ortho and remote metalation – cross coupling strategies. Total synthesis of the naturally occurring fluorenone dengibsinin and the azafluoranthene alkaloid imeluteine

Abstract

The total synthesis of the naturally occurring fluorenone, dengibsinin (7a), and the azafluoranthene alkaloid, imeluteine (30e), is described. Using combined ortho metalation – cross coupling sequences that terminate in Friedel–Crafts (18b → 6d) and remote metalation (21a,b → 6c,d) reactions, the synthesis of fluorenone dimethyl ethers 6c and 6d is reported. 6c and 6d were shown not to be identical to dengibsinin dimethyl ether and dengibsin dimethyl ether, respectively, derived from the natural products. This work, together with synthetic and structural evidence from Sargent and Talapatra, led to the reassignment of structures for dengibsinin (7a) and dengibsin (7b). Using the metalation – cross coupling approach, the synthesis of the reassigned dengibsinin (7a) is presented. Two alternate unsuccessful approaches to imeluteine are briefly described (31 + 32 and 33 + 34). The successful approach incorporates the remote metalation – double cyclization, 43 → 44, as the key step.