E200 Regioselective Photocyclization Reactions of 3-Allyloxy-6-chloro-2- (thiophen-3-yl)-4H-chromen-4-one: Solvent Effect Vol 50 Ramesh C. Kamboj, * Dinesh Kumar, Rita Arora, Surinder Berar, and Geeta Sharma Department of Chemistry, Kurukshetra University, Kurukshetra 136119, India *E-mail: rckamboj@rediffmail.com Received January 16, 2011 DOI 10.1002/jhet.1006 Published online 2 April 2013 in Wiley Online Library (wileyonlinelibrary.com). The photo-irradiation of thienylchromenone resulted in the regioselective cyclization which is exclusively controlled by the nature of solvent used as reaction medium. Compared to nonpolar medium, polar solvent furnished a diverse array of novel angular tetracyclic photoproducts with gem-dihydro functionality and exocyclic double bonds on the fused pyran ring, which is unprecedented to best of our knowledge. J. Heterocyclic Chem., 50, E200 (2013). INTRODUCTION Regioselective photochemical reactions have become key topics in organic photochemistry. Many research efforts have been directed to obtain regioselective photocyclized products [1]. In most cases, because several types of photoproducts were produced, controlling the selectivity of the product is very hard. In our earlier study, an attempt was made to study the regioselective photocyclization of 3-alkoxy-chromones bearing a 3-furyl [2] group at C-2, possessing two sites for clipping of 1,4-biradical. In that case, the photoproducts were furnished only by clipping at C-2 0 position of furyl moiety and no photoproduct formation by clipping at C-4 0 (Fig. 1) was observed in methanolic media. And also, no isomeric product was realized through the photo-activation of furan moiety [3,4], which itself is a good chromophore (Scheme 1). We irradiated the chromenone having 3-thienyl group at 2-position (in place of 3-furyl) and also studied the effect of reaction media (benzene/methanol) to further explore the possibility of regioselective photocyclization in such type of molecules. Here, the interesting results were obtained. The photolysis of 3-allyloxy-6-chloro-2-(thiophen-3-yl)- 4H-chromen-4-one 1 in a nonpolar solvent, that is, benzene, effectively afforded the cyclized and cyclodehydrogenated photoproducts by the clipping of initially formed 1,4-biradical (A) with the C-2 0 position of the thiophene ring (Scheme 2). The photolysis of same in a polar solvent, that is, methanol, selectively produced entirely different type of cyclized and cyclodehydrogenated photoproducts enclosed the gem- dihydro functionality and exocyclic double bonds onto the fused pyran ring, respectively, by clipping of 1,4-biradical (B) to the C-4 0 position of the thienyl group that is unfore- seen in the studied chromenone (Scheme 3); furthermore, this indicated that the product selectivity in thienylchrome- none 1 is perfectly controlled by the reaction media. RESULTS AND DISCUSSIONS The required substrate for photolysis, 3-allyloxy-6-chloro- 2-(thiophen-3-yl)-4H-chromen-4-one 1 was synthesized by the literature procedure [2] and examined toward photolysis in both polar (methanol) and nonpolar (benzene) solvents (Scheme 1). The photo-irradiations of 1 in benzene, a nonpolar solvent, with pyrex filtered UV-light from 125 W high-pressure mercury lamp under nitrogen atmosphere produced the photoproducts 2a and 2b (Scheme 1). The formation of products 2a and 2b, in benzene, plausibly occurs through the clipping of 1,4-biradical (A) with C-2 0 position (Scheme 2). The structure of compounds 2a and 2b was unequivocally establish by their spectral data when compared with our earlier studies [4,5]. To study the photochemical behavior of 1 in a polar media, it was photolysed under identical conditions in its methanolic solution (1.0 mM) that furnished the exclusive diverse photoproducts 2c, 2d as major and a minor product 2b. The structure and stereochemistry were suggested by the spectral data. The compound 2c in its IR spectrum exhibited a strong absorption band at 1628 cm 1 that may be assigned to the C═O of the pyrone moiety. The © 2013 HeteroCorporation