Bicondensation Adducts of Salicylic Aldehydes on Benzoheterocycloammonium Salts. Potential Applications to hermographic Recording Process

Abstract

The condensation in a basic medium of a 2, 3-dimethylbenzothiazolium salt with salicylic aldehyde, ortho- vanilline or 2-hydroxy-l-naphthaldehyde does not lead to the normal spiropyran or merocyanine, but gives a bicondensed spirodihydropyran. In the case of salicylic aldehyde substituted by an electron-withdrawing group, the condensation leads to a permanent open-form product.

Taking notice of these observations and of similar results obtained by different authors with other heterocyclic series, an explanation and a mechanism are proposed to explain the formation of bicondensed compounds.

The excellent thermo-chemical reaction of such bicondensed benzothiazoline products has allowed us to develop a thermographic recording process. Optimization and realization of this process are described.