Synthesis and characterization of abietadiene, levopimaradiene, palustradiene, and neoabietadiene: hydrocarbon precursors of the abietane diterpene resin acids

The abietane diterpenes—abietadiene, levopimaradiene, palustradiene, and neoabietadiene (1b–4b)—were prepared from the corresponding resin acids by diazomethane esterifications, LiAlH4 reductions, tosylations, and Zn/NaI reductions. Abietadiene was also obtained less efficiently by catechol borane reduction of abietadienal tosylhydrazone and Li/NH3 reduction of its 18-phenylthio derivative. These conjugated dienes were characterized by chromatographic properties (HPLC, TLC, GC), MS fragmentation patterns, optical rotations, and UV, IR, 1H NMR, and 13C NMR data. Assignments for the 1H and 13C NMR spectra were made by COSY, DEPT, HMQC, HMBC, NOE data, H–H coupling analysis, and comparisons with literature data. The four diterpenes were identified as cyclization products of recombinant abietadiene synthase, supporting their likely role in the biosynthesis of the abietane resin acids in conifer oleoresin.