Preparations of Spiropyrans

The condensation of the two halves of a BIPS or other spiropyran involving a nitrogen heterocycle generally goes rapidly and smoothly. In most cases the 

Normally the hydroxyaldehyde and Fischer s base or corresponding quaternary salt are used in equimolar amounts. Some hydroxyaldehydes readily form the poorly soluble, nonphotochromic dicondensed 3,4-dihydroBIPS having a second Fischer s base moiety as a substituent in the 4-position. Salicylaldehyde itself does this, as do many 3-alkoxy (especially methoxy) salicylaldehydes containing additional substituents. To minimize the formation of the dicondensed product, add the Fischer s base slowly to a 5-10% excess of the aldehyde. The readily soluble excess aldehyde is easily removed from the (usually) poorly soluble desired BIPS. 

If the desired BIPS is much less soluble in the reaction solvent than the reactants and side products, as is often the case, it can be obtained highly pure and in high yield. Thus, for example, the procedure described in section 1.5.1 gives a 95-98% yield of 8-ethoxy-6-nitroBIPS of 99+% purity. More typically, this procedure 

Although ethanol is the usual reaction solvent, lower ketones such as 2-butanone are also useful solvents if an intermediate contains a group reactive to alcohols, e.g., l-(2-carboxyethyl)Fischer s Base hydroiodide.70 In aromatic solvents such as toluene, the condensation proceeds well and the water formed may be removed by azeatroping, but the product generally is easily soluble, requiring concentration of the reaction mixture. Separation of pure spiropyran from colored impurities of similar solubility may then be tedious. 

The preparation, filtration, washing, and drying of the spiropyran should, of course, be carried out in the absence of direct or diffuse daylight, and if possible under incandescent or gold (minus blue) fluorescent lamps. There is sufficient UV radiation available from ordinary fluorescent lamps to significantly color a still-damp filter cake of a sensitive compound. 

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