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 BF 3 ·Et 2 O. 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 CH 2 O‐addition/rearrangement sequence ( Scheme 2 ).