A Flexible Stereoselective Synthesis of the Spirosesquiterpenes (±)‐β‐Acorenol, (±)‐β‐Acoradiene, (±)‐Acorenone‐B and (±)‐Acorenone via an Intramolecular Ene‐Reaction

The racemic spirosesquiterpenes β‐acorenol ( 1 ), β‐acoradiene ( 2 ), acorenone‐B ( 3 ) and acorenone ( 4 ) ( Scheme 2 ) have been synthesized in a simple, flexible and highly stereoselective manner from the ester 5 . The key step ( Schemes 3 and 4 ), an intramolecular thermal ene reaction of the 1,6‐diene 6 , proceeded with 100% endo ‐selectivity to give the separable and interconvertible epimers 7a and 7b . Transformation of the ‘ trans ’‐ester 7a to (±)‐ 1 and (±)‐ 2 via the enone 9 ( Scheme 5 ) involved either a thermal retro ‐ene reaction 10 → 12 or, alternatively, an acid‐catalysed elimination 11 → 13 + 14 followed by conversion to the 2‐propanols 16 and 17 and their reduction with sodium in ammonia into 1 which was then dehydrated to 2 . The conversion of the ‘ cis ’‐ester 7b to either 3 ( Scheme 6 ) or 4 ( Scheme 7 ) was accomplished by transforming firstly the carbethoxy group to an isopropyl group via 7b → 18 → 19 → 20 , oxidation of 20 to 21 , then alkylative 1,2‐enone transposition 21 → 22 → 23 → 3 . By regioselective hydroboration and oxidation, the same precursor 20 gave a single ketone 25 which was subjected to the regioselective sulfenylation‐alkylation‐desulfenylation sequence 25 → 26 → 27 → 4 .