Two pentalenanoids, pentalenene (4) and pentalenic acid (5) were synthesized from humulene via biosynthetic pathway. Humulene on treatment with Hg(NO_3)_2 followed by KBr gave 10-bromomercuri-3,6-secoprotoilludane derivatives 12 and 13 and these on demercuration or under Whitesides's reaction conditions (O_2, NaBH_4, DMF) furnished the ethers, 9 and 10, or hydroxy ethers, 14, 15, 17 and 18, respectively. Cleavage of ether linkage in these compounds resulted in the formation of 3,6-secoprotoilluden-3-ol (11) or 3,10-diol (16) depending upon the starting ethers. These two compounds, 11 and 16, which are equivalent to cation 3 and the corresponding cation with hydroxyl group at C-10, are the key intermediates not only in the chemical synthesis but also in the biosynthesis of pentalenanoids. Pentalenene (4) was obtained by the formolysis of 11 while a corresponding compound with hydroxyl group at C-10, 24, was furnished by the treatment of 16 with BF_3-OEt_2. The allylic oxidation of 13-Me of 24 yielded methyl pentalenate (29). The compound 11 was converted to methoxy ketone 31 which was expected to serve as the intermediate for the synthesis of pentalenanoids with a lactone moiety in ring C through the enone 33 as shown in Fig 5. We have already obtained 39 and 41 from which the syntheses of pentalenolactone E, G and H are in progress.
