AbstractThe power of cycloadditions for ring formation derives from the potential for chemo‐, regio‐, diastereo‐, and enantioselectivity. Trimethylenemethane (TMM) and its equivalents offer the possibility of extending such benefits to the synthesis of five‐membered rings. Three types of conjunctive reagents appear to be promising candidates: (1) selected 4‐alky‐lidene‐4,5‐dihydro‐3H‐pyrazoles, (2) 2‐[(trimethylsilyl)methyl]allyl esters and halides, and (3) alkylidenecyclopropanes. Thermal reactions, especially intramolecular ones, effect the cycloaddition of the pyrazoles to olefins–especially electron‐poor olefins. The latter two precursors require a catalyst, notably a palladium complex, to unlock the synthetic potential. With the 2‐[(trimethylsilyl)methyl]allyl esters, a general chemo‐, regio‐, and diastereoselective cycloaddition with typical Diels‐Alder dienophiles forms methylenecyclopentanes in both inter‐ and intramolecular fashion. A tin analogue allows extension of this cycloaddition to aldehydes, and, in a related sequence, to ketones and imines to form 3‐methylenete‐trahydrofurans and 3‐methylenepyrrolidines with excellent diastereoselectivity. The 2‐[(tri‐methylsilyl)methyl]allyl esters also serve to effect net methylenecyclopentane annulation to cyclic enols. Such adducts can further undergo three‐carbon intercalation or combined ring contraction‐spiroannulation simply by the use of either a nucleophilic or electrophilic trigger. Alkylidenecyclopropanes undergo cycloadditions to both electron‐rich and electron‐poor olefins when stimulated by metal catalysts, especially palladium. A complementary regiochemistry to the approach based upon silicon conjunctive reagents is observed. The availability of these cycloadditions provides