matrix below 50 K; oligomerizes or polymerizes at higher temperatures. The strain energy released by breaking the axial bond was estimated as 75 kcal/mol. [3.1.1]Propellane, C7H10, CAS number (Gassman, 1980; Szeimies, 1992; Anderson, 2022). Several synthetic procedures are established making it accessible on scales useful for synthesis to derive bicyclo[3.1.1]heptanes which are proposed as isosteres for meta-substituted arene systems. [3.2.1]Propellane or tricyclo[3.2.1.01,5]octane, C8H12, CAS number (K. Wiberg and G. Burgmaier, 1969). Isolable. Has inverted tetrahedral geometry at the shared carbons. Estimated strain energy of 60 kcal/mol. Remarkably resistant to thermolysis; polymerizes in diphenyl ether solution with halflife of about 20 hours at 195 °C. It reacts spontaneously with oxygen at room temperature to give a copolymer with –O–O– bridges. [4.1.1]Propellane, C8H12, CAS number (D. Hamon, V. Trennery, 1981) Isolable. [2.2.2]Propellane or tricyclo[2.2.2.01,4]octane, C8H12, CAS number (P. Eaton and G. Temme, 1973). This propellane is unstable, too, due to the three cyclobutane-like rings and the highly distorted bond angles (three of them nearly 90°, the other three nearly 120°) at the axial carbons. Its strain energy is estimated to be 93 kcal/mol (390 kJ/mol). [3.3.3]Propellane, C11H18, CAS number . It is a stable solid that melts at 130 °C. It was synthesized in 1978 by Robert W. Weber and James M. Cook who developed a general synthetic route for all [n, 3, 3]propellanes, with n ≥ 3: [4.3.3]Propellane, C12H20, CAS number (R. Weber and J. Cook, 1978). A stable solid that melts at 100–101 °C. [6.3.3]Propellane, C14H24, CAS number (R. Weber and J. Cook, 1978). An oily liquid that boils at 275–277 °C. [10.3.3]Propellane, C18H32, CAS number (S. Yang and J. Cook, 1976). A stable solid that sublimes at 33–34 °C. Propellane derivatives 1,3-Dehydroadamantane, C10H14 (Pincock and Torupka, 1969). This compound is formally derived from adamantane by removing two hydrogens and adding an internal bond. It can be viewed as [3.3.1]propellane (whose axis would be the new bond), with an extra methylene bridge between its two larger "propeller blades". It is unstable and reactive and can be polymerized. 2,4-Methano-2,4-dehydroadamantane: C11H14 (Majerski, 1980) It can be interpreted as a adamantyl-caged [3.1.1]propellane derivative. A general reactivity profile was investigated showing similarities to the omniphilic behaviour of [1.1.1]propellane. Propellane natural products Dichrocephone B, a sesquiterpenoid with a [3.3.3]propellane core was isolated in 2008 from Dichrocephala benthamii. It was first synthesized in 2018 using a general strategy for the synthesis of carbocyclic propellanes from 1,3-cycloalkanediones.