Enantiospecific synthesis of the phospholipase A2 inhibitors (-)-cinatrin C1 and (+)-cinatrin C3

Article abstract of DOI:10.1039/b308028e

The enantiospecific synthesis of (-)-cinatrin C₁ (3) and (-)-cinatrin C₃ (5) from the D-arabinose derivative 9 is described. The stereochemistry at C₂ was introduced via a chelation-controlled addition of a carbanion to a-hydroxy ketone 8. The best selectivity was achieved by use of the Grignard reagent derived from trimethylsilylacetylene. Transformation of the terminal alkyne into methyl ester 17 followed by acetal hydrolysis and selective lactol oxidation gave cinatrin C₁ dimethyl ester (7). Base hydrolysis and acid induced relactonization then gave a 1 : 1 mixture of cinatrins C₁ (3) and C₃ (5).

Full text of DOI:10.1039/b308028e

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Enantiospecific synthesis of the phospholipase A2 inhibitors
(؊)-cinatrin C1 and (؉)-cinatrin C3
Anthony N. Cuzzupe, Romina Di Florio, Jonathan M. White and Mark A. Rizzacasa*
School of Chemistry, The University of Melbourne, Victoria, 3010, Australia.
E-mail: masr@unimelb.edu.au
Received 16th July 2003, Accepted 1st September 2003
First published as an Advance Article on the web 22nd September 2003
The enantiospecific synthesis of (Ϫ)-cinatrin C1 (3) and (ϩ)-cinatrin C3 (5) from the -arabinose derivative 9 is
described. The stereochemistry at C2 was introduced via a chelation-controlled addition of a carbanion to α-hydroxy
ketone 8. The best selectivity was achieved by use of the Grignard reagent derived from trimethylsilylacetylene.
Transformation of the terminal alkyne into methyl ester 17 followed by acetal hydrolysis and selective lactol
oxidation gave cinatrin C1 dimethyl ester (7). Base hydrolysis and acid induced relactonization then gave a 1 : 1
mixture of cinatrins C1 (3) and C3 (5).
esters, which also served to confirm the number of carboxylic
Introduction
acid groups present.4 The relative configuration of cinatrin C3
Endogenous eicosanoids such as the prostanoids (prosta-
(5) was determined from its single crystal X-ray structure while
the absolute stereochemistry was obtained by CD spectroscopy.
In 1997, Evans and co-workers reported the first total
glandins and thromboxanes) and leukotrienes, are powerful
mediators in all types of inflammation.1 The main source of
eicosanoids is arachidonic acid, a 20 carbon unsaturated fatty
acid. The bulk of arachidonate in mammalian cells is found
esterified in the fatty acyl chains of glycerophospholipids,
synthesis of (Ϫ)-cinatrin C1 (3) and (ϩ)-cinatrin C3 (5) using a
tartrate aldol methodology.5 This work revealed the structures
of the cinatrins to be enantiomeric to those originally
almost exclusively in the 2-position.2 The rate limiting step in
reported.4 The novel structure and biological activity of this
the biosynthesis of eicosanoids is catalysed by the lipolytic
group of natural products makes them interesting targets, and
enzyme, phospholipase A2 (PLA2) which specifically hydrolyses
we have embarked on a synthetic program towards these
the 2-acyl position of a glycerophospholipid.1 PLA2 also
compounds. In 2002 we reported the first enantiospecific
mediates the formation of lyso-PAF, a precursor to platelet
activating factor (PAF). PAF is released from many inflam-
matory cells when activated and is responsible for producing
synthesis of (Ϫ)-cinatrin B (2) from -arabinose.6 We now
report the enantiospecific synthesis of (Ϫ)-cinatrin C1 (3) and
(ϩ)-cinatrin C3 (5) from a common intermediate used in the
most phenomena of inflammation.3
(Ϫ)-cinatrin B (2) synthesis.6
In a screening program for microbial products which possess
pharmacological activity, Itazaki and co-workers isolated a
Results and discussion
family of PLA2 inhibitors, cinatrins A (1), B (2), C1 (3), C2 (4)
and C3 (5), from the fermentation broth of the microorganism
Circinotrichum falcatisporum RF-641.4 Since inhibition of the
enzymatic activity of PLA2 could be therapeutically beneficial,
the cinatrins show potential for use as anti-inflammatory agents.
Retrosynthetic analysis
Structural studies on cinatrins C1 (3) and C3 (5) revealed that
the two natural products could be obtained in a 1 : 1 ratio by
acidification of the seco acid salt 6 produced by hydrolysis of
either 3 or 5 with aqueous sodium hydroxide (Scheme 1).4
Scheme 1 Conversion of 3 or 5 into a 1 : 1 mixture of 3 and 5.4
In light of the above result, it was envisaged that synthesis of
the methyl ester of either natural product might provide access
to both 3 and 5. Therefore, it was proposed that cinatrins C1 (3)
and C3 (5) could be obtained by hydrolysis and acidification
of the cinatrin C1 dimethyl ester (7)4 (Scheme 2). A chelation
controlled7 carbanion addition to α-hydroxy ketone 8 in prefer-
ence to the ester functionality should provide a method for the
The structures of all members of the cinatrin family were
determined by elemental analysis, SI-mass spectrometry, vari-
ous NMR techniques and analysis of the respective methyl
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 5 7 2 – 3 5 7 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Scheme 4 Reagents and conditions: (a) 60 psi H2, Pd(OH)2/C, MeOH,
88%; (b) TESCl, imidazole, DMF, 84%; (c) (i) Dess–Martin
periodinane, CH2Cl2, pyridine; (ii) 48% HF/H2O/MeCN, rt.
Scheme 2 Retrosynthetic analysis of cinatrins C1 (3) and C3 (5).
introduction of C2† acid functionality. An analagous addition
reaction was utilized successfully in the synthesis of cinatrin B
(2)6 where the free hydroxyl group delivered an appropriate
nucleophile to the ketone from the β-face. The α-hydroxy
ketone 8 could be synthesised from ester 9 which in turn is
obtained from -arabinose.6
Protection of the less hindered alcohol in 12 by treatment
with TESCl and imidazole provided monoether 13. Oxidation
with Dess–Martin periodinane10 buffered with pyridine
followed by HF induced desilylation gave labile α-hydroxy
ketone 8, which was utilized immediately in the addition
reaction.
With the α-hydroxy ketone 8 in hand, attention then turned
to introduction of the C2 acid functionality. A number of
reagents were trialed for the chelation-controlled addition11–13
to 8 as outlined in Scheme 5. Treatment of 8 with vinylmag-
nesium bromide in THF at Ϫ78 ЊC gave a separable mixture
of C2 epimeric alkenes 14 and 15 in good overall yield. These
were obtained in a 1.6:1 ratio favouring 14, which was found to
possess correct configuration at the newly formed stereocentre
(see Scheme). On the other hand, treatment of 8 with lithium
trimethylsilylacetylide followed by desilylation and partial
reduction of the resultant alkyne with hydrogen gas in the
presence of Lindlar catalyst gave 14 with a higher ds but in
lower yield. Treatment of 8 with the Grignard reagent derived
from TMS acetylene in THF at Ϫ78–0 ЊC followed by desilyl-
ation and partial reduction gave 14 in highest ds and excellent
overall yield. The stereoselectivity of this reaction was sensitive
to the rate of addition of a solution of the anion to ketone 8
and how long the reaction was maintained at low temperature.
Protection of the less hindered alcohol by treatment of minor
epimer 15 with TBSOTf and 2,6-lutidine provided monoether
16 (Scheme 5). Observation of an NOE between the vinylic
Enantiospecific synthesis of cinatrins C1 (3) and C3 (5)
The route towards cinatrins C1 (3) and C3 (5) began with the
known methyl ester 9, available in eight steps from the chiral
pool starting material -arabinose (Scheme 3).6 The key step in
the synthesis of 9 involved an Ireland–Claisen rearrangement
of chiral ester 10 in the presence of a β-leaving group, by treat-
ment with TMSCl/NEt3 and LDA at Ϫ100 ЊC.8,9 Warming to
room temperature followed by base hydrolysis and esterification
afforded esters 9 and 11 in excellent yield and in 73 : 27 ratio.
The stereochemistry of the major ester 9 was assigned on the
basis of an NOE observed between the allylic protons and H2
in the NOESY spectrum of 9 (Scheme 3).6
Debenzylation and saturation of the double bond in 9 by
exposure to 60 psi H2 gas and Pd(OH)2 gave diol 12 as a
crystalline solid (Scheme 4). Initial work carried out in the
enantiomeric series derived from -arabinose provided ent-12,
the enantiomer of diol 12 (Fig. 1) using an identical sequence.
An X-ray structure of ent-12 was obtained which confirmed the
stereochemistry of diol 12 and therefore 9 to be as depicted in
Scheme 3.
Scheme 5 Reagents and conditions: (a) H2C᎐CHMgBr, THF, Ϫ78 ЊC,
᎐
᎐
59% overall from 13, Ratio 1.6 : 1, 14 : 15; (b) (i) TMSC᎐CLi, THF,
᎐
Fig. 1 X-Ray structure of ent-12. Some protons are omitted for
clarity.
Ϫ78 ЊC; (ii) TBAF, THF, rt; (iii) H2, Lindlar, MeOH, 27% overall from
᎐
13, Ratio 3 : 1, 14 : 15; (c) (i) TMSC᎐CMgBr, THF, Ϫ78–0 ЊC; (ii)
᎐
TBAF, THF, rt; (iii) H2, Lindlar, MeOH, 57% overall from 13, Ratio 5 :
1, 14 : 15; (d) TBSOTf, 2,6-lutidine, 0 ЊC, 79%.
† Cinatrin numbering is used throughout.
Scheme 3 Reagents and conditions: (a) LDA, TMSCl/NEt3, HMPA/THF, Ϫ100 ЊC–rt then aqueous NaOH, CH2N2, 97%, Ratio 73 : 27, 9 : 11.6
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 5 7 2 – 3 5 7 7
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1
Table 1 Comparison of H NMR data δH (400 MHz, CD3OD) for
Table 3 Comparison of 13C NMR data δC (100 MHz, d6-DMSO) for
natural4 and synthetic cinatrins C1 (3) and C3 (5)
natural5 and synthetic cinatrin C1 (3)
Position
Natural (m, J/Hz)
Synthetic (m, J/Hz)
Cinatrin C1 (3)
Cinatrin C3 (5)
Position
Natural
Synthetic
Natural
Synthetic
C1H
4.69 (s)
4.73 (s)
C4H
C4H
C5H
C5H
(CH2)9CH3
CH3
2.14 (app. t, 8.4)
1.70 (m)
1.50 (m)
1.39 (m)
1.28 (m)
2.16 (dd, 13.6, 4.4)
1.68 (dd, 13.6, 4.8)
1.50 (m)
1.37 (m)
1.28 (m)
1
1-CO
2
2-CO
3
3-CO
4
5
6
7
8
9
10
11
12
13
14
15
73.1
173.6
84.0
170.4
86.5
170.6
30.8
23.5
28.7
28.8
29.2
29.0
29.0
29.0
28.7
31.3
22.1
13.9
73.0
173.2
83.9
170.0
86.4
170.2
30.8
23.5
28.7
28.9
29.2
29.0
29.0
29.0
28.7
31.3
22.1
13.9
79.4
167.5
81.3
170.4
78.7
174.6
30.5
21.0
29.6
28.9
29.0
29.0
29.0
29.0
28.7
31.2
22.0
13.9
79.5
167.5
81.4
170.5
78.8
174.6
30.6
21.1
29.7
29.0
29.0
29.0
29.0
29.1
28.7
31.3
22.1
14.0
0.89 (app. t, 6.9)
0.89 (app. t, 6.8)
1
Table 2 Comparison of H NMR data δH (400 MHz, CD3OD) for
natural5 and synthetic cinatrin C3 (5)
Position
Natural (m, J/Hz)
Synthetic (m, J/Hz)
C1H
C4H2
C5H2
(CH2)9CH3
CH3
5.40 (s)
5.40 (s)
1.83 (app. t, 8.4)
1.55–1.41 (m)
1.28 (m)
1.83 (app. t, 8.4)
1.56–1.42 (m)
1.28 (m)
0.89 (app. t, 6.9)
0.89 (app. t, 6.8)
Experimental
General
proton and H1 confirmed that ether 16 derived from 15
possesses incorrect configuration at C2. This in turn confirmed
major epimer 14 had the desired configuration.
Optical rotations were recorded in a 10 cm microcell. High
resolution mass spectra (HRMS) electrospray ionisation (ESI)
were run at Monash University, Clayton, Victoria. Proton
nuclear magnetic resonance (1H NMR, 300 and 400 MHz) and
proton decoupled carbon nuclear magnetic resonance spectra
(13C NMR, 75.5 and 100 MHz) were recorded for deuterio-
chloroform solutions with residual chloroform as internal
standard unless otherwise stated. Microanalyses were carried
out by the Campbell Microanalytical Laboratory at the
University of Otago, Dunedin, New Zealand. Analytical thin
layer chromatography (TLC) was conducted on aluminium
backed 2 mm thick silica gel GF254. Compounds were visualised
with solutions of 20% w/w phosphomolybdic acid in ethanol,
20% w/w potassium permanganate in water or under UV (365
nm). Anhydrous THF and diethyl ether were distilled from
sodium benzophenone ketyl and sodium metal under a nitrogen
atmosphere. Petrol refers to the fraction boiling at 40–60 ЊC. All
other commercial reagents were used as received. All air and
moisture sensitive reactions were performed in glassware that
was either flame dried under an atmosphere of dry argon or
oven dried at 150 ЊC.
The final steps of the synthesis are outlined in Scheme 6.
Cleavage of the terminal alkene in 14 by ozonolysis followed
by oxidation with sodium chlorite and methylation yielded
the methyl ester 17. One-step oxidation14 to form the lactone
failed, so a two step sequence was employed. Acid hydrolysis of
the methyl ketal followed by selective lactol oxidation15 gave
(Ϫ)-cinatrin C1 dimethyl ester (7). Some of the C1 epimer 18
was also isolated. The data obtained for synthetic 7 compared
well with that reported for the dimethyl ester derived from
natural cinatrin C1 (3).4 Base hydrolysis of 7 followed by
relactonization and purification by reverse-phase HPLC
afforded (Ϫ)-cinatrin C1 (3) and (ϩ)-cinatrin C3 (5) in a 1 : 1
ratio. The data obtained for the synthetic compounds com-
pared well with that for the natural products4,5 (see Tables 1, 2
and 3). In addition, reverse phase HPLC retention times of the
synthetic samples of 3 and 5 compared well with those of the
natural materials.
Synthesis
Methyl methyl-4-C-dodecanyl-ꢀ-L-xylofuranosiduronate (12).
To a solution of the dibenzyl ether 96 (463 mg, 0.859 mmol) in
MeOH (14 mL) was added Pd(OH)2 on C, 20 wt% Pd (dry
basis) (158 mg, 0.297 mmol) and the resulting suspension was
stirred vigorously under a H2 atmosphere at 60 psi for 22 h. The
suspension was then filtered through Celite and the eluent was
concentrated under reduced pressure. The residue was purified
by flash chromatography with 40% EtOAc/petrol as eluent to
give the diol 12 (302 mg, 97%) as a crystalline solid: mp 60–61
ЊC; [α]2D4 ϩ75.5 (c 0.41, CHCl3) (Found: C, 63.11; H, 9.86%;
C19H36O6 requires C, 63.30; H, 10.07%); νmax (Nujol) 3451,
2915, 1721, 1466, 1219, 1096, 1052, 967, 722 cmϪ1; δH (300
MHz, CDCl3) 0.88 (t, J = 6.3 Hz, 3H), 1.24 (m, 20H), 1.59 (br s,
1H), 1.97 (m, 2H), 3.35 (d, J = 11.1 Hz, 1H), 3.51 (s, 3H), 3.78
(s, 3H), 3.92 (d, J = 10.0 Hz, 1H), 4.19 (br s, 1H), 4.96 (s, 1H);
δC (75.5 MHz, CDCl3) 14.0, 22.6, 24.0, 29.2, 29.4, 29.46, 29.53,
29.6, 31.8, 37.3, 52.0, 55.3, 80.1, 81.8, 91.7, 109.3, 173.1.
Scheme 6 Reagents and conditions: (a) (i) O3, Me2S, CH2Cl2/MeOH,
Ϫ78 ЊC; (ii) NaClO2/NaH2PO4ؒH2O; (iii) excess CH2N2, 33% overall;
(b) 80% aq. AcOH/10% aq. HCl, 100 ЊC, 3.5 h; (ii) I2/CaCO3, MeOH/
H20, 70 ЊC, 16 h, 41% 7; 10% 18 for 2 steps; (c) 2 M aq. NaOH/THF,
40 ЊC, 22 h then 2 M aq. HCl, 68%.
In conclusion, we have completed the total synthesis of
(Ϫ)-cinatrin C1 (3) and (ϩ)-cinatrin C3 (5) from the common
precursor, cinatrin C1 dimethyl ester (7). This was synthesised
from a common intermediate also utilised for the synthesis
of (Ϫ)-cinatrin B (2).6 A key step was the chelation-controlled
addition to an α-hydroxy ketone in the presence of an ester to
introduce the C2 acid functionality.
Methyl
methyl-2-O-(triethylsilyl)-4-C-dodecanyl-ꢀ-L-xylo-
furanosiduronate (13). To a solution of the diol 12 (406 mg,
1.13 mmol) and imidazole (152.8 mg, 2.24 mmol) in DMF
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 5 7 2 – 3 5 7 7
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(11 mL) at Ϫ40 ЊC under argon was added chlorotriethylsilane
(207 µL, 1.23 mmol) and the solution was stirred for 1 h. Water
and Et2O were added and the aqueous phase was extracted with
Et2O. The combined extracts were washed with water, brine
and dried (MgSO4). The solvent was removed under reduced
pressure and the residue was purified by flash chromatography
with 5–10% EtOAc/petrol as eluent to give the TES ether 13
(451 mg, 84%) as a colourless oil: [α]1D9 ϩ44.2 (c 1.04, CHCl3);
νmax (thin film) 3510, 2927, 2856, 1758, 1464, 1115 cmϪ1; δH (300
MHz, CDCl3) 0.62 (q, J = 7.8 Hz, 6H), 0.86 (t, J = 6.6 Hz, 3H),
0.95 (t, J = 7.8 Hz, 9H), 1.06–1.34 (m, 19H), 1.42–1.56 (m, 1H),
1.95 (m, 2H), 3.22 (d, J = 11.1 Hz, 1H), 3.49 (s, 3H), 3.75
(s, 3H), 3.80 (d, J = 11.1 Hz, 1H), 4.07 (d, J = 1.2 Hz, 1H), 4.86
(s, 1H); δC (75.5 MHz, CDCl3) 4.6, 6.6, 14.1, 22.7, 24.1, 29.3,
29.5, 29.6, 29.7, 31.9, 37.1, 52.0, 55.2, 80.5, 82.2, 93.2, 110.0,
172.7; HRMS (ESI) m/z: Calc. for C25H50NaO6Si [M ϩ Na]ϩ
497.3274, found 497.3274.
MHz, CDCl3) 0.86 (t, J = 6.6 Hz, 3H), 0.96–1.13 (m, 1H),
1.23 (m, 18H), 1.38–1.55 (m, 1H), 1.86 (m, 2H), 3.52 (s, 3H),
3.68 (s, 3H), 3.99 (d, J = 2.7 Hz, 1H), 4.96 (d, J = 3.0 Hz,
1H), 5.18 (d, J = 10.8 Hz, 1H), 5.43 (d, J = 17.1 Hz, 1H),
5.95 (dd, J = 17.1, 10.8 Hz, 1H); δC (75.5 MHz, CDCl3) 14.1,
22.6, 23.8, 29.3, 29.4, 29.5, 29.6, 29.64, 29.9, 31.9, 33.4, 52.0,
56.3, 79.6, 81.1, 91.9, 108.2, 114.9, 137.0, 172.6; HRMS (ESI)
m/z: Calc. for C21H38NaO6 [M ϩ Na]ϩ 409.2566, found
409.2573.
Method B. To a solution of trimethylsilylacetylene (73 µL,
0.525 mmol) in dry THF (1.1 mL) at Ϫ78 ЊC under argon was
added n-BuLi in hexanes (197 µL, 0.473 mmol, 2.4 M)
dropwise. The solution was stirred at Ϫ78 ЊC for 30 min and a
solution of the crude ketone 8 (40 mg) in dry THF (0.5 mL) was
added via cannula. The reaction was then stirred for a further
15 min and quenched with saturated aqueous NH4Cl. The
aqueous phase was extracted with Et2O and the combined
extracts were washed with water, brine and dried (MgSO4). The
solvent was removed under reduced pressure and the residue
was purified by flash chromatography with 10–20% EtOAc/
petrol as eluent to give an inseparable 3 : 1 mixture of TMS
acetylenes as a yellow oil (17.6 mg, 37% from 13). To a solution
of the mixture of TMS acetylenes (17.6 mg, 0.039 mmol) in
THF (1 mL) at rt was added TBAFؒ3H2O (17.8 mg, 0.056
mmol) and the mixture was stirred for 1 h. Water was added
and the aqueous phase was extracted with Et2O and the com-
bined extracts were washed with water, brine and dried
(MgSO4). The solvent was removed under reduced pressure and
the residue was dissolved in MeOH (1 mL). Lindlar catalyst
(5 mg, 2.38 µmol) was added and the suspension was stirred
under a H2 atmosphere for 1.5 h. The suspension was filtered
through Celite and the eluent was concentrated under reduced
pressure to give a mixture of alkenes 14 and 15 which were
separated by flash chromatography. Elution with 20% EtOAc/
petrol gave 14 (8.3 mg, 21% from 13) and 15 (2.5 mg, 6% from
13) as pale yellow oils.
Method C. To a solution of ethylmagnesiumbromide (1.0 M
in THF, 4.2 mL, 4.2 mmol) in THF (0.68 mL) at 0 ЊC under
argon was added TMS acetylene (0.6 mL, 4.25 mmol) and the
mixture was stirred for 30 min before being allowed to warm to
30 ЊC and stirred for a further 10 min. The mixture was then
cooled to Ϫ78 ЊC and a solution of the crude ketone 8 (265 mg)
in THF (1.36 mL) was added dropwise by cannula and the
mixture was stirred for 3 h. The mixture was allowed to warm to
0 ЊC and stirring was continued for a further 10 min then
quenched with saturated aqueous NH4Cl. The aqueous phase
was extracted with Et2O and the combined extracts were
washed with water, brine and dried (MgSO4). The solvent was
removed under reduced pressure and the residue was dissolved
in THF (13 mL). TBAFؒ3H2O (320 mg, 1.01 mmol) was added
and the mixture was stirred at rt for 1 h. Water was added and
the aqueous phase was extracted with Et2O and the combined
extracts were washed with water, brine and dried (MgSO4). The
solvent was removed under reduced pressure and the residue
was purified by flash chromatography with 30% EtOAc/petrol
as eluent to give an 83 : 17 inseparable mixture of alkynes as a
yellow oil (179 mg, 66% from 13). To a solution of the alkynes
(179 mg, 0.466 mmol) in MeOH (8.4 mL) was added Lindlar
catalyst (33.6 mg, 0.016 mmol) and the suspension was stirred
under a H2 atmosphere for 1 h. The suspension was filtered
through Celite and the eluent was concentrated under reduced
pressure to give a mixture of alkenes 14 and 15 which were
separated by flash chromatography. Elution with 20% EtOAc/
petrol gave 14 (128 mg, 47% from 13) and 15 (28 mg, 10% from
13) as pale yellow oils.
Methyl
methyl-4-C-dodecanyl-D-erythro-3-pentulofurano-
siduronate (8). To a solution of the alcohol 13 (333 mg, 0.701
mmol) in CH2Cl2 (8.6 mL) at 0 ЊC under argon was added
pyridine (103 µL, 3.5 mmol) followed by Dess–Martin reagent
(0.59 g, 1.4 mmol). The mixture was allowed to warm to rt
and stirred for 1 h. Et2O (30 mL), saturated aqueous NaHCO3
(30 mL) and 1.5 M aqueous Na2S2O3 (30 mL) were added and
the mixture was stirred until two clear layers formed. The aque-
ous phase was extracted with Et2O and the combined extracts
were washed with water, saturated aqueous CuSO4, water, brine
and dried (MgSO4). The solvent was removed under reduced
pressure to give the ketone as a yellow oil (340 mg) which was
treated with a solution (7 mL) containing MeCN (19 mL),
water (2 mL) and 48% aqueous HF (1 mL) and the mixture was
stirred vigorously at rt for 1 h. Et2O was added and the reaction
was quenched with saturated aqueous NaHCO3. The aqueous
phase was extracted with Et2O and the combined extracts were
washed with water, brine and dried (MgSO4). The solvent was
removed under reduced pressure to give the α-hydroxy ketone 8
as a yellow oil (265 mg) which was utilized immediately in the
next step.
Methyl
methyl-3-C-vinyl-4-C-dodecanyl-ꢀ-L-xylofurano-
siduronate (14) and methyl methyl-3-C-vinyl-4-C-dodecanyl-ꢀ-L-
ribofuranosiduronate (15). Method A. To a solution of the crude
ketone 8 (708 mg) in THF (11.1 mL) at Ϫ78 ЊC under argon
was added vinylmagnesiumbromide (1.0 M solution in THF,
13.9 mL, 13.9 mmol). The mixture was stirred at Ϫ78 ЊC for 1 h
after which time more vinylmagnesiumbromide was added
(1.0 M solution in THF, 5.9 mL, 5.9 mmol). Stirring was
continued for 1 h before the mixture was allowed to warm to
0 ЊC and quenched with saturated aqueous NH4Cl. The
aqueous phase was extracted with Et2O and the combined
extracts were washed with water, brine and dried (MgSO4). The
solvent was removed under reduced pressure to give a mixture
of alkenes 14 and 15 in a 1.6 : 1 ratio which were separated by
flash chromatography. Elution with 20% EtOAc/petrol gave the
major C2 epimer 14 as a pale yellow oil (290 mg, 39% from 13):
[α]1D7 ϩ42.8 (c 1.16, CHCl3); νmax (thin film) 3474, 2926, 2855,
1755, 1738, 1445, 1105, 1037 cmϪ1; δH (300 MHz, CDCl3) 0.86
(t, J = 6.6 Hz, 3H), 0.94–1.11 (m, 1H), 1.23 (m, 18H), 1.44–1.60
(m, 1H), 1.79–1.91 (m, 2H), 2.47 (d, J = 4.2 Hz, 1H), 3.55
(s, 3H), 3.72 (s, 3H), 3.93 (d, J = 3.9 Hz, 1H), 4.17 (s, 1H), 5.00
(s, 1H), 5.41 (dd, J = 11.0, 1.8 Hz, 1H), 5.61 (dd, J = 17.1, 1.8
Hz, 1H), 6.13 (dd, J = 17.1, 11.1 Hz, 1H); δC (75.5 MHz,
CDCl3) 14.1, 22.7, 24.4, 29.3, 29.4, 29.5, 29.62, 29.64, 29.8,
31.9, 36.5, 51.9, 55.4, 82.4, 82.7, 95.6, 108.4, 118.2, 131.9, 172.1;
HRMS (ESI) m/z: Calc. for C21H38NaO6 [M ϩ Na]ϩ 409.2566,
found 409.2565.
Methyl methyl-2-O-(tert-butyldimethylsilyl)-3-C-vinyl-4-C-
dodecanyl-ꢀ-L-ribofuranosiduronate (16). To a solution of the
alcohol 15 (30 mg, 0.084 mmol) and 2,6-lutidine (58.7 µL, 0.504
mmol) in CH2Cl2 (1.09 mL) at 0 ЊC under argon was added
Further elution afforded the minor C2 epimer 15 as a pale
yellow oil (140 mg, 19% from 13): [α]1D9 ϩ31.5 (c 1.03, CHCl3);
νmax (thin film) 3468, 2925, 2855, 1737, 1467, 1109 cmϪ1; δH (300
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 5 7 2 – 3 5 7 7
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tert-butyldimethylsilyl trifluoromethanesulfonate (57.9 µL,
0.252 mmol) and the solution was stirred for 40 min. Saturated
aqueous NaHCO3 was added and the aqueous phase was
extracted with Et2O. The combined extracts were washed with
water, saturated aqueous CuSO4, water, brine and dried
(MgSO4). The solvent was removed under reduced pressure and
the residue was purified by flash chromatography with 10%
EtOAc/petrol as eluent to give the TBS ether 16 as a colourless
oil (33 mg, 79%): [α]2D2 ϩ12.1 (c 1.12, CHCl3); νmax (thin film)
2926, 2855, 1799, 1749, 1440, 1275 cmϪ1; δH (400 MHz,
d6-DMSO) 0.84 (t, J = 6.0 Hz, 3H), 1.23 (m, 20H), 1.40 (m, 1H),
1.69 (m, 1H), 3.65 (s, 3H), 3.72 (s, 3H), 5.50 (s, 1H), 6.40 (s, 1H),
6.72 (s, 1H); δC (75.5 MHz, CDCl3) 14.1, 21.6, 22.7, 29.33,
29.35, 29.5, 29.58, 29.62, 29.63, 29.7, 31.9. 32.0, 52.9, 54.2,
78.4, 80.9, 166.4, 170.8, 173.7; HRMS (ESI) m/z: Calc. for
C20H34NaO8 [M ϩ Na]ϩ 425.2151, found 425.2146.
Further elution gave cinatrin C1 dimethyl ester (7) as a
colourless oil (42.2 mg, 41%): [α]2D0 Ϫ5.9 (c 0.31, CHCl3); νmax
3524, 2919, 2851, 1747, 1460, 1251, 1127, 1061, 1023 cmϪ1
;
(thin film) 3466, 2926, 2855, 1801, 1749, 1439, 1261, 1122 cmϪ1
;
δH (300 MHz, CDCl3) 0.07 (s, 3H), 0.10 (s, 3H), 0.87 (t, J = 6.6
Hz, 3H), 0.89 (s, 9H), 1.23 (m, 20H), 1.85 (m, 2H), 3.14 (s, 1H),
3.50 (s, 3H), 3.69 (s, 3H), 3.94 (d, J = 3.0 Hz, 1H), 4.83
(d, J = 3.0 Hz, 1H), 5.16 (dd, J = 10.8, 1.8 Hz, 1H), 5.47 (dd,
J = 17.1, 1.8 Hz, 1H), 5.89 (dd, J = 17.1, 10.8 Hz, 1H);
δC (75.5 MHz, CDCl3) Ϫ5.0, Ϫ4.7, 14.1, 18.0, 22.7, 23.7,
25.6, 29.3, 29.4, 29.5, 29.6, 29.7, 30.0, 31.9, 33.0, 51.9, 56.4,
80.9, 81.3, 91.8, 108.8, 115.2, 137.0, 172.6; HRMS (ESI)
m/z: Calc. for C27H52O6SiNa [M ϩ Na]ϩ 523.3431, found
523.3428.
δH (400 MHz, d6-DMSO) 0.84 (t, J = 6.6 Hz, 3H), 1.22 (m,
20H), 1.42 (m, 1H), 2.05 (m, 1H), 3.69 (s, 3H), 3.71 (s, 3H), 4.50
(d, J = 6.8 Hz, 1H), 6.59 (d, J = 6.8 Hz, 1H), 7.07 (s, 1H);
δC (75.5 MHz, d6-DMSO) 13.9, 22.0, 23.4, 28.5, 28.6, 28.7, 28.9,
30.7, 31.2, 52.3, 52.8, 73.0, 84.4, 86.8, 168.8, 169.0, 172.6;
HRMS (ESI) m/z: Calc. for C20H34NaO8 [M ϩ Na]ϩ 425.2151,
found 425.2148.
Cinatrin C1 (3) and C3 (5). A solution of the dimethyl ester (7)
(15.5 mg, 0.038 mmol) in THF (0.5 mL) and 2 M aqueous
NaOH (0.5 mL, 1.0 mmol) was stirred at 40 ЊC for 22 h. The
solution was acidified with 2 M aqueous HCl (0.7 mL,
1.4 mmol) and stirring was continued for a further 1 h. The
solution was allowed to cool to rt and stirring was continued for
a further 3.5 h. Water was added and the mixture was extracted
with EtOAc. The combined extracts were washed with brine
and dried (Na2SO4). The solvent was removed under reduced
pressure to give a mixture of cinatrin C1 (3) and cinatrin C3 (5)
in a 1 : 1 ratio by integration of the 1H NMR spectrum.
Purification by preparative reverse phase HPLC (C-18 5 µm,
250 × 10 mm, 0.1% TFA–80% MeCN/H2O as eluent, flow rate:
2 mL minϪ1; synthetic 3, tR 7.33 min; natural 3, tR 7.37 min;
synthetic 5, tR 8.06 min; natural 5, tR 8.12 min) gave cinatrin C1
(3) as a white powder (5.1 mg, 36%): [α]2D6 Ϫ1.6 (c 0.11, MeOH);
lit.,4 [α]2D4 Ϫ11.2 (c 0.31, MeOH); See Tables 1 and 3 for NMR
data; HRMS (ESI) m/z: Calc. for C18H29O8 [M Ϫ H]Ϫ 373.1862,
found 373.1863. Further elution gave cinatrin C3 (5) as a white
powder (4.5 mg, 32%): [α]2D7 ϩ59.7 (c 0.26, MeOH); lit.,5 [α]2D3
ϩ73.2 (c 0.17, MeOH); See Tables 2 and 3 for NMR data;
HRMS (ESI) m/z: Calc. for C18H29O8 [M Ϫ H]Ϫ 373.1862,
found 373.1864.
Methyl methyl-3-C-carboxymethyl-4-C-dodecanyl-ꢀ-L-xylo-
furanosiduronate (17). Ozone gas was bubbled through a solu-
tion of the alkene 14 (152 mg, 0.39 mmol) in CH2Cl2 (8.82 mL)
and MeOH (306 µL) at Ϫ78 ЊC until a pale blue colour per-
sisted. Me2S (145 µL, 1.95 mmol) was added and the solution
was allowed to warm to rt and stirring was continued for a
further 20 min. Water and Et2O were added and the aqueous
phase was extracted with Et2O. The combined extracts were
washed with water, brine and dried (MgSO4) and the solvent
was removed under reduced pressure. The resulting crude
aldehyde (178 mg) was dissolved in tert-butanol (2.59 mL) and
treated with 2-methyl-2-butene (414 µL). A solution of
NaH2PO4ؒH2O (108.8 mg, 0.79 mmol) and NaClO2 (142.6 mg,
1.59 mmol) in water (0.91 mL) was then added and the mixture
was stirred at rt for 16 h. Water and EtOAc were added and the
aqueous phase was extracted with EtOAc and the combined
extracts were washed with water, brine and dried (MgSO4). The
solvent was removed under reduced pressure and the residue
was treated with an excess of CH2N2. Purification by flash
chromatography with 20–30% EtOAc/petrol as eluent gave the
dimethyl ester 17 as a yellow oil (54.3 mg, 33%): [α]1D9 ϩ26.9
(c 1.09, CHCl3); νmax (thin film) 3464, 2927, 2855, 1743, 1440,
1238, 1126, 1020 cmϪ1; δH (300 MHz, CDCl3) 0.87 (t, J = 6.6 Hz,
3H), 0.96–1.12 (m, 1H), 1.23 (m, 18H), 1.48–1.62 (m, 1H), 1.70
(m, 1H), 1.82 (m, 1H), 3.57 (s, 3H), 3.79 (s, 3H), 3.91 (s, 3H),
4.27 (s, 1H), 4.29 (br s, 1H), 4.50 (br s, 1H), 5.07 (s, 1H); δC (75.5
MHz, CDCl3) 14.1, 22.7, 24.4, 29.3, 29.4, 29.5, 29.56, 29.61,
29.7, 31.9, 35.8, 52.4, 53.3, 55.9, 81.1, 82.8, 93.8, 108.6, 170.9,
171.2; HRMS (ESI) m/z: Calc. for C21H38NaO8 [M ϩ Na]ϩ
441.2464, found 441.2467.
Crystallography
Data for ent-12 was collected on an Enraf-Nonius CAD-4
diffractometer using Ni-filtered Cu-Kα radiation (λ = 1.5418 Å.
The data were corrected for Lorentz and polarization effects16
for absorption (ABSORB)17 and extinction.18 The structure
was solved by direct methods (SHELXS-86)19 and refined
on F 2 (SHELXL-97).18 ent-12 Exists in the solid state with
two independent molecules in the asymmetric unit. All non-
hydrogens were refined anisotropically, while the hydrogen
atoms were constrained in idealised positions. Thermal
ellipsoids plots were generated using the program
ORTEP-3.20 All programs were implemented within the suite
WINGX.20
Cinatrin C1 dimethyl ester (7). A solution of the methyl ketal
17 (106 mg, 0.253 mmol) in 80% aqueous acetic acid (12.2 mL)
and 10% aqueous HCl (428 µL) was heated at 100 ЊC for 3.5 h
then cooled to rt. Water was added and the aqueous phase was
extracted with Et2O and the combined extracts were washed
with water, saturated aqueous NaHCO3, brine and dried
(MgSO4). The solvent was removed under reduced pressure
and the residue was dissolved in MeOH (1.58 mL) and water
(159 µL). CaCO3 (190 mg, 1.90 mmol) and I2 (394 mg, 1.52
mmol) were then added and the resulting brown suspension was
stirred at 70 ЊC for 16 h then cooled to rt. Water was added and
the aqueous phase was extracted with Et2O. The combined
extracts were washed with 1.5 M aqueous Na2S2O3, brine and
dried (MgSO4). The solvent was removed under reduced
pressure and the residue was purified by flash chromatography
with 30–40% EtOAc/petrol as eluent to give the C2 epimer of
cinatrin C1 dimethyl ester (18) as a colourless oil (10.1 mg,
10%): [α]2D0 ϩ55.4 (c 0.43, CHCl3); νmax (thin film) 3473, 2957,
Crystal data for ent-12
C19H36O6, M = 360.48, T = 298 (1) K, λ = 1.5418 Å, triclinic,
space group P1, a = 5.7655(3), b = 9.7432(4), c = 19.338(1) Å,
α = 83.041(5), β = 85.348(6), γ = 85.954(4)Њ, V = 1072.72(9) Å3,
Z = 2, Dx = 1.116 Mg mϪ3, µ(Cu-Kα) = 0.663 mmϪ1, F(000) =
396, crystal size 0.60 × 0.15 × 0.08 mm, max. min. transmission
0.96 and 0.86 respectively, 4048 reflections measured, 4048
independent reflections and the final wR(F 2) was 0.1033 and
final R was 0.0398 for 2196 unique data with [I > 2σ(I )].
CCDC reference number 214283.
See http://www.rsc.org/suppdata/ob/b3/b308028e/ for crystal-
lographic data in CIF or other electronic format.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 5 7 2 – 3 5 7 7
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9 R. K. Mann, J. G. Parsons and M. A. Rizzacasa, J. Chem. Soc.,
Perkin Trans. 1, 1998, 1283.
Acknowledgements
10 D. B. Dess and J. C. Martin, J. Am. Chem. Soc., 1991, 113,
We thank Dr Toshiyuki Kamigauchi (Shinogi Research
Laboratories, Osaka, Japan) for authentic samples of cinatrin
C1 (3) and cinatrin C3 (5) as well as copies of the NMR spectra
of cinatrin C1 dimethyl ester (7). This work was financially
supported by the Australian Research Council Large Grants
Scheme.
7277.
11 D. A. Evans, S. W. Kaldor, T. K. Jones, J. Clardy and T. Stout, J. Am.
Chem. Soc., 1990, 112, 7001.
12 W. C. Still and J. H. McDonald, III, Tetrahedron Lett., 1980, 21,
1031.
13 W. C. Still and J. A. Schneider, Tetrahedron Lett., 1980, 21,
1035.
14 P. A. Grieco, T. Oguri and Y. Yokoyama, Tetrahedron Lett., 1978, 20,
419.
15 E. J. Corey, M. Kang, M. C. Desai, A. K. Ghosh and I. N. Houpis,
J. Am. Chem. Soc., 1988, 110, 649.
16 R. W. Gable, B. F. Hoskins, A. Linden, I. A. S. McDonald and
R. J. Steen, PROCESS_DATA. A Program for the Processing of
CAD-4 Diffractometer Data, The University of Melbourne,
Australia, 1993.
17 A. L. Spek, PLATON for windows. A Program for Absorption
Corrections, University of Utrecht, The Netherlands, 1999.
18 G. M. Sheldrick, SHELXL-97; A Program for the Refinement of
Crystal Structures, University of Göttingen, Germany, 1997.
19 G. M. Sheldrick, SHELXS-86; Crystallographic Computing 3,
University of Göttingen, Germany, 1986.
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