The rapid spread of the acquired immunodeficiency syndrome (AIDS) epidemic has driven efforts to develop therapeutic agents targeting the human immunodeficiency virus (HIV) protease, an enzyme essential for viral maturation. Peptidomimetic inhibitors, while potent, suffer from low oral bioavailability, prompting the search for non-peptidic alternatives. Building on previous work identifying phenprocoumon as an HIV protease inhibitory template and developing first- (U-96988) and second-generation (U-103017) clinical candidates via structure-based design, we report a structure-activity relationship (SAR) study of sulfonamide-containing 5,6-dihydro-4-hydroxy-2-pyrones. A series of these compounds were synthesized, and their HIV protease inhibitory and antiviral activities were evaluated. Compound V, bearing a 1-methylimidazol-4-yl sulfonamide group, exhibited high potency (Ki = 1 nM, IC50 = 0.5 µM). Resolution of its four stereoisomers revealed that the 3RR,6R diastereomer (Vb) was the most active. Further optimization with substituted pyridyl sulfonamides yielded compounds VIb and VIIb, which showed exceptional activity (Ki < 10 pM, IC50 = 30 nM in HIV-1IIIB-infected H9 cells). Compound VIIb (U-140690) was selected as a candidate due to its high potency against HIV-1/2 proteases, selectivity over other aspartyl proteases (e.g., human pepsin, cathepsins D/E), efficacy against AZT-resistant clinical isolates, and moderate oral bioavailability (30% in rats). Preclinical and safety studies are underway to advance U-140690 as a third-generation HIV protease inhibitor for AIDS treatment.