Eight-membered ring formation

A good yield of 492 was obtained by reaction of 369 with dithiol 491 (Eq. 47). This reaction is probably the only example of the eight-membered ring formation by denitrocyclization reaction described so far (91ZOR185). 

N—Li bond energies could perhaps be similar for both forms. But repulsions also are important. Eight-membered ring formation allows Li6 (and N6-) centers to be widely spaced. In contrast, Li—Li distances in the stack are much shorter, both within each ring (now only four-membered) and, especially, between rings (Table XI). 

I) Five-membered ring formation, (2) Six-membered ring formation, (3) Seven- and eight-membered ring formation, and finally (4) Tandem cyclization. 

We have already discussed the effect of the ligand on the selectivity of eight-membered ring formation (Section IV,B). Table XI illustrates that this is also strongly dependent on the number of methyl substituents. 

Eight-membered Ring Formation Using Sml2... 

Matsuda T, Fujimoto A, Ishibashi M, Murakami M (2004) Eight-membered ring formation via olefin insertion into a carbon-carbon single bond. Chem Lett 33(7) 876-877... 

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]. 

The precise nature of the adhesion of the polyelectrolyte cements to untreated dental enamel and dentine has yet to be established. The earliest theory was due to Smith (1968) who speculated that the polyacrylate chains of the cement formed a chelate with calcium ions contained in the hydroxyapatite-like mineral in enamel and dentine. Beech (1973) considered this unhkely since it involved the formation of an eight-membered ring. Beech studied the interaction between PAA and hydroxyapatite, identified the formation of polyacrylate and so considered that adsorption was due to ionic attraction. 

Reaction of Hg(02CCF3)2 or Hg(03SCF3)2 with a series of dibenzylcarbinols gave exo cyclization for formation of five-, six-, and seven-, but not eight-membered rings.109... 

The reaction in Entry 8 was conducted in excess refluxing vinyl f-butyl ether, using 1.1 equivalent of Hg(OAc)2 to catalyze the exchange reaction. In Entry 9 a thermal reaction leads to formation of an eight-membered ring. 

This process allowed, for example, formation of the angulary fused tricycle 1-13 containing a five-, six-, and eight-membered ring from precursor 1-12 in 64% yield (Scheme 1.4) [Id]. 

Besides the formation of carbenes from diazo compounds and the hydroformyla-tion, rhodium (as described previously for palladium) has also been used as catalyst in domino processes involving cycloadditions. Thus, Evans and coworkers developed a new Rh(I)-catalyzed [4+2+2] cycloaddition for the synthesis of eight-membered rings as 6/2-105 using a lithium salt of N-tosylpropargylamines as 6/2-104, allyl carbonates and 1,3-butadiene (Scheme 6/2.22) [221]. The first step is an al-... 

Scheme 6.74 Formation of seven- and eight-membered rings by ring-closing metathesis reactions.

The unusually facile formation of a disulfonium dication from sulfide 10 is the result of stereochemical features of the eight-membered ring, which favor the formation of a transannular bond.31 According to X-ray data (see in Chapter 7.1 Table 1), the distance between the two sulfur atoms in 1,5-dithiacyclooctane 10 is smaller than the sum of their van der Waals radii (3.75 A), which results in a strong non-bonded interaction between the atoms confirmed by photoelectron spectroscopy and mass spectrometry.32 33 This interaction and the sulfur-sulfur distance can be decreased as a result of bond formation with an electronegative substituent as in sulfoxide 13 or sulfoximine 14.34,35... 

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