The present article provides a personalized account of our recent work on the synthesis of substituted and fused five-membered heterocycles using various organosulfur building blocks, derived primarily through base-mediated condensation of active methylene compounds with (het)aryl/alkyl dithioesters, which have not been previously explored. The first part describes the ring-opening transformations of 4-[(methylthio)-(het)aryl)-methylene]-2-phenyl -5-oxazolones, leading to the synthesis of functionalized oxazoles, thiazoles, and bisoxazoles. The second part focuses on the synthesis of substituted benzothiophenes, indoles, and benzofurans, as well as their hetero-fused analogs. These compounds are synthesized via transition metal catalyzed intramolecular Cheteroatom (CS, CN, CO) bond formation (via cross-coupling or CH bond functionalization) of various reactive organosulfur intermediates, derived from base-mediated condensation of 2-bromo(het)arylacetonitriles, acetates, or desoxybenzoins or the corresponding 2-unsubstituted precursors. The third part highlights the synthetic applications of a new class of previously unexplored organosulfur building blocks, namely, unsymmetrically substituted 1,3-bis(het)aryl-1,3-monothioketones, derived via base-mediated condensation of ketones with (het)aryl/alkyl dithioesters, for regioselective synthesis of substituted pyrazoles, isoxazoles, thiophenes, imidazoles, and benzothiophenes. 1. Introduction 2. 4-[(Methylthio)-het(aryl)-methylene]-2-phenyl/2-(2-thienyl)-5-loxazolones: Synthesis of Oxazoles, Thiazoles, and Bisoxazoles 3. Synthesis of Benzothiophenes, Indoles, and Benzofurans via Transition Metal-Catalyzed Intramolecular C-Heteroatom Bond Formation 4. 1,3-Bis(het)arylmonothio-1,3-diketones: Versatile Intermediates for Regioselective Synthesis of Five-Membered Heterocycles 5. Conclusion