A three-carbon (n+1+2) ring expansion method for the synthesis of macrocyclic enones. Application to muscone synthesis

The three-carbon ring expansion methodology commences with the preparation of a cyclic allene (C9, C11, C13), readily available from the corresponding cycloalkene via dibromocarbene addition and subsequent treatment with methyllithium. Dichloroketene addition to the cyclic allene regioselectively provides the [2+2] cycloadduct, which is reductively dechlorinated with zinc in methanol. The resulting cyclobutanone is then catalytically hydrogenated; cyclobutanone ring opening is affected with trimethylsilyl iodide; immediate dehydroiodination of the resulting β-iodocycloalkanone with diazabicycloundecane (DBU) provides the corresponding macrocyclic enone. The 15-membered enone was converted to d,l-muscone with (CH3)2CuLi.