Shifts chromonics

Monosubstituted 3-bromochroman-4-ones are converted into chromones on photolysis. For certain 2,2-disubstituted derivatives, dehydrohalogenation is accompanied by a 1,2-shift of a 2-substituent to afford a 2,3-disubstituled chromone <96H(43)339>. 

Pyran-4-one (56a) and its benzo derivative (chromone) show chemical properties in agreement with substantial jr-electron delocalization and consistent with a betaine structure 56b (Scheme 27). Experimental data have therefore generated numerous theoretical studies on the aromaticity of pyranones, which have been extensively reviewed.219 Earlier studies suggested that chemical shifts and coupling constants... 

I, 2-alkyl shift provides a convenient route for the synthesis of chromones 335 (Eq. 46), tetrahydroxanthones (336, n = 1) and higher homologs (336, n = 2) (Eq. 47). Both heat and ultrasonic conditions work well to effect this alkyl shift (94SC2637). 

A methyl group is commonly encountered at C-2 in chromones. Its effect is to shift the signal from H-3 upheld (67IJC93). 

Chromones with hydroxy or methoxy groups in the aromatic ring show high intensity bands at 240-260 and 290-360 nm. Both bands are red-shifted relative to chromone and in sodium methoxide solution these shifts are enhanced as a result of ionization of the phenolic groups (76JHC211). 

Calculated 7r- electron densities and orbital energies, obtained for chromone using a semi-empirical PPP method, were shown to correlate well with H NMR chemical shifts and polarographic half-wave potentials (78lJC(A)53l). 

The conversion of 2-methyl-3-(2-methylbenzoyl)chromone into the benzoxanthone has been achieved photochemically in high yield (75JCS(Pl)l845). Initial photoenolization is followed by a [l,7]-sigmatropic shift and cyclization. 

Several reports describing the 170 NMR of pyrans and derivatives have appeared, including a study in which the natural abundance 170 NMR data for lactones such as pyranone were collected and the relationships between 170 chemical shifts and structure were discussed of <1989H(29)301>. It is possible to distinguish between polyfunctionalized coumarins and chromones by 170 NMR <1993CPB211>. 

Table 5 Influence of replacing the O atoms in a chromone with S on 1H NMR chemical shifts <5 J (Hz)...

The selective replacement of O by S in the series chromone —> thiochromone thione results in a downfield shift in the signal for the CF3 group (Figure 47). Coupling between H-3 and the fluorine atoms is only evident for the thiochromone system <2006RCB523>. 

Electron densities and SCF orbital energies for chromone and some derivatives calculated by the PPP method have been correlated with H shifts and polarographic half-wave potentials (78IJC(A)53l). 

The ketone functional group appears in a wide variety of natural products ranging from the simple terpenes to complex macrocycles and polyfunctional molecules. This section will focus on simple, naturally-occurring ketones, largely from the terpene family, which illustrate relationships between structure and 1 0 NMR chemical shifts. Other more complex ketone containing structures will be examined later according to their natural product family classification, e.g. chromones, flavones, etc. 

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