Journal of Natural Products Vol. 5 1 No. 2, pp. 2 7 5 - 2 8 1 , Mar-Apr 1988
275
I
VARIABILIN AND RELATED COMPOUNDS FROM A SPONGE OF THE GENUS SARCOTRAGUS COLINJ. BARROW,JOHN W. BLUNT,+MURRAYH.G. MUNRO," and NIGELB. PERRY Department of Chemistry, University of Canterbury, Chistchurch, New Zealand, and Harbor Branch Oceunographic InstitdonlSeuPham Project, Fort Pierre, Flmida 3 3 4 5 0
ABSTRACT.-The bioactivity-directed analysis of the extract from a sponge of the genus Sarrotragus led to the isolation of a series of bioactive sesterterpenes, of which variabilin [la) was the major component. The sesterterpenes 2a, 3, and ha, along with the related C,, furanoterpene 151, were present in lesser amounts. The unequivocal assignment of the stereochemistry of the 20,21 double bond in variabilin as 202 was achieved through examination of the 22-0methyl derivative Ib of variabilin and the isolation of the variabilin isomer 2a with the 20E configuration.
Sponges of the order Dictyoceratida have yielded a wide range of new sesterterpenes, many of which contain both furan and tetronic acid functional groups (1,2). Typical of these furanosesterterpene tetronic acids is variabilin [la],which was first isolated from the sponge Ircinia variabilis ( 3 ) . This compound is antimicrobial and has since been reported to be cytotoxic (4). In the original communication, neither the stereochemistry at the double bonds nor the absolute configuration at C-18 was defined. Subsequently, 7E and 12E configurations were assigned ( 5 ) , leaving the stereochemistry at the exocyclic double bond unknown. Variabilin has now been identified as the major component responsible for the bioactivity of an extract from a sponge of the genus Sarcotragus (Thorectidae, Dictyoceratida). This isolation arose from a search for potential antiviral and antitumor agents from New Zealand marine invertebrates. Variabilin [la]is a major component in all New Zealand collections of sponges of the genera Ircinia, Psamrnocinia, and Sarcotragus (6). Investigations of the 'H- and l3C-nmr spectra of the 22-0-methyl derivative of variabilin have now revealed the stereochemistry of the remaining double bond as 202. This assignment was aided by the isolation of the 20E isomer 2a as a minor component from the same sponge. Three other terpenes, including another double bond isomer 3, a hydroxy derivative of variabilin 4a,and a C, furanoterpene 5 have also been isolated from the Sarcotragus sp. RESULTS AND DISCUSSION An MeOHItoluene extract of a sponge, identified as Sarcotragus sp. I, displayed significant in vitro antiviral activity against HwpessimplexTypeI and Polio Type I, with little cytotoxicity to the tumor cells used in the antiviral assay. Bioassay-directed Si gel chromatography of this extract yielded variabilin {la]as the major bioactive component. However, the pure compound displayed cytotoxic rather than antiviral properties. The cause of this change in bioactivity is not yet understood. The related sesterterpenes 2a, 3, and ha, along with the related C,, furanoterpene 5, were cytotoxic at similar levels. The identity of variabilin was confirmed by comparison of its uv, ir, I3C- and 'Hnmr spectra with those reported previously (3,5), and from its mass spectrum. Full assignments of the 'H-nmr spectrum (Table 1) and I3C-nmr spectrum (Table 2) were achieved through the use of homonuclear (COSY) and heteronuclear (HETCOR and XCORFE) correlation 2D spectroscopy. These assignments are in agreement with published results for variabilin and other furanosesterterpene tetronic acids ( 1). In order to determine the C-20,21 double bond geometry, 22-0-methyl variabilin
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Journal of Natural Products
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la R=H l b R=Me
0
0 2a
R=H
2b R=Me 0
IC
OCH,
0
3 OH
4b R=Me
OR
0
[lb] was prepared. Treatment of variabilin with CH,N, gave two methylation products l b and ICin the ratio 3:1 (7). Difference nOe 'H-nmr experiments on the major methylation product Ib showed enhancement of the H-20 doublet as well as the tetronic acid methyl signal upon irradiation of the methoxyl signal. Irradiation of the H-20 doublet gave an enhancement of the methoxyl peak intensity. This allowed the exocyclic tetronic acid double bond of the 22-0-methyl derivative l b and, hence, that of variabilin itself, to be assigned as 202.
Mar-Apr 19881 Barrow et a!. : Variabilin and Related Compounds
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TABLE2.
"C-nrnr Data for Compounds 1-5." Compound
- -- - -
Orbon
la
2a
IC
I
1 2 3 4 5
6 7
8 9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 OMe
142.47 111.09 124.98 138.78 25.04 28.44 123.72 135.71 16.04 39.55' 26.57 124.41 134.76 15.80 39.68' 25.68 36.57 30.95 20.60 117.17 142.96 162.47 99.06 172.54 6.06
143.20 111.76 125.60 139.60 25.92 29.45 124.74' 136.07d 16.50' 40.64 27.56 125.26' 135.31d 16.69' 40.48 26.65 37.42 31.79 21.29 116.85 144.18 163.65 100.00 173.2 6.82
142.51 111.12 125.01 138.82 25.04 28.42 123.75 135.73 16.05 39.54' 26.52 124.37 134.89 15.83 39.67' 25.70 37.26 30.11 21.62 121.37 142.14 162.29 101.61
-f
6.12
142.50 111.09 125.00 138.80 25.06 28.46 123.72 135.76 16.05 39.57' 26.54 124.30 134.91 15.86 39.69' 25.72 36.68 30.81 20.69 115.24 142.70 161.97 98.99 171.10 8.57 58.80
142.53 111.09 124.97 138.81 25.04 28.44 123.75 135.71 16.07 39.53' 26.56 124.37 134.88 15.86 39.70' 25.66 37.26 29.90 21.60 120.79 142.16 163.14 101.63 170.62 9.03 58.90
142.5 111.1 125.0 138.8 25.0 28.5 123.8 135.7 16.0 39.5' 26.6 124.5 134.7 15.8 39.7' 25.7 36.3 30.8 20.3 119.5 146.7 183.8 m.1 175.6 4.0 56.4
3
3b
4a
142.55 111.14 125.04 138.83 25.06 28.42 123.70 135.87' 15.96 39.55 25.72 31.39 135.82' 23.38 124.52 26.31 37.46 30.79 20.55 116.11 142.67
143.21 111.74 125.61 139.60 25.88 29.39 124.72' 136. 13d 16.52 40.37 27.18 32.20 136.08d 24.11 125.66' 26.77 38.41 31.52 21.27 114.68 144.30
142.75
99.f3
100.45
--
-i
-i
-
-i
6.06
6.64
4b
5
142.59
142.54 111.09 125.01 138.84 25.08 28.47 123.17 135.77 16.05 39.65' 26.55 124.27 134.94 15.89 39.69' 25.21 36.24 30.11 19.70 41.21 186.74
-
1io.m 110.87 124.86 138.85 25.16 24.42 41.26' 73.98 26.56 41.21' 22.55 124.34 135.63 15.96 39.30 25.52 36.51 30.74 20.59 115.79 142.05 162.36 99.27 172.10 6.22
124.89 138.73 25.31 24.51 41.69' 72.72 26.92 41.52' 22.62 124.30 135.19 15.92 39.56 25.69 36.70 30.80 20.75 115.18 142.59 161.84 98.96 171.10 8.70 58.85
'Values in pprn. relarive to = 0.00 for (TMS) in CDCI, solutions b ~" , ~" , solution.
