For addition of sulfur ylides to enones , 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 ketones , esters , and amides ( the example below involves a Weinreb amide ) . With further conjugated systems 1 @,@ 6 @-@ addition tends to predominate over 1 @,@ 4 @-@ addition .