Tridehydro 18 annulene

The electronic spectrum of the fractions containing the pure tridehydro [18]annulene exhibits the strongest absorption maximum (in benzene) at 342 nm. (e 155,000) and the spectroscopic yield, based on the molar extinction coefficient, is 1.17 g. (2.40% from 1,5-hexadiyne). The yield of tridehydro[18]annulene in the mixed fractions, based on the 342 nm. maximum,is 0.27g.(0.55%). The tridehydro [18]annulene is best stored in solution in the refrigerator. 

Tridehydro[18]annulene here and in the sequel refers to the symmetrical isomer shown in the formula, admixed with smaller quantities of an unsymmetrical isomer and tetradehydro[18]annulene.6 These can be separated by chromatography on alumina coated with 20% silver nitrate, but this is unnecessary for the synthesis of [18]annulene sinceallthree substances give this annulene on catalytic hydrogenation.8... 

For the full electronic spectra of tridehydro[18]annulene and tetradehydro[24]annulene in isooctane, see Reference 3. 

The submitters reported in their original procedure that some additional amounts of [18]annulene and tridehydro[18]annulene were obtained by chromatography of the mother liquors. 

The method of synthesis is essentially that described by Sondheimer and Wolovsky3 (preparation of tridehydro[18]annulene) and by Sondheimer, Wolovsky, and Amiel2 (hydrogenation of tridehydro[18]-annulene to [18]annulene). It has been simplified, since [18]annulene is... 

Hydrogenation of tridehydro[18]annulene (3) to [18]annulene (4) is carried out with a suspension of palladium catalyst in benzene in a hydrogenation apparatus. The yield of [18]annulene is 0.63% overall from 1,5-hexadiyne.1... 

Wolovsky describes the oxidative coupling of 1,5-hexadiyne, followed by prototropic rearrangement by heating with potassium /-butoxide in f-butanol-benzene at 100°. The resulting mixture of unsaturated hydrocarbons was easily separated by elution chromatography on a column of alumina impregnated with 20% silver nitrate into the tetradehydro[18]annulene (111) and the two isomeric tridehydro[18]annulenes (1) and (11). 

The possibility of varying the sequence of a given number of trans and cis double bonds around a ring (within limits set by the positions of the acetylene bonds) has been referred to for 1,3,7,13-tetradehydro-[18]annulene, 16. In the case of l,7,13-tridehydro-[18]annulene 110> this phenomenon has lead to the isolation of two distinct isomers 107> of this compound, isomer I, 18, and isomer II, 19 (see Table 4). 

Among the higher annulenes, a mono-charged analogue has been reported in one case, that of the seventeen-carbon macrocycle 63 (18 tu 17 C) (see Table 13) 205). This anion is a counterpart of the tridehydro-[18]annulenes, and contains 18 out-of-plane 7t-electrons. Like the neutral higher annulenes, it possesses both inner and outer protons, the reson-... 

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