Reassignment of the absolute configuration of plakinidone from the sponge consortium Plakortis halichondrioides-Xestospongia deweerdtae using a combination of synthesis and a chiroptical approach

Article abstract of DOI:10.1016/j.tetasy.2016.03.011

Recent work by Wu et al. in connection with the first synthesis of the marine natural product plakinidone revealed that the most salient feature of its purported structure, a six-membered perlactone moiety, was in fact a five-membered lactone, i.e. a 3-me

Full text of DOI:10.1016/j.tetasy.2016.03.011

Tetrahedron: Asymmetry xxx (2016) xxx–xxx
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Reassignment of the absolute configuration of plakinidone from
the sponge consortium Plakortis halichondrioides–Xestospongia
deweerdtae using a combination of synthesis and a chiroptical
approach
Carlos Jiménez-Romero a, Joanna E. Rode b, Abimael D. Rodríguez a,
⇑
a Molecular Sciences Research Center, University of Puerto Rico, 1390 Ponce de León Avenue, San Juan 00926, Puerto Rico
b Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Recent work by Wu et al. in connection with the first synthesis of the marine natural product plakinidone
revealed that the most salient feature of its purported structure, a six-membered perlactone moiety, was
in fact a five-membered lactone, i.e. a 3-methyl-4-hydroxy-2(5H)-furanone or tetronic acid ring. With the
planar structure of plakinidone confidently revised, we undertook a new investigation to unambiguously
establish its absolute configuration. Upon preparing two stable derivatives 1 and 5 from a sample of
naturally occurring plakinidone extracted from the sponge association Plakortis halichondrioides–
Xestospongia deweerdtae, the absolute configuration was assigned by synthesis and vibrational and
electronic circular dichroism (VCD and ECD) measurements in combination with density functional
theory calculations at the B3LYP/aug-cc-pVDZ/PCM(CH3CN) level of theory. Our combined efforts and
the agreement between the experimental and calculated VCD/ECD spectra of 1 revealed that the absolute
configuration of plakinidone was in fact (11S,17R) and not the formerly reported (11S,17S) diastereomer
assigned by Wu et al. Therefore, we propose that natural plakinidone is accurately represented by
structure 12.
Received 18 March 2016
Accepted 23 March 2016
Available online xxxx
Ó 2016 Elsevier Ltd. All rights reserved.
23
23
1. Introduction
CH3
CH3
7
7
11
11
In 1991, during an investigation into the chemical composition
of the Caribbean sponge Plakortis angulospiculatus, Kushland and
Faulkner reported the discovery of a new natural product which
they named plakinidone.1 This interesting p-hydroxyphenyl
polyketide was heralded as the first natural product that incorporated
a six-membered-ring perlactone moiety.2 The purported structure
for plakinidone was established exclusively on the basis of conven-
tional spectroscopic data (IR, EIMS and 1D NMR) without any
stereostructural or chemical characterization work (Fig. 1).
In 2015, while attempting the first synthesis of plakinidone, Wu
et al. found evidence that the previous assignment of this natural
product as a perlactone was mistaken and thus plakinidone was
structurally revised to a five-membered lactone (Fig. 1).3 As part
of a strategy to scrutinize the absolute structure, they synthesized
three out of the four possible stereoisomers for plakinidone and
compared their 13C NMR and optical rotation data with the natural
product. Neither the specific rotation nor the 13C NMR data
1
1
14
22
CH3
14
22
4
4
OH
CH3
OH
21
CH3
OH
OH
21
CH3
17
17
20
O
O
20
O
O
O
(a) original (Faulkner et al.)
(b) reassigned (Wu et al.)
Figure 1. Planar structures and numbering system for plakinidone: (a) original and
(b) reassigned.
recorded were compatible with natural plakinidone. Soon there-
after, they realized that the secondary products arising from the
facile air oxidation of the synthetic plakinidones accounted for
these discrepancies. They argued that the tetronic acid moiety in
these compounds absorbs oxygen, thus making them unstable,
with a tendency to decompose into analogues of varying specific
rotation. Also, we suspect that the susceptibility of natural
⇑
Corresponding author.
E-mail address: abimael.rodriguez1@upr.edu (A.D. Rodríguez).
http://dx.doi.org/10.1016/j.tetasy.2016.03.011
0957-4166/Ó 2016 Elsevier Ltd. All rights reserved.
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C. Jiménez-Romero et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
plakinidone to air oxidation was not entirely realized by Faulkner
suggesting that the reported specific rotation should be called into
question.1 To complicate matters further, Wu et al. did not have
natural plakinidone and were restricted in their studies by having
only two of the three synthetic plakinidones prepared in pure form.
Perhaps unbeknownst to these researchers, the position of the 13C
NMR peaks about the tetronic acid moiety (as well as the tautomer
distribution of the furanone forms) varies somewhat with temper-
ature, the polarity of the solvent, and the compound concentration,
which would help to partially explain the discrepancies observed
and CHCl3. Concentration of the CHCl3 extract in vacuo afforded
the crude dark brown oil (8.0 g) used in this investigation. Repeti-
tive reversed-phase chromatography afforded 926 mg of pure
plakinidone whose 1H and 13C NMR properties in CDCl3 were in
agreement with the published values.1 Following exclusion of air
from the NMR solvent, higher quality NMR spectra were obtained
in CD3OD as plakinidone was sparingly soluble in CDCl3. The speci-
fic rotation data are relevant and include [
a
]
D = +7.9 (c 0.61,
CH3OH) of Faulkner’s sample1 versus [
a
]D = À13.1 (c 0.61, CH3OH)
of our carefully handled sample. Since the original structure
assignment of plakinidone was based solely on 1D NMR and MS
data, we sought to collect additional 2D-NMR data in support of
in the 13C NMR and [ data.4 We contend that the seemingly
a]
D
ambiguous nature of the data encountered by Wu et al. following
this approach made his attempts at assigning the absolute
configuration of plakinidone quite vulnerable to error.5 In spite of
all these drawbacks, his team tentatively assigned the absolute
configuration as (11S,17S) on the basis of the estimation of
oxidation-related changes in the optical activity of all the synthetic
stereoisomers prepared.
the Wu’s revised planar structure.3 Thus,
a combination of
1H–1H-COSY, HSQC, HSQC-TOCSY, and HMBC NMR experiments
allowed for the complete 1H and 13C-signal assignments. Although
1H and 13C NMR data of plakinidone were essentially the same as
those reported,1 for comparative purposes they are given in Table 1.
Of particular importance was a HSQC-TOCSY 13C–1H-shift-correlation
experiment with plakinidone, which allowed us to unambiguously
place the secondary methyl group at C-11. As the locus of the
H3-23 group in the 1991 disclosure was based solely on mass
spectrometric evidence for mechanisms of fragmentation, we
repeated the EIMS measurements and confirmed the constitution
of plakinidone in accordance with the recently revised structure.1,3
In view of these obstacles, the re-isolation of plakinidone
represented a convenient opportunity to provide not only a
detailed structure elucidation using modern 2D NMR spectroscopic
techniques (1H–1H COSY, HSQC, HSQC-TOCSY, and HMBC), but also
to determine its stereostructural features in a manner that is
essentially unconcerned with the exact specific rotation and 13C
NMR chemical shift values. Briefly, our basic strategy called for
the preparation of two stable derivatives of natural plakinidone,
1 and 5, that are not only impervious to air-oxidation but amenable
to total synthesis, and vibrational and electronic circular dichroism
(VCD and ECD) studies. Chiroptical techniques are increasingly
employed for the absolute configuration assignment of chiral
molecules through comparison of experimental spectra with
theoretical predictions.6,7 This approach has proved to be particu-
larly useful in the case of natural products, an important source
for drug discovery often exhibiting molecular flexibility, multiple
functional groups, or chromophores.8 To increase the confidence
level of the stereochemical analysis, the use of different chiroptical
techniques is postulated.9–17 On the other hand, the determination
of the absolute configuration of molecules with many stereogenic
centers by chiroptical methods is still a challenge, and is rather
limited to rigid molecules.8,15,18–21 In the case of diastereomers,
there is no spectra mirror relationship as for enantiomers,
and therefore, in order to unambiguously assign the absolute con-
figuration, the compared experimental and calculated data have to
be of very good quality. Nevertheless, successful attempts exist for
the absolute configuration assignment of large and flexible
molecules.17,22,23 In such cases, the use of other methods such as
the synthesis of model compounds based on well-established
reactions or NMR studies, are helpful for structure elucidation.
Herein, the assignment of the C11 center of natural plakinidone 1
was ascertained by enantioselective syntheses of fragment 5.
Furthermore, for the configuration assignment of the C17 center
of 1, two independent techniques, vibrational and electronic
circular dichroism (VCD and ECD), were applied and the interpre-
tation of the experimental data was based on DFT calculations.
2.2. Methylation of natural plakinidone
Due to plakinidone’s tendency for air oxidation, we conducted
the derivatization as quickly as possible.3 As shown in Scheme 1,
the natural product was treated with diazomethane to furnish in
almost quantitative yield a 1.3:1 mixture of dimethylated adducts
1
and 2, which were easy to separate using conventional
chromatographic methods. Structural clarification of the new
substances followed from their physical and spectroscopic proper-
ties, including 1D and 2D NMR studies, IR, UV, and LR-EIMS
measurements. Compounds 1 and 2 were highly soluble in CDCl3
and practically inert under these conditions, which made them
suitable for the determination of the configuration at C-17 using
VCD and ECD measurements. In the end, we chose 1 over 2 as
the best target for chiroptical studies since the former compound
was more abundant and both its molecular arrangement and
specific rotation data matched more closely those of natural
plakinidone.
In order to pursue the absolute configuration of the more
remote C-11 stereogenic center we next decided to synthesize 5,
a plakinidone derivative devoid of the five-membered lactone
(Scheme 1). This fragment possesses a single stereogenic center
and was expected to be impervious to air oxidation and accessible
by enantioselective total syntheses. After repeated failures, we
found that stirring natural plakinidone for 3 days, without any pre-
cautions to exclude air, led to an unstable mixture of products that
decomposed spontaneously to give a 1:1 mixture of 3 in 67% yield.
Subsequent degradation of 3 with LDA/THF afforded p-hydroxy-
phenyl ketone 4 in 73% yield, which upon treatment with diazo-
methane afforded the end product 5 in 72% yield. The specific
rotation for 5 was subsequently determined to be +8.6 (c 1.1,
CHCl3).
2. Results and discussion
2.1. Isolation of natural plakinidone
2.3. Enantioselective synthesis of plakinidone-derived fragment 5
Samples of a sponge consortium constituted by Plakortis hali-
chondrioides and Xestospongia deweerdtae specimens from Puerto
Rico were collected by members of our research team at a depth
of 90–100 ft.24 The crude extract of a small portion of the consor-
tium containing mainly Plakortis halichondrioides was extracted
with CHCl3–MeOH. After solvent removal, the dark gum obtained
was suspended in H2O and extracted exhaustively with n-hexane
The C1–C17 (including C22 and C23) fragment 5, obtained by
the deliberate decomposition of natural plakinidone (Scheme 1),
was synthesized as shown in Scheme 2. Commercially available
(S)-citronellol 6 was treated with CBr4/PPh3 to furnish the known
citronellyl bromide 7 in 63% yield. Alkylation of 4-ethylnylanisole
with 7 gave 8 (40%), which upon treatment with MMPP/MeOH
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C. Jiménez-Romero et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
3
Table 1
1H NMR and 13C NMR data for plakinidone from different sources
a
b
c
d
e
Proton
dH
Carbon (mult.)
dC
dC
dC
dC
1
2
3
6
7
8
9
6.95 (2H, d, J = 8.4 Hz)
6.67 (2H, d, J = 8.4 Hz)
1 (CH)
2 (CH)
3 (C)
6 (C)
7 (CH2)
8 (CH2)
9 (CH2)
10 (CH2)
11 (CH)
12 (CH2)
13 (CH2)
14 (CH2)
15 (CH2)
129.3 (2 Â C)
115.0 (2 Â C)
154.0
134.4
34.7
31.7
26.1
36.9
32.5
36.9
26.7
29.9
23.1
129.3 (2 Â C)
115.0 (2 Â C)
154.0
134.2
34.7
31.7
26.1
36.5
32.5
36.4
26.7
29.9
23.0
130.2 (2 Â C)
116.0 (2 Â C)
156.2
134.8
36.0
33.3
27.6
38.0
33.8
38.0
27.9
30.8
24.1
130.2 (2 Â C)
116.0 (2 Â C)
156.2
134.8
36.0
33.3
27.6
38.0
33.8
38.0
27.9
30.8
24.1
2.48 (2H, t, J = 7.6 Hz)
1.51 (2H, m)
1.25–1.20 (2H, br m)
1.28 (1H, m), 1.07 (1H, m)
1.36 (1H, m)
1.28 (1H, m), 1.07 (1H, m)
1.25–1.20 (2H, br m)
1.25–1.20 (2H, br m)
1.25–1.20 (1H, br m)
1.10 (1H, br m)
10a,b
11
12a,b
13
14
15a
15b
16
17
18
19
20
21
22
23
1.72 (2H, m)
16 (CH2)
17 (C)
18 (C)
19 (C)
20 (C)
21 (CH3)
22 (CH3)
23 (CH3)
36.6
83.9
178.0
95.5
176.2
5.7
36.6
83.9
178.4
95.2
176.3
5.8
37.5
85.1
181.1
95.5
178.1
5.9
37.5
84.9
179.5
96.4
177.5
5.9
1.64 (3H, s)
1.39 (3H, s)
0.82 (3H, d, J = 6.5 Hz)
23.0
19.8
23.1
19.8
23.7
20.1
23.7
20.1
a
b
c
Data (500 MHz, CD3OD) for natural plakinidone from this work.
Data (50 MHz, CDCl3) for natural plakinidone taken from Kushland, D. M.; Faulkner, D. J. J. Nat. Prod. 1991, 54, 1451–1454.
Data (125 MHz, CDCl3) for natural plakinidone from this work. Two drops of CD3OD were added to assist with sample solubilization.
Data (125 MHz, CD3OD) for natural plakinidone from this work.
d
e
Data (125 MHz, CD3OD) for the synthetic enantiomer of plakinidone, i.e., ent-12, taken from Xu, Z.-J.; Tan, D.-X.; Wu, Y. Org. Lett. 2015, 17, 5092–5095.
CH3
CH3
CH3
11
CH2N2 in Et2O
22
CH3
+
OH
r.t., 72h
92%
OCH3
O
CH3
CH3
OH
OCH3
OCH3
17
17
17
CH3
CH3
CH3
OCH3
O
O
O
O
O
1
(52%)
2
natural plakinidone (planar structure)
(40%)
20= -5.2 (c 1.6, CHCl )
[
20= +35.4 (c 1.1, CHCl3)
α
]
D
[
]
20= -13.1 (c 0.61, CH3OH)
α
α
[
]
D
3
D
air oxidation
stir in CHCl3-MeOH (1:1)
for 3 days
67%
23
CH3
CH3
11
1) LDA in THF
0oC to r.t., 2h (73%)
O
O
CH3
2) CH2N2 in Et2O
r.t., 8h (72%)
22
17 CH3
OH
OH
OR
19
O
CH3
O
20 = +8.0 (c 1.6, CHCl )
4
5
α
α
R = H
R = CH3
[
[
]
]
D
3
3
20 = +8.6 (c 1.1, CHCl3)
D
(1:1 epimers)
Scheme 1. Derivatizations of natural plakinidone into 1–5 and the specific rotation data of the end products.
afforded 9 in 95% yield as a 1:1 mixture of epoxides. Subsequent
treatment with NaIO4/THF/H2O led to the desired aldehyde 10 in
reaction with methyl vinyl ketone. Finally, hydrogenation over
Pd/C afforded the end product (11S)-5 in 42% yield; the specific
75% yield, which was further transformed into b,
c
-unsaturated
rotation [
[a]
D
a]
20 = +7.3 (c 1.1, CHCl3), was compatible with the
D
ketone 11 (71%) using an In/InCl3-mediated cross-coupling
20 = +8.6 (c 1.1, CHCl3) value for 5 (Scheme 1) obtained from
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4
C. Jiménez-Romero et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
CH3
CH3
CH3
CH3
CH3
n-BuLi
4-ethynylanisole
MMPP
R
CH3
THF, reflux
40%
MeOH
95%
H
8
(S)-(-)- 6 R = OH
CBr4
PPh3
63%
H3CO
(S)-(-)- 7 R = Br
CH3
O
CH3
CH3
1) methyl vinyl ketone,
In/InCl3
THF-H2O
CH3
NaIO4
THF-H2O
O
H
H
2) HCl (1.0M)
71%
75%
H3CO
9
10
H3CO
CH3
11
)
CH3
11
)
(
S
(
R
CH3
O
H2, Pd/C
CH3
acetone
42%
OCH3
17 CH3
O
OCH3
17 CH3
O
11
H3CO
(11S)-5
]
20= +7.3 (c 1.1, CHCl3)
D
(11R)-5
[
]
20= -7.8 (c 1.1, CHCl )
α
[
α
D
3
Scheme 2. Synthesis of the (11S)-enantiomer of plakinidone product 5.
natural plakinidone. Furthermore, enantiomer (11R)-5 (Scheme 2),
Conflex25–27 and ComputeVOA,28 were used with the MMFF94s
force field and the 5 kcal/mol energy window. However, the
molecular mechanics favor chain bending conformers whereas
the aliphatic chains adopt rather a not-bent zig-zag structure.
Therefore, additional linear chain conformers were generated by
synthesized from commercially available (R)-citronellyl bromide in
20
a parallel way (see the Supplementary data), showed a [
a
]
value
D
of À7.8 (c 1.1, CHCl3), thus confidently establishing that natural
plakinidone must have an (11S)-absolute configuration.
freezing the v2–v10 chain dihedral angles and allowing the rotation
2.4. Assignment of the configuration of the C17 center of
along the remaining single bonds. The obtained conformers were
further optimized by using the DFT hybrid Becke three-parameter
Lee–Yang–Parr (B3LYP) functional,29,30 the aug-cc-pVDZ basis set31
and polarizable continuum model (PCM), which accounted for the
solvent effect.32,33 The relative abundances were calculated based
plakinidone derivative
1 by combined experimental and
theoretical analysis of ECD and VCD spectra
2.4.1. Conformational analysis
Natural plakinidone derivative 1 contains two stereogenic C11
and C17 carbon centers. The configuration of the C11 center was
assigned through synthetic studies as (S). To assign the C17 center
configuration, the experimental VCD and ECD spectra of 1 were
measured in CH3CN and compared with the calculated ones. At
the beginning, the conformational analysis of the (11S,17S)- and
(11S,17R)-epimers of 1 was explored through systematic rotation
on the
The most stable structures of the (11S,17S)- and (11S,17R)-
conformers of 1 are of zig-zag chain (the v1 v10 dihedral angles
are almost 180°, Table 1SD), however one of them, 11S,17S_c5,
has the chain bent (Fig. 3). The main geometrical differences
between the conformers come from orientation of the furan-
2-one ring and the methoxy group in the benzene moiety (defined
by the v1 and v12 dihedral angles, respectively, Table 1SD). The
Cartesian coordinates of the most stable systems are shown in
Tables 2SD and 3SD.
DG298 values relative to the most stable conformer.
–
about the single bonds by changing v1–v13 and five methyl groups
dihedral angles (Fig. 2). With this aim, two independent programs,
2.4.2. Assignment of the C17 center configuration based on the
ECD and VCD spectra
The experimental VCD and ECD spectra of 1 were measured in
CH3CN and compared with the calculated ones (Fig. 4). The VCD
spectrum of each individual conformer was calculated at the
B3LYP/aug-cc-pVDZ/PCM(CH3CN) level. The final spectra accounts
for the room temperature Boltzmann weights (DG298) of the most
stable conformers of the (11S,17S)- and the (11S,17R)-epimers of 1
(Fig. 3 and Table 4SD) and are converted to Lorentzian bands with a
4 cm–1 half-width at half-peak height. On the other hand, the
Figure 2. The conformational analysis of 1 explored by changing the indicated
v1–v13 dihedral angles and rotation of the five methyl groups.
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C. Jiménez-Romero et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
(11S,17S)-epimer conformers
5
11S,17S_c1 (0.0; 35.4%)
11S,17S_c2 (0.02; 34.2%)
11S,17S_c4 (1.11; 5.4%)
11S,17S_c3 (0.31; 20.9%)
11S,17S_c5 (1.28; 4.1%)
(11S,17R)-epimer conformers
11S,17R_c1 (0.0; 40%)
11S,17R_c3 (0.40; 21%)
11S,17R_c2 (0.39; 21%)
11S,17R_c4 (0.80; 10%)
11S,17R_c5 (1.37; 4%)
11S,17R_c6 (1.37; 4%)
Figure 3. The structures of the most stable 11S,17S and 11S,17R epimers of 1. In parentheses the relative to the most stable conformer Gibbs free energies (kcal/mol) and
population factors (including only the depicted systems) calculated at the B3LYP/aug-cc-pVDZ/PCM(CH3CN) theory level.
theoretical ECD and UV spectra were simulated with the time-
dependent DFT (TD-DFT) method, which represents a good balance
between accuracy and computational efficiency.34 The Coulomb-
attenuating CAM-B3LYP35 functional was chosen due to its
proper population factors, which is directly related with the
computational level used. For such a conformationally flexible
system as 1, the B3LYP/aug-cc-pVDZ/PCM(CH3CN) level was a com-
promise between accuracy and computational cost and efficiency.
It should be noted that in the case of analysis of the chiroptical
spectra of the studied system, a rotation of the CH3 group in the
furan-2-one moiety by ca. 30° affects the ECD spectra above
200 nm and VCD spectra in the 1800–1400 cmÀ1 range (Fig. 3SD).
In the ECD spectrum, this manifests in the couplet bands of the
opposite signs at ca. 215 nm in the two, CH3 non-rotated and
rotated, structures respectively. Also, in the calculated VCD
spectrum, opposite sign bands occur in the 1800–1400 cmÀ1 range.
Thus, in order to avoid misinterpretation of the C17 center config-
uration assignment, only the ECD spectrum below 200 nm and
VCD spectrum in the 1400–1000 cmÀ1 range were considered.
The comparison of the experimental and calculated ECD and VCD
spectra is shown in Figure 4, while the calculated numerical data
are gathered in Tables 5SD–7SD.
documented superiority over B3LYP functional.36
A Gaussian
band-shape was applied with 0.30 eV as a half-height width and
the calculated UV–Vis and ECD spectra were shifted by the value
of difference of maximum absorption of the experimental and
calculated UV–Vis spectra.37 All calculations were performed by
using the Gaussian 09 package of programs.38
Previously, analysis of the enone properties has been shown to
be intricate due to conformation complexity and UV transitions
composed of multiple (not pure) excitations.39,40 This also holds
true in the case of the studied molecule. Comparison of the ECD
spectra of individual conformers shows that there is no character-
istic band (with the unchanged position and sign), which could be
used for the C17 center configuration assignment (Figs. 1SD and
2SD). Moreover, the calculated ECD spectra of some (11S,17R)-
conformers are close mirror images of the (11S,17S)-epimer
structures (for example 11S,17R_c1 vs 11S,17R_c3 or 11S,17R_c2
vs 11S,17R_c3, Fig. 1SD). This also holds true for some of the VCD
bands, yet the differences are not so striking (Fig. 1SD). Therefore,
the reliable simulation of the weighted spectrum depends on the
The experimental and simulated UV spectra of the (11S,17R)-
and (11S,17S)-epimers show high compliance in the position
and intensity of the bands at ca. 195 and 225 nm. The third
experimental UV band at ca. 280 nm is not reproduced by the
calculations properly. The simulated ECD spectra are reliable in
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6
C. Jiménez-Romero et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
Figure 4. Comparison of the experimental (red) and DFT/aug-cc-pVDZ/PCM(CH3CN) population-weighted spectra of 1 measured in CH3CN.
synthetic plakinidones
natural plakinidone
HO
HO
HO
CH3
CH3
CH3
11
11
11
(R)
(R)
CH3
)
17
O
(S)
CH3
( )
R
17
O
(
S
CH3
)
17
O
(
R
H3C
H3C
H3C
O
O
O
OH
OH
OH
12
13
12
(11R,17S)-plakinidone ent-
(11R,17R)-plakinidone
(11S,17R)-plakinidone
[
]
26 = +1.2
[
]
26= -4.4
D
[
20 = -13.1
α
]
D
α
α
D
Figure 5. Structures for natural plakinidone 12 (herein), synthetic plakinidone stereoisomers ent-12 and 13 (Wu’s work) and the specific rotation data measured at c = 0.61 in
MeOH after degassing with N2 to exclude unwanted air oxidation.
the 170–200 nm range where two Cotton effects (CE) at ca. 188 and
198 nm were registered. The ECD experimental bands are better
reflected by the simulated spectrum of the (11S,17R)-epimer since
the bands are of the same sign. The ECD CEs of the (11S,17S)-
epimer are predicted to be of the opposite sign to the measured
one. Thus, based on the ECD method, the (R)-configuration was
assigned to the C17 center.
The other chiroptical method, VCD, also confirms the (R)-config-
uration at the C17 atom (Fig. 4), yet the fitting is not so sticking as
for the ECD spectrum. Comparison between the experimental
and calculated VCD spectra was analyzed by using the Com-
pareVOA41 program and the confidence level of the (11S,17R)-
population-weighted VCD spectrum was equal to 71% when the
1400–1000 cmÀ1 spectral range was applied.
3. Conclusion
In spite of the air-sensitive nature of natural plakinidone and its
synthetic congeners, we have demonstrated that the absolute
configuration assignment could be achieved unambiguously by
transforming the natural product into very stable derivatives
that are amenable to chiroptical spectral analysis (i.e. 1) and
enantioselective syntheses (i.e. 5). From the evidence gathered,
we have unambiguously established the absolute configuration of
natural plakinidone as (11S,17R). Thus, we propose that natural
plakinidone is accurately represented by structure 12 (Fig. 5).
Coincidentally, one of the two pure stereoisomers synthesized by
Wu et al. (i.e. ent-12) is enantiomeric with our proposed structure
for natural plakinidone (the other one being 13, which is clearly
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C. Jiménez-Romero et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
7
diastereomeric with 12).42 As expected, the absolute configuration
of ent-12 is opposite according to the sign of the specific rotation,
although the apparent specific rotation is too small in magnitude.
Barring substantial deviations due to unsuspecting air oxidation
or the diverging temperatures (i.e. 20 °C vs 26 °C), the fact that
the apparent value +1.2 for synthetic ent-12 deviates from the
À13.1 value for natural plakinidone 12 by a factor of about 10,
can only be explained by human error or the use of improperly
(3 Â 2 L). Concentration of the CHCl3 extract under reduced pres-
sure yielded 8.0 g of a dark brown oil, which was chromatographed
over C18 reversed-phase silica gel (100 g) using mixtures of
MeOH–H2O of increasing polarity (70–100%). A total of 10 fractions
(I–X) were generated on the basis of TLC and 1H NMR analyses.
Careful scrutiny of the combined spectroscopic data revealed that
fractions V and VI consisted of pure plakinidone (926 mg, 0.23%
yield). Except for the specific rotation data, our isolate proved
identical with material isolated by Faulkner with regards to all
analytical and spectroscopic data.1 After degassing the solvent by
bubbling N2 (to avoid unwanted air-oxidation), the specific rota-
tion of our natural plakinidone was determined to be À13.1
(c 0.61, CH3OH), while the corresponding data for Faulkner’s
sample (most likely recorded without taking any precautions to
exclude air) was +7.9 (c 0.61, CH3OH).
calibrated instrumentation during [
a
]
determinations (Fig. 5).
D
Notwithstanding, the observation that the 13C NMR spectra of
these compounds in CD3OD are nearly identical (Table 1) supports
our contention that they are indeed enantiomers of one another.43
4. Experimental
4.1. General
4.3. Methylation of natural plakinidone and purification of
methylated products 1 and 2
Optical rotations were measured in CHCl3 or CH3OH at 589 nm
with a Rudolph Research Analytical Autopol IV using a 10-cm
microcell. Infrared and UV spectra were measured with a Nicolet
To a fresh solution of natural plakinidone (80 mg, 0.214 mmol)
in CHCl3 (5 mL) was added a solution of diazomethane in ether
(25 mL), and the resulting mixture was stirred at 25 °C for 72 h.
The reaction mixture was concentrated in vacuo, and the oily
residue was chromatographed by silica gel (4.0 g) column
chromatography eluting with an increasing polarity of CHCl3–n-
Hex (1:1 ? 6:4) to yield pure compounds 1 (44.5 mg, 52% yield)
and 2 (36 mg, 40% yield), respectively.
Magna 750 FT-IR and
a Shimadzu UV-2401 PC UV–Visible
spectrophotometer, respectively. 1D and 2D NMR spectral data
were generated with a 500 MHz (Bruker DRX-500) and 700 MHz
(Bruker AscendTM 700) FT-NMR spectrometers. Chemical shifts are
expressed in ppm and were referenced to the solvent residual peak
as internal standard: CD3OD (dH 3.30; dC 49.00) or CDCl3 (dH 7.26;
dC 77.00). Multiplicities in the 1H NMR spectra are described as
follows: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet,
and br = broad, and coupling constants are reported in Hertz. 13C
4.3.1. Derivative 1
Colorless oil; [
a]
20 = À5.2 (c 1.6, CHCl3); UV (CH3OH) kmax (log
e)
D
NMR multiplicities were obtained from a DEPT-135 experiment.
226 (4.31), 201 (4.13) nm; IR (thin film) mmax 2929, 2856, 1877
2,3
HMBC parameters were optimized for
J
CH
= 6 and 8 Hz. Mass
(weak), 1751, 1671, 1512, 1460, 1315, 1246, 1037, 986, 820,
spectrometric analyses were performed at the Mass Spectrometry
Laboratory, School of Chemical Sciences, University of Illinois with
a Waters VG 70-VSE (EI+, 70 eV) and a Waters Q-TOF Ultima (ESI+,
50/50 H2O/CH3CN with 0.1% formic acid). GC–MS analyses were
recorded at 70 eV using a Hewlett Packard 5890 MS Chem Station
coupled with a Hewlett Packard 5972 mass detector. The GC was
equipped with a 30 m  0.25 mm special performance capillary
column (HP-5MS) of polymethyl siloxane crosslinked with 5%
phenyl methylpolysiloxane. Column chromatography was per-
formed either on C18 reversed-phase silica gel (35–75 mesh) or
normal-phase silica gel (35–75 mesh). TLC analyses were carried
out using glass C18 reversed-phase silica gel plates. Spots were
visualized by exposure to I2 vapors or heating the plates sprayed
with phosphomolybdic acid (PMA) solution in EtOH. Commercially
available reagents were purchased and used as received unless
stated otherwise. Diazomethane was prepared from DiazaldÒ as
previously described.44 All of the reactions requiring anhydrous
conditions were conducted in flame-dried glass apparatus under
an argon atmosphere. All solvents used were spectral grade and
freshly distilled prior to use.
762 cmÀ1 1H NMR (CDCl3, 500 MHz) d 7.09 (2H, d, J = 8.4 Hz,
;
H-1), 6.82 (2H, d, J = 8.4 Hz, H-2), 4.10 (3H, s, OCH3), 3.78 (3H, s,
OCH3), 2.54 (2H, t, J = 7.6 Hz, H2-7), 1.71–1.62 (2H, m, H2-16),
2.00 (3H, s, H3-21), 1.54 (2H, m, H2-8), 1.38 (3H, s, H3-22), 1.34
(1H, m, H-11), 1.25 (9H, br envelope, H-15a, H2-14, H2-13, H-12a,
H-10a, H2-9), 1.08 (3H, m, H-15b, H-12b, H-10b), 0.82 (3H, d,
J = 6.5 Hz, H3-23); 13C NMR (CDCl3, 125 MHz) d 176.0 (C, C-18),
174.3 (C, C-20), 157.5 (C, C-3), 135.0 (C, C-6), 129.2 (2 Â CH, C-1),
113.6 (2 Â CH, C-2), 96.0 (C, C-19), 83.0 (C, C-17), 58.8 (OCH3),
55.2 (OCH3), 36.9 (CH2, C-16), 36.8 (CH2, C-12), 36.8 (CH2, C-10),
35.0 (CH2, C-7), 32.6 (CH, C-11), 32.0 (CH2, C-8), 29.8 (CH2, C-14),
26.8 (CH2, C-13), 26.7 (CH2, C-9), 23.5 (CH3, C-22), 23.1 (CH2,
C-15), 19.6 (CH3, C-23), 8.4 (CH3, C-21); LR-EIMS m/z (relative inten-
sity) 402 (8) [M+], 347 (6), 177 (27), 149 (100), 121 (32), 55 (65).
4.3.2. Derivative 2
Colorless oil; [
(log ) 264 (3.93), 224 (3.71), 201 (4.86) nm; IR (thin film) mmax
2928, 2856, 1701, 1612, 1512, 1477, 1395, 1369, 1246, 1198,
1037, 820 cmÀ1 1H NMR (CDCl3, 500 MHz)
7.09 (2H, d,
a]
20 = +35.4 (c 1.10, CHCl3); UV (CH3OH) kmax
D
e
;
d
J = 8.4 Hz, H-1), 6.82 (2H, d, J = 8.4 Hz, H-2), 4.00 (3H, s, OCH3),
3.78 (3H, s, OCH3), 2.54 (2H, t, J = 7.6 Hz, H2-7), 1.74 (2H, m,
H2-16), 1.58 (3H, s, H3-21), 1.55 (2H, m, H2-8), 1.39 (3H, s,
H3-22), 1.31 (1H, m, H-11), 1.25 (10H, br envelope, H2-15, H2-14,
H2-13, H-12a, H-10a, H2-9), 1.08 (2H, m, H-12b, H-10b), 0.82
(3H, d, J = 6.5 Hz, H3-23); 13C NMR (CDCl3, 125 MHz) d 201.0 (C,
C-18), 179.3 (C, C-20), 157.6 (C, C-3), 135.0 (C, C-6), 129.2
(2 Â CH, C-1), 113.6 (2 Â CH, C-2), 93.0 (C, C-19), 87.4 (C, C-17),
55.6 (OCH3), 55.2 (OCH3), 36.3 (CH2, C-16), 37.0 (CH2, C-10), 36.8
(CH2, C-12), 35.0 (CH2, C-7), 32.6 (CH, C-11), 32.1 (CH2, C-8), 29.9
(CH2, C-14), 26.8 (CH2, C-13), 26.7 (CH2, C-9), 23.1 (CH2, C-15),
23.0 (CH3, C-22), 19.6 (CH3, C-23), 4.0 (CH3, C-21); LR-EIMS m/z
(relative intensity) 402 (31) [M+], 253 (3), 211 (5) 177 (9), 155
(26), 149 (35), 142 (73), 121 (100).
4.2. Isolation of natural plakinidone
Fresh specimens of the sponge Plakortis halichondrioides
(Wilson, 1902) (phylum Porifera; class Demospongiae; subclass
Homoscleromorpha; order Homosclerophorida; family Plakinidae)
in association with Xestospongia deweerdtae were collected by
hand using scuba at depths of 90–100 ft off Mona Island, Puerto
Rico.24 The organism was frozen and lyophilized prior to extrac-
tion. The dry specimens (395 g) were cut into small pieces and
blended in a mixture of CHCl3–MeOH (1:1) (11 Â 1 L). After
filtration, the crude extract was concentrated and stored under
vacuum to yield a dark gum (100 g), which was suspended in
H2O (2 L) and extracted with n-hexane (3 Â 2 L) and CHCl3
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8
C. Jiménez-Romero et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
4.4. Air oxidation of natural plakinidone
26.5 (CH2, C-9), 23.9 (CH2, C-15), 19.7 (CH3, C-23); LR-EIMS m/z
(relative intensity) 290 (21) [M+], 185 (21), 107 (100), 91 (29).
A fresh solution of natural plakinidone (40 mg, 0.107 mmol) in a
1:1 mixture of CHCl3–MeOH (5 mL) was stirred in an open round
flask for 3 days. The solvent was then evaporated and the product
was purified by silica gel flash chromatography eluting with
CHCl3–MeOH (9:1). The product obtained 3 (28 mg, 67% yield)
consisted of a 1:1 mixture of C-19 epimers. Further purifica-
tion through a short plug of silica gel (0.8 g) using mixtures of
CHCl3–MeOH (98:2; 96:4; 92:6) allowed these epimers, 3a
(10 mg) and 3b (12 mg), to be completely separated.
4.6. Methylation of p-hydroxyphenyl ketone
diazomethane
4
with
To a solution of p-hydroxyphenyl ketone 4 (20 mg, 0.069 mmol)
in CHCl3 (2 mL) was added a solution of diazomethane in ether
(10 mL), and the resulting mixture was stirred at 25 °C for 8 h.
The reaction mixture was then concentrated in vacuo and the oily
residue obtained was purified by silica gel (0.7 g) column chro-
matography with an isocratic mixture of CHCl3–n-hexanes (7:3)
to afford pure compound 5 (15 mg, 72% yield). Colorless oil;
4.4.1. Epimeric alcohol 3a
Colorless oil; [
a
]
20 = À20.1 (c 1.0, CHCl3); IR (thin film) mmax
[
a
]
20 = +8.6 (c 1.1, CHCl3); UV (CH3OH) kmax (log
D
e
) 224 (4.03),
200 (4.15) nm; IR (thin film) mmax 2927, 2854, 1717, 1613, 1584,
1513, 1464, 1359, 1300, 1246, 1171, 1038, 820 cmÀ1 1H NMR
D
3406, 2928, 2855, 1800, 1759, 1614, 1597, 1515, 1456, 1377,
1218, 1114, 954, 828, 758 cmÀ1
;
1H NMR (500 MHz, CDCl3) d
;
7.03 (2H, d, J = 8.4 Hz), 6.75 (2H, d, J = 8.4 Hz), 2.53 (2H, t,
J = 7.4 Hz), 1.86–1.80 (2H, m), 1.59 (3H, s), 1.55 (3H, s), 1.54 (2H,
m), 1.36–1.16 (2H, m), 1.33 (1H, m), 1.23 (8H, m), 1.06 (2H, m),
0.81 (3H, d, J = 6.6 Hz); 13C NMR (125 MHz, CDCl3) d 210.7 (C),
173.7 (C), 153.4 (C), 135.0 (C), 129.4 (2 Â CH), 115.1 (2 Â CH),
90.4 (C), 69.7 (C), 38.1 (CH2), 36.7 (CH2), 36.6 (CH2), 34.9 (CH2),
32.6 (CH), 31.7 (CH2), 29.8 (CH2), 26.5 (CH2), 26.3 (CH2), 23.8
(CH3), 23.3 (CH3), 23.2 (CH2), 19.8 (CH3); LR-EIMS m/z (relative
intensity) 390 (12) [M+], 372 (6), 318 (12), 304 (17), 291 (24),
290 (100), 275 (6), 232 (12), 133 (25).
(CDCl3, 500 MHz) d 7.09 (2H, d, J = 8.4 Hz, H-1), 6.82 (2H, d,
J = 8.4 Hz, H-2), 3.79 (3H, s, OCH3), 2.54 (2H, t, J = 7.6 Hz, H2-7),
2.41 (2H, t, J = 7.4 Hz, H2-16), 2.13 (3H, s, H3-22), 1.56 (4H, m,
H2-15, H2-8), 1.36 (1H, m, H-11), 1.28 (2H, m, H-12a, H-10a),
1.26 (6H, br envelope, H2-14, H2-13, H2-9), 1.10 (2H, m, H-12b,
H-10b), 0.83 (3H, d, J = 6.5 Hz, H3-23); 13C NMR (CDCl3, 125 MHz)
d 209.4 (C, C-17), 157.5 (C, C-3), 135.0 (C, C-6), 129.2 (2 Â CH,
C-1), 113.6 (2 Â CH, C-2), 55.2 (OCH3), 43.8 (CH2, C-16), 36.8
(2 Â CH2, C-12 and C-10), 35.1 (CH2, C-7), 32.6 (CH, C-11), 32.1
(CH2, C-8), 29.9 (CH3, C-22), 29.5 (CH2, C-14), 26.8 (CH2, C-13),
26.7 (CH2, C-9), 23.9 (CH3, C-15), 19.6 (CH3, C-23); LR-EIMS m/z
(relative intensity) 304 (20) [M+], 155 (6), 121 (100), 107 (6), 91
(15); HR-EIMS calcd for C20H32O2: 304.2402, found: 304.2398.
4.4.2. Epimeric alcohol 3b
Colorless oil; [
a
]
20 = À23.1 (c 1.2, CHCl3); IR (thin film) mmax
D
3405, 2928, 2855, 1799, 1758, 1614, 1515, 1457, 1377, 1285,
1115, 952, 828, 764, 668 cmÀ1
;
1H NMR (500 MHz, CDCl3) d 7.04
4.7. Bromination of (S)-(À)-citronellol 6 with CBr4/PPh3
(2H, d, J = 8.2 Hz), 6.75 (2H, d, J = 8.2 Hz), 2.52 (2H, t, J = 7.6 Hz),
1.82 (2H, m), 1.54 (3H, s), 1.54 (2H, m), 1.53 (3H, s), 1.40 (1H,
m), 1.34 (1H, m), 1.26 (9H, m), 1.06 (2H, m), 0.82 (3H, d,
J = 6.5 Hz); 13C NMR (125 MHz, CDCl3) d 210.1 (C), 173.6 (C),
153.5 (C), 135.5 (C), 129.6 (2 Â CH), 115.1 (2 Â CH), 90.5 (C), 69.3
(C), 37.3 (CH2), 36.7 (CH2), 36.6 (CH2), 34.9 (CH2), 32.6 (CH), 31.9
(CH2), 29.8 (CH2), 26.6 (CH2), 26.4 (CH2), 23.5 (CH2), 23.0 (CH3),
21.9 (CH3), 19.7 (CH3); LR-EIMS m/z (relative intensity) 390 (100)
[M+], 344 (54), 318 (42), 304 (23), 300 (40), 291 (27), 290 (88),
275 (26), 133 (96).
A solution of (S)-citronellol 6 (500 mg, 3.2 mmol) and CBr4 (1.60 g,
4.8 mmol) in CH2Cl2 (5 mL) was cooled to 0 °C. Small portions of PPh3
(1.26 g, 4.8 mol) were then added over 5 min with vigorous stirring.
The solution, which turned into a light red color, was stirred for
another 2 h at 25 °C after which time the reaction mixture was con-
centrated in vacuo. The addition of n-hexane (50 mL) caused the pre-
cipitation of a white solid (Ph3PO), which was filtered through a short
column of silica gel (3.0 g) and rinsed with n-hexane (150 mL). Rotary
evaporation of the material eluted afforded the known (S)-citronellyl
bromide 7 as a pure colorless liquid (440 mg, 63% yield).
4.5. Synthesis of p-hydroxyphenyl ketone 4
4.8. Alkylation of 4-ethynylanisole with (S)-citronellyl bromide 745
A mixture of epimeric alcohols 3 (25 mg, 0.064 mmol) was
placed in a flame-dried flask and dissolved in THF (5 mL), cooled
to 0 °C, and then treated carefully with lithium diisopropylamide
(12.6 mg, 0.118 mmol, 2 equiv). The reaction was stirred at 0 °C
for 30 min, then warmed to 25 °C for 2 h. Shortly thereafter, the
reaction was quenched with water (10 mL), and the reaction
product extracted with EtOAc (3 Â 15 mL), and concentrated by
rotoevaporation. The crude oil obtained was passed through a
short plug of silica gel (1.0 g) using CHCl3–n-hexanes (9:1) to afford
A 0.4 M solution of 4-ethynylanisole in anhydrous THF (4.5 mL)
was treated with a 2.5 M solution of n-BuLi in n-hexanes (1.0 mL,
1.4 equiv) and added dropwise at 25 °C over 5 min before heating
at reflux for 3 h. The mixture was cooled to 25 °C before adding a
solution of (S)-citronellyl bromide 7 (393 mg, 1.8 mmol, 1.0 equiv)
in dry THF (4.5 mL). The reaction was refluxed for 12 h, cooled to
25 °C, quenched with H2O (10 mL), and extracted with EtOAc
(3 Â 25 mL). The combined organic extracts were dried (MgSO4),
filtered and concentrated to give a dark gum, which was purified
by silica gel (3.0 g) flash chromatography using 100% n-hexanes
pure compound 4 (13.5 mg, 73% yield). Colorless oil; [
(c 1.6, CHCl3); UV (CH3OH) kmax (log ) 224 (3.56), 200 (3.70) nm;
IR (thin film) mmax 3379 (br), 2926, 2854, 1706, 1614, 1515, 1462,
1375, 1262, 1171, 1112, 825 cmÀ1 1H NMR (CDCl3, 500 MHz) d
a]
20 = +8.0
D
e
to afford alkyne 8 (117 mg, 40% yield). Colorless oil; [
a]
20 = À11.6
D
;
(c 1.31, CHCl3); IR (thin film) mmax 2956, 2930, 2854, 2109, 1720,
7.03 (2H, d, J = 8.4 Hz, H-1), 6.75 (2H, d, J = 8.4 Hz, H-2), 2.53 (2H,
t, J = 7.6 Hz, H2-7), 2.41 (2H, t, J = 7.4 Hz, H2-16), 2.14 (3H, s,
H3-22), 1.55 (4H, m, H2-15, H2-8), 1.35 (1H, m, H-11), 1.26 (8H,
br envelope, H2-14, H2-13, H-12a, H-10a, H2-9), 1.09 (2H, m,
H-12b, H-10b), 0.82 (3H, d, J = 6.5 Hz, H3-23); 13C NMR (CDCl3,
125 MHz) d 209.6 (C, C-17), 153.5 (C, C-6), 135.1 (C, C-6), 129.4
(2 Â CH, C-1), 115.1 (2 Â CH, C-2), 43.8 (CH2, C-16), 36.8 (CH2,
C-12), 36.7 (CH2, C-10), 35.0 (CH2, C-7), 32.6 (CH, C-11), 31.9
(CH2, C-8), 29.8 (CH3, C-22), 29.5 (CH2, C-14), 26.8 (CH2, C-13),
1607, 1510, 1500, 1463, 1441, 1378, 1288, 1247, 1173, 1054,
1021, 832, 812, 658 cmÀ1 1H NMR (500 MHz, CDCl3) d 7.33 (2H,
;
d, J = 8.8 Hz), 6.81 (2H, d, J = 8.8 Hz), 5.12 (1H, dt, J = 7.1, 1.4 Hz),
3.79 (3H, s, OCH3), 2.40 (2H, m), 2.01 (2H, m), 1.69 (3H, s), 1.64–
1.40 (2H, m), 1.62 (3H, s), 1.64 (1H, m), 1.40–1.20 (2H, m), 0.93
(3H, d, J = 6.6 Hz); 13C NMR (125 MHz, CDCl3) d 158.9 (C), 132.8
(2 Â CH), 131.2 (C), 124.8 (CH), 116.3 (C), 113.8 (2 Â CH), 88.8
(C), 80.1 (C), 55.2 (OCH3), 36.7 (CH2), 35.9 (CH2), 31.7 (CH), 25.7
(CH3), 25.4 (CH2), 19.2 (CH3), 17.6 (CH3), 17.1 (CH2); GC–MS tR
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C. Jiménez-Romero et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
9
23.6 min (98.7%); MS (70 eV, EI): m/z (relative intensity) 270 (14)
[M+], 199 (12), 187 (13), 185 (15), 171 (14), 159 (18), 147 (34),
145 (79), 130 (17), 128 (16), 121 (60), 115 (48), 107 (23), 102
(55), 93 (18), 91 (36), 89 (16), 81 (18), 77 (28), 69 (100).
0.130 mmol, 0.5 equiv), and methyl vinyl ketone (66
l
L,
0.783 mmol, 3 equiv) in THF–H2O (4:1, 2 mL) was stirred at 25 °C
for 24 h. After the addition of 1 M HCl (1.0 mL), the reaction mixture
was stirred for 1.5 h and extracted with EtOAc (3 Â 15 mL). The com-
bined organic phase was washed with brine, dried (MgSO4), and con-
centrated under reduced pressure to afford a residue that was passed
through a short plug of silica gel (0.7 g) with a mixture of n-hexanes–
4.9. Epoxidation of compound
monoperoxyphthalate (MMPP)46
8
with magnesium
EtOAc (85:15) to yield b,c-unsaturated ketone 11 (40 mg, 71% yield).
To a solution of alkyne 8 (190 mg, 0.703 mmol, 1 equiv) in
MeOH (5.0 mL) was added MMPP at 80% (965 mg, 1.05 mmol,
1.5 equiv). After stirring at 25 °C for 6 h, the solvent was removed
under reduced pressure, the reaction mixture was diluted with sat-
urated NaHCO3 (15 mL), and extracted with EtOAc (3 Â 15 mL).
The combined organic extracts were dried (MgSO4), concentrated,
and purified by silica gel (2.0 g) flash chromatography eluting with
n-hexane–EtAOc (95:5) to afford a 1:1 mixture of the expected
epoxides 9 (191 mg, 95% yield). Colorless oil; IR (thin film) mmax
2957, 2927, 2855, 2361, 1727, 1607, 1510, 1463, 1379, 1289,
Colorless oil; [a D
]
20 = À13.0 (c 0.70, CHCl3); IR (thin film) mmax 2930,
2870, 2045, 1716, 1606, 1510, 1464, 1361, 1289, 1248, 1174, 1106,
1074, 1034, 833, 536 cmÀ1 1H NMR (500 MHz, CDCl3) d 7.31 (2H,
;
d, J = 8.8 Hz), 6.80 (2H, d, J = 8.8 Hz), 5.54 (2H, m), 3.79 (3H, s,
OCH3), 3.08 (2H, d, J = 5.2 Hz), 2.39 (2H, m), 2.12 (3H, s), 2.05 (2H,
m), 1.61 (1H, m), 1.61–1.41 (2H, m), 1.41–1.24 (2H, m), 0.92 (3H, d,
J = 6.5 Hz); 13C NMR (125 MHz, CDCl3) d 207.5 (C), 158.9 (C), 135.3
(CH), 132.8 (2 Â CH), 121.7 (CH), 116.2 (C), 113.8 (2 Â CH), 88.7 (C),
80.2 (C), 55.2 (OCH3), 47.6 (CH2), 36.0 (CH2), 35.8 (CH2), 31.5 (CH),
30.0 (CH2), 29.2 (CH3), 19.0 (CH3), 17.1 (CH2); LR-ESIMS m/z 299.2
[M+H]+, m/z 321.2 [M+Na]+.
1247, 1173, 1107, 1024, 1034, 832 cmÀ1 1H NMR (500 MHz,
;
CDCl3) d 7.32 (2H, d, J = 8.8 Hz), 6.80 (2H, d, J = 8.8 Hz), 3.79 (3H,
s, OCH3), 2.71 (1H, dt, J = 6.5, 2.1 Hz), 2.41 (2H, m), 1.67 (1H, m),
1.67–1.45 (4H, m), 1.55 (2H, m), 1.30 (3H, s), 1.27 (3H, s), 0.94
(3H, d, J = 6.5 Hz); 13C NMR (125 MHz, CDCl3) d 159.0 (C), 132.8
(2 Â CH), 116.1 (C), 113.8 (2 Â CH), 88.5 (C), 80.3 (C), 64.6/64.5
(CH), 58.3/55.2 (C), 55.2 (OCH3), 35.8/35.7 (CH2), 33.2/33.1 (CH2),
31.9/31.8 (CH), 26.4/26.3 (CH2), 24.9 (2 Â CH3), 19.1/19.0 (CH3),
18.7/18.6 (CH3), 17.1/17.0 (CH2); GC–MS: tR 24.9 min (98.0%); MS
(70 eV, EI): m/z (relative intensity) 286 (3) [M+], 188 (10), 187
(38), 172 (11), 159 (14), 158 (16), 147 (46), 146 (17), 145 (92),
134 (18), 128 (15), 121 (38), 115 (60), 102 (73), 91 (37), 77 (31),
69 (21), 63 (29), 57 (100); GC–MS tR 25.3 min (97.8%); MS
(70 eV, EI): m/z (relative intensity) 286 (3) [M+], 187 (11), 185
(15), 160 (30), 159 (27), 158 (22), 147 (18), 146 (15), 145 (100),
130 (21), 129 (15), 128 (19), 121 (50), 115 (56), 103 (17), 102
(77), 91 (27), 89 (20), 79 (11), 76 (30), 71 (83), 65 (16), 63 (24).
4.12. Hydrogenation of b,
ketone (11S)-5
c-unsaturated ketone 11 to methyl
A mixture of b,c-unsaturated ketone 11 (30 mg, 0.101 mmol)
and 10% Pd on charcoal in dry acetone (5 mL) was stirred under
H2 (1 atm) at 25 °C for 24 h. The reaction mixture was evacuated,
then filtered through a short pad of Celite and concentrated under
reduced pressure to give the crude extract. Purification through a
short plug of silica gel (0.5 g) with mixtures of n-hexanes–EtOAc
of increasing polarity (100:0; 98:2; 94:6) gave pure methyl ketone
(11S)-5 (13 mg, 42% yield). Colorless oil; [
IR (thin film)
a]
D
20 = +7.3 (c 1.1, CHCl3);
m
max 2928, 2856, 1719, 1612, 1513, 1464, 1376, 1359,
1246, 1177, 1122, 1073, 1038, 820 cmÀ1; GC–MS tR 26.0 min
(99.7%); MS (70 eV, EI): m/z (relative intensity) 304 (6.0) [M+],
134 (4), 122 (9), 121 (100), 91 (8), 78 (9), 77 (11). This synthetic
enantiomer proved identical with natural plakinidone-derived
fragment 5 (Scheme 1) in all analytical and spectroscopic regards.
4.10. Oxidation of 9 with sodium periodate (NaIO4)47
A mixture of epoxides 9 (190 mg, 0.664 mmol, 1 equiv) and NaIO4
(284 mg, 1.33 mmol, 2 equiv) in THF–H2O (1:1 v/v, 5 mL) was stirred
at 25 °C for 12 h. After diluting with H2O (15 mL), the aqueous layer
was extracted with EtOAc (3 Â 15 mL), the combined organic
extracts were washed with H2O and brine, dried (MgSO4), and con-
centrated under reduced pressure. The crude oil left was purified
by silica gel (2.0 g) column chromatography using n-hexanes–EtOAc
(90:10) to afford aldehyde 10 (121 mg, 75% yield). Colorless oil;
4.13. VCD measurements
The VCD and IR spectra of 1 dissolved in CH3CN were measured
using the ChiralIR-2X FT-VCD spectrometer from BioTools Inc. The
spectra were collected in the 2000–1000 cmÀ1 range at a resolu-
tion of 4 cmÀ1. The spectrometer was equipped with dual sources
and dual ZnSe photoelastic modulators (PEMs) optimized at
1400 cmÀ1. A solution with a concentration of ca. 0.14 M was
[a]D
20 = À5.8 (c 0.52, CHCl3); IR (thin film) mmax 2957, 2931, 2872,
measured in a BaF2 cell with a path length of 99.7 lm. The final
2721, 2361, 1725, 1607, 1510, 1464, 1380, 1289, 1247, 1173, 1106,
spectrum was averaged from 9 blocks, each of 2048 interferomet-
ric scans (1 block accumulated for 40 minutes). Baseline correction
using the spectrum of the relevant solvent obtained under the
same conditions was performed.
1073, 1034, 833 cmÀ1 1H NMR (500 MHz, CDCl3) d 9.79 (1H, t,
;
J = 3.5 Hz), 7.31 (2H, d, J = 8.8 Hz), 6.81 (2H, d, J = 8.8 Hz), 3.79 (3H,
s, OCH3), 2.47 (2H, m), 2.42 (2H, m), 1.72–1.50 (2H, m), 1.65 (1H,
m), 1.65–1.45 (2H, m), 0.94 (3H, d, J = 6.5 Hz); 13C NMR (125 MHz,
CDCl3) d 202.7 (CH), 159.0 (C), 132.8 (2 Â CH), 116.0 (C), 113.8
(2 Â CH), 88.2 (C), 80.4 (C), 55.2 (OCH3), 41.6 (CH2), 35.5 (CH2),
31.6 (CH), 28.5 (CH2), 18.9 (CH3), 17.1 (CH2); GC–MS tR 21.8 min
(99.5%); MS (70 eV, EI): m/z (relative intensity) 244 (10) [M+], 202
(7), 201 (4), 200 (5), 199 (5), 188 (20), 187 (33), 185 (12), 173 (11),
172 (11), 171 (5), 161 (3), 160 (9), 159 (23), 158 (17), 147 (29),
145 (100), 144 (15), 134 (20), 128 (18), 121 (40), 115 (61), 102
(85), 91 (36), 89 (28), 79 (19), 76 (36), 65 (20), 62 (42).
4.14. ECD measurements
The ECD and UV spectra of 1 in CH3CN (ꢀ1.7Á10À4 M) were
recorded in a quartz cell with a path length of 0.1 cm between
300 and 180 nm at 25 °C on a Jasco J-815 spectrometer. The spec-
trum was recorded using 100 nm/min scanning speed, a step size
of 0.2 nm, a bandwidth of 1 nm, a response time of 0.5 s, and an
accumulation of 10 scans. The spectra were background corrected
using respective solvents recorded under the same conditions.
4.11. In/InCl3-mediated cross-coupling reaction of 10 with
methyl vinyl ketone48
Acknowledgments
A mixture of aldehyde 10 (63.7 mg, 0.261 mmol, 1 equiv), In
(-100 mesh, 60 mg, 0.522 mmol, 2 equiv), InCl3 (30 mg,
We are grateful to Jan Vicente and José C. Asencio for collecting
the sponge, Christian Morales and Nashbly Montano for assistance
Please cite this article in press as: Jiménez-Romero, C.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.03.011

10
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during GC–MS analyses, and Yeiry M. Pérez for technical support
during the sponge extraction and isolation procedures. Dr. Marcin
Górecki from the Institute of Organic Chemistry, Polish Academy of
Sciences, is acknowledged for his very kind help with the ECD
measurements. The calculations were performed at the Interdisci-
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University of Warsaw (G19-4) and the PL-Grid Infrastructure.
Financial support to C.J.-R was provided by the IFARHU-SENACYT
Program of the Republic of Panama. This research was supported
by the NIH Grant 1SC1GM086271-01A1 awarded to A. D.
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of natural plakinidone, experimental details for the synthesis of
enantiomer (11R)-5, Figs. 1SD–5SD, Tables 1SD–7SD, and under-
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found, in the online version, at http://dx.doi.org/10.1016/j.tetasy.
2016.03.011.
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Please cite this article in press as: Jiménez-Romero, C.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.03.011