Ring-opening metathesis regioselective reactions

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

Facile, regioselective ring opening-cross-metathesis reactions between unsymmet-rical norbornene derivatives and electron-rich alkenes in the presence of the second-generation Grubbs catalyst have been reported to generate highly substituted furans and pyrroles.114... 

Scheme 207). Diazametallacycles such as 831 are key intermediates involved in imine metathesis catalyzed by zirconocene imido complexes,628 the authentic samples of which were synthesized independently by the reaction of 825 with 2 equiv. of the corresponding imine.629 The zirconocene imido complex 825 also ring-opens epoxides such as styrene oxide regioselectively to give metallacycle 832 (Scheme 207).630... 

Further studies of the regioselective tandem ring opening/cross metathesis of 2-emlo-substituted 7-oxanorbomenes with electron-rich olefins, a process first described in 2004, were reported. Reaction of the 2-exo isomer, like that of the 2-endo isomer, was also found to... 

In 2004, Kozmin et al. reported the total synthesis of bistramide A (69), a marine natural product with potent cytotoxicity [67]. In this synthesis (Fig. 21) the authors developed a novel approach featuring a sequential ring opening/selective alkene cross metathesis of highly strained cyclopropenone ketal (70) with terminal alkenes (71) and (72). After extensive reaction optimization, this process was successfully achieved using Grubbs II catalyst. For the cross metathesis reaction, a remarkable regioselectivity was observed in the presence of two different alkene moieties. 

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