Semisynthesis of labdane diterpene metabolites from the nudibranch Pleurobranchaea meckelii

Article abstract of DOI:10.1016/j.tet.2007.09.039

Two isomeric labdane aldehyde metabolites (1 and 2), first isolated from the skin of the Notaspidean nudibranch Pleurobranchaea meckelii, were synthesized in six steps from manool in 19 and 6% overall isolated yields, respectively.

Full text of DOI:10.1016/j.tet.2007.09.039

Tetrahedron 63 (2007) 12179–12184
Semisynthesis of labdane diterpene metabolites from the
nudibranch Pleurobranchaea meckelii
*
Albert W. W. van Wyk and Michael T. Davies-Coleman
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
Received 10 July 2007; revised 3 September 2007; accepted 20 September 2007
Available online 23 September 2007
Abstract—Two isomeric labdane aldehyde metabolites (1 and 2), first isolated from the skin of the Notaspidean nudibranch Pleurobranchaea
meckelii, were synthesized in six steps from manool in 19 and 6% overall isolated yields, respectively.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
source of 1 and 2 could not be identified and their presence
in the skin of P. meckelli seemed to suggest that these two
compounds may be the products of de novo biosynthesis.
A defensive role for 1 and 2 in the chemical ecology of
P. meckelli was not established experimentally but rather
implied from the presence of a,b-unsaturated aldehyde moi-
eties in these two compounds.5 Bioactivity studies of marine
natural products are frequently hampered by the paucities of
these compounds that are often available for these studies.7
The semisynthesis of selected bioactive diterpene marine
natural products from commercially available terrestrial
plant diterpenes provides one possible solution to the bio-
active marine diterpene supply problem and we report here
the semisynthesis (Scheme 1) of 1 and 2 from commercially
available manool (3).
Marine sea slugs or nudibranchs are generally conspicuously
brightly coloured, slow moving invertebrate animals, which
do not possess any visible means of physical protection
against predation, e.g., a shell or sharp spines.1 Most nudi-
branchs have accordingly evolved complex chemical de-
fence strategies to deter potential predators. The most
commonly observed form of chemical defence is the seques-
tration by nudibranchs of toxic secondary metabolites from
their diet of other marine invertebrates such as sponges or
octocorals.2 The sequestered metabolites are stored in the
nudibranch’s dorsal mantle tissues, with the aposematic col-
ouration of the nudibranch acting as a warning to potential
predators of the nudibranch’s toxic status.1,2 A less common
chemical defence strategy, and one that predominates in
many cold water nudibranch species, is de novo biosynthesis
of toxic secondary metabolites and their storage in special-
ized glands in the nudibranch’s skin for release as feeding
deterrents in the presence of a predator.3,4
Cimino and co-workers alluded to the instability of 1 and 2
and the implications this instability had for structure elucida-
tion and further chemical ecology studies.5 The relative
instability of many bioactive marine natural products
through facile degradation processes, including isomeriza-
tion on exposure to sunlight and/or chromatographic media,
is often problematic. Accordingly, a desirable semisynthetic
strategy would be one that can quantitatively deliver a rela-
tively unstable marine natural product for bioactivity studies
from a stable precursor without prior chromatographic puri-
fication. We identified labd-13E-ene-8b,15-diol (4) and
labd-13Z-ene-8b,15-diol (5) as suitable stable precursors
of 1 and 2, respectively, with MnO2 oxidation of the former
two compounds providing a facile route to 1 and 2.
Nudibranchs belonging to the order Notaspidea, e.g., the
Mediterranean nudibranch Pleurobranchaea meckelii,
secrete an acidic mucus as an additional defence strategy
to deter predation.5,6 An acetone extract of several speci-
mens of P. meckelli, collected by SCUBA from the Gulf
of Naples, also afforded two isomeric labdane diterpene
aldehyde metabolites: labd-13E-ene-8b-ol-15-al (1) and
labd-13Z-ene-8b-ol-15-al (2).5 Diterpene metabolites are
commonly sequestered by marine opisthobranch molluscs
from their marine invertebrate diet.6 However, a dietary
Only two non-stereoselective semisyntheses of 4 and 5 have
been reported. Popa et al.8 synthesized 4 and 5, and their
respective 8a-epimers, via an initial series of successive
MnO2 and Ag2O oxidations of 3 followed by a regioselective
epoxidation of the D8,17 olefin in the final oxidation product
Keywords: Manool; Labdane diterpene; Marine; Nudibranch; Pleurobran-
chaea meckelii.
* Corresponding author. Tel.: +27 46 603 8924; fax: +27 46 622 5109;
e-mail: m.davies-coleman@ru.ac.za
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.09.039

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A. W. W. van Wyk, M. T. Davies-Coleman / Tetrahedron 63 (2007) 12179–12184
CHO
OH
COOEt
16
O
15
13
20
1
17
O
O
b
8
a
c
78%
5
74%
90%
18
3
19
6
7
8
Δ13
Δ13
E
9
Z
d
81%
COOEt
OH
CHO
OH
OH
OH
f
e
90%
66%
1
Δ13
Δ13
E
13 Δ13
14 Δ13
E
Z
4
Δ13
Δ13
E
2
Z
5
Z
Scheme 1. (a) PCC, CH2Cl2; (b) (i) O3, CH2Cl2, ꢀ78 ꢁC; (ii) Ph3P, CH2Cl2; (c) triethylphosphonoacetate, NaH, THF, 0 ꢁC; (d) MeMgCl, Et2O, ꢀ20 ꢁC; (e)
LAH, THF, 0 ꢁC; (f) MnO2, CH2Cl2.
(methyl labd-8(17),13-dien-15-oate) and LAH reduction of
the resultant a- and b-epoxides. The D13 geometric isomers
of 4 and 5 were not separated and the spectroscopic data
obtained in support of the chemical structures of the semi-
synthetic products were limited. A biomimetic cyclization
of (E,E,E)-geranylgeranyl acetate in the presence of a mer-
cury triflate/N,N-dimethylaniline cylization reagent com-
plex afforded racemic 4 in good yield.9 The semisynthesis
reported here therefore represents the first stereoselective
synthesis of 4 and 5 and their corresponding aldehyde oxida-
tion products 1 and 2.
a,b-unsaturated aldehyde (7). Reductive ozonolysis of E/
Z-7 afforded 6 in an overall yield (70%) from 3 comparable
with yields reported previously for this compound.13–15
Regioselective olefination of the more accessible methyl
ketone functionality proceeded smoothly via a Horner–
Wadsworth–Emmons (HWE) modification of the Wittig
reaction.17 The stereoselectivity of the HWE reaction is
diminished when trisubstituted alkenes are prepared from
ketones18 and a 3:1 ratio of ethyl 17-norlabd-13-E-en-8-one-
15-oate (8) and ethyl 17-norlabd-13-Z-en-8-one-15-oate (9)
was obtained in 74% overall yield. The two geometric
isomers were readily separated via normal phase HPLC
(hexane–EtOAc 6:1) and confirmation of the configuration
of the E D13 olefin in 8 and Z D13 olefin in 9 was pro-
vided by the 13C chemical shift of the vinylic methyl
(C-16) carbon resonance (dC 19.0 and 25.1, respectively)
and the allylic methylene carbon (C-12) signal (dC 40.2
and 32.6, respectively).19 Two minor tricyclic products
from the HWE reaction, ethyl 17-norabiet-13(15)-E-en-
8b-ol-16-oate (10) and ethyl 17-norabiet-13(15)-Z-en-8b-
ol-16-oate (11), were also isolated in 9% combined yield.
We have previously shown that these two compounds are
formed from 8-hydroxy-13-podocarpanone (12), an intra-
molecular aldol condensation product readily generated
from 6 in the presence of excess NaH.20
2. Results and discussion
Commercially available (+)-manool (3) is a common pre-
cursor regularly used for the semisyntheses of both marine
and terrestrial natural products.10–12 Various reported
approaches to the synthesis of diketone (6) from 3 have
included oxidation of the latter compound with either a com-
bination of OsO4/HIO5 at pH 6 or KMnO4 followed by sub-
sequent ozonolysis.13–15 Our preliminary synthesis of 6 from
3 (Scheme 1) was achieved via an initial high yielding oxi-
dative rearrangement of 316 to give a 2:1 mixture (as deter-
mined by the aldehyde proton signal intensities in the
NMR spectrum of the mixture) of the E/Z isomers of the
OH
COOEt
O
COOEt
OH
OH
H
OH
H
OH
10
11
12
15

A. W. W. van Wyk, M. T. Davies-Coleman / Tetrahedron 63 (2007) 12179–12184
12181
Chemoselective methylation of the C-8 ketone in 8 and 9
proceeded smoothly under standard Grignard conditions.
The methyl nucleophile attacked the endocyclic ketone
with Re-facial selectivity to afford ethyl labd-13E-en-8b-
ol-15-oate (13) and ethyl labd-13Z-en-8b-ol-15-oate (14),
respectively. Interestingly, the yield of 13 and 14 was en-
hanced when methylmagnesium chloride (81%), as opposed
to either methylmagnesium bromide (70%) or methyl lith-
ium (51%), was used in the nucleophilic methyl addition
reaction. The 8b-axial orientation of the alcohol substituent
in 13 and 14 was supported by threefold spectroscopic evi-
dence, viz. the deshielding of the H3-20 angular methyl pro-
tons (dH 0.94), the shielding of C-6 (dC 18.6) and a downfield
shift of the C-17 methyl carbon resonance (dC 30.6) in the 1H
and 13C NMR spectra of these two compounds. The shield-
ing of the C-6 resonance is attributed to a g-gauche effect
associated with a b-axially orientated hydroxyl functionality
at C-8.21 Conversely, an 8b-methyl substituent, e.g., in 15
shields the H3-20 protons (dH 0.80), the C-17 methyl carbon
resonance is shifted upfield (dH 24.0) while the C-6 reso-
nance is shifted downfield dC 20.5.19
1 ([a]D +25.5) and 2 ([a]D +24.1) recorded in benzene but
also opposite in sign and magnitude to the specific rotation
(recorded in chloroform) of both the reduction product
precursor, 4, derived from 1 isolated from P. meckelii,5 and
our semisynthetic 4. Interestingly, a negative specific rota-
tion ([a]D ꢀ8.5, c 0.085, CHCl3) was reported for 15, the
8a-epimer of 4.19 While epimerization at C-8 is an unlikely
explanation for the discrepancy between the signs of the
specific rotations of the naturally occurring and synthetic 1
and 2, we believe that this discrepancy may be related to
the facile isomerization and instability of the a,b-aldehydes
to both chromatographic media and solution in chloroform.
3. Conclusion
Two unstable marine a,b-unsaturated labdane diterpene
aldehydes, 1 and 2, originally isolated from the Mediterra-
nean nudibranch P. meckelli, have been successfully synthe-
sized from commercially available 3 via the stable alcohol
precursors 4 and 5. A discrepancy in the sign of the specific
rotations has been identified between the naturally occurring
and semisynthetic 1 and 2.
LAH reduction of esters 13 and 14 gave the corresponding
diols 4 and 5, respectively. Although the specific rotation
of synthetic (+)-4 ([a]D +21) was lower in magnitude than
that established by Cimino and co-workers5 ([a]D +32),
for the reduction product of the nudibranch derived 1, it
was significantly different from the specific rotations
reported for labd-13E-ene-8b,15-diol isolated from terres-
trial plants ([a]D 0 and +99).22,23 The negative specific rota-
tion previously reported for ent-labd-13E-ene-8b,15-diol
([a]D ꢀ32)24,25 is in agreement with the established labdane
stereochemistry of semisynthetic 4 and supports that pro-
posed by Cimino and co-workers for marine-derived 4.
4. Experimental
4.1. General
Melting points were determined using a Reichert hot stage
microscope and are uncorrected. Optical rotations were
measured using a Perkin–Elmer 141 polarimeter calibrated
at the sodium D line (598 nm). IR spectra were recorded
on a Perkin Elmer Spectrum 2000 FT-IR spectrometer
with the compounds as films (neat) on NaCl discs. NMR
spectra were acquired on Bruker 400 MHz Avance and
600 MHz Avance II spectrometers using standard pulse se-
quences. Chemical shifts are reported in parts per million,
referenced to residual solvent resonances (CDCl3 dH 7.25,
dC 77.2, C6D6 dH 7.15, dC 128.02), and coupling constants
are reported in hertz. HRFABMS data were obtained on
a JEOL SX102 spectrometer. Reactions where exclusion
of water was necessary were performed in flame dried glass-
ware under Ar. Just prior to their use Et2O and THF were dis-
tilled from sodium metal/benzophenone ketyl and CH2Cl2
from CaH. General laboratory solvents were distilled before
use. Reactions were monitored by thin layer chromato-
graphy (DC-Plastikfolien Kieselgel 60 F254 plates) and visu-
alized under UV light and developed by spraying with either
10% concd H2SO4 in methanol or iodine. Kieselgel 60 (230–
400 mesh) was used for initial flash chromatographic sepa-
rations. Semi-preparative HPLC was performed using
a Whatman’s Magnum 9 Partisil 10 column (10 mm i.d.,
The target aldehyde 1 was quantitatively procured by stirring
a dichloromethane solution of 4 with MnO2 (10 equiv) over-
night. Filtration through Celite gave a 25:1 mixture of 1:2
(i.e., >95% E isomeric purity from NMR analysis). The ratio
of 1:2 changed substantially (15:1) after an attempt was
made to remove the small amount of 2 in the isomeric mix-
ture using normal phase HPLC (hexane–EtOAc 4:1) and this
isomerization was suggestive of the instability of these com-
pounds to silica chromatography. An analogous MnO2 oxi-
dation of 5 gave 2:1 in a 50:1 ratio (i.e., >98% Z isomeric
purity from NMR analysis) which, after normal phase
HPLC, also reverted to a mixture of 11:1 (Z/E) geometric
isomers. The 1H and 13C NMR data of the 25:1 mixture con-
sisting predominantly of 1 (acquired in CDCl3) were consis-
tent in all respects with the spectroscopic data published for
the corresponding P. meckelii metabolite.5 In our hands the
1H NMR spectrum of the isomeric mixture, containing
almost exclusively 2, showed evidence of initial partial
isomerization followed by total degradation (to form an in-
separable mixture of products) on standing in a solution of
CDCl3 (12 h). Accordingly, the NMR data for the Z isomer
dominated mixture were obtained in deuterated benzene.
We consequently also avoided using chloroform for record-
ing the specific rotations of 1 and 2. Paradoxically, Cimino
and co-workers5 reported negative specific rotations for
chloroform solutions of 1 ([a]D ꢀ35.2) and 2 ([a]D ꢀ28.2)
isolated from P. meckelii which is not only at variance
with the specific rotations we obtained for our semisynthetic
length 50 cm) with an eluent flow rate of 4 mL minꢀ1
.
4.2. Labd-8(17),13E-diene-15-al and its 13Z isomer (7)
A solution of manool 3 (170 mg, 0.586 mmol) and pyridi-
nium chlorochromate (417 mg, 1.93 mmol, 3.3 equiv) in
CH2Cl2 (8 mL) was stirred (26 h) at room temperature.
Et2O (8 mL) was added, and the formation of an orange pre-
cipitate noted. The solution was then filtered through a Celite
plug and the filtrate concentrated in vacuo to yield a dark

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A. W. W. van Wyk, M. T. Davies-Coleman / Tetrahedron 63 (2007) 12179–12184
brown oil (195 mg). The oil was purified on a silica column
(hexane–EtOAc 15:1) to yield an inseparable mixture of the
E and Z isomers of 7 in a 2:1 ratio (154 mg, 90% yield) as
a pale yellow oil; 1H and 13C NMR data consistent with lit-
erature values;26 HRFABMS m/z 289.2532 [M+H]+ (calcd
for C20H33O2, 289.2531).
(CDCl3, 100 MHz) d 14.3 (q, –OCH2CH3); 14.7 (q, C-20);
18.7 (q, C-16); 19.0 (t, C-2); 19.6 (t, C-11); 21.7 (q, C-19);
24.1 (q, C-6); 33.5 (q, C-18); 33.7 (s, C-4); 39.3 (t, C-1);
40.2 (t, C-12); 41.9 (t, C-3); 42.6 (t, C-7); 42.6 (s, C-10);
54.3 (d, C-5); 59.4 (t, –OCH2CH3); 63.2 (d, C-9); 115.7
(d, C-14); 160.1 (s, C-13); 166.8 (s, C-15); 211.7 (s, C-8);
HRFABMS m/z 336.2663 [M]+ (calcd for C21H36O3
336.2664).
4.3. 15,16,17-Trinorlabdane-8,13-dione (6)
The isomeric mixture of 7 (100 mg, 0.35 mmol) was dis-
solved in anhydrous CH2Cl2 (5 mL) and cooled to ꢀ78 ꢁC.
A steady stream of O3 was bubbled through the solution
(10 min) until the solution had turned a pale blue colour
whereupon N2 was bubbled through the solution to remove
excess O3. After the addition of triphenyl phosphine
(548 mg, 2.09 mmol, 6 equiv) the solution was stirred at am-
bient temperature (4 h). The solution was cooled to 0 ꢁC,
H2O2 (30%, 1 mL) added and stirring continued (0.5 h).
The solution was finally washed with water (3ꢂ5 mL), and
the combined organic extracts dried (anhydrous MgSO4)
and concentrated. Subsequent purification of the concen-
trated organic material on a silica column (EtOAc–hexane
1:1) afforded 6 (72 mg, 0.27 mmol, 78%): colourless oil;
1H and 13C NMR data consistent with literature
values;15,27,28 [a]2D0 ꢀ28.3 (c 1.3, CHCl3) lit.15 ꢀ11; IR
(film) nmax 2940, 1709, 1698, 1355, 1164 cmꢀ1; HRFABMS
m/z 265.2168 [M]+ (calcd for C17H28O2 265.2168).
4.4.2. Ethyl 17-norlabd-13-Z-en-8-one-15-oate (9). Pale
yellow oil; [a]D25 ꢀ56.6 (c 0.9, CHCl3); IR (film) nmax
1
2945, 1712, 1646, 1244, 1168 cmꢀ1; H NMR (400 MHz,
CDCl3) d 0.69 (3H, s, H3-20); 0.83 (3H, s, H3-19); 0.95
(3H, s, H3-18); 1.19 (1H, m, H-1a); 1.25 (3H, t, J¼6.6 Hz,
–OCH2CH3); 1.23 (1H, m, H-3a); 1.35 (1H, m, H-11a);
1.43 (1H, m, H-3); 1.48 (1H, m, H-5); 1.52 (2H, m, H2-2);
1.63 (2H, m, H2-6); 1.73 (1H, m, H-1b); 1.89 (3H, s, H3-
16); 1.90 (1H, m, H-11b); 2.03 (1H, m, H-6b); 2.13 (1H,
m, H-9); 2.11 (1H, m, H-12a); 2.34 (2H, m, H2-7); 2.85
(1H, m, H-12b); 4.11 (2H, dd, J¼13.2, 6.4 Hz, –OCH2CH3);
5.60 (1H, br s, H-14); 13C NMR (100 MHz, CDCl3) d 14.4
(q, –OCH2CH3); 14.7 (q, C-20); 19.0 (t, C-2); 20.0 (t,
C-11); 21.7 (q, C-19); 24.1 (t, C-6); 25.0 (q, C-16); 32.6
(t, C-12); 33.5 (q, C-18); 33.6 (s, C-4); 39.1 (t, C-1); 41.9
(t, C-3); 42.6 (t, C-7); 42.7 (s, C-10); 54.2 (d, C-5); 59.5
(t, –OCH2CH3); 63.6 (d, C-9); 116.3 (d, C-14); 160.5 (s,
C-13); 166.4 (s, C-15); 212.0 (s, C-8); HRFABMS m/z
336.2664 [M]+ (calcd for C21H36O3 336.2664).
4.4. Ethyl 17-norlabd-13-E-en-8-one-15-oate (8), ethyl
17-norlabd-13-Z-en-8-one-15-oate (9), ethyl 17-nora-
biet-13(15)-E-en-8b-ol-16-oate (10) and ethyl
17-norabiet-13(15)-Z-en-8b-ol-16-oate (11)
4.4.3. Ethyl 17-norabiet-13(15)-E-en-8b-ol-16-oate (10).
White crystals (from Et2O); mp 110–111 ꢁC; [a]D34 ꢀ68.1
(c 1.8, CHCl3); IR, 1H and 13C NMR data consistent with lit-
erature values;20 HRFABMS m/z 334.2509 [M]+ (calcd for
C21H34O3 334.2508).
To a solution of triethylphosphonoacetate (1660 mL,
8.37 mmol, 3 equiv) in anhydrous THF (5 mL) was added
NaH (60%, 140 mg, 2.5 equiv) and stirred for 30 min before
addition to a 0 ꢁC solution of 6 (737 mg, 2.79 mmol). The
solution was warmed to ambient temperature and stirred
overnight, whereupon HCl (1 M, 3 mL) was added and the
solution concentrated. The resultant yellow oil was taken
up in Et2O (5 mL) and washed with H2O (3ꢂ5 mL). The or-
ganic fraction was dried (MgSO4), concentrated and excess
triethylphosphonoacetate removed by flash silica chromato-
graphy (10:1 hexane–EtOAc). The mixture of HWE-prod-
ucts obtained from flash chromatography (927 mg) was
further purified with normal phase HPLC (hexane–EtOAc
14:1) to yield 8 (409 mg, 55%), 9 (175 mg, 19%), 10
(62 mg, 7%) and 11 (18 mg, 2%).
4.4.4. Ethyl 17-norabiet-13(15)-Z-en-8b-ol-16-oate (11).
1
Yellow oil; [a]D34 +66.3 (c 0.8, CHCl3); IR, H and 13C
NMR data consistent with literature values;20 HRFABMS
m/z 335.2582 [M+H]+ (calcd for C21H35O3 335.2588).
4.5. Ethyl labd-13E-en-8b-ol-15-oate (13) and ethyl
labd-13Z-en-8b-ol-15-oate (14)
This method is representative for the Grignard methylation
of either 8 or 9. A solution of 8 (76 mg, 0.23 mmol) in anhy-
drous Et2O (5 mL) was cooled to ꢀ20 ꢁC. An ether solution
of MeMgCl (3 M, 96 mL, 1.3 equiv) was slowly added in
a dropwise fashion with vigorous stirring. The solution
was maintained at ꢀ20 ꢁC (1 h), warmed to 0 ꢁC (3 h) and
excess MeMgCl quenched through the addition of three
drops of H2O. The reaction mixture was further acidified
with HCl (1 M, 2 mL) and stirred until the solution had be-
come clear. The Et2O solution was washed with H2O
(3ꢂ5 mL), dried (MgSO4) and concentrated in vacuo to
yield a pale yellow oil. Subsequent purification of this oil
via normal phase HPLC (hexane–EtOAc 6:1) afforded
exclusively 13 with an average yield of 81%.
4.4.1. Ethyl 17-norlabd-13-E-en-8-one-15-oate (8). Pale
yellow oil; [a]D34 ꢀ20.2 (c 0.5, CHCl3); IR (film) nmax
2948, 1713, 1642, 1218, 1146 cmꢀ1 1H NMR (CDCl3,
;
400 MHz) d 0.71 (3H, s, H3-20); 0.84 (3H, s, H3-19); 0.94
(3H, s, H3-18); 1.14 (1H, ddd, J¼13.2, 12.2, 4.9 Hz, H-
1a); 1.26 (3H, t, J¼7.1 Hz, –OCH2CH3); 1.23 (1H, m, H-
3a); 1.36 (2H, m, H2-11); 1.44 (1H, m, H-3b); 1.46 (1H,
m, H-5); 1.50 (2H, m, H2-2); 1.64 (1H, dddd, J¼13.3,
13.1, 13.0, 5.0 Hz, H-6a); 1.74 (1H, m, H-1b); 1.87 (1H,
m, H-12a); 1.99 (1H, m, H-9); 2.05 (1H, m, H-6b); 2.13
(3H, d, J¼1.2 Hz, H3-16); 2.27 (1H, ddd, J¼13.3, 7.2,
6.8 Hz, H-7a); 2.18 (1H, m, H-12b); 2.41 (1H, ddd,
J¼13.2, 4.8, 2.1 Hz, H-7b); 4.13 (2H, dq, J¼7.1, 0.5 Hz,
–OCH2CH3); 5.61 (1H, dd, J¼2.1, 1.2 Hz, H-14); 13C NMR
4.5.1. Ethyl labd-13E-en-8b-ol-15-oate (13). Pale yellow
oil; [a]3D4 +24.8 (c 2.2, CHCl3); IR (film) nmax 3498, 2937,
1712, 1454, 1163 cmꢀ1 1H NMR (CDCl3, 400 MHz)
;
d 0.79 (1H, dd, J¼4.4, 2.7 Hz, H-5); 0.81 (3H, s, H3-19);
0.85 (1H, m, H-9); 0.86 (3H, s, H3-18); 0.87 (1H, m, H-1a);

A. W. W. van Wyk, M. T. Davies-Coleman / Tetrahedron 63 (2007) 12179–12184
12183
0.94 (3H, s, H3-20); 1.12 (1H, m, H-3a); 1.13 (3H, s, H-17);
1.26 (3H, t, J¼7.1 Hz, –OCH2CH3); 1.36 (1H, m, H-3b);
1.41 (2H, m, H-2a and H-11a); 1.48 (1H, m, H-6a);1.49
(1H, m, H-7a); 1.50 (1H, m, H-6b); 1.56 (1H, m, H-2b);
1.57 (1H, m, H-11b); 1.66 (1H, m, H-1b); 1.75 (1H, m, H-
7b); 2.12 (3H, d, J¼1.3 Hz, H3-16); 2.14 (1H, m, H-12a);
2.15 (1H, m, H-12b); 4.14 (2H, q, J¼7.1 Hz, –OCH2CH3);
5.67 (1H, dd, J¼2.3, 1.1 Hz, H-14); 13C NMR (CDCl3,
100 MHz) d 14.3 (q, –OCH2CH3); 15.1 (q, C-20); 18.1 (t,
C-6); 18.3 (t, C-2); 19.0 (q, C-16); 21.6 (q, C-19); 23.6 (t,
C-11); 30.6 (q, C-17); 33.2 (s, C-4); 33.4 (q, C-18); 39.0
(s, C-10); 39.2 (t, C-1); 42.0 (t, C-3); 42.3 (t, C-7); 44.6 (t,
C-12); 55.9 (d, C-5); 58.8 (d, C-9); 59.5 (t, –OCH2CH3);
73.1 (s, C-8); 115.3 (d, C-14); 160.2 (s, C-13); 166.9 (s,
C-15); HRFABMS m/z 351.2895 [M+H]+ (calcd for
C22H39O3 351.2899).
J¼13.5, 3.4 Hz, H-2b); 1.67 (1H, m, H-1b); 1.69 (3H, s,
H3-16); 1.77 (1H, m, H-7b); 2.02 (1H, m, H-12a); 2.05
(1H, m, H-12b); 4.14 (1H, d, J¼6.9 Hz, H2-15); 5.41 (1H,
dt, J¼6.9, 1.2 Hz, H-14); 13C NMR (CDCl3, 150 MHz)
d 15.1 (q, C-20); 16.4 (q, C-2); 18.2 (t, C-2); 18.3 (t, C-6);
21.6 (q, C-19); 23.9 (t, C-6); 30.6 (q, C-17); 33.2 (s, C-4);
33.4 (q, C-18); 38.9 (s, C-10); 39.1 (t, C-1); 42.0 (t, C-3);
42.2 (t, C-7); 43.3 (t, C-12); 55.9 (d, C-5); 58.8 (d, C-9);
59.4 (t, C-15); 73.2 (s, C-8); 123.1 (d, C-14); 140.4 (s,
C-13); HRFABMS m/z 308.2715 [M]+ (calcd for
C20H36O2 308.2715).
4.6.2. Labd-13Z-ene-8b,15-diol (5). White amorphous
powder; [a]3D4 +24.3 (c 1.0, CHCl3); IR (film) nmax 3395,
1
2930, 1182, 909, 758 cmꢀ1; H NMR (CDCl3, 600 MHz,)
d 0.79 (1H, dd, J¼2.7, 1.6 Hz, H-9); 0.81 (3H, s, H3-19);
0.84 (1H, m, H-5); 0.86 (3H, s, H3-18); 0.89 (1H, m, H-
1a); 0.93 (3H, s, H3-20); 1.14 (1H, m, H-3a); 1.17 (3H, s,
H3-17); 1.31 (1H, m, H-11a); 1.39 (1H, m, H-3b); 1.41
(1H, m, H-2a); 1.45 (1H, m, H-11b); 1.48 (1H, m, H-7a);
1.50 (1H, m, H-6a); 1.52 (1H, m, H-6b); 1.60 (1H, dt,
J¼13.4, 3.1 Hz, H-2b); 1.69 (1H, ddd, J¼12.3, 4.3,
3.0 Hz, H-1b); 1.76 (1H, m, H-7b); 1.77 (3H, d, J¼0.9 Hz,
H3-16); 2.05 (1H, m, H-12a); 2.08 (1H, m, H-12b); 4.13
(2H, d, J¼6.8 Hz, H2-15); 5.37 (1H, t, J¼7.1 Hz, H-14);
13C NMR (CDCl3, 150 MHz) d 15.1 (q, C-20); 18.2 (t,
C-2); 18.3 (t, C-6); 21.6 (q, C-19); 23.6 (q, C-16); 24.5
(t, C-11); 30.6 (q, C-17); 33.3 (s, C-4); 33.4 (q, C-18);
36.0 (t, C-12); 38.9 (s, C-10); 39.1 (t, C-1); 42.0 (t, C-3);
42.2 (t, C-7); 55.9 (d, C-5); 59.28 (t, C-15); 59.32 (d, C-9);
73.1 (s, C-8); 123.6 (d, C-14); 140.9 (s, C-13); HRFABMS
m/z 308.2716 [M]+ (calcd for C20H36O2 308.2715).
4.5.2. Ethyl labd-13Z-en-8b-ol-15-oate (14). Pale yellow
oil; [a]3D4 +15.1 (c 0.7, CHCl3); IR (film) nmax 3461, 2923,
1709, 1451, 1159 cmꢀ1 1H NMR (CDCl3, 400 MHz)
;
d 0.82 (3H, s, H3-19); 0.84 (1H, m, H-5); 0.85 (1H, m, H-
9); 0.86 (3H, s, H3-18); 0.94 (1H, ddd, J¼14.0, 6.8,
4.0 Hz, H-1a); 0.94 (3H, s, H3-20); 1.14 (1H, ddd, J¼13.9,
6.6, 4.2 Hz, H-3a); 1.24 (3H, s, H3-17); 1.25 (3H, t,
J¼7.1 Hz, –OCH2CH3); 1.37 (1H, m, H-3b); 1.41 (1H, m,
H-11a); 1.49 (2H, m, H-6a and H-7a); 1.51 (2H, m, H-6b
and H-11b); 1.60 (1H, m, H-2b); 1.75 (2H, m, H-1b and
H-7b); 1.90 (3H, d, J¼2.8 Hz, H3-16); 2.65 (1H, ddd,
J¼12.4, 5.8, 5.3 Hz, H-12a); 2.75 (1H, ddd, J¼12.4, 6.2,
2.7 Hz); 4.12 (2H, q, J¼7.1 Hz, –OCH2CH3); 5.60 (1H, d,
J¼1.3 Hz, H-14); 13C NMR (CDCl3, 100 MHz) d 14.4 (q,
–OCH2CH3); 15.1 (q, C-20); 18.2 (t, C-2); 18.3 (t, C-6);
21.7 (q, C-19); 23.8 (t, C-11); 25.1 (q, C-16); 30.6 (q,
C-17); 33.3 (s, C-10); 33.4 (q, C-18); 36.7 (t, C-12); 39.0
(s, C-10); 39.1 (t, C-1); 42.0 (t, C-3); 42.4 (t, C-7); 55.9
(d, C-5); 59.3 (t, –OCH2CH3); 59.4 (d, C-9); 115.9 (s,
C-14); 159.9 (s, C-13); 166.2 (s, C-15); HRFABMS m/z
351.2895 [M+H]+ (calcd for C22H39O3 351.2899).
4.7. Labd-13E-ene-8b-ol-15-al (1) and labd-13Z-ene-
8b-ol-15-al (2)
This method is representative of the MnO2 oxidation of both
4 and 5. The allylic alcohol 4 (40.8 mg, 0.136 mmol) was
dissolved in anhydrous CH2Cl2 (2 mL) and finely powdered
activated MnO2 (86.9 mg, 1.36 mmol, 10 equiv) was added.
The solution was stirred at ambient temperature overnight
(16 h) and then filtered through Celite 545 to give 1 as a white
amorphous powder (>95% purity from NMR) in 90% over-
all yield.
4.6. Labd-13E-ene-8b,15-diol (4) and labd-13Z-ene-
8b,15-diol (5)
This method is representative for the LAH reduction of
either 13 or 14. Compound 13 (42.1 mg, 0.12 mmol) was
dissolved in anhydrous Et2O (2 mL) and cooled to 0 ꢁC.
LAH (13.5 mg, 0.361 mmol, 3 equiv) was added and stirring
continued (0.5 h) after which the solution was warmed to
ambient temperature over a period of 2 h. HCl (1 M,
3 mL) was added slowly and the ether solution washed
with H2O (3ꢂ2 mL). The organic fraction was then dried
(MgSO4) and concentrated to yield a white amorphous pow-
der which was purified by normal phase HPLC (hexane–
EtOAc 1:1) to yield 4 (66%).
4.7.1. Labd-13E-ene-8b-ol-15-al (1). White amorphous
powder; [a]D34 +25.5 (c 1.4, C6H6); IR (film) nmax 3481,
1
2921, 1668, 1454, 1382 cmꢀ1; H NMR (C6D6, 600 MHz)
d 0.41 (1H, dd, J¼4.6, 2.6 Hz, H-9); 0.63 (1H, dd, J¼12.2,
2.4 Hz, H-5); 0.65 (1H, m, H-1a); 0.85 (3H, s, H3-19);
0.86 (3H, s, H3-17); 0.88 (3H, s, H3-18); 0.95 (3H, s, H3-
20); 1.10 (1H, ddd, J¼14.2, 6.6, 3.8 Hz, H-3a); 1.18 (2H,
m, H-2a and H-7a); 1.20 (1H, m, H-11a); 1.23 (1H, m, H-
6a); 1.33 (1H, m, H-6b); 1.37 (1H, ddd, J¼11.8, 3.8,
1.4 Hz, H-3b); 1.43 (1H, m, H-11b); 1.48 (1H, dd, J¼3.5,
1.0 Hz, H-1b); 1.49 (1H, m, H-2b); 1.53 (1H, ddd, J¼13.5,
3.5, 3.3 Hz, H-7a); 1.60 (3H, d, J¼1.3 Hz, H3-16); 1.80
(1H, m, H-12a); 1.84 (m, 1H, H-12b); 5.93 (1H, ddd,
J¼7.8, 2.5, 1.3 Hz, H-14); 9.91 (1H, d, J¼7.8 Hz, H-15);
13C NMR (C6D6, 150 MHz) d 15.3 (q, C-20); 17.0 (q,
C-16); 18.61 (t, C-2); 18.62 (t, C-6); 21.9 (q, C-19); 23.5
(t, C-11); 30.8 (q, C-17); 33.4 (s, C-4); 33.7 (q, C-18);
39.2 (s, C-10); 39.3 (t, C-1); 42.4 (t, C-3); 42.6 (t, C-7);
4.6.1. Labd-13E-ene-8b,15-diol (4). White amorphous
powder; [a]3D4 +20.8 (c 1.5, CHCl3), lit.5 +32; IR (film)
1
nmax 3420, 2921, 1189, 912, 758 cmꢀ1; H NMR (CDCl3,
600 MHz) d 0.79 (1H, dd, J¼4.3, 2.7 Hz, H-9); 0.81 (3H,
s, H3-19); 0.85 (1H, m, H-5); 0.88 (1H, m, H-1a); 0.86
(3H, s, H3-18); 0.94 (3H, s, H3-20); 1.13 (3H, s, H3-17);
1.14 (1H, m, H-3a); 1.38 (1H, m, H-3b); 1.40 (1H, m, H-
11a); 1.42 (1H, m, H-2a); 1.51 (1H, m, H-7a); 1.52 (2H,
m, H-6a and H-11b); 1.54 (1H, m, H-6b); 1.59 (1H, dt,

12184
A. W. W. van Wyk, M. T. Davies-Coleman / Tetrahedron 63 (2007) 12179–12184
4. Gavagnin, M.; Fontana, A.; Ciavatta, M. L.; Cimino, G. Ital.
J. Zool. 2000, 67, 101–109.
5. Ciavatta, M. L.; Villani, G.; Trivelloni, E.; Cimino, G.
Tetrahedron Lett. 1995, 36, 8673–8676.
44.4 (t, C-12); 56.0 (d, C-5); 58.8 (d, C-9); 72.4 (s, C-8);
127.3 (d, C-14); 162.1 (s, C-13); 190.0 (d, C-15); HRFABMS
m/z 306.2559 [M]+ (calcd for C20H34O2 306.2559).
4.7.2. Labd-13Z-ene-8b-ol-15-al (2). Pale yellow oil; [a]D34
+24.1 (c 0.9, C6H6); IR (film) nmax 3410, 2915, 1661, 1447,
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1381 cmꢀ1; H NMR (C6D6, 600 MHz) d 0.43 (1H, dd,
J¼4.4, 2.6 Hz, H-9); 0.63 (1H, dd, J¼12.1, 2.3 Hz, H-5);
0.68 (1H, ddd, J¼13.2, 6.4, 3.4 Hz, H-1a); 0.83 (3H, s,
H3-19); 0.88 (3H, s, H3-17); 0.89 (3H, s, H3-18); 0.93 (3H,
s, H3-20); 1.10 (1H, ddd, J¼13.5, 6.6, 4.0 Hz, H-3a); 1.17
(1H, m, H-7a); 1.20 (1H, m, H-2a); 1.25 (1H, m, H-11a);
1.32 (1H, m, H-6a); 1.35 (1H, m, H-6b); 1.36 (1H, m, H-
3b); 1.46 (1H, m, H-11b); 1.47 (1H, m, H-7b); 1.48 (1H,
m, H-2b); 1.49 (3H, d, J¼1.3 Hz, H3-16); 1.52 (1H, m, H-
1b); 2.17 (1H, ddd, J¼10.9, 6.2, 5.8 Hz, H-12a); 2.19 (1H,
ddd, J¼10.9, 6.0, 5.8 Hz, H-12b); 5.81 (1H, d, J¼7.9 Hz,
H-14); 10.08 (1H, d, J¼7.9 Hz, H-15); 13C NMR (C6D6,
150 MHz) d 15.3 (q, C-20); 18.57 (t, C-2); 18.60 (t, C-6);
21.9 (q, C-19); 24.5 (q, C-16); 25.1 (t, C-11); 30.9 (q,
C-17); 33.4 (s, C-4); 33.6 (q, C-18); 36.3 (t, C-12); 39.2 (s,
C-10); 39.4 (t, C-1); 42.3 (t, C-7); 42.6 (t, C-3); 55.9 (d,
C-5); 59.3 (d, C-9); 72.3 (s, C-8); 128.3 (d, C-14); 162.9
(s, C-13); 189.2 (d, C-15); HRFABMS m/z 306.2559 [M]+
(calcd for C20H34O2 306.2559).
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Acknowledgements
We would like to thank Professor Brian Robinson of West-
chem Industries, New Zealand for the generous donation
of manool. Financial assistance from Rhodes University,
the South African National Research Foundation (NRF),
the Department of Environmental Affairs and Tourism and
DAAD (Ph.D. bursary A.W.W.vW), is gratefully acknow-
ledged.
€
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Supplementary data
Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.tet.2007.09.039.
References and notes
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