One-Pot Aldol Cascade for the Preparation of Isospiropyrans, Flavylium Salts, and bis-Spiropyrans

We probed tandem aldol condensations of sixteen o-hydroxyacetophenones, carrying electron-withdrawing and -donating groups at positions 4 and 5, using five molar equivalents of SiCl₄ in anhydrous ethanol. Substrates carrying electron-withdrawing groups (EWGs) (0 < ∑σ > 0.63) populated the equilibria with isospiropyrans (12–74% yield), while those carrying electron-donating groups (EDGs) (∑σ < −0.31) gave flavylium salts (50–80%) or thermochromic bis-spiropyrans (73%). The results are of interest for developing novel organic materials possessing switchable photochromic and thermochromic characteristics.