Bismethine compounds

Ciba-Geigy [23] has investigated other methine pigments, including bismethine compounds with one or two nonhalogenated isoindolinone ring systems. 

Only symmetrical and asymmetrical bismethine compounds of the general structure 2 have made their way into the market. 

The NMR spectra of compounds 4-8 were consistent with the structures assigned. This evidence together with the great similarities in UV and IR spectra discussed above clearly established that the degradations had not caused any major skeletal changes. The facile aromatiza-tion characteristic of the morphine alkaloids appears to be blocked in this case by the five-membered ring and spiro carbon atom C-6. This explains the stability of cancentrine methine methiodide to potassium hydroxide and the need for the much stronger base to effect the second elimination to the bismethine 8 (3, 4). 

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