Novel Photochromic Naphthopyran DerivativesV/ANew General Synthetic Methodology

A new method for the synthesis of 2H-naphtho[1,2-b]pyrans in good yield has been developed. The pyrans were synthesised by double lithiation of 3-bromo-1-naphthol, reaction with an αβ-unsaturated aldehyde or ketone to give substituted hydroxymethylnaphthol intermediates, followed by cyclodehydration. Reaction of lithium 2-lithio-1-naphthoxide with heteroaryl aryl ketones has given 2H-naphtho[1,2-b]pyrans fused to a benzannelated heterocyclic ring system. The photochromic and tribochromic properties of these new compounds are intrinsically linked to the nature of the heteroatom, and to its position in the molecule with respect to the pyran oxygen. Reaction of lithium 2-lithio-1-naphthoxide with heteroaryl methyl ketones did not yield fused benzannelated heterocyclic naphthopyrans, but gave heteroaryl vinylnaphthol derivatives by an alternative dehydration mechanism. They are photochromic, presumably by ketoenol tautomerism. The colourless photochromic compounds show colour changes to yellow, orange or red.