Spiro compounds are organic compounds that contain at least two molecular rings with one common atom. The bicyclic compounds are the simplest spiro compounds having two rings connected with defining a single common atom.
Spiro compounds are of two types based on the non-carbon atom present. If a spiro compound has all carbon atoms, then it is called carbocyclic (all carbon). Alternatively, if it has one or more non-carbon atoms, then it is heterocyclic. The common atom that connects rings in spiro compound is called spiro atom, which is generally a quaternary carbon.
As already mentioned, a bicyclic molecule has two joined rings. They are found in many important biological molecules. In a bicyclic molecule, the atoms at which two rings are joined are called bridgehead carbons. A bicyclic compound can also be carbocyclic or heterocyclic. Also, the two rings can both be aliphatic or can be aromatic or a mixture of aliphatic and aromatic compounds. Natural bicyclic compounds examples are α-thujene and camphor.
Types of Bicyclic Compounds
Based on the ring junction there are three types of bicyclic compounds.
Spirocyclic Compounds: Here, the two rings are connected with a single atom called spiro atom. Example for such a compound is Spiropyran which has photochromic properties and hence has applications in medical and technological field.
Structure of Spirocyclic Compounds
Fused/Condensed Bicyclic Compounds: In this type, two rings share two adjacent atoms which means they share one covalent bond. When the bridgehead carbon atoms are adjacent it is called fused.
Bridged Bicyclic Compounds: Here, two rings share two or more atoms and a bridge is formed between two bridgehead atoms since they are not adjacent. Example is Camphor.
Structure of Fused and Bridged Bicyclic Compounds
Naming of Bicyclo Compound and Spiro Compounds
Bicyclic Compounds Nomenclature
Naming of bicyclic molecules is done based on the IUPAC nomenclature. The procedure for bicyclic compound nomenclature is as follows:
The total number of atoms in all rings together gives the root name of the compound and sometimes. It is followed by a suffix denoting the functional group with the highest priority.
The carbon chain numbering always begins at one bridgehead atom (i.e., where the rings meet), and then it continues along the carbon chain along the longest path, to the next bridgehead atom. Numbering is then continued along the second longest path and so on.
The prefix ‘bicyclo’ is attached to Fused and bridged bicyclic and the prefix ‘spiro’ is given to spirocyclic compounds.
A pair of square brackets with digits that denote the number of carbon atoms between each of the bridgehead atoms is added in between the prefix and the suffix. These digits are written in descending order and are separated by periods.
For example, the norbornane carbon frame has a total of 7 atoms, so the root name is heptane. This molecule has two paths of 2 carbon atoms and a third path of 1 carbon atom between the two bridgehead carbons, so the brackets are filled in descending order: $[2.2.1]$. The prefix bicyclo gives the total name of the compound as bicyclo $[2.2.1]$ heptane.
Structure of Norborane (Bicyclo $[2.2.1]$ Heptane)
The fused bicyclic compounds have the same method of nomenclature as stated above. Here the exception is the third path between the two bridgehead atoms now has zero atoms. Hence, they have a ‘0’ included in the brackets. Example, decalin or bicyclo $[4.4.0]$ decane.
Structure of Decalin or Bicyclo $[4.4.0]$ Decane
The heterocyclic molecule DABCO (1,4-diazabicyclo $[2.2.2]$ octane) or triethylenediamine or TEDA has formula N2( C2H4)3. It has 8 atoms in its bridged structure and thus the root name is octane. Here Nitrogen is the two bridgehead atoms instead of carbon atoms. Hence, the name has an additional prefix 1, 4-diaza and the whole name becomes 1, 4-diazabicyclo $[2.2.2]$ octane.
Structure of 1, 4-Diazabicyclo $[2.2.2]$ Octane
Spiro Compounds Nomenclature
Spiro Compounds have a prefix spiro which represent two rings with a spiro junction.
Prefix is followed by a square bracket comprising the number of atoms in the smaller ring then the number of atoms in the larger ring, separated by a period, excluding the spiroatom.
Position-numbering is done by beginning with an atom of the smaller ring which is near to the spiroatom around the atoms of that ring, then the spiroatom itself, then around the atoms of the larger ring.
The compound shown below is called 1-bromo-3-chlorospiro $[4.5]$ decan-7-ol,
Structure of 1-bromo-3-chlorospiro $[4.5]$ decan-7-ol
Spiro and Bicyclic Compounds Examples
Spiro compounds like glycerin acetal cyclic ketones are used as plasticizers. Spiro compounds also find applications in perfumery, intermediate products in the synthesis of epoxy, in manufacture of medicine. Spiro compounds like Spiropyran are used as photochromic materials. Leuco dyes are spiro forms of lactones and oxazines as they display chromism (It means they change from their colorless form to color form reversibly in solution)
Bicyclic compounds having bridgehead phosphorus have found applications in biological and industrial applications. They are used as pesticides, antibiotics, chemotherapy agents, weed control agents, flameproofing agents, plastics stabilizers, lubricants, surfactants, metal extractants, water softeners etc.
Organic compounds that have two or more rings joined at common atoms are called polycyclic compounds. When two rings join, they form bicyclic compounds. In a bicyclic compound, the atoms at which two rings are joined are known as bridgehead carbons. If the bridgehead carbons of a bicyclic compound are adjacent, then it is called a fused bicyclic compound. Conversely, when the bridgehead carbons are not nearby, the compound is known as a bridged bicyclic compound.
In spirocyclic compounds, the two rings are connected with a single atom called a spiro atom. They find different applications in biological, medicinal and industrial fields.