Ring-closing metathesis reactions Subject

Representatives of the bridged sulfone system 70 have been subjected to ruthenium catalysed ring-closing metathesis reactions (Grubbs catalyst) and shown to afford, in low yields, a few selected cyclic dimers and trimers, of all the possibilities available. The diastereoselectivities observed were rationalised in terms of kinetic control involved with internal ruthenium/sulfonyl oxygen coordination . 

Deslongchamps and co-workers approach to the synthesis of Kempane diterpenes employed a ring-closing metathesis strategy in the construction of the 13-membered ring carbocycle, which was subsequently subjected to a transannular Diels-Alder reaction to produce the tricyclic... 

The transition-metal-catalyzed olefin metathesis is a still recent, but already well-established, development, which has evolved toward a highly useful synthetic tool [27]. The ring-closing variant of this reaction ]ring-closing metathesis (RCM)] has proved to be very suitable for the preparation of carbo- and heterocycles of any ring size, except for those that are strained ]28, 29]. In the case of 5,6-dihydropyran-2-ones, RCM of homoaUyl acrylates has very often been used for the direct creation of this heterocyclic system, most particularly in the past decade (Figure 2.4). Allyl esters of fi,y-unsaturated acids may also be subjected to RCM with the formation of P,y-unsaturated 8-lactones, which can be easily isomerized by acid or base catalysis to their a,fi-unsaturated counterparts ]6, 30-39]. 

Grigg et al. reported a successful four-component domino reaction for the synthesis of functionalized dienes 316 from aryl iodides, allyl amine derivative, allene, and carbon monoxide [110] (Scheme 6.83). Carbon monoxide could insert into the C—Pd bond of arylpalladium(II) iodides to generate a carbonylpalladium species, which is followed by allenylation to form n-allylpalladium species. Finally, the attack of the nitrogen nucleophile produces the product observed. The products of this domino multicomponent reaction could be subjected efficiently to ring-closing metathesis in the presence of Grubbs second-generation catalyst. 

The marine natural product ascidiatrienolide A 220 is a strong inhibitor of phospholipase A2. Compound 220 and the closely related didemnilactones 221-223 feature a common hydroxy-substituted anti to the ring oxygen) (Z)-nonenolide core. Lactone 219, which constituted the key intermediate in a previous total synthesis of 220 and can also be elaborated to lactones 221-223, has been the subject of an interesting study by Flirstner s group, that revealed once more the very subtle and cooperative influence of different parameters on the stereochemical course of metathesis reactions. Thus, it was shown that the ElZ ratio obtained in an RCM step is not only dependent on the relative configuration of the cyclization substrate, but also on the chosen catalyst (Scheme 41). When applied to the... 

Glycals can also be obtained from suitable substrates, by ring-closing olefin metathesis reactions. In a general approach to variously linked C-disaccharides, illustrated in O Scheme 48, nonreducing-end glycals are typical intermediates, finally subjected to hydroboration or dihy-droxylation to afford the desired C-analogs of O-disaccharides [238,239]. 

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