Enantiospecific Synthesis of a Novel Rearranged Eunicellane Diterpenoid by SmI₂-Mediated Cyclization

Abstract

Aiming at the assembly of marine-derived diterpenoids, the synthesis and cyclization of α-geranylated carvones was investigated. 3-Hydroxyalkylation of side-chain hydrogenated carvone with geraniol-derived aldehydes gave access to diterpenoid allyl phosphates. It was shown that retro-aldol fragmentation of ring-hydrogenated 3-hydroxyalkylcarvones is surprisingly facile, because the preferred conformation resembles a Zimmerman-Traxler type transition-state. The hitherto unknown rearranged eunicellane skeleton can be obtained in one step by treatment of an α,β-unsaturated diterpenoid with samarium diiodide generated in situ in THF. NOESY­-based structure analysis revealed the presence of an ansa bridge across a twist-boat six-membered ring.

Key words

diterpenoids - retro-aldol reaction - samarium diiodide - isoeunicellane - NOESY-based analysis

References

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14

The high value for ^³ J _OH-7H (11-12 Hz) in all eight examples reflects the antiperiplanar arrangement of the two protons. The ^³ J _3H-7H coupling constants are very small (1-2 Hz) as expected for cis-fused six-membered rings. ^³ J _3H-4H coupling constants show values between 11 and 12 Hz due to diaxial arrangement.

15

^³ J _OH-CH Coupling constants vary from 3 to 9 Hz indicating that hydrogen-bridged six-membered rings are not as dominant as in the cyclohexanone case. The ^³ J _3H-4H coupling constants of cyclohexenone-type carvones reach values between 2 and 5 Hz. The absence of NOESY correlations between any of the diastereotopic protons 5a-H/5b-H and
3-H rules out axial positioning of 3-H. ^³ J coupling constants between 8 and 10 Hz are consistent with an antiperiplanar arrangement of 3-H and the carbinol-H of the side chain.