Synthesis and structural revision of calafianin, a member of the spiroisoxazole family isolated from the marine sponge, Aplysina gerardogreeni

Article abstract of DOI:10.1016/j.tetlet.2004.12.084

Calafianin 1 was successfully synthesized by employing a spiroisoxazoline compound, which was produced by electrochemical oxidation of the oximino-phenol derivative 7 followed by reduction. This investigation revealed a structural revision of 1, the trans

Full text of DOI:10.1016/j.tetlet.2004.12.084

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 1083–1086
Synthesis and structural revision of calafianin, a member
of the spiroisoxazole family isolated from the marine sponge,
Aplysina gerardogreeni
Takahisa Ogamino and Shigeru Nishiyama*
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku,
Yokohama 223-8522, Japan
Received 19 November 2004; revised 8 December 2004; accepted 20 December 2004
Available online 8 January 2005
Abstract—Calafianin 1 was successfully synthesized by employing a spiroisoxazoline compound, which was produced by electro-
chemical oxidation of the oximino-phenol derivative 7 followed by reduction. This investigation revealed a structural revision of
1, the trans-relationship of two oxygen atoms between the epoxide and the isoxazoline.
Ó 2004 Elsevier Ltd. All rights reserved.
Calafianin 1,1 isolated from the sponge, Aplysina gerardo-
greeni n. sp (Aplysinidae), is a member of such spiro-
isoxazoline natural products as aerothionin 2,2 homo-
aerothionin 3, and aerophobin 1 4 (Fig. 1).3 In
contrast to diverse biological activities of the spiroisox-
azolines involving antimicrobial, cytotoxic, and antiin-
flammatory activities, calafianin 1 has been reported
not to have significant biological activity. Previously, 1
and 2 were examined against multidrug-resistant clinical
isolates of M. tuberculosis H37Rv,4 whereas 2 was
potently active, 1 showed no significant activity in spite
of its closely related structure to 2. No activity will make
it possible to express a specific activity against new bio-
assay system. Accordingly, such natural product will be
a promising candidate possessing high biological selec-
tivity. Against such background, synthetic studies
toward 1 were initiated for acquisition of further biolog-
ical information.
Calafianin 1 consists of the spiroisoxazoline moiety and
the cyclohexenone carrying the epoxide. The relative ste-
reochemistry of the C-1, C-2, C-6 centers on the cyclo-
hexenone was deduced to possess the cis-relationship
of two oxygen atoms between the epoxide and the iso-
xazoline by the NOE correlation between H-1 and H-7
protons.1
Figure 1. Proposed structure of calafianin 1 and related natural
products.
In this context, we have accomplished the synthesis of
these spiroisoxazoline natural products employing phe-
nolic oxidation of oximino-phenol 7 with TTN5 and
anodic oxidation6 as key steps. The following Zn(BH4)2
reduction of spirodienone 8 produced, provided the spi-
roisoxazoline derivatives involving natural trans-form 5
and unnatural cis-form 6 (Scheme 1).7
Keywords: Aplysina gerardogreeni; Calafianin; Spiroisoxazoline; Bromo-
phenylpyruvic acid.
*
Corresponding author. Tel./fax: +81 45 566 1717; e-mail:
nisiyama@chem.keio.ac.jp
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.12.084

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T. Ogamino, S. Nishiyama / Tetrahedron Letters 46 (2005) 1083–1086
Scheme 1. Synthesis of the spiroisoxazoline structure.
1
We describe herein the findings obtained in the synthesis
of calafianin 1 by utilizing the trans- and cis-spiroisoxaz-
olines 5 and 6.
Unfortunately, comparison of the H NMR data of the
synthetic sample with reported data1 indicated a clear
difference in the region of H-2, H-5, and H-7. Christo-
phersen et al. mentioned determination of the relative
stereochemistry, based on the spectroscopic evidence:
in particular, the NOE correlation between H-1 and
H-7 indicated that the H-1 proton exists at the same side
of C-7. In contrast, a series of spiroisoxazoline natural
products, such as aerothionin 2, homoaerothionin 3
and aerophobin-1 4, have a trans vicinal relationship
between a hydroxy group and an oxygen atom in the
spiroisoxazoline unit (Fig. 1). A hypothesis of the bio-
synthesis indicated that spiroisoxazoline A is prepared
from p-methoxy phenyl derivative B by epoxidation, fol-
lowed by intramolecular nucleophilic attack to the epox-
ide of the arene oxide C (Scheme 3).10
At the outset, synthesis of methyl ester 10 carrying a
partial-structure of calafianin 1, was performed. The
cis-spiroisoxazoline 6 was submitted to cleavage of
methyl enol ether under acidic conditions to give 9,
which was basified with DBU to afford the epoxy deriv-
ative 10 in 32% yield (Scheme 2). A main reason of the
low yield might be the labile property of the cis-spirois-
oxazoline 6 under the acidic or basic conditions, which
gave the spirodienone 118 by dehydration, along with
7 by a ring-opening of the isoxazoline moiety as byprod-
ucts. However, this synthetic approach starting from cis-
alcohol 6, might be an effective suitable method for
stereoselective construction of the epoxide moiety. The
proposed structure of calafianin 19 was successfully
obtained by condensation of carbonic acid 12 with 1,4-
diaminobutane in 12% yield.13
On the basis of these considerations, we expected 1
might possess the relative stereochemistry of 13, which
has trans-relationship between the epoxy group and
Scheme 2. Synthesis of proposed structure of calafianin 1.
Scheme 3. Biosynthetic hypothesis of natural spiroisoxazolines.

T. Ogamino, S. Nishiyama / Tetrahedron Letters 46 (2005) 1083–1086
1085
Scheme 4. Synthesis of 13, revised structure of calafianin 1.
Scheme 5. Synthesis of 13, prepared through aerothionin 2.
the oxygen atom in the spiroisoxazoline ring, if this nat-
ural product was prepared through the same biosyn-
thetic cascade as those of other spiroisoxazolines. This
hypothesis prompted us to synthesize 13 from 5.
into other framework of natural product. This synthesis
enabled the structural revision of 1 to 13, which has
trans-relationship between the epoxy group and the oxy-
gen atom in the spiroisoxazoline ring.
The spiroisoxazoline 5 was treated under acidic condi-
tions to give the bromohydroxy derivative 14 as a diaste-
reomeric mixture (ca. 2:1). The epoxy derivative 15,11
which has trans-relationship between two oxygen atoms,
was synthesized from 14 by intramolecular cyclization
under DBU conditions in high yield, without production
of spirodienone 11 as a by-product. The carbonic acid
16, prepared from 15 with hydrolysis, was submitted
to the same procedure as in the case of 1 to afford
13,12,13 the spectral data of which was superimposable
to those reported.1,14 Especially, NOE correlation be-
tween H-1 and H-7 was observed. Accordingly, the rel-
ative stereochemistry of calafianin 1 should be revised to
13, as shown in Scheme 4. Alternatively, 13 was ob-
tained from aerothionin 2,2 which was prepared from
55a,b in two steps (Scheme 5). In this reaction, prepara-
tion of 2 under acidic conditions succeeded upon using
strong acid, methane sulfonic acid, while acetic acid ef-
fected no reaction.
Acknowledgements
This work was supported by Grant-in-Aid for the 21st
Century COE program ÔKeio Life Conjugate Chemis-
tryÕ, as well as Scientific Research C from the Ministry
of Education, Culture, Sports, Science, and Technology,
Japan. The authors are grateful to the COE program for
financial support to T.O.
References and notes
1. Encarnacion, R. D.; Sandoval, E.; Malmstrom, J.; Chris-
tophersen, C. J. Nat. Prod. 2000, 63, 874–875.
2. Fattorusso, E.; Minale, L.; Sodano, G.; Moody, K.;
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5. (a) Nishiyama, S.; Yamamura, S. Tetrahedron Lett. 1983,
24, 3351–3352; (b) Nishiyama, S.; Yamamura, S. Bull.
Chem. Soc. Jpn. 1985, 58, 3453–3456.
In conclusion, the proposed structure of calafianin 1 and
its stereo isomer 13 were successfully synthesized by
using the spiroisoxazolines 5 and 6 as an efficient dem-
onstration of conversion of spiroisoxazoline structure

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6. (a) Ogamino, T.; Ishikawa, Y.; Nishiyama, S. Heterocycles
2003, 61, 73–78; (b) Ogamino, T.; Nishiyama, S. Tetrahe-
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Hoshino, O. J. Chem. Soc., Chem. Commun. 1994, 443–
444; (b) Murakata, M.; Yamada, K.; Hoshino, O.
Tetrahedron 1996, 52, 14713–14722; (c) Murakata, M.;
Tamura, M.; Hoshino, O. J. Org. Chem. 1997, 62, 4428–
4433; (d) Murakata, M.; Yamada, K.; Hoshino, O.
Heterocycles 1998, 47, 921–931; (e) Wasserman, H. H.;
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12. Compound 13: IR (film) 3410, 1701, 1670 cmÀ1
;
1H NMR (400 MHz, DMSO-d6) d 1.48 (4H, br s),
3.20 (4H, br s), 3.60 (2H, d, J = 18.0 Hz), 3.67 (2H,
d, J = 18.0 Hz), 3.92 (2H, d, J = 3.6 Hz), 4.12 (2H,
dd, J = 2.4, 4.0 Hz), 7.48 (2H, d, J = 2.4 Hz), 8.64
(2H, t, J = 6.0 Hz); 13C NMR (100 MHz, DMSO-d6)
d 26.3, 38.5, 43.3, 52.9, 56.8, 83.9, 122.7, 143.7, 154.8,
158.2, 185.9.
13. The reason of the low yield of this condensation reaction
may be the instability of carbonic acid 12 or 16. In the case
of 16, the nitrile 17 was easily produced by decarboxyl-
ation and the following heterolysis of the N–O bond, as a
by-product. Improvement of the condition is in
progress.
14. Synthetic product 13 might be a diastereomer mixture.
Unfortunately, up to now we have not been able to
accomplish their separation. This problem was also
encountered in the synthesis of aerothionin 2.5
8. Forrester, A. R.; Thomson, R. H.; Woo, S. O. J. Chem.
Soc., Perkin Trans. 1 1975, 4, 2340–2348.
9. Synthetic sample of 1: IR (film) 3375, 1703, 1666 cmÀ1; 1H
NMR (400 MHz, DMSO-d6) d 1.52 (4H, br s), 3.19 (4H,
br s), 3.41 (2H, d, J = 18.4 Hz), 3.56 (2H, d, J = 18.4 Hz),
3.81 (2H, d, J = 4.0 Hz), 4.12 (2H, d, J = 2.8, 4.0 Hz), 7.29
(2H, d, J = 2.8 Hz), 8.61 (2H, t, J = 6.0 Hz); 13C NMR
(100 MHz, DMSO-d6) d 26.3, 38.5, 43.3, 51.8, 55.0, 85.4,
119.7, 144.2, 154.6, 159.7, 185.8.