Observations on the Synthesis of Photochromic Naphthopyrans

1‐Naphthol reacts with 1,1‐diarylprop‐2‐yn‐1‐ols 5a , b , under alumina catalysis, by two pathways to give the photochromic naphtho[1,2‐ b ]pyrans 6a , b , together with the propenylidenenaphthalenones 7a , b , representatives of a new class of merocyanine dyes. With 2‐methyl‐1‐naphthol, formation of the photochrome is suppressed; the only products are merocyanines 7c , d . The cyclocondensation of 2‐naphthol with 5a , b proceeds much more efficiently, to give the naphtho[2,1‐ b ]pyrans 14a , b . Pyran formation is not suppressed from either 1‐bromo‐ or 1‐(4‐methoxyphenyl)‐2‐naphthol; reaction with 5a , b merely results in expulsion of the C‐1 substituent. An alternative pathway supervenes in the reaction of 1‐methyl‐2‐naphthol with 5a to give the benz[ e ]indanone 17 , the constitution of which was determined by X‐ray crystallography. Reaction of the 1,3,3‐triarylpropynols 19a , b with 1‐naphthol affords the naphthopyrans 20 together with merocyanines 21 , whilst the isomeric pyrans 23 are efficiently produced from 2‐naphthol. The configuration of merocyanines 7a and 21a was unequivocally established by X‐ray crystallography. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)