Hexamethyl-2,4-cyclohexadienone

Hexamethyl-2,4-cyclohexadienone in ether, through which methylamine has been bubbled 45 min. at room temp., irradiated 3.5 hrs. with the Pyrex-filtered light of a 450 w. Hanovia lamp dienamide (Y 89%) in glacial acetic acid flushed with Ng, satd. with HCl, isoamyl nitrite added dropwise, and heated 4.5 hrs. at 100° -> permethyl-2-pyrone imine (Y 67%) heated 4 hrs. at 160-180° with 5 3 aq. 20%-NaOH-methanol permethyl-2-pyridone (Y 74%). H. Hart, D. A. Dickinson, and W. Y. Li, Tetrah. Let. 1975, 2253 ring opening s. a. G. Quinkert et al., B. 109, 1332 (1976). 

A soln. of peroxytrifluoroacetic acid (from 90%-H2O2 and trifluoroacetic anhydride) in methylene chloride added dropwise at 0° during 45 min. to a soln. of hexamethylbenzene in the same solvent with simultaneous introduction of BFg, stirring continued 35 min. 2,3,4,5,6,6-hexamethyl-2,4-cyclohexadienone. Y 93%. A. J. Waring and H. Hart, Am. Soc. 86, 1454 (1964). 

Heavily substituted o-dienones, such as 2,3,4,5,6,6-hexamethyl and 2,3,4,6,6-pentamethyl cyclohexadienone, upon irradiation give not the ketene but a substituted bicyclic compound of the following type (62)<45-54) ... 

In the monocyclic series, the 2,4-cyclohexadienones rearrange to photoproducts of potential synthetic value. However, for efficient 1,2-acyl migration, only a relatively small number of substrates are suitable. These must be highly substituted, for example (48). Whereas on direct excitation in methanol, cleavage to the isomeric ketenes (49) (4> 2 0.42) predominates, the remarkably stereoselective 1,2-acyl shift to the bicyclohexenone (50) is found either in trifluoroethanol or when the dienone is adsorbed on silica gel. The conversion to (50) is followed by a reversible phototransformation to the cross-conjugated dienone (51) and accompanied by aromatization to (52) to a minor extent. Such reactivity has also been verified for tetra- and penta-methylated 2,4-cyclohexadienones. The only photoreaction of the hexamethylated homolog, on the other hand, is ketene formation. ° ... 

The photolysis of hexafluorobenzene in the presence of oxygen yields the corresponding Dewar benzene and its epoxide the latter may act as a dienophile and thermally rearrange to a perfluoro-l,3-cyclohexadienone (67)66). This reaction is in marked contrast to the reaction of hexamethyl Dewar benzene (see Eq. 56). 

Hexahydrobenzophenanthrene, 402, 403 Hexahydrotetrazine, 399 Hexamethylbenzene, 125, 168 HexamethyI-2,4-cyclohexadienone, 168 Hexamethyl Dewar benzene, 23, 90 Hexamethyldisilazane, 159, 207-208, 370 Hexamethylenetetramine, 208... 

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