Asymmetric total synthesis of AK-toxins

Article abstract of DOI:10.1016/S0040-4020(02)00117-5

A practical total synthesis of AK-toxins (AK-toxin I, 1; II, 2), host-specific toxins against the Japanese pear, has been achieved in 10% total yield, starting from the key intermediate 6. The (2E,4Z,6E)-conjugated triene system was successfully construct

Full text of DOI:10.1016/S0040-4020(02)00117-5

TETRAHEDRON
Pergamon
Tetrahedron 58 (2002) 2351±2358
Asymmetric total synthesis of AK-toxins
Ippei Uemura,p Hisashi Miyagawa and Tamio Ueno
Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
Received 26 December 2001; accepted 29 January 2002
AbstractÐA practical total synthesis of AK-toxins (AK-toxin I, 1; II, 2), host-speci®c toxins against the Japanese pear, has been achieved in
10% total yield, starting from the key intermediate 6. The (2E,4Z,6E)-conjugated triene system was successfully constructed by the use of the
Stille coupling reaction between the (E,Z)-bromodiene (17) and (E)-stannylacrylate (21). The (E,Z)-con®guration of 17 was established with
excellent geometrical purity by palladium-catalyzed stereoselective hydrogenolysis of gem-dibromide (15), which was derived from 6 via the
Wittig±Horner reactions. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
AK-toxins are esters consisting of a phenylalanine deriva-
tive and a hydroxydecatrienoic acid. The latter has two
characteristic substructures, a conjugated triene carboxylic
acid with (2E,4Z,6E)-con®guration (C1±C7) and a chiral
methylglycidol (C8±C10) with two neighboring chiral
centers. A number of researchers have been intrigued by
the precise stereochemistry required for the synthesis of
these compounds and the insuf®cient availability of a
natural source. Accordingly, several efforts have been
made to achieve the complete chemical synthesis of
AK-toxins. Irie et al. pioneered in this ®eld, and achieved
the ®rst total synthesis of the methyl ester of 2.9±12 They
used the acetonide 3, which was derived from ascorbic acid,
as the chiral source for establishing the con®guration at C8
of AK-toxins (Scheme 1). They also prepared all the
possible stereoisomers of an AK-toxin analogs, to demon-
strate that the con®guration of C8 and C9 was critical for the
phytotoxicity.13 In another synthetic strategy adopted by
Ando etal. 14 the stereochemistry of C8 and C9 was derived
from the chiral intermediate 4, which was prepared from
d-fructose.15 Taking advantage of its stereochemistry,
compound 4 was also used in the synthesis of [3H]-labeled
AK-toxin I methyl ester by Okada et al.16 However,
compound 4 is a poor starting material, since the conversion
of d-fructose to 4 requires extended reaction times (2±3
months) to provide even low yields (ca. 10%). In contrast
to these strategies relying on natural chiral sources, Crombie
et al. tried to chemically construct the stereostructure by
utilizing Sharpless asymmetric epoxidation (SAE) for the
kinetic resolution of the racemic tetraene alcohol 5.17±19 The
targeted glycidol derivative, however, was obtained only
with moderate stereoselectivity and in a low yield. Thus,
all the preceding methods of AK-toxin preparation were
unsatisfactory for our needs, both in terms of stereo-
selectivity and total yield, affording only small amounts of
AK-toxins with great dif®culty. In order to establish a more
facile method of preparation, we recently developed an
effective and stereoselective synthesis of a chiral glycidol
AK-toxins (AK-toxin I, 1; II, 2) were ®rstisolaetd as hos-t
speci®c toxins (HSTs) produced by Alternaira alternata
Japanese pear pathotype, the causal fungus of black spot
disease in the Japanese pear.1±3 The structures of the toxins
were elucidated by Nakashima et al. in 1982 as shown in
Fig. 1.4,5 They cause severe necrosis on the fruit and the leaf
of the susceptible cultivars of Japanese pear at an extra-
ordinary low concentration,6 resulting in large scale harvest
damage. Microscopic analysis suggests that the primary site
of action for AK-toxins is on the plasma membrane of the
pear cells.7,8 However the exact mechanism of their toxicity
is not yet clear. The dif®culty in elucidating the mechanism
of action is partly due to the fact that the toxins can only be
isolated from the natural source in small amounts. Thus,
their practical preparation method has been the major
requirement for elucidating the mode of action.
Figure 1. Structure of AK-toxins.
Keywords: AK-toxin; host-speci®c toxin; asymmetric total synthesis;
palladium-catalyzed stereoselective hydrogenolysis; Stille reaction.
Corresponding author. Tel: 181-75-753-6117; fax: 181-75-753-6123;
e-mail: uemura@colorado.kais.kyoto-u.ac.jp
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00117-5

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I. Uemura et al. / Tetrahedron 58 (2002) 2351±2358
Scheme 1. Preceding strategies for the preparation of AK-toxins.
intermediate 6, of which con®guration agrees with those of
AK-toxins.20 In this paper, we report the transformation of 6
to achieve the total synthesis of AK-toxin I and II.
unreacted.20 This reaction condition turned out to be the
optimum, since a prolonged heating to complete the reaction
caused a substantial reduction in yield, probably due to the
hydrolysis of the oxirane ring of 12. Other deprotection
methods using Lewis acids, such as boron trichloride or
zinc bromide, were all ineffective, giving complex mixture
of products.24,25 Oxidation of 12 was effected by Dess±
Martin periodinane,26,27 affording the corresponding
aldehyde 13 in a stereochemically pure form and in an
excellent yield (91%). Our preceding study showed that
this transformation could not affect the stereochemistry at
the carbon next to the carbonyl group.20 The Horner±
Wadsworth±Emmons reaction of 13, with diethyl
2-oxoethylphosphonate under the conditions developed by
Masamune provided the a,b-unsaturated aldehyde 14 in
geometrically pure form in 54% yield.28±30 The E-orienta-
tion of the introduced double bond was con®rmed by the
coupling constant of ole®nic protons (JH2,H3ˆ15.7 Hz). Any
diastereomeric byproduct was not found even in the crude
2. Results and discussion
Our synthetic plan to achieve the conjugated triene
system of 6 is outlined in Scheme 2. We employed the
Pd(0)-catalyzed hydrogenolysis to stereoselectively
construct the (Z)-alkenyl unitfrom a gem-dibromide
(7),21,22 which was prepared from 6 via the transformation
based on the Wittig±Horner reaction. The conjugated
system of the resultant bromodiene (8) was then extended
to a triene (10) by the Stille reaction with a stannylacrylate
(9).23
First, the secondary hydroxyl group of 6 was protected as
t-butyldiphenylsilyl (TBDPS) ether (Scheme 3). Treatment
of 6 with TBDPS chloride, imidazole, and a catalytic
amountof 4-(dimehtylamino)pyridine in DMF afforded
the corresponding glycidyl ether 11. The trityl group of 11
was removed in 90% acetic acid at 508C to give the primary
alcohol 12 in 75% yield with small amount of 11 remained
1
productby H NMR analysis. This implies that the absolute
con®guration of 13 was unchanged under this reaction
condition, though an excess amount of base (6 equiv. N,N-
diisopropylethylamine) was used. This retention of con-
®guration is probably attributed to the sterical hindrance
Scheme 2. Synthetic strategy from the key intermediate 6.

I. Uemura et al. / Tetrahedron 58 (2002) 2351±2358
2353
Scheme 3. Reagents and conditions: (a) TBDPSCl, imidazole, DMAP, DMF, rt, overnight; (b) 90% aq. AcOH, 508C, 3 h; (c) Dess±Martin periodinane,
CH2Cl2, rt, 30 min; (d) LiCl, N,N-diisopropylethylamine, (EtO)2P(O)CH2CHO, CH3CN, rt, overnight; (e) CBr4, PPh3, CH2Cl2, 2208C, 1 h; (f) Pd(OAc)2,
PPh3, (n-Bu)3SnH, benzene, rt, 1.5 h; (g) (n-Bu)4NF, THF, rt, overnight.
around the carbon bearing the TBDPSO group, as well as
the relatively weak basicity of N,N-diisopropylethylamine.
Compound 14 was then treated with dibromomethyl-
enetriphenylphosphorane, which had been generated in
situ from triphenylphosphine and tetrabromomethane,31 to
give the gem-dibromide (15) in 92% yield. The conversion
of 15 to monobromide (16) was performed by the
palladium-catalyzed stereoselective hydrogenolysis using
the procedure recommended by Uenishi et al.21,22 Palladium
acetate (5 mol%) and triphenylphoshine (20 mol%) had
been mixed to generate Pd(PPh)4 as a catalyst, to which
15 and tributyltin hydride were added sequentially. The
reaction proceeded smoothly to provide a single product,
of which NMR spectra showed that the desired Z-alkenyl
unitwas successfully constructed ( JH6,H7ˆ6.3 Hz). The key
element of this reaction is the purity of the hydride source.
When slightly cloudy tributyltin hydride was used, longer
reaction time was required for completion and a signi®cant
reduction in yield was observed (89%, 1.5 h!67%, 3 h) due
to a formation of a large number of byproducts. The TBDPS
group of the resultant monobromide 16 was removed with
tetrabutylammonium ¯uoride in THF to afford the
substituted glycidol 17, which was then subjected to the
Stille reaction.
stannylacrylate (21), was synthesized as shown in Scheme
4. Methyl (E)-3-tributylstannylacrylate (19) was obtained
from commercially available methyl propiolate (18).32
Alkaline hydrolysis of 19 with aqueous sodium hydroxide
in 1,4-dioxane gave the acid (20), which was then esteri®ed
without puri®cation by using 2-(trimethylsilyl)ethanol and
DCC to trimethylsilylethyl (TMSE) ester (21). The yield
was 72% over 2 steps. The effectiveness of TMSE group
as the protective group of terminal carboxyl moiety in
AK-toxin synthesis has also been demonstrated by Ando
14
etal.
These two fragments were coupled by the Stille reaction
(Scheme 5).23 Compound 17 and 21 were treated with
10 mol% Pd(PPh3)2Cl2 in DMF with mild heating (508C),
which was necessary to complete the reaction within reason-
able time. Puri®cation of the crude product afforded the
targeted (2E,4Z,6E)-triene (22) and its all-trans isomer
(23), which were separable by ¯ash column chromato-
graphy. The yield was 95% and the isomeric ratio was 8:1
in favor of 22. Their geometrical con®gurations were
con®rmed to be as such, based on the coupling constants
1
in H NMR spectrum (JH3,H4ˆ11.3 Hz for 22, 14.3 Hz for
23). Since the yield of 22 was excellentand 23 was success-
fully removed, further optimization of this reaction was not
attempted.
The coupling partner of 17, 2-(trimethylsilyl)ethyl (E)-
Scheme 4. Reagents and conditions: (a) NaOH, H2O, 1,4-dioxane, 608C, 3 h; (b) DCC, 4-(dimethylamino)pyridine, 2-(trimethylsilyl)ethanol, CH2Cl2, rt ,
overnight.

2354
I. Uemura et al. / Tetrahedron 58 (2002) 2351±2358
Scheme 5. Reagents and conditions: (a) Pd(PPh3)2Cl2, 21, DMF, 508C, 5.5 h; (b) DCC, 4-pyrrolidinopyridine, (2S,3S)-N-Fmoc-3-methylphenylalanine or
(2S)-N-Fmoc-phenylalanine, EtOAc, rt, 1 h; (c) piperidine, CH2Cl2, rt, 5 h; (d) Ac2O, pyridine, CH2Cl2, rt, 30 min; (e) (n-Bu)4NF, THF, rt, 5 h.
The hydroxyl group of 22 was then acylated with (2S,3S)-N-
Fmoc-3-methylphenylalanine or (2S)-N-Fmoc-phenyl-
alanine using DCC as the condensing agent.33 Although
the structures of the resultant N-Fmoc derivatives (24, 25)
by the excellent stereoselectivity, availability of 6, and ease
of product isolation without the use of HPLC. We have
obtained 1 and 2 in stereochemically pure form on a several
hundred milligram scale, and further scale-up is possible.
With these synthetic AK-toxins in hand, we are now
attempting to elucidate their mode of action against the
Japanese pear.
1
were con®rmed by H NMR, a small amountof impuriites
could not be removed even after chromatographic puri®ca-
tion. Since they were found to be unstable, these semi-
puri®ed materials were immediately used for the next
reaction without further puri®cation. Treatment of 24 and
25 with piperidine in CH2Cl2 afforded the free amines (26,
27) along with the dibenzofulvene and its piperidine adduct.
This mixture was directly subjected to acetylation using
acetic anhydride and pyridine in CH2Cl2, after most of the
piperidine, which was expected to interfere with the acetyl-
ation, was removed by evaporation. The TMSE esters
AK-toxin I and II (28, 29) were thus obtained in good yields
(75 and 89%, respectively). Neither recemization at C20 nor
geometrical isomerization occurred during this transforma-
tion. Finally, deprotection of the terminal carboxyl group
was effected with tetrabutylammonium ¯uoride in THF to
give the free acids (30, 31), of which 1H NMR spectra were
practically identical with the literature data of naturally
occuring compounds. Speci®c rotation of 30 and 31 were
also in good agreement with the reported values, indicating
that the stereochemically pure AK-toxins were successfully
synthesized.
4. Experimental
4.1. General
1H and 13C NMR spectra were measured on a Bruker
1
AC-300 spectrometer (300 MHz for H; 75 MHz for 13C),
using tetramethylsilane as the internal standard. The
solvents used are noted in each section below. Chemical
shifts are reported in d value by ppm unitand ht e coupling
patterns are represented by the characters (sˆsinglet,
dˆdoublet, tˆtriplet, mˆmultiplet, and combination of
them). Optical rotations were measured on a JASCO
DIP-1000 polarimeter. Elemental analyses were performed
at the Microanalytical Center of Kyoto University. High-
resolution mass spectra (HRMS) were taken on a JEOL
JMS-600H instrument.
Reactions were carried out under inert atmosphere (Ar or
N2) in oven-dried glassware. Reaction solvents were
dehydrated prior to use according to the standard protocols.
Commercial reagents were used as purchased without
further puri®cation, except for N,N-diisopropyethylamine,
which was distilled over CaH2 prior to use. Diethyl
2-oxoethylphosphonate and (2S,3S)-N-Fmoc-3-methyl-
3. Conclusion
An ef®cient total synthesis of AK-toxins (1, 2) has been
achieved with an overall yield of 10%, requiring 12 steps
starting from intermediate 6. This procedure is highlighted

I. Uemura et al. / Tetrahedron 58 (2002) 2351±2358
2355
phenylalanine were prepared by the known procedure.30,33
Preparation of compounds 6, 11±13 was reported in our
previous paper.20 Flash column chromatography was
performed on silica gel (Merck Kieselgel 60).
acetate (105 mg, 0.47 mmol) and triphenylphosphine
(492 mg, 1.90 mmol) were stirred in benzene (15 ml) to
form a cloudy yellow solution in 30 min. By adding a solu-
tion of 15 (5.04 g, 9.4 mmol) in benzene (15 ml), a clear
orange solution was obtained. To this solution was added
dropwise tributyltin hydride (3.15 g, 10.8 mmol) over
30 min. In the course of addition, slight evolution of gas
(H2) was observed. After 1.5 h, the reaction mixture was
diluted with Et2O (200 ml) and washed with saturated
aqueous KF (100 ml), water, and brine. The organic solution
was dried over anhydrous MgSO4, ®ltered, and concentrated
to an oil, which was puri®ed by ¯ash column chromato-
graphy (hexane/EtOAcˆ14:1, v/v) to provide 16 (3.81 g,
4.1.1. (2E,4R,5S)-4-(tert-Butyldiphenylsilyl)oxy-5,6-epoxy-
5-methylhex-2-enal (14). To a stirred suspension of LiCl
(2.54 g, 60 mmol) in CH3CN (65 ml) was added N,N-diiso-
propylethylamine (21.0 ml, 120 mmol) and a solution of
diethyl 2-oxoethylphosphonate (5.40 g, 30 mmol) in
CH3CN (10 ml). After 30 min, a solution of 13 in CH3CN
(10 ml) was introduced, and the resultant yellow solution
was stirred overnight. The solution was poured onto
saturated aqueous solution of NH4Cl (200 ml) with vigorous
stirring to afford a biphasic mixture, which was extracted
wtih EtOAc (200 ml£3). The combined organic extract was
washed with 1N aqueous HCl (200 ml), saturated aqueous
NaHCO3 (200 ml), and brine (200 ml). After dehydration
with anhydrous Na2SO4, the solution was ®ltered and
concentrated to a brown oil, which was puri®ed by ¯ash
column chromatography (hexane/EtOAcˆ9:1, v/v) to afford
14 (4.11 g, 11 mmol, 54% yield) as a colorless oil. 1H NMR
(CDCl3) 1.10 (s, 9H), 1.27 (s, 3H), 1.87 (d, Jˆ4.6 Hz, 1H),
2.25 (d, Jˆ4.6 Hz, 1H), 3.98 (dd, Jˆ3.9, 1.7 Hz, 1H), 6.47
(ddd, Jˆ15.7, 8.0, 1.7 Hz, 1H), 6.85 (dd, Jˆ15.7, 3.9 Hz,
1H), 7.34±7.47 (m, 6H), 7.57±7.70 (m, 4H), 9.58 (d,
Jˆ8.0 Hz, 1H); 13C NMR (CDCl3) 15.7, 19.4, 27.0, 54.6,
57.6, 76.1, 127.8, 127.9, 130.2, 132.4, 132.6, 133.2, 134.8,
1
8.3 mmol, 89% yield) as a colorless oil. H NMR (CDCl3)
1.11 (s, 9H), 1.30 (s, 3H), 2.02 (d, Jˆ4.8 Hz, 1H), 2.30 (d,
Jˆ4.8 Hz, 1H), 3.81 (d, Jˆ5.0 Hz, 1H), 5.95 (dd, Jˆ14.6,
5.0 Hz, 1H), 6.14 (d, Jˆ6.3 Hz, 1H), 6.59 (dd, Jˆ10.4,
6.3 Hz, 1H), 6.67 (dd, Jˆ14.6, 10.4 Hz, 1H), 7.33±7.45
(m, 6H), 7.62±7.71 (m, 4H); 13C NMR (CDCl3) 16.0,
19.4, 27.0, 54.0, 58.1, 76.6, 108.4, 127.2, 127.6, 129.9
29
(£2), 131.8, 132.4, 133.1, 135.9, 136.1; [a]D ˆ230.6 (c
1.18, EtOH); Anal. Found: C, 63.17, H, 6.54. Calcd for
C24H29BrO2Si: C, 63.01, H, 6.39.
4.1.4. (4E,6Z,2R,3S)-7-Bromo-1,2-epoxy-2-methylhepta-
4,6-dien-3-ol (17). To a solution of 16 (3.60 g, 7.9 mmol)
in THF (40 ml) was added dropwise a 1 M solution of tetra-
butylammonium ¯uoride in THF (16 ml, 16 mmol), and
stirred overnight. The reaction mixture was diluted with
EtOAc (200 ml), washed with saturated aqueous NH4Cl
(100 ml) and brine (100 ml). After being dried over
anhydrous MgSO4, the solution was ®ltered and concen-
trated. The residual oil was puri®ed by ¯ash column
chromatography (hexane/EtOAcˆ3:1, v/v) to give 17
(1.47 g, 6.7 mmol, 85% yield) as a white waxy solid. This
material was suf®ciently pure for the next reaction, though
further puri®cation by recrystallizaion (hexane/ethyl
acetate) gave colorless needles. Mp 598C; 1H NMR
(CDCl3) 1.37 (s, 3H), 1.45 (s, 1H), 2.63 (d, Jˆ4.7 Hz,
1H), 2.93 (d, Jˆ4.7 Hz, 1H), 4.22 (d, Jˆ6.9 Hz, 1H), 5.86
(dd, Jˆ14.6, 6.9 Hz, 1H), 6.23 (d, Jˆ6.2 Hz, 1H), 6.65 (dd,
Jˆ10.2, 6.2 Hz, 1H), 6.71 (dd, Jˆ14.6, 10.2 Hz, 1H); 13C
NMR (CDCl3) 17.9, 50.2, 58.6, 72.7, 109.3, 128.5, 131.6,
29
135.7, 135.8, 155.6, 193.2; [a]D ˆ221.2 (c 1.30, EtOH);
Anal. Found: C, 72.19, H, 7.71. Calcd for C23H28O3Si: C,
72.59, H, 7.42.
4.1.2. (4E,2S,3R)-7,7-Dibromo-3-(tert-butyldiphenylsilyl)-
oxy-1,2-epoxy-2-methylhepta-4,6-diene (15). A solution
of tetrabromomethane (4.97 g, 15 mmol) in CH2Cl2
(10 ml) was added dropwise to a stirred solution of tri-
phenylphosphine (7.87 g, 30 mmol) in CH2Cl2 (60 ml),
keeping the internal temperature below 2158C. After
30 min, a solution of 14 (4.00 g, 11 mmol) in CH2Cl2
(5 ml) was added slowly, and stirring was continued for
1 h. To the resultant orange solution was added triethyl-
amine (3 ml). The reaction mixture was then warmed up
t o 08C. Saturated aqueous Na2CO3 (4 ml) was added to
the solution, which was then stirred vigorously for several
minutes. This two-phase mixture was poured onto Et2O
(200 ml) and the resultant suspension was ®ltered through
a pad of silica gel (B 60£20 mm2). The ®ltrate was concen-
trated to crystallize triphenylphosphine oxide, which was
then ®ltered off. The clear ®ltrate was concentrated and
puri®ed by ¯ash column chromatography (hexane/
EtOAcˆ14:1, v/v) to afford 15 as a colorless oil (5.16 g,
9.6 mmol, 92% yield). 1H NMR (CDCl3) 1.10 (s, 9H), 1.28
(s, 3H), 1.98 (d, Jˆ4.8 Hz, 1H), 2.29 (d, Jˆ4.8 Hz, 1H),
3.75 (dd, Jˆ5.0, 1.4 Hz, 1H), 5.92 (dd, Jˆ15.4, 5.0 Hz,
1H), 6.38 (ddd, Jˆ15.4, 10.3, 1.4 Hz, 1H), 6.92 (d, Jˆ
10.3 Hz, 1H), 7.33±7.47 (m, 6H), 7.60±7.70 (m, 4H); 13C
NMR (CDCl3) 15.9, 19.4, 27.0, 54.2, 58.0, 76.5, 91.6, 127.7
(£2), 128.1, 129.9, 130.0, 133.0, 133.6, 135.8, 135.9, 136.1,
26
134.7; [a]D ˆ173.1 (c 1.02, EtOH); Anal. Found: C,
43.67, H, 4.87. Calcd for C8H11BrO2: C, 43.86, H, 5.06.
4.1.5. 2-(Trimethylsilyl)ethyl (E)-3-(tributylstannyl)-
acrylate (21). A 10% w/v aqueous NaOH (30 ml) was
poured onto a solution of methyl (E)-3-(tributylstannyl)-
acrylate (19, 4.92 g, 13 mmol) in 1,4-dioxane (30 ml), and
the biphasic mixture was vigorously stirred at 608C. After
2 h, the mixture was acidi®ed with 1N aqueous HCl and
extracted with EtOAc (100 ml£3). The combined extract
was washed with brine (150 ml), dried over anhydrous
MgSO4, ®ltered, and concentrated. To the solution of the
residual oil in CH2Cl2 (40 ml) was added 2-(trimethylsilyl)-
ethanol (1.77 g, 15 mmol), 4-(dimethylamino)pyridine
(0.12 g, 1 mmol), and a solution of 1,3-dicyclohexylcarbo-
diimide (3.09 g, 15 mmol) in CH2Cl2 (5 ml). After being
stirred overnight, the resultant suspension was diluted with
Et2O (200 ml) and the precipitated dicyclohexylurea was
®ltered off. The ®ltrate was washed with 1N aqueous HCl
(100 ml), saturated aqueous NaHCO3 (100 ml), and brine
29
136.2; [a]D ˆ248.9 (c 1.02, EtOH); Anal. Found: C,
53.83, H, 5.38. Calcd for C24H28Br2O2Si: C, 53.74, H, 5.26.
4.1.3. (4E,6Z,2S,3R)-7-Bromo-3-(tert-butyldiphenylsilyl)-
oxy-1,2-epoxy-2-methylhepta-4,6-diene (16). Palladium

2356
I. Uemura et al. / Tetrahedron 58 (2002) 2351±2358
(100 ml). The organic layer was dried over anhydrous
MgSO4, ®ltered, and concentrated to an oil, which was
puri®ed by ¯ash column chromatography (hexane/
Et2Oˆ39:1, v/v) to provide 21 (4.38 g, 9.5 mmol, 72%) as
solution was ®ltered and concentrated to give a crude oil.
Puri®cation by ¯ash column chromatography (hexane/
Et2Oˆ3:2, v/v) afforded 24 (1.4 g) as a viscous oil with
some impurities detected on TLC. This material was
immediately used for the next reaction without further
1
a colorless oil. H NMR (CDCl3) 0.53 (s, 9H), 0.89 (t,
1
Jˆ7.2 Hz, 9H), 0.94±1.00 (m, 6H), 1.01±1.07 (m, 2H),
1.25±1.37 (m, 6H), 1.45±1.59 (m, 6H), 4.22±4.27 (m,
2H), 6.28 (d, Jˆ19.4 Hz, 1H), 7.72 (d, Jˆ19.4 Hz, 1H);
13C NMR (CDCl3) 21.5, 9.6, 13.7, 17.4, 27.2, 29.0, 62.6,
136.7, 152.1, 165.0; Anal. Found: C, 52.02, H, 9.32. Calcd
for C20H42O2SiSn: C, 52.07, H, 9.18.
puri®cation. H NMR (CDCl3) 0.06 (s, 9H), 1.01±1.06 (m,
2H), 1.27 (s, 3H), 1.36 (d, Jˆ7.1 Hz, 3H), 2.57 (d,
Jˆ4.7 Hz, 1H), 2.71 (d, Jˆ4.7 Hz, 1H), 3.35±3.44 (m,
1H), 4.18±4.28 (m, 3H), 4.33 (dd, Jˆ10.5, 6.9 Hz, 1H),
4.44 (dd, Jˆ10.5, 7.2 Hz, 1H), 4.65 (dd, Jˆ8.7, 5.1 Hz,
1H), 5.08 (d, Jˆ8.7 Hz, 1H), 5.28 (d, Jˆ7.9 Hz, 1H), 5.76
(dd, Jˆ15.2, 7.9 Hz, 1H), 5.92 (d, Jˆ15.1 Hz, 1H), 6.17 (t,
Jˆ11.1 Hz, 1H), 6.27 (t, Jˆ10.9 Hz, 1H), 6.89 (dd, Jˆ15.0,
10.9 Hz, 1H), 7.13 (d, Jˆ6.8 Hz, 2H), 7.20±7.33 (m, 5H),
7.40 (t, Jˆ7.5 Hz, 2H), 7.54 (t, Jˆ6.6 Hz, 2H), 7.68 (dd,
Jˆ15.1, 11.4 Hz, 1H), 7.76 (d, Jˆ7.5 Hz, 2H); 13C NMR
(CDCl3) 21.4, 17.3, 17.4, 17.6, 42.0, 47.2, 52.3, 56.5, 59.1,
62.8, 67.1, 77.1, 120.0, 123.5, 125.0, 125.2, 127.1, 127.7,
127.8, 128.6, 128.9, 130.1, 130.4, 134.8, 137.9, 140.3,
4.1.6. 2-(Trimethylsilyl)ethyl (2E,4Z,6E,8R,9S)-9,10-
epoxy-8-hydroxy-9-methyldeca-2,4,6-trienoate (22). To
a
solution of dichlorobis(triphenylphophine)palladium
(248 mg, 0.35 mmol) and 17 (744 mg, 3.5 mmol) in DMF
(20 ml) was added a solution of 21 (2.30 g, 5.0 mmol) in
DMF (5 ml) with stirring at 508C. After 5.5 h, the reaction
mixture was poured onto water (150 ml) with vigorous stir-
ring. The resultant suspension was extracted with Et2O
(100 ml£3). The combined extract was washed with
saturated aqueous NaHCO3 (150 ml) and brine (150 ml),
dried over anhydrous MgSO4, ®ltered, and concentrated.
The resultant crude oil was puri®ed by ¯ash column chro-
matography (hexane/Et2Oˆ1:1, v/v) to provide 22 (540 mg,
1.7 mmol, 51%), along with a 3:1 mixture of 22 and 23
(460 mg, 1.5 mmol, 44%). The isomeric ratio of 22 to 23
was 8:1 in total. The latter fraction was further puri®ed by
¯ash column chromatography using the same mobile phase.
26
141.3, 143.7, 143.9.; [a]D ˆ180 (c 0.56, EtOH); Anal.
Found: C, 70.92, H, 6.85, N, 2.01. Calcd for
C41H47NO7Si: C, 70.97, H, 6.83, N, 2.02.
4.1.8. 2-(Trimethylsilyl)ethyl (2E,4Z,6E,8R,9S,20S)-9,10-
epoxy-8-[20-(9-¯uorenylmethoxycarbamino)-30-phenyl-
propanoyloxy]-9-methyldeca-2,4,6-trienoate (25). Com-
pound 25 was synthesized from (2S)-N-Fmoc-
phenylalanine (0.92 g, 2.4 mmol) and 22 (568 mg,
1.8 mmol) in a similar manner to that used for the prepara-
tion of 24, using 1,3-dicyclohexylcarbodiimide (0.57 g,
2.7 mmol) and 4-pyrrolidinopyridine (20 mg, 0.13 mmol).
The semipuri®ed 25 (1.5 g) was subjected to the next reac-
1
Compound 22: H NMR (CDCl3) 0.64 (s, 9H), 1.01±1.07
(m, 2H), 1.38 (s, 3H), 2.35 (s, 1H), 2.64 (d, Jˆ4.7 Hz, 1H),
2.93 (d, Jˆ4.7 Hz, 1H), 4.22±4.29 (m, 3H), 5.83 (dd,
Jˆ15.1, 6.7 Hz, 1H), 5.91 (d, Jˆ15.2 Hz, 1H), 6.13 (t,
Jˆ11.3 Hz, 1H), 6.32 (t, Jˆ11.3 Hz, 1H), 6.95 (dd,
Jˆ15.1 Hz, 11.4 Hz, 1H), 7.74 (ddd, Jˆ15.2, 11.7, 0.7 Hz,
1H); 13C NMR (CDCl3) 21.4, 17.4, 18.1, 50.1, 58.8, 62.7,
72.6, 122.7, 127.3, 127.4, 135.2, 135.7, 138.4, 167.1;
1
tion without further puri®cation. H NMR (CDCl3) 0.59 (s,
9H), 1.01±1.07 (m, 2H), 1.29 (s, 3H), 2.59 (d, Jˆ4.6 Hz,
1H), 2.73 (d, Jˆ4.6 Hz, 1H), 3.08±3.16 (m, 2H), 4.18±4.29
(m, 3H), 4.35 (dd, Jˆ10.6, 6.9 Hz, 1H), 4.44 (dd, Jˆ10.5,
7.2 Hz, 1H), 4.67±4.74 (m, 1H), 5.26 (d, Jˆ7.9 Hz, 1H),
5.28 (d, Jˆ7.7 Hz, 1H), 5.68 (dd, Jˆ14.9, 7.6 Hz, 1H), 5.93
(d, Jˆ15.1 Hz, 1H), 6.17 (t, Jˆ11.0 Hz, 1H), 6.25 (t,
Jˆ10.9 Hz, 1H), 6.85 (dd, Jˆ15.0, 10.6 Hz, 1H), 7.09 (d,
Jˆ7.0 Hz, 2H), 7.20±7.33 (m, 5H), 7.30 (t, Jˆ7.4 Hz, 2H),
7.40 (dd, Jˆ7.4 Hz, 2H), 7.68 (dd, Jˆ15.1 Hz, 1H), 7.76 (d,
Jˆ7.5 Hz, 2H); 13C NMR (CDCl3) 21.4, 17.4 (£2), 38.2,
47.2, 52.2, 54.8, 56.5, 62.8, 67.1, 77.1, 120.0, 123.5, 125.1
(£2), 127.1, 127.2, 127.7, 128.6, 128.7, 129.4, 130.0, 130.2,
27
[a]D ˆ184.0 (c 1.02, EtOH); HRMS (EI) m/z Found:
310.1610. Calcd for C16H26O4Si: 310.1600; Compound
23: H NMR (CDCl3) 0.05 (s, 9H), 1.01±1.07 (m, 2H),
1
1.37 (s, 3H), 2.27 (s, 1H), 2.63 (d, Jˆ4.7 Hz, 1H), 2.92 (d,
Jˆ4.7 Hz, 1H), 4.22±4.29 (m, 3H), 5.84 (dd, Jˆ14.4,
6.6 Hz, 1H), 5.90 (d, Jˆ15.2 Hz, 1H), 6.35 (dd, Jˆ14.3,
11.1 Hz, 1H), 6.45 (ddd, Jˆ14.3, 10.7, 1.1 Hz, 1H), 6.55
(dd, Jˆ14.3, 10.7 Hz), 7.28 (dd, Jˆ15.2, 11.1 Hz, 1H);
13C NMR (CDCl3) 21.4, 17.4, 18.1, 50.1, 58.8, 62.6,
72.5, 122.0, 131.0, 132.1, 134.8, 139.1, 143.8, 167.1;
26
134.8, 135.4, 138.0, 141.3, 143.7, 143.8; [a]D ˆ167.2 (c
1.10, EtOH); Anal. Found: C, 70.73, H, 6.63, N, 2.26. Calcd
for C40H45NO7Si: C, 70.66, H, 6.67, N, 2.06.
26
[a]D ˆ149 (c 0.82, EtOH); HRMS (EI) m/z Found:
310.1615. Calcd for C16H26O4Si: 310.1600.
4.1.9. 2-(Trimethylsilyl)ethyl (2E,4Z,6E,8R,9S,20S,30S)-
8-(20-acetylamino-30-phenylbutanoyloxy)-9,10-epoxy-9-
methyldeca-2,4,6-trienoate: AK-toxin I TMSE ester (28).
Piperidine (2.0 ml, 20 mmol) was added to a solution of 24
(1.4 g) in CH2Cl2 (30 ml) and the reaction mixture was
stirred for 5 h. The resultant pale yellow solution was
diluted with toluene (100 ml) and evaporated to dryness to
remove the excess piperidine (repeated twice). The semi-
solid material thus obtained was dissolved in CH2Cl2
(30 ml), followed by the addition of pyridine (3.0 ml,
37 mmol) and acetic anhydride (1.0 ml, 11 mmol). After
30 min, the solution was diluted with EtOAc (200 ml),
and washed with 1N aqueous HCl (100 ml£2), saturated
aqueous NaHCO3 (100 ml), and brine (100 ml). The organic
4.1.7. 2-(Trimethylsilyl)ethyl (2E,4Z,6E,8R,9S,20S,30S)-
9,10-epoxy-8-[20-(9-¯uorenylmethoxycarbamino)-30-
phenylbutanoyloxy]-9-methyldeca-2,4,6-trienoate (24).
1,3-Dicyclohexylcarbodiimide (0.53 g, 2.6 mmol) in
EtOAc (12 ml) was added to a solution of (2S,3S)-N-
Fmoc-3-methylphenylalanine (0.89 g, 2.2 mmol), 4-pyrroli-
dinopyridine (20 mg, 0.13 mmol), and 22 (531 mg,
1.7 mmol) with stirring. After 1 h, the resultant white
suspension was diluted with Et2O (100 ml) and stirred
vigorously for several minutes. The white precipitate was
®ltered off, and the ®ltrate was washed with aqueous 1N
HCl (50 ml), aqueous saturated NaHCO3 (50 ml), and
brine (50 ml). After drying over anhydrous MgSO4, t he

I. Uemura et al. / Tetrahedron 58 (2002) 2351±2358
2357
phase was dried over anhydrous MgSO4, ®ltered, and
concentrated. The residual oil was puri®ed by ¯ash column
chromatography (hexane/ethyl acetateˆ1:1, v/v) to provide
66.51, H, 6.70, N, 3.36, Calcd for C23H27NO6: C, 66.81,
H, 6.58, N, 3.39.
1
4.1.12. (2E,4Z,6E,8R,9S,20S)-8-(20-Acetylamino-30-phenyl-
propanoyloxy)-9,10-epoxy-9-methyldeca-2,4,6-trienoic
acid (31): AK-toxin II (2). Compound 31 was synthesized
from 29 (712 mg, 1.4 mmol) as described in the preparation
of 28, using 1 M tetrabutylammonium ¯uride in THF (6 ml,
6 mmol). A 352 mg (0.88 mmol, 62%) of 31 was obtained
as a white powder. 1H NMR (CD3OD) 1.29 (s, 3H), 1.94 (s,
3H), 2.61 (d, Jˆ4.8 Hz, 1H), 2.75 (d, Jˆ4.8 Hz, 1H), 3.00
(dd, Jˆ13.7, 8.2 Hz, 1H), 3.10 (dd, Jˆ13.7, 6.9 Hz, 1H),
4.67 (dd, Jˆ8.1, 7.0 Hz, 1H), 5.22 (d, Jˆ7.0 Hz, 1H), 5.71
(dd, Jˆ15.2, 7.0 Hz, 1H), 5.95 (d, Jˆ15.1 Hz, 1H), 6.22 (t,
Jˆ11.1 Hz, 1H), 6.33 (t, Jˆ10.9 Hz, 1H), 6.81 (dd, Jˆ15.1,
11.4 Hz, 1H), 7.15±7.29 (m, 5H), 7.74 (dd, Jˆ15.1,
11.3 Hz, 1H); 13C NMR (CD3OD) 17.6, 22.2, 38.5, 53.1,
55.7, 57.8, 77.7, 124.0, 128.0, 129.2, 129.6, 130.2, 130.3,
132.3, 136.7, 137.9, 140.3, 170.3, 172.1, 173.2;
28 (657 mg, 1.3 mmol, 75% from 22) as a viscous oil. H
NMR (CDCl3) 0.06 (s, 9H), 1.01±1.07 (m, 2H), 1.30 (s, 3H),
1.35 (d, Jˆ7.1 Hz, 3H), 1.98 (s, 3H), 2.60 (d, Jˆ4.7 Hz,
1H), 2.72 (d, Jˆ4.7 Hz, 1H), 3.32±3.40 (m, 1H), 4.23±
4.29 (m, 2H), 4.86 (dd, Jˆ8.2, 5.8 Hz, 1H), 5.27 (d,
Jˆ7.9 Hz, 1H), 5.69 (d, Jˆ8.1 Hz, 1H), 5.78 (dd, Jˆ15.2,
7.9 Hz, 1H), 5.94 (d, Jˆ15.1 Hz, 1H), 6.19 (t, Jˆ11.1 Hz,
1H), 6.30 (t, Jˆ10.9 Hz, 1H), 6.89 (dd, Jˆ15.1, 11.1 Hz,
1H), 7.13±7.16 (m, 2H), 7.20±7.31 (m, 3H), 7.69 (dd,
Jˆ15.2, 11.6 Hz, 1H); 13C NMR (CDCl3) 21.5, 17.3,
17.4, 17.7, 23.1, 41.9, 52.3, 56.5, 57.3, 62.7, 77.2, 123.5,
127.4, 127.7 (£2), 128.7, 128.8, 130.2, 134.8, 138.0, 140.6,
25
166.9, 170.0, 170.5; [a]D ˆ1112 (c 0.80, EtOH); HRMS
(FAB) m/z Found: 513.2563. Calcd for C28H39NO6Si:
513.2547.
21
4.1.10. 2-(Trimethylsilyl)ethyl (2E,4Z,6E,8R,9S,20S)-8-
(20-acetylamino-30-phenylpropanoyloxy)-9,10-epoxy-9-
methyldeca-2,4,6-trienoate: AK-toxin II TMSE ester
(29). Compound 29 was synthesized from 25 (1.5 g) in a
similar manner to that used for the preparation of 28, using
piperidine (2.0 ml, 20 mmol), pyridine (3.0 ml, 37 mmol),
and acetic anhydride (1.0 ml, 11 mmol). Puri®cation by
¯ash column chromatography (hexane/EtOAcˆ2:3, v/v)
afforded 29 (818 mg, 1.6 mmol, 89% from 22) as a viscous
oil. 1H NMR (CDCl3) 0.06 (s, 9H), 1.02±1.07 (m, 2H), 1.31
(s, 3H), 2.01 (s, 3H), 2.61 (d, Jˆ4.7 Hz, 1H), 2.74 (d,
Jˆ4.7 Hz, 1H), 3.12 (d, Jˆ6.1 Hz, 2H), 4.24±4.29 (m,
2H), 4.91 (dt, Jˆ7.6, 6.1 Hz, 1H), 5.27 (d, Jˆ7.8 Hz, 1H),
5.68 (dd, Jˆ15.2, 7.9 Hz, 1H), 5.91 (d, Jˆ7.4 Hz, 1H), 5.94
(d, Jˆ15.2 Hz, 1H), 6.19 (t, Jˆ10.9 Hz, 1H), 6.28 (t,
Jˆ10.8 Hz, 1H), 6.84 (dd, Jˆ15.2, 10.8 Hz, 1H), 7.07±
7.10 (m, 2H), 7.20±7.26 (m, 3H), 7.68 (dd, Jˆ15.2,
11.1 Hz, 1H); 13C NMR (CDCl3) 21.4, 17.4 (£2), 23.1,
37.9, 52.2, 53.2, 56.5, 62.8, 77.0, 123.5, 127.2, 128.6,
128.8, 129.4, 130.0, 130.2, 134.8, 135.5, 138.0, 166.9,
[a]D ˆ1125 (c 0.12, MeOH); Anal. Found: C, 66.10, H,
6.28, N, 3.53, Calcd for C22H25NO6: C, 66.15, H, 6.31, N,
3.51.
Acknowledgements
The authors thank Dr Kazuhiro Irie for taking high-
resolution mass spectra of our synthetic compounds.
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1H), 6.35 (t, Jˆ11.1 Hz, 1H), 7.15±7.31 (m, 5H), 7.77 (dd,
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