text:  The development of an enantioselective <unk> <unk> is a desirable addition to the repertoire of <unk> <unk> reactions . To that end , several variations have been developed utilizing chiral auxiliaries and chiral catalysts . <unk> <unk> are also known , in which extant <unk> direct the <unk> . Almost all of the attempts are based on the idea of <unk> ; selecting one direction for the vinyl groups to " rotate " in turn sets the stereochemistry as shown below . 
  