Pyrans, diaryl, synthesis

A better method of promoting fade in 2,2-diaryl (and other) 2//-naphtho[1.2-6]pyrans is to introduce a substituent at the 5-position. Such substitution produces steric crowding in the trans quinoidal open form (see Figure 3.14). The steric crowding will result in a destabilization of the colored species and a more rapid conversion (fading) to the colorless ground state. Methyl has been introduced in the 5-position for this purpose.59 The synthesis of the starting material for these 5,6-... 

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. 

The synthesis of 1-heteroaryl-l-arylprop-2-yn-l-ols or the 1,1-diheteroaryl analogues is fundamental to the introduction of a heterocycle into the gem diaryl unit of the naphthopyran isomers. The route is outlined in Scheme 10 small variations are encountered in the technique used for the nucleophilic attack of the alkyne unit on the carbonyl substrate <96USP5552091, 01DP(49)65, 03EJ01220>. The data in Table 4 indicate that red shifts of Xmax follow the introduction of furyl, thienyl and 2,2-bithienyl groups at C-3 of 3H-naphtho[2,l -Z)]pyrans. 

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