FURO-FUSED 2H-CHROMENES - SYNTHESIS AND PHOTOCHROMIC PROPERTIES

New photochromic chromenes annulated with a furan ring have been synth esized. Thus, suitable heterocyclic phenols react with different propa rgylic alcohols in acidic medium to give the corresponding ethers, whi ch cyclize into benzopyrans by thermal Claisen rearrangement. This syn thetic approach was found to lead to a mixture of linear and angular c hromenes that is strictly related to the nature of the phenol. However , regiospecificity could be obtained by reacting beta-phenylcinnamalde hyde, in refluxing aprotic nonpolar solvents, with titanium(IV) salts of the former phenols. Electrocyclization of intermediately generated o-quinoid structures occurs on the cw position towards the heterocycli c junction. All compounds exhibit photochromic behavior at room temper ature. Furo-fused benzopyrans are particularly interesting with respec t to naphthopyran parents in view of the bathochromically shifted and broadened absorption spectra of photoinduced forms. This trend is conf irmed by the spectral data of several heterocyclospiro(7H-furo[3,2-f] chromenes). The achievable color depends significantly on the relative position of annulation on the chromenic moiety and substitution on th e sp(3) carbon atom.