Allylmalonate as an Activator Subunit for the Initiation of Relay Ring-Closing Metathesis Reactions

摘要

Our research group has previously described a total synthesis of (+)-gigantecin (5, Scheme 1) in which the importance of the sequencing of the competitive ring-closing metathesis (RCM) and cross-metathesis events was demonstrated.Significantly, when using the Hoveyda-Grubbs second-generation initiator (HG2), the RCM reaction of the key substrate 2 (Scheme 1) gave the 11-membered cyclic alkene 1 rather than the desired silacycloheptene derivative that would have arisen from closure of C15 with C16. In contrast, initial cross-metathesis of 2 with 3 to form the C7=C8 bond with subsequent RCM gave 4, which was additionally manipulated in a straightforward fashion to give 5.