Reaction Eight-membered rings

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]. 

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 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... 

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). 

Besides simple condensation reactions, valence isomerization reactions, in particular, arc used for the synthesis of unsaturated, eight-membered-ring azaheterocycles. These isomeriz-ations mainly involve rearrangements of nitrogen-containing bicyclo[4.2.0]octatriene or semi-bullvalene systems. 

The reaction of several substituted imidazo[4,5-c/]-, pyrazolo[3,4-r/]- and triazolo[4,5-zf]pyrid-azines 3 with ynamines, in competition with [4 + 2] cycloaddition, leads to [2 + 2] derivatives 4, which rearrange to l,2-diazocines5.7 8 The reaction seems to be sensitive to the substituents, as replacement of the electron-withdrawing group R on the pyridazine ring of the pyrazolo compound (A = N, B = CH) by chlorine completely inhibits both the [4 + 2] and [2 + 2] cycloaddition reactions. The X-ray structure of the imidazo derivative 5 (R = Ms, A = CH, B = N) reveals a tub conformation of the eight-membered ring. 

An important method for the synthesis of 1,3-diazocines consists of the reaction of pyrimidines 1 with ynamines to give [2 + 2] cycloadducts, which rearrange to the eight-membered-ring compounds 2 by valence tautomerization.2-3... 

In the case of 1,2-dibenzoylbenzene systems 3, however, it is possible to obtain the 1,4-diazocine derivatives 4 on reaction with benzene-1,2-diamines.13 - 18 The eight-membered ring exists in a tub conformation, which exhibits such a high inversion barrier that in the case of unsym-metrically substituted systems it is possible to isolate the corresponding enantiomers. 

TABLE 6.1 Relative Rate Constants at 50°C (Eight-Membered Ring = 1) for the Reaction... 

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. 

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