The enzymatic activation of 3,4-dehydrolysine and subsequent formation of the 12-membered syringolin macrolactam were investigated. The timing of the desaturation was elucidated through the analysis of the initial adenylation domain of SylD. The SylD-TTE didomain was characterized and demonstrated to be the catalyst for formation of 12-membered macrocycles. When the SylD thioesterase domain was reacted with a family of acyclic CoA both natural and unnatural macrocycles were generated.