Diversity-Oriented Synthesis (DOS) principles crystallized around 1962–1979 as researchers pursued maximum structural diversity by leveraging cycloaddition and rearrangement strategies to rapidly assemble heterocyclic scaffolds and natural product–like cores. A general reagent toolbox enabling diverse bond formations—including nucleophilic acylation, oxidation-reduction condensations, and activated alkynes—supported broad exploration of chemical space. Emphasis on stereoselectivity and mechanistic insight in both intermolecular and intramolecular cycloadditions unified methodological development into scalable routes for diverse skeletal frameworks. Historical Significance: Foundational transformations such as 1,3-dipolar cycloadditions introduced versatile five-membered heterocycles; the Ene reaction broadened C=C functionalization, enabling varied skeletons and stereocontrolled products. Intramolecular [4+2] and [3+2] cycloadditions provided rapid access to crowded polycyclic frameworks in a single sequence, informing library-style diversification. The nickel-phosphine–catalyzed cross-coupling of Grignard reagents with halides expanded modular diversification to biaryl and alkenyl motifs, foreshadowing cross-coupling frameworks central to later diversity-oriented strategies. Collectively, these breakthroughs established a mechanistic toolbox and a versatile repertoire of transformations that early DOS-inspired thinking would formalize into expansive exploration of chemical space.