Fenestrane
A fenestrane in organic chemistry is a type of chemical compound with a central quaternary carbon atom which serves as a common vertex for four fused carbocycles. They can be regarded as spiro compounds twice over. Because of their inherent strain and instability, fenestranes are of theoretical interest to chemists. The name—proposed in 1972 by Vlasios Georgian and Martin Saltzman—is derived from the Latin word for window, fenestra. Georgian had intended that "fenestrane" solely referred to [4.4.4.4]fenestrane, whose skeletal structure looks like windows, and Kenneth B. Wiberg called that specific structure "windowpane". The term fenestrane has since become generalized to refer to the whole class of molecules that have various other ring-sizes. Georgian recommended rosettane for the class, based on the structural appearance as a rosette of flowers.
Structures within this class of chemicals can be named according to the number of atoms in each ring in addition to the systematic nomenclature of IUPAC naming rules. The smallest member of the family, consisting of four fused cyclopropane rings, is [3.3.3.3]fenestrane, which has systematic name tetracyclo[2.1.0.01,3.02,5]pentane and is also called pyramidane. The next symmetric member, [4.4.4.4]fenestrane, has four cyclobutane rings fused, and has systematic name tetracyclo[3.3.1.03,9.07,9]nonane. The rings need not all be the same size as each other, so [4.4.4.5]fenestrane has three cyclobutane rings and one cyclopentane ring. Other structural modifications vary the name as usual in systematic nomenclature, so a [4.6.4.6]fenestradiene has two cyclobutane rings and two cyclohexane rings in an alternating pattern and two alkene units in the ring structure.
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