text:  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 <unk> for a leaving group and direct oxidation of an <unk> . <unk> ( <unk> ) - ( <unk> <unk> ) ( <unk> ) and N @-@ <unk> are the most common <unk> sources of oxygen implemented in this process . One advantage of using N @-@ <unk> is that higher <unk> induction is almost invariably observed relative to <unk> and other <unk> . High yield ( 77 @-@ 91 % ) and <unk> ( 95 : 5 - 99 : 1 ) are reported for α @-@ <unk> with the Evans ' <unk> auxiliary with N @-@ <unk> as the <unk> . <unk> induction has been demonstrated with many other <unk> <unk> and <unk> with <unk> <unk> , including <unk> and <unk> . 
  