Nucleophilic addition reactions of benzoxanthone derivatives with collapse of aromaticity

3,8-Bis(methoxymethoxy)benzo[b]xanthene-12-one (1a) was reacted with n-butyllithium at -78 degrees C to afford the expected 1,2-adduct 1b and also the 1,4-adduct 1c. Using 2-Me-THF as the solvent afforded predominantly the 1,2-adduct 1b in 66% yield and the 1,4-adduct 1c in 5% yield. In contrast, in THF, the 1,4-adduct 1c was obtained as the major product in 47% yield and the 1,2-adduct 1b was obtained in 22% yield. As a result of many experiments, it was found that these peculiar regioselectivity is due to a variety of factors such as the kind of solvents, the organometallic species, and the reaction substrates. Among them, the difference in regioselectivity between THF and 2-Me-THF was explained using theoretical calculations. (C) 2020 Elsevier Ltd. All rights reserved.