A convergent synthesis of SylA was developed and consists of the synthesis of a fully functionalized macrocycle, which is subsequently coupled with a urea moiety. For cyclization, ring-closing metathesis of a conformationally preorganized precursor was employed. The established synthetic route was then applied to the synthesis of SylA derivatives by using various peptidic side chains for decoration of the SylA macrocycle. The resulting collection of SylA analogues was tested for proteasome inhibition, revealing PEGylated SylA derivatives as the most potent proteasome inhibitors.