Metathesis ring closure/opening

Wipf et al. <2005OL4483> elaborated a new metathesis method including ring opening in order to generate dynamic combinatorial libraries. The transformation is shown in Scheme 12. The essence of this method is the recognition that an equilibrium takes place with 93 and an aldehyde in aqueous conditions at pH 4 in a phosphate buffer. With the help of this transformation, a series of new R2-substituted products 94 - not available via direct ring closure - have been synthesized. 

These complexes are suitable for the ring-opening metathesis polymerization of functionalized norbomenes in water,148 as well as ring-closure reactions via ethylene elimination.149 The versatility of these catalysts has given rise to extensive applications in organic synthesis,150151 including, for example, amino acid derivatives.152... 

A family of phosphine-free ruthenium-based olefin metathesis catalysts has been developed over the last few years. First, work done independently by Hoveyda and Blechert resulted in the H2Mes isopropoxybenzylidene (4b), a highly active air-stable ruthenium (pre)catalyst for olefin metathesis (Scheme 4). Hoveyda described (4b) as a recyclable monomeric catalyst also with high activity for ring-opening, ring-closure, and cross metathesis that tolerates... 

While the mechanishc principles were settled some time ago [4], structurally defined, tailored catalysts came along only recently, especially for functionalized olefins. Olefin metathesis, previously a most parameter-sensitive reaction, is no longer restricted to simple olefins, and it am now also be conducted in polar solvents. Ring-opening metathesis (ROMP), aciycHc a,< -diene metathesis (ADMET), ring-closure metathesis (RCM), and just simple metathesis are the main applications. 

Three novel stereo- and regioselective schemes for the total synthesis of (+ )-brefeldin A 440 have been accomplished. Each of them exploit intermolec-ular nitrile oxide cycloaddition for constructing the open chain and introducing substituents, but differ in subsequent stages. The first (480) and the second (481) use intramolecular cycloaddition for the macrocycle closure. However, in the second scheme INOC is followed by C=C bond cis-trans-isomerization. In the third scheme (481) intermolecular cycloaddition is followed by ring closing metathesis as the key step. 

Another useful variation is to effect the opening of one ring and the closure of another in the same operation. An example is shown in eqn. (19). The net result is the opening of the 7-membered ring and the closure of the less strained 6-membered ring. The function of the ethene is to prevent the occurrence of an intermolecular metathesis reaction leading to the formation of dimer [78]. Many other examples will be found in ref. [79]. 

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