Stereotopic Molecular Faces

We now examine, in detail, aspects of vectoselectivity at the eleven fundamental stereotopic molecular faces. 

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In order to discuss facioselectivity concisely, cate needs to classify molecular faces (half-spaces). - In 1975, we categorized time-resolved/time-averaged planar stereotopic molecular faces into homotopic, enantiotopic, and diastereotopic classes." ... 

Figure 12.1. Complete Classification of Stereotopic Molecular Faces...

To clarify selectivity at faces of planar molecular fragments, or facioselectivity, we presented a complete classification of all eleven types of stereotopic molecular faces. W e defined the different modes of facioselectivity viz. facioaselectivity, faciononselectivity and stereofacioselectivity at each type of molecular face. We also discussed difacioselectivity for conjunctive processes involving the interactions of two molecular faces. 

The complete description of processes taking place at stereotopic molecular faces involves three fundamental aspects - facioselectivity, vectoselectivity and anguloselectivity. In order to evaluate the interrationships of the underlying three concepts above, we now consider (1,2)-ligogenic processes for all eleven types of molecular stereofaces. 

The term half-space concisely represents the region in space, either above or below a flat molecular moiety. Stereotopic or paired half-spaces are separated by a surface that may or may not be planar. The alternative term molecular face is more descriptive of a given molecular structure. Two stereotopic molecular faces are separated by a surface that is assumed to coincide with the plane of a finite, flat molecular framework. We use molecular face, in preference to half-space, as it is intimately linked to the term facioselectivity. Ring systems in carbohydrates, heterocyclic bases, steroids, porphyrins (see refs. 129(a), 129(b) above) are also said to have molecular faces, albeit in a generalized sense. 

At the end of Volume 2, we append a generalized system for assigning specific stereodescriptors to stereotopic/paired polycentric planar molecular faces (half-spaces) (Addendum C), and a designation of paired stereotopic molecular faces and stereotopic ligands (at tetrahedral and trigonal carbon atoms) (Addendum D). 

Finally, in Chapter 18 we present an alternative, universal stereochemical classification of chemical transformations based on (a) overall loss, (b) no loss/gain, and (c) overall gain of chirotopic atoms we label these chirotopoprocesses as chirotopolysis, chirotopomutation and chirotopogenesis, respectively. Further subclassification is carried out using the dual criteria of rotativity (expected optical activity) and stereoselectivity (preferential formation of one stereoisomer over another). We also introduce and define the novel concepts of chiroselectivity and chirospecificity. Finally, the merits of the classification of chirotopoprocesses are discussed, and the stereotopoprocesses and chirotopoprocesses are correlated in relation to the stereotopic molecular faces. 

XIV. Stereotopoprocesses vs. Chirotopoprocesses in Relation to Stereotopic Molecular Faces. 159... 

Figure 15.2. Achiroastereogenic (o), Achirostereogenic (s), Chiroastereogenic (o ) and Chirostereogenic (s ) Atoms in Stereotopic Molecular Faces...

In the previous chapter, we established the relationships between stereotopoprocesses, on the one hand, and stereotopic molecular faces, on the other. We now proceed to show, similarly, the relationships between chirotopoprocesses and each of the stereotopic molecular faces. Tables... 

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