Ring opening-closing metathesis

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)... 

More complex small molecules can also be made by metathesis cascades and tandem reaction sequences involving olehn metathesis components [41], The examples illustrated in Fig. 4.13 include inter- and intramolecular enyne metathesis between an olefin and an alkyne [42], ring-opening cross metathesis to form new substituted acyclic olefins [43], ring-opening ring-closing sequences... 

It is clear that ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM) and ring-opening cross metathesis (RO/CM) are interesting methods to... 

Driven by the release of the ring strain inherent in 10, ring opening cross metathesis with 6 proceeded to give the 1 1 adduct 11 in near quantitative yield. The derived cross-linked silyl ether 12 underwent smooth ring-closing metathesis to the dienone 1. 

By use of Ru-based carbenes such as [Ru]-Ia, II, III, various carbo- and heterocycles can be synthesized from the corresponding enyne precursors. The metathesis is carried out not only by ring-closing enyne metathesis but also by dienyne metathesis, cross enyne metathesis, and ring-opening enyne metathesis.In this section, the synthetic applications of enyne metathesis reactions, including skeletal reorganization reactions and hetero-enyne metathesis, to natural products and related compounds are described. 

When a mixture of alkenes 1 and 2 or an unsymmetrically substituted alkene 3 is treated with an appropriate transition-metal catalyst, a mixture of products (including fi/Z-isomers) from apparent interchange of alkylidene moieties is obtained by a process called alkene metathesis. With the development of new catalysts in recent years, alkene metathesis has become a useful synthetic method. Special synthetic applications are, for example, ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROM) (see below). 

The reversal of ring-closing metathesis, namely ring-opening metathesis, is also a synthetically useful reaction. With strained (small-ring) cycloalkenes, e.g. 12, the equilibrium of the reaction lies on the side of the open-chain product 13 ... 

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