Sensitized Photooxygenation of Tinosponone, a Clerodane Diterpene from Tinospora Cordifolia

The reaction of tinosponone (1) with singlet oxygen was studied by using different combinations of photosensitizers (i.e. rose bengal, methylene blue, riboflavin and benzophenone), solvents (i.e. benzene, chloroform, acetone, acetonitrile and methanol) and singlet oxygen scavengers (i.e. DABCO and sodium azide). Two major products (3S,4aS.4bS,8R,8aR,10aR)-8-Hydroxy-3-(5'-hydroxy-2'-oxo-2',5'-dihydrofuran-3'-yl)-4a,8a-dimethyl-3,4,8,8a,9,10-hexahydro-10aH-benzo[f]isochromene-1,5(4aH,4bH)-dione (2) and (3S,4aS,4bS,8R,8aR,10aR)-8-Hydroxy-4a,8a-dimethyl-3-((1'R)-3'-oxo-4',6'-dioxa-hicyclo[3.1.0]hexan-1'-yl)-3,4,8,8a,9,10-hexahydro-10aH-benzo[f]isochmmene-1,5(4aH,4bH)-dione (3) were isolated in all the solvents except methanol. In methanol a single product (3S,4aS,4bS,8R,8aR,10aR)-8-Hydroxy-3-(5'-hydroperoxy-2'-methoxy-2',5'-dihydrofuran-3'-yl)-4a,8a-dimethyl-3,4,8,8a,9,10-hexahydro-10aH-benzo[f]isochromene-1,5(4aH,4bH)-dione (4) was obtained. All products were characterized on the basis of IR, 1 H NMR, 1 3 C NMR and elemental analysis studies. The formation of products was explained by photooxidation of tinosponone. Effects of different solvents with the variation of added singlet oxygen sensitizers and singlet oxygen scavangers were observed on the yield of photooxidation products and were correlated to the rate of singlet oxygen formation.