Synthesis of (3S,3′S)- and meso-stereoisomers of alloxanthin and determination of absolute configuration of alloxanthin isolated from aquatic animals

Article abstract of DOI:10.3390/md12052623

In order to determine the absolute configuration of naturally occurring alloxanthin, a HPLC analytical method for three stereoisomers 1a-c was established by using a chiral column. Two authentic samples, (3S,3?S)- and meso-stereoisomers 1b and 1c, were ch

Full text of DOI:10.3390/md12052623

Mar. Drugs 2014, 12, 2623-2632; doi:10.3390/md12052623
OPEN ACCESS
marine drugs
ISSN 1660-3397
www.mdpi.com/journal/marinedrugs
Article
Synthesis of (3S,3′S)- and meso-Stereoisomers of Alloxanthin
and Determination of Absolute Configuration of Alloxanthin
Isolated from Aquatic Animals
Yumiko Yamano 1,*, Takashi Maoka 2 and Akimori Wada 1
1
Kobe Pharmaceutical University, Motoyamakita-machi, Higashinada-ku, Kobe 658-8558, Japan;
E-Mail: a-wada@kobepharma-u.ac.jp
2
Research Institute for Production Development, 15 Shimogamo-morimoto-cho, Sakyo-ku,
Kyoto 606-0805, Japan; E-Mail: maoka@mbox.kyoto-inet.or.jp
* Author to whom correspondence should be addressed; E-Mail: y-yamano@kobepharma-u.ac.jp;
Tel./Fax.: +81-78-441-7562.
Received: 20 March 2014; in revised form: 15 April 2014 / Accepted: 15 April 2014 /
Published: 8 May 2014
Abstract: In order to determine the absolute configuration of naturally occurring
alloxanthin, a HPLC analytical method for three stereoisomers 1a–c was established by
using a chiral column. Two authentic samples, (3S,3′S)- and meso-stereoisomers 1b and 1c,
were chemically synthesized according to the method previously developed for
(3R,3′R)-alloxanthin (1a). Application of this method to various alloxanthin specimens of
aquatic animals demonstrated that those isolated from shellfishes, tunicates, and crucian
carp are identical with (3R,3′R)-stereoisomer 1a, and unexpectedly those from lake shrimp,
catfish, biwa goby, and biwa trout are mixtures of three stereoisomers of 1a–c.
Keywords:
carotenoid;
alloxanthin;
synthesis;
chiral
HPLC
separation;
absolute configuration
1. Introduction
Alloxanthin (1) (Figure 1) was first isolated from Cryptomonas algae [1] and its structure was
determined to be 7,8,7′,8′-tetreradehydro-β,β-carotene-3,3′-diol by MS, IR and 1H-NMR
spectroscopies [2]. Additionally, cynthiaxanthin [3] from the tunicate Cynthia rorezi (Halocynthia
rorezi) and pectenoxanthin [4] from giant scallop Pecten maximus were isolated by Japanese scientists.

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In 1967, Campbel et al. demonstrated that these two carotenoids were identical with alloxanthin [5].
Therefore, cynthiaxanthin and pectenoxanthin were synonyms of alloxanthin. The absolute
configuration of alloxanthin isolated form algae was deduced to be 3R,3′R by X-ray analysis of
degradation product of fucoxanthin and in view of biogenetic grounds [6]. Bartlett et al. reported that
the ORD spectra of alloxanthin specimens from Cryptomonas algae and tunicate showed an identical
shape each other and that both specimens are assumed to have an identical absolute configuration [7].
Figure 1. Structures of stereoisomers of alloxanthin (1a–c) and other related carotenoids.
Since then, alloxanthin was isolated from several aquatic animals, such as shellfishes [8,9],
starfishes [10], tunicates [11,12] and freshwater fishes [13,14], etc. These alloxanthin specimens
showed similar non-conservative CD with weak negative Cotton effects.
Carotenoids such as astaxanthin, zeaxanthin, lutein, and tunaxanthin in animals are known to exist
as a mixture of stereoisomers. Namely, astaxanthin in crustaceans and marine fishes exists as a mixture
of three stereoisomers at C3 and C3′-positions [15,16]. Zeaxanthin [17], lutein [18], and
tunaxanthin [19] in marine fishes also consist of these stereoisomers. Their absolute configurations
were determined by CD spectra and chiral HPLC analyses. Due to its non-conservative CD, absolute
configurations of alloxanthin in several origins could not be determined exactly by CD spectra.
In order to determine the absolute configuration of naturally occurring alloxanthin, we synthesized
stereoisomers of alloxanthin (1a–c) and established a HPLC analytical method using a chiral column.
Applying this method, the absolute configurations of alloxanthin specimens isolated from shellfishes,
tunicates and fishes were investigated. Here, we describe these results.

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2. Results and Discussion
2.1. Synthesis of (3S,3′S)-Alloxanthin (1b) and meso-Alloxanthin (1c)
We previously reported [20] stereoselective total synthesis of (3R,3′R)-alloxanthin (1a) by use of
C15-acetylenic tri-n-butylphosphonium salt 5a (Scheme 1) as a versatile synthon for syntheses of
acetylenic carotenoids. This time, (3S,3′S)-alloxanthin (1b) and its meso-stereoisomer 1c were newly
synthesized using (3S)-phosphonium salt 5b, which was prepared from 3-epi-actinol 6 [21] in the same
procedure [20] as preparation of (3R)-one 5a.
Scheme 1. Synthesis of C15-acetylenic tri-n-butylphosphonium salts 5a and 5b.
Reagents: (a) (i) TMSCl, Et3N, DMAP, (ii) TMSC≡CH, nBuLi, then aq. KOH, (iii) Ac2O, pyridine,
(iv) CuSO4, xylene, reflux (Dean-Stark), (v) LiAlH4; (b) TESCl, Et3N, DMAP, (c) (i) vinyl
bromide 6, Pd(PPh3)4, CuI, BHT, iPr2NH, (ii) MsCl, LiCl, γ-collidine, (iii) PnBu3, Et3N.
Compound 6 was converted into terminal alkyne 3b via the addition of lithium acetylide in 72%
yield over six steps. The high enantiomeric purity of 3b (99% ee) was confirmed by HPLC analysis
[CHIRALPAK AY-H; Daicel, 2-PrOH–n-hexane (5:95)]. Compound 3b was then transformed into the
phoshonium salt 5b via Sonogashira cross-coupling of the triethylsilyl (TES)-protected terminal alkyne
4b with vinylbromide 6 in 59% over four steps.
Wittig condensation of C10-dialdehyde 7 with excess amount of (3S)-phosphonium salt 5b in the
presence of sodium methoxide in dichloromethane at room temperature and subsequent desilylation
stereoselectively provided (3S,3′S)-alloxanthin (1b) (Scheme 2). On the other hand, meso-alloxanthin
(1c) was synthesized via condensation between (3S)-phosphonium salt 5b and (3R)-C25-acetylenic
apocarotenal 8, which was prepared by Wittig reaction of C10-dialdehyde 7 with (3R)-phosphonium
salt 5a in the presence of sodium methoxide in dichloromethane at 0 °C.

Mar. Drugs 2014, 12
Scheme 2. Synthesis of three stereoisomers of alloxanthin (1a–c).
2626
CD spectrum of (3S,3′S)-alloxanthin (1b) showed an antisymetrical curve having week Cotton
effects to that of previously synthesized [20] (3R,3′R)-alloxanthin (1a) as shown in Figure 2.
Figure 2. CD spectra in Et2O–isopentane–EtOH (5:5:2) of synthesized
(3R,3′R)-alloxanthin (1a) and (3S,3′S)-alloxanthin (1b).
2.2. Determination of Absolute Configuration of Alloxanthin Isolated from Aquatic Animals by HPLC
In order to determine the absolute configuration of naturally occurring alloxanthin, a HPLC
analytical method for three stereoisomers 1a–c was investigated. As a result, three synthetic
stereoisomers of alloxanthin can be separated using a chiral column (CHIRALPAK AD-H; Daicel) as
shown in Figure 3.
Next, alloxanthin specimens isolated from scallop Mizuhopecten yessoensis, oyster Crassostrea
gigas, pacific pearl oyster Pinctada margaritifera, freshwater bivalve Unio douglasiae, tunicate
Halocynthia roretzi, and crucian carp Carassius auratus grandoculis were subjected to the HPLC
method to find that these consist of only (3R,3′R)-stereoisomer 1a. On the other hand, alloxanthin
specimens isolated from lake shrimp Palaemon paucidens, catfish Silurus asotus, biwa goby
Gymnogobius isaza, and biwa trout Oncorhynchus masou rhodurus consisted of three stereoisomers
1a–c (Table 1).

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Figure 3. HPLC elution profile of a mixture of three stereoisomers of alloxanthin (1a–c).
2627
Column: CHIRALPAK AD-H 0.46 × 25 cm (Daicel, Tokyo, Japan); eluent: 2-PrOH–n-hexane
(4:96); flow rate: 0.6 mL/min; temperature: 23 °C; detection: 450 nm.
Table 1. Occurrence and percentage composition of alloxanthin stereoisomers in aquatic animals.
3R,3′R
3S,3′S
meso
Species
1a
1b
1c
Shellfish
Scallop
Mizuhopecten yessoensis
Crassostrea gigas
100
100
100
100
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
Oyster
Pacific pearl oyster
Freshwater bivalves
Tunicate
Pinctada margaritifera
Unio douglasiae
Sea squirt
Halocynthia roretzi
Palaemon paucidens
100
n.d.
9.6
n.d.
Crustacean
Lake shrimp
Fish
53.7
36.7
Crucian carp
Biwa goby
Biwa trout
Catfish
Carassius auratus grandoculis
Gymnogobius isaza
100
91.4
n.d.
0.9
n.d.
7.7
Oncorhynchus masou rhodurus
Silurus asotus
>99.9
82.9
trace
1.5
trace
15.6
n.d.: not detected.
Previously, one of the authors reported that zeaxanthin in plants, shellfishes, and tunicates consisted
of only (3R,3′R)-stereoisomer, whereas zeaxanthin in fishes consisted of three stereoisomers [17].
Similar results were obtained in the case of alloxanthin in aquatic animals. Alloxanthin is de novo
synthesized in Chryptophyceae and Euglenophyceae micro algae [22]. However, origin of alloxanthin
in aquatic animals was remained uncertain. Patrali et al. (1989) [22] and Liaaen-Jensen (1998) [23]
reported that alloxanthin in Mytilus edulis might be a terminal metabolite of fucoxanthin through
intermediates, halocynthiaxanthin and isomytiloxanthin, based on observation in feeding experiment.
However, conversion of isomytiloxanthin into alloxanthin is too complex and there were no direct

Mar. Drugs 2014, 12
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evidences for the conversion, especially in aquatic animals. In our experience, isomytiloxanthin has
not been isolated from these animals [24].
Shellfishes (bivalves) and tunicates are filter-feeders, which accumulate carotenoids from micro
algae. Therefore, alloxanthin in these animals is assumed to originate from Chryptophyceae and
Euglenophyceae micro algae, etc. Thus, these alloxanthin specimes consist of only (3R,3′R)-stereoisomer.
Crucian carp is omnivorous and feeds not only animal planktons belonging to Cladocera but also micro
algae. Therefore, alloxanthin in crucian carp is also assumed to originate from micro algae. On the
other hand, alloxanthin in lake shrimp, catfish, biwa goby, and biwa trout exist as a mixture of three
stereoisomres. These crustacean and fishes are carnivorous. Especially, lake shrimp contains a large
amount of (3S,3′S)- and meso-alloxanthin (Table 1). Lake shrimp is a one of the major food of catfish
and biwa trout. Therefore, (3S,3′S)- and meso-alloxanthin in these fishes might be originated from lake
shrimp. However, origin of (3S,3′S)- and meso-alloxanthin in lake shrimp is uncertain.
Catfish is a top predator in Japanese freshwater ecosystems. Catfish ingests astaxanthin from
crustaceans whose astaxanthin exists as a mixture of three stereoisomers. Catfish can convert
astaxanthin into zeaxanthin [24]. Therefore, zeaxanthin in catfish exists as a mixture of three
stereoisomers. Although the origin of stereoisomers of alloxanthin in catfish is uncertain, it might be
naturally formed by epimerization of 7,8,7′,8′-tetradehydroastaxanthin originated from crustacean at
C3 and C3′-positions and subsequent reduction at C4 and C4′-positions. Further studies are need to
reveal the origin of (3S,3′S)- and meso-alloxanthin in crustaceans and fishes.
This is the first report of the occurrence of (3S,3′S) and meso-alloxanthin in nature.
3. Experimental Section
3.1. General
IR spectrum was measured on a Perkin-Elmer FT-IR spectrometer (Perkin-Elmer, Yokohama,
1
Japan), spectrum 100. H and 13C NMR spectra were determined on a Varian Gemini-300
superconducting FT-NMR spectrometer (Agilent Technologies, Santa Clara, CA, USA) and the
chemical shifts were referenced to tetramethylsilane. Mass spectrum was taken on a Thermo Fisher
Scientific Exactive spectrometer (Thermo Fisher Scientific, Bremen, Germany). CD spectra were
measured on a Shimadzu-AVIN 62A DS circular dichroism spectrometer (Shimadzu, Kyoto, Japan).
HPLC analyses were performed on Simadzu-LC-20AT instrument (Shimadzu, Kyoto, Japan) with a
photodiode array detector (Waters 996, Tokyo, Japan) and column oven (GL Sciences Model 552,
Tokyo, Japan).
NMR assignments are given using the carotenoid nmbering system.
3.2. Synthesis of (3S,3′S)-Alloxanthin (1b) and meso-Alloxanthin (1c)
In the same procedure [20] as preparation of (3R)-phosphonium salt 5a and (3R,3′R)-alloxanthin
(1a), (3S)-5b and (3S,3′S)-alloxanthin (1b) were prepared. Spectral data except for optical data of
compounds 1b, 3b, 4b and 5b were identical with the corresponding previous reported [20]
enantiomers 1a, 3a, 4a and 5a.
(3S,3′S)-Alloxanthin (1b): HRMS (ESI) m/z calcd for C40H53O2 [M + H]+ 565.4040, found 565.4038.

Mar. Drugs 2014, 12
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26
Compound 3b: [α]D 102.9 (c 1.03, MeOH); HRMS (ESI) m/z calcd for C11H17O [M + H]+
165.1274, found 165.1277.
23
Compound 4b: [α]D 68.1 (c 1.00, MeOH); HRMS (ESI) m/z calcd for C17H31OSi [M + H]+
279.2139, found 279.2139.
Compound 5b: HRMS (ESI) m/z calcd for C33H62OPSi [M − Cl]+ 533.4302, found 533.4293.
meso-Alloxanthin (1c) was synthesized via condensation between 5b and (3R)-C25-acetylenic
apocarotenal 8, which was prepared by Wittig reaction of C10-dialdehyde 7 with 5a as follows.
(2E,4E,6E,8E,10E)-2,7,11-trimethyl-13-[(R)-2,6,6-trimethyl-4-triethylsilyloxycyclohex-1-en-1-yl]
trideca-2,4,6,8,10-pentaen-12-ynal (8). NaOMe (1 M in MeOH; 1.2 mL, 1.2 mmol) was added to a
solution of the (3R)-phosphonium salt 5a (409 mg, 0.73 mmol) and C10-dialdehyde 7 (100 mg,
0.61 mmol) in CH2Cl2 (10 mL) at 0 °C. After being stirred at 0 °C for 15 min, the mixture was poured
into saturated aq. NH4Cl and extracted with AcOEt. The extracts were washed with brine, dried over
Na2SO4 and evaporated to afford a residue, which was purified by flash column chromatography
(AcOEt–n-hexane, 1:4) to give the (3R)-C25-acetylenic apocarotenal 8 (165 mg, 57%) as an orange
viscous oil: UV-VIS λmax (EtOH)/nm 420; IR νmax (CHCl3)/cm−1 2170 (C≡C), 1663 (conj. CHO), 1610
and 1599 (split) (C=C), 1552 (C=C); 1H-NMR (CDCl3, 300 MHz) δ 0.61 (6H, q, J = 8 Hz, SiCH2 × 3),
0.97 (9H, t, J = 8 Hz, CH2CH3 × 3), 1.14 and 1.18 (each 3H, s, 1-gem-Me), 1.49 (1H, t, J = 12 Hz,
2-Hβ), 1.74 (1H, ddd, J = 12, 3.5, 2 Hz, 2-Hα), 1.89 (3H), 1.91 (3H) and 2.03 (6H) (each s, 5-Me, 9-Me,
13-Me and 13′-Me), 2.11 (1H, br dd, J = 17.5, 9.5 Hz, 4-Hβ), 2.30 (1H, br dd, J = 17.5, 5.5 Hz, 4-Hα),
3.94 (1H, m, 3-H), 6.32 (1H, br d, J = 12 Hz, 14-H), 6.37 (1H, d, J = 15 Hz, 12-H), 6.46 (1H, br d,
J = 11.5 Hz, 10-H), 6.66 (1H, dd, J = 15, 11.5 Hz, 11-H), 6.70 (1H, dd, J = 14.5, 11.5 Hz, 15′-H), 6.96
13
(1H, br d, J = 11.5 Hz, 14′-H), 7.03 (1H, dd, J = 14.5, 12 Hz, 15-H), 9.46 (1H, s, CHO); C-NMR
(CDCl3, 75 MHz) δ 4.82 (C × 3), 6.83 (C × 3), 9.59, 12.96, 18.17, 22.53, 28.61, 30.45, 36.53, 42.11,
47.04, 65.01, 90.10, 98.16, 121.15, 123.84, 126.60, 127.73, 131.75, 134.51, 137.02, 137.07, 137.47,
138.70, 141.26, 148.75, 194.45; HRMS (ESI) m/z calcd for C31H47O2Si (MH)+ 479.3340, found 479.3347.
Preparation of meso-alloxanthin (1c). NaOMe (1 M in MeOH; 0.24 mL, 0.24 mmol) was added to a
solution of the (3S)-phosphonium salt 5b (113 mg, 0.20 mmol) and (3R)-C25-acetylenic apocarotenal 8
(59 mg, 0.12 mmol) in CH2Cl2 (10 mL) at room temperature. After being stirred for further 15 min, the
mixture was poured into saturated aq. NH4Cl and extracted with AcOEt. The extracts were washed
with brine, dried over Na2SO4 and evaporated to afford a residue, which was purified by flash column
chromatography (AcOEt–n-hexane, 1:4) to give the TES-protected condensed product. Subsequently,
to a solution of this condensed product in dry THF (5 mL) were added AcOH (1 M in THF; 0.20 mL,
0.20 mmol) and then tetrabutylammonium fluoride (TBAF) (1 M in THF, 0.40 mL, 0.40 mmol). After
being stirred at room temperature for 2 h, the mixture was concentrated to give a residue, which was
purified by flash column chromatography (AcOEt–n-hexane–MeOH, 50:45:5) to provide meso-alloxanthin
(1c) (70 mg, quant.) as red solids. Its spectral data were identical with those of (3R,3′R)-alloxanthin
(1a) [20]. HRMS (ESI) m/z calcd for C40H53O2 [M + H]+ 565.4040, found 565.4033.

Mar. Drugs 2014, 12
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3.3. Configurational Analysis of Natural Alloxanthin
3.3.1. Animal Materials
Scallop Mizuhopecten yessoensis was provided from Hokkaido Research Organization, Abashiri
Fisheries Research Institute, Hokkaido, Japan. Oyster Crassostrea gigas, and sea squirt Halocynthia
roretzi were purchased from fisheries market at Kyoto city. Pacific pearl oyster Pinctada margaritifera
was provided from a pearl aquaculture industry, Ishigaki city, Okinawa Prefecture. Freshwater bivalve
Unio douglasiae, crucian carp Carassius auratus grandoculis, and catfish Silurus asotus were
purchased from Katata fisheries cooperative, Shiga Prefecture. Biwa trout Oncorhynchus masou
rhodurus was purchased from Nango Fisheries Center, Shiga Prefecture. Biwa goby Gymnogobius
isaza and lake shrimp Palaemon paucidens were purchased from fisheries market at Maibara city.
3.3.2. Isolation of Alloxanthin from Aquatic Animals
According to our routine methods, carotenoid was extracted with acetone from animal tissue. The
extract was partitioned between Et2O–n-hexane (1:1) and water in separating funnel. The organic
phase was evaporated and saponified with 5% KOH/MeOH at room temperature for 2 h. Then,
unsaponifiable compounds were extracted with Et2O–n-hexane (1:1, v/v) from the reaction mixture by
addition of water. The organic layer was dried over Na2SO4 and evaporated. The residue was subjected
to silica gel column chromatography increasing percentage of Et2O in n-hexane. The fraction eluted
with Et2O was subjected to HPLC on silica gel with acetone–n-hexane (3:7) to afford alloxanthin.
1
Purity of alloxanthin was checked by UV-Vis, H-NMR, and MS spectral data. Then alloxanthin
obtained from aquatic animals was subject to configurational analysis using a chiral column
described above.
4. Conclusions
In conclusion, we synthesized stereoisomers of alloxanthin (1a–c) and established a HPLC
analytical method using a chiral column to identify them for naturally occurring alloxanthin.
Application of this method to various alloxanthin specimens of aquatic animals demonstrated that
those isolated from shellfishes, tunicates, and crucian carp are identical with (3R,3′R)-stereoisomer 1a,
and unexpectedly those from lake shrimp, catfish, biwa goby, and biwa trout are mixtures of three
stereoisomers of 1a–c. This is the first report of the occurrence of (3S,3′S) and meso-alloxanthin in
nature. The analytical method can be a powerful tool to identify stereoisomers of alloxanthin in nature
in a straightforward manner.
Acknowledgments
We thank M. Kurimoto and M Shoji for technical assistance.

Mar. Drugs 2014, 12
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Author Contributions
Basic idea of the research was proposed by three authors collaboratively. The synthetic and
analytical experiments were designed and performed by Y. Yamano. The isolation of natural products
was designed and carried out by T. Maoka.
Conflicts of Interest
The authors declare no conflict of interest.
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