Some interesting reactions of gelomulides, the natural diterpene lactones

Some interesting chemical reactions involving the characteristic features and confirming the proposed stereostructures of gelomulides A-F (1-6), the closely related diterpene lactones from the leaves of Gelonium multiflorum (Fam. Euphorbiaceae) and some chemical correlations were carried out. Thus, 3 upon treatment with DDQ gave 4 which was also produced when 6 was treated with pyridine or methanolic NaHCO3. Zn dust/AcOH (reflux) or PPh3/I-2 deepoxidized 1 and 4 to 13 and 14 respectively. PPh3/I-2 converted 6 to the corresponding olefin 15 which upon refluxing with pyridine afforded 14, also obtained directly from 6 by refluxing with Zn dust/ AcOH. NaBH4 reduction of 3 and 6 gave the corresponding tetrahydro derivatives 18 and 21; additionally, 1 and 6 gave 19 and 20 respectively. Compound 1 underwent base-catalyzed opening of the lactone and the epoxide rings, followed by ring C aromatization to give 16. Similarly, under three different acidic conditions, 4 and 6 gave the same product 17. Catalytic hydrogenation of different gelomulides caused the hydrogenolysis of the allylic epoxide bond and reduction of the Delta(1,2) bond (if present) but the 3-CO group remained unaffected. The formation of the products have been rationalized in terms of the assigned stereostructures and molecular conformations of the gelomulides.