New synthetic methodologies, Ti(III)-promoted N-O bond reductions and Pd(0)/InI-mediated allylations, are developed with acylnitroso-derived hetero-Diels-Alder adducts to provide relevant syn-1,4-disubstituted cyclopentene intermediates. The carbocyclic scaffolds are further functionalized to afford biologically significant molecules, including carbocyclic uracil polyoxin C analogs, carbocyclic aminonucleosides and (-)-epi-4'-carbocyclic puromycin. Chapter two explores Pd(0)/InI-mediated allylations with acylnitroso-derived hetero-Diels-Alder adducts in the presence of diverse electrophiles. Eschenmoser's salt, 4-acetoxy-2-azetidinone, and formaldehyde (formed in situ from Eschenmoser's salt) serve as appropriate electrophiles for the indium(III)-mediated allylic additions. The resultant functionalized cyclopentenes are key intermediates in the syntheses of biologically significant molecules. In chapter three, titanocene monochloride (Cp2TiCl) is described as an alternative source of Ti(III) for the selective reductions of the N-O bonds of acylnitroso-derived hetero-Diels-Alder adducts, N-hydroxy carbamates and hydroxamic acids under mild conditions. Additionally, N-O bonds may be reduced with catalytic amounts of Cp2TiCl. The synthesis of carbocyclic polyoxin C analogs is discussed in chapter four. A ̢-lactam-derived carbocyclic scaffold is used as a key intermediate in the preparation of unprecedented carbocyclic nucleosides. An N-methylthio protecting group is required for the key transformation to the uracil moiety. In chapter five, hydroxymethyl(cyclopentenyl) derivatives are used as intermediates in the syntheses of carbocyclic aminonucleosides and (-)-epi-4'-carbocyclic puromycin. The cis-2',3'-aminoalcohol moiety is installed by employing an osmium-catalyzed tethered aminohydroxylation reaction to afford functionalized carbocyclic substrates with complete regio- and diastereocontrol. In conclusion, acylnitroso-derived hetero-Diels-Alder adducts are versatile substrates that may be elaborated to provide functionalized cyclopentene scaffolds. These carbocyclic platforms serve as key intermediates in the syntheses of biologically relevant molecules.