text:  <unk> @-@ directed <unk> <unk> can exhibit induced diastereoselectivity in this way . In the example below , the <unk> @-@ group acts to direct the <unk> by preventing the distant <unk> from rotating " towards " it via unfavorable <unk> interaction . In this way the silicon acts as a <unk> auxiliary . ( The starting material is not <unk> but the retention of enantiomeric excess suggests that the auxiliary directs the <unk> . ) 
  