introduction of the hydroxyl group into the molecule. While nootkatone readily affords the hydration product 2 in homogenous dilute acid, analysis (by successive column and thin layer chromatography) of extracts of grapefruit juices which had been stored at various temperatures showed that no significant amounts of 2 had been formed. However, thin layer chromatography analysis of extracts from cloud particles and supernatant liquor established that the nootkatone is mainly associated with the former and as such is not exposed to the action of aqueous citric acid.
ACKNOWLEDGEMENTS We thank Firmenich et Cie for a gift of ( +)-nootkatone, Mr R. Rosen for technical assistance and the Directors of Cadbury Schweppes Ltd for permission to publish this work.
REFERENCES BAXTER,R. L., LAURIE,W. A. & MCHALE,D. (1978). Transformationsof monoterpenoidsin aqueous acids. The reactionsoflinalool, geraniol,nerol and their acetatesin aqueouscitricacid, Tetrahedron,
34, p. 2195. HIKINO,H., SUZUKI,N. & TAKEMOTO,T. (1968). Structureand absoluteconfigurationof kusunol, Chem. Pharm. Bull. (Tokyo), 16, 832.
THE HYDRATION OF NOOTKATONE IN AQUEOUS ACIDS
321
M ACLEOD, W. D. (1966). Nootkatone, grapefruit flavour and the citrus industry, CaliJl Citrograph, 51, 120. MACLEOD, W. D. & BUIGUES,N. M. (1964). Sesquiterpenes. !. Nootkatone, a new grapefruit flavour constituent, J. Food Sci., 29, 565. TERANISHt, R. (1971). Odour and molecular structure. In Gustation and olJaction ((Eds.), Ohloff, G. & Thomas, A. F.) London, Academic Press, 165 77. R. L. BAXTER & D . MCHALE,
( R e c e i v e d : 7 J u l y , 1978)
Group Research Laboratory, Cadbury Schweppes Ltd, Garrick Rd., London N W 9 6 A N , Great Britain