Scalable synthesis of the aroma compounds d6-β-ionone and d6-β-cyclocitral for use as internal standards in stable isotope dilution assays

Article abstract of DOI:10.1016/j.tetlet.2020.152642

C₁₃ Norisoprenoids are important aroma compounds in wine, giving positive attributes to the overall wine aroma even when found at very low levels. β-Ionone is considered one of the most important aroma compounds giving violet, woody and raspberry aromas to wine, fruits and vegetables in which it is found. Due to its potent aroma at low levels, precise analytical methods are desired for its quantification. Stable isotope dilution assay (SIDA) is one of the most important of these methods but requires the use of isotopically labelled standards. Herein, we describe the scalable synthesis of d₆-β-ionone and d₆-β-cyclocitral, another aroma compound with smokey and fruity notes, starting from the relatively inexpensive deuterated starting material d₆-acetone.

Full text of DOI:10.1016/j.tetlet.2020.152642

Tetrahedron Letters 61 (2020) 152642
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Scalable synthesis of the aroma compounds d6-b-ionone and
d6-b-cyclocitral for use as internal standards in stable isotope
dilution assays
a,b,c,
Shabnam Mosaferi a, Rebecca E. Jelley a, Bruno Fedrizzi a,b, , David Barker
⇑
⇑
a School of Chemical Sciences, University of Auckland, 23 Symonds St, Auckland, New Zealand
b Centre for Green Chemical Science, School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand
c MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand
a r t i c l e i n f o
a b s t r a c t
Article history:
C13 Norisoprenoids are important aroma compounds in wine, giving positive attributes to the overall
wine aroma even when found at very low levels. b-Ionone is considered one of the most important aroma
compounds giving violet, woody and raspberry aromas to wine, fruits and vegetables in which it is found.
Due to its potent aroma at low levels, precise analytical methods are desired for its quantification. Stable
isotope dilution assay (SIDA) is one of the most important of these methods but requires the use of iso-
topically labelled standards. Herein, we describe the scalable synthesis of d6-b-ionone and d6-b-cycloci-
tral, another aroma compound with smokey and fruity notes, starting from the relatively inexpensive
deuterated starting material d6-acetone.
Received 13 September 2020
Revised 4 November 2020
Accepted 8 November 2020
Available online 16 November 2020
Keywords:
Aroma compounds
Labelled standards
b-Ionone
Ó 2020 Elsevier Ltd. All rights reserved.
Stable isotope dilution assays
Introduction
and identified in Tokaj Aszu wines made from white grapes by
Schreier and co-workers in 1976 [12]. b-Ionone has been associ-
Aroma is considered to be one of the key factors in determining
the quality of a wine [1–3]. Aroma compounds can be present in
grapes during ripening as free volatile species or as non-volatile
precursors [4]. Monoterpenes, norisoprenoids, benzenoids, and lac-
tones are examples of classes of compounds that contribute to the
varietal aroma of wine and are present in grapes as odourless pre-
cursors or glycosides [5]. C13 Compounds are the most abundant
natural norisoprenoids and are thought to contribute favourable
aroma in wines, even at very low concentrations [6–8]. C13-Noriso-
prenoids constitute an essential component of the aroma profile in
non-floral grapes such as Cabernet sauvignon [9], Syrah [10], Sau-
vignon blanc [11] and Pinot noir [7].
ated with violet, woody and raspberry sensory descriptors and
has an odour threshold of 30 ng L-1 in water [15]. Studies have
demonstrated that b-ionone and its derivatives can exhibit anti-
inflammatory, antifungal, antibacterial, anti-proliferative, anti-
metastatic and anti-cancer pharmaceutical properties [16].
Due to the important contribution of b-ionone to the aroma
profile of wines, a number of sensitive analytical techniques have
been developed for the quantification of this compound. These
methods, with a focus on the wine matrix, include stir bar sorptive
extraction with liquid desorption followed by large volume injec-
tion coupled to gas chromatography–quadrupole mass spectrome-
try (SBSE-LD/LVI-GC–qMS) [17], headspace solid phase
microextraction (SPME) coupled with gas-chromatography/mass-
spectrometry (GC–MS) [18], stable isotope dilution assay (SIDA)
[18], stir bar sorptive extraction (SBSE) followed by a thermal des-
orption-gas chromatography–mass spectrometry analysis [19],
automated headspace solid-phase microextraction (HS-SPME)
combined with gas chromatography-ion trap/mass spectrometry
(GC-IT/MS) [20], chromatography–olfactometry (GC-O) [21] and
stir bar sorptive extraction gas chromatography mass spectrome-
try (SBSE-GC–MS) [7]. The development of SIDA has contributed
significantly to improving the accuracy and ease of determination
of trace concentrations of compounds by limiting variability
related to the sample preparation and the matrix [18,22,23]. This
b-Ionone
[(2,2,6-trimethyl-1,3-cyclohexen-1-yl)-3-buten-2-
one] is considered as one of the most important volatile C13-noriso-
prenoids from an aroma point of view [12]. The structure of this
aroma compound consists of the megastigmane backbone with a
keto group at C(9) (Fig. 1) [13]. b-Ionone and its derivatives are
widely present in fruits, vegetables and grains containing b-caro-
tene [14]. It was first isolated in 1929 from Boronia megastigma
⇑
Corresponding authors at: School of Chemical Sciences, University of Auckland,
23 Symonds St, Auckland, New Zealand.
E-mail addresses: b.fedrizzi@auckland.ac.nz (B. Fedrizzi), d.barker@auckland.ac.
nz (D. Barker).
https://doi.org/10.1016/j.tetlet.2020.152642
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.

S. Mosaferi, R.E. Jelley, B. Fedrizzi et al.
Tetrahedron Letters 61 (2020) 152642
It was envisaged that deuterated b-ionone could be prepared
using the inexpensive and readily available deuterated solvent
d6-acetone, which would have the added benefit of introducing
six deuterium atoms. In addition, the use of d6-acetone versus
d3-iodomethane as the source of deuteration is approximately
ten times more cost efficient.
Result and discussion
The proposed synthesis was first investigated using unlabeled
acetone to establish the methodology. Acetone (1) was converted
to unsaturated ester 2 in 88% yield using triethyl phosphonoac-
etate (Scheme 1). Reduction of ester 2 with LiAlH4 gave allylic alco-
hol 3, which was immediately converted to dimethylallyl bromide
4 in 70% yield using HBr. The dianion of ethyl acetoacetate was
formed using NaH and n-BuLi, which was reacted with dimethylal-
lyl bromide 4 to give b-ketoester 5 in 60% yield [28]. Cyclisation of
b-ketoester 5 was achieved easily using tin-(IV) chloride, providing
cyclic b-ketoester 6 in 88% yield [29]. Following formation of the
required cyclohexane ring in cyclic b-ketoester 6, a Wittig reaction,
using methylene(triphenyl)phosphorane, afforded methylenecy-
clohexane ester 7 in 85% yield [30]. Reduction of ester 7 using
DIBAL-H provided primary alcohol 8 in a moderate 62% yield
[31]. LiAlH4 was less effective than DIBAL-H at yielding 7 from 8;
even when excess LiAlH4 was used, starting material consistently
remained. Alcohol 8 was oxidised using Dess-Martin periodinane
(DMP) to give the desired aldehyde 9. Rearrangement of the alkene
in aldehyde 9 using DBU gave conjugated aldehyde 10, which itself
Fig. 1. b-Ionone structure.
approach is based on using isotopically labelled compounds as
analytical standards. In previously reported synthetic methods,
b-ionone has been converted to a deuterated analogue through
the inclusion of three deuterium atoms at the C-10 methyl group
[24]. This approach tends to require long reaction times and/or
results in low yields. However, more importantly, the deuterium
atoms at this position can be readily lost during mass spectrometry
fragmentation, which poses a hindrance to some experimental
designs [23,25]. In another reported method, the toxic and highly
volatile reagent d3-iodomethane is employed as the source of deu-
terium; this reaction results in deuteration of the methyl groups on
the cyclohexene ring [26,27].
Scheme 1. Synthesis of b-cyclocitral and b-ionone. Reagents and conditions: (a) triethyl phosphonoacetate (1.3 equiv.), NaH (1.2 equiv.), THF, 0 °C, 24 h, 88%; (b) LiAlH4 (1.5
equiv.), Et2O, 0 °C, 3 h, 72%; (c) HBr 48% (1.2 equiv.), CH2Cl2, 0 °C, 2 h, 70%; (d) NaH (2 equiv.), ethyl acetoacetate (1 equiv.), 1.6 M n-BuLi (1.6 equiv.), THF, 0 °C, 21 h, 60%; (e)
SnCl4 (1.6 equiv.), CH2Cl2, 0 °C, 24 h, 88%; (f) KOtBu (2.3 equiv.), Ph3P+MeBr- (2.2 equiv.), THF, 60 °C, 23 h, 85%; (g) 1 M DIBAL-H (2.5 equiv.), CH2Cl2, À78 °C, 2 h, 62%; (h) DMP
(1.1 equiv.), CH2Cl2, rt, 2 h, 75%; (i) DBU (3 equiv.), CH2Cl2, 0 °C, 24 h, 60%; (j) 10% aqueous KOH (8 equiv.), acetone, 60 °C, 22 h, 40%.
Scheme 2. Synthesis of d6-b-ionone. Reagents and conditions; (a) triethyl phosphonoacetate (1.3 equiv.), NaH (1.2 equiv.), THF, 0 °C, 24 h, 85%; (b) LiAlH4 (1.5 equiv.), Et2O,
0 °C, 3 h, 77%; (c) HBr 48% (1.2 equiv.), CH2Cl2, 0 °C, 2 h, 69%; (d) NaH (2 equiv.), ethyl acetoacetate (1 equiv.), 1.6 M n-BuLi (1.6 equiv.), THF, 0 °C, 21 h, 58%; (e) SnCl4 (1.6
equiv.), CH2Cl2, 0 °C, 24 h, 86%; (f) KOtBu (2.3 equiv.), Ph3P+MeBr- (2.2 equiv.), THF, 60 °C, 23 h, 82%; (g) 1 M DIBAL-H (2.5 equiv.), CH2Cl2, À78 °C, 2 h, 68%; (h) DMP (1.1
equiv.), CH2Cl2, rt, 2 h, 71%; (i) DBU (3 equiv.), CH2Cl2, 0 °C, 24 h, 65%; (j) 10% aqueous KOH (8 equiv.), acetone, 60 °C, 22 h, 38%.
2

S. Mosaferi, R.E. Jelley, B. Fedrizzi et al.
Tetrahedron Letters 61 (2020) 152642
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With the synthesis of b-ionone achieved, the same procedure
was subsequently performed for the synthesis of deuterated b-
ionone. Using d6-acetone 1a rather than acetone 1 gave d6-b-
ionone (Scheme 2).
In conclusion, a robust, easily scalable and efficient synthetic
procedure for the syntheses of d6-b-ionone and d6-b-cyclocitral
has been established. These isotopically labelled analogues can
be used as analytical standards for accurate quantitative analyses
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152642.
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