Chromanones dehydrogenation

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

The direct conversion of a chromanone to a chromone by dehydrogenation has been achieved using palladium on charcoal <74CR(C)(279)l5l> and with selenium dioxide (65JCS2743), but both reagents appear to be somewhat fickle. 

The most representative systems of the [4,3-b]indolo series (75 and 79-84) are prepared by dehydrogenation of their dihydro products (Scheme 10), which can be obtained from the corresponding chromanones, thiochroma-nones or seleninochromanones, and aryl hydrazines by Fischer s indol... 

Flavanones undergo a variety of transformations by HTI or other hypervalent iodine reagents. In methanol, at room temperature, they were dehydrogenated to the corresponding flavones [40], A similar dehydrogenation occurred in chromanones [41]. Dramatic solvent effects occurred when methanol was replaced by other solvents. Thus, in trimethyl orthoformate ring-contraction was the main pathway and methyl 2-aryl-2,3-dihydrobenzofuran-3-carboxylates the major products, accompanied by substantial amounts of 3-methoxyflavones [42] ... 

Thallium trinitrate has been shown to be an efficient reagent for the dehydrogenation of chroma-nones and flavanones (Table 10, entry 5). the reaction being carried out in methanol at room temperature. The addition of perchloric acid to chromanone oxidations enhances the yields and reaction rates by promotion of enolization, but qiparently was without effect on flavanones. The ease with which chimanones and similar compound may be dehydrogenated has also permitted unusual procedures such as trimethylsilyl chloride/acetic anhydride and photolysis to be used (Table 10, entries 6 and... 

Chromenes.- Continuing interest in the precocenes and their biological action has stimulated further work on their synthesis. Precocene III (82) has been synthesized from a phenol, 3-methylbut-2-enal and titanium(IV) ethoxide93 and precocenes I and II were obtained by cyclization of the relevant 3-methoxyphenol with isoprene and dehydrogenating the resultant chroman with DDQ.94 Several new positional isomers of precocenes and related compounds have been synthesized and screened for insecticidal activity in a rice crop. Chromanones [such as (83)] were reduced with LAH to the chromanol which was efficiently dehydrated with a trace of toluene-4-sulphonic acid. Under some conditions, reduction gave dimeric... 

The total synthesis of the natural chromone pestalotiopsone A includes a microwave-promoted aldol condensation and oxa-Michael cyclization to construct the chromanone core, followed by 2-iodoxybenzoic acid (IBX)-induced dehydrogenation to afford the desired chromone. The synthesis of pestalotiopsones B, C, and F is also accomplished using similar approaches (13OBC1109). 1,3,5-Trimethoxybenzene is the starting material for the... 

Pd-catalyzed oxidative cross-coupling of arenes with chromanones was reported by the Hong group [36]. These reactions proceed through in situ formation of the chromones via Pd-catalyzed dehydrogenation of the starting chromanones (Scheme 24.40, step i). Subsequent coupling of the aUcene moiety in the chromones with the arenes affords the final products. 

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