Structure revision and assignment of absolute stereochemistry of a marine C21 bisfuranoterpene

Article abstract of DOI:10.1021/np000477o

The C₂₁ bisfuranoterpene (-)-isotetradehydrofurospongin-1 (6), previously isolated from a Western Australian Spongia sp., has been reisolated from a specimen of Spirastrella papilosa collected during scientific trawling operations in the Great Australian Bight. A 2D NMR analysis of 6 has prompted reassignment of the published structure 5, while degradation and chiral HPLC analysis have allowed determination of the absolute stereochemistry.

Full text of DOI:10.1021/np000477o

6
38
J . Nat. Prod. 2001, 64, 638-639
Str u ctu r e Revision a n d Assign m en t of Absolu te Ster eoch em istr y of a Ma r in e C21
Bisfu r a n oter p en e
,
†
†
†
‡
Robert J . Capon,* Alister J enkins, Francis Rooney, and Emilio L. Ghisalberti
School of Chemistry, University of Melbourne, Parkville, Victoria, 3010, Australia, and Department of Organic Chemistry,
University of Western Australia, Nedlands, 6009, Australia
Received October 5, 2000
The C21 bisfuranoterpene (-)-isotetradehydrofurospongin-1 (6), previously isolated from a Western
Australian Spongia sp., has been reisolated from a specimen of Spirastrella papilosa collected during
scientific trawling operations in the Great Australian Bight. A 2D NMR analysis of 6 has prompted
reassignment of the published structure 5, while degradation and chiral HPLC analysis have allowed
determination of the absolute stereochemistry.
Marine sponges are the source of a unique class of
metabolite collectively known as C21 bisfuranoterpenes.
Early examples included (+)-furospongin-1 (1) from a
Mediterranean Spongia officinalis,1 as well as (-)-
,2
3
3,4
untenospongin A (2) and (-)-untenospongin B (3) from
an Okinawan Hippospongia sp. It is worthwhile noting that
(
-)-untenospongin B (3) is very closely related to the prior
published sponge metabolite (+)-tetradehydrofurospongin-1
5
(
4). Indeed, the structure originally assigned to (+)-
tetradehydrofurospongin-1 (4) was recently revised to that
shown.6 This reassignment, first foreshadowed by Van
7
Altena et al. in 1989, requires that 3 and 4 be enantiomers.
On the basis of spectroscopic anomalies similar to those
noted for 4, Van Altena et al.7 also queried the ∆
regiochemistry of the double bond in the structure origi-
nally deduced for 5. The C21 bisfuranoterpene 5 was first
reported from a Western Australian Spongia sp. in 1982,8
and at that time ozonolytic degradation yielded citramalic
acid, an observation that was interpreted as requiring a
6,7
6
,7
∆
double bond. In this report we describe the reisolation
of 5 from a southern Australian sponge, Spirastrella
papilosa (Spirastrellidae), and the subsequent structure
revision to 6. The revised structure 6 accommodates all
earlier spectroscopic and degradative observations and
addresses the concerns raised by Van Altena et al. We also
assign the absolute stereochemistry to 6 by chiral HPLC
analysis of the citramalic acid recovered from oxidative
degradation. In light of its isomeric relationship with 3 and
4
and in the absence of a preexisiting trivial name we
propose the name (-)-isotetradehydrofurospongin-1 for 6.
The EtOH extract of Spirastrella papilosa collected
during scientific trawling operations in the Great Austra-
lian Bight, Australia, yielded the known C21 bisfuranoter-
pene (-)-isotetradehydrofurospongin-1 (5). Spectroscopic
comparison with the authentic sample of 5 obtained during
an earlier investigation of a Western Australian Spongia
8
sp. confirmed this assignment, while 2D NMR analysis
(
Table 1) provided evidence to support structure revision
of 5 to 6. Noteworthy COSY correlations include those
between the 8-Me (H -9) and both H -7 and H-10 and also
between the 13-Me (H -14) and both H -12 and H-15.
3
2
permitted the reassignment of NMR data, in particular
3
2
resonances associated with ∆5 and ∆
,6
10,11
. In total these
Together with a gHMBC correlation between H-5 and C-3
these observations require repositioning both disubstituted
observations require revision of the structure for (-)-
isotetradehydrofurospongin-1 (6) to that shown (less ster-
eochemistry).
While assignment of E stereochemistry about ∆ and
are secure from J 5,6 (15.6 Hz) and the C NMR
chemical shift for C-14 (15.9 ppm) respectivelysarguments
double bonds to ∆5 and ∆
,6
10,11
. The 2D NMR analysis also
*
To whom correspondence should be addressed. Tel: 61 3 8344 6468.
5,6
Fax: 61 3 8347 5180. E-mail: r.capon@unimelb.edu.au.
∆1
3,15
13
†
University of Melbourne.
University of Western Australia.
‡
1
0.1021/np000477o CCC: $20.00
© 2001 American Chemical Society and American Society of Pharmacognosy
Published on Web 04/19/2001

Notes
J ournal of Natural Products, 2001, Vol. 64, No. 5 639
Ta ble 1. NMR (CDCl3) Data for (-)-Isotetradehydrofurospongin-1 (6)a
13Cd δ
1Ha δ (m, J (Hz))
COSYb 1H- H
1
gHMBCc 1H-13
C
no.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
142.9
107.4
123.1
139.9
123.3
137.9
46.2
7.34 (s)
6.49 (s)
H-2, H-4
H-1, H-4
C-2, C-3, C-4
C-1, C-3, C-4
7.37 (s)
H-1, H-2
H-6, H-7
H-5, H-7
H-6, H-9
C-1, C-2, C-3
C-3, C-6, C-7
C-5, C-7
6.31 (d, 15.6)
5.90 (dt, 15.6, 7.6)
2.37 (d, 7.6)
C-5, C-6, C-8, C-9
71.8
27.9
1.31 (s)
H-7, H-10
C-7, C-8, C-10
C-8, C-9, C-12
C-8, C-9, C-12
0
1
2
3
4
5
6
7
8
9
0
1
137.2
126.4
42.0
5.49 (m)
5.49 (m)
2.70 (d, 4.4)
H-12, H-9
H-12, H-9
H-10 /H-11, H-14
C-10, C-11, C-13, C-14
133.6
15.9
124.6
28.2
24.9
124.1
138.7
111.0
142.9
1.57 (s)
H-12, H-15
H-16, H-14
H-15, H-17
H-16
C-12, C-13, C-15
5.19 (t, 7.2)
2.24 (dt, 7.2, 6.8)
2.44 (t, 6.8)
C-12, C-13, C-14, C-16, C-17
C-15, C-17
C-15, C-16, C-18, C-19, C-20
7.20 (s)
6.27 (s)
7.34 (s)
5.38 (s)
H-20, H-21
H-19, H-21
H-19, H-20
C-18, C-20, C-21
C-18, C-19, C-21
C-18, C-19, C-20
OH
a
13C NMR assignments supported by gHMQC and DEPT 90° and 135° NMR. b
c
400 MHz. 600 MHz.
8
presented in the original publication swe can now confirm
and concentrated in vacuo, and the residue was purified by
silica HPLC (2 mL/min elution with 15% EtOAc/hexane
through a 5 µm Phenomenex silica 250 × 10 mm column) to
yield (R)-citramalic acid (5 mg, 25%), identical in all respects
to an authentic sample. This material (3 mg) in Et O (2 mL)
2
was treated with an excess of ethereal diazomethane at 0 °C
and stirred for 2 h. The ethereal diazomethane was removed
an E stereochemistry about ∆1
0,11
on the basis of NOESY
-12. Earlier analysis8
correlations between H-10 and H
2
based on computer simulation of the second-order H-10/
H-11 H NMR multiplet supports assignment of a ∆
1
10,11
E
stereochemistry.
Whereas earlier investigations8 had established that
ozonolytic degradation of 6 yielded citramalic acid, detected
by GC/MS analysis of the corresponding ester, dimethyl
citramalate, no attempt was made to determine the enan-
tiomeric purity or absolute stereochemistry of this chiral
degradation product. In repeating this degradation, we
2
under a stream of N and concentrated in vacuo to yield a
product that was chromatographed on silica HPLC (2 mL/min
elution with 10% EtOAc/hexane through a 5 µm Phenomenex
silica 250 × 10 mm column) to give (R)-dimethyl citramalate
1
(7) (2 mg, 90%) as a clear oil: [R]
D
-32.0° (c 0.1, CHCl
3
); H
NMR (400 MHz, CDCl
d, J ) 16.2, Ha-3), 2.66 (d, J ) 16.2, Hb-3), 2.65 (s, H
5.18 (s, OH); ESIMS (30 kV) m/z 199 (M + Na); HRESIMS
m/z 199.0569 (C Na requires 199.0582).
3
) δ 3.77 (s, OCH
3
), 3.65 (s, OCH
3
), 2.96
(
3
-2),
recovered (R)-dimethyl citramalate (7), which was con-
1
firmed by H NMR, [R]
D
, and chiral HPLC comparison to
7
12 5
H O
authentic samples of both (R)- and (S)-dimethyl citra-
malate. Thus the complete stereostructure for (-)-isotet-
radehydrofurospongin-1 (6) can be assigned as shown.
Separate as well as coelution of authentic samples of (-)-
R)- and (+)-(S)-dimethyl citramalate through chiral HPLC
0.1 mL/min elution with 20% EtOAc/hexane through a 5 µm
(
(
Phenomenex, Chirex, (R)-1-(R-naphyhyl)ethylamine derivative
of (S)-valine 50 × 3.2 mm column), with PDA and ESIMS
detection, yielded retention times of 2.32 and 4.01 min for the
respective enantiomers. Elution of dimethyl citramalate (7)
derived from oxidative degradation of 6 revealed a single
retention time of 2.32 min, confirming that 7, and hence 6,
was enantiomerically pure with an R stereochemsitry.
Exp er im en ta l Section
Gen er a l Exp er im en ta l P r oced u r es. See ref 9.
An im a l Ma ter ia l. A specimen of sponge identified as
Spirastrella papilosa (Museum of Victoria Registry Number
F80010) was collected by epibenthic sled during scientific
trawling operations aboard the RV Franklin in the Great
Australian Bight (33° 07.06′ S, 124° 22.94′ E).
Ack n ow led gm en t. We are very grateful to the CSIRO
Division of Oceanography and the crew and scientific staff of
the RV Frankin for support in specimen collection. The
assistance of L. Goudie in specimen collection and taxonomic
classification is also greatly appreciated. This research was
funded in part by the Australian Research Council.
Extr a ction a n d Isola tion . The sponge was extracted with
EtOH and the concentrated extract partitioned into CH
MeOH-, and H O-soluble fractions. The CH Cl -soluble fraction
2 2
Cl -,
2
2
2
was subjected to rapid silica filtration using a 10% stepwise
gradient from hexane to EtOAc. The fraction eluting with 15%
EtOAc/hexane was further purified by silica HPLC (2 mL/min
1
1
5% EtOAc/hexane through a Phenomenex 5 µm silica 250 ×
Refer en ces a n d Notes
0 mm column) to yield (-)-isotetradehydrofurospongin-1 (6)
(8 mg, 0.074%). An authentic sample of 6 was also available
(1) Kobayashi, M.; Chavakula, R.; Murata, O.; Sarma, N. S. J . Chem.
Res (S) 1992, 366-367.
(100 mg, ELG).
(
2) Cimino, G.; De Stefano, S.; Minale, L.; Fattorusso, E. Tetrahedron
(
D
-)-Isotetr a d eh yd r ofu r osp on gin -1 (6): pale yellow oil;
1
971, 27, 4673-4679.
-
1
[R]
-10° (c 0.1, CHCl
3
); IR (film) νmax 3400 and 1690 cm
;
(3) Umeyama, A.; Shoji, N.; Arihara, S.; Ohizumi, Y.; Kobayashi, J . Aust.
J . Chem. 1989, 42, 459-462.
1
13
UV (EtOH) λmax (log ꢀ) 227 (4.6); H and C NMR data, see
(
(
(
(
4) Kobayashi, J .; Shinonaga, H.; Shigemori, H.; Sasaki, T. Chem. Pharm.
Table 1; ESIMS m/z 295 [M + H]; HREIMS m/z 295.2062 (calc
Bull. 1993, 41 (2), 381-382.
for C21
26 2
H O , 295.2062). Identical in all respects with the
5) Kazlauskas, R.; Murphy, P. T.; Quinn, R. J .; Wells, R. J . Tetrahedron
Lett. 1976, 16, 1331-1332.
8
original authentic sample of 6.
6) Fontana, A.; Albarella, L.; Scognamigilo, G.; Uriz, M.; Cimino, G. J .
Nat. Prod. 1996, 59, 869-872.
7) Van Altena, I.; Miller, D. A. Aust. J . Chem. 1989, 42, 2181-2190.
2 2
Ozon olysis of 6. A sample of 6 (45 mg) in CH Cl (20 mL)
3 2
at -78 °C was treated with a stream of O /O until the solution
turned pale blue. The resulting ozonide was allowed to warm
to room temperature, then quenched with J ones' reagent (10
(8) Capon, R. J .; Ghisalberti, E. L.; J efferies, P. R. Experentia 1982, 38,
1444-1445.
(
9) Urban, S.; Capon, R. J .; Hooper, J . N. A. Aust. J . Chem. 1994, 47,
drops), and allowed to stir overnight. After dilution with H
40 mL) the reaction mixture was extracted with Et
O (3 ×
0 mL), the ethereal phase was dried with anhydrous MgSO
2
O
2
279-2282.
(
4
2
4
NP000477O