Biomimetic conversion of (-)-fusoxypyridone and (-)-oxysporidinone to (-)-sambutoxin: Further evidence for the structure of the tricyclic pyridone alkaloid, (-)-fusoxypyridone

Article abstract of DOI:10.1016/j.bmcl.2011.02.091

Biomimetic-type reactions of the tricyclic pyridone alkaloid, (-)-fusoxypyridone [(-)-4,6′-anhydrooxysporidinone] (1), recently encountered in an endophytic strain of Fusarium oxysporum, and (-)-oxysporidinone (2) afforded (-)-sambutoxin (3) and an analog

Full text of DOI:10.1016/j.bmcl.2011.02.091

Bioorganic & Medicinal Chemistry Letters 21 (2011) 2327–2329
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Biomimetic conversion of (ꢀ)-fusoxypyridone and (ꢀ)-oxysporidinone
to (ꢀ)-sambutoxin: Further evidence for the structure of the tricyclic
pyridone alkaloid, (ꢀ)-fusoxypyridone
⇑
E. M. Kithsiri Wijeratne, A. A. Leslie Gunatilaka
Southwest Center for Natural Products Research and Commercialization, Office of Arid Lands Studies, School of Natural Resources and The Environment, College of
Agriculture and Life Sciences, The University of Arizona, 250 E. Valencia Road, Tucson, AZ 85706-6800, USA
a r t i c l e i n f o
a b s t r a c t
Biomimetic-type reactions of the tricyclic pyridone alkaloid, (ꢀ)-fusoxypyridone [(ꢀ)-4,60-anhydrooxyspo-
ridinone] (1), recently encountered in an endophytic strain of Fusarium oxysporum, and (ꢀ)-oxysporidinone
(2) afforded (ꢀ)-sambutoxin (3) and an analogue of 1, identified as(ꢀ)-10(60)-dehydro-4,60-anhydrooxyspo-
ridinone (4), thus confirming the structure previously proposed for 1 and suggesting that 1–3 bear the same
relative stereochemistry. Oxidation of 4 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yielded a
hitherto unknown sambutoxin analogue, (ꢀ)-4,20-anhydrosambutoxin (5).
Article history:
Received 8 January 2011
Revised 16 February 2011
Accepted 22 February 2011
Available online 16 March 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Biomimetic reaction
4-Hydroxy-2-pyridone alkaloids
(ꢀ)-Fusoxypyridone
(ꢀ)-Oxyporidinone
(ꢀ)-4,20-Anhydrosambutoxin
R1
4-Hydroxy-2-pyridones are a small group of fungal-derived
alkaloids bearing the central 2-pyridone ring linked to two substit-
uents at C-3 and C-5 positions. The members of this group display a
range of biological activities and include (ꢀ)-4,60-anhydrooxyspo-
ridinone (1)1 [now named as (ꢀ)-fusoxypyridone], (ꢀ)-oxysporid-
inone (2),1–3 (ꢀ)-6-deoxy-oxysporidinone,1 oxysporidinone
dimethylketal,2 6-epi-oxysporidinone,2 (ꢀ)-sambutoxin (3),2
N-demethyl-sambutoxin,2 apiosporamide,4 ilicicolin,5 funiculosin,6
liporins A7 and B,8 fischerin (YM 215343),9 tenellin,10 militari-
none,11 fusarinones A–C,12 asparidones A and B,13 and fusapyri-
dons A (7) and B (8).14 Among these, the N-methylated members,
(ꢀ)-oxysporidinone (2), (ꢀ)-sambutoxin (3), fusapyridon A (7),
and funiculosin are of particular interest as they exhibit anti-
fungal,2 mycotoxic,15 antibacterial,14 and antiviral6 activities,
respectively. We have recently encountered the first tricyclic
N-methyl-4-hydroxy-2-pyridone alkaloid, (ꢀ)-fusoxypyridone (1),
in an endophytic strain of Fusarium oxysporum.1 Subsequent to
our report two analogues of 1, fusapyridons A (7) and B (8) have
been encountered in another endophytic fungal strain Fusarium
sp. YG-45.14 The structure of 1 was elucidated by the application
of extensive 2D NMR techniques.1
4'
6'
H
H
O
19
14
21
7
4
1'
R2O
5
3
16
22
11
O
18
O
N
20
23
1 R1 = O; R2 = H
7 R1 = O; R2 = Me
8 R1 = OMe,OMe; R2 = Me
R2O
H
H
OH
OH
H
OR2
H
1'
5
4'
11
O
OH
R1
6'
O
O
O
N
R1
O
O
N
2
H
3 R2 = H
6 R2 = Me
H
O
HO
H
O
R1
H
O
O
N
O
R1
4
O
N
R1
=
5
⇑
Corresponding author. Tel.: +1 520 621 9932; fax: +1 520 621 8378.
E-mail address: leslieg@ag.arizona.edu (A.A.L. Gunatilaka).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.02.091

2328
E. M. Kithsiri Wijeratne, A. A. Leslie Gunatilaka / Bioorg. Med. Chem. Lett. 21 (2011) 2327–2329
O
We have now investigated biomimetic-type reactions of both 1
H
O
and (ꢀ)-oxysporidinone (2) which resulted in the formation of
(ꢀ)-sambutoxin (3) and the tricyclic analogue, (ꢀ)-10(60)-dehy-
dro-4,60-anhydro-oxysporidinone (4), providing further evidence
for the previously proposed structure and stereochemistry of
(ꢀ)-fusoxypyridone (1) and stereochemical disposition of the
methyl groups of the trimethylheptenyl substituent at C-11 of
(ꢀ)-oxysporidinone (2). (ꢀ)-Fusoxypyridone (1) on treatment with
p-TsOH (p-toluenesulfonic acid) in toluene at 25 °C yielded two
products,16 one of which was identified as (ꢀ)-sambutoxin (3) by
H
OH
N
+
O
R
2
1
R
HO
HO
HO
HO
O
O
N
-
H2O
O
-
H2O
O
Aromatization
O
OH
R
R
R
direct comparison (mp, [a
]D, and 1H NMR) with an authentic sam-
DDQ
O
O
O
N
N
N
ple.2 The HRFABMS of the more polar product 4 exhibited its
[M+H]+ at m/z 454.2948, consistent with the molecular formula
5
3
4
C28H40O4N, suggesting that it is a dehydration product of 1
Scheme 1. Proposed pathway for biomimetic-type reactions of 1 and 2 giving 3–5.
(C28H42O5N). 1H and 13C NMR spectroscopic data of 4 were similar
to those of 11 except for the chemical shifts of protons and carbons
in the disubstituted cyclohexanone ring. In the 13C NMR spectrum
of 4, when compared with (ꢀ)-fusoxypyridone (1), the signals due
to C-10 and C-60 were shifted from d 77.2 to 109.8 and from d 92.0
to 150.5, respectively, indicating that a molecule of H2O has been
lost from these positions in 1 to form a double bond. Compound
4 was thus identified as (ꢀ)-10(60)-dehydro-4,60-anhydro-oxyspo-
ridinone which was further confirmed by detailed analysis of its
HMBC data (Fig. 1).
Although the gross structure of (ꢀ)-oxysporidinone (2), includ-
ing the connectivity and the relative stereochemistry of the trisub-
stituted tetrahydropyran, has been determined by extensive
application of 2D NMR techniques,2,3 stereochemistry of the
1,3-dimethyl array at C-14 and C-16 and its absolute configuration
have remained undefined. The relative configurations at C-14 and
C-16 of some naturally occurring 4-hydroxy-2-pyridone alkaloids
have been deduced by comparison with experimental and calcu-
lated 13C NMR shifts for anti- and syn-isomers of model compounds
having comparable partial structures.15 However, this method is
known to suffer from a major disadvantage that it is only predic-
tive and not conclusive.17 Considering the structural relationship
between (ꢀ)-oxysporidinone (2) and (ꢀ)-fusoxypyridone (1), it
was of interest to see if a similar biomimetic-type transformation
of 2 would lead to (ꢀ)-sambutoxin (3). As expected, the treatment
of (ꢀ)-oxysporidinone (2) with p-TsOH yielded 3 and (ꢀ)-10(60)-
dehydro-4,60-anhydrooxysporidinone (4) (Scheme 1).16
indicated the absence of the six aliphatic protons assigned to the
disubstituted cyclohexenone ring; instead it showed the presence
of three aromatic protons suggesting that compound 4 on treat-
ment with DDQ had undergone dehydrogenation followed by aro-
matization to give (ꢀ)-4,20-anhydosambutoxin (5) (Scheme 1).
Methylation of (ꢀ)-sambutoxin (3) with Me2SO4/K2CO3 gave its
new analogue (ꢀ)-dimethylsambutoxin (6) as the only product,
structure of which was confirmed by analysis of its spectral data
(1H and 13C NMR and HRFABMS).
Acknowledgments
This work was supported by a grant from the National Cancer
Institute (Grant No. R01CA090265-06A1). We are also thankful to
Dr. N. P. D. Nanayakkara (University of Mississippi) for providing
samples and copies of spectroscopic data of 2 and 3.
References and notes
1. Zhan, J.; Burns, A. M.; Liu, M. X.; Faeth, S. H.; Gunatilaka, A. A. L. J. Nat. Prod.
2007, 70, 227.
2. Jayasinghe, L.; Abbas, H. K.; Jacob, M. R.; Herath, W. H. M. W.; Nanayakkara, N.
P. D. J. Nat. Prod. 2006, 69, 439.
3. Breinholt, J.; Ludvigsen, S.; Rassing, B. R.; Rosendahl, C. N. J. Nat. Prod. 1997, 60,
33.
4. Alfatafta, A. A.; Gloer, J. B.; Scott, J. A.; Malloh, D. J. Nat. Prod. 1994, 57, 1696.
5. Matsumoto, M.; Minato, H. Tetrahedron Lett. 1976, 42, 3827.
6. Ando, K.; Suzuki, S.; Saeki, T.; Tamura, G.; Arima, K. J. Antibiot. 1969, 22, 189.
7. TePaske, M. R.; Gloer, J. B.; Wicklow, D. T.; Dowd, P. F. Tetrahedron Lett. 1991,
32, 5687.
8. Zhang, C.; Jin, L.; Mondie, B.; Mitchell, S. S.; Castelhano, A. L.; Cai, W.;
Bergenhem, N. Bioorg. Med. Chem. Lett. 2003, 13, 1433.
9. Fujimoto, H.; Ikeda, M.; Yamamoto, K.; Yamazaki, M. J. Nat. Prod. 1993, 56,
1268.
10. Wat, C. K.; McInnes, A. G.; Smith, D. G.; Wright, J. L. C.; Vining, L. C. Can. J. Chem.
1977, 55, 4090.
11. Schmidt, K.; Gunther, W.; Stoyanova, S.; Schubert, B.; Li, Z.; Hamburger, M. Org.
Lett. 2002, 4, 197.
Formation of (ꢀ)-sambutoxin (3) from both (ꢀ)-fusoxypyridone
(1) and (ꢀ)-oxysporidinone (2) suggests that the polyketide-de-
rived trimethylheptenyl substituent at C-11 of the tetrahydropyran
ring of 1 and 2 displays the anti-1,3-dimethyl relationship as in
(ꢀ)-sambutoxin (3). Since the relative and absolute stereochemis-
try of the optical antipode of 3 namely, (+)-sambutoxin, has been
confirmed by an enantiocontrolled total synthesis,18 the relative
stereochemistry of the C-11 trimethylheptenyl substituent of com-
pounds 1–4 should be as shown.
12. Cheng, Y.; Schneider, B.; Riese, U.; Schubert, B.; Li, Z.; Hamburger, M. J. Nat.
Prod. 2004, 67, 1854.
13. Bergmann, S.; Schumann, J.; Scherlach, K.; Lange, C.; Brakhage, A. A.; Hertweck,
C. Nat. Chem. Biol. 2007, 3, 213.
14. Tsuchinari, M.; Shimanuki, K.; Hiramatsu, F.; Muratama, T.; Koseki, T.; Shiono,
Y. Z. Naturforsch., B: Chem. Sci. 2007, 62, 1203.
Oxidation of 4 with DDQ gave 5 as the only product. HRMS data
of this compound suggested the molecular formula C28H37O4N. 1H
NMR spectroscopic data of 5, when compared with that of 3,
15. Cheng, Y.; Schneider, B.; Riese, U.; Schubert, B.; Li, Z.; Hamburger, M. J. Nat.
Prod. 2006, 69, 436.
16. Experimental: p-TsOH and DDQ were purchased from Aldrich Chemical Co.
(Milwaukee, WI) and all solvents from VWR Scientific (San Francisco, CA). All
reactions were carried out in an atmosphere of N2. Melting points were
determined on an electrothermal mp apparatus and were uncorrected. Optical
rotations were measured with JASCO Dip-370 polarimeter. UV spectra were
recorded with a Shimadzu UV-1601 spectrophotometer. 1H NMR and 13C NMR
spectra were obtained on Bruker DRX-500. Low resolution and high resolution
H
O
H
O
H
H
H
CH3
H
H
O
R
MS were recorded on a Shimadzu LC–MS QP8000
spectrometers, respectively.
a and an IonSpec FT mass
O
H
N
Reaction of (ꢀ)-fusoxypyridone (1) with p-TsOH: To a stirred solution of (ꢀ)-
fusoxypyridone (1, 3.0 mg) in toluene (0.5 mL) was added p-TsOH (1 crystal).
After 30 min, toluene was removed under reduced pressure and the residue
was separated on preparative TLC (silica gel) using hexane/acetone (1:2.5) as
CH3
Figure 1. Selected HMBC correlations for 4.

E. M. Kithsiri Wijeratne, A. A. Leslie Gunatilaka / Bioorg. Med. Chem. Lett. 21 (2011) 2327–2329
2329
eluant to give (ꢀ)-sambutoxin (3) (1.1 mg, 38%) and 4 (1.6 mg, 56%).
J = 8.3, 2.2 Hz, H-50), 5.14 (1H, d, J = 9.9 Hz, H-13), 5.02 (1H, dd, J = 11.7, 2.3 Hz, H-
7), 3.63 (3H, s, N–CH3), 3.49 (1H, d, J = 9.8 Hz, H-11), 2.45 (1H, m, H-14), 2.16 (1H,
m, H-8a), 1.93 (1H, dq, J = 13.2, 3.4 Hz, H-9a), 1.76 (1H, m, H-10), 1.73 (3H, s, H3-
20), 1.68 (1H, m, H-8b), 1.40 (1H, m, H-9b), 1.35 (1H, m, H-17a), 1.32 (1H, m, H-
16a), 1.17 (1H, m, H-15a), 1.04 (1H, m, H-15b), 1.03 (1H, m, H-17b), 0.86 (3H, d,
J = 6.7 Hz, H3-21), 0.84 (3H, m, H3-18), 0.82 (3H, d, J = 6.3 Hz, H3-22), 0.76 (3H, d,
J = 6.6 Hz, H3-19); HRFABMS m/z 452.2795 [M+H]+ (calcd for C28H38O4N,
452.2801).
(ꢀ)-10(60)-Dehydro-4,60-anhydrooxysporidinone 4: mp 175–177 °C; [
a]D ꢀ65.8
(c 0.3, CH3OH); UV (EtOH) kmax (log e H
) 336 (3.53), 264 (4.79), 230 (4.79) nm; 1
NMR (CDCl3) d 7.34 (1H, s, H-6), 5.12 (1H, d, J = 9.1 Hz, H-13), 4.98 (1H, d,
J = 8.7 Hz, H-7), 3.62 (3H, s, N-CH3), 3.52 (2H, br s, H2-50) 3.46 (1H, d, J = 9.9 Hz,
H-11), 2.77 (2H, t, J = 6.2 Hz, H2-20), 2.70 (2H, t, J = 6.2 Hz, H2-30), 2.43 (1H, m,
H-14), 2.01 (1H, dq, J = 13.8, 3.6 Hz, H-8a), 1.90 (1H, m, H-9a), 1.77 (1H, dq,
J = 13.2, 2.5 Hz, H-8b), 1.70 (1H, m, H-10), 1.65 (3H, d, J = 1.0 Hz, H3-20), 1.38
(1H, m, H-9b), 1.35 (1H, m, H-17a), 1.32 (1H, m, H-16), 1.17 (1H, m, H-15a),
1.03 (2H, m, H-15b, H-17b), 0.86 (3H, d, J = 6.4 Hz, H3-21), 0.83 (3H, m, H3-18),
0.82 (3H, d, J = 6.4 Hz, H3-22), 0.73 (3H, d, J = 6.6 Hz, H3-19); 13C NMR (CDCl3) d
204.8 (C, C-40), 161.9 (C, C-2), 161.2 (C, C-4), 150.4 (C, C-60), 136.2 (CH, C-13),
132.1 (C, C-12), 126.2 (CH, C-6), 114.6 (C, C-5), 112.1 (C, C-3), 109.8 (C, C-10),
91.4 (CH, C-11), 72.8 (CH, C-7), 44.9 (CH2, C-15), 38.8 (CH3, C-23), 38.7 (CH2, C-
50), 38.4 (CH2, C-30), 32.9 (CH2, C-9), 31.9 (CH, C-16), 31.8 (CH, C-10), 30.2 (CH2,
C-8), 29.5 (CH2, C-14), 29.2 (CH2, C-17), 20.6 (CH3, C-21), 19.5 (CH3, C-22), 17.9
(CH3, C-19), 17.8 (CH2, C-20), 11.3 (CH3, C-18), 11.2 (CH3, C-20); HRFABMS m/z
454.2948 [M+H]+ (calcd for C28H40O4N, 454.2957).
Methylation of (ꢀ)-sambutoxin 3: To a solution of (ꢀ)-sambutoxin (3) (2.0 mg)
in acetone (0.6 mL) were added K2CO3 (20 mg) and Me2SO4 (20 lL). After 3 h
under reflux (TLC control), the reaction mixture was filtered, the filtrate
evaporated under reduced pressure, and the resulting residue was purified on
prep TLC using hexane/acetone (1:2) as eluant to give 6 (2.1 mg, 98%) as a white
amorphous powder; [a]D ꢀ92.4 (c 0.8, CH3OH); UV kmax (log e) 315 (3.61), 260
(4.14), 215 (4.11) nm; 1H NMR (CDCl3) d 7.29 (2H, d, J = 8.6 Hz, H-20 and H-60),
7.12 (1H, s, H-6), 6.90 (2H, d, J = 8.6 Hz, H-30 and H-50), 5.11 (1H, d, J = 9.0 Hz, H-
13), 4.96 (1H, d, J = 11.6 Hz, H-7), 3.82 (3H, s, OCH3), 3.49 (3H, s, OCH3), 3.41 (1H,
d, J = 9.8 Hz, H-11), 3.38 (3H, s, N–CH3), 2.39 (1H, m, H-14a), 2.37 (1H, m, H-8a),
1.89 (1H, dd, J = 12.9, 3.0 Hz, H-9a), 1.67 (1H, m, H-10), 1.62 (3H, s, H3-20), 1.56
(1H, m, H-8b), 1.38 (1H, m, H-9b), 1.30 (1H, m, H-16), 1.30 (1H, m, H-17a), 1.16
(1H, m, H-15a), 1.02 (2H, m, H-15b, H-17b), 0.84 (3H, d, J = 6.5 Hz, H3-21), 0.81
Reaction of (ꢀ)-oxysporidinone with p-TsOH: To
a
solution of (ꢀ)-
oxysporidinone (2) (5.0 mg) in toluene (0.5 mL) was added p-TsOH (1 crystal)
and stirred for 30 min (TLC control). Solvent was then removed under reduced
pressure and the residue was separated on preparative TLC (silica gel) using
hexane/acetone (1:2.5) as eluant to give (ꢀ)-sambutoxin (3) (1.8 mg, 39%) and
(ꢀ)-10(60)-dehydro-4,60-anhydro-oxysporidinone (4) (2.7 mg, 58%).
(3H, d, J = 6.6 Hz, H3-22), 0.80 (3H, m, H3-18), 0.70 (3H, d, J = 6.5 Hz, H3-19); 13
C
NMR (CDCl3) d 165.7 (C, C-4), 162.8 (C, C-2), 158.9 (C, C-40), 136.8 (CH, C-13),
136.3 (CH, C-6), 132.1 (C, C-12), 129.7 (CH, C-20 and C-60), 127.0 (C, C-10), 122.4 (C,
C-5), 117.5 (C, C-3), 114.0 (CH, C-30 and C-50), 92.2 (CH, C-11), 73.7 (CH, C-7), 44.9
(CH2, C-15), 37.6 (CH3, C-23), 33.5 (CH2, C-9), 32.0 (CH, C-10), 31.8 (CH, C-16),
29.5 (CH2, C-8), 29.3 (CH2, C-17), 29.1 (CH, C-14), 20.6 (CH3, C-21), 19.5 (CH3, C-
22), 18.0 (CH3, C-19), 11.6 (CH3, C-20), 11.2 (CH3, C-18); HRFABMS m/z 482.3247
[M+H]+ (calcd for C30H44O4N, 482.3270).
Oxidation of 4 to (ꢀ)-4,20-anhydosambutoxin 5: To a stirred solution of 4
(0.5 mg) in 1,4-dioxane (0.2 mL) was added DDQ (3.0 mg). After 3 h at 25 °C (TLC
control), the reaction mixture was diluted with EtOAc (20 mL), washed with
water (3x15 mL), dried (Na2SO4), and evaporated. The residue thus obtained was
purified by prep. TLC using hexane/acetone (1:2.5) as eluant to give 5 (0.3 mg,
60%) as a white amorphous powder; [
(log
) 338 (3.71), 310 (3.89), 258 (4.32), 211 (4.29) nm; 1H NMR (CDCl3) d 7.68
(1H, s, H-6), 7.37 (1H, d, J = 8.3 Hz, H-60), 6.86(1H, d, J = 2.2 Hz, H-30), 6.68 (1H, dd,
a
]
D ꢀ78.2 (c 0.2, CH3OH); UV (EtOH) kmax
17. Stahl, M.; Schopfer, U.; Frenking, G.; Hoffmann, R. W. J. Org. Chem. 1996, 61,
8083.
18. Williams, D. R.; Turske, R. A. Org. Lett. 2000, 2, 3217.
e