Synthesis and absolute configuration of (+)-pseudodeflectusin: Structural revision of aspergione B

Article abstract of DOI:10.1002/ejoc.200600702

We report herein the total synthesis and determination of the absolute configuration of (+)-pseudodeflectusin. The total synthesis of (+)-pseudodeflectusin starting from o-anisic aid was achieved in 11 total steps with an overall yield of 2.0%. The ¹

Full text of DOI:10.1002/ejoc.200600702

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200600702
Synthesis and Absolute Configuration of (+)-Pseudodeflectusin: Structural
Revision of Aspergione B
Fumiyo Saito,[a] Kouji Kuramochi,[b] Atsuo Nakazaki,[a] Yoshiyuki Mizushina,[c]
Fumio Sugawara,[b,d] and Susumu Kobayashi*[a,d]
Keywords: Total Synthesis / Natural products / Aromatic compounds / Carbonyl compounds / Stereochemistry
We report herein the total synthesis and determination of the
absolute configuration of (+)-pseudodeflectusin. The total
synthesis of (+)-pseudodeflectusin starting from o-anisic aid
was achieved in 11 total steps with an overall yield of 2.0%.
The 1H- and 13C NMR spectroscopic data of our synthetic
pseudodeflectusin was identical to that of the natural com-
pound. The absolute configuration of (+)-pseudodeflectusin
was determined by chiral HPLC and X-ray crystallographic
analyses. We also synthesized the proposed structure of as-
pergione B, whose 1H- and 13C NMR spectroscopic data is
identical to that of pseudodeflectusin. The 1H- and 13C NMR
spectra of our synthetic aspergione B were different from
those of the natural compound reported by Proksch et al. Our
results confirm that aspergione B and pseudodeflectusin are,
in fact, the same compound.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Pseudodeflectusin (1), isolated from the culture broth of
Aspergillus pseudodeflectus, exhibits cytotoxicity against se-
veral human cancer cell lines derived from the stomach
(NUGC-3), cervix (HeLa-S3), and peripheral blood (HL-
60). Pseudodeflectusin is an isochroman derivative, which
consists of a cyclic hemiacetal and a methylethylidene group
at a furanone ring.[1] Although 1 contains a cyclic hemiace-
tal, which might exist in a reversible mixture of two isomers,
the compound was isolated as a single isomer. However, the
absolute configurations of the stereogenic centers at C-7
and C-9 were not determined.
Figure 1. Structure of pseudodeflectusin (1) and aspergione B (2).
To clarify the structure of 1 and to assign the absolute
configurations at C-7 and C-9, we started the synthetic
studies of pseudodeflectusin.
Ochratoxin A, which has a similar benzoisochroman
skeleton to pseudodeflectusin, has been isolated from some
strains of Aspergillus ochraceus (Figure 2).[3–5] The absolute
configuration at C-3 of ochratoxin A has been determined
to be (R) by comparison of the optical rotation of the de-
graded product with (R)-(–)-mellein.[4,6] Therefore, we rea-
soned that the absolute configuration at C-7 of pseudode-
flectusin was also (R).
Proksch et al. isolated aspergione B (2) from the fungus
Aspergillus versicolor, as a tricyclic chromone derivative
with a dihydropyran ring.[2] Both 1 and 2 (Figure 1) are
tricyclic hemiacetals. Compound 1 has a 2-(1-methylethyl-
idene)dihydrofuranone moiety, whereas compound 2 has a
2,3-dimethyl-4-pyrone moiety. The reported NMR spectro-
scopic data (1H- and 13C NMR spectra) of 2 is in good
accordance with that of 1.
[a] Faculty of Pharmaceutical Sciences, Tokyo University of Sci-
ence,
2641 Yamazaki, Noda, Chiba, 278-8510, Japan
Fax: +81-4-7121-3671
E-mail: kobayash@rs.noda.tus.ac.jp
[b] Department of Applied Biological Science, Faculty of Science
and Technology, Tokyo University of Science,
2641 Yamazaki, Noda, Chiba, 278-8510, Japan
[c] Department of Nutritional Science, Kobe-Gakuin University,
Nishi-Ku, Kobe, Hyogo 651-2180, Japan
Figure 2. Mellein and ochratoxin A.
Our synthetic approach for the preparation of (+)-
pseudodeflectusin is outlined in Scheme 1;[7] mellein (5) was
the key intermediate for its preparation. ortho Lithiation of
amide 3 and alkylation of (R)-(+)-propylene oxide led to
alcohol 4. Cyclization of 4 and demethylation led to 5.
[d] Frontier Research Center for Genome and Drug Discovery, To-
kyo University of Science,
2641Yamazaki, Noda, Chiba, 278-8510, Japan
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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Absolute Configuration of (+)-Pseudodeflectusin
SHORT COMMUNICATION
Mellein (5) was converted into furanone 6, followed by con-
densation with acetone to afford 7. Finally, reduction of 7
generated 1.
Scheme 1. Synthetic approach for the preparation of pseudode-
flectusin.
The synthesis of (–)-mellein (5) is shown in Scheme 2.
The condensation of o-anisic acid (8) with tert-butylamine
and DCC in the presence of DMAP afforded benzamide 3
in 75% yield. Treatment of 3 with two equiv. of nBuLi in
the presence of TMEDA led to the formation of the orange-
colored dianion, which opened (R)-(+)-propylene oxide to
afford secondary alcohol (–)-4 in 61% yield.[8,9] Acid-in-
duced (pTsOH·H2O) intramolecular attack of the newly
generated alcohol at the amide group of (–)-4 led to δ-lac-
tone (–)-9 in 72% yield.[9] The methyl group in (–)-9 was
cleanly removed by the exposure of (–)-9 to BCl3 in CH2Cl2
to obtain (–)-mellein [(–)-5] in 95% yield.
Scheme 3. Reagents and conditions: a) Cl2CHOCH3, TiCl4/
CH2Cl2, –10 °C, 88%; b) NaCN, MnO2/CH3OH, 91%; c) K2CO3,
BrCH2CO2CH3/DMF, 50 °C, 98%; d) NaOCH3/CH3OH, reflux,
93%; e) LiOHH2O/DMSO, H2O, 75 °C, 80%; f) pTsOHH2O/ace-
tone, reflux, 7; 49%, 14; 24% (9% recovery of 6); g) MsCl, DMAP/
pyridine, 72%; h) DIBAL (2.5 equiv.)/THF, –78 °C, 28% (49% re-
covery of 7).
For the preparation of the methylethylidene moiety, the
reaction of (–)-6 with pTsOH·H2O in acetone afforded lac-
tone (–)-7 and alcohol 14 in 49% and 24% yield, respec-
tively. Alcohol 14 was converted into (–)-7 by its reaction
with MsCl in pyridine in 72% yield.
Finally, the reduction of lactone (–)-7 with DIBAL in
THF at –78 °C afforded (+)-pseudodeflectusin (1) in 28%
yield, accompanied by recovered (–)-7 in 49% yield.[13]
1
The H- and 13C NMR spectra of synthetic pseudode-
flectusin (1) agreed with those of natural pseudodeflectusin
(1) in all respects.[1] However, the optical rotation of syn-
thetic 1 {[α]2D3 = +63.7 (c = 0.08, MeOH)} was higher than
that of natural 1 {[α]2D3 = +11 (c = 0.18, MeOH)}.[1] Thus,
we prepared ( )-pseudofectusin by using ( ) propylene ox-
ide according to the same procedure, and used synthetic
( )-1 as an authentic sample for HPLC analysis. Synthetic
(+)-1, ( )-1, and natural 1 were subjected to chiral HPLC
analysis [Daicel Chiralpak OD, 30 °C, 254 nm, hexane/iP-
Scheme 2. Reagents and conditions: a) tBuNH2, DCC, DMAP/
CH2Cl2, 75%; b) nBuLi, TMEDA, (R)-(+)-propylene oxide/THF, –
78 °C, 61% (32% recovery of 3); c) pTsOH·H2O/toluene, reflux,
72%; d) BCl3/CH2Cl2, –78 °C to 0 °C, 95 %.
Treatment of (–)-5 with titanium(IV) chloride and rOH = 15:1, 0.5 mL/min]. The HPLC analysis revealed that
Cl2CHOCH3 afforded aldehyde (–)-10 in 88% yield ac- the (7R)-isomer [(+)-1] (tR = 55.8 min) eluted after the (7S)-
companied by the p-formyl isomer in 8% yield isomer [(–)-1] (tR = 36.8 min), which confirmed that natural
(Scheme 3).[10] Aldehyde (–)-10 was converted in 91% yield 1 has the (7R)-configuration. The analysis also indicated
into proposed methyl ester (–)-11 with the use of sodium that the optical purity of our synthetic (+)-1 was 99% ee.
cyanide and MnO2 in MeOH according to Corey’s pro- Because the (7S)-enantiomer was not detected in the sample
cedure.[11] Resulting phenol (–)-11 was then converted into of natural 1, the low value for the optical rotation for natu-
ether (–)-12 by means of methyl bromoacetate and potas- ral 1 must be due to the presence of impurities in the sam-
sium carbonate in 98% yield. The cyclization of (–)-12 with ple.
sodium methoxide in MeOH gave β-keto ester (–)-13, which
was treated with lithium hydroxide in aqueous DMSO at ously determined by X-ray crystallography.[14] Figure 3
75 °C to produce furanone (–)-6 in 80% yield.[12]
shows an ORTEP drawing of (+)-1. The anti relationship
The configuration of the C-9 stereocenter was unambigu-
Eur. J. Org. Chem. 2006, 4796–4799
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F. Saito, K. Kuramochi, A. Nakazaki, Y. Mizushina, F. Sugawara, S. Kobayashi
SHORT COMMUNICATION
between the methyl group at C-7 and the hydroxy group at Table 1. 1H- and 13C NMR spectra of natural aspergione B (2) and
synthetic aspergione B [( )-2].
C-9 was established, which indicates that (+)-1 has the (9S)-
configuration. As a result, we confirmed that the absolute
configuration of natural pseudodeflectusin (1) is (7R,9S).
Figure 3. ORTEP drawing of (+)-1.
In order to ascertain the structure of natural aspergione
B, we prepared ( )-2 from ( )-10 (Scheme 4). The addition
of ethylmagnesium bromide to aldehyde ( )-10 gave a 1:1
diastereomeric mixture of the alcohol, which was oxidized
by MnO2 to afford ketone ( )-15 in 2 steps. Treatment of
( )-15 with Ac2O and DBU in pyridine heated at 60 °C
produced ( )-16 in 53% yield.[15] The reduction of the lac-
Conclusion
tone moiety in ( )-16 with DIBAL in CH2Cl2 afforded ( )-
2, which is the proposed structure of aspergione B.
In conclusion, we have achieved the total synthesis of
(+)-pseudodeflectusin (1) from o-anisic acid in 11 steps with
1
an overall yield of 2.0%. The H- and 13C NMR spectro-
scopic data of our synthetic pseudodeflectusin is identical
to that of natural pseudodeflectusin. Chiral HPLC analysis
of our synthetic (+)-1, ( )-1, and natural 1, together with
X-ray crystallographic analysis of synthetic (+)-1, con-
firmed the absolute configuration of natural 1 to be
(7R,9S). Finally, the proposed structure of aspergione B (2),
whose NMR spectroscopic data were reported to be quite
similar to those of pseudodeflectusin, was also synthesized.
The NMR spectroscopic data of synthetic ( )-2 is different
from that reported by Proksch et al. for natural 2. Our re-
sults confirm that the structure of aspergione B is identical
to that of pseudodeflectusin.
Scheme 4. Reagents and conditions: a) EtMgBr/THF, –5 °C;
b) MnO2/CH2Cl2, 52% in 2 steps; c) Ac2O, DBU, pyridine, 60 °C,
53%; d) DIBAL/CH2Cl2, –78 °C, 92 %.
The H- and 13C NMR spectroscopic data of our syn-
1
Experimental Section
thetic ( )-2 were different from those of natural 2 reported
by Proksch et al. (Table 1).[2] In particular, the chemical
shifts of 6-H, 7-H, 15-H, and 16-H were different between
the two samples (Table 1).[1,2] In the 13C NMR spectro-
scopic data, chemical shifts of C-2, C-3, C-10, and C-16 for
our synthetic ( )-2 were quite different from those of natu-
For details see Supporting Information.
Supporting Information (see footnote on the first page of this arti-
cle): 1H NMR spectra of natural pseudodeflectusin (1) and syn-
thetic (–)-1, chiral HPLC analysis of synthetic (+)-1, ( )-1, and
natural 1, and crystal data and measurement conditions of (+)-1.
ral 2. In the HMBC spectrum of our synthetic ( )-2, Synthesis of compounds: (–)-4, (–)-5, (–)-6, (–)-7, (–)-9, (–)-10, (–)-
11, (–)-12, (–)-13, (+)-1, ( )-15, ( )-16, and ( )-2.
HMBC correlations from 15-H to C-2 and C-3, as well as
from 16-H to C-2, C-3, and C-4 were observed. Although
Proksch et al. reported that the HMBC correlations from
15-H to C-16, and from 16-H to C-15 were observed for
natural aspergione B, these correlations were not observed
in synthetic ( )-2. Taken together, our observations con-
firmed that the structure of aspergione B is identical to that
of pseudodeflectusin.
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Absolute Configuration of (+)-Pseudodeflectusin
SHORT COMMUNICATION
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Received: August 11, 2006
Published Online: September 11, 2006
Eur. J. Org. Chem. 2006, 4796–4799
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