Lissoclinotoxin A, a new antibiotic 1,2,3-trithiane derivative, was isolated from the Tunicate Lmoelimm pe1f5ratum. The structure elucldabon was based upon spectral dataThe phylum Tunicates has proven to be a rich source for the isolation of structurally original and biologically active products such as eudistomins 1, ulithtacyclamtdes2, didemnins 3, bistratamidesh, grossularins5. In the course of our program devoted to the search of bioactive compounds in this phylum we found, m a preliminary screening, that the methanolic extract of Li.w.dmum petjiirutum Venil, 1871 (Didemnidae) collected in Dinard (France) exhibited potent antimicrobial and antrfungal acttvtttes (S. aureus, 0 inhibition : 15mm/3OO~g). Fractionation of the extract and purtfication of the active compounds were monitored by an antimicrobial assay.The lyophtlized animals were extracted first with CH2C12, then with MeOH. Only the methanolic extract exhibited a marked antimicrobial activity. A s&age1 preparative separation of the methanolic extract, using chloroform with increastng amounts of methanol as eluent, gave a most acttve fraction, eluted with chloroformmethanol 8/2. Preparative HPLC (S102 Partistl SlO; chloroform-methanol 8/2) led to the isolation of the major active compound, lissoclmotoxin A, b, as an amorphous yellow sohd, m.p. 245.250°C ; UV. hmax (nm, ethanol): 215 (E 10440), 246 (E 8874); (yield , 0.01% dry weight), HRMS (M+ 260 9945) furnished the formula C9HllNO2S3 (6). EI mass spectrum: 261 (M+), 244 (Mf-NH3) 197 (M+-2s). This product was poorly soluble in the common organic solvents. However, the lH NMR could be performed in d6-DMSG and showed the presence of one aromatic proton (6 6.97 ppm), a methoxyl (8 3.63) and two coupled methme groups (6 3.27 and 3.08 ppm). In order to gain more information on the structure we prepared some derivatives.First, we attempted to obtain an acetylated derivative. When acetylation was performed in a l/5 mixture of pyrtdine and acetic anhydride, a diacetyl derivative fi was obtamed as the major product isolated (yellow oil, 32% yield). The *H NMR (see Table 1) favoured the presence of an 0-acetyl group (6 2.36 ppm) and an Nacetyl one (6 1.96 ppm, and 6 5.30 : NH-AC); this NH is coupled with the methme group at 6 2.81 ppm, itself coupled with one at 6 3 42, suggesting the presence of a -CH2-CH2-NH-CO-CH3 moiety. The HRMS (m/e 303.0054, M+-AC)' indicates again the presence of three sulfur atoms.However when the acetylation was run in DMF with acetic anhydride and DMAP (r. t., 18h), the major product was a tenacetylated denvative 2, m.p. 5557'C (26% yield). In 1H NMR (see Table 3), two additionalacetyl groups (d 2.37 and 2.34 ppm) corresponding to two S-CO-~~38 are present. HRMS (m/e 355.0549, M+-Ac)~ and elemental analysis9 indicated the presence of only two sulfur atoms.The 1H and l3C NMR data of the diacetylated and tetraacetylated derivatives (see Tables l-4) were consistent with the presence of an aromatic nucleus bearing the following substituents: one OH, one OMe, two sulfur atoms, and one -CH2-CH2-NH2 group in lissoclinotoxin AThe NOE observed for the tetracetylated derivative are shown in fig 1. Although of low intensities, these data allowed us to assign structure 2 to this derivative. Owing to the fact that the tetracetylated derivative contain& only two snlfur atoms and the diacetylated three sulfur atoms (as lisssclinotoxin A), we decked that lissoclinotoxin A must possess a trisulfur bridge and hence should be assigned structure la. We have to notice that only the loss of two sulfur atoms was observed in the EI as well as in the CI mass spectra.The natural aithiane lisscclinotoxin A h, exhibits potent antnnicrobial and antifungal activities in vitro . The CM1 against a variety of Gram(+) and Gram(-) bacterial strains arc in the range 0.6 to 0.1 pg/ml (Staphylococclrs aureus, Streptococcus faecalu, Citrobacter spp , Klebssiella spp., Escherischia colt, Enterobucter spp., Salmonella, Proteus) and also towards the pathogemc fungi Candida albicans (4Opg/ml) and 7'richosporon mentugrophytes (20 clg/ml). Lissoclinotoxin A has a modest cytotoxlclty towards L 1210 leukemia cells (ID 50 : 4pg/ml) and is toxic in vwo for mce (LD 50 < 50 mg/kg).Only three natural l-2-3 mthtanes have so far been reported (10,11,12). However a remarkable example related to Ia, 2, has been isolated from another tunicate Aplidium sp I*. Thts uithiane also prossesses antibacterial and antifungal activiues