Stereoselective Conversion of Campholene- to Necrodane-Type Monoterpenes. Novel Access to (−)-(R,R)- and (R,S)-α-Necrodol and the Enantiomeric γ-Necrodols

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 1, Table), both readily available from (−)‐campholenyl acetate ((−)‐i) by an efficient stereoselective synthesis. The thermodynamically preferred (−)‐(R)‐γ‐necrodol ((−)‐5) becomes the major product (≥ 80% yield) after either prolonged treatment with LEDA or exposure of α‐ and β‐necrodols to BF3·Et2O. In an alternative route, (+)‐5 is prepared starting from (+)‐campholenal ((+)‐ii) via Pd‐catalysed decarbonylation to (−)‐(S)‐1,4,5,5‐tetramethylcyclopent‐l‐ene ((−)‐6) and subsequent application of an acid‐catalysed CH2O‐addition/rearrangement sequence (Scheme 2).