Synthesis of Benzo[b]fluorenone Nuclei of Stealthins

Two routes, one based on a Michael-initiated aldol condensation and the other on an intramoleculer carbonyl-ene reaction, have been found to be feasible for an entry to benzo[]fluorenones. Reaction of 4,9-dimethoxybenz[]indenone with nitromethane in the presence of DBU gave the corresponding Michael adduct, which afforded 2-methyl-5,10-dimethoxybenzo[b]fluorenone on reaction with methacrolein under a variety of basic conditions. Similarly, 2-methallyl-4,9-dimethoxybenz[]indenone reacted with nitromethane to give the corresponding Michael adduct, Nef reaction of which furnished 3-formyl-2-methyl-4,9-dimethoxybenz[]indanone. This underwent ene-cyclization under the influence of SnCl4. 5H2O, and yielded 2-methyl-5,10-dimethoxybenzo[]fluorenone.