Reductive lithiation of cyclic benzofused ethers: a source of oxygen-functionalised organolithium compounds

The reaction of 2,3-dihydrobenzofuran () with lithium and a catalytic amount of 4,4′-di-tert-butylbiphenyl (DTBB, 5%) in THF at 0°C for 1.5 h, followed by addition of an electrophile [E+=H2O, tBuCHO, PhCH2CHO, Ph(CH2)2CHO, PhCHO, furfural, Me2CO, Et2CO, cyclopentanone, cyclohexanone, cyclooctanone, (−)-menthone] in THF at −78°C gives, after hydrolysis, compounds . Some diols are easily transformed into 2-substituted chromans under acidic reaction conditions. The reductive lithiation of chroman () at 20°C for 3 h leads exclusively to the intermediate , which is formed through a dearylation process, and isomerises to the apparently more stable benzylic intermediate . The reaction of these intermediates with different electrophiles {E+=tBuCHO, PhCHO, furfural, Me2CO, [CH3(CH2)4]2CO, cyclopentanone, cyclohexanone, (−)-menthone, Ph2CO}, at −78°C in THF leads, after hydrolysis, to a mixture of regioisomers and . The reaction of 2,3-benzofuran () with an excess of lithium and a catalytic amount of DTBB (5%) in THF at 0°C for 45 min leads to dianionic intermediate through a dealkylation process, which after hydrolysis gives 2-vinylphenol . In the case of 4H-chromene (), reductive opening is performed at 20°C for 45 min, a mixture of dearylation and dealkylation intermediates and , respectively, is obtained (2:1 ratio) giving, after hydrolysis, 3-phenylpropanal () and 2-allylphenol ().