 For addition of sulfur ylides to <unk> , higher 1 @,@ 4 @-@ selectivity is typically obtained with sulfoxonium reagents than with sulfonium reagents . Many electron @-@ withdrawing groups have been shown compatible with the reaction including <unk> , <unk> , and <unk> ( the example below involves a <unk> <unk> ) . With further conjugated systems 1 @,@ 6 @-@ addition tends to predominate over 1 @,@ 4 @-@ addition . 
