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.