Eight-membered rings

Eight-membered CNS heterocycles of the type Ar2C2N4S2Ph2 (104) are generated by the cyclocondensation of trisilylated benzamidines (4-XC6H4)CN2(SiMe)3 (X=Br, CF3) with PhSCl in a 1 3 molar ratio at very low temperatures. The 16-membered ring (4-BrC6H4)4C4N8S4Ph4 (105) is obtained as a minor product. 

A wide range of derivatives of the bicylic ring systems RCN5S3 (108, R=alkyl, aryl, CF3, NR 2, Q) have been prepared and structurally characterized. Two synthetic routes are available (a) reaction of S,5 -dichlorodithiatriazines with (Me3SiN)2S (Equation 4) and (b) reactions of trisilylated benzamidines with (NSC1)3. 

The molecular structure of the bicyclic framework in 108 is best described as a dithiatriazine bridged by an -N=S=N- group. The aryl-substituted derivatives 108 (R=aryl) form 2 1 inclusion complexes with aromatic fluorocarbons or hydrocarbons. In solution, the phenyl derivative 108 (R=Ph) is fluxional.  

Eight-membered Rings.—The eight-membered ring in the cyclo-octatetraenide salt 

Eight-membered Rings.—Using variable temperature n.m.r., c ,cw-cyclo-octa-1,3-diene was shown to exist as a nearly equal mixture of the twist-boat (11) and the twist-boat-chair (12) conformations. The degenerate interconversion of the twist-boat 

Force-field calculations indicate that twist-boat conformations are the most stable conformations of cis,cts-cyclo-octa-l,S-diene and its dibenzo-derivative, e.gf. (16) for the diene itself. The lower energy (AG = 4.4 kcal mol ) and higher energy (AG = 4.9 kcal mol ) equilibration processes detected by n.m.r. for cis,cis-cyclo-octa-l,S-diene were assigned to the equilibration of twist-boat conformations via a boat conformation and to the interconversion of boat conformations by ring inversion via a chair conformation, respectively. For the dibenzo-compound the boat conformations were shown to interconvert by pseudorotation via the twist conformation, calculated AG = 7.0 kcal mol The n.m.r. behaviour of the deuteriated dibenzo-cyclo-octa-1,5-diene (17) was interpreted in terms of chair and twist-boat conformations.  

spectra of cis,cis,cis-cyclo-octa-l,3,5-triene at low temperatures is consistent with a process that interconverts a chiral twist-boat conformer with its 

Tokuma, S. Koda, S. Tsubouchi, and Y. Moiimoto, Bull. Chem. Soc. Japan, 1975,48,294. 

E ht-membered Rings.—The geometric constraints in eight-membered rings are discussed.  

Low-temperature n.m.r. studies show that cis- and trans-l,5-diacetoxy- 

The shifts induced by Ag in the spectra of cyclo-octene, cyclo-octadienes, 

Ring-inversion barriers in (8 X = pyrrolidine or OH) (AG = 70.2 and 81.5 kJ mol, respectively), as determined by n.m.r. spectroscopy, are considerably greater than those of other eight-membered rings. Barriers to ring inversion in benzocyclo-octatetraene and a benzocyclo-octatriene have been determined. Theoretical and experimental studies of ring inversion in tetrabenzocyclo-octatetraenes are also reported. (+)-(9) shows no change in 

2 Cyclic Sulfur Liiides 6.2.1 Eight-membered rings 

The formal replacement of a sulfur atom in cyclo-S generates S7NH (6.1), the first member of a series of cyclic sulfur imides that includes the three diimides 1,3-, 1,4-, and 1,5-S6(NH)2 (6.2-6.4), two triimides 1,3,5-and 1,3,6-S5(NH)3, and S4N4H4 (6.5). Isomers of these ring systems containing adjacent NH groups are not known. 

The cyclic sulfur imides readily undergo condensation reactions in the presence of a base, e.g., pyridine. For example, the reaction of S7NH with sulfur halides, S Cla or SOCI2, produces the series (S7N)2Sx (x = 1, 2, 3, 5), or (S7N)2S0, respectively. The bicyclic compound S11N2 (6.6) is obtained by treatment of 1,3-S6(NH)2 with The reaction 

Cyclo-octane-1,5-dione dioxime has been shown by an A -ray structure determination to have the mirror-symmetric boat-chair conformation (18).  

Aryl derivatives of 101 (R=Ar) are planar, and the bond lengths indicate a fully delocalized 10 re-electron aromatic system. By contrast, the (dimethyl-amino) derivative 102 adopts a folded structure. Dithiatetrazocines with exo-cyclic N(Me)Bu groups exhibit both cisltrans and ring inversion isomerism.257 

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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). 

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 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... 

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. ... 

The 1,5-isomers 13.3 (E = S) are colourless, air-stable solids. They are prepared by the cyclocondensation reaction of R2PN2(SiMe3)3 with sulfur dichloride or thionyl chloride. A similar cyclocondensation process, using a mixture of SeCU and Sc2Cl2 as a source of selenium, produces a mixture of the isomers 13.2 and 13.3 (E = Se, R = Ph). The structures of 13.3 (E = S, R = alkyl, aryl) are folded eight-membered rings with a cross-ring S S distance of ca. 2.50 This structural... 

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