 Silicon @-@ directed <unk> <unk> can exhibit induced diastereoselectivity in this way . In the example below , the silyl @-@ group acts to direct the <unk> by preventing the distant alkene from rotating " towards " it via unfavorable steric 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> . ) 
