Stereoselective Oxazaborolidine-Borane Reduction of Biphenyl Alkyl Diketones-Lignin Models: Enantiopure Dehydrodiapocynol Derivatives

Asymmetric reduction of two conformationally flexible biphenyl alkyl diketones 9 and 10 with (R)-oxazaborolidine-3-borane system was successfully carried out and the corresponding biphenyl alcohols 11 and 12 were obtained in high yield and e.e. with predominance of the homochiral (S,S) dicarbinols. The absolute configuration of diastereopure dehydrodiapocynol derivative (S,S)-14 was assigned by crystallographic analysis which confirms the known stereochemical course of CBS-catalysed reduction of ketones and gives useful information on spatial arrangement.