Divergent synthesis of the co-isolated mycotoxins longianone, isopatulin, and (Z)-ascladiol via furan oxidation

Article abstract of DOI:10.1021/jo900855e

(Chemical Equation Presented) Longianone and the biosynthetically related mycotoxins isopatulin and (Z)-ascladiol were prepared following a divergent route from a readily available furan diol. The route toward longianone features an unprecedented TBAF-pro

Full text of DOI:10.1021/jo900855e

pubs.acs.org/joc
SCHEME 1. Proposed Biosynthesis of Longianone (1), Patulin
(2), Isopatulin (3), and (E)/(Z)-Ascladiol (6)
Divergent Synthesis of the Co-isolated Mycotoxins
Longianone, Isopatulin, and (Z)-Ascladiol via Furan
Oxidation
Ioannis N. Lykakis, Ioannis-Panayotis Zaravinos,
Christos Raptis, and Manolis Stratakis*
Department of Chemistry, University of Crete, Voutes 71003,
Iraklion, Greece
stratakis@chemistry.uoc.gr
Received April 29, 2009
pathway of 5 affords isopatulin (3). An enzyme-catalyzed
reduction/oxidation sequence on isopatulin yields patulin
(2)4 via the intermediate formation of (E)-ascladiol5 (6-E).
(E)-Ascladiol isomerizes partially to (Z)-ascladiol (6-Z)6
during the bacterial degradation of patulin, as well as
through a nonenzymatic pathway catalyzed by sulfhydryl
compounds.5 Ascladiols are also potent toxins; however,
their activity has not been fully investigated6 due to the lack
of adequate quantities.
Longianone and the biosynthetically related mycotoxins
isopatulin and (Z)-ascladiol were prepared following a
divergent route from a readily available furan diol. The
route toward longianone features an unprecedented
TBAF-promoted intramolecular oxa-Michael reaction
to a conjugated keto enoate, and the oxidation of dihy-
drolongianone to longianone with stabilized IBX. The
route to isopatulin features a chemoenzymatic synthesis
of (Z)-ascladiol, and the regioselective oxidation of (Z)-
ascladiol to isopatulin with MnO2.
The synthesis of 1-3 has attracted in the past the interest
of organic chemists. Woodward was the first to report the
structure elucidation and the synthesis of patulin (2),7
a
synthetic landmark at that time,8 yet, in a very low yield.
Patulin also has been synthesized by the groups of Patten-
den,9 Tada,10 Riguera,11 and Boukouvalas.12 Isopatulin (3)
has been synthesized twice,13 while 10 years ago Steel14
reported the only known synthesis of longianone (1), using
tetronic acid as starting material and featuring a free radical
cyclization (Bu3SnH, AIBN) of an enyne, as a key step, to
construct the spirocyclic carbon skeleton of 1.
Longianone1 (1), patulin2 (2), and isopatulin3 or neopa-
tulin (3) are isomeric metabolites that are produced by the
fungus Xylaria longiana. Isopatulin and primarily patulin
have an unusual complex chemistry for compounds of such a
low molecular weight. They have been extensively studied
because of their antibiotic properties and potent toxicity if
present on fruits and grains. It has been established4 that
longianone, patulin, and isopatulin arise from 6-methylsa-
licylic acid. Following several biosynthetic steps, 6-methyl-
salicylic acid transforms to phyllostine (4), which hydrolyzes
to 5 (Scheme 1). The postulated highly functionalized acyclic
intermediate 5 leads to longianone via an oxa-Michael/
lactonization sequence. Longianone has been postulated4
as an intermediate during the biosynthesis of secosyrins and
syringolides. An alternative intramolecular lactonization
It is indicative that there is no connectivity among the
existing synthetic approaches to longianone, patulin, and
isopatulin, despite their common biosynthetic origin. We
(5) Sekiguchi, J.; Shimamoto, T.; Yamada, Y.; Gaucher, G. M. Appl.
Environ. Microbiol. 1983, 45, 1939–1942.
(6) Ricelli, A.; Baruzzi, F.; Solfrizzo, M.; Morea, M.; Fanizzi, F. P. Appl.
Environ. Microbiol. 2007, 73, 785–792.
(7) Woodward, R. B.; Singh, G. J. Am. Chem. Soc. 1950, 72, 1428–1428.
(8) Meinwald, J. J. Org. Chem. 2009, 74, 1813–1825.
(9) Gill, G. B.; Pattenden, G.; Stapleton, A. Tetrahedron Lett. 1988, 29,
2875–2878.
(1) Edwards, R. L.; Maitland, D. J.; Oliver, C. L.; Pacey, M. S.; Shields,
L.; Whalley, A. J. S. J. Chem. Soc., Perkin Trans. 1 1999, 715–719.
(2) Engel, G.; Teuber, M. In Mycoxins-Production, Isolation, Separation
and Purification; Betina, V., Eds.; Elsevier: Amsterdam, The Netherlands,
1984; pp 291-314.
(10) Tada, M.; Ohtsu, K.; Chiba, K. Chem. Pharm. Bull. 1994, 42, 2167–
2169.
(11) Seijas, J. A.; Tato, M. P. V.; Estevez, R.; Castedo, L.; Riguera, R.
Heterocycles 1989, 29, 181–184.
(12) Boukouvalas, J.; Maltais, F. Tetrahedron Lett. 1995, 36, 7175–7176.
(13) (a) Bennett, M.; Gill, G. B.; Pattenden, G.; Shuker, A. J.; Stapleton,
A. J. Chem. Soc., Perkin Trans. 1 1991, 929–937. (b) Boukouvalas, J.;
Maltais, F. Tetrahedron Lett. 1994, 35, 5769–5770.
(14) Steel, P. G. Chem. Commun. 1999, 2257–2258.
(3) Sekiguchi, J.; Gaucher, G. M.; Yamada, Y. Tetrahedron Lett. 1979,
20, 41–42.
(4) Goss, R. J. M.; Fuchser, J.; O’Hagan, D. Chem. Commun. 1999, 2255–
2256.
DOI: 10.1021/jo900855e
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Published on Web 07/07/2009
J. Org. Chem. 2009, 74, 6339–6342 6339
2009 American Chemical Society

Lykakis et al.
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SCHEME 2. Retrosynthetic Analysis for the Acyclic Inter-
mediate 5
nonpolar solvents, is essentially the reduced aldehyde form
of 7. To undertake the cyclization of 10 to the [5,5]-spiro-
cyclic skeleton of longianone, we treated 10 with 1.0 equiv of
CH2N2 to form 11 in high yield (90%). The oxa-Michael
cyclization of keto enoate 11 to the dihydrofuran-3(2H)-one
12 was surprisingly achieved in 93% isolated yield after
column chromatography by reacting with 1.0 equiv of TBAF
in THF (25 °C, 30 min). While compound 12 can be isolated,
direct acidification of the crude reaction mixture (one-pot)
with p-TsOH (for 30 min) provides the desired spirobicyclic
13 (dihydrolongianone)1,14 in 87% yield over the two steps.
All attempts to achieve the transformation of 11 to 13 by
using acidic20 (p-TsOH, HCl, BF3) or basic catalysts21 in
various solvents provided after prolonged reaction times
complex mixtures of products with 13 seen as a minor
product in some cases. While TBAF has been used as a
promoter in intramolecular oxa-Michael reactions of eno-
ates,22 to the best of our knowledge this is the first example of
an oxa-Michael reaction to a conjugated keto enoate. It is
notable that the TBAF-promoted spirocyclization is free of
byproduct, and proceeds relatively fast despite the steric
hindrance imposed by the hydroxymethylene substituent.
Our next challenge was the oxidation (dehydrogenation)
of 13 to longianone (1). In his synthetic approach to long-
ianone, Steel14 prepared 13 and succeeded in the introduc-
tion of the double bond within two steps (PhSeCl, O3), yet in
a very low overall yield (<17%). Trying to develop an
alternative more efficient methodology, we attempted the
two-step procedure described by Rovis and Orellana23 dur-
ing the introduction of a double bond in a structurally similar
spirocyclic system (TESOTf, Ph3CþBF4-). Our efforts were
unsuccessful, despite several attempts and modifications.
Indeed, Steel14 had already indicated difficulties in generat-
ing the enolate of 13 in agreement with our failures. Subse-
quently, we turned our attention to the direct oxidation of 13
to 1 using IBX.24 Notably, there are no examples in the
literature regarding the IBX oxidation of the 5-membered
dihydrofuran-3(2H)-ones. Testing the oxidation of 13, we
found that by using 3 equiv of IBX in DMSO the reaction
proceeds relatively fast (∼70% conversion after 1 h), yet with
a <60% mass balance (GC analysis with an internal
standard). After 12 h no starting material could be detected,
yet the reaction mass balance, and accordingly the product
yield, drops dramatically to <10%, indicative that IBX
causes decomposition of the product. After extensive experi-
mentation we found that stabilized IBX25 (SIBX) is signifi-
cantly more effective. Thus, in the reaction 13 with 2.0 equiv
of SIBX (DMSO, 80 °C) a 70% conversion was achieved
after 1.5 h while the reaction mass balance was maintained
high (>90%). Then the reaction mixture was carefully chroma-
tographed to separate longianone (slightly more polar, TLC)
from unreacted 13. Under those reaction conditions, a 47%
SCHEME 3. Synthesis of Longianone (1) from Furan Diol 9
envisioned a common divergent synthetic route to 1-3 and 6
on the basis of the biosynthetic scenario presented in
Scheme 1. Thus, the synthesis of the postulated acyclic
intermediate 5 was targeted. Compound 5 is essentially the
acyclic form of the γ-hydroxybutenolide 7, which could
derive from the suitable oxidation15 of furan 8 (Scheme 2).
While furan 8 with its hydroxyl protected as TBS silyl ether is
a known compound,16 all attempts to reproduce its synthesis
in an acceptable yield failed. The major obstacle was the
ortho-metalation of the precursor TBS-protected 3-furyl-
methanol, which proceeds in extremely low yield despite
several modifications which were attempted. A similar ob-
servation has been noted by Katsumura and co-workers.17
The failure to prepare 8 led us to attempt the synthetic
approach to 1-3 and 6, slightly modifying the retrosynthetic
analysis presented in Scheme 2. Thus, we decided to use the
readily available furan 910 as the starting material, and
perform an additional oxidation at
a
latter stage18
(Scheme 3). Furan 9 can be easily prepared in multigram
scale and in ∼80% yield by condensation of dimethyl 3-
oxoglutarate with chloroacetaldehyde followed by reduction
of the resulting diester with LiAlH4. The NaClO2-mediated19
oxidation of 9 cleanly afforded the γ-hydroxybutenolide 10
in 86% isolated yield. Compound 10, which is insoluble in
(15) Recent selected examples of natural product syntheses featuring
furan oxidation: (a) Vassilikogiannakis, G.; Stratakis, M. Angew. Chem.,
Int. Ed. 2003, 42, 5465–5468. (b) Margaros, I.; Montagnon, T.; Vassiliko-
giannakis, G. Org. Lett. 2007, 9, 5585–5588. (c) Margaros, I.; Vassilikogian-
nakis, G. J. Org. Chem. 2008, 73, 2021–2023. (d) Boukouvalas, J.; Wang, J.
X.; Marion, O.; Ndzi, B. J. Org. Chem. 2006, 71, 6670–6673. (e) Helliwell, M.;
Karim, S.; Parmee, E. R.; Thomas, E. J. Org. Biomol. Chem. 2005, 3, 3636–
3653. (f) Sofikiti, N.; Tofi, M.; Montagnon, T.; Vassilikogiannakis, G.;
Stratakis, M. Org. Lett. 2005, 7, 2357–2359. (g) Montagnon, T.; Tofi, M.;
Vassilikogiannakis, G. Acc. Chem. Res. 2008, 41, 1001–1011.
(16) Paterson, I.; Gardner, M.; Banks, B. J. Tetrahedron 1989, 45, 5283–
5292.
(17) Katsumura, S.; Fujiwara, S.; Isoe, S. Tetrahedron Lett. 1987, 28,
1191–1194.
(18) Although furan 9 could be considered as a precursor to 8, it is
known10 that its oxidation with MnO2 occurs exclusively on the side chain of
C-3.
(20) Intramolecular oxa-Michael reactions of enediones have been re-
cently shown to proceed efficiently under p-TsOH catalysis: Tofi, M.;
Koltsida, K.; Vassilikogiannakis, G. Org. Lett. 2009, 11, 313–316.
(21) Wong, H. N. C. Pure Appl. Chem. 1996, 68, 335–344.
(22) (a) Bates, R. H.; Shotwell, J. B.; Roush, W. R. Org. Lett. 2008, 10,
4343–4346. (b) Kubota, T.; Tsuda, M.; Kobayashi, J. Org. Lett. 2001, 3,
1363–1366.
(23) Orellana, A.; Rovis, T. Chem. Commun. 2008, 730–732.
(24) Nicolaou, K. C.; Montagnon, T.; Baran, P. S.; Zhong, Y.-L. J. Am.
Chem. Soc. 2002, 124, 2245–2258.
(19) (a) Annangudi, S. P.; Sun, M.; Salomon, R. G. Synlett 2005, 1468–
1470. (b) Clive, D. L. J.; Minaruzzaman; Ou, L. J. Org. Chem. 2005, 70, 3318–
3320.
(25) Ozanne, A.; Pouysegu, L.; Depernet, D.; Francois, B.; Quideau, S.
Org. Lett. 2003, 5, 2903–2906.
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SCHEME 4. Synthesis of (Z )-Ascladiol (6-Z ) and Isopatulin (3)
for 2-3 min accelerates substantially the transformation of
17-Z to 3. Isopatulin was purified by recrystallization (white
solid) from ethyl acetate in 78% isolated yield. We observed
that 3 is in equilibrium with its open forms 17-Z and 17-E,
with the lactol form (isopatulin) being predominant (ca.
90%). A similar observation was noted by Pattenden and
co-workers13a in their isopatulin synthesis.
In conclusion, we have presented a simple, efficient, and
bioinspired route to longianone, (Z)-ascladiol, and isopatu-
lin using a divergent route from an easily accessible furan.
The current syntheses complement the work of Tada,10 who
used furan 9 as the starting material to synthesize patulin.
Experimental Section
5-Hydroxy-5-(2-hydroxyethyl)-4-(hydroxymethyl)furan-2(5H)-
one (10). To a solution of furan 9 (400 mg, 2.8 mmol) in
t-BuOH/H2O (5/1) (12 mL) were added in one portion at 20 °C
NaClO2 (760 mg, 8.4 mmol) and NaH2PO4 (650 mg, 4.2 mmol).
The reaction was monitored by TLC and after 3 h it was
complete. Ethyl acetate (30 mL) was added and the solvents
were evaporated under vacuum. The residue was washed with
acetone (3 ꢀ 10 mL), and after evaporation of the solvents,
the γ-hydroxybutenolide 10 was isolated in 86% yield as color-
less syrup. 1H NMR (300 MHz, acetone-d6) δ 5.91 (t, J=2.0 Hz,
1H), 4.54 (dd, J1=16.0 Hz, J2=2.0 Hz, 1H), 4.45 (dd, J1=16.0 Hz,
J2 = 2.0 Hz, 1H), 3.81 (m, 2H), 3.68 (m, 2H), 2.27 (m, 1H), 1.99
(m, 1H). 13C NMR (75 MHz, acetone-d6) δ 172.2, 169.7, 115.3,
106.4, 57.4, 57.2, 38.9. MS (EI) 174 (1%), 156 (5%), 144 (4%),
138 (10%), 129 (9%), 110 (26%), 98 (9%), 83 (13%), 66 (12%),
55 (100%). HRMS (CI) calcd for C7H10O5 þ H 175.06080,
found 175.06065.
(Z)-Methyl 6-Hydroxy-3-(hydroxymethyl)-4-oxohex-2-eno-
ate (11). To a solution of 10 (210 mg, 1.2 mmol) in acetone
(3 mL) was added dropwise at 0 °C a solution of CH2N2 in ether
until a slightly yellow color persisted. The solvents were evapo-
rated, and the residue was purified by column chromatography
(hexane:EtOAc:acetone 5:1:1) to afford 199 mg of 11 (90%
yield). 1H NMR (300 MHz, CDCl3) δ 5.96 (t, J = 2.0 Hz, 1H),
4.35 (d, J = 2.0 Hz, 2H), 3.96 (t, J = 5.0 Hz, 2H), 3.72 (s, 3H),
2.89 (t, J = 5.0 Hz, 2H). 13C NMR (75 MHz, CDCl3) δ 207.6,
165.8, 159.7, 116.1, 62.9, 57.8, 52.1, 45.1. MS (EI) 170 (2%), 157
(17%), 143 (42%), 127 (11%), 115 (15%), 98 (5%), 83 (44%),
73 (19%), 55 (100%). HRMS (CI) calcd for C8H12O5 þ H
189.07635, found 189.07630.
yield of longianone (61% based on the recovered starting
material) was obtained.
Having synthesized longianone, we envisioned the dehy-
dration of butenolide 10 to form (E)- and (Z)-ascladiol. All
attempts to perform an acid-catalyzed dehydration of 10
failed as the expected ascladiol is extremely sensitive under
basic or acidic conditions (see the discussion below). Thus,
we decided to use as starting material the diacetate of 9.
Furan diol 9 was acetylated to form diacetate 14 in 96%
yield, and subsequently 14 was oxidized with NaClO2 to
form the γ-hydroxybutenolide 15 in 88% yield (Scheme 4).
The crude 15 underwent facile dehydration with P2O5 in
benzene26 (60 °C, 15 min) to (Z)-ascladiol diacetate 16 in
82% yield after column chromatography. The (E)-isomer
was formed only in traces. The selective formation of the 16
could be rationalized in terms of its higher thermodynamic
stability compared to the more hindered (E)-isomer. The
hydrolysis of 16 to (Z)-ascladiol (6-Z) seemed quite straight-
forward; however, all attempts by chemical means, either
under basic (K2CO3/MeOH, LiOH/THF, etc.) or acidic
conditions, proved unsuccessful with formation of mixtures
of unidentified products in very poor yield. We then decided
to perform an enzymatic hydrolysis. Treatment of 16 with a
commercially available lipase from Candida cylindracea in a
buffered aqueous solution (pH 7.2) led after 12 h to its clean
hydrolysis to (Z)-ascladiol (6-Z) in 71% isolated yield. (Z)-
Ascladiol is relatively unstable and partially transforms on
standing (>50% after a week) into a mixture of nonidenti-
fied compounds. Subsequently, 6-Z was oxidized with
20 equiv of activated MnO2 (DCM/acetone = 3/1, 20 °C,
30 min) to form exclusively 17-Z. This regioselective oxida-
tion can be rationalized due to the higher acidity of the allylic
hydrogen atoms next to the hydroxyl at the 20-position of the
side chain, as the exocyclic double bond is the most ele-
ctron-deficient. On standing in CDCl3 or acetone-d6 17-Z
slowly isomerizes to isopatulin (∼20-25% conversion after
24 h). Irradiation of the solution with a 300 W xenon lamp
1,7-Dioxaspiro[4.4]nonane-4,8-dione (13). To a solution of
ester 11 (176 mg, 1.0 mmol) in THF (2 mL) was added dropwise
at 20 °C a solution of TBAF (1 M in THF, 1.1 mL, 1.1 mmol).
After 30 min, the solvent was evaporated under vacuum, and the
residue was purified by column chromatography (hexane:
EtOAc 1:1) to afford 164 mg of 12 (93% yield). 1H NMR
(300 MHz, CDCl3) δ 4.32 (m, 2H), 3.66 (s, 3H), 3.63 (dd,
J1 = 11.5 Hz, J2 = 8.0 Hz, 1H), 3.53 (dd, J1 = 11.5 Hz, J2 =
5.0 Hz, 1H), 2.84 (m, 1H), 2.83 (d, J = 16.5 Hz, 1H), 2.69 (d, J =
16.5 Hz, 1H), 2.59 (m, 1H), 2.13 (t, J = 8.0 Hz, 1H, -OH). 13
C
NMR (75 MHz, CDCl3) δ 215.7, 170.5, 81.8, 65.7, 64.8, 52.0,
38.8, 36.6. MS (EI) 157 (7%), 143 (72%), 127 (27%), 115 (23%),
97 (10%), 83 (61%), 67 (26%), 55 (100%). Acidification of the
above crude reaction mixture to pH ∼4 with p-TsOH afforded
after 30 min the spirobicyclic 13 (dihydrolongianone), which
was purified after evaporation of the solvents without extracting
by column chromatography (hexane:EtOAc 2:1) to afford
136 mg of 13 (94% yield). 1H NMR (300 MHz, CDCl3) δ 4.34
(d, J = 11.0 Hz, 1H), 4.29 (d, J = 11.0 Hz, 1H), 4.23 (m, 2H),
2.79 (d, J = 17.5 Hz, 1H), 2.63 (d, J = 8.0 Hz, 1H), 2.62 (d, J =
17.5 Hz, 1H), 2.61 (d, J = 8.0 Hz, 1H). 13C NMR (75 MHz,
CDCl3) δ 211.9, 173.1, 84.3, 74.2, 63.2, 37.7, 35.9. MS (EI) 156
(26) Cane, D.; Ukachukwu, V. C. J. Org. Chem. 1985, 50, 2195–2198.
J. Org. Chem. Vol. 74, No. 16, 2009 6341

Lykakis et al.
JOCNote
(Mþ, 27%), 126 (89%), 116 (13%), 106 (11%), 98 (100%), 94
(5%), 85 (8%), 78 (15%), 72 (29%), 56 (18%). The above two-
step process can be performed in one pot (87% yield) without
isolating 12.
(75 MHz, CDCl3) δ 170.5, 169.9, 167.1, 152.5, 148.8, 118.0,
106.3, 58.2, 57.3, 20.6, 20.4. MS (EI) 240 (Mþ, 1%), 198 (1%),
180 (21%), 169 (4%), 156 (6%), 138 (100%), 127 (3%), 110
(67%), 93 (10%), 67 (21%), 55 (37%). HRMS (ESI) calcd for
C11H12O6 þ Na 263.05238, found 263.05261.
Longianone (1). To a 0.5 M solution of dihydrolongianone 13
(30 mg, 0.21 mmol) in dry DMSO (0.2 mL) was added 2 equiv of
stabilized IBX (0.43 mmol) and the mixture was heated to 80 °C.
The reaction progress was monitored by GC, and after 90 min
longianone had been formed in ∼70% relative yield. Ethyl
acetate was added, and the reaction mixture was extracted with
a saturated solution of NaHCO3. The organic residue was
carefully purified by column chromatography (hexane:EtOAc
gradually from 10:1 to 5:1), affording 14 mg of longianone (1)
and 7 mg of 13 (61% yield of 1 based on recovered 13). 1H NMR
(300 MHz, CDCl3) δ 8.31 (d, J=2.5 Hz, 1H), 5.82 (d, J=2.5 Hz,
1H), 4.43 (d, J = 11.0 Hz, 1H), 4.39 (d, J = 11.0 Hz, 1H), 3.04
(d, J = 18.0 Hz, 1H), 2.71 (d, J = 18.0 Hz, 1H). 13C NMR
(75 MHz, CDCl3) δ 199.3, 177.6, 172.3, 106.8, 89.4, 73.9, 37.6.
MS (EI) 154 (Mþ, 20%), 136 (8%), 124 (27%), 110 (11%), 96
(52%), 71 (39%), 54 (100%).
(2-(2-Acetoxyethyl)furan-3-yl)methyl Acetate (14). 14 was
prepared in 96% yield by acetylating furan 9 with acetic
anhydride (K2CO3, DMAP in ethyl acetate). 1H NMR
(300 MHz, CDCl3) δ 7.29 (d, J = 2.0 Hz, 1H), 6.36 (d, J =
2.0 Hz, 1H), 4.92 (s, 2H), 4.27 (t, J = 7.0 Hz, 2H), 3.02 (t, J =
7.0 Hz, 2H), 2.05 (s, 3H), 2.03 (s, 3H). 13C NMR (75 MHz,
CDCl3) δ 170.9, 170.8, 150.2, 141.4, 116.2, 111.5, 62.3, 57.5,
25.9, 20.9, 20.8. MS (EI) 226 (Mþ <1%), 184 (2%), 166 (79%),
153 (6%), 137 (8%), 124 (100%), 107 (50%), 95 (51%), 78
(62%), 65 (22%), 55 (13%). HRMS (ESI) calcd for C11H14O5 þ
Na 249.07304, found 249.07334.
(Z)-Ascladiol (6-Z). Diacetate 16 (60 mg, 0.25 mmol) was
added into a suspension of lipase from Candida cylindracea (150
mg) in 2 mL of H2O buffered with NaH2PO4. The reaction
mixture was stirred at 25 °C and monitored by TLC. After 12 h,
the reaction mixture was extracted with ethyl acetate (5 ꢀ 5 mL).
The combined organic extracts were dried over MgSO4, the
organic solvents were removed under vacuum, and the residue
was purified by column chromatography (hexane:EtOAc 1:2),
1
to afford 28 mg of (Z)-ascladiol (6-Z) in 72% yield. H NMR
(300 MHz, acetone-d6) δ 6.18 (s, 1H), 5.54 (t, J = 6.5 Hz, 1H),
4.76 (br s, 1H, -OH), 4.63 (s, 2H), 4.37 (d, J = 6.5 Hz, 2H), 4.30
(br s, 1H, -OH). 13C NMR (75 MHz, acetone-d6) δ 168.1, 160.4,
147.0, 114.9, 112.0, 56.3, 56.1. MS (EI) 138 (Mþ - H2O, 41%),
124 (2%), 110 (61%), 84 (39%), 68 (63%), 55 (100%).
Isopatulin (3). To solution of (Z)-ascladiol 6-Z (25 mg,
0.15 mmol) in a 2 mL mixture of dichloromethane/acetone (3/
1) was added activated MnO2 (265 mg). The resulting slurry was
stirred at 20 °C and monitored by TLC. After 30 min the
reaction mixture was filtered through a short pad of Celite
and the solids were washed with ethyl acetate. The combined
organic extracts were concentrated under vacuum and the
residue was purified by column chromatography (hexane:
EtOAc 1:2), affording 17 mg of aldehyde 17-Z (71% yield). 1H
NMR (300 MHz, CDCl3) δ 10.20 (d, J = 8.0 Hz, 1H), 6.47 (t,
J = 1.0 Hz, 1H), 5.60 (d, J = 8.0 Hz, 1H), 4.69 (d, J = 1.0 Hz,
1
2H). H NMR (300 MHz, acetone-d6) δ 10.21 (d, J = 8.0 Hz,
(2-(2-Acetoxyethyl)-2-hydroxy-5-oxo-2,5-dihydrofuran-3-yl)
methyl Acetate (15). To a solution of 14 (225 mg, 1.0 mmol) in
t-BuOH/H2O (5/1) (5 mL) were added in one portion and at
20 °CNaClO2 (0.3 g, 3.35 mmol) and NaH2PO4 (260 mg, 1.7 mmol).
The reaction was monitored by TLC, and after 7 h the starting
material had disappeared. Ethyl acetate (10 mL) was added, the
solvents were evaporated under vacuum, and the residue was
washed with acetone (2 ꢀ 5 mL). The γ-hydroxybutenolide 15
(227 mg) was isolated after evaporation of the solvents in 88%
yield. 1H NMR (300 MHz, CDCl3) δ 5.96 (m, 1H), 4.92 (m, 2H),
4.21 (t, J = 6.0 Hz, 2H), 2.39 (m, 1H), 2.13 (s, 3H), 2.10 (m, 1H),
2.05 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 171.0, 170.4, 169.0,
164.6, 117.8, 105.5, 59.2, 58.7, 35.7, 20.7, 20.5. MS (EI) 156
(6%), 144 (12%), 138 (14%), 101 (42%), 94 (27%), 84 (70%), 55
(100%). HRMS (ESI) calcd for C11H14O7 þ Na 281.06314,
found 281.06318.
1H), 6.63 (m, 1H), 5.85 (d, J = 8.0 Hz, 1H), 4.79 (d, J = 1.0 Hz,
2H). 13C NMR (75 MHz, CDCl3) δ 188.6, 166.1, 159.1, 158.6,
119.2, 106.9, 57.1. MS (EI) 154 (Mþ, 1%), 136 (Mþ - H2O,
49%), 123 (19%), 118 (13%), 97 (9%), 69 (100%), 55 (65%). On
standing in CDCl3 or acetone-d6, aldehyde 17-Z slowly iso-
merizes to isopatulin (3) (∼25% conversion after 24 h). Irradia-
tion with a 300 W xenon lamp for 2-3 min accelerates
substantially the transformation of 17-Z to 3. Isopatulin is in
equilibrium with its open forms 17-Z and 17-E, with the lactol
form being predominant (>90%). 1H NMR of isopatulin (300
MHz, CDCl3) δ 5.89 (m, 1H), 5.86 (m, 1H), 5.80 (m, 1H), 5.06
(dd, J1 = 16.5 Hz, J2 = 2.0 Hz, 1H), 4.74 (dd, J1 = 16.5 Hz,
J2 = 2.0 Hz, 1H). 13C NMR of isopatulin (75 MHz, CDCl3) δ
168.2, 150.7, 149.4, 110.2, 106.3, 89.4, 57.4. MS (EI) 154 (Mþ,
12%), 136 (Mþ - H2O, 29%), 126 (24%), 110 (87%), 97 (23%),
82 (38%), 69 (95%), 55 (100%).
(Z)-Ascladiol Diacetate (16). To a solution of γ-hydroxybu-
tenolide 15 (130 mg, 0.5 mmol) in benzene (5 mL) was added in
one portion at 60 °C 4 equiv of P2O5 (0.37 g, 2 mmol). After 15
min the reaction mixture was filtered and the solids were washed
with ethyl acetate (3 ꢀ 3 mL). The residue after the evaporation
of the combined solvent extracts was purified by column chro-
matography (hexane:EtOAc 4:1), affording 16 (99 mg) in 82%
Acknowledgment. This project was partially supported by
the ELKE, University of Crete. We thank professor I.
Smonou for providing the lipase from Candida cylindracea,
and Dr. C. Rabalakos for obtaining some HRMS.
1
yield as a single diastereomer. H NMR (300 MHz, CDCl3) δ
6.18 (s, 1H), 5.39 (t, J = 7.0 Hz, 1H), 4.97 (d, J = 1.5 Hz, 2H),
Supporting Information Available: Copies of 1H, 13C NMR,
MS, and HRMS of key compounds and reactions. This material
is available free of charge via the Internet at http://pubs.acs.org.
4.85 (d, J = 7.0 Hz, 2H), 2.11 (s, 3H), 2.04 (s, 3H). 13C NMR
6342 J. Org. Chem. Vol. 74, No. 16, 2009