Synthesis of analogues of citranaxanthin and their activity in free radical scavenging

Article abstract of DOI:10.3184/174751916X14580338203147

Citranaxanthin and its analogues were synthesised via a C₅ unit elongation to substituted conjugated polyenes. Their free radical scavenging activity was measured by 1,1-diphenyl-2-picrylhydrazinyl spectrophotometric methods. Results indicated that the new compounds exhibited antioxidant activities. Three new analogues had stronger antioxidant activity than citranaxanthin.

Full text of DOI:10.3184/174751916X14580338203147

JOURNAL OF CHEMICAL RESEARCH 2016
VOL. 40 MAY, 257–260
RESEARCH PAPER 257
Synthesis of analogues of citranaxanthin and their activity in free
radical scavenging
Shaofeng Zhang, Yuan Liu and Juan Luo*
College of Chemistry, Sichuan University, Chengdu 610064, P.R. China
Citranaxanthin and its analogues were synthesised via a C5 unit elongation to substituted conjugated polyenes. Their free radical
scavenging activity was measured by 1,1-diphenyl-2-picrylhydrazinyl spectrophotometric methods. Results indicated that the new
compounds exhibited antioxidant activities. Three new analogues had stronger antioxidant activity than citranaxanthin.
Keywords: citranaxanthin, antioxidant activity, β-apo-8’-carotenal
Citranaxanthin (CTX) (1) (Fig. 1) is the first known naturally
occurring carotenoid with a methyl ketone grouping in the side
chain.1 It has free-radical scavenging ability2–4 and can be used
in pharmaceuticals, pigments, feed additives and aquaculture.5–6
Related carotenoids play an important role in the prevention
of certain human diseases such as cardiovascular disease and
cancer.7–9 Higher nutritional uptake of CTX may increase macular
pigment density and thereby lower the risk of age-related macular
degeneration (AMD). AMD is the main cause of loss of vision in
western countries.10–11
In 1996, some CTX analogues were synthesised and used in
egg colouring. They have a low deposition efficiency in yolks.12 In
the present work, a series of new CTX analogues were designed
and synthesised by condensation of methyl ketones with β-apo-
8’-carotenal (12) or β-apo-12’-carotenal (11). The structures of
the new compounds were characterised by NMR, HRMS, IR
and UV-Vis. The free radical scavenging activity of the samples
was measured using a WD-2102A automatic enzyme instrument
with 1,1-diphenyl-2-picrylhydrazinyl (DPPH) as the organic free
radical.
Results and discussion
Synthesis of β-apo-8’-carotenal (12) and β-apo-12’-carotenal (11)
Isoprene was used as a starting material (Scheme 1).
O
Fig. 1 Structure of citranaxanthin (CTX) (1).
1 equiv. hexamet-
hylenetetramine
EtOH, H2O(1:1)
40 ℃, 6h
0.65 equiv. NBS
Br
AcOH, rt, 24h
O
OAc
OAc
2 equiv. AcOH,
60 ℃, 8h
4(50%)
2
3(60%)
1.1 equiv.
Neopentyl glycol
0.01 mmol%p-TSA
reflux
toluene, 3h
SOCl2, Na2SO4
Et2O, DMF
O
KOH, CH3OH
60℃,2h
O
OAc
O
OH
Cl
O
O
O
0 ℃, 6h
5(90%)
6(80%)
8(95%)
10 equiv. MnO2
CH2Cl2, rt, 2h
P(OEt)3
reflux,10h
O
O
O
OEt
OEt
O
P
PPh3 HSO4
O
+
O
10
7(83%)
9
1.5 equiv. CH3ONa
MeOH, rt
t-BuOK,THF
0 ℃, 0.5h
O
1M HCl
11(62%)
O
12(70%)
Scheme 1 Synthesis of β-apo-12’-carotenal (11) and β-apo-8’-carotenal (12).
* Correspondent. E-mail: loyis-luo@163.com

258 JOURNAL OF CHEMICAL RESEARCH 2016
Compound 3 was synthesised from the reaction of 2 with
N-bromosuccinimide (NBS) in 60% yield.13 Ethanol was used
as the solvent in the synthesis of 4 from 3, to assist isolation.14
Compounds 5, 6 and 8 were successfully synthesised in 90%,
80% and 95% yields respectively.15 The above products were
purified by distillation under reduced pressure. Compound 6
was then oxidised with manganese dioxide to form the aldehyde
7 in 83% yield.16
sample and 200 μL ethanol was used for zero setting.23 Distilled
water (100 μL) added to DPPH (200 μL, 0.1 mM) without
addition of the test sample was used as a blank control reaction.
A lower absorbance of the reaction mixture indicated greater
free radical scavenging activity. The scavenging activity on
DPPH radical (SADPPH) is defined as:
SADPPH = (1– As /Ac) × 100%
The Wittig salt 10 was prepared by the equimolar addition of
triphenylphosphonium sulfate to a stirred solution of vitamin
A acetate (provided by Guangzhou Juyuan Bio Chem Co. Ltd)
in dry methanol.17 β-Apo-12’-carotenal (11) was prepared from
7 with the Wittig salt 10 in 62% yield. Only 11 was obtained.18
Compound 8 was refluxed with triethyl phosphite for 10 h to
give 9. The phosphate reacted with potassium tert-butoxide and
β-apo-12’-carotenal (11) to give β-apo-8’-carotenal (12) in 70%
yield.
where Ac is the absorbance of the blank control reaction and As
is the absorbance in the presence of samples.
All synthetic new compounds and the known carotenoids
showed DPPH radical scavenging effects (Fig. 2). The DPPH
radical scavenging effects of the carotenoids decreased in the
order of 14a > 14b > β-carotene > 12 > 14c > CTX > 13c >
13b > 13a and were 54.20%, 53.17%, 41.30%, 39.02%, 36.96%,
36.88%, 25.65%, 21.98% and 21.75% in 100 μL of solution.
Compounds 14a and 14b had stronger antioxidant activity than
citranaxanthin, β-carotene and compound 12.
Synthesis of the analogues of CTX
The target compounds 13a–c and 14a–c were synthesised by
reaction of β-apo-8’-carotenal (12) or β-apo-12’-carotenal (11)
with methyl ketones and KOH at room temperature in high
yields. The dark red crystals of the products were purified by
silica column chromatography (Scheme 2).19
Conclusion
A series of new analogues of citranaxanthin was designed and
synthesised by condensation of methyl ketones with 11 or 12 and
their structures were confirmed. The free radical scavenging
activity test showed that among 6 target compounds, 14a and
14b had stronger antioxidant activity than β-carotene and 12
and 14c had stronger antioxidant activity than CTX. Among
All the new compounds were characterised by NMR, HRMS,
1
IR and UV-Vis. H NMR showed the corresponding olefins
with a chemical shift of 6 to 8 ppm. 13C NMR showed the
corresponding number of carbons and the peaks of carbonyl
group carbon atoms (≈190 ppm) were found. The separation of
cis–trans isomers was not possible by column chromatography
because of similar retention times between them in different
solvents.20 The target compounds were unstable at high
temperature.21
Determination of DPPH radical scavenging activity
The free radical scavenging activity of samples on 1,1-diphenyl-
2-picrylhydrazinyl (DPPH) was obtained using the method of
Sharma and Bhat (2009) with slight modifications.22 Briefly,
a DPPH working solution (200 μM) was freshly prepared
in methanol (spectroscopic grade) by stirring for at least
15–20 min and was used within 3 h of preparation. A solution
of DPPH (200 μL, 0.1 mM) was added to 100 μL of a 10 mg
L–1 sample. The mixture was shaken and left to stand at room
temperature in the dark. After 30 min, the absorbance of the
mixture was measured at 517 nm. A solution of 100 μL test
Fig. 2 The DPPH radical scavenging activity of the target compounds.
O
2equiv.KOH
O
+
R
CH3OH, Et2O 1:1
11 n=0
12 n=1
n
O
R
n
O
R1=
R2=
R3=
O
13c n=0
14c n=1
13a n=0
14a n=1
13b n=0
14b n=1
Scheme 2 Synthesis of the analogues of CTX.

JOURNAL OF CHEMICAL RESEARCH 2016 259
129.27, 127.36, 126.35, 122.78, 39.76, 34.42, 33.27, 29.12, 21.93, 19.39,
13.09, 12.96, 12.86, 9.81; UV (ethanol) max: 464 nm; HRMS calcd for
C30H40ONa: [M + Na]+: 439.2977; found: 439.2973.
the synthesised target compounds, 14a had the best antioxidant
activity. The target compounds with long chains have a better
ability to eliminate free radicals.
Synthesis of the target compounds 13a–c, 14a–c and citranaxanthin
(1); general procedure
Experimental
Reagents and starting materials obtained from commercial suppliers
were used without further purification unless otherwise stated. The
IR spectra were recorded on a PerkinElemer 16PC-FT spectrometer.
NMR spectra were recorded on a Varian Unity Inova-400 spectrometer
(Varian Inc., Palo Alto, CA, USA) with CDCl3 as the solvent and
tetramethylsilane (TMS) as the internal standard. Mass spectra (MS)
were obtained with a LCMS-IT-TOF spectrometer (Shimadzu, Japan)
using electrospray ionisation (ESI). UV-Vis spectra were obtained on
a UV3600 (Shimadzu, Japan) instrument. Flash chromatography was
performed on > 300 mesh silica gel. Melting points were determined
using a XRC-1 melting point apparatus (Sichuan University Instrument
Inc., Chengdu, China) and were uncorrected.
β-Apo-8’-carotenal (12) (1 mmol) or β-apo-12’-carotenal (11)
(1 mmol) was added to a mixture of the methyl ketone (2 mmol) and
KOH (2 mmol) in 20 mL of diethyl ether/methanol (1:1) at 0 °C.
Under ultrasound, the mixture was stirred for 3 min. Then the mixture
was stirred for 18 h in the dark at room temperature in a nitrogen
atmosphere. Acetic acid (5 mL) was added to the solution obtained
above and the mixture was stirred at room temperature for 0.5 h. The
resulting mixture was diluted with water (30 mL) and extracted with
Et2O (3 × 25 mL). The organic layer was washed three times with
saturated sodium chloride and dried with Na2SO4 and concentrated in
vacuum. The residue was purified by silica column chromatography
(5% ethyl acetate in petroleum ether) to give the target compounds in
75–85% yield.
Preparation of intermediates of 3–10
5,9,14,18-Tetramethyl-20-(2,6,6-trimethylcyclohex-1-enyl)icosa-
3,5,7,9,11,13,15,17,19-nonaen-2-one (1): Yield 83%; m.p.: 110–118 °C;
IR (KBr): 2918, 1657 (C=O), 1588 (C=C), 1513, 1442, 1360, 1255,
The intermediates 3–9 were prepared from isoprene according to the
literature. Compound 10 was also prepared according to the literature
(see Results and discussion section).
1
1187 cm–1; H NMR (400 MHz, CDCl3): δ 7.23 (d, J = 15.8 Hz, 1H),
2,7,11-Trimethyl-13-(2,6,6-trimethylcyclohex-1-enyl)trideca-
2,4,6,8,10,12-hexaenal (11)
6.75–6.49 (m, 6H), 6.34 (dd, J = 14.0, 9.8 Hz, 2H), 6.19 (m, 5H), 2.32
(s, 3H, CH3), 2.02 (m, 2H, CH2), 1.98 (s, 6H, 2 × CH3), 1.95 (s, 3H,
CH3), 1.73 (s, 3H, CH3), 1.51–1.42 (m, 2H, CH2), 1.23–1.31 (m, 2H,
CH2), 1.05 (s, 6H，2 × CH3); 13C NMR (101 MHz, CDCl3): δ 198.61,
148.08, 142.44, 140.69, 138.04, 137.95, 137.83, 137.14, 136.72, 135.89,
135.61, 133.57, 132.26, 131.98, 130.87, 129.65, 127.16, 125.91, 125.60,
124.05, 39.81, 34.43, 33.28, 29.13, 27.61, 21.92, 19.41, 13.04, 12.94,
12.88, 12.82; UV (ethanol) max: 472 nm; HRMS calcd for C33H44ONa:
[M + Na]+: 479.3290; found: 479.3290.
Sodium methoxide (0.81 g, 15 mmol) was added to a solution of 10
(6.28 g, 10 mmol) in dry methanol (30 mL) at 0 °C and the mixture was
stirred about for 0.5 h. A solution of 7 (0.92 g, 5 mmol) in dry methanol
(5 mL) was added and the mixture was stirred for 6 h. A solution of 1
M HCl (5 mL) was added to the mixture obtained above and this was
stirred at room temperature for 3 h. The reaction mixture was diluted
with water (25 mL) and extracted with Et2O (3 × 25 mL). The organic
layer was dried over Na2SO4 and the solvent was removed in vacuum
to give the C25-aldehyde 11 (1.08 g, 62%). This was purified through
silica column chromatography (5% ethyl acetate in petroleum ether)
to give: m.p. 86–89 °C; IR (KBr): 2927, 1662 (C=O), 1607 (C=C),
6,11,15-Trimethyl-17- (2,6,6-trimethylcyclohex-1-enyl) -1-
phenylheptadeca-1,4,6,8,10,12,14,16-octaen-3-one (13a): Yield 80%;
m.p. 36–48 °C; IR (KBr): 2925, 1666 (C=O), 1609 (C=C), 1449, 1360,
1
1257, 1182, 1102 cm–1; H NMR (400 MHz, CDCl3): δ 7.71–7.65 (m,
1
1545, 1442, 1382, 1184 cm–1; H NMR (400 MHz, CDCl3): δ 9.45 (d,
1H), 7.59 (dt, J = 5.2, 3.2 Hz, 2H), 7.47 (d, J = 15.4 Hz, 1H), 7.42–7.36
(m, 3H), 7.03 (d, J = 15.9 Hz, 1H), 6.87–6.79 (m, 1H), 6.74 (dd, J = 14.9,
11.5 Hz, 1H), 6.63 (dd, J = 7.9, 5.3 Hz, 2H), 6.54–6.48 (m, 1H), 6.36
(d, J = 14.9 Hz, 1H), 6.30–6.24 (m, 1H), 6.21–6.11 (m, 3H), 2.04 (m,
2H, CH2), 2.02 (s, 3H, CH3), 2.00 (s, 3H, CH3), 1.99 (s, 3H, CH3), 1.72
(s, 3H, CH3), 1.61 (m, 2H, CH2), 1.45 (m, 2H, CH2), 1.03 (s, 6H, 2 ×
CH3); 13C NMR (101 MHz, CDCl3): δ 188.89, 147.86, 142.51, 141.12,
139.66, 137.99, 137.75, 137.31, 136.89, 135.16, 134.55, 134.27, 131.83,
130.74, 130.37, 129.78, 129.03, 129.01, 128.43, 127.49, 126.74, 125.99,
124.30, 39.76, 34.41, 33.26, 29.11, 21.92, 19.38, 13.13, 12.96, 12.84; UV
(ethanol) max: 327, 472 nm; HRMS calcd for C35H42ONa: [M + Na]+:
501.3133; found: 501.3139.
J = 4.5 Hz, 1H), 7.08–6.92 (m, 2H), 6.79 (dd, J = 14.6 Hz, 1H), 6.67
(dd, J = 14.6, 11.8 Hz, 1H), 6.40–6.33 (m, 1H), 6.32–6.26 (m, 1H),
6.26–6.20 (m, 1H), 6.15 (td, J = 11.1, 4.6 Hz, 2H), 2.03 (m, 2H, CH2),
1.99 (s, 6H, CH3), 1.87 (s, 3H, CH3), 1.71 (s, 3H), 1.61 (m, 2H, CH2),
1.48–1.44 (m, 2H, CH2), 1.02 (s, 6H, 2 × CH3); 13C NMR (101 MHz,
CDCl3): δ 194.59, 149.21, 149.13, 141.93, 138.09, 137.91, 137.61,
136.87, 136.42, 130.92, 130.50, 129.95, 127.96, 127.84, 127.37, 39.74,
34.40, 33.26, 29.10, 21.91, 19.36, 13.20, 12.99, 9.73; UV (ethanol) max:
426 nm; HRMS calcd for C25H34ONa: [M + Na]+: 373.2507; found:
373.2509.
2,6,11,15-Tetramethyl-17-(2,6,6-trimethylcyclohex-1-enyl)heptadeca-
1- (2,6-Dimethoxyphenyl) - 6,11,15-trimethyl-17- (2,6,6 -
trimethylcyclohex-1-enyl)heptadeca-1,4,6,8,10,12,14,16-octaen-3-
one (13b): Yield 78%; m.p. 90–95 °C; IR (KBr): 2923, 1598 (C=O),
2,4,6,8,10,12,14,16-octaenal (12)
The phosphate 9 (0.50 g, 1.63 mmol) was added to a solution of t-BuOK
(0.19 g, 1.70 mmol) in dry THF (15 mL). The mixture was stirred at
room temperature for 30 min. Then a solution of 11 (0.35 g, 1 mmol)
in dry THF (15 mL) was added and the mixture was stirred for 18 h
in the dark at room temperature in a nitrogen atmosphere. A solution
of 1 M HCl (20 mL) was added to the solution obtained above and the
mixture was stirred at room temperature for 3 h. The reaction mixture
was diluted with water (30 mL) and extracted with Et2O (3 × 25 mL).
The organic layer was dried with Na2SO4 and concentrated in vacuum.
The residue was purified by silica column chromatography (2.5% ethyl
acetate in petroleum ether) to give: m.p. 126–128 °C; IR (KBr): 2922,
1667 (C=O), 1609 (C=C), 1514, 1401, 1380, 1357, 1182 cm–1; 1H NMR
(400 MHz, CDCl3): δ 9.45 (s, 1H), 6.95 (dd, J = 10.7, 1.0 Hz, 1H),
6.81–6.57 (m, 5H), 6.45 (d, J = 11.6 Hz, 1H), 6.36 (d, J = 14.9 Hz, 1H),
6.27 (d, J = 11.7 Hz, 1H), 6.19–6.08 (m, 3H), 2.02 (m, 2H, CH2), 2.00
(s, 6H, 2 × CH3), 1.98 (s, 3H, CH3), 1.90 (s, 3H), 1.72 (s, 3H), 1.65–1.57
(m, 2H, CH2), 1.49–1.44 (m, 2H, CH2), 1.03 (s, 6H, 2 × CH3); 13C NMR
(101 MHz, CDCl3): δ 194.74, 149.54, 146.13, 138.80, 138.00, 137.78,
137.76, 137.06, 136.98, 136.81, 135.29, 133.18, 132.03, 130.79, 129.76,
1
1637 (C=C), 1473, 1324, 1225, 1181, 1107 cm–1; H NMR (400 MHz,
CDCl3): δ 8.14 (d, J = 16.2 Hz, 1H), 7.45 (dd, J = 15.8, 11.8 Hz, 2H),
7.31–7.27 (m, 1H), 6.85–6.52 (m, 7H), 6.36 (d, J = 14.9 Hz, 1H), 6.28
(d, J = 11.7 Hz, 1H), 6.25–6.09 (m, 3H), 3.90 (s, 6H, 2 × OCH3),
2.04 (m, 2H, CH2), 2.01 (s, 3H, CH3), 2.01 (s, 3H, CH3), 1.99 (s, 3H,
CH3), 1.72 (s, 3H, CH3), 1.64–1.60 (m, 2H, CH2), 1.49–1.44 (m, 2H,
CH2), 1.04 (s, 6H, 2 × CH3); 13C NMR (101 MHz, CDCl3): δ 190.60,
160.27, 146.75, 139.08, 137.88, 137.67, 137.01, 136.86, 134.52, 133.81,
133.66, 131.82, 131.25, 130.66, 129.63, 129.08, 128.99, 127.25, 126.37,
124.96, 112.88, 103.72, 55.85, 39.64, 34.30, 33.15, 29.00, 21.81, 19.27,
13.00, 12.85, 12.81; UV (ethanol) max: 322, 474 nm; HRMS calcd for
C37H46O3Na: [M + Na]+: 561.3345; found: 561.3341.
8,13,17-Trimethyl-19- (2,6,6-trimethylcyclohex-1-enyl) -1-
phenylnonadeca-1,3,6,8,10,12,14,16,18-nonaen-5-one (13c): Yield
in 75%. m.p. 32–36 °C; IR (KBr): 2920, 1652 (C=O), 1616 (C=C),
1446, 1360, 1253 cm–1; 1H NMR (400 MHz, CDCl3): δ 7.48 (dd, J = 3.5,
1.7 Hz, 3H), 7.35–7.29 (m, 4H), 6.92 (ddt, J = 19.4, 14.0, 4.5 Hz, 4H),

260 JOURNAL OF CHEMICAL RESEARCH 2016
6.66–6.54 (m, 3H), 6.36 (d, J = 14.9 Hz, 1H), 6.31–6.22 (m, 2H),
6.21–6.11 (m, 3H), 2.04 (m, 2H, CH2), 2.02 (s, 3H, CH3), 1.99 (s, 6H, 2
× CH3), 1.72 (s, 3H, CH3), 1.64–1.58 (m, 2H, CH2), 1.50–1.43 (m, 2H,
CH2), 1.03 (s, 6H, 2 × CH3); 13C NMR (101 MHz, CDCl3): δ 188.85,
147.43, 143.09, 142.55, 141.51, 141.13, 140.85, 139.49, 137.88, 137.65,
137.18, 136.80, 136.25, 136.15, 134.35, 134.22, 131.73, 130.65, 129.67,
129.55, 129.10, 129.05, 127.02, 124.20, 39.65, 34.30, 33.16, 29.74, 29.02,
21.82, 19.28, 19.18, 13.03, 12.86, 12.73; UV (ethanol) max: 324, 477
nm; HRMS calcd for C37H45O: [M + H]+: 505.3470; found: 505.3461.
6,10,15,19-Tetramethyl-21-(2,6,6-trimethylcyclohex-1-enyl)-1-
phenylhenicosa-1,4,6,8,10,12,14,16,18,20-decaen-3-one (14a): Yield
85%. m.p. 86–96 °C; IR (KBr): 2914, 1658 (C=O), 1587 (C=C), 1442,
124.26, 124.15, 39.77, 34.41, 33.26, 29.84, 29.12, 21.92, 19.39, 14.27,
13.04, 12.93, 12.90, 12.87; UV (ethanol) max: 335, 477 nm; HRMS
calcd for C42H51O: [M + H]+: 571.3940; found: 571.3941.
Electronic Supplementary Information
TheESI(1HNMRand13CNMRspectraandtheHRMSofβ-apo-
8’-carotenal (12), β-apo-12’-carotenal (11), citranaxanthin (1)
and compounds 13a–c and 14a–c) is available through:
stl.publisher.ingentaconnect.com/content/stl/jcr/supp-data
We are grateful to the comprehensive training platform of
the specialised laboratory, College of Chemistry, Sichuan
University for NMR, HRMS, IR and UV-Vis analysis.
1
1360, 1256, 1186 cm–1; H NMR (400 MHz, CDCl3): δ 7.53–7.33 (m,
7H), 6.97 (dd, J = 9.6, 6.9 Hz, 2H), 6.74–6.55 (m, 6H), 6.45 (d, J = 15.4
Hz, 1H), 6.37 (dd, J = 12.8, 6.8 Hz, 2H), 6.29–6.23 (m, 1H), 6.15 (dd,
J = 14.1, 9.8 Hz, 2H), 2.02 (m, 2H), 2.01 (s, 3H, CH3), 1.99 (s, 6H, 2
× CH3), 1.98 (s, 3H, CH3), 1.72 (s, 3H, CH3), 1.63–1.60 (m, 2H, CH2),
1.50–1.44 (m, 2H, CH2), 1.03 (s, 6H, 2 × CH3); 13C NMR (101 MHz,
CDCl3): δ 189.04, 148.17, 142.68, 142.57, 141.49, 137.99, 137.96, 137.79,
137.12, 136.67, 135.92, 135.80, 135.11, 133.92, 132.28, 132.05, 130.87,
130.36, 129.65, 129.61, 129.00, 128.42, 127.42, 127.11, 126.93, 125.89,
124.06, 71.69, 39.76, 34.38, 33.23, 29.80, 29.09, 21.88, 21.44, 19.37,
13.00, 12.89, 12.86, 12.83; UV (ethanol) max: 316, 498 nm; HRMS
calcd for C40H49O: [M + H]+: 545.3783; found: 545.3776.
1-(2,6-Dimethoxyphenyl)-6,10,15,19-tetramethyl-21-(2,6,6-
trimethylcyclohex-1-enyl)henicosa-1,4,6,8,10,12,14,16,18,20-decaen-
3-one (14b): Yield 80%; m.p. 110–124 °C; IR (KBr): 2926, 1595
(C=O), 1588 (C=C), 1474, 1383, 1257, 1180, 1108 cm–1; 1H NMR (400
MHz, CDCl3): δ 8.15 (d, J = 16.2 Hz, 1H), 7.49 (dd, J = 21.5, 5.3 Hz,
2H), 7.28 (t, J = 6.3 Hz, 1H), 6.69 (dd, J = 18.4, 7.6 Hz, 3H), 6.62 (d,
J = 8.4 Hz, 2H), 6.58 (d, J = 8.4 Hz, 4H), 6.40–6.32 (m, 2H), 6.24 (dd,
J = 22.6, 8.6 Hz, 2H), 6.19–6.12 (m, 2H), 3.90 (d, J = 7.5 Hz, 6H, 2 ×
OCH3), 2.03 (s, 3H, CH3), 2.01 (m, 2H, CH2), 1.99(s, 6H, 2 × CH3),
1.98 (s, 3H, CH3), 1.74 (s, 3H), 1.66–1.59 (m, 2H), 1.48 (m, 2H, CH2),
1.04 (s, 6H, 2 × CH3); 13C NMR (101 MHz, CDCl3): δ 190.64, 160.27,
146.95, 141.84, 140.29, 137.91, 137.72, 137.66, 137.07, 136.50, 134.20,
133.64, 132.22, 131.64, 131.22, 130.78, 129.63, 129.51, 128.99, 126.96,
125.67, 124.79, 124.27, 103.75, 58.47, 55.84, 39.66, 34.29, 33.14, 29.71,
28.99, 21.78, 19.27, 18.44, 12.90, 12.85, 12.80, 12.75; UV (ethanol)
max: 342, 476 nm; HRMS calcd for C42H52O3Na: [M + Na]+: 627.3814;
found: 627.3812.
Received 13 December 2015; accepted 19 January 2016
Paper 1503770 doi: 10.3184/174751916X14580338203147
Published online: 18 April 2016
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