Refining the chemical structure of functionalized pyrrolidine-based inhibitors of Golgi α- mannosidase II (GMII) to optimize binding affinity provided a lead molecule that demonstrated nanomolar competitive inhibition of α-mannosidases II and an optimal fit in the active site of Drosophila GMII by X-ray crystallography. Esters of this lead compound also inhibited the growth of human glioblastoma and brain-derived endothelial cells more than the growth of non-tumoral human fibroblasts, suggesting their potential for anti-cancer therapy.