Synthesis of the Alleged Natural Monoterpenoid α‐Santolinenone

Authentic α‐santolinenone ( = (+)‐(4R)‐1(7)‐p‐menthen‐2‐one; (+)‐1) is made available for the the first time in 30% overall yield from (+)‐(4R)‐p‐menthene ((+)‐2) via the diastereoisomeric allylic alcohols (+)‐4a/(+)‐4b, which are oxidized to (+)‐1 with Ag2CO3/Celite. Yields are good, except for the last stage; indeed, only alcohol (+)‐4a, with equatorial OH‐group, undergoes oxidation, and (+)‐1 is partly substracted via a hetero Diels‐Alder dimerization giving a mixture of the diastereoisomeric dihydropyrans (+)‐5a/(+)‐5b. When Cr(VI) reagents ae used, (+)‐4a/(+)‐4b mainly give phellandral (6) and carvotanacetone (7), NnO2 reacts too sluggishly with (+)‐4a/(+)‐4b. A camphor pyrolyzate, previously thought to be 1 must be a different compound, probably 7.