 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 <unk> <unk> and <unk> <unk> . <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 " <unk> " in turn sets the <unk> as shown below . 
