Application of the α-Alynone Cyclization: Synthesis of rac-Modhephene

Article abstract of DOI:10.1002/hlca.19810640417

rac-Modephene 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 of the construction of the <3.3.0>-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 reaction shown in the Scheme 3.The further elaboration and separation of the cyclization product 4 and 14 to rac-modhephene 1 and its epimer 27 are outlined in the Scheme 5.