Synthesis of photochromic 3-arylvinyl-3H-naphtho[2,1-b]pyrans: An unexpected one-step annulation to cyclopenta[b]naphtho[1,2-d]furans

The synthesis and photochromic evaluation of 3-arylvinyl-3H-naphtho[2,1-b]pyrans is discussed. Under acid catalysis, 1,3-diarylpent-1-en-4-yn-3-ols undergo a cascade reaction with variously substituted naphthols involving the formation of an intermediate photochromic naphthopyran which, under the reaction conditions, subsequently undergoes an electrocyclic ring-opening and a double intramolecular cyclisation in a Nazarov-type reaction to afford [5,5,6,6] cyclopentene fused naphthofurans; the sequence constitutes a new, one-pot, atom efficient approach to the cyclopenta[b]naphtho[1,2-d]furan unit.