First synthesis of 3-S-glutathionylhexanal-d8 and its bisulfite adduct

Article abstract of DOI:10.1016/j.tetlet.2020.152100

3-Sulfanylhexan-1-ol (3SH) is an impact odorant of white wines, imparting tropical fruit aromas. A reliable synthetic pathway to 3-S-glutathionylhexanal (glut-3SH-al), a precursor to 3SH that has not been intensively studied, was developed starting from 1-butanol. Application of this synthesis to 1-butanol-d₁₀, conserved eight deuteriums, producing glut-3SH-al-d₈, which can be used as an internal standard for future work on the occurrence and evolution of glut-3SH-al in wine systems. Additionally, both glut-3SH-SO₃ and glut-3SH-SO₃-d₈ were synthesised from the corresponding aldehyde, enabling further study of the role of these bisulfite adducts in 3SH biogenesis.

Full text of DOI:10.1016/j.tetlet.2020.152100

Tetrahedron Letters 61 (2020) 152100
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
First synthesis of 3-S-glutathionylhexanal-d and its bisulfite adduct
8
a
a
a,b,
⇑
Jennifer R. Muhl , Lisa I. Pilkington , Rebecca C. Deed
a
School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
b
a r t i c l e i n f o
a b s t r a c t
Article history:
3-Sulfanylhexan-1-ol (3SH) is an impact odorant of white wines, imparting tropical fruit aromas. A reli-
able synthetic pathway to 3-S-glutathionylhexanal (glut-3SH-al), a precursor to 3SH that has not been
intensively studied, was developed starting from 1-butanol. Application of this synthesis to 1-butanol-
Received 19 April 2020
Revised 26 May 2020
Accepted 29 May 2020
Available online 2 June 2020
d
10, conserved eight deuteriums, producing glut-3SH-al-d
for future work on the occurrence and evolution of glut-3SH-al in wine systems. Additionally, both
glut-3SH-SO and glut-3SH-SO -d were synthesised from the corresponding aldehyde, enabling further
study of the role of these bisulfite adducts in 3SH biogenesis.
8
, which can be used as an internal standard
3
3
8
Keywords:
3
-S-Glutathionylhexanal
Ó 2020 Elsevier Ltd. All rights reserved.
Isotopic labelling
Thiol precursors
Total synthesis
Wine
Key impact odorants of white wines, particularly those made
from Vitis vinifera L. cv. Sauvignon blanc grapes, are three volatile
thiol compounds 4-methylsulfanylpentan-2-one (1, 4MSP), 3-sul-
fanylhexan-1-ol (2, 3SH), and 3-sulfanylhexyl acetate (3, 3SHA)
tial for further elucidation of 3SH 2 in wine than what is currently
established [9].
One group of precursors to 3SH 2 which have been under-exam-
ined are the glutathione conjugate of E-2-hexenal 6, the aldehyde
derivative of glut-3SH 4, 3-S-glutathionylhexanal (7, glut-3SH-al),
(
see Fig. 1) [1–4].
These compounds contribute significantly to the aromatic pro-
3
and its bisulfite adduct (8, glut-3SH-SO ), highlighted in Scheme 1
file of white wines and manipulation of the concentration of these
compounds results in measurable changes to wine aroma [5].
Of particular interest are 3SH 2 and 3SHA 3 which show a 95%
correlation between their concentration and the associated sen-
sory descriptors of grapefruit, and sweet, sweaty passionfruit,
which are desirable characters in wines [2]. In wine systems, 3SH
and 3SHA exist as a scalemic mixture of the R and S enantiomers,
with each enantiomer having slightly different perception thresh-
olds and aromas [6]. Volatile thiol aroma compounds such as
[8–11].
Glut-3SH-al 7 has been previously identified in both juice and
finished wine [8,10,11]. Clark and Deed [8] found that glut-3SH-
al 7 was produced in appreciable levels by the chemical addition
of glutathione to E-2-hexenal 6 in grape must. Although the syn-
3
thesis of glut-3SH-al 7 and glut-3SH-SO 8 have been previously
reported, these synthetic routes began from commercially-avail-
able E-2-hexenal 6, which is not readily available in deuterated
form [10,11]. The lack of commercially-available deuterated E-2-
hexenal 6 precludes the synthesis of highly-deuterated analogues
of glut-3SH-al 7, which are important for further elucidation of
the role of glut-3SH-al 7 in the formation of 3SH 2. While glut-
3
SH 2 are produced during fermentation from precursors present
in the grape must, including glutathione and/or cysteine conju-
gates of C-6 compounds such as 3-S-glutathionylhexanol (4, glut-
3
SH) and 3-S-cysteinylhexanol (5, cys-3SH) [7]. As with the volatile
3SH-al-d
E-2-hexenal-d
8
has been reportedly identified in grape must spiked with
6-d8, the synthesis and full characterisation of this
thiols, these precursors exist in a mixture of diastereomers with
variable stereochemistry at the C-3 position of the C-6 moiety
8
compound has not yet been reported [12]. This work reports the
[
[
6]. Formation of these precursors, and their conversion to 3SH 2
6] occurs through a combination of enzymatic and chemical reac-
development of a synthetic pathway to glut-3SH-al 7 and its
highly-deuterated analogue, glut-3SH-al-d
bisulfite adducts. We are the first to report the synthesis of glut-
3SH-al-d 7-d and glut-3SH-SO -d 8-d
8 8
7-d , along with their
tions, leading to a complex pathway (see Scheme 1) [8]. The cur-
rent understanding of this pathway explains less than 50% of the
final 3SH 2 concentrations in wine, suggesting that there is poten-
8
8
3
8
8
.
Synthesis of glut-3SH-al 7 began with the generation of butanal
and, subsequently, E-2-ethyl hexenoate 11a from commercially-
9
available 1-butanol 10 (see Scheme 2) as reported by Jelley and co-
workers [13]. Butanal 9 was obtained through a Swern oxidation
⇑
Corresponding author.
E-mail address: rebecca.deed@auckland.ac.nz (R.C. Deed).
https://doi.org/10.1016/j.tetlet.2020.152100
040-4039/Ó 2020 Elsevier Ltd. All rights reserved.
0

2
J.R. Muhl et al. / Tetrahedron Letters 61 (2020) 152100
8 8
HRMS data for glut-3SH-al-d 7-d confirmed the inclusion of eight
1
13
deuterium atoms, while H and C NMR spectra were in agree-
ment with the spectra for glut-3SH-al 7, apart from missing peaks
1
2
corresponding to replacement of H with H on C-6, C-5, C-4, and
C-3.
Additionally, the fragmentation pattern observed in the HRMS
spectrum for glut-3SH-al-d
reported by Capone and Jeffery [12], confirming their tentative
identification of glut-3SH-al-d 7-d in juice from grapes spiked
with E-2-hexenal-d 6-d
8 8
7-d was in agreement with that
Fig. 1. The structures of key thiol aroma compounds.
8
8
8
8
.
The synthesis of the bisulfite adduct of glut-3SH-al 7, glut-3SH-
with oxalyl chloride at À78 °C and was subsequently reacted with-
SO 8 has been previously reported in the literature, along with its
identification in samples of grape juice (and wine) [8,10,11]. How-
3
out further purification due to its volatility. A selective Wittig ole-
fination between butanal
9
and (carbethoxymethylene)
ever, the isotopically labelled version, glut-3SH-SO -d8 8-d8 has
3
triphenylphosphorane was carried out and purification by flash
never been reported, and will be of great use in analytical studies
chromatography yielded both E-ethyl 2-hexenoate 11a and Z-ethyl
investigating the interconversion of glut-3SH-SO3 8 and glut-
3SH-al 7 in grape juice and wine. Both glut-3SH-SO3 8 and glut-
3SH-SO -d 8-d8 were synthesised for NMR analysis by addition
2
-hexenoate 11b, in separate fractions, in an isomeric ratio of 95:5.
Reduction of E-ethyl 2-hexenoate 11a to E-2-hexen-1-ol 12 was
achieved using diisobutylaluminium hydride (DIBAL) at À78 °C
and reaction side products were removed by washing with a satu-
rated solution of Rochelle’s salt. Purification was carried out using
flash chromatography and the resulting alcohol was obtained in
3
8
of sodium metabisulfite to a sample of purified glut-3SH-al 7 in
D O 30 min prior to analysis – the NMR of the synthesised product
2
glut-3SH-SO3 8 was in accordance with previously reported data
[10], while the NMR of the deuterated analogue glut-3SH-SO -d8
3
7
9% yield, before being oxidised to the corresponding aldehyde
8-d8 was in agreement with its non-deuterated counterpart 8.
using pyridinium chlorochromate. E-2-hexenal 6 was obtained as
a pale-yellow solution, and the crude product was used directly
in the next reaction due to the volatility of the product. Addition
of the glutathione moiety was carried out as described by Thibon
and co-workers [10] and the resulting pale orange powder was col-
lected. Portions of the crude product were purified on prepacked
Supelclean ENVI -18 SPE tubes with an ethanol/water solvent
system. Fractions containing 7 were freeze-dried and the resulting
white powder was characterised as 7 (23%). Both NMR and HRMS
data of glut-3SH-al 7 was in agreement with previous literature
For further analysis, the bisulfite adducts were synthesised by mix-
ing an aqueous solution of Na S O (1 equivalent) with an aqueous
solution of the aldehyde. This enabled MS analysis of the product,
which confirmed the synthesis of 8. HRMS of 8 was in agreement
with the data reported by Thibon and co-workers [10], as was 8-
d8, though with a mass increase of 8.
2
2 5
TM
TM
In summary, we report the synthesis of glut-3SH-al 7 using a
novel pathway, beginning from 1-butanol 9, as well as the first
total synthesis of glut-3SH-al-d 7-d , a critical compound for the
8
8
analysis of 3SH 2 formation in wines. The new synthetic approach
to generate glut-3SH-al 7 from 1-butanol 9 had not been previ-
ously reported and permitted the synthesis of the heavily deuter-
[
10].
The deuterated analogues of these compounds were obtained
through the same reaction conditions, beginning from 1-butanol-
10 9-d10, with comparable yields for every step (see Scheme 3).
ated glut-3SH-al-d8 7-d8 from 1-butanol-d10 9-d10
.
The
d
incorporation of eight deuterium atoms in glut-3SH-al-d8 7-d8
Scheme 1. Current understanding of 3SH biogenesis pathways in wine. Compounds synthesised in this work are highlighted in red.

J.R. Muhl et al. / Tetrahedron Letters 61 (2020) 152100
3
Scheme 2. Synthesis of compound 7 and 8, starting with butanol 10.
8 8
Scheme 3. Synthesis of compounds 7-d and 8-d starting from commercially available butanol-d10 10-d10.
reported here has the potential to be extremely useful for mecha-
nistic studies on glut-3SH-al 7 breakdown in wine, as well as the
analysis of glut-3SH-al 7 concentrations, since the loss of a single
deuterium atom will not result in the compound being indistin-
guishable from the natural glut-3SH-al 7. Additionally, the deu-
terium atoms are incorporated in the hexenal moiety, the section
of interest in the breakdown of glut-3SH-al 7 during fermentation.
8 8
Use of glut-3SH-al-d 7-d for metabolomic studies would result in
all breakdown products through to 3SH containing the eight deu-
terium atoms and therefore able to be distinguished from the nat-
ural products. Synthesis and isolation of glut-3SH-al-d
8 8
7-d
enables the use of this compound as an internal standard for fur-
ther studies of the formation of 3SH 2 via metabolism of the
glut-3SH-al-d 7-d . Additionally, this enabled the synthesis of
8 8

4
J.R. Muhl et al. / Tetrahedron Letters 61 (2020) 152100
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Funding information
The authors acknowledge a University of Auckland Postgradu-
ate Honours Scholarship and a University of Auckland Doctoral
Scholarship that helped fund this research (J. R. M.). Additionally,
the authors received internal funding from the School of Chemical
Sciences (University of Auckland) for this work.
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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Acknowledgment
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The authors would like to thank Dr Andrew Clark for his guid-
ance and the University of Auckland for funding support.
[10] C. Thibon, C. Böcker, S. Shinkaruk, V. Moine, P. Darriet, D. Dubourdieu,
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Appendix A. Supplementary data
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