Naphthopyran moiety

Photochromic performance was enhanced when the naphthopyran moiety was located in the middle rather than at the terminus. A middle disposition of the naphthopyran provides better encapsulation by the polymeric arms giving improved coloration and decoloration rates and higher optical density in the colored open-state [26]. 

The presence of the enamino moiety in 2-amino-4H-pyrans accoimts for their ability to undergo recyclizations into various pyridones, 1,4-dihy-dropyridines, and 2H-pyrones-2. To some extent, properties of 2-amino-4H-pyrans in reactions with nucleophiles can be compared to those of pyrillium salts (68T5059,80T697) because they also tend to form recyclized products. Reactions proceed in the presence of bases or acids. Naphthopyrans 133 form 2-alkoxypyridines 261 on the action of sodium alcoholates or ethanolic NaOH (79M115) (Scheme 101). 

The annellation of a benzene ring to the 5 and 6 positions of the benzopyran moiety in CH2, instead of the pyridine ring in CHI, does not essentially affect the structure of the naphthopyran fragment. The pyran rings in CHI and CH2 are nonplanar with folding along O- -C(2) and O- -0(3) in a similar manner as in SPs and SOs. The Cspiro-0 bond in CHI and CH2 is elongated, and the 0-CPh bond is shortened (Table 5). 

The angular isomers, 2 and 3, have received much attention since they display good photochromic properties in solution under ambient conditions. Further structural diversity has been achieved by the fusion of aromatic and heterocyclic moieties onto 2Ff-[l]benzopyran and the isomeric naphthopyrans. This review discusses the consequences of the reversible opening of the pyran ring in such compounds under the influence of UV-irradiation and draws together the information reported in the scientific and patent literature concerning the synthesis and photochromic properties of these molecules. Particular attention is paid to the... 

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