 α @-@ <unk> , or <unk> , are an important synthetic motifs present in many natural <unk> α @-@ <unk> have been synthesized in many ways , including reduction of α @-@ <unk> , substitution of a hydroxyl for a leaving group and direct oxidation of an <unk> . <unk> ( pyridine ) - ( <unk> <unk> ) ( <unk> ) and N @-@ <unk> are the most common electrophilic sources of oxygen implemented in this process . One advantage of using N @-@ <unk> is that higher chiral induction is almost invariably observed relative to <unk> and other oxidants . High yield ( 77 @-@ 91 % ) and dr ( 95 : 5 - 99 : 1 ) are reported for α @-@ hydroxylation with the Evans ' chiral auxiliary with N @-@ <unk> as the <unk> . Chiral induction has been demonstrated with many other chiral ketones and ketones with chiral auxiliaries , including <unk> and <unk> . 
