Tandem reactions ring-rearrangement metathesis

Fukuyama and coworkers [48] have also utilized an olefin additive in the optimization of a crucial ring-rearrangement metathesis sequence to form the key 5,6-ring system in the synthesis of (-)-isoschizogamine (Scheme 12.27). The tandem metathesis reaction of the norbornene 87 was difficult due to the substitution pattern of norbornene skeleton, which restricted the approach of the Ru catalyst. For example, the treatment of norbornene 87 with the second-generation Hoveyda - Grubbs catalyst 4 at reflux in benzene provided lactone 89 in only 24% isolated yield. To optimize this reaction, the use ofthe modified Hoveyda-Grubbs catalyst 88 [49] and the addition of 20 equiv of 1,6-heptadiene as an additive were quired. The desired lactone 89 was then obtained in 73% yield. 

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