Reductive opening of 2,7-dihydrodinaphthoxepine and thiepine: easy regioselective preparation of 2,2′-difunctionalised binaphthyls

Abstract

The lithiation of 2,7-dihydrodinaphthoheteroepines ( 5 ) with 2.2 equiv of lithium naphthalenide in THF at −78 °C gives dianionic intermediates 8 , which by reaction with different electrophiles [H 2O, D 2O, t BuCHO, Me 2CO, Et 2CO, (CH 2) 4CO, (CH 2) 5CO] at the same temperature, followed by hydrolysis, leads to unsymmetrically 2,2′-disubstituted binaphthyls 6 . When the lithiation is performed with an excess of lithium in the presence of a catalytic amount of 4,4′-di- tert-butylbiphenyl (DTBB, 10 mol %), a double reductive cleavage takes place to give dianionic intermediate 9 , which by reaction with different electrophiles [H 2O, Me 2CO, Et 2CO, (CH 2) 4CO, (CH 2) 5CO], followed by hydrolysis with water, yields symmetrically 2,2′-disubstituted binaphthyls 7 . In the case of starting from ( R)- 5a , the reductive opening by treatment with 2.2 equiv of lithium naphthalenide followed by reaction with H 2O or (CH 2) 5CO as electrophiles and final hydrolysis, leads to enantiomerically pure compounds ( R)- 6aa and ( R)- 6af , respectively.