The Absolute Configurations and Biogenesis of Some New Halogenated Chamigrenes from the Sea Hare Aplysia dactylomela

Four new halogenated chamigrenes have been isolated from the sea hare, the (E)‐ and (Z)‐9‐(bromomethylidene)‐1,2,5‐trimethylspiro[5.5]undeca‐1,7‐dien‐3‐ones (11 and 10, resp.) the structures of which were deduced by NMR spectroscopy and by comparison with a known relative, the (8R,9R)‐8‐bromo‐9‐chloro‐5,5,9‐trimethyl‐1‐methylidenespiro[5,5]undec‐3‐en‐2‐one (12), and its (11 R)‐11‐acetoxy‐substituted derivative 13. Their structures and absolute configurations were determined and deduced by X‐ray using the absolute‐structure parameter. The configurations were remarkable in being enantiomeric to those of the tertiary chlorochamigrenes isolated so far. Consequently, a more general scheme is proposed to account for their biogenesis.