The sesquiterpene isocyanide __ . __ stylotelline isolated from the marine sponge Stylotella sp. was assigned the structure la (absolute stereochemistry) on the basis of spectral - essentially 2D-NME-- and chemical data. Terpenoid isocyanides have been found previously in marine sponges, but still represent a relatively rare class of natural products showing cytotoxic, antibiotic or antifeedant properties. 1 We report here the isolation and structure determination of a new sesquiterpene isocyanide, stylotelline la, from the sponge Stylotella sp. 2 The sponge was freeze-dried and continuously extracted with methylene chloride. Silica gel chromatography of the crude extract gave la as an oil - (yield 0.16% of the sponge dry weight).Stylotelline la, [aID-47' (CHC13, c = 1.7) had the molecular formula C16H2SN (MS and l3 C NMR) and showed IR absorption at 2140 cm -l (i&C). Its planar structure was readily assigned on the basis of NMR data.In the 13 C spectrum, multiplicity of the carbons was established by a J modulation sequence3 (4C, 3CH, 5CH2, 4CH3). Attribution of the carbons in the 1.0~ field part of the spectrum was straightforward (C-5, C-6, C-7, NC as shown in Fig. 1). In the broad band decoupled spectrum 13 C-5 and 13C-16 bound to nitrogen appeared as triplets coupled to 14N (J z 4 H2).4 In the high field part of the spectrum, heteronuclear 6-6 correlation (lH-13C) allowed identification of the pairs of l3 C and directly bonded nuclei. Proton connectivities were further deduced from the 11 H- H COSY spectrum (Fig. 2). H-6 (not shown in Fig. 2) appeared as a multiplet at 6 5.44 showing a long range coupling with H-8.Fig.1 - 13C NMR data of 12 The planar structure of - la was confirmed and the absolute configuration at C-10 established by facile elimination of the tertiary isocyanide group c1 to the double bond (MeOH, HCl l%, 30 min, rt) giving the known diene (+)-6-selinene 2. 2 was identified by comparing its properties (Cal,, IR, 1H and l3 C NMR) with those of an authentic sample prepared from eudesmols. 5,6 The same diene was also found in the least polar chromatographic fractions of the crude extract, but could have been formed secondarly from - la during conservation or extraction of the sponge since elimination of the 8s? group proceeded slowly in the presence of traces of acid.The remaining stereochemical assignments were deduced from NMR data. The value of the coupling constant 3J3 4 = 12 Hz required an axial orientation for H-4. Proton 4 appeared also'coupled to nitrogen (3JN H_4 = 3 Hz).~ Evidence for the trans junction C-5 - C-10 came, on the one' hand, from a long-range COSY spectrum' of la (Fig. - 2) showing a coupling between H-3ax and CH3-14 establishing their trans diaxial orientation and, on the other hand, from the observation of a NOE between H-3ax and NH in compound - lb {F 124'-', [a] +122' (CHC13, c = 0.5} readily prepared from - la (AcOH, ether, 16h, rt). H-3ax was identified in lb 1 - H NMR spectrum by decoupling from H-4 .The formamide lb was not found in the sponge extract, although cooccu- rence of isocyanides with the corresponding formamides and also isothiocyanates has often been observed in other sponges. 1