Oxidative processes in the australian marine sponge plakinastrella clathrata: Isolation of plakortolides with oxidatively modified side chains

Article abstract of DOI:10.1021/np200619q

Sixteen new cyclic peroxides (1-16) with a plakortolide skeleton and the methyl ester derivative of a didehydroplakinic acid (17) were isolated from the Australian sponge Plakinastrella clathrata Kirkpatrick, 1900. Structural elucidation and configurational assignments were based on spectroscopic analysis and comparison with data for previously isolated plakortolides and revealed both phenyl- and methyl-terminating side chains attached to the plakortolide core. Plakortoperoxides A-D (5-8) each contained a second 1,2-dioxine ring; a cis configuration for the side chain endoperoxide ring was determined by a low-temperature NMR study and by comparison of chemical shift values with those of reported compounds. An enantioselective HPLC study compared natural plakortoperoxide A with a synthetic sample prepared by cyclization of plakortolide P with singlet oxygen and revealed that the natural sample was a mixture of cis diastereomers at C-15/C18. Four other cyclic peroxides (9-12) possessed a C₉-truncated side chain terminating in a formyl or carboxylic acid functionality, suggesting that these metabolites may have been formed by oxidative cleavage of the Δ^9,10 bond of diene-functionalized plakortolides. A final group of four metabolites (13-16) with hydroxy or the rare hydroperoxy functionality unexpectedly revealed a C₈ side chain, while the ester (17) represents further structural variation within the growing family of cyclic peroxy sponge metabolites.

Full text of DOI:10.1021/np200619q

Article
pubs.acs.org/jnp
Oxidative Processes in the Australian Marine Sponge Plakinastrella
clathrata: Isolation of Plakortolides with Oxidatively Modified Side
Chains
Ken W. L. Yong,† Lynette K. Lambert,‡ Patricia Y. Hayes,† James J. De Voss,† and Mary J. Garson*,†
†School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia
‡Centre for Advanced Imaging, The University of Queensland, Brisbane, QLD 4072, Australia
S
* Supporting Information
ABSTRACT: Sixteen new cyclic peroxides (1−16) with a plakortolide
skeleton and the methyl ester derivative of a didehydroplakinic acid (17) were
isolated from the Australian sponge Plakinastrella clathrata Kirkpatrick, 1900.
Structural elucidation and configurational assignments were based on
spectroscopic analysis and comparison with data for previously isolated
plakortolides and revealed both phenyl- and methyl-terminating side chains
attached to the plakortolide core. Plakortoperoxides A−D (5−8) each
contained a second 1,2-dioxine ring; a cis configuration for the side chain
endoperoxide ring was determined by a low-temperature NMR study and by
comparison of chemical shift values with those of reported compounds. An
enantioselective HPLC study compared natural plakortoperoxide A with a
synthetic sample prepared by cyclization of plakortolide P with singlet oxygen
and revealed that the natural sample was a mixture of cis diastereomers at C-
15/C18. Four other cyclic peroxides (9−12) possessed a C9-truncated side chain terminating in a formyl or carboxylic acid
functionality, suggesting that these metabolites may have been formed by oxidative cleavage of the Δ9,10 bond of diene-
functionalized plakortolides. A final group of four metabolites (13−16) with hydroxy or the rare hydroperoxy functionality
unexpectedly revealed a C8 side chain, while the ester (17) represents further structural variation within the growing family of
cyclic peroxy sponge metabolites.
yclic peroxides are a distinctive suite of bioactive
metabolites isolated from marine sponges, notably those
have a C8 side chain terminating in either a hydroxy or the rare
hydroperoxy functionality. An additional ester metabolite (17),
named by us as a didehydroplakinic acid derivative, lacks the
lactone ring associated with the plakortolide core. The
biological activities associated with the various metabolites
will be reported separately.
C
from the genera Plakortis and Plakinastrella.1 Recently we
reported nine new plakortolide metabolites (K−S), each
possessing a Ph(CH2)n− side chain (n = 10 or 12), together
with their reduced (seco) derivatives and some stereochemically
related plakortone ethers, from the Australian marine sponge
Plakinastrella clathrata Kirkpatrick, 1900. In our study, we
addressed the relative and absolute configurations of the
metabolites and revealed that diastereomeric plakortolides in
this particular sponge specimen uniformly had a 6S
configuration and, thereby, differed in configuration at the C-
3/C-4 centers. We also provided evidence that plakortone
ethers derive from seco-plakortolides under mild conditions.2
We now describe an additional set of 16 plakortolide
metabolites (1−16) from the same sponge specimen. Three of
these plakortolides revealed a Ph(CH2)12− side chain, while
five others had a side chain either C15 or C17 in length and
terminating in a methyl group; additionally, four of these
metabolites had an endoperoxide functionality in the side chain.
The remaining eight plakortolides contained truncated side
chains apparently resulting from side chain oxidative processes.
These included two pairs of diastereomeric plakortolides that
have a C9 side chain terminating in either a carboxylic acid or
an aldehyde and two pairs of diastereomeric plakortolides that
RESULTS AND DISCUSSION
■
Structural and Stereochemical Studies. Fractions from
the CH2Cl2/MeOH (1:1) extract of P. clathrata2 were
subjected to NP-HPLC (hexanes/EtOAc) or to RP-HPLC
(CH3CN/H2O) to give metabolites 1−17.
Plakortolides. Plakortolide T (1) was isolated as a colorless
oil and had a formula of C26H38O4 inferred from HR-ESIMS.
1
The metabolite showed H NMR and HSQC data (Tables 1
and 2) for a phenyl ring [δ H 7.32 (2H), 7.26 (2H), 7.16 (1H);
δC 138.4 (s), 128.5 (d), 126.8 (d), 126.0 (d)], two methyl
groups [δH 1.37 (s), 1.27 (s); δC 26.2, 22.4], one oxygenated
methine [δH 4.43 (d); δC 81.1], two methylenes with a
diagnostic AB signal pattern [δ H 2.89, 2.60; δC 34.3 and δH
Special Issue: Special Issue in Honor of Gordon M. Cragg
Received: July 25, 2011
Published: November 3, 2011
© 2011 American Chemical Society and
American Society of Pharmacognosy
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Journal of Natural Products
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Chart 1
2.15, 1.69; δC 40.6], and a cluster of methylenes, as well as
HMBC correlations, that were all consistent with a plakortolide
skeleton. In the side chain, there was an E-olefinic group [δH
6.35, 6.20 (J = 16.0 Hz); δC 129.8, 131.3] that was located next
to the phenyl ring by HMBC correlations from H-18 at δH 6.35
to C-19 (δC 138.4) and C-20 (δC 126.0). The relative
configuration was defined by consideration of the chemical
shifts for the bicyclic core that closely matched data for
plakortolide L (18) rather than for the diastereomeric
plakortolide K (19) (for 1 C-7, δC 41.0; C-24, δH 1.27; δC
22.4 vs 18: C-7, δC 40.8; C-24, δH 1.27; δC 22.2 or 19: C-7 δC
36.6; C-24, δ H 1.18; δC 24.4). NOESY data, notably
correlations between H-3/H-5b and H-5a/H-24, were fully
consistent with this relative configuration.2 Plakortolide T is
thus the 17,18-didehydro analogue of plakortolide L.
The related plakortolide U (2) of molecular formula
C26H36O4 by HR-ESIMS had an identical core to 1 from
1H/13C NMR data and differed only in the unsaturation pattern
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in the ω-phenyl-terminating C12 side chain. The signals for the
E-olefinic group of 1 were replaced by signals at δH 5.51 (dt, H-
15), 6.14 (t, H-16), 7.04 (dd, H-17), and 6.50 (d, H-18),
assigned as a 15Z,17E-diene from coupling constant data
(JH‑15/H‑16 = 11.5 Hz; JH‑17/H‑18 = 15.5 Hz); a chemical shift
value of δC 27.9 for C-14 agreed with the C-15 Z configuration.
Plakortolide U therefore had the opposite configuration of the
C-15/C-16 double bond compared to the previously reported
plakortolide P (20).2 The relative configuration was again
peroxy compounds, 7 and 8, could not be separated, either by
HPLC or by silver nitrate-based chromatography, and so were
characterized as a mixture.
The first diperoxide, named plakortoperoxide A (5), had a
molecular formula of C26H36O6 by HR-ESIMS and exhibited all
the anticipated NMR features associated with a plakortolide
core linked by an alkyl chain to a phenyl group. The trans
arrangement of the C-4 and C-6 methyl groups in 5 followed
from 13C NMR comparison with other plakortolides. However,
four additional signals, at δH 4.54 (m, H-15), 6.09 (dt, H-16),
1
deduced from the H and 13C NMR values for the C-6 methyl
1
group and for C-7. The absolute configuration shown for both
1 and 2 was selected on the basis that a 6S configuration had
previously been confirmed by Mosher analysis in four different
plakortolide metabolites that were representative of both
diastereomeric series.2 We thus considered it biosynthetically
reasonable that the absolute configuration would be consistent
among the entire group of phenyl-group-terminating plakorto-
lide metabolites isolated from the single specimen of P.
clathrata used in our study.
6.05 (dt, H-17), and 5.49 (q, H-18), were apparent in the H
NMR spectrum, and comparison of these chemical shifts with
those of model cyclic peroxides5,6 suggested a second 1,2-
dioxine moiety. The associated 13C NMR signals [δC 78.3 (C-
15), 128.7 (C-16), 126.2 (C-17), and 80.0 (C-18)] were
located using HSQC data, while HMBC correlations from H-20
to C-18 and from H-18 to δC 128.4 (C-20) positioned the
peroxy ring adjacent to the phenyl ring.
The relative configuration of the side chain endoperoxide
moiety was suggested as cis since the chemical shift value for H-
15 (δH 4.54) was close to that observed for the equivalent
position in cis-3,6-dipropyl-1,2-dioxa-4-cyclohexene (δ H 4.66)5a
or cis-3,6-dibutyl-1,2-dioxa-4-cyclohexene (δ H 4.44)5b and to
data for synthetic model peroxides prepared by us. The cis or
trans configuration of endoperoxides can be distinguished by
The sponge extract further contained a range of plakortolides
that did not possess a phenyl end group. The first of these was
plakortolide V (3), with a molecular formula of C23H42O4 from
1
HR-ESIMS, supporting a saturated C15 side chain. The H
NMR data revealed signals for a terminal methyl group (δ H
0.86, t) and a long-chain alkyl side chain, but in all other
respects the 1H/13C NMR and HMBC data confirmed a
plakortolide skeleton. The relative configuration of 3 followed
directly from comparison of the chemical shift values for C-7
and for the C-6 methyl group (here numbered as C-23) with
those of plakortolide L. NOESY data showed correlations
between H-3/H-5b and between H-5a/C-6Me as expected
given the trans arrangement of methyl groups. The absolute
configuration of 3 was determined by reductive cleavage of the
peroxy ring using H2/Pd/C to give diol 21, which was esterified
at C-3 as the (R)- and (S)-O-methyl mandelate (MPA) esters
22a/22b;3,4 the ΔδRS values were negative for H2-2 and
positive for H2-5, H2-7, H3-22, and H3-23, consistent with a 3S
configuration. Together with the relative configurational
information, this Mosher analysis confirmed 3 as 3S,4S,6S-
configured, thus belonging to the same configurational series as
the phenyl end group-functionalized plakortolides.
1
low-temperature H NMR study since in the cis stereoisomer
two half-chair conformations are possible, whereas in the trans
isomer a single half-chair conformer is favored (Figure 1).5 A
Figure 1. Half-chair conformations of cis- and trans-1,4-disubstituted
endoperoxides.
portion of the spectrum (25 °C) of 5 in acetone-d6 is shown in
Figure 2a; in contrast, at −87 °C (Figure 2b), two sets of NMR
signals in a ratio of ∼1:1 were apparent for H-15 and H-18 of 5,
consistent with the interconversion of two half-chairs for the
side chain endoperoxide ring. These pairs of NMR signals
coalesced at −60 °C. The spectrum at −87 °C also revealed
two sets of signals for H-2a/b, H-3, H-5a/b, and the 4- and 6-
methyl groups compared to the spectrum recorded at 25 °C.
The conformational implications for the plakortolide core are
currently under evaluation. Cyclization of plakortolide P (20),
isolated in our earlier study,2 with singlet oxygen in DCM at 5
°C in the presence of rose bengal bis(triethylammonium) salt
as photosensitizer (Scheme 1) led to a synthetic sample of
plakortoperoxide A (5) in 62% yield and with spectroscopic
data identical to natural material. The synthetic material could
be separated into the two diastereomers, 5a and 5b (1:1 ratio),
by enantioselective HPLC using a DAICEL Chiralpak AD
column with UV detection at 254 nm; this experiment served as
a standard for the stereochemical analysis of natural 5, which
then revealed that the naturally isolated material was likewise a
mixture (∼1:1) of the two diastereomers 5a and 5b. The two
Plakortolide W (4), with the molecular formula C23H38O4 by
HR-ESIMS, also had a C15 side chain (δH 0.89 (t, H-21)), but
differed from 3 in that side chain unsaturation was present.
Signals at δH 5.63 (dt, H-15), 6.27 (ddd, H-16), 5.93 (t, H-17),
and 5.28 (dq, H-18) were assigned to a 15E,17Z-diene from
coupling constant data (JH‑15/H‑16 = 15.2 Hz; JH‑17/H‑18 = 10.9
Hz). The 13C values for C-14 and C-19 further confirmed the
double-bond configurations (C-14: δC 33.0; C-19: δC 29.8). In
the HMBC spectrum, there were correlations from the terminal
methyl (H-21) to C-20 (δC 23.0) and C-19 (δC 29.8), from H-
18 to C-15 (δC 134.6), C-16 (δC 125.6), and C-20, and from
H-17 to C-19, while 1D-TOCSY data showed correlations
through H-20 and H-19 to H-18 upon irradiation of H-21. The
relative configuration of the bicyclic ring was again established
1
from the H and 13C NMR values for C-7 and the C-6 methyl
group. Plakortolide W thus has the opposite configuration for
each side chain double bond compared to plakortolide U.
Plakortoperoxides. Four metabolites isolated from the P.
clathrata extract each had an additional two oxygen atoms in
their molecular formula, suggestive of a second 1,2-dioxine ring.
Two of these metabolites, namely, 5 and 6, were successfully
purified by repetitive HPLC; however the remaining two
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1
Figure 2. H NMR spectra of plakortoperoxide A (5) in acetone-d6 at (a) 25 °C; (b) −87 °C.
Scheme 1. Preparation of Synthetic Plakortoperoxide A (5) as a Mixture (1:1) of Diastereomers 5a and 5b for Comparison with
Natural Material
1
individual diastereomers were named plakortoperoxides A1 and
A2, respectively.
C (7), the side chain endoperoxide was apparent from H
NMR signals at δH 4.46 (m, H-15), 5.90 (m, H-16), 5.84 (m,
H-17), and 4.84 (m, H-18) and was adjacent to an E-olefinic
moiety [δH 5.49 (m, H-19), 5.76 (dt, H-20); JH‑19/H‑20 = 15.3
Hz]. In the HMBC, there were cross-peaks from the terminal
methyl (δH 0.88, H-23) to C-22 (δC 21.7) and C-21 (δC 34.2),
from H-22 to C-20 (δC 137.0), and from H-21 to C-19 (δ C
126.0). Additional HMBC cross-peaks from H-20 to C-18 at δC
78.7 confirmed the location of the 1,2-dioxine moiety between
C-15 and C-18. The assignment was consistent with the
observed TOCSY spin system from H-23 to H-19. The
downfield value for C-18 in 7 of δC 78.7 compared to δC 74.4
in 6 supported the configurational assignments, although
unusually the opposite trend was noted in the proton values
(H-18 in 7 is δH 4.84 compared to δH 5.24 in 6). In
plakortoperoxide D (8), TOCSY correlations observed from H-
23 (δH 0.91) to H-22, H-21, and an oxymethine at δH 4.46 (H-
20) revealed that the endoperoxide ring and alkene moiety
were reversed compared to 7. The 15.3 Hz coupling between
H-15 and H-16, together with a chemical shift value of δC 32.2
for C-14, supported a 15E double bond. HMBC correlations
from both H-15 and H-16 to methylene carbons of the side
chain were in accordance with the structure of 8. A cis
configuration for the side chain endoperoxide was apparent
from the chemical shift values of H-15 (δ H 4.46) in 7 and of H-
20 (δH 4.46) in 8, while in each metabolite the plakortolide
core had a trans arrangement of the C-4 and C-6 methyl groups
by 13C NMR. Each plakortoperoxide was assumed to be a
diastereomeric mixture.
The second endoperoxide, named plakortoperoxide B (6),
had a molecular formula of C25H40O6 by HR-ESIMS and,
therefore, had a different carbon chain length from any of the
other plakortolides isolated from P. clathrata. Inspection of the
1H/13C NMR data revealed the absence of a phenyl ring;
instead signals at δH 0.90 for H-23 supported a methyl
terminus, and on the basis of the MS data, there was a C17 side
chain. 1D-TOCSY correlations from H-23 through H-22 and
from H-21 to H-20 (δH 5.66) and H-19 (δH 5.39) established a
C-19/C-20 olefinic group, assigned as Z from the 10.6 Hz
coupling constant. Signals at δH 4.42 (m, H-15), 5.90 (dt, H-
16), 5.79 (t, H-17), and 5.24 (brd, H-18) were for an
endoperoxide moiety that could be positioned by HMBC
correlations from H-20 to C-18 (δ C 74.5), together with
correlations from H-18 to C-20 (δ C 135.7) and to C-16 (δC
127.9) as adjacent to the Z-olefin. The cis configuration of the
side chain endoperoxide was inferred from chemical shift
comparison with H-15 of 5 and of model endoperoxides,5 while
the relative configuration of the plakortolide ring was again
inferred from a 13C NMR chemical shift comparison with co-
occurring plakortolides. Although the stereochemical homoge-
neity was not investigated by enantioselective HPLC,
plakortoperoxide B is also likely to be a mixture of C-15/C-
18-cis-diastereomers that could individually be named as
plakortoperoxides B1 and B2.
The remaining pair of plakortoperoxides was characterized as
a 1:1 mixture. A molecular formula of C25H40O6 was apparent
by HR-ESIMS, supporting a C17 side chain. In plakortoperoxide
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a b
,
1
Table 3. H NMR Assignments for Side Chain-Oxidized Plakortolides 9−16
position
2a
9
10
11
12
13
14
15
16
2.88, dd (18.5,
6.0)
2.89, dd (18.5,
6.1)
2.89, dd (18.5,
5.9)
2.89, dd (18.6,
6.2)
2.89, dd (18.5,
6.0)
2.89, dd (18.5,
6.0)
2.89, dd (18.5,
5.7)
2.89, dd (18.5,
6.2)
2b
3
2.53, brd (18.5) 2.59, brd (18.5) 2.54, brd (18.3) 2.60, brd (18.6) 2.53, brd (18.5) 2.60, brd (18.5) 2.54, brd (18.5) 2.60, brd (18.5)
4.45, d (6.0)
2.24, d (14.9)
1.63, d (14.9)
1.70, m
4.43, d (6.1)
2.14, d (14.8)
1.68, d (14.8)
1.51, m
4.46, d (5.9)
2.25, d (14.9)
1.63, d (14.9)
1.70, m
4.44, d (6.2)
2.15, d (14.9)
1.68, d (14.9)
1.47, m
4.45, d (6.0)
2.24, d (15.0)
1.63, d (15.0)
4.42, d (6.0)
2.16, d (15.0)
1.68, d (15.0)
4.45, d (5.7)
2.25, d (15.0)
1.64, d (15.0)
4.43, d (6.2)
2.15, d (14.5)
1.68, d (14.5)
1.49, m
5a
5b
7
c
c
c
n.d.
n.d.
n.d.
1.51, m
1.46, m
1.52, m
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
8
1.25, m
1.25, m
1.25, m
1.25, m
1.25, m
1.25, m
1.28, m
1.28, m
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
9
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.22−1.32, m
1.23−1.30, m
1.23−1.30, m
1.23−1.30, m
1.23−1.30, m
1.23−1.30, m
1.23−1.30, m
1.23−1.30, m
1.23−1.30, m
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
10
11
12
13
14
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
de
,
c
c
de
,
de
,
1.62, m
1.62, m
1.61, m
1.61, m
n.d.
n.d.
1.61, m
1.61, m
2.32, t (7.5)
2.32, t (7.5)
2.40, td (7.5,
1.8)
2.40, td (7.5,
1.8)
3.61, brt (6.5)
3.61, brt (6.5)
4.00, t (6.7)
4.00, t (6.7)
15
9.74, t (1.8)
1.36, s
9.74, t (1.8)
1.37, s
1.36, s
1.18, s
1.37, s
1.27, s
1.35, s
1.17, s
1.37, s
1.26, s
16
1.35, s
1.16, s
1.36, s
1.25, s
17
1.17, s
1.26, s
f
f
−OOH
7.32
7.33
a
b
c
d
Chemical shifts (ppm) referenced to CHCl3 (δH 7.24). Frequency at 500 MHz. Not detected, as obscured by other signals. Unresolved chemical
e
f
shifts due to overlapping signals. Signal multiplicity unresolved due to overlapping signals. Interchangeable.
a b
,
Table 4. 13C NMR Assignments for Side Chain-Oxidized Plakortolides 9−16
position
9
10
174.1
11
173.9
12
174.1
13
174.0
14
174.1
15
174.1
16
174.1
1
174.0
34.1
80.8
82.5
40.3
80.1
36.8
23.0
2
34.3
81.2
34.2
80.7
34.2
81.1
34.1
80.6
82.5
40.3
80.1
36.8
23.0
34.3
33.9
34.1
3
81.2
80.6
80.9
4
82.8
82.3
82.7
82.8
82.3
82.5
5
40.6
40.3
40.7
40.6
40.1
40.5
6
80.2
79.9
79.9
80.2
80.0
79.9
7
41.0
36.8
40.9
41.0
36.6
40.8
8
23.0
23.2
23.2
23.0
22.8
22.8
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
9
29.0−29.9
29.0−29.9
29.0−29.9
29.0−29.9
24.7
29.0−29.9
29.0−29.9
29.0−29.9
29.0−29.9
24.7
28.8−29.9
28.8−29.9
28.8−29.9
28.8−29.9
22.0
28.8−29.9
28.8−29.9
28.8−29.9
28.8−29.9
22.0
29.0−29.9
29.0−29.9
29.0−29.9
29.0−29.9
32.5
29.0−29.9
29.0−29.9
29.0−29.9
29.0−29.9
32.5
29.1−29.9
29.1−29.9
29.1−29.9
25.7
29.1−29.9
29.1−29.9
29.1−29.9
25.7
c
c
c
10
11
12
13
14
15
16
17
27.3
27.3
33.7
33.7
43.7
43.7
62.9
62.9
77.0
77.0
177.4
177.4
25.9
202.9
25.9
202.9
25.9
25.8
25.8
25.7
25.7
25.9
24.7
22.3
24.5
22.2
24.9
22.5
24.8
22.2
a
b
c
Chemical shifts (ppm) referenced to CDCl3 (δC 77.0). Frequency at 500 MHz. Unresolved chemical shifts taken from HSQC experiments.
Side Chain-Truncated Plakortolides. The final group of
plakortolide metabolites isolated from the Plakinastrella extract
all showed evidence of side chain oxidation. Following
repetitive RP-HPLC, four fractions were obtained, each of
which was a mixture of diastereomers. The first fraction, a 1:2
mixture of diastereomers 9 and 10 by NMR, had a formula of
C17H28O6 inferred from HR-ESIMS data, supporting a C9 side
chain, and the two individual structures were named
carboxyplakortolides-1 and -2. The second fraction, containing
diastereomers 11 and 12 in a 1:2 ratio, named formylplakorto-
lides-1 and -2, respectively, likewise had a C9 side chain from
MS data that corresponded to a molecular formula of
consistent with a plakortolide skeleton. The 1H NMR spectrum
of metabolites 9 and 10 revealed a downfield signal (δH 2.32)
that showed an HMBC signal to a carboxyl group (δ C 177.4).
Isomers 11 and 12 showed side chain NMR signals (11: δH
9.74; δC 202.9; 12: δH 9.74; δC 202.9) that were diagnostic of
an aldehyde and that showed HMBC correlations from an
adjacent downfield triplet proton signal (δ H 2.40 for both 11
1
and 12). Metabolites 9 and 11 showed similar H/13C NMR
data, respectively, for the bicyclic core as plakortolide K (9 and
11; C-7, δC 36.8 for each vs δC 36.6 for plakortolide K; C-6
Me; δH 1.16, δC 24.9 and δH 1.17, δC 24.8, respectively, vs δH
1.18; δC 24.2 for plakortolide K). Similarly the data for 10 and
12 matched those of plakortolide L (10 and 12; C-7, δC 41.0
and 40.9, respectively, vs δC 40.8 for plakortolide L; C-6 Me;
δH 1.25; δC 22.5 and δH 1.26; δC 22.2, respectively, vs δH 1.27;
δC 22.2 for plakortolide L).2 Inspection of NOESY data
1
C17H28O5. These four metabolites showed H/13C NMR data
(Tables 3 and 4) as well as HMBC correlations from H-3 (at
δH 4.45, 4.43, 4.46, and 4.44 in 9−12, respectively) to the
lactone C-1, to C-4, and to the C-4 Me, which were fully
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Journal of Natural Products
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revealed correlations between H-3/H-5b and H-5b/C-6Me for
9 and between H-3/H-5b and H-5a/C-6Me for 10, as expected
for cis and trans methyl-configured plakortolides, respectively.2
The two remaining fractions yielded sets of diastereomers
(13−16) each possessing a C8 side chain by HR-ESIMS data.
Diastereomers 13 and 14 (1:2 ratio), named hydroxyplakorto-
lides-1 and -2, had the molecular formula C16H28O5. NMR
signals at δH 3.61 linked to a carbon at δC 62.9 suggested a
hydroxy functionality. The pair of diastereomers 15 and 16 (3:5
ratio), with a molecular formula of C16H28O6, had an additional
oxygen atom compared to 13 and 14, implying a hydroperoxy
functionality. 1H NMR signals at δH 4.00 (t, J = 6.0 Hz) linked
to a carbon at δC 77.0 were observed in the side chain of both
15 and 16 and matched literature data for the terminal
methylene of primary alkyl hydroperoxides.7 There were also
two singlets at δH 7.32 and 7.33 that could be assigned to the
hydroperoxy protons.8 Reduction of the hydroperoxy group to
the corresponding alcohol was not attempted owing to the
small amount of material available and the presence of the
metal or Ph3P-sensitive plakortolide core.9 The two hydro-
peroxides were named hydroperoxyplakortolides-1 and -2,
respectively. These are the first examples of primary alkyl
hydroperoxides isolated from marine sponges, although we
note that a primary allylic hydroperoxide has been reported
functionalized acid isolated from the New Guinea sponge
Callyspongia sp.10
Biosynthesis. In our earlier publication,2 we presented a
biosynthetic route to the diastereomeric plakortolides that
involves nucleophilic attack of a 6S-hydroperoxy group onto C-
3 of an α,β-unsaturated carboxylic acid derivative. Attack of the
carboxylic group onto the carbocation derived from proto-
nation of the C-4/C-5 alkene then generates the bicyclic
system, with the stereochemical outcome at C-4 necessarily
linked to that at C-3 owing to the steric constraints of the fused
lactone. This biosynthetic proposal contrasts with the generally
accepted mechanism for endoperoxide formation involving
Diels−Alder addition of singlet oxygen to a diene. Experimental
studies have shown that Diels−Alder reactions with singlet
oxygen are not always concerted5a and that both E,E and E,Z
dienes generate cis adducts.5,6 Our detailed study of P. clathrata
has provided plakortolides with side chains containing E,E-
dienes (plakortolides O−R2) as well as an E,Z-diene in
plakortolides U (2) and W (4); thus plakortolide U (2) is a
plausible precursor of plakortoperoxide A (5), while each of
plakortoperoxides B−D presumably arises from a plakortolide
with a Δ15,17,19 triene functionality. Although we did not isolate
any directly, evidence for plakortolides with side chain trienes
1
was noticed in some HPLC fractions on comparison of H
from the Okinawan cnidarian Anthopleura pacifica Uchida.7a
A
NMR data with those of the recently reported epiplakinic acid
F methyl ester.11 It is difficult to assess whether the
plakortoperoxides are genuine natural products or artifacts of
side chain oxidation during isolation. The literature reports one
other example of a diperoxy polyketide, notably isolated along
with some related dienes.12 Some of the new plakortolides
isolated in our study possessed a methyl end group rather than
the phenyl end group, and so their biosynthesis likely involves
acetate rather than phenylacetate as a chain starter unit.
Carboxylic acids 9 and 10 and aldehydes 11 and 12, all with a
C9 side chain, apparently derive by oxidative cleavage of the
Δ15,16 double bond of a parent plakortolide. For comparison,
10-carboxy-11,12,13,14-tetranorplakortide Q has recently been
reported along with plakortide Q and an 11,12-didehydro-13-
oxo analogue from a Plakortis sp.,13 while manadoperoxide C
might derive from oxidative cleavage of the Δ12,13 double bond
of the co-occurring manadoperoxides A and B.3 However C−C
single bond cleavage is known and commonly involves a
cytochrome P450-catalyzed oxidation proceeding via a diol
intermediate.14
More intriguing from a biosynthetic perspective are the
alcohols 13 and 14, and particularly the hydroperoxides 15 and
16 with a C8 side chain. In Nature, hydroperoxides may be
formed by an “ene” reaction involving molecular oxygen or by
radical abstraction of a hydrogen atom followed by addition of
molecular oxygen.9 These pathways generate a double bond
adjacent to the position of hydroperoxidation in both terpene15
and fatty acid/oxylipins.15,16 In plant species, and perhaps also
in this Plakinastrella sponge, hydroperoxide reductases often
detoxify reactive peroxy products by converting them to
inactive alcohols.17 In plants and diatoms, hydroperoxide lyases
release unsaturated aldehydes with a shortened chain length
that are implicated in the ecological success of the host
system.16 However the absence of any alkene functionality in
the side chains of hydroperoxy compounds 15 and 16 defies all
of these established biosynthetic pathways. When considered
together with the stereoselective hydroperoxidation at C-6 of
plakortolide precursors that was explored in our earlier paper,2
the presence of such a diverse suite of oxidatively modified
comparison of 13C NMR data confirmed the relative
configurations of 13 and 15 matched those of plakortolide K,
while 14 and 16 had the same relative configuration as
plakortolide L.
Isolation of a Methyl Ester of a Didehydroplakinic Acid. A
nonpolar compound, 17, eluted before the various plakortolide
metabolites in Si flash chromatography and gave a sodiated
molecular ion peak (m/z 449.2653) by HR-ESIMS that
1
corresponded to a molecular formula of C27H38O4. H and
HSQC spectra showed the expected signals for a phenyl ring
[δH 7.35 (2H), 7.27 (2H), and 7.17 (1H); δC 128.5, 126.6, and
126.0], an oxy-methine signal (δH 4.88; δC 79.9), and two
characteristic AB methylene groups (δ H 2.70, dd and 2.89, br;
δC 39.4 and δH 2.15, d and 1.69, d; δC 40.4) in addition to the
cluster of methylene signals typical for an alkyl side chain. In
contrast to the various plakortolides, there was only one methyl
group (δH 1.16; δC 22.2), but instead there were signals for an
exomethylene group (δH 4.91 and 4.75; δC 111.3) and a
methoxy group (δH 3.68, δC 51.6). The methoxy group and the
signal at δH 2.70 (H-2) showed HMBC correlations to an ester
carbonyl (δC 171.0, C-1), revealing that the plakortolide
lactone ring had been opened. HMBC correlations from the
exomethylene (δH 4.91 and 4.75) to the C-4 quaternary carbon
(δC 141.0), to C-3 (δC 79.9), and to the isolated C-5
methylene carbon (δC 40.4) placed the exocyclic double bond
at C-4, while correlations from the methyl signal at δH 1.16 to
C-5 (δC 40.4), C-7 (δC 36.9), and C-6 (δC 82.5) all confirmed
a C-6 methyl group. In the side chain, there were also
characteristic signals (δH 6.72, 6.42, 6.19, and 5.79; δC 135.8,
130.2, 129.7, and 129.2) for a conjugated (E,E)-diene system
that matched data for the reported plakortolides O-R;2 the
diene was positioned next to the terminal phenyl ring from
HMBC data. The absolute configuration at C-6 was tentatively
assigned as S on the basis of the conserved stereochemistry of
the biosynthetic precursor for the “phenyl” group of
plakortolides. The configuration at C-3 was not determined.
Ester 17 shows structural resemblance to a cytotoxic peroxy-
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Journal of Natural Products
Article
plakortoperoxides C (7) and D (8) (0.5 mg) followed by
plakortoperoxide B (6) (0.1 mg). Fraction HP10g was chromato-
graphed on analytical RP-HPLC using a solvent gradient of 85%
CH3CN/15% H2O to 96% CH3CN/4% H2O over 10 min, followed
by 96% CH3CN/4% H2O for 10 min, yielding plakortolide U (2) (0.3
mg). Fraction HP10j was chromatographed on NP-HPLC using
hexanes/EtOAc (85:15) to give plakortolide W (4) (0.8 mg) and
plakortolide T (1) (2.8 mg). Finally HP10p was rechromatographed
on NP-HPLC using hexanes/EtOAc (3:1) to afford plakortoperoxide
A (5) (0.8 mg).
plakortolide structures hints at an extraordinary peroxidative-
based enzymatic chemistry in this marine sponge species that
warrants further investigation.
CONCLUSIONS
■
Four new plakortolides (1−4) with either a side chain diene
functionality or a saturated side chain have been isolated from
the Australian sponge P. clathrata Kirkpatrick, 1900, and
characterized by extensive 2D NMR studies. An additional four
metabolites, the plakortoperoxides A−D (5−8), all shared a
second peroxy ring in their side chain, and the relative
configuration of the side chain endoperoxide was deduced as cis
from mechanistic considerations, by low-temperature NMR
study, and by comparison with literature models. Each
plakortoperoxide was shown to be a mixture of two
diastereomers by enantioselective HPLC. A number of these
peroxy metabolites possessed a methyl end group rather than
the common phenyl end group, and so their biosynthesis likely
involves acetate rather than phenylacetate as a chain starter
unit. A final group of eight metabolites with a plakortolide core
contained a truncated side chain, either C9 in length and
terminating in either a carboxylic acid (9, 10) or a formyl (11,
12) functionality or alternatively C8 in length and terminating
in either a hydroxy (13, 14) or the rare hydroperoxy (15, 16)
group. The sponge extract also contained an ester (17), which
represents an additional structural variation in the complex
family of peroxy-containing metabolites.
Plakortolide T (1): colorless oil; [α]D −15 (c 0.05, CHCl3); 1H and
13C NMR (CDCl3, 500 MHz), see Table 1 and Table 2; HR-ESIMS
m/z 437.2663 [M + Na]+ (calcd for C26H38NaO4, 437.2662).
Plakortolide U (2): colorless oil; [α]D +26 (c 0.02, CHCl3); 1H and
13C NMR (CDCl3, 500 MHz), see Table 1 and Table 2; HR-ESIMS
m/z 435.2508 [M + Na]+ (calcd for C26H36NaO4, 435.2506).
Plakortolide V (3): colorless oil; [α]D +15 (c 0.05, CHCl3); 1H and
13C NMR (CDCl3, 500 MHz), see Table 1 and Table 2; HR-ESIMS
m/z 405.2991 [M + Na]+ (calcd for C23H42NaO4, 405.2975).
Plakortolide W (4): colorless oil; [α]D −7 (c 0.05, CHCl3); 1H and
13C NMR (CDCl3, 500 MHz), see Table 1 and Table 2; HR-ESIMS
m/z 401.2662 [M + Na]+ (calcd for C23H38NaO4, 401.2662).
Plakortoperoxide A (5): colorless oil; [α]D −13 (c 0.06, CHCl3);
1
1H and 13C NMR (CDCl3, 500 MHz), see Table 1 and Table 2; H
and 13C NMR (acetone-d6, 500 MHz) δH 7.41 (2H, m, H-20), 7.39
(1H, m, H-22), 7.36 (2H, m, H-21), 6.19 (1H, ddd, J = 10.3, 2.5, 2.5
Hz, H-16), 6.11 (1H, ddd, J = 10.3, 2.3, 2.3 Hz, H-17), 5.51 (1H, dd, J
= 2.5, 2.3 Hz, H-18), 4.55 (1H, d, J = 6.2 Hz, H-3), 4.44 (1H, m, H-
15), 3.14 (1H, dd, J = 18.5, 6.2 Hz, H-2a), 2.40 (1H, d, J = 18.5 Hz, H-
2b), 2.14 (1H, d, J = 14.9 Hz, H-5a), 1.80 (1H, d, J = 14.9 Hz, H-5b),
1.72 (1H, m, H-14a), 1.56 (1H, m, H-14b), 1.52 (2H, m, H-7), 1.41
(3H, s, H-23), 1.38−1.28 (10H, m, H-9 to H-13), 1.35−1.32 (2H, m,
H-8), 1.23 (3H, s, H-24); δC 174.4 (C-1), 138.9 (C-19), 129.4 (C-16),
129.1 (C-20), 128.9 (C-21), 128.9 (C-22), 127.0 (C-17), 83.0 (C-4),
81.8 (C-3), 80.5 (C-18), 80.1 (C-6), 78.7 (C-15), 41.4 (C-7), 40.8 (C-
5), 34.5 (C-2), 33.4 (C-14), 31.0−29.0 (C-9 to C-13), 25.4 (C-23),
23.4 (C-8), 22.5 (C-24); HR-ESIMS m/z 467.2404 [M + Na]+ (calcd
for C26H36NaO6, 467.2404).
EXPERIMENTAL SECTION
■
General Experimental Procedures. These have been reported
previously.2 NMR spectra obtained in acetone-d6 at room temperature
were internally referenced to acetone (1H, 2.05 ppm; 13C, 29.8 ppm).
Analytical enantioselective HPLC was performed on an Agilent 1200
Series liquid chromatograph system equipped with both a UV detector
(set at 254 nm) and an ALP detector (advanced laser polarimeter,
PDR-Chiral Inc.) using a Chiralpak AD column (4.6 × 250 mm,
DAICEL Chemical Co.) and with isocratic iPrOH/hexanes (3:17) at a
flow rate of 0.5 mL per min.
Plakortoperoxide B (6): colorless oil; [α]D −51 (c 0.01, CHCl3);
1H and 13C NMR (CDCl3, 500 MHz), see Table 1 and Table 2; HR-
ESIMS m/z 459.2719 [M + Na]+ (calcd for C25H40NaO6, 459.2717).
Mixture of plakortoperoxide C (7) and plakortoperoxide D (8):
Biological Material. This has been described previously.2
1
colorless oil; H and 13C NMR (CDCl3, 500 MHz), see Table 1 and
Table 2; HR-ESIMS m/z 459.2719 [M + Na]+ (calcd for
C25H40NaO6, 459.2717).
Extraction and Purification. A portion of the EtOAc extract
from P. clathrata2 was subjected to NP flash chromatography with
gradient elution (hexanes → CH2Cl2 → EtOAc → MeOH) to give 20
combined fractions (based on TLC profiles) and coded NP1−NP20.
Fraction NP7, which eluted from 100% CH2Cl2, was further purified
using analytical RP-HPLC with 100% CH3CN to give the ester (17)
(0.4 mg). Fraction NP9, which eluted from 100% CH2Cl2 to CH2Cl2/
EtOAc (5:1), was subjected to additional NP flash chromatography
with gradient elution (hexanes → EtOAc) to give five fractions, coded
as NP9a to NP9e. Fraction NP9a was purified by NP-HPLC using
EtOAc/hexanes (3:1) to give plakortolide W (3) (0.6 mg). Fraction
NP9c was also purified by NP-HPLC using EtOAc/hexanes (3:1) to
afford a mixture (1:2 by NMR) of formylplakortolide-1 (11) and -2
(12) (1.0 mg) and a mixture (3:5 by NMR) of hydroperoxyplakorto-
lide-1 (15) and -2 (16) (0.5 mg). Fraction NP9e was purified by RP-
HPLC using gradient elution (50% CH3CN/50% H2O to 100%
CH3CN over 30 min), yielding first a mixture (1:2) of
carboxyplakortolide-1 (9) and -2 (10) (1.6 mg), followed by a
mixture (1:2) of hydroxyplakortolide-1 (13) and -2 (14) (0.5 mg).
Fraction NP10, which eluted from CH2Cl2/EtOAc (3:1), was further
chromatographed using RP-HPLC with a solvent gradient of 95%
CH3CN/5% H2O to 100% CH3CN over 10 min followed by 100%
CH3CN for 30 min to give 16 HPLC fractions (HP10a−HP10p)
based on the UV profiles. Fraction HP10c was chromatographed on
analytical RP-HPLC using a solvent gradient of 85% CH3CN/15%
H2O to 95% CH3CN/5% H2O over 10 min, followed by 95%
CH3CN/5% H2O for 10 min to afford a mixture (1:1) of
Mixture of carboxyplakortolide-1 (9) and carboxyplakortolide-2
(10): colorless oil; 1H and 13C NMR (CDCl3, 500 MHz), see Table 3
and Table 4; HR-ESIMS m/z 351.1773 [M + Na]+ (calcd for
C17H28NaO6, 351.1778).
Mixture of formylplakortolide-1 (11) and formylplakortolide-2
(12): colorless oil; 1H and 13C NMR (CDCl3, 500 MHz), see Table 3
and Table 4; HR-ESIMS m/z 335.1818 [M + Na]+ (calcd for
C17H28NaO5, 335.1829).
Mixture of hydroxyplakortolide-1 (13) and hydroxyplakortolide-
2 (14): colorless oil; 1H and 13C NMR (CDCl3, 500 MHz), see Table
3 and Table 4; HR-ESIMS m/z 323.1829 [M + Na]+ (calcd for
C16H28NaO5, 323.1829).
Mixture of hydroperoxyplakortolide-1 (15) and hydroperoxypla-
kortolide-2 (16): colorless oil; 1H and 13C NMR (CDCl3, 500 MHz),
see Table 3 and Table 4; HR-ESIMS m/z 339.1784 [M + Na]+ (calcd
for C16H28NaO6, 339.1778).
Ester 17: colorless oil; [α]D −4 (c 0.02, CHCl3); 1H and 13C NMR
(CDCl3, 500 MHz), see Table 1 and Table 2; HR-ESIMS m/z
449.2653 [M + Na]+ (calcd for C27H38NaO4, 449.2662).
Reductive Cleavage of Plakortolide V (3). A mixture of
plakortolide V (3) (0.8 mg, 2.1 μmol) and 10% Pd on charcoal in
EtOAc (1 mL) was stirred under H2 at room temperature for 16 h.
The reaction mixture was filtered through a short plug of silica (0.8 g),
eluting with 2 mL of CHCl3, then dried under reduced pressure to
obtain a synthetic sample of seco-plakortolide V (21) (0.6 mg, 75%).
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Journal of Natural Products
Article
seco-Plakortolide V (21): colorless oil; [α]D +15 (c 0.05,
(ANU) for valuable discussions. The sponge collection was
made with the assistance of staff of ScubaWorld, Mooloolaba.
1
CHCl3); H and 13C NMR (CDCl3, 500 MHz) δH 4.17 (1H, dd, J
= 6.8, 1.8 Hz, H-3), 2.90 (1H, dd, J = 18.2, 6.8 Hz, H-2a), 2.53 (1H,
dd, J = 18.2, 1.8 Hz, H-2b), 2.17 (1H, d, J = 15.0 Hz, H-5a), 2.08 (1H,
d, J = 15.0 Hz, H-5b), 1.62 (1H, m, H-7a), 1.52 (1H, m, H-7b), 1.42
(3H, s, H-22), 1.34 (3H, s, H-23), 1.22−1.31 (26H, m, H-8 to H-20),
(0.86 (3H, t, J = 6.8 Hz, H-21); δC 174.8 (C-1), 89.7 (C-4), 73.7 (C-
3), 73.1 (C-6), 43.6 (C-5), 43.5 (C-7), 37.8 (C-2), 31.7 (C-19), 29.0−
29.9 (C-9 to C-18), 29.9 (C-23), 26.6 (C-22), 24.2 (C-8), 22.5 (C-
20), 13.9 (C-21); HR-ESIMS m/z 407.3134 (calcd for C23H44NaO4,
407.3132).
DEDICATION
■
Dedicated to Dr. Gordon M. Cragg, formerly Chief, Natural
Products Branch, National Cancer Institute, Frederick, Mary-
land, for his pioneering work on the development of natural
product anticancer agents.
REFERENCES
■
575.
Preparation of MPA Esters of seco-Plakortolide V (22a/
22b). The sample of seco-plakortolide V (21) (0.6 mg) obtained from
the catalytic (Pd/C) reduction of plakortolide V was divided in half,
and each sample (approximately 0.3 mg) was treated with either (R)-
or (S)-MPA (1 mg, 2 equiv), followed by DCC (1 mg, 2 equiv) and
DMAP (0.6 mg, 2 equiv) in dry CH2Cl2 (0.5 mL). Each reaction was
stirred overnight at room temperature before filtering through a small
plug of silica (0.8 g) eluting with CHCl3 (5 mL). The solvent was
removed in vacuo, and the MPA esters were individually purified by
RP-HPLC using an analytical column and eluting with CH3CN/H2O
(9:1) to give the (R)-MPA ester (22a) (0.1 mg, 30%) and (S)-MPA
ester (22b) (0.1 mg, 30%), respectively.
(1) Rahm, F.; Hayes, P. Y.; Kitching, W. Heterocycles 2004, 64, 523−
(2) Yong, K. W. L.; De Voss, J. J.; Hooper, J. N. A.; Garson, M. J. J.
Nat. Prod. 2011, 74, 194−207.
(3) Fattorusso, C.; Persico, M.; Calcinai, B.; Cerrano, C.; Parapini, S.;
Taramelli, D.; Novellino, E.; Romano, A.; Scala, A.; Fattorusso, E;
Taglialatela-Scafati, O. J. Nat. Prod. 2010, 73, 1138−1145.
(4) Varoglu, M.; Peters, B. M.; Crews, P. J. Nat. Prod. 1998, 61, 27−
36.
(5) (a) O’Shea, K. E.; Foote, C. S. J. Am. Chem. Soc. 1988, 110,
7167−7170. (b) Suzuki, M.; Ohtake, H.; Kameya, Y.; Hamanaka, N.;
Noyori, R. J. Org. Chem. 1989, 54, 5292−5302. (c) Matsumoto, M.;
Dobashi, S.; Kuroda., K.; Kondo, K. Tetrahedron 1985, 41, 2147−
2154.
(6) Avery, T. D.; Culbert, J. A.; Taylor, D. K. Org. Biomol. Chem.
2006, 4, 323−330.
1
(R)-MPA ester (22a): colorless oil; H NMR (CDCl3, 500 MHz)
δH 7.33−7.42 (5H, m, MPA phenyl protons), 5.19 (1H, dd, J = 6.4,
1.8 Hz, H-3), 4.79 (1H, s, CH of MPA), 3.39 (3H, s, OMe), 2.94 (1H,
dd, J = 18.6, 6.4 Hz, H-2a), 2.25 (1H, dd, J = 18.6, 1.8 Hz, H-2b), 1.91
(1H, d, J = 15.0 Hz, H-5a), 1.76 (1H, d, J = 15.0 Hz, H-5b), 1.55 (3H,
s, H-22), 1.20−1.32 (28H, m, H-7 to H-22), 1.13 (3H, s, H-23), 0.86
(3H, t, J = 7.1 Hz, H-21); HR-ESIMS m/z 555.3666 (calcd for
C32H52NaO6, 555.3656).
(7) (a) Zheng, G.-C.; Ichikawa, A.; Ishitsuka., M. O.; Kusumi, T.;
Yamamoto, H.; Kakisawa, H. J. Org. Chem. 1990, 55, 3677−3679.
(b) Dussault, P.; Sahli, A. J. Org. Chem. 1992, 57, 1009−1012.
(8) Harwood, L. M.; Robertson, J.; Swallow, S. Synlett 1999, 9,
1359−1362.
1
(S)-MPA ester (22b): colorless oil; H NMR (CDCl3, 500 MHz)
δH 7.33−7.42 (5H, m, MPA phenyl protons), 5.19 (1H, dd, J = 6.3,
1.3 Hz, H-3), 4.74 (1H, s, CH of MPA), 3.36 (3H, s, OMe), 3.03 (1H,
dd, J = 18.5, 6.3 Hz, H-2a), 2.48 (1H, dd, J = 18.5, 1.3 Hz, H-2b), 1.58
(1H, d, J = 15.0 Hz, H-5a), 1.48 (3H, s, H-22), 1.40 (1H, d, J = 15.0
Hz, H-5b), 1.20−1.30 (28H, m, H-7 to H-22), 0.86 (3H, t, J = 7.0 Hz,
H-21), 0.83 (3H, s, H-23); HR-ESIMS m/z 555.3664 (calcd for
C32H52NaO6, 555.3656).
(9) (a) Jin, H.-X.; Liu, H.-H.; Zhang, Q.; Wu, Y. J. Org. Chem. 2005,
70, 4240−4247. (b) Balci, M. Chem. Rev. 1981, 81, 91−108.
(10) Toth, S. I.; Schmitz, F. J. J. Nat. Prod. 1994, 57, 123−127.
́
́
(11) Jimenez-Romero, C.; Ortiz, I.; Vicente, J.; Vera, B.; Rodriguez,
A. D.; Nam, S.; Jove, R. J. Nat. Prod. 2010, 73, 1694−1700.
(12) Rudi, A.; Talpir, R.; Kashman, Y.; Benayahu, Y.; Schleyer, M. J.
Nat. Prod. 1993, 56, 2178−2182.
Biomimetic Synthesis of Plakortoperoxide A (5) and
Comparison with Natural Material. A solution of plakortolide
P2 (20) (9.4 mg) in anhydrous DCM (2 mL) at 5 °C was irradiated
with light from a 500 W tungsten-halogen lamp in the presence of rose
bengal bis(triethylammonium) salt (0.1 equiv), and oxygen bubbled
through the solution for 3 h. The solution was concentrated to
dryness, and the residue purified by NP flash chromatography using
CHCl3 to give a synthetic sample of plakortoperoxide A (5) (6.3 mg,
(13) Feng, Y.; Davis, R. A.; Sykes, M.; Avery, V. M.; Camp, D.;
Quinn, R. J. J. Nat. Prod. 2010, 73, 716−719.
(14) (a) Cryle, M. J.; De Voss, J. J. J. Chem. Soc., Chem. Commun.
2004, 86−87. (b) Cryle, M. J.; Schlichting, I. Proc. Natl. Acad. Sci.
2008, 105, 15696−15701.
(15) Casteel, D. A. Nat. Prod. Rep. 1999, 16, 55−73.
(16) (a) Pohnert, G. ChemBioChem. 2005, 6, 946−959. (b) d’Ippo-
lito, G.; Cutignano, A.; Briante, R.; Febbraio, F.; Cimino, G.; Fontana,
A. Org. Biomol. Chem. 2005, 3, 4065−4070.
(17) LeBlanc, J.; Davidson, R. J.; Hoffman, P. S. J. Bacteriol. 2006,
188, 6235−6244.
1
62%), [α]D −23 (c 0.09, CHCl3), identical to natural material by H
NMR and MS data.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures S1−S28. 1H and selected 2D NMR data for
1
compounds 1−17 and H NMR spectra for compounds 21
and 22a/b. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel: +61-7-3365 3605. Fax: +61-7-3365 4273. E-mail: m.
garson@uq.edu.au.
■
ACKNOWLEDGMENTS
■
We thank the Australian Research Council and The University
of Queensland for financial support, Dr. A. Piggott (IMB) and
Mr. G. McFarlane (SCMB) for HR-ESIMS measurements, Dr.
C. M. Williams and Mr. P. M. Mirzayans (SCMB) for advice
about the photochemical reactions, and Prof. M. G. Banwell
360
dx.doi.org/10.1021/np200619q|J. Nat. Prod. 2012, 75, 351−360