Beilstein Journal of Organic Chemistry Beilstein Journal of Organic Chemistry Beilstein Journal of Organic Chemistry Synthesis of Spiropyrans: H-abstractions in 3-cycloalkenyloxybenzopyrans 2. Results

A photochemical route for the synthesis of some benzopyronospiropyrans from 2-furyl-3-cycloalkenyloxybenzopyrones involving H-abstraction is reported. How a methyl group on the furyl ring affects the product formation is also investigated. Spiropyrans exhibit photochromism due to the pho-toequilibrium with their open chain analogues merocy-anins – a property that makes them a material of choice in digital storage technology. [1-5] Some spiropyrans have been found to have antiphlogistic, spasmolytic, antidia-betic and antifeedant activities amongst others. [6-10] The methods available for the synthesis of spiropyrans include enamine eliminations, reductive/thermal cyclisations and [4+2] cycloadditions. [11-16] These methods being specific in nature for a particular spirocyclic compound, offer limited synthetic utility. In the past, we have investigated the use of photochemical methods for the synthesis of organic molecules like vinyl ethers, [17] pyranopyrones [18,19] etc. and in continuation of that work we published a preliminary report on photochemical synthesis of spiropyrans. [20] To extend and examine the scope of this methodology, in this communication, we report the synthesis of spiropyrans bearing dihydrofuryls and acylcyclo-propanes. The substrates 1 and 2 required for this study were obtained through the alkylation of


Background
Spiropyrans exhibit photochromism due to the photoequilibrium with their open chain analogues merocyanins -a property that makes them a material of choice in digital storage technology. [1][2][3][4][5] Some spiropyrans have been found to have antiphlogistic, spasmolytic, antidiabetic and antifeedant activities amongst others. [6][7][8][9][10] The methods available for the synthesis of spiropyrans include enamine eliminations, reductive/thermal cyclisations and [4+2] cycloadditions. [11][12][13][14][15][16] These methods being specific in nature for a particular spirocyclic compound, offer limited synthetic utility. In the past, we have investigated the use of photochemical methods for the synthesis of organic molecules like vinyl ethers, [17] pyranopyrones [18,19] etc. and in continuation of that work we published a preliminary report on photochemical synthesis of spiropyrans. [20] To extend and examine the scope of this methodology, in this communication, we report the synthesis of spiropyrans bearing dihydrofuryls and acylcyclopropanes.

Scheme 1: Synthesis of 3-Cycloalkenyloxybenzopyrans.
The irradiation of 1c in methanol with pyrex filtered light from a 125W Hg lamp produced two products 5c and 6c along with some 3.

Discussion
The structures of the photoproducts 5c and 6c were consistent with their spectral parameters [see additional file 1]; both the spiropyrans exhibited the presence of ions at m/z 155 and m/z 202 values in their mass spectra; these ions could very well be the result of rDA fragmentation of 5c and 6c. In the mass spectrum of 6c, the base peak was at m/z 327 (M + -CHO).
The spiropyrans 6 (R = H) bearing cyclopropanes are the secondary photoproducts formed as a result of the further reorganization of 5 (R = H), through a ring contractionring expansion mechanism. [22,23] The formation of acetylcyclopropane compound 7 (R = CH 3 ) from 2 (R = CH 3 ) can be explained likewise. However, it is pertinent to mention here that when a large amount of 2b was photolysed and the photolysate was carefully chromatographed, a very small amount (<3%) of photoproduct 5 (R = CH 3 ) could be isolated (vide experimental). The reason for such behaviour of 2 (R = CH 3

Conclusion
In conclusion this photochemical method can be of utility for synthesizing a variety of pyronospiropyrans bearing both dihydrofuryls and acylcyclopropanes. The substitution on the furan ring makes the primary photoproducts more amenable to cleavage.