Naphthopyran group

The colored form of spironaphthopyran 32 absorbs at A,max of ca. 450 m,70 and the closed spiro form is colorless, which has no absorption band above 400nm. Bulky substituent group is especially important for photochromic sunglass. Introduction of the spiroadamantane or spirobi-cyclo[3.3.1]heptane into the 2-position of naphthopyran increases the resistance to photo-fatigue reaction, since endocyclic double bond induced by 1,7-hydrogen shift in the colored form cannot be formed in 2-adamantyl or 2-bicycloheptanyl group. 

Figure 1.8 UV-visible spectra of ORMOSIL-embedded naphthopyrans with different modifying groups show that samples prepared with iBu and Ph groups lead to more significant shifts to the UV compared with samples prepared with the same relative amount of Me groups. (Reproduced from ref. 16,... 

The photochromic compounds of potential interest, based on the 2//-chromene ring system, are the 2//-benzopyrans (1.18) or the three isomeric naphthopyrans (1.19-1.21). However, 2H-naphtho[2,3-( ]pyrans (1.21) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R and can be part of a carbocyclic spiro ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups. 

In this section, the three simple reduced pyrans (200), (201) and (202) are initially considered. Fused alicyclic analogues are included in this group. A discussion of the benzologues, chroman (203) and isochroman (204), and the reduced naphthopyrans follows. 

Benzopy rans are, as a group, less photochromic than 3//-naphtho[2,l-Z>]pyrans and 2//-naphtho l,2-/> pyrans having the same substituents,8 and they are mote fatigue prone. An additional problem with the 2//-1-benzopy rans is that they are less responsive to solar radiation due to their UV absorptions being at lower wavelengths than the naphthopyrans. As a result of these problems, researchers have modified the parent compound in an attempt to provide molecules with commercial potential. 

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. 

Spirofindoline-benzopyrans], spiro[indoline-naphthopyrans], and spiro[indo-line-naphthoxazines] can rather readily undergo electron-transfer processes in either direction i.e they can be readily oxidized and reduced. Because the presence of nitro groups in their molecular framework normally enhances the photochromaticity of these compounds, several combined electrochemical and EPR studies have been carried out on a variety of mono- and dinitro-substituted spiroindolinic photochromes.52 56-60... 

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. 

In a similar manner, the TV -methylated pyrano[2,3-6]carbazoles 28 and the [3,2-6] analogues 29 exhibit photochromism at room temperature. The single absorption band shown by the [2,3-6] series is red shifted with respect to naphtho[2,l-6]pyran and the intensity is also greater. The colourability is particularly enhanced by the introduction of a 2-thienyl group at the 8-position. Again, faster fading characteristics are observed relative to the parent diphenyl substituted naphthopyrans <01HCA1163>. 

Topics which have formed the subjects of reviews this year include photoinduced organic synthesis, photoisomerisations involving super-cyclophanes, regioselec-tive and stereoselective [2+2] photocycloadditions, position- and stereoselective photocyclisation, the photochemistry of indoles, five-membered heterocyclic compounds of the indigo group, pyrazoles and isothiazoles, and heterocyclic N-oxides, photochromic reactions of naphthopyran derivatives, photodegradation reactions of photochromic spirooxazines and 2H-chromenes, ° and chiral photo-chromic compounds. ... 

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