Abstract
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[b]fluorenones. Reaction of 4,9-dimethoxybenz[f]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[f]indenone reacted with nitromethane to give the corresponding Michael adduct, Nef reaction of which furnished 3-formyl-2-methyl-4,9-dimethoxybenz[f]indanone. This underwent ene-cyclization under the influence of SnCl4. 5H2O, and yielded 2-methyl-5,10-dimethoxybenzo[b]fluorenone.
Highlights
Stealthins A (1a) and B (1b), isolated from Streptomyces viridochromogenes as potent radical scavengers, are the first known members of natural benzo[b]fluorenones [1]. Interest in this group of compounds grew considerably due to the identification of structurally allied natural products, stealthin C (1c) [2], kinafluorenone (2) [3], prekinamycin (3) [4], and kinamycin antibiotics [5] (Figure 1)
Their synthesis became an active area of research since 1996 [6,7,8,9,10,11,12,13,14,15,16,17,18,19]
In line with the Ishikawa approach [19], we intended to explore the chemistry of benz[ f ]indenones (e.g., 5b) to establish new synthetic routes to functionalized benzo[b]fluorenones [20]
Summary
Stealthins A (1a) and B (1b), isolated from Streptomyces viridochromogenes as potent radical scavengers, are the first known members of natural benzo[b]fluorenones [1]. Interest in this group of compounds grew considerably due to the identification of structurally allied natural products, stealthin C (1c) [2], kinafluorenone (2) [3], prekinamycin (3) [4], and kinamycin antibiotics (e.g., kinamycin D, 4) [5] (Figure 1). We report regiospecific construction of the D-ring of benzo[b]fluorenones (e.g., 6) from the corresponding benz[ f ]indenones
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