Chemo-enzymatic synthesis of new chiral bipyridyl alcohols and their atropisomeric N,N-dioxides

New chiral bipyridyl alcohols and the corresponding N,N-dioxide derivatives were obtained with high enantiomeric excess by a chemo-enzymatic procedure. Metalation of commercial 6,6'-dimethyl-2,2'-bipyridine followed by reaction with acetaldehyde furnished an inseparable 1:1 mixture of dl- and meso- 6,6'-bis(2-hydroxypropyl)-2,2'-bipyridine ((±)-7a and meso-7b) that was subjected to lipasecatalysed enantioselective esterification with vinyl acetate in organic solvent. Immobilised Mucor Miehei lipase (Lipozyme ® IM) displayed high stereoselectivity and unreacted alcohol (S,S)-7a, monoacetyl derivative (R,S)-6 and diacetate (R,R)-9 were obtained in high optical purities as the result of the simultaneous resolution of the racemate and desymmetrization of meso isomer. After separation, the chiral compounds were converted into the corresponding N,N-dioxide derivatives with additional axial chirality stereoselectively induced during the N,N-oxidation and some considerations about their stereochemistry and atropisomeric stability were deduced by circular dichroism spectroscopy.