A series of benzofuranquinones, analogues of the marine metabolite annulin A, has been synthesized and evaluated as inhibitors of human indoleamine 2,3-dioxygenase (IDO). The synthesis was carried out by copper(II)-mediated reaction of bromobenzoquinones with 1,3-dicarbonyl compounds followed by functional group interconversions. The most potent compounds were 5-methoxy-2-methylbenzofuranquinones containing a CH₂OR group at position-3 with IC₅₀ ∼ 0.2 mM. The corresponding hydroquinones were inactive. Compounds based on the benzimidazolequinone framework are also active IDO inhibitors. The quinones do not generate significant levels of oxidative stress at concentrations that inhibit IDO.