Tropone ring, condensed

Benzotropones (XXXIV) were first prepared more than seventy years ago utilising a condensation reaction between phthalaldehyde and acetonedicarboxylic acid [122-124]. It is interesting to note that this preparative method involved the use of a condensation reaction leading directly to a tropone ring system, for simple tropones have not been made in this way. Variants of this method have been used to prepare a number of benzotronones a recent optimised method provides yields of 70% [237]. Other more recent methods use o-di(bromomethyl)benzene as starting material [238]. 

Another example of bromotropone contraction is derived from Nozoe s benzotropazine chemistry [Scheme 146 89H(29)1459]. Condensation of tropone 552 with further o-aminothiophenol (117b) gave benzenoid compound 554 instead of the expected tris(benzazino)tropylidene. Presumably, this condensation takes place via initial cyclization (to benzothiazine), then cyclocondensation (to spirobenzothiazoline), followed by tautomerization (to norcaradiene 553), ring contraction, and dehydrogenation. 

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