Naphthopyran intermediate

When the annellated heteroaromatic group is a five-member ring (43, 44), the properties are best described as intermediate between a naphtho and benzopyran. The standard method of synthesis for diaryl naphthopyrans (acid-catalyzed condensation between a diaryl propargyl alcohol and a naphthol, Scheme 8) is reported not to work for heteroaromatic fused phenols. This is true especially in cases where the heteroaromatic phenol contains a (basic) nitrogen. Alternatively, these pyrans can be prepared by the method of Casiraghi69 outlined in Scheme 16. 

Intramolecular Diels-Alder reactions have been used to make chiral naphthopyrans, as illustrated by the conversion of (17) into (18) on thermolysis. Intermediate (17) was prepared from di-O-acetyl-D-xylal by a sequence involving a carba-Ferrier reaction, and the epimer of (17) at the asterisked stereocentre gave the analogous epimer of (18), with the same stereochemistry at the ring junctions. 3 The glucose-derived diene (19)... 

---