Stereoselective synthesis by Olefin Metathesis and characterization of η-carotene (7,8,7′,8′-tetrahydro-β,β-carotene)

Article abstract of DOI:10.1021/np300230t

The purported structure of the elusive η-carotene (7,8,7′, 8′-tetrahydro-β,β-carotene), a natural C₄₀ carotenoid first detected in the berries of Lonicera japonica and in citrus fruits sixty years ago, has been synthesized by olefin cross-metat

Full text of DOI:10.1021/np300230t

Article
pubs.acs.org/jnp
Stereoselective Synthesis by Olefin Metathesis and Characterization
of η-Carotene (7,8,7′,8′-tetrahydro-β,β-carotene)
Noelia Fontan, Rosana Alvarez,* and Angel R. de Lera*
́
́
Departamento de Química Organica, Facultade de Química, Universidade de Vigo, 36310 Vigo, Spain
S
* Supporting Information
ABSTRACT: The purported structure of the elusive η-carotene (7,8,7′,8′-
tetrahydro-β,β-carotene), a natural C40 carotenoid first detected in the berries of
Lonicera japonica and in citrus fruits sixty years ago, has been synthesized by olefin
cross-metathesis/dimerization of a C21 polyene derived from trans-7,8-dihydror-
etinal, thus allowing the full characterization of this highly unstable natural product.
number of the more than 700 different carotenoids
and we may use their antioxidant properties for prevention of
certain cardiovascular diseases and cancer.10
A
isolated thus far1−3 are incompletely characterized due to
their instability or as a consequence of the scarcity of material
isolated from the natural source. 7,8,7',8'-Tetrahydro-β,β-
carotene (η-carotene; 1g) (Figure 1) stands out from the
subgroup of carotenoids whose identity remains uncertain. It
was isolated sixty years ago as a very minor carotenoid (1.3%)
from berries of Lonicera japonica4 and later also from the
flavedo of the fruit of the trigeneric hybrid Sinton citrangequat5
and other citrus fruits6−8 and in the Siluridae family of catfish.9
However, only UV absorption data and insufficient (signals in
In the past few years palladium-catalyzed cross-coupling
processes11 (primarily Negishi,12,13 Stille,14 and Suzuki15) for
C−C single bond formation have found a niche in the polyene
field, as they complement traditional C−C double bond
condensation methods such as Wittig, Horner−Wadsworth−
Emmons (HWE), and Julia−Kocienski olefination reac-
tions.16,17 Olefin metathesis18−21 is a powerful tactic for the
preparation of a great variety of natural products22,23 and its use
has been extended to the synthesis of the conjugated
carotenoid polyene chain.24,25 Four fully conjugated C40
undecaene carotenoids, namely, β,β-carotene (1a), (R,R)-
zeaxanthin (1b), rac-iso-zeaxanthin (1c), and lycopene (ψ,ψ-
carotene; 1d)25 (Figure 1), and two symmetrical conjugated
nonaene carotenoids, namely, violaxanthin (1e) and mimulax-
anthin (1f),24 have been prepared by metathesis−dimerization
of precursors 2a−f.
In this C21 + C21 − C2 = C40 approach to carotenoids 1, the
C21 heptaene/hexaene precursors 2 are obtained by Wittig
olefination of trans-retinal and ring/chain-modified or acyclic
analogues.25 Application of the same strategy to η-carotene
(1g) requires the preparation of stereochemically homogeneous
nonconjugated hexaene 2g, itself obtained from 7,8-dihydror-
etinal (12) (Scheme 1). Compounds with the general 7,8-
dihydroretinal structure but with 11-cis geometry have been
described previously,26 but no synthesis of the trans isomer has
been reported.
9
1
the aliphatic region, δ ∼1.01−2.12 ppm) H NMR data have
been reported; thus, no confirmation of its structure has been
provided. Moreover, the presence of η-carotene in natural
sources appears to depend upon the ripening period of the
fruits, and therefore “a full investigation into this pigment must
await the availability of larger amounts of ripe Lonicera berries
at a time when it is feasible to examine them”.4 Synthesis is
invaluable in providing larger amounts of material to fully
characterize and confirm or correct the proposed structures of
natural products.
The inherent instability of carotenoids derives from the
reactive nature of the highly conjugated polyene skeleton when
subjected to a variety of reaction conditions. In contrast, the
producing organisms exquisitely handle the basic C40
tetraterpenoid and also the diverse carotenoids generated by
biosynthetic modifications including cyclizations, oxidations,
dehydrogenations, and rearrangement reactions. The photo-
physical and photochemical properties of these polyenes are
responsible for much of the coloration in Nature, as well as for
the photoprotection of plants and microorganisms, where they
also function as accessory pigments in photosynthesis.10 Unable
to biosynthesize carotenoids, humans ingest them in the diet,
Received: March 26, 2012
Published: May 15, 2012
© 2012 American Chemical Society and
American Society of Pharmacognosy
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Figure 1. Symmetrical carotenoids 1a−f previously prepared by acyclic cross-metathesis/dimerization and the proposed structure of η-carotene (1g).
a
Scheme 1
a
Reagents and conditions: (a) n-BuLi, DMPU, THF, −78 °C, 47 h, 90%, 9:1 E/Z; (b) DIBAL-H, THF, −78 °C, 30 min, 97%; (c) MnO2, Na2CO3,
CH2Cl2, 25 °C, 3 h, 85%; (d) n-BuLi, DMPU, THF, −78 to 25 °C, 2 h, 97%, 20:3:1 mixture of (2E,4E,6E)/(2E,4E,6Z)/(2Z,4E,6E) isomers; (e)
DIBAL-H, THF, −78 °C, 45 min, 91%; (f) MnO2, Na2CO3, CH2Cl2, 25 °C, 1 h, 82%; (g) CH3PPh3Br, NaHMDS or n-BuLi, HMPA, THF, −78 °C,
2 h, 80%; (h) 2nd-generation Grubbs' catalyst 3, CH2Cl2, 50 °C, 6.5 h (1g, 60%; 13, 9%; 14, 5%; 2g, 9%).
bromide (13) using NaHMDS or n-BuLi as base and HMPA or
DMPU as cosolvent afforded hexaene 2g.
RESULTS AND DISCUSSION
■
The synthesis of trans-7,8-dihydroretinal (12)26 (Scheme 1)
started with the HWE reaction of commercial 7,8-dihydro-β-
ionone (4) (which can also be obtained from β-ionone using
Bu3SnH and catalytic amounts of Pd(PPh3)427) and 2-
(diethoxyphosphoryl)acetate (5) using n-BuLi as base in
THF and DMPU as cosolvent. Ester 6 was obtained in 90%
yield as a 9:1 mixture of double-bond isomers, which were
separated by chromatography. Subsequent reduction of 6 with
DIBAL-H, followed by MnO2 oxidation, provided aldehyde 8 in
good yield as a mixture of all-trans/9-cis isomers in a ≥37:1
ratio. A second HWE olefination with ethyl (E)-4-(diethox-
yphosphoryl)-3-methylbut-2-enoate (9) led in excellent yield
(97%) to ester 10 as a 20:3:1 mixture of (2E,4E,6E)/
(2E,4E,6Z)/(2Z,4E,6E) isomers. Subsequent reduction of the
purified trans isomer with DIBAL-H in THF followed by MnO2
oxidation afforded trans-7,8-dihydroretinal (12) in good yield.
Wittig olefination of 12 with methyltriphenylphosphonium
Polyene 2g was subjected to the optimized conditions for
cross-metathesis described for β,β-carotene (1a),25 yielding
7,8,7′,8′-tetrahydro-β,β-carotene (η-carotene; 1g) in 60% yield
together with small amounts of unreacted 2g (9%) and the
shorter nonconjugated diene 13 (9%) and triene 14 (5%)
products, which must form through alternative metathesis of
other double bonds in the starting pentaene or in 1g. The
structures of 13 and 14 were confirmed by independent
synthesis by Wittig reaction of 4 and 8, respectively. Similar to
previous examples (1a−f), the greater reactivity of the
ruthenium carbenes with the terminal monosubstituted double
bond when compared to the sterically more hindered tetra-, tri-,
and disubstituted double bonds of 2g produced the desired
symmetrical polyene 1g with remarkable site-selectivity and
trans stereoselectivity. When nonstereochemically homoge-
neous trans/cis mixtures of 2g were used in the metathesis
reaction, the mixtures of double-bond isomers of 1g obtained
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C−H), 1717 (s, CO), 1648 (m), 1442 (w), 1236 (m), 1200 (m),
proved difficult to separate by HPLC (Develosil C30-UG or
Spherisorb CN columns). Moreover, the reactivity of the allylic
positions makes 1g highly prone to oxidation and decom-
position. In fact, attempts to acquire MS data (ESI+ or EI
ionization) of 1g were complicated by the appearance of a [M +
16] peak corresponding to an oxidized derivative. Likewise,
samples left under air showed red-shifted UV spectra indicative
of allylic oxidations.
The UV spectrum of synthetic 1g [λmax in acetone (ε) 380
(103 500), 402 (131 500), and 427 (121 000)] was coincidental
with that published for the product isolated from natural
sources.4,6−8 The extinction coefficients, although not reported
in the original disclosure, are also similar to those described for
the acyclic analogue 7,8,7′,8′-tetrahydro-ψ,ψ-carotene (ζ-
carotene),2,28,29 and therefore we surmise that the proposed
natural product structure must correspond to 1g. From the
biogenetic perspective30 it is possible that, in the natural
sources, ζ-carotene functions as a substrate for lycopene β-
cyclase to first produce monocyclic dihydro-β-zeacarotene and
then dicyclic η-carotene. In fact, both η-carotene and ζ-carotene
were isolated as products of the lycopene cyclase gene crtY
from the epiphytic bacterium Erwinia uredovora expressed in E.
coli.31
1
1156 (s) cm−1; H NMR (C6D6, 400 MHz) δ 5.75 (1H, s, H2), 4.03
(2H, q, J = 7.1 Hz, CO2CH2CH3), 2.92−2.88 (2H, m, CH2), 2.30−
2.26 (2H, m, CH2), 1.91 (2H, t, J = 6.0 Hz, CH2), 1.81 (3H, s, CH3),
1.62 (3H, d, J = 1.2 Hz, CH3), 1.59−1.55 (2H, m, CH2), 1.48−1.45
(2H, m, CH2), 1.19 (6H, s, 2 × CH3), 1.00 (3H, t, J = 7.1 Hz,
CO2CH2CH3); 13C NMR (C6D6, 100 MHz) δ 166.0 (C), 160.1 (C),
137.1 (C), 128.2 (C), 116.5 (CH), 59.4 (CH2), 40.4 (CH2), 35.4 (C),
34.4 (CH2), 33.3 (CH2), 28.9 (CH3, 2×), 27.5 (CH2), 25.0 (CH3),
20.1 (CH3), 20.0 (CH2), 14.4 (CH2); HRMS (ESI+) m/z 265.2167
(calcd for C17H29O2 [M + H]+, 265.2162). Double-bond geometry
assignment was based on NOE measurements.
General Procedure for the DIBAL-H Reduction. To a solution
of E-6 (0.337 g, 1.274 mmol) in THF (12.8 mL) was added dropwise
diisobutylaluminum hydride (5.1 mL, 1 M in hexane, 5.1 mmol) at
−78 °C. After stirring for 30 min at the same temperature, a saturated
aqueous solution of potassium sodium (+)-tartrate was added, and the
mixture was stirred for 4 h. The mixture was extracted with ethyl ether
(1×), and the aqueous layer was saturated with NaCl and then
extracted with ethyl ether (3×). The organic extracts were dried
(Na2SO4), and the solvent was evaporated, affording 0.271 g (97%) of
an oil identified as (E)-3-methyl-5-(2′,6′,6′-trimethylcyclohex-1′-en-1′-
yl)pent-2-en-1-ol (E-7), which was used in the next reaction without
further purification.
General Procedure for the MnO2 Oxidation. To a cooled (0
°C) solution of (E)-3-methyl-5-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)-
pent-2-en-1-ol (E-7) (0.263 g, 1.181 mmol) obtained above in THF
(23 mL) were added activated manganese dioxide (1.912 g, 21.256
mmol) and sodium carbonate (2.253 g, 21.256 mmol). The reaction
mixture was stirred at 25 °C for 5 h and then filtered through a pad of
Celite. The solvent was evaporated to afford 0.220 g (85%) of a white
solid identified as (E)-3-methyl-5-(2′,6′,6′-trimethylcyclohex-1′-en-1′-
yl)pent-2-enal (E-8).
In summary, we have achieved the first total synthesis of the
purported structure of 7,8,7′,8′-tetrahydro-β,β-carotene (η-
carotene) using the acyclic cross-metathesis/dimerization
reaction, thus making possible complete spectroscopic
characterization of this nonaene with a central conjugated
chain of seven double bonds.
EXPERIMENTAL SECTION
General Experimental Procedures. For general procedures, see
ref 25.
(E)-3-Methyl-5-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)pent-2-enal
(E-8): white solid; UV (MeOH) λmax 237 nm; IR (NaCl) ν 2930 (m,
C−H), 2867 (m, C−H), 1675 (s, CO), 1445 (w), 1192 (w), 1119
■
1
(w) cm−1; H NMR (C6D6, 400 MHz) δ 9.92 (1H, d, J = 7.7 Hz,
General Procedure for the Horner−Wadsworth−Emmons
Reaction. A cooled (0 °C) solution of ethyl 2-(diethoxyphosphoryl)-
acetate (5, 0.94 g, 4.192 mmol) in THF (2.3 mL) was treated with n-
BuLi (2.56 mL, 1.5 M in hexane, 3.843 mmol) and DMPU (6.98 mL,
57.722 mmol) and stirred for 30 min. The mixture was cooled to −78
°C, and a solution of 4-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)butan-2-
one (4, 0.34 g, 1.747 mmol) in THF (4.7 mL) was added. The
resulting mixture was stirred for 30 min at −78 °C and then allowed to
warm to 25 °C for 47 h. The reaction mixture was quenched with
H2O, and the aqueous layer was extracted with ethyl ether (3×). The
combined organic layers were washed with water (2×) and brine (2×)
and dried (Na2SO4), and the solvent was evaporated. The residue was
purified by column chromatography (CC) (silica gel, 98:2 hexane/
EtOAc) to afford 0.378 g (82%) of a colorless oil identified as ethyl
(E)-3-methyl-5-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)pent-2-enoate
(E-6) and 0.039 g (8%) of ethyl (Z)-3-methyl-5-(2′,6′,6′-trimethylcy-
clohex-1′-en-1′-yl)pent-2-enoate (Z-6). An aliquot of the mixture of
isomers was purified by HPLC (Prep Novapak HR silica, 6 μm, 19 ×
300 mm, 98/2 hexane/EtOAc, 3 mL/min; tR(Z) = 23.1 min and tR(E)
= 27.0 min).
Ethyl (E)-3-methyl-5-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)pent-
2-enoate (E-6): colorless oil; IR (NaCl) ν 2928 (m, C−H), 2865 (m,
C−H), 1717 (s, CO), 1649 (m), 1445 (w), 1222 (s), 1144 (s)
cm−1; 1H NMR (C6D6, 400 MHz) δ 5.93 (1H, s, H2), 4.08 (2H, q, J =
7.1 Hz, CO2CH2CH3), 2.29 (3H, d, J = 1.0 Hz, CH3), 2.07 (4H, app s,
2 × CH2), 1.83 (2H, t, J = 6.2 Hz, CH2), 1.56−1.49 (2H, m, CH2),
1.47 (3H, s, CH3), 1.40−1.37 (2H, m, CH2), 1.04 (3H, t, J = 7.1 Hz,
CO2CH2CH3), 0.94 (6H, s, 2 × CH3); 13C NMR (C6D6, 100 MHz) δ
166.6 (C), 160.2 (C), 136.5 (C), 127.9 (C), 115.8 (CH), 59.4 (CH2),
41.7 (CH2), 40.1 (CH2), 35.2 (C), 33.0 (CH2), 28.7 (CH3, 2×), 27.4
(CH2), 19.9 (CH3), 19.9 (CH2), 18.9 (CH3), 14.5 (CH3); HRMS
(ESI+) m/z 265.2159 (calcd for C17H29O2 [M + H]+, 265.2162).
Ethyl (Z)-3-methyl-5-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)pent-
2-enoate (Z-6): colorless oil; IR (NaCl) ν 2927 (m, C−H), 2864 (m,
CHO), 5.93 (1H, d, J = 7.7 Hz, H2), 1.94 (4H, app s, 2 × CH2), 1.82
(2H, t, J = 6.3 Hz, CH2), 1.58 (3H, d, J = 1.2 Hz, CH3), 1.55−1.50
(2H, m, CH2), 1.40−1.37 (3H, s, CH3), 1.4−1.3 (2H, m, CH2), 0.93
(6H, s, 2 × CH3); 13C NMR (C6D6, 100 MHz) δ 190.0 (CH), 162.1
(C), 136.3 (C), 128.1 (C), 127.2 (CH), 41.2 (CH2), 40.1 (CH2), 35.2
(C), 33.0 (CH2), 28.7 (CH3, 2×), 26.9 (CH2), 19.9 (CH3), 19.9
(CH2), 17.0 (CH3); HRMS (ESI+) m/z 221.1897 (calcd for C15H25O
[M + H]+, 221.1899).
Ethyl (2E,4E,6E)-(3,7-Dimethyl-9-(2′,6′,6′-trimethylcyclohex-
1′-en-1′-yl)nona-2,4,6-trienoate (2E,4E,6E-10). Following the
general procedure for the HWE reaction, the reaction of E-9 (0.308
g, 1.168 mmol), n-BuLi (0.78 mL, 1.37 M in hexane, 1.07 mmol),
DMPU (0.33 mL, 2.724 mmol), and (E)-3-methyl-5-(2′,6′,6′-
trimethylcyclohex-1′-en-1′-yl)pent-2-enal (E-8) (0.214 g, 0.973
mmol) in THF (1.75 mL) afforded, after purification by CC (silica
gel, from 97.5:2.5 hexane/Et3N to 97.5:2.5 hexane/EtOAc), 0.312 g
(97%) of a colorless oil identified as a 20:3:1 mixture of (2E,4E,6E)/
(2E,4E,6Z)/(2Z,4E,6E) isomers of the title compound, which were
separated by HPLC (Prep Novapak HR silica 60 Å, 19 × 300 mm, 98/
2 hexane/EtOAc, 3 mL/min; tR(2Z,4E,6E) = 24.5 min, tR(2E,4E,6Z) =
26.4 min, and tR(2E,4E,6E) = 28.7 min).
Ethyl (2E,4E,6E)-3,7-dimethyl-9-(2′,6′,6′-trimethylcyclohex-1′-en-
1′-yl)nona-2,4,6-trienoate (2E,4E,6E-10): colorless oil; UV (MeOH)
λmax 314 nm; IR (NaCl) ν 2925 (m, C−H), 2865 (m, C−H), 1711 (s,
1
CO), 1603 (s), 1441 (w), 1237 (m), 1148 (s) cm−1; H NMR
(C6D6, 400 MHz) δ 6.81 (1H, dd, J = 15.2, 11.0 Hz, H5), 6.11 (1H, d,
J = 15.2 Hz, H4), 5.99 (1H, d, J = 11.0 Hz, H6), 5.98 (1H, s, H2), 4.08
(2H, q, J = 7.1 Hz, CO2CH2CH3), 2.49 (3H, s, CH3), 2.18 (4H, app s,
2 × CH2), 1.89 (2H, t, J = 6.2 Hz, CH2), 1.65 (3H, s, CH3), 1.61 (3H,
s, CH3), 1.69−1.55 (2H, m, CH2), 1.46−1.43 (2H, m, CH2), 1.05
(6H, m, 2 × CH3), 1.01 (3H, t, J = 7.1 Hz, CO2CH2CH3); 13C NMR
(C6D6, 100 MHz) δ 166.9 (C), 153.1 (C), 144 (C), 136.9 (C), 134.2
(CH), 131.1 (CH), 127.7 (C), 125.2 (CH), 119.0 (CH), 59.6 (CH2),
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41.3 (CH2), 40.2 (CH2), 35.3 (C), 33.1 (CH2), 28.8 (CH3, 2×), 28.0
(CH2), 20.0 (CH3), 19.9 (CH2), 17.0 (CH3), 14.5 (CH3), 13.9
(CH3); HRMS (ESI+) m/z 331.2631 (calcd for C22H35O2 [M + H]+,
331.2632). Double-bond geometry assignment was based on NOE
measurements.
Ethyl (2E,4E,6Z)-3,7-dimethyl-9-(2′,6′,6′-trimethylcyclohex-1′-en-
1′-yl)nona-2,4,6-trienoate (2E,4E,6Z-10): colorless oil; 1H NMR
(C6D6, 400 MHz) δ 7.05−6.98 (1H, m, H5), 6.13 (1H, d, J = 15.3 Hz,
H4), 5.98 (1H, s, H2), 5.85 (1H, d, J = 10.8 Hz, H6), 4.08 (2H, q, J =
7.1 Hz, CO2CH2CH3), 2.54 (3H, d, J = 1.0 Hz, CH3), 2.28−2.24 (2H,
m, CH2), 2.10−2.06 (2H, m, CH2), 1.87−1.81 (2H, m, CH2), 1.74
(3H, s, CH3), 1.59 (3H, s, CH3), 1.57−1.53 (2H, m, CH2), 1.43−1.40
(2H, m, CH2), 1.05 (6H, m, 2 × CH3), 1.01 (3H, t, J = 7.1 Hz,
CO2CH2CH3).
Ethyl (2Z,4E,6E)-3,7-dimethyl-9-(2′,6′,6′-trimethylcyclohex-1′-en-
1′-yl)nona-2,4,6-trienoate (2Z,4E,6E-10): colorless oil; 1H NMR
(C6D6, 400 MHz) δ 8.42 (1H, d, J = 15.6 Hz, H4), 6.86 (1H, dd, J =
15.5, 11.0 Hz, H5), 6.25 (1H, d, J = 11.0 Hz, H6), 5.77 (1H, s, H2),
4.05 (2H, q, J = 7.1 Hz, CO2CH2CH3), 2.14 (4H, app s, 2 × CH2),
1.88 (2H, t, J = 6.4 Hz, CH2), 1.77 (3H, s, CH3), 1.69 (3H, s, CH3),
1.58−1.55 (5H, m, CH3 + CH2), 1.45−1.42 (2H, m, CH2), 1.05 (6H,
m, 2 × CH3), 1.01 (3H, t, J = 7.1 Hz, CO2CH2CH3).
(2E,4E,6E)-3,7-Dimethyl-9-(2′,6′,6′-trimethylcyclohex-1′-en-
1′-yl)nona-2,4,6-trienal (2E,4E,6E-12). Following the general
procedure for the DIBAL-H reduction, the reaction of 2E,4E,6E-10
(0.31 g, 0.938 mmol) and diisobutylaluminum hydride (3.75 mL, 1 M
in hexane, 3.75 mmol) in THF (9.4 mL) afforded, after purification by
column chromatography (silica gel, from 95:2:3 hexane/EtOAc/Et3N
to 80:20 hexane/EtOAc), 0.246 g (91%) of (2E,4E,6E)-3,7-dimethyl-
9-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)nona-2,4,6-trien-1-ol
(2E,4E,6E-11).
λmax 321, 307 nm; IR (NaCl) ν 3040 (w, C−H), 2927 (s, C−H), 2865
(m, C−H), 1610 (w), 1471 (w), 1441 (w), 983 (m), 954 (m), 897
1
(m) cm−1; H NMR (C6D6, 400 MHz) δ 6.75−6.60 (2H, m, H5′
+
H9′), 6.32 (1H, d, J = 15.2 Hz, H6′), 6.17 (2H, d, J = 11.1 Hz, H4′ +
H8′), 5.20 (1H, d, J = 16.7 Hz, H10a′), 5.07 (1H, d, J = 10.2 Hz, H10b′),
2.24 (4H, app s, 2 × CH2), 1.90 (2H, t, J = 6.2 Hz, CH2), 1.81 (3H, s,
CH3), 1.77 (3H, s, CH3), 1.63 (3H, s, CH3), 1.61−1.55 (2H, m, CH2),
1.47−1.44 (2H, m, CH2), 1.06 (6H, s, 2 × CH3); 13C NMR (C6D6,
100 MHz) δ 139.9 (C), 137.1 (C), 136.5 (C), 135.8 (CH), 133.8
(CH), 131.6 (CH), 127.5 (C), 125.9 (CH), 125.7 (CH), 117.0 (CH2),
41.3 (CH2), 40.2 (CH2), 35.3 (C), 33.1 (CH2), 28.8 (CH3, 2×), 28.2
(CH2), 20.1 (CH3), 20.0 (CH2), 17.0 (CH3), 12.8 (CH3); MS (EI+)
m/z (%) 284 [M]+ (16), 147 (100), 137 (42), 119 (56), 105 (80), 95
(62), 91 (43); HRMS (EI+) m/z 284.2507 (calcd for C21H32,
284.2504).
7,8,7',8'-Tetrahydro-β,β-carotene (1g). To a degassed solution
of (3′E,5′E,7′E-2g) (0.047 g, 0.165 mmol) in CH2Cl2 (3.1 mL) was
added Grubbs' second-generation catalyst (0.014 g, 0.016 mmol).
After stirring for 5 h at 50 °C, the reaction mixture was filtered
through Celite, which was washed with acetone, and the solvent was
evaporated. The residue was purified by CC (C18 silica gel, from 90:10
CH3CN/CH2Cl2 to 70:30 CH3CN/CH2Cl2 containing small amounts
of BHT, butylated hydroxytoluene (2,6-di-tert-butyl-p-cresol)) to
afford 0.027 g (60%) of a reddish solid identified as 7,8,7′,8′-
tetrahydro-β,β-carotene (1g) and a mixture, which was purified by a
second CC (silica gel, from hexane to 99:1 hexane/EtOAc) to afford,
in order of elution, 2.8 mg (9%) of an oil identified as 1,3,3-trimethyl-
2-(3′-methylbut-3′-en-1′-yl)cyclohex-1-ene (13), 2.0 mg (5%) of
another oil identified as (E)-1,3,3-trimethyl-2-(3′-methylhexa-3′,5′-
dien-1′-yl)cyclohex-1-ene (E-14), and 4.1 mg (9% recovered) of
starting material.
Following the general procedure for the MnO2 oxidation, the
reaction of 2E,4E,6E-11 (0.245 g, 0.849 mmol), manganese dioxide
(1.375 g, 15.288 mmol), and sodium carbonate (1.62 g, 15.287 mmol)
in THF (16.3 mL) afforded 0.199 g (82%) of a white solid identified
as (2E,4E,6E)-3,7-dimethyl-9-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)-
nona-2,4,6-trienal (2E,4E,6E-12), which was used in the next step
without further purification.
7,8,7′,8′-Tetrahydro-β,β-carotene (1g): reddish solid; UV (ace-
tone) λmax (log ε) 380 (5.01), 402 (5.12), 427 (5.08) nm; (hexane)
λmax 378, 400, 426 nm. IR (NaCl) ν 3027 (w, C−H), 2924 (s, C−H),
1
2851 (m, C−H), 1441 (m), 1061 (m) cm−1; H NMR (C6D6, 400
MHz) δ 6.72−6.66 (4H, m, 2H11 + 2H15), 6.45 (2H, d, J = 15.0 Hz,
2H12), 6.35 (2H, d, J = 9.6 Hz, 2H14), 6.25 (2H, d, J = 11.0 Hz, 2H10),
2.28−2.25 (8H, m, 4 × CH2), 1.92−1.89 (10H, m, 2 × CH3 + 2 ×
CH2), 1.81 (6H, s, 2 × CH3), 1.64 (6H, s, 2 × CH3), 1.62−1.55 (4H,
m, 2 × CH2), 1.47−1.45 (4H, m, 2 × CH2), 1.08 (6H, s, 2 × CH3)
(signals of BHT stabilizer are not reported); 13C NMR (C6D6, 100
MHz) δ 139.9 (C), 137.2 (C), 136.4 (C), 136.2 (CH), 132.3 (CH),
130.3 (CH), 127.5 (C), 126.2 (CH), 125.4 (CH), 41.4 (CH2), 40.2
(CH2), 35.3 (C), 33.1 (CH2), 28.9 (CH3, 2×), 28.3 (CH2), 20.1
(CH3), 20.0 (CH2), 17.0 (CH3), 13.0 (CH3) (signals of BHT
stabilizer are not reported); HRMS (ESI+) m/z 540.4703 (calcd for
C40H60 [M]+, 540.4689).
(2E,4E,6E)-3,7-Dimethyl-9-(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)-
nona-2,4,6-trienal (2E,4E,6E-12): white solid; UV (MeOH) λmax 341
nm; IR (NaCl) ν 2927 (m, C−H), 2865 (w, C−H), 1661 (s, CO),
1
1630 (w), 1594 (m) cm−1; H NMR (C6D6, 400 MHz) δ 10.03 (1H,
d, J = 7.9 Hz, CHO), 6.73 (1H, dd, J = 15.1, 11.3 Hz, H5), 6.00−5.94
(3H, m, H2 + H4 + H6), 2.18 (4H, app s, 2 × CH2), 1.90 (2H, t, J = 6.0
Hz, CH2), 1.74 (3H, s, CH3), 1.65 (3H, s, CH3), 1.61 (3H, s, CH3),
1.58−1.55 (2H, m, CH2), 1.47−1.44 (2H, m, CH2), 1.05 (6H, s, 2 ×
CH3); 13C NMR (C6D6, 100 MHz) δ 189.9 (CH), 153.6 (C), 145.5
(C), 136.8 (C), 133.7 (CH), 132.0 (CH), 129.4 (CH), 127.8 (C),
125.1 (CH), 41.3 (CH2), 40.2 (CH2), 35.3 (C), 33.1 (CH2), 28.8
(CH3, 2×), 27.9 (CH2), 20.0 (CH3), 20.1 (CH2), 17.1 (CH3), 12.6
(CH3); HRMS (ESI+) m/z 287.2365 (calcd. for C20H31O ([M + H]+),
287.2369).
(3′E,5′E,7′E)-2-(3′,7′-Dimethyldeca-3′,5′,7′,9′-tetraen-1′-yl)-
1,3,3-trimethylcyclohex-1-ene (3′E,5′E,7′E-2g). To a suspension
of methyltriphenylphosphonium bromide (0.227 g, 0.635 mmol) in
THF (0.6 mL) were added dropwise n-BuLi (0.25 mL, 2.27 M in
hexane, 0.564 mmol) and DMPU (0.07 mL, 0.529 mmol), and the
resulting mixture was stirred for 30 min at 25 °C. The mixture was
cooled to −78 °C, and a solution of (2E,4E,6E)-3,7-dimethyl-9-
(2′,6′,6′-trimethylcyclohex-1′-en-1′-yl)nona-2,4,6-trienal (0.101 g, 0.353
mmol) in THF 4.5 mL) was added. After stirring for 1 h at −78 °C the
reaction was allowed to warm to 25 °C for 1 h, and hexane was added.
The combined organic layers were washed with brine (3×) and water
(3×) and dried (Na2SO4), and the solvent was evaporated. The
residue was purified by CC (silica gel, from 99:1 hexane/Et3N to 99:1
hexane/EtOAc) to afford 0.080 g (80%) of a yellow oil identified as
(3′E,5′E,7′E)-2-(3′,7′-dimethyldeca-3′,5′,7′,9′-tetraen-1′-yl)-1,3,3-trime-
thylcyclohex-1-ene (3′E,5′E,7′E-2g).
9
1
The H NMR data coincide with those described previously for
signals corresponding to the aliphatic region of this compound in
CDCl3: δ 2.12 (8H, m), 1.97 (6H, s), 1.86 (6H, s), 1.66 (6H, s), 1.01
(12H, s).
1,3,3-Trimethyl-2-(3′-methylbut-3′-en-1′-yl)cyclohex-1-ene (13):
colorless oil; IR (NaCl) ν 2966 (s, C−H), 2929 (s, C−H), 2909 (s,
1
C−H), 2865 (m), 1649 (w), 1472 (w), 1455 (w), 885 (m) cm−1; H
NMR (C6D6, 400 MHz) δ 4.88 (1H, s, H4a′), 4.83 (1H, s, H4B′), 2.25−
2.14 (4H, m, 2 × CH2), 1.90 (2H, t, J = 6.3 Hz, CH2), 1.72 (3H, s,
CH3), 1.58−1.53 (5H, m, CH3 + CH2), 1.45−1.42 (2H, m, CH2), 1.05
(6H, t, J = 0.5 Hz, 2 × CH3); 13C NMR (C6D6, 100 MHz) δ 146.5
(C), 137.3 (C), 127.3 (C), 109.8 (CH2), 40.3 (CH2), 38.9 (CH2),
35.3 (C), 33.1 (CH2), 28.8 (CH3, 2×), 27.8 (CH2), 22.7 (CH3), 20.0
(CH2), 19.9 (CH3); MS (EI+) m/z 192 [M]+ (13), 177 (20), 137
(83), 121 (18), 107 (22), 95 (100), 93 (21), 91 (19), 81 (49), 79
(20); HRMS (EI+) m/z 192.1882 (calcd. for C14H24, 192.1878).
This compound was alternatively synthesized from dihydro-β-
ionone 4: Following the general procedure for the Wittig reaction, the
reaction of methyltriphenylphosphonium bromide (0.652 g, 1.825
mmol) with n-BuLi (1.10 mL, 1.46 M in hexane, 1.622 mmol), DMPU
(0.18 mL, 1.521 mmol), and 4-(2′,6′,6′-trimethylcyclohex-1′-en-1′-
yl)butan-2-one) (4) (0.197 g, 1.014 mmol) in THF (15 mL) afforded,
after purification by CC (silica gel, from 95:3:2 hexane/EtOAc/Et3N
(3′E,5′E,7′E)-2-(3′,7′-Dimethyldeca-3′,5′,7′,9′-tetraen-1′-yl)-1,3,3-
trimethylcyclohex-1-ene (3′E,5′E,7′E-2g): yellow oil; UV (MeOH)
978
dx.doi.org/10.1021/np300230t | J. Nat. Prod. 2012, 75, 975−979

Journal of Natural Products
Article
to 95:5 hexane/EtOAc), 0.120 g (62%) of a colorless oil identified as
1,3,3-trimethyl-2-(3′-methylbut-3′-en-1′-yl)cyclohex-1-ene (13).
(E)-1,3,3-Trimethyl-2-(3′-methylhexa-3′,5′-dien-1-yl)cyclohex-1-
ene (E-14): colorless oil; UV (MeOH) λmax 237 nm; IR (NaCl) ν 3083
(w, C−H), 2952 (s, C−H), 2928 (s, C−H), 2865 (m, C−H), 2829
(w, C−H), 1650 (w), 1472 (w), 895 (m) cm−1; 1H NMR (C6D6, 400
MHz) δ 6.64 (1H, ddd, J = 16.8, 10.8, 10.2 Hz, H5′), 6.08 (1H, d, J =
10.8 Hz, H4′), 5.17 (1H, d, J = 16.8 Hz, H6a′), 5.03 (1H, d, J = 10.2 Hz,
H6b′), 2.18 (4H, app s, 2 × CH2), 1.87 (2H, t, J = 6.1 Hz, CH2), 1.68
(3H, s, CH3), 1.59−1.53 (5H, m, CH3 + CH2), 1.45−1.42 (2H, m,
CH2), 1.03 (6H, s, 2 × CH3); 13C NMR (C6D6, 100 MHz) δ 139.9
(C), 137.1 (C), 133.9 (CH), 127.4 (C), 125.8 (CH), 114.9 (CH2),
40.9 (CH2), 40.2 (CH2), 35.3 (C), 33.1 (CH2), 28.8 (CH3, 2×), 28.0
(CH2), 20.0 (CH3), 19.9 (CH2), 16.7 (CH3) ppm; MS (EI+) m/z 218
[M]+ (3), 137 (100), 95 (97), 81 (54), 79 (22); HRMS (EI+) m/z
218.2042 (calcd for C16H26, 218.2035).
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This compound was alternatively synthesized from dihydro-β-
ionylideneacetaldehyde (8). Following the general procedure for the
Wittig reaction, the reaction of methyltriphenylphosphonium bromide
(0.277 g, 0.776 mmol) with n-BuLi (0.47 mL, 1.46 M in hexane, 0.690
mmol), DMPU (0.08 mL, 0.646 mmol), and (E)-3-methyl-5-(2′,6′,6′-
trimethylcyclohex-1′-en-1′-yl)pent-2-enal (E-8) (0.095 g, 0.431 mmol)
in THF (6.5 mL) afforded, after purification by CC (silica gel
saturated with 2% Et3N, 99:1 hexane/EtOAc), 0.055 g (59%) of a
colorless oil identified as (E)-1,3,3-trimethyl-2-(3′-methylhexa-3′,5′-
dien-1′-yl)cyclohex-1-ene (E-14).
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ASSOCIATED CONTENT
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(27) Nilsson, Y. I. M.; Aranyos, A.; Andersson, P. G.; Backvall, J.-E.;
̈
S
* Supporting Information
Parrain, J.-L.; Ploteau, C.; Quintard, J.-P. J. Org. Chem. 1996, 61,
1825−1829.
Copies of spectra of the compounds described in the text. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
■
́
(30) Ruiz-Sola, M. A.; Rodríguez-Concepcion, M. The Arabidopsis
Corresponding Author
Book 2012, 10, e0158.
*Tel: +34-986-812316. Fax: +34-986-818622. E-mail: qolera@
uvigo.es (A.R.L.); Tel: +34-986-812632. Fax: +34-986-818622.
E-mail: rar@uvigo.es (R.A.).
(31) Takaichi, S.; Sandmann, G.; Schnurr, G.; Satomi, Y.; Suzuki, A.;
Misawa, N. Eur. J. Biochem. 1996, 241, 291−296.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
The authors are grateful to the Ministerio de Economia y
Competitividad-Spain (SAF2010-17935-FEDER-Fondos Eu-
ropeos para el Desarrollo Regional; FPU Fellowship to N.F.)
and Xunta de Galicia (Grant 08CSA052383PR from DXI+D+i;
́
Consolidacion 2006/15 and from DXPCTSUG; INBIOMED)
for financial support.
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