09-18-2021-0806 - Spiro compounds

 Spiro compounds have at least two molecular rings with only one common atom. The simplest spiro compounds are bicyclic(having just two rings), or have a bicyclic portion as part of the larger ring system, in either case with the two rings connected through the defining single common atom.^{[3]: SP-0  [4]: 653, 839} The one common atom connecting the participating rings distinguishes spiro compounds from other bicyclics: from isolated ring compounds like biphenyl that have no connecting atoms, from fused ring compounds like decalin having two rings linked by two adjacent atoms, and from bridged ring compounds like norbornane with two rings linked by two non-adjacent atoms.^{[5][4]: 653ff  : 839ff}

A fully carbocyclic spiro compound.Spiropentadiene, an esoteric hydrocarbon, in a representation where all carbons and hydrogens are shown explicitly. The two cyclopropene rings are each planar, but are perpendicular to each other.

Spiro compounds may be fully carbocyclic (all carbon) or heterocyclic (having one or more non-carbon atom). One common type of spiro compound encountered in educational settings is a heterocyclic one— the acetal formed by reaction of a diol with a cyclic ketone. The common atom that connects the two (or sometimes three) rings is called the spiro atom;^[3]: SP-0 in carbocyclic spiro compounds like spiro[5.5]undecane (see image at right), the spiro-atom is a quaternary carbon, and as the -ane ending implies, these are the types of molecules to which the name spirane was first applied (though it is now used general of all spiro compounds).^{[6]: 1138ff} Likewise, a tetravalent neutral silicon or positively charged quaternary nitrogen atom (ammonium cation) can be the spiro center in these compounds, and many of these have been prepared and described.^{[6]: 1139f [citation needed]} The 2-3 rings being joined are most often different in nature, though they, on occasion, be identical [e.g., spiro[5.5]undecane, just shown, and spiropentadiene, at right]. Although sketches of organic structures makes spiro compounds appear planar, they are not; for instance, a spiro compound with a pair of three-membered cyclopropene rings connected in spiro fashion (image below) has been given the popular misnomer of being a bow tie structure, when it is not flat or planar like a bow tie. This can be stated another way, saying that the best-fit planes to each ring are often perpendicular or are otherwise non-coplanar to one another.^{[4]: 319f.846f}

Spiro compounds are present throughout the natural world, some cases of which have been exploited to provide tool compounds for biomedical study and to serve as scaffolds for the design of therapeutic agents with novel shapes.^{[citation needed]} As well, the spiro motif is present in various practical compound types (such as dyes), as well as in a wide variety of oligo- and polymeric materials designs, for the unique shapes and properties the spiro center imparts, e.g., in the design of electronically active materials in particular.^{[citation needed]} In both cases, the presence of the spiro center, often with four distinct groups attached, and with its unique aspects of chirality, adds unique challenges to the chemical synthesis of each compound type.^{[citation needed]}

A fully carbo-cyclic spiro compound.Spiro[5.5]undecane,^[2]again shown in line-angle representation and in apparent planarity, masking the actual most populated chair conformation of each ring, and their actual orthogonalorientation to one another.

^{https://en.wikipedia.org/wiki/Spiro_compound}