 <unk> reagents have been less successful , with most variations suffering from poor yield , poor <unk> , or both . There are also issues with substrate scope , most having limitations with methylene transfer and <unk> <unk> . The trouble stems from the need for a nucleophilic sulfide that efficiently generates the ylide which can also act as a good leaving group to form the <unk> . Since the factors underlying these <unk> are at odds , tuning of the catalyst properties has proven difficult . <unk> below are several of the most successful <unk> along with the yields and enantiomeric excess for their use in synthesis of ( E ) <unk> oxide . 
