The brown alga, Pachydictyon coriaceum, produces various type of diterpenes such as 1-4. Chromatographic separation of the methanol extract of this seaweed afforded six new diterpenes, 7, 8, 14, 17, 18, and 19. Acetylcoriacenone (7) is a novel diterpene, which possesses a unique cyclobutenone moiety. Its structure has been deduced on the basis of the spectral data, as well as chemical conversions. Auto-oxidation of 7 gives an epoxide 10, the stereochemical features of which have been elucidated as 12 by the analysis of the 400MHz ^1H-NMR spectrum. Isoacetylcoriacenone (8) is a C-19 epimer of 7. Pachylactone (14) includes a cyclopropane moiety, and its conformation has been deduced as in 15 from its characteristic ^1H-NMR spectrum; the methine proton at C-6 shows the unusual upfield signal at δ 0.85, shielded by both of C_1=C_2 and the cyclopropane ring, and also the signal due to H-10 in the side chain appears at δ 2.33, de-shielded by C=O at C-18, which shows that rotation about C_9-C_<10> axis is restricted so that 10-H is located close to the carbonyl group. Detection of NOE's between H-4b and H-19b, and between H-7a and H-9 supports the conformation 15. The structures of three other new diterpenes, 17, 18, and 19, have been elucidated by spectroscopic analyses.
