Stereoselective conversion of campholene- to necrodane-type monoterpenes. Novel access to (-)-(R,R)- and (R,S)-α-Necrodol and the Enantiomeric γ-Necrodols

Article abstract of DOI:10.1002/hlca.19910740310

Naturally occurring (-)-(R,R)-α -necrodol ((-)-1) and its C(4)-epimer (-)-2 are obtained in 84 and 44% yields, respectively, by lithium ethylenediamide (LEDA) treatment of the corresponding β-necrodols (-)-3 and (-)-4 (Scheme I , Table), both readily available from (-)-campholenyl acetate ((-)-i) by an efficient stereoselective synthesis. The thermodynamically preferred (-)-(R)-y -necrodol ((-)-5) becomes the major product ( ≥ 80 % yield) after either prolonged treatment with LEDA or exposure of α- and β-necrodols to BF₃Et₂O. In an alternative route, (+)-5 is prepared starting from (+)-campholenal ((+)-ii) via Pd-catalysed decarbonylation to (-)-(S)-1,4,5,5-tetramethylcyclopent-1-ene ((-)-6) and subsequent application of an acid-catalysed CH₂O-addition/ rearrangement sequence (Scheme 2).