Ring-Closing Olefin Metathesis RCM

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

Another synthetic application of olefin metathesis using a thioacetal-titanocene(II) system is the ring-closing olefin metathesis (RCM) of carbene complexes possessing an olefin moiety, e. g. 33 (Scheme 14.17). The success of the RCM apparently depends on the substituents at the carbon—carbon double bond (i. e. the substituent(s) on the resulting carbene complex 34). 

These reactions involve metallate rearrangements , migratory insertion and transition metal-catalysed vinylic substitution reactions. They also perform well in applications in natural product synthesis . Many useful synthetic possibilities arise from application of ring-closing olefin metathesis (RCM) to unsaturated homoaldol products and their derivatives by means of the Grubbs catalyst 3942 4-286 Equation 105 presents some examples. ... 

For the cyclization to the 16-membered macrolactone structure of epothilones C and D (= desoxyepothilones A and B, resp. [26]), three different strategies have been used successfully so far (1) Ring-closing olefin metathesis (RCM) between C12 and Cl3. RCM is a comparably new method in total synthesis and underwent enor-... 

The formation of the macrocycle (O Scheme 26), which had been a serious problem, was achieved hy ring-closing olefin metathesis (RCM). RCM of orthoester 136 was performed... 

Olefin metathesis has emerged as a powerful tool for the preparation of cyclic organic compounds. Metathesis involves the redistribution of carbon-carbon double bonds in the presence of metal carbene complexes ([M]=CR2). The reaction of these metal carbenes with a,co-dienes leads to well-defined carbocyclic systems in what is termed ring-closing olefin metathesis (RCM). ... 

The chapter ends with a rather miscellaneous collection of reactions such as one example of an electrocyclic ring closure 19. There are more important examples of ring closing olefin metathesis (RCM) and other organometallic reactions such as the Pauson-Khand reaction and co-trimerisation. 

In recent years, ring-closing olefin metathesis (RCM) has attracted organic chemists as a versatile cyclization method via carbon-carbon bond formation, and had a great impact on the synthesis of natural products [29,30]. In the syntheses of dihydropyrans of marine macrolides, allyl homoallyl ethers 28 have been cyclized to dihydropyrans 29 via RCM. This methodology has been seen in the syntheses of laulimalide by Mulzer [31,32], Crim-mins [33], and Nelson [34] (Scheme 9). 

Metathesis is the most important and useful reaction in recent synthetic organic chemistry [1]. In ring-closing olefin metathesis (RCM), formally, bond fission of two double bonds occurs and the new double bond is formed at the same time to produce a cyclic compound (Equation 6.1). 

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