Eunicellane diterpenes with a C(7)C( ) (D 7,16 ; see 17 ± 19), (Z)-C( )C( ) ((7Z)D 7,8 ; see 20 ± 23), and (Z)-C( )C( ) ((6Z)D 6,7 ; see 10) bond, an uncommon feature in the case of extensive functionalization at the cyclohexane ring and the latter exhibiting uncommon configurations, were isolated from the gorgonian Eunicella cavolinii from Marseille (Figs. and). The gorgonian Eunicella singularis ( Eunicella stricta) from the same area gave (7Z)D 7,8 , D 7,16 , and (6E)D 6,7 analogs 24, 25, and 13, respectively (Fig. and Scheme). The (6E)D 6,7 moiety of 13 ± characterized by a slow 1808 conformational flipping (Fig. ) that results in broadening of NMR signals ± makes the macrocycle highly strained. This may explain the spontaneous conversion of 13 to the 6-methoxy-7-hydroxy derivative 14 in the presence of MeOH at À 208 in the dark (Scheme). The isomeric, deacylated analogue 10 showed only little broadening of NMR signals and proved stable, in accordance with the less strained nature of this compound (Fig. ).
Helvetica Chimica Acta ± Vol. 83 (2000) Helvetica Chimica Acta ± Vol. 83 (2000) and the relative configurations. In particular, NOE experiments established that the sp 3 C-atoms of 10 have the same configuration as the corresponding C-atoms of the massileunicellins A ± C (7 ± 9) [1]. The (6Z) configuration for 10 is based on d(C( )) 27.38 (in CDCl 3 ), a value typical for a deshielded Me C-atom at a trisubstituted CC bond and in accordance with d(C( )) 28.87 reported for eunicellane 11, previously isolated from the gorgonian Muricella sp. from south Korean waters . All (6E) eunicellane diterpenes so far isolated ± including the new 13 (see below, Table ) isolated from E. singularis ± are characterized by d(C( )) values comprised between 18 and 21 ppm 4 ) 5 ). These conclusions find further support from related families of diterpenoids, where the methyl C-atom at the trisubstituted CC bond with (Z)configuration resonates downfield with respect to the (E)-isomer. This is the case for the asbestinanes, which have the same O-bridged macrocycle of the eunicellanes [11a], the briaranes, where there is no O-bridge [11b], and the xenicanes, where the macrocycle is nine-membered [11c]. The configuration of analogs, which are only known in the (Z) configuration, was assigned by X-ray diffraction data; these analogs showed consistently relatively low-field d(C( )) resonances 6 ). This is the case of briaranes called stecholides [11d] and of certain other briaranes [11e].