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)