Optically Active Spiropyrans

Little has been reported about the preparation and photochromic behavior of optically active spiropyrans, despite an old suggestion to examine photocoloration with circularly polarized light. 

Chiral 2,4-dimethyl-3-(l-phenylethyl)thiazolium perchlorates were prepared from the available chiral 1-phenylethylamines and used to prepare optically active monomethinecyanines, but not spiropyrans.195 

3 -Phenyl-6-nitroBIPS was easily resolved (at the 3 -carbon atom, not the 2 -spiro position) by reaction in ethanol with either (+) or (-) 10-camphorsulfonic acid. The first fraction to crystallize in each case was the salt of the open form. Both salts had a mp of 237°C and their solutions, after standing to thermally equilibrate at the 2 -carbon atom, showed equal but opposite rotations, [oc]d25 = -201° for the salt from the (+) sulfonic acid and +201° for the salt from the (-) acid. After treatment of these salts with aqueous sodium acetate and benzene, the respective spiropyrans, having melting points of 201-202°C and specific rotations of -11° and +11° for thermally equilibrated solutions, were recovered from the benzene extract. Similar attempts to resolve several other 3 -phenylBIPS with the camphorsulfonic or 3-bromocamphor-8-sulfonic acids were only pardy successful.7 

The resolution of 3-phenylFischer s base itself would make available the chiral base for the preparation of not only indolinospiropyrans, but also for cyanine, merocyanine, and styryl dyes. Several attempts to effect this resolution using the camphorsulfonic, bromocamphorsulfonic, and dibenzoyltartaric acids in lower alcohols, 1,2,-dimethoxyethane and tetrahydrofuran, were unsuccessful in giving a clean resolution. Perseverance and luck are of prime importance. 

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There continues to be a high level of interest shown in photochromic systems. The optically active l,T-bi-2-naphthol gives an optically active intramolecular addition product (37) on irradiation, and on prolonged exposure, cycloreversion occurs which gives evidence for a photoequilibrium between the asymmetric molecules (Cavazza et al). A number of reports within the year describe a variety of aspects of the well-known thermally-reversible spiroindoline-oxazine to photo-merocyanine conversion. For example, the process is sensitised by triplet cam-phorquinone (Favaro et al.), the influence of complexation on indoline and phenanthroline spiropyrans with transition and rare earth metal ions has been described (Atabekyan et al.), and the photochromism of other derivatives in water using vesicles and y-cyclodextrin is reported to be faster than in methanol with the process most favoured in the vesicles (Ishiwatari et al.). 

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