Cyclooctane ring

H and C NMR spectra were applied to elucidate the electronic structure of the dianions in the dilithium arene compounds formed with anthracene, phenanthrene and 1,2,3,4-dibenzocyclooctatetraene (115). Although the three dianions have 16- 7r isoelec-tronic systems, the former two differ due to their charge distributiou, and 115 due to the steric strain in the cyclooctane ring, interfering with a coplanar configuration... 

A cyclooctane ring ( benzocaine ) has also been obtained in moderate yield by this... 

Cyclopentadienylcobalt(l) complexes react with 1,2,3-selenadiazoles 16 annulated with unsaturated cyclooctane rings to afford fluxional complexes 23 <1987JOMC39, 19890M800>, as illustrated in Equation (3). Reaction of these... 

A crown-family conformation also cannot explain the nmr spectrum of the acetonide of five-membered ring fused to the cyclooctane ring. 46) A temperature-independent spectrum would be predicted, because a im s-fused five-membered ring can only be located at equatorial positions in crown-family conformations, and ring inversion is therefore prohibited. The nmr spectrum of this compound is actually strongly temperature-dependent at about — 70°C, thus excluding any conformation in the crown family, at least as the sole conformation. 

The least known conformational family is the boat-boat. It seems that a very special substitution pattern is required to make conformations in this family more stable than those of the crown and boat-chair. Highly fluorinat-ed cyclooctanes possibly exist in the twist-boat or boat-boat conformations however, additional evidence is badly needed to support the conformational assignments which have been made. A boat-boat conformation (in the cyclooctane nomenclature) does occur in [3.3.1]bicyclononane, which contains a 1,5-bridged cyclooctane ring. 3)... 

The nickel-mediated [4 -i- 4] cycloaddition strategy has also provided a concise and stereocontroUed route into the sequiterpene lactone (+)-asteiiscanolide (128). The basic features of this approach are outlined in Scheme 17. The critical [4 + 4] cycloaddition st occurred under standard conditions to give the key intermediate in 67% yield. Clearly, the intramolecular version of the nickel-catalyzed diene cy-clodimerization has been established as a powerful and highly-selective protocol for the synthesis of cyclooctane ring systems and should find extensive qjplication to natural product synthesis. 

Another common strategy for construction of fused cyclooctanones is to first build a fused cyclobutanone by [2 + 2] cycloaddition of a vinylketene to a cycloalkene. Equation (58) illustrates this approach with Paquette s synthesis of the tricyclic skeleton of the ophiobolins. Cyclobutanone (108) is assembled by addition of a vinylketene to cyclopentadiene. Cyclopentenyllithium then adds to the less-hindered face of (108), and the lithium alkoxide undergoes a spontaneous anionic oxy-Cope rearrangement to afford the central cyclooctane ring. 

The stereochemical outcome of the reaction is rationalized by a twist-boat conformation of the cyclooctane ring in the transition state. 

Three methods have been developed to fuse a cyclooctane ring onto a pre-existing cyclic system. These annulations have again been organized according to our conventions for describing intramolecular allylsilane additions to conjugated dienones. 

In the structure of 1,3,5,7-tetramethyIenecycIooctane, ideally the effect of endo-spiroconjugation should manifest itself in the geometry of the skeletal cyclooctane ring system the fourtrigonal atoms in the ring should... 

---