lldione. whichhas been produced efficiently by means of electrochemical method.
Pentalenene belongs to a group of triquinane sesquiterpenesl and is attractive to synthetic chemists.2f We describe herein a synthesis of (f)-pentalenene u&g electrochemical method as a key step. The known phenol (1) (418 mg)4 was subjected to anodic oxidation [CCE: 71.8 mA (+540 -1500 mV vs. SCE; ca. 2 F/mol] using MeOH (30 ml) -AcGH (20 ml) containing LiC104 (400 mg) under argon, diluted with large amounts of toluene (1000 ml), and then concentrated under reduced pressure at 50 OC to afford the desired tricyclic compound (2) and its epimer at C2-position in 64 and 16% yields, respectively.4 The former was reduced with DIBAL-H and then acetylated to give a triacetate (3)s in 94% overall yield. The compound (3) was hydrolyzed with oxalic acid and treated again with Ac20 -pyridhte to afford a ketone (4),5 which was further subjected to Grignard reaction followed by selective benzylation to give rise to a monobenxyl ether (5)s in 77% overall yield in 4 steps.
In the next step, 5 was oxidized with PCC and then with Pb(OAc)4 in MeGH to afford a bicyclic compound (6),5 which was further hydrolyzed to yield a diketone (7),5 as seen in Scheme 1. On intramolecular aldol condensation followed by catalytic hydrogenation, 7 was readily converted into a tricyclic compound (8),5 in high yield. Dehydration of 8 followed by acid-catalyzed isomerization gave rise to the known ketone (9),3~6 which had been already transformed to (f)-pentalenene (lo)? This research has been supported in part by grants from the Ministry of Education, Science and Culture as well as the Kurata Foundation, to which grateful acknowledgment is made.