ROM-CM Sequences

Because of the reversible character of the metathesis itself, it is possible to perform ring-opening metathesis (ROM) of endocyclic olefins if the process is thermodynamically favored. This particular type of tandem process is especially atom-economical (as no ethylene is released), although the negative entropy 

---

This reaction was first reported by Grubbs and his group [44], An ROM/CM sequence between cyclooctene and one of the unsaturated ketones of 66 is followed by... 

ROM-CM Sequence The ROM reaction of a cycloalkene engaged in the subsequent CM reaction with an olefinic partner provides the straightforward procedure for the synthesis of diene products having the isolated double bonds. This sequence is applicable not only to relatively strained cycloalkenes but also to unstrained compounds such as cyclohexene. The ROM-CM sequence of norbor-nene compounds or cyclopropene ketals with various olefins is reported for the preparations of natural products. 

In the final synthesis a silicon tethered olefin was used as a protecting group that facilitates a RCM-ROM-RCM sequence. An alternative strategy was to carry out ROM-RCM with TBS protected alcohol (Scheme 20) and subsequent CM, but this strategy was low yielding. 

The Blechert group also applied the tandem RRM process with CM (RCM-ROM-RCM sequence) to the synthesis of (—)-lasubine 11 (141, Scheme 24.36), an alkaloid extracted from the leaves of Lagerstroemia subcostata Koehne. ° This strategy requires a careful choice of substrates and catalysts to avoid the CM reaction before the RRM reaction takes place. Therefore, cyclopentenone 138 was chosen as a starting material from the outcome of their study. In the presence of [Ru]-II (5 mol%), the treatment... 

Cross-metathesis of Alkyne with Cycloalkene (ROM-CM-RCM Sequence) A cascade metathesis with cycloalkene compounds is an interesting reaction because the different ring-size cyclic diene is formed from the starting cycloalkene via multiple processes in... 

Fig. la—d Typical alkene metathesis reactions ring-closing (RCM) and ring-opening (ROM) metathesis (a), diene cross metathesis (CM, b), ROM-RCM (c), and ROM-double RCM (d) sequences (ring-rearrangement reactions, RRM)... 

The synthetic potential of RRM can be amphfied with the introduction of a third olefin, permitting the access to much more complex structures in one single transformation, by a further intermolecular CM or a second RCM. As discussed above, two mechanistic pathways can be envisaged. The metathesis cascade may involve an initial formation of a ruthenium-alkyhdene complex at the terminal olefin, or alternatively, the sequence may begin with [2+2]-cydoaddition of the ruthenium carbene onto endocychc double bond, followed by ROM-RCM or CM. Neither of these possibihties can be ruled out, and the occurrence of one or the other may be influenced according to ring strain, steric and electronic properties of the corresponding olefins, and also the reaction conditions employed (Scheme 11.22). 

---