Anwendung der α‐Alkinon‐Cyclisierung: Synthese von rac ‐Modhephen

Application of the α‐Alkynone Cyclization: Synthesis of rac ‐Modhephene rac ‐Modhephene 1 , the first sesquiterpene with a propellane C‐skeleton and its epimer rac ‐ epi ‐modhephene 27 , were synthesized starting from bicyclo[3.3.0]oct‐1(5)‐en‐2‐one ( 2 ). The key step in the construction of the [3.3.3]‐propellane system is an application of the α‐alkynone cyclization, namely 3 → 4 and 11 → 14 . The preferred formation of the propellanes 4 and 14 in this step shows that the insertion of the postulated alkylidene carbene intermediate into tertiary C,H‐bonds outweighs the one into the secondary ring‐C,H‐bonds leading to 12/13 and 15/16 , respectively. The two starting materials for the α‐alkynone cyclization, 3 and 11 , were prepared from 2 by the reactions shown in Scheme 3 . The further elaboration and separation of the cyclization products 4 and 14 to rac ‐modhephene 1 and its epimer 27 are outlined in Scheme 5 .