A chemoenzymatic total synthesis of the undecenolide (-)-cladospolide C

Article abstract of DOI:10.1071/CH05074

The (-)-enantiomer, ent-3, of the natural product (+)-cladospolide C (3) has been prepared for the first time using the monochiral cis-1,2- dihydrocatechol 5 as starting material. Key steps include coupling of the derived acid 6 with the enzymatically gen

Full text of DOI:10.1071/CH05074

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2005, 58, 511–516
A Chemoenzymatic Total Synthesis of the Undecenolide
(−)-Cladospolide C
Martin G. Banwell,A,B David T. J. Loong, and Anthony C. Willis
A
A
A
Research School of Chemistry, Institute of Advanced Studies, Australian National University,
Canberra ACT 0200, Australia.
Corresponding author. Email: mgb@rsc.anu.edu.au
B
The (−)-enantiomer, ent-3, of the natural product (+)-cladospolide C (3) has been prepared for the first time using
the monochiral cis-1,2-dihydrocatechol 5 as starting material. Key steps include coupling of the derived acid 6 with
the enzymatically generated (S)-(+)-4-penten-2-ol (7) and ring-closing metathesis (RCM) of the resultant doubly
unsaturated ester 8 to give lactone 9. The structure of this last compound has been confirmed by single-crystal
X-ray analysis. This work has established that the absolute configuration of (+)-cladospolide C has been correctly
assigned and is as illustrated in structure 3.
Manuscript received: 15 March 2005.
Final version: 4 May 2005.
Introduction
M. racemosus and P. oryzae with IC50 values of 0.1 and
−1
[4]
2
9 µg mL , respectively. Compounds 1 and 2 have also
The natural products 1–4 (Fig. 1) are all 12-membered
macrolides derived from various Cladosporium species of
fungi and are known as cladospolides A–D, respectively.
Each possesses interesting biological properties. For exam-
ple, the first three members of the class, namely compounds
[7]
been patented as inhibitors of allergy and inflammation.
Cladospolide A (1) has been the subject of several syn-
thetic studies with full details of the shortest of the three
distinct routes to this compound having been reported by
[
1–6]
[
8]
us in 2004, and involving an eleven-step sequence start-
ing from the cis-1,2-dihydrocatechol 5 (Fig. 2). This last
compound is an enantiomerically pure material available in
multi-gram quantities through the whole-cell biotransfor-
mation of chlorobenzene using the genetically engineered
organism E. coli JM19 (pDTG601) that overexpresses the
toluene dioxygenase (TDO) enzyme system responsible for
1
–3, inhibit shoot elongation in rice seedlings and cla-
dospolide C (3) does so without causing signs of necrosis,
thus suggesting that this macrolide, at least, inhibits gib-
[
3]
berellin biosynthesis. Fascinatingly, cladospolide A also
inhibits root elongation in lettuce seedlings, while cla-
dospolide B appears to have the opposite effect.[ Although
the structure of cladospolide D remains to be fully defined,
it has been shown to exhibit antimicrobial activity against
2]
[
9]
this fascinating conversion. The pivotal step in our synthe-
sis of compound 1 involved a ring-closing metathesis (RCM)
of an ester that incorporated terminal alkene moieties in both
the carboxylic acid and alcohol residues. This led to the
corresponding unsaturated nonenolide, which was then sub-
jected to a two-carbon homologation sequence involving,
among other things, a Horner–Wadsworth–Emmons (HWE)
reactionandwhichultimatelydeliveredtheundecenolidering
system associated with the target natural product 1. This
ordering of events was pursued because attempts to form
5
4
OH
OH
S
R
S
R
R
5
R
HO
3
O
OH
11
O
1
2
O
1
2
O
1
2
OH
OH
5
OH
O
R
1
R
OH
R
11
O
O
1
2
OH
2
O
O
Cl
3
4
5
Fig. 1.
Fig. 2.
10.1071/CH05074
©
CSIRO 2005
0004-9425/05/070511

5
12
M. G. Banwell, D. T. J. Loong and A. C. Willis
OMe
OMe
O
O
ref. 8
steps
5
7
HO
O
2
,4,6-trichlorobenzoyl
6
chloride,
Et3N, DMAP, PhMe,
ЊC, 3 h
0
ϩ
92%
(from 6)
OMe
OMe
OMe
O
O
Grubbs’ I cat.,
DCM, 40ЊC, 8 h
O
O
O
O
OH
86% at 72%
conversion
2؅
OMe
O
O
7
8
9
H , Pd on C,
EtOH, 18ЊC, 14 h
2
91%
OMe
OMe
O
trimethyl
phosphonoacetate
NaH, THF,
OMe
OMe
OMe
OMe
O
O
O
DIBAL-H, PhMe,
Ϫ78ЊC, 0.5 h
HO
0
to 18ЊC, 1 h
O
O
O
MeO
53%
O
(
from 10)
OH
O
at 84% conversion
O
12
11
10
NaOH, EtOH
H2O, 18ЊC, 3 h
ϩ
then H3O
OMe
OMe
OMe
O
O
2,4,6-trichlorobenzoyl-
chloride, Et3N, DMAP,
PhMe, 112ЊC, 1 h
O
O
OH
TiCl , DCM,
18ЊC, 1 h
4
HO
OH
79%
86%
OMe
HO
(from 12)
O
O
O
O
O
13
14
ent-3
Scheme 1.
the 12-membered lactone ring directly through the use of
RCM procedures failed.[ Recent extensions of this type
of chemistry have allowed us to complete the first total
work served to establish that the absolute stereochemistry of
the natural product had been incorrectly assigned and is, in
fact, as illustrated in structure 2. Such an outcome meant that
it was all the more critical to establish syntheses of the two
remaining members of the series, namely cladospolides C (3)
and D (4), in order to secure the absolute configurations of
these compounds. Herein we report on the first total synthesis
of (−)-cladospolide C (ent-3) in a manner that allows for the
unequivocal assignment of the stereochemistry of both this
material and its naturally occurring enantiomer.
8]
[10]
synthesis of (−)-cladospolide B (ent-2). Once again, com-
pound 5 served as the starting material in this work although
a Mitsunobu reaction was now required to invert the stereo-
chemistry at C2. Furthermore, a mid-stage RCM reaction
was again carried out to generate the relevant nonenolide
and this was followed by a HWE reaction to introduce the
remaining two carbons required in the target ring system,
albeit with an E-configuration about the C C bond. The
E-isomer of target ent-2 was then obtained after macro-
lactonization. In the final step of the reaction sequence, this
isomer was subjected to a photo-rearrangement reaction,
thereby delivering (−)-cladosplide B (ent-2), which consti-
tutes the enantiomer of the natural product. Importantly, this
Results and Discussion
The reaction sequence leading to (−)-cladospolide C (ent-3)
is shown in Scheme 1 and exploits the carboxylic acid
6 obtained in seven steps from cis-1,2-dihydrocatechol 5

Total Synthesis of Cladospolide C
513
◦
tetrachloride in dichloromethane (DCM) at 0 C for 1 h and
then treated with aqueous NaHCO3. Subsequent extractive
work-up delivered (−)-cladospolide C (ent-3) as a crystalline
solid in 86% yield. The melting point of this material was
the same as that reported for the natural product. Moreover,
C6
C5
C21
C7
C4
O16
C22
C3
O20
1
13
the H and C NMR spectroscopic data were in excellent
agreement with those recorded for the naturally occurring
enantiomer (seeTable 1).The infrared spectrum of compound
ent-3 exhibited absorption maxima at νmax 3410, 1715, and
C8
C15
C14
C2
O10
C9
C1
O17
−1
1
647 cm as would be expected for a compound incorpo-
C11
O13
rating hydroxyl ester carbonyl and carbonyl conjugated C C
O12
C18
C19
moieties. The specific rotation of macrolide ent-3, as mea-
−
1
sured in MeOH at a concentration of 0.8 g 100 mL , was
−
84. This compares with a value of +59.7 (c 0.4 in MeOH)
Fig. 3. Anisotropic displacement ellipsoid plot of compound 9 with
labelling of selected atoms. Ellipsoids show 50% probability levels.
Hydrogen atoms are drawn as circles with small radii.
reported for the natural product. These data confirm that the
absolute configuration of (+)-cladospolide C (3) has been
correctly assigned but the distinct difference in magnitude of
these two rotations suggests that the natural product may be
contaminated with some other material that has a very large
and negative rotation.
during the course of establishing a total synthesis of (−)-
cladospolide B (ent-2).[ Thus, Yamaguchi-type esterifica-
tion of acid 6 with (S)-(+)-4-penten-2-ol (7), itself readily
available through enzymatic resolution of the correspond-
8]
[13]
The enantiomers of compounds 5 and 7, namely ent-5
[
8b]
and ent-7,
are both readily available materials so the work
described here can also be regarded as constituting a for-
ing racemic alcohol using Candida antarctica lipase B
(
mal total synthesis of (+)-cladospolide C (3). Furthermore,
CALB),[ afforded the doubly unsaturated ester 8 as the
8b]
and as noted earlier,
[10]
the present work, when considered
pivotal substrate for a RCM reaction. Exposure of this last
compound to Grubbs’first-generation catalyst[ resulted in
the smooth and selective formation of the Z-configured and
crystalline lactone 9 (96% at 72% conversion), which was
subjected to single-crystal X-ray analysis (see Fig. 3 and
Experimental). RCM product 9 was accompanied by only
small quantities (approx. 2%) of the same types of head-to-
tail and head-to-head ‘dimers’encountered during the course
together with our earlier syntheses of (−)-cladospolide A
11]
[8] [10]
(1), (−)-cladospolide B (ent-2),
and the 18-membered
[
12]
lactone (+)-aspicillin,
serves to emphasize the value of
cis-1,2-dihydrocatechols as building blocks in the assem-
bly of various macrolides. This value arises from a capacity
to ‘stitch’ such compounds into larger ring systems through
the sequential application of ozonolysis, methylenation, and
RCM reactions. Work aimed at extending such protocols to
the synthesis of other biologically active macrolides, includ-
ing cladospolide D, is now underway in these laboratories.
Results will be reported in due course.
[10]
of our synthesis of (−)-cladospolide B. This outcome con-
trasts somewhat with observations made during the course of
ꢀ
our earlier work when the C2 epimer of diene 8 was subjected
to exactly the same reaction conditions. Under such circum-
stances, the desired monomeric compound was obtained in
only 66% yield and the dimeric species in approximately
Experimental
5
% yield. These variations in outcome serve to highlight the
General Procedures
significant influence seemingly rather minor changes in sub-
stituent or stereochemistry can exert on the yield of products
arising from RCM reactions that lead to macrolides.
Unless otherwise specified, proton (1H) and carbon (13C) NMR spec-
tra were recorded on a Varian Gemini 300 or Varian Mercury 300
spectrometer operating at 300 MHz for proton and 75 MHz for carbon
[
8b,12]
nuclei. Chemical shifts were recorded as δ values in parts per million
Hydrogenation of the unsaturated lactone 9 at one atmo-
sphere and in the presence of 10% palladium on carbon
afforded the corresponding saturated system 10 (91%). This
was immediately subjected to DIBAL-H reduction to afford
the expected lactol 11 which was, without spectroscopic char-
acterization, engaged in a HWE reaction. In this manner, the
E-configured α, β-unsaturated ester 12 [53% (at 84% con-
version) from 10] was obtained as a clear, colourless oil.
Saponification of this last compound with sodium hydrox-
ide followed by acidic workup then gave the corresponding
free acid 13 which served as the substrate for a Yam-
aguchi macrolactonization reaction and, thus, afforded the
bis-acetal protected form, 14 (79% from ester 12), of the final
target. In the last step of the reaction sequence, com-
pound 14 was exposed to 1.2 molar equivalents of titanium
◦
(
ppm). Spectra were acquired in deuterated chloroform (CDCl3) at 20 C
1
unless otherwise stated. For H NMR spectra recorded in CDCl3, the
peak due to residual CHCl3 (δH 7.26) was used as the internal refer-
ence, while the central peak (δC 77.0) of the CDCl3 triplet was used as
the reference for proton-decoupled 13C NMR spectra. H NMR spec-
troscopic data are recorded as follows: chemical shift (δH) [relative
integral, multiplicity, coupling constant(s) J (Hz)] whereby multiplic-
ity is defined as: s for singlet; d for doublet; t for triplet; q for quartet
or quintet; m for multiplet, or combinations thereof. Infrared spectra
1
(
νmax) were recorded on either a Perkin–Elmer 1800 Fourier-transform
infrared spectrophotometer or a Perkin–Elmer Spectrum One instru-
ment. Samples were analyzed as KBr disks (for solids) or as thin films
on KBr plates (for liquids/oils). Low- and high-resolution mass spec-
tra were recorded on an AUTOSPEC spectrometer. Optical rotations
were determined on a Perkin–Elmer 241 polarimeter at the sodium
D line (598 nm) using spectroscopic-grade chloroform (unless other-
◦
−1
)
wise specified) at 20 C and at the concentrations (c, g 100 mL

5
14
M. G. Banwell, D. T. J. Loong and A. C. Willis
Table 1. Comparison of the 1H and 13C NMR spectroscopic data derived from synthetic
A,B
(
−)-cladospolide C (ent-3) and its natural enantiomer (+)-cladospolide C (3)
δH
δC
Synthetically derived ent-3
Natural product 3
Synthetically
Natural
derived ent-3
product 3
6
6
4
3
3
3
1
1
.80 (1H, dd, J 9.1, 15.8)
.03 (1H, dd, J 0.6, 15.8)
.94 (1H, ddq, J 2.5, 6.3, 8.8)
.96 (1H, dd, J 7.8, 9.1)
.54 (1H, ddd, J 1.8, 7.8, 9.3)
.40–3.05 (2H, br s)
6.82 (1H, dd, J 9.0, 16.0)
6.06 (1H, d, J 16.0)
4.98 (1H, ddq, J 2.5, 6.3. 9.0)
3.98 (1H, dd, J 7.5, 9.0)
3.56 (1H, ddd, J 2.5, 7.5, 7.5)
Not observed
167.0 (C)C
145.6 (CH)
124.1 (CH)
77.4 (CH)
76.4 (CH)
74.5 (CH)
33.8 (CH2)
32.0 (CH2)
166.8
145.3
124.5
77.3
76.4
74.4
34.0
32.0
27.3
24.5
24.1
20.7
.70–1.02 (10H, complex m)
.29 (3H, d, J 6.3)
1.80–1.02 (10H, complex m)
1.31 (3H, d, J 6.3)
2
2
2
2
7.2 (CH2)
4.4 (CH2)
3.9 (CH2)
0.8 (CH3)
A
B
C
All data recorded in CDCl3 at 300 MHz (for H NMR spectra) or 75 MHz (for 13C NMR spectra).
Data listed for (+)-cladospolide (3) have been extracted from reference [3].
DEPT experiments were used to establish these assignments.
1
indicated. Measurements were carried out in a cell with a path length
of 1 dm. Melting points (mp) were recorded on a Reichert hot-stage
apparatus and are uncorrected. Elemental analyses were performed
by the Australian National University Microanalytical Services Unit
based in the Research School of Chemistry, Australian National Uni-
versity, Canberra. Analytical thin-layer chromatography (TLC) was
conducted on aluminium-backed 0.2 mm thick silica gel 60 F254 plates
and the combined organic fractions were washed with water (1 × 20 mL)
then brine (1 × 20 mL) before being dried (MgSO4), filtered, and con-
centrated under reduced pressure to give a yellow oil. Subjection of this
material to flash chromatography (1 : 9 v/v ethyl acetate/hexane elution)
followed by concentration of the relevant fractions (RF 0.4 in 1 : 4 v/v
ethyl acetate/hexane) gave the title compound 8 (2.80 g, 92%) as a clear
+
•
and colourless oil, [α]D −147 (c 0.7) (Found: M 342.2037. C18H30O6
+
•
−1
(
Merck), and the chromatograms were visualized under a 254 nm UV
lamp and/or by treatment with an anisaldehyde–sulfuric acid–ethanol
3/4.5/200 mL)dip, or, occasionally, withaphosphomolybdicacid–ceric
requires M 342.2042). νmax/cm 3078, 2980, 2949, 2834, 1744,
1642, 1451, 1378, 1284, 1183, 1120, 1082, 1039, 994, 974, 915, 888,
852. δH (CDCl3, 300 MHz) 5.84–5.65 (2H, complex m), 5.10–4.90 (5H,
complex m), 4.06 (1H, d, J 9.8), 3.86 (1H, m), 3.26 (3H, s), 3.23 (3H, s),
2.33 (3H, m), 2.08 (1H, m), 1.53 (2H, m), 1.32 (3H, s), 1.27 (3H, s),
1.23 (3H, d, J 6.3). δC (CDCl3, 75 MHz) 168.4 (C), 137.9 (CH), 133.3
(CH), 117.9 (CH2), 114.8 (CH2), 98.8 (C), 98.5 (C), 72.6 (CH), 71.1
(CH), 67.8 (CH), 48.1 (CH3), 47.9 (CH3), 40.0 (CH2), 29.7 (CH2), 28.9
(CH2), 19.3 (CH3), 17.6 (CH3), 17.3 (CH3). m/z (EI, 70 eV) 342 (<2%,
(
sulfate–sulfuric acid–water (37.5 g/7.5 g/37.5 mL/720 mL) dip followed
by heating. Flash chromatography was conducted according to the
[
14]
method of Still and co-workers
using silica gel 60 (mesh size 0.040–
0
.063 mm) as the stationary phase and the analytical reagent (AR) grade
solvents indicated. Many starting materials and reagents were avail-
able from the Aldrich or EGA–Chemie and were used as supplied,
or, in the case of stable liquids, simply distilled. Drying agents and
other inorganic salts were purchased from Ajax or BDH Chemicals.
Reactions employing air- and/or moisture-sensitive reagents and inter-
mediates were carried out under an atmosphere of dry, oxygen-free
nitrogen in flame-dried apparatus. Tetrahydrofuran (THF) and diethyl
ether were dried using sodium metal and then distilled, as required, from
sodium benzophenone ketyl. Methanol was distilled from magnesium
methoxide. Dichloromethane was distilled from calcium hydride. N,N-
Dimethylformamide (DMF) was heated at reflux over calcium hydride
for 16 h then distilled and stored over 4 Å molecular sieves. Organic
solutions obtained from workup of reaction mixtures were dried with
anhydrous magnesium sulfate (MgSO4) then filtered and concentrated
under reduced pressure on a rotary evaporator with the water bath
+
•
• +
M
), 311 (33, [M − CH3O ] ), 257 (17), 194 (11), 153 (12), 126 (60),
116 (49), 109 (100), 101 (75), 97 (33).
(Z,2R,3R,4aR,7S,12aS)-2,3,7,8,12,12a-Hexahydro-2,3-
dimethoxy-2,3,7-trimethyl-4aH-[1,4]-dioxino[2,3-c]
oxecin-5(11H)-one 9
A solution of ester 8 (83 mg, 0.24 mmol) in CH2Cl2 (240 mL) was
deoxygenated by being heated at reflux under an atmosphere of nitro-
[
11]
gen for 4 h. Grubbs’first-generation catalyst (17 mg, 0.02 mmol) was
thenaddedtothecooledsolutionwhichwas, onceagain, heatedatreflux.
After 8 h, the reaction mixture was cooled, treated with DMSO (0.9 mL),
◦
and allowed to stand at 18 C for 16 h. Distillation, at atmospheric pres-
◦
temperature generally not exceeding 40 C.
sure, was used to remove excess solvent and the black residue thus
obtained was dissolved in diethyl ether (50 mL) and washed with H2O
(2 × 10 mL) then brine (1 × 10 mL) before being dried (MgSO4), fil-
tered, and concentrated under reduced pressure to give a light-yellow
oil. Subjection of this material to flash chromatography (1 : 4 v/v ethyl
acetate/hexane elution) afforded three fractions, A–C.
Concentration of fraction A (RF 0.4) afforded the starting ester 8
(23 mg, 28% recovery) as a clear, colourless oil that was identical, in all
respects, with authentic material.
Synthetic Studies
ꢀ
ꢀ
(
5
S)-Pent-4 -en-2 -yl (2R,3S,5R,6R)-3-(But-3-enyl)-5,6-dimethoxy-
,6-dimethyl-1,4-dioxane-2-carboxylate 8
[
10]
A solution of acid 6
(2.47 g, 9.0 mmol) in toluene (90 mL)
◦
was cooled to 0 C and treated sequentially with DMAP (1.10 g,
9
mmol), (S)-(+)-4-penten-2-ol (7) (1.80 mL, 18.0 mmol), and tri-
ethylamine (1.90 mL, 14.0 mmol). 2,4,6-Trichlorobenzoyl chloride
Concentration of fraction B (RF 0.3) gave the title compound 9
◦
(
1.40 mL, 9.0 mmol) was then added dropwise to the reaction mixture
(47 mg, 86% at 72% conversion) as colourless plates, mp 118–122 C,
◦
• +
and the ensuing white slurry was maintained at 0 C for 3 h before being
quenched with NaHCO3 (20 mL of a saturated aqueous solution). The
separated aqueous layer was extracted with diethyl ether (3 × 100 mL)
[α]D −165 (c 0.5) (Found: C 60.8, H 8.2%, [M − CH3O ] 283.1547.
C16H26O6 requires C 61.1, H 8.3%, 283.1545). δH (CDCl3, 300 MHz)
5.57 (2H, m), 5.10 (1H, m), 3.99 (2H, m), 3.29 (3H, s), 3.26 (3H, s), 2.83

Total Synthesis of Cladospolide C
515
(
1
9
1H, m), 2.36–1.67 (5H, complex m), 1.34 (3H, d, J 6.7), 1.30 (3H, s),
.27 (3H, s). δC (CDCl3, 75 MHz) 168.5 (C), 133.9 (CH), 123.3 (CH),
8.7 (C), 98.2 (C), 75.2 (CH), 70.2 (CH), 67.5 (CH), 48.1 (CH3), 47.8
to give a light-yellow oil. Subjection of this material to flash chro-
matography (3 : 7 v/v ethyl acetate/hexane elution) afforded two major
fractions, A and B.
(
CH3), 31.5 (CH2), 30.1 (CH2), 23.1 (CH2), 18.6 (CH3), 17.7 (CH3),
Concentration of fraction A (RF 0.3) gave the starting lactone 10
(177 mg, 16% recovery) as a white crystalline solid that was identical,
in all respects, with authentic material.
−
1
1
1
2
1
7.1 (CH3). νmax/cm 2944, 2835, 1733, 1459, 1378, 1280, 1205,
•
+
121, 1036, 888, 853, 828, 741. m/z (EI, 70 eV) 313 (<5%, [M − H ] ),
•
+
• +
97 (<5, [M − HO ] ), 283 (12, [M − CH3O ] ), 166 (10), 122 (50),
Concentration of fraction B (RF 0.1) gave the title compound 12
(565 mg, 53% at 84% conversion) as a clear, colourless oil, [α]D −133 (c
01 (48), 96 (30), 81 (20), 78 (38), 68 (100).
•
+
Concentration of fraction C (RF 0.1) gave an inseparable mixture of
3.1) (Found: [M − CH3O ] 343.2118. C19H34O7 requires 343.2121).
1
−
3432, 2938, 2858, 1727, 1659, 1438, 1375, 1307, 1197,
two compounds (2 mg), which were tentatively identified as the head-
to-tail and head-to-head ‘dimers’derived from twofold cross-metathesis
νmax/cm
1125, 1037, 981, 888, 851. δH (CDCl3, 300 MHz) 6.84 (1H, dd, J 5.8
and 15.7), 6.18 (1H, dd, J 1.5 and 15.7), 4.13 (1H, ddd, J 1.5, 5.8, and
9.6), 3.76 (1H, m), 3.72 (3H, s), 3.50 (1H, m), 3.22 (6H, s), 1.64 (1H,
br s), 1.50–1.22 (10H, complex m), 1.29 (3H, d, J 2.5), 1.17 (3H, s),
1.15 (3H, s). δC (CDCl3, 75 MHz) 166.5 (C), 142.9 (CH), 123.0 (CH),
98.6 (C), 98.5 (C), 71.6 (CH), 70.6 (CH), 67.8 (CH), 51.5 (CH3), 47.8
•
+
of the starting diene 8 (Found: [M − CH3O ] 597.3274. C32H52O12
−
1
requires 597.3275). νmax/cm 2945, 1734, 1453, 1377, 1281, 1122,
•
+
1
(
1
040, 972, 889, 733. m/z (EI, 70 eV) 597 (<5%, [M − CH3O ] ), 566
3), 565 (8), 416 (19), 332 (13), 205 (24), 148 (24), 122 (46), 116 (64),
01 (92), 81 (100).
(
CH3), 47.7 (CH3), 39.0 (CH2), 30.4 (CH2), 29.4 (CH2), 25.5 (CH2),
2
4.9 (CH2), 23.3 (CH3), 17.5 (CH3), 17.4 (CH3). m/z (EI, 70 eV) 343
(
4
2R,3R,4aR,7S,12aS)-Octahydro-2,3-dimethoxy-2,3,7-trimethyl-
aH-[1,4]dioxino[2,3-c]oxecin-5(7H)-one 10
• +
(
8%, [M − CH3O ] ), 311 (11), 225 (10), 194 (11), 138 (12), 117 (30),
116 (100), 101 (65).
Palladium on charcoal (118 mg, 10 wt%) was added to a magneti-
cally stirred solution of the unsaturated lactone 9 (1.18 g, 3.8 mmol) in
absolute ethanol (40 mL). A balloon of dihydrogen was attached and the
reaction vessel was evacuated and flushed twice with dihydrogen. The
resultant black suspension was stirred vigorously under an atmosphere
Compound 14
A magnetically stirred solution of methyl ester 12 (533 g, 1.42 mmol)
in ethanol (5 mL) was treated with NaOH (5 mL of a 1.5 M aqueous solu-
tion, 7.5 mmol). The ensuing mixture was allowed to stir at 18 C for 3 h
◦
◦
of dihydrogen at 18 C for 14 h, then filtered through a pad of Celite,
before being diluted with water (10 mL), acidified to pH 3 (with 1 M
aqueous HCl), and extracted with DCM (3 × 30 mL). The combined
organic phases were dried (MgSO4), filtered, and concentrated under
reduced pressure to give the carboxylic acid 13 as a light-yellow oil.
This unstable material was immediately dissolved in THF (28 mL) that
contained triethylamine (238 µL, 1.71 mmol), the resultant solution was
and the filtrate was concentrated under reduced pressure to give a grey-
yellow oil. Subjection of this material to flash chromatography (1 : 9 v/v
diethyl ether/hexane elution) and concentration of the relevant fractions
(
(
0
RF 0.3 in 1 : 4 v/v ethyl acetate/hexane) gave the title compound 10
◦
1.08 g, 91%) as white, crystalline plates, mp 112–113 C, [α]D −137 (c
•
+
.5) (Found: [M − CH3O ] 285.1705. C16H28O6 requires 285.1702).
◦
cooled to 0 C and, while being stirred magnetically, was treated with
−1
νmax/cm
2944, 2834, 1736, 1455, 1377, 1275, 1211, 1120, 1037,
2
,4,6-trichlorobenzoyl chloride (222 µL, 1.42 mmol) then allowed to
1
005, 854, 749. δH (CDCl3, 300 MHz) 4.96 (1H, m), 4.17 (1H, m), 4.02
1H, d, J 9.9), 3.31 (3H, s), 3.27 (3H, s), 1.89 (1H, m), 1.70 (1H, m),
.59–1.42 (6H, complex m), 1.33 (3H, d, J 6.5), 1.32 (3H, s), 1.27 (3H,
s), 1.15 (1H, m), 0.95 (1H, m). δC (CDCl3, 75 MHz) 168.4 (C), 99.0 (C),
8.8 (C), 75.1 (CH), 74.7 (CH), 66.1 (CH), 48.1 (CH3), 47.9 (CH3), 32.5
CH2), 29.9 (CH2), 25.2 (CH2), 23.9 (CH2), 23.3 (CH3), 19.7 (CH2),
◦
warm to 18 C. After 16 h, TLC analysis indicated the complete con-
sumption of starting materials. As a consequence, the reaction mixture
was diluted with toluene (640 mL) and then added by cannula, over
approximately 0.5 h, to a solution of DMAP (869 mg, 7.11 mmol) in
refluxing toluene (70 mL). The ensuing mixture was heated at reflux
for 1 h then cooled, quenched with NaHCO3 (100 mL of a saturated
aqueous solution), and extracted with diethyl ether (3 × 100 mL). The
combined organic fractions were washed with water (1 × 20 mL) and
brine (1 × 20 mL) then dried (MgSO4), filtered, and concentrated under
reduced pressure to give a light-yellow oil. Subjection of this mate-
rial to flash chromatography (1 : 9 v/v ethyl acetate/hexane elution) and
concentration of the relevant fractions (RF 0.2) gave the title macro-
(
1
9
(
1
1
•
+
7.8 (CH3), 17.2 (CH3). m/z (EI, 70 eV) 285 (38%, [M − CH3O ] ),
68 (28), 139 (18), 116 (51), 108 (21), 101 (100), 100 (52), 81 (68).
Methyl ( E)-3-{(2S,3S,5R,6R)-3-[( S)-6-Hydroxyheptyl]-5,6-
dimethoxy-5,6-dimethyl-1,4-dioxan-2-yl}acrylate 12
DIBAL-H (4.2 mL of a 1 M solution in hexane, 4.2 mmol) was added
lactone 14 (385 mg, 79%) as a clear, colourless oil, [α]D −229 (c
in portions over 1 h to a magnetically stirred solution of lactone 10
• +
0
.4) (Found: [M − CH3O ] 311.1856. C18H30O6 requires 311.1858).
◦
(
1.08 g, 3.42 mmol) in toluene (34 mL) maintained at −78 C under
−1
νmax/cm 2941, 1721, 1457, 1375, 1256, 1122, 1036, 993, 862. δH
an atmosphere of nitrogen. Stirring was continued for 0.5 h then the
reaction mixture was treated with NaK tartrate (20 mL of a 1 M aque-
ous solution). The resultant mixture was warmed to 0 C over 10 min
(
CDCl3, 300 MHz) 6.80 (1H, dd, J 9.1 and 15.7), 6.07 (1H, dd, J 0.4
and 15.7), 4.94 (1H, m), 4.07 (1H, app. t, J ≈ 9.1), 3.65 (1H, m), 3.26
◦
(
(
(
(
1
(
3H, s), 3.24 (3H, s), 1.72–1.00 (10H, complex m), 1.27 (9H, br s). δC
CDCl3, 75 MHz) 166.7 (C), 142.8 (CH), 126.0 (CH), 99.3 (C), 98.3
C), 74.4 (CH), 73.5 (CH), 71.5 (CH), 48.0 (CH3), 47.8 (CH3), 34.3
CH2), 29.6 (CH2), 27.1 (CH2), 25.4 (CH2), 24.2 (CH2), 20.6 (CH3),
and then extracted with ethyl acetate (3 × 50 mL). Hydrochloric acid
(
a few drops of a 1 M aqueous solution) was added to disperse the emul-
sion formed during extraction. The combined organic fractions were
washed with brine (1 × 5 mL) before being dried (MgSO4), filtered,
and concentrated under reduced pressure to give lactol 11 as an unsta-
ble, yellow oil that was used immediately in the next step of the reaction
sequence. Thus, a magnetically stirred suspension of NaH (270 mg of
a 60% dispersion in mineral oil, 6.75 mmol) in THF (34 mL) main-
• +
7.7 (CH3), 17.4 (CH3). m/z (EI, 70 eV) 343 (<1%, [M + H ] ), 311
• +
20, [M − CH3O ] ), 117 (52), 116 (100), 101 (93).
(
−)-Cladospolide C (ent-3)
A magnetically stirred solution of bis-acetal 14 (344 mg, 1.0 mmol) in
◦
tained at 0 C under an atmosphere of nitrogen was treated dropwise with
◦
trimethyl phosphonoacetate (1.10 mL, 6.8 mmol). The ensuing mixture
CH2Cl2 (10 mL) was cooled to 0 C and treated with freshly distilled
◦
was warmed to 18 C over 0.5 h and then a solution of lactol 11 (obtained
TiCl4 (130 µL, 1.2 mmol).The resultant yellow solution was maintained
at this temperature for 1 h then NaHCO3 (10 mL of a saturated aqueous
solution) was added. After 0.5 h, the resultant white slurry was filtered
through a pad of Celite and the filtrate was extracted with ethyl acetate
(3 × 20 mL). The combined organic fractions were washed with water
(1 × 10 mL) and brine (1 × 10 mL) before being dried (MgSO4), fil-
tered, and concentrated under reduced pressure to give a light-yellow
oil. Subjection of this material to flash chromatography (ethyl acetate
◦
as described above) in THF (34 mL) was added by cannula at 0 C. After
being allowed to re-warm to 18 C over 1 h, the reaction mixture was
◦
diluted with ethyl acetate (30 mL) and quenched with NH4Cl (30 mL
of a saturated aqueous solution). The separated aqueous fraction was
extracted with ethyl acetate (3 × 50 mL) and the combined organic frac-
tions were washed with water (1 × 10 mL) and brine (1 × 10 mL) before
being dried (MgSO4), filtered, and concentrated under reduced pressure

5
16
M. G. Banwell, D. T. J. Loong and A. C. Willis
elution) followed by concentration of the relevant fractions (RF 0.3 in
[4] H. Zhang, H. Tomoda, N. Tabata, H. Miura, M. Namikoshi,
Y. Yamaguchi, R. Masuma, S. Omura, J. Antibiot. 2001, 54, 635.
[5] (a) A. Hirota, H. Sakai, A. Isogai,Y. Kitano, T. Ashida, H. Hirota,
T. Takahashi, Agric. Biol. Chem. 1985, 49, 903.
7
8
9
: 3 v/v ethyl acetate/hexane) gave ent-cladospolide C (ent-3) (198 mg,
◦
[3]
6%) as white needles, mp 90–91 C {lit. [for (+)-cladospolide C]
◦
[3]
0–91 C}, [α]D −84 (c 0.8 in MeOH) {lit. [α]D [for (+)-cladospolide
+
C] +59.7 (c 0.4 in MeOH)} (Found: [M + H] 229.1440. C12H20O4
(b) H. Hirota, A. Hirota, H. Sakai, A. Isogai, T. Takahashi, Bull.
Chem. Soc. Jpn. 1985, 58, 2147.
−
1
requires 229.1440). νmax/cm 3410, 2936, 2865, 1715, 1647, 1459,
1
3
2
354, 1263, 1166, 1123, 1036, 995, 869, 728, 667, 605. δH (CDCl3,
[6] C. J. Smith, D. Abbanat, V. S. Bernan, W. M. Maiese, M. Green-
stein, J. Jompa, A. Tahir, C. M. Ireland, J. Nat. Prod. 2000, 63,
142. doi:10.1021/NP990361W
00 MHz) seeTable 1. δC (CDCl3, 75 MHz) seeTable 1. m/z (EI, 70 eV)
•
+
• +
29 (<1%, [M + H ] ), 211 (2, [M − HO ] ), 184 (5), 127 (21), 109
(
63), 102 (85), 84 (87), 57 (73), 55 (100).
[7] F. Hansske, O. H. Wilhelms, H. Anke, German Pat. DE 3906214
1
990.
[
8] (a) M. G. Banwell, K. A. Jolliffe, D. T. J. Loong, K. J. McRae,
F. Vounatsos, J. Chem. Soc., Perkin Trans. 1, 2002, 22.
(b) M. G. Banwell, D. T. J. Loong, Org. Biomol. Chem. 2004, 2,
2050. doi:10.1039/B401829J
Crystallographic Studies
Crystal data for 9: C16H26O6, M 314.38, T 200(1) K, monoclinic,
space group P21, Z 2, a 6.9026(2), b 12.7895(4), c 10.1239(3) Å,
◦
3
−3
β 109.540(2) , V 842.27(4) Å , Dc 1.240 g cm , 2012 unique data
[9] For reviews on methods for generating cis-1,2-dihydrocatechols
by microbial dihydroxylation of the corresponding aromatics, as
well as the synthetic applications of these metabolities, see:
◦
2θmax 55 ), 1580 with I > 2.0σ(I), R 0.0292, Rw 0.0340, S 1.0781.
(
(
3
a) T. Hudlicky, D. Gonzalez, D. T. Gibson, Aldrichim. Acta 1999,
2, 35.
Structure Determination
Images were measured on a Nonius Kappa CCD diffractome-
(b) M. G. Banwell, A. J. Edwards, G. J. Harfoot, K. A. Jolliffe,
M. D. McLeod, K. J. McRae, S. G. Stewart, M. Vögtle, PureAppl.
Chem. 2003, 75, 223.
ter (MoKα, graphite monochromator, λ 0.71073 Å) and data were
[
15]
extracted using the DENZO package.
Structure solution was
[
16]
by direct methods (SIR92). The structure of compound 9 was
refined using the CRYSTALS program package.
(c) R. A. Johnson, Org. React. 2004, 63, 117.
[
17]
Atomic coordi-
[10] K. A. B. Austin, M. G. Banwell, D. T. J. Loong, A. D. Rae,
A. C. Willis, Org. Biomol. Chem. 2005, 3, 1081. doi:10.1039/
B417685E
nates, bond lengths and angles, and displacement parameters have
been deposited at the Cambridge Crystallographic Data Centre
(
CCDC reference number 265981). These data can be obtained free
[11] E. L. Dias, S. T. Nguyen, R. H. Grubbs, J. Am. Chem. Soc. 1997,
119, 3887. doi:10.1021/JA963136Z
[12] M. G. Banwell, K. J. McRae, Org. Lett. 2000, 2, 3583.
doi:10.1021/OL006457V
[13] C. C. R. Allen, C. E. Walker, N. D. Sharma, N. A. Kerley,
D. R. Boyd, H. Dalton, Biocatal. Biotrans. 2002, 20, 257.
doi:10.1080/10242420290029454
of charge from www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting the Cambridge Crys-
tallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033.
Acknowledgments
[14] W. C. Still, M. Kahn, A. Mitra, J. Org. Chem. 1978, 43, 2923.
doi:10.1021/JO00408A041
Dr Gregg Whited (Genencor International Inc., Palo Alto,
CA) and Professor Tomas Hudlicky (Brock University) are
warmly thanked for providing us with generous quantities of
compound 5.
[
15] DENZO-SMN: Z. Otwinowski, W. Minor, Processing of X-Ray
Diffraction Data Collected in Oscillation Mode, in Methods
in Enzymology, Volume 276: Macromolecular Crystallography,
Part A (Eds C. W. Carter, Jr, R. M. Sweets) 1997, pp. 307–326
(
Academic Press: New York, NY).
[
16] A. Altomare, M. Cascarano, C. Giacovazzo, A. Guagliardi,
M. C. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 1994,
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