Eight-membered ring systems

As depicted in Eqs. 6.5—6.7, kinetic resolution of a variety of cyclic allylic ethers is effected by asymmetric Zr-catalyzed carbomagnesation. Importantly, besides six-membered ethers, seven- and eight-membered ring systems can readily be resolved by the Zr-catalyzed protocol. 

Cycloadditions of bisdienes. This reaction, catalyzed efficiently by Ni(COD)2-P(C6Hs)3 (usually 1 2), provides a useful route to the eight-membered ring system found in taxane diterpenes. 

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

In connection with the behavior of the eight-membered ring system, it is interesting to mention that the uncatalyzed thermolysis of the open-chain tetraene ether 230 in toluene at 150 °C (11 h) gives rise to a mixture of four intramolecular Diels-Alder products 231-234 in 80% total yield (equation 74)88. The thermolysis of dimethyl homologue 235 (toluene, 150 °C, 11 h, 81%) affords the c/s-fused cyclohexene derivative 236 and... 

Gilbertson and co-workers were also able to facilitate the rhodium-catalyzed [4-i-2-1-2] carbocyclization reaction with a substrate having an all-carbon tether (Eq. 13). This methodology has been extended to the asymmetric rhodium-catalyzed [4-t 2-1-2] reaction (Eq. 14). Although the exact origin of asymmetric induction was not discussed, the ability to accomplish the asymmetric rhodium-catalyzed [4-i-2-1-2] reaction provides a novel approach to eight-membered ring systems. 

Synthesis of Methyl-Substituted 1,5-Cyclooctadienes We will first consider the formation of substituted eight-membered ring systems using the dimerization catalyst (nickel-ligand). Two routes are available either two molecules of substituted butadiene may dimerize... 

The diversity-oriented synthesis of enantiomerically pure seven- and eight-membered ring systems was reported from easily accessible naturally occurring. Y-amino acids and their readily prepared derivatives as chiral synthons <2007JC0321>. Intramolecular Mitsunobu reaction was used as a key transformation to construct the 1,4-diazocines 224 (Scheme 27) and 1,4-oxazocines 225 (Scheme 28) <2007JC0321>. 

Four main low-energy conformational families of eight-membered ring systems, consisting of the boat-chair, crown, boat, and chair forms, were surveyed in CHEC-II(1996). They are discussed in the foregoing sections of this chapter in conjunction with the theoretical and experimental structural studies of triheterocines (see sections 14.08.2 and 14.08.3). 

Cyclization of silylated vinylcyclopropanol 42 upon treatment of la at -78 °C afforded bicyclo[4.4.0]decane 43 (Sch. 33) [60]. This cyclization protocol provides a new route to six-, seven- and eight-membered ring systems attached to a spirobuta-none. 

An intramolecular version of this acylation of homoallylsilanes leads to 2-cyclopropylcycloal-kanones 10. This reaction gives five-, six-, seven- and even eight-membered ring systems in a highly regiocontrolled manner. In all these cases, the 2-cyclopropyl ketones 7 were isolated as the sole product of intramolecular acylation. . 2-Cyclopropylcyclobutanones were not ob-... 

As depicted in Eqs (3)-(5), kinetic resolution of a variety of cyclic allylic ethers is effected by asymmetric Zr-catalyzed carbomagnesation. Importantly, in addition to six-membered ethers, seven- and eight-membered ring systems can be readily resolved by the Zr-catalyzed protocol. It is worthy of note that the powerful Ti-catalyzed asymmetric epoxidation procedure of Sharpless [15] is often used in the preparation of optically pure acyclic allylic alcohols through the catalytic kinetic resolution of easily accessible racemic mixtures [16]. When the catalytic epoxidation is applied to cyclic allylic substrates, reaction rates are retarded and lower levels of enantioselectivity are observed. Ru-catalyzed asymmetric hydrogenation has been employed by Noyori to effect resolution of five-... 

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