Enantioselective construction of biaryl part in the synthesis of stegane related compounds

A Pd-mediated intramolecular aryl-aryl coupling reaction of phenyl benzoate derivatives were examined to form benzo[c]chromen-6-ones, and then enantioselective lactone-opening reaction with a borane-oxazaborolidine combination was carried out. The resulting biphenyl was transformed into a key intermediate for the stegane related compounds. The absolute configuration of the biphenyl is also discussed. Stegane and related compounds are important because of their interesting biological activities such as antileukemic properties.1 One of the most outstanding features of their chemical structures is an unsymmetrical 2,2’-disubstituted biphenyl moiety with an axial chirality (Figure 1). For the formation of such a biphenyl part in the syntheses of the stegane families, several approaches have been attempted such as photocyclization,2 Suzuki coupling,3 oxidative biaryl coupling,4 the SNAr reaction,5 Ullmann coupling,6 and the [2+2+2] three-component cyclization reaction.7