Meso Chemical Symmetry, Definition by Molecular Modelling and Application to Inositols

Plane of symmetry is a physical chemical phenomenon and an instrument. Geometrical plane of symmetry indicates two identical achiral halves. In a slightly modified form, it is applied also in chemistry. In geometry it cuts a point or a line, while in chemistry it cuts a cloud of electrons (a bond) or one or more atoms. The second type, chemical plane of symmetry, named also mirror plane of symmetry, indicates two enantiomeric chiral halves uniformly linked with each other or uniformly linked on a suitable matrix. Compounds characterized by a mirror plane of symmetry have been designated meso. Meso compounds designated in this way by Cahn-Ingold-Prelog rules do not change the latter assertion: one can assert that molecules of this group are formed of two imaginary enantiomeric halves separated by an imaginary mirror plane of symmetry. From the definition of meso compounds one can infer, by molecular modelling, that alternative dimerization of the two enantiomeric halves, between them or on the initial matrix, would produce two chiral enantiomeric products. However, inositols, considered meso by numerous authors, present spectacular and unexpected surprises.