Eight-membered

Two moles of diphenylacetylene insert into the benzyl methyl sulfide complex 481 to afford the eight-membered heterocycle 482[440j. The cinnolinium Salt 483 is prepared by the insertion of alkynes into the azobenzene com-plex[44l]. 

Synthesis of camptothecin (163) is another example[133]. The iboga alkaloid analog 164 has been synthesized smoothly by the intramolecular coupling of iodoindole and unsaturated ester to form an eight-membered ring. Af-Methyl protection of the indole is important for a smooth reaction[134]. An efficient construction of the multifunctionalized skeleton 165 of congeners of FR900482 has been achieved[135]. 

No 0-allylation is observed in formation of the six-membered ring compound 79 by intramolecular allylation of the /3-keto ester 78(15,57]. Intramolecular allylation is useful for lactone fonnation. On the other hand, exclusive formation of the eight-membered ring lactone 81 from 80 may be in part derived from the preference for the nucleophile to attack the less substituted terminus of the allyl system[58]. 

Eight-membered rings may be formed thermally or photochemically (see PhotochemicalTOCHNOLOGy). Excellent yields can be obtained with ferric acetylacetonate—triethylaluminum—dipyridyl (43). 

There are also apertures with eight-membered oxygen rings in this zeoHte. 

The TIPDS derivative can be induced to isomerize from the thermodynamically less stable eight-membered ring to the more stable seven-membered ring derivative. The isomerization occurs only in DMF. 

Examine the following thermochemical data pertaining to hydrogenation of unsaturated eight-membered ring hydrocarbons to give cyclooctane ... 

The huge difReretice in rate that results fhun the alternative placement of oxygen in the eight-membered rmgs reflects the relative stability of the various oxonium ions that result fiom pairicipariaQ. The ion 16 is much mote favorable than 14 or 15. 

The crystal structure of the potassium salt of 1,3,5,7-tetramethylcyclootatetraene dianion has been determined by X-ray dififaction. ° The eight-membered ring is planar, with aromatic C—C bond lengths of about 1.41 A without significant alternation. The spectroscopic and structural studies lead to the conclusion that the cyclooctatetraene dianion is a stabilized delocalized structure. 

In the case of cycloheptenone and larger rings, the main initial photoproducts are the -cycloalkenones produced by photoisomerization. In the case of the seven- and eight-membered rings, the double bonds are sufficiently strained that rapid reactions follow. In nonnucleophilic solvents dimerization occurs, whereas in nucleophilic solvents addition occurs. ... 

An elegant application of Se NMR spectroscopy, in conjunction with N NMR spectroscopy, involves the detection of the thermally unstable eight-membered rings (RSeN)4 (3.13) from the reactions of a mixture of seleninic anhydrides (RSeO)20 (R=Ph, Pr) and N-enriched... 

LUMO (tc ) transition. The 1,3- and 1,5- isomers of the eight-membered R2P(NSN)2PR2 (R = Me, Ph) rings, 3.27 and 3.28, respectively, exhibit characteristically different colours. The 1,3-isomer 3.27 is dark orange with an absorption maximum at ca. 460 nm, whereas the 1,5 isomer has a very pale yellow colour. In the absence of a... 

The [S4N4] cation 5.18 is a planar eight-membered ring with equal S-N bond lengths ( >4 ) of ca. 1.55 A. It is a fully delocalized ten r-electron system with a strong r-network. ... 

Cyclic Sulfur Liiides 6.2.1 Eight-membered rings... 

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