2/7-Chromene electrocyclic ring opening

Pyrans and napthopyrans (chromenes) are photochromic compounds that undergo photochemically induced electrocyclic ring opening to give colored ortho-quinone methides.95-98 For example, chromene 153 opens on irradiation to give 154 (Eq. 1.41). 

In the above cases, the photoreactivity of indole, coumarin, pyridine, and so forth, does not compete effectively with PFR, which becomes the major reaction. However, in the case of the chromene depicted in Scheme 28, the electrocyclic ring opening prevails over PFR. By contrast, the analogous chromane undergoes clearly acyl migration [86]. 

The 4-benzylpyran (501) rearranges on irriadiation or thermally into the 2-benzyl isomer (502) which yields the benzene (503) via electrocyclic ring opening. Irradiation of 2//-chromenes (504) gives an intense red color due to the quininoid photoproduct (505). 

The 4-benzyl 1,4-dihydropyran 520 rearranges on irriadiation, or thermally to the 2-benzyl isomer 521 which yields 522 via electrocyclic ring opening. Irradiation of 277-chromenes 523 gives an intensely red photoproduct 524. 

The photochemical ring opening of chrom-3-enes (29) has been shown to produce o-quinoneallides (30). These have been trapped as the phenols (31) and (32) by their in situ reduction with lithium aluminum hydride (67JPC4045). The reverse process, thermal electrocyclic ring closure of the allide, is a facile reaction and hence any synthesis of an o -quinoneallide is effectively a synthesis of a chrom-3-ene. Many chromene syntheses are considered to proceed through the intermediacy of a quinoneallide and all of these are... 

The photochromic properties of 2/7-chromene derivatives has generated much interest in recent years. Under UV irradiation these molecules can undergo reversible electrocyclic opening of the pyran ring to afford colored ortho-quinone methides <2005T11730, 2005T1681>. 

---