Configuration, conformation, and reactivity of highly functionalized eunicellane diterpenes isolated from the gorgonians Eunicella cavolinii and Eunicella singularis from Marseille

Eunicellane diterpenes with a C(7) = C(16) (Delta(7,16); see 17 - 19), (Z)- C(7) = C(8) ((7Z)Delta(7,8); see 20 - 23), and (Z)-C(6) = C(7) ((6Z)Delta(6 ,7); see 10) bond, an uncommon feature in the case of extensive functionali zation at the cyclohexane ring and the latter exhibiting uncommon configura tions, were isolated from the gorgonian Eunicella cavolinii from Marseille (Figs. 5 and 2). The gorgonian Eunicella singularis (= Eunicella stricta) f rom the same area gave (72)Delta(7,8), Delta(7,16), and (6E)Delta(6,7) anal ogs 24, 25, and 13, respectively (Fig. 5 and Scheme). The (6E)Delta(6,7) mo iety of 13 - characterized by a slow 180 degrees conformational flipping (F ig. 3) that results in broadening of NMR signals - makes the macrocycle hig hly strained. This may explain the spontaneous conversion of 13 to the 6-me thoxy-7-hydroxy derivative 14 in the presence of MeOH at - 20 degrees in th e dark (Scheme). The isomeric, deacylated analogue 10 showed only little br oadening of NMF signals and proved stable, in accordance with the less stra ined nature of this compound (Fig. 4).