Bridged compounds

A bridged compound is a chemical compound, generally an organic compound, but also possibly inorganic, that has two or more rings (a ring system) that contains a bridge—a single atom or an unbranched chain of atoms (or even just a valence bond) that connect two 'bridgehead' atoms. The bridgehead atoms are defined as any atom that is not a hydrogen, and that is part of the skeletal framework of the molecule that is bonded to three or more other skeletal atoms.:VB-1 Clayden simplifies this description, saying that bridged bicyclic compounds 'are just what the name implies— a bridge of is thrown across from one side of the ring to the other,':839 examples of which are shown at left and right, and below. The presence of the bridge connecting the bridgehead atoms, which are most often two non-adjacent atoms, distinguishes bridged compounds from fused ring compounds that have two rings linked by two adjacent atoms, and from spiro compounds that have two rings linked by a single atom.:653ff :839ff A bridged compound is a chemical compound, generally an organic compound, but also possibly inorganic, that has two or more rings (a ring system) that contains a bridge—a single atom or an unbranched chain of atoms (or even just a valence bond) that connect two 'bridgehead' atoms. The bridgehead atoms are defined as any atom that is not a hydrogen, and that is part of the skeletal framework of the molecule that is bonded to three or more other skeletal atoms.:VB-1 Clayden simplifies this description, saying that bridged bicyclic compounds 'are just what the name implies— a bridge of is thrown across from one side of the ring to the other,':839 examples of which are shown at left and right, and below. The presence of the bridge connecting the bridgehead atoms, which are most often two non-adjacent atoms, distinguishes bridged compounds from fused ring compounds that have two rings linked by two adjacent atoms, and from spiro compounds that have two rings linked by a single atom.:653ff :839ff Bridged compounds may be fully carbocyclic (all carbon) or heterocyclic (having one or more non-carbon atoms). The principal and bridging rings are almost always different in nature, though they can be identical. Although sketches of organic structures may make bridged compounds appear planar, they are not; for instance, comparing the 3D ball-and-stick model of norbornane at left, with the image shown at right, makes clear the bridging ring ensures that the molecule is not planar. Due to geometric constraints imposed by the bridge, double bonds typically do not occur at bridgeheads. This is known as Bredt's rule, named after German chemist Julius Bredt. However, for larger ring systems, such geometric constraints are relaxed, allowing for double bonds at bridgeheads. Natural products that contain bridgehead double bonds (i.e. anti-Bredt double bonds) has been reviewed. The nomenclature of bridged compounds was established by the IUPAC based on the system introduced by von Baeyer. Some important concepts and definitions for bridged compounds are as follows. Examples include: norbornane, shown above, along with other alicyclic (purely hydrocarbon) examples in the Nomenclature section; adamantane and its amine analogs memantine (shown), amantadine (shown), and rimantadine (the latter two historic influenza drugs); the heteroatom bridgehead examples hexamine (shown) and 1,4- Diazabicyclooctane (DABCO, shown in two representations); morphan, an example having a heteroatom in a bridge (shown); biperiden; and methenamine.