Diastereomer, definition

Since the two reactants cis and trans butenes are stereoisomers, being diastereomers, the product from cis is a meso compound and that from the trans give a pair of enantiomers, by definition both the reactions are stereospecific. 

On the other hand, selective, usually applied to a synthesis, means that of all the possible isomers only one isomer is obtained. However, if the reaction product was/is a mixture of isomers one could speak then of the "degree of selectivity". Since usually one of the isomers will be the predominant isomer, we may say that the reaction (or the synthesis) is selective with respect to this particular isomer. As in the case of "specificity", we may refer to "regioselectivity" or to "stereoselectivity" (either diastereoselectivity or enantioselectivity) and may say, for instance, that a synthesis is 80% diastereoselective. According to the most updated terminology "diastereomers" are all the "stereoisomers" that are not "enantiomers", so geometrical isomers are also included in such a definition. 

Definitions It is important to define precisely the stereochemical terms that will be employed in this discussion. The term racemization has often been used loosely by chemists to describe any situation in which a mixture of enantiomers or diastereomers is produced as a result of an amide-bond-forming reaction, without regard to the ratio of stereoisomers formed. For the purposes of this discussion though, the term racemization will be used to describe the situation leading toward the formation of an exact 1 1 mixture of stereoisomers. Racemization, therefore, is a process that occurs to a collection of molecules, and can happen to a single residue or to one residue in a peptide sequence (Scheme 1). This is a macroscopic event, as the result is detected subsequent to the amide bond formation. 

As stated earlier, this technique relies essentially on the formation of covalently bonded diastereomers derived from a pair of chiral analytes (SAs pair of enantiomers) which have been converted to a pair of diastereomers using an optically pure chiral derivatizing agent (CDA) which, in this case, serves as a chiral selector (SO). In this context the definition of " optical purity of the CDA is critical (see Section 3.2.1.2.) and has to be evaluated by complementary methods. 

Enantioselective (enzymatic) cyclization of 1 could lead to either enantiomer 2 a or 3 a, depending on which face of the internal double bond is attacked at C-6 by the cation derived from the allylic pyrophosphate unit. Re- or Si-face cyclization (see the detailed discussion in Section 4.3.4.1.2.1., p 442 of 1 b would, however, lead to the diastereomers 2b and 3b. respectively. A thorough analysis of the NMR spectrum of the cyclization product of 1 b definitely showed that 2b was formed thus proving the absolute configuration of (-)-fl-irans-berga-motene to be 2a185. 

Using a chiral column, coated with a definite modified cyclodextrin as the chiral stationary phase, the elution orders of furanoid and pyranoid linalool oxides are not comparable [11, 12]. Consistently, the chromatographic behaviour of diastereomers and/or enantiomers on modified cyclodextrins is not predictable (Fig. 17.1, Table 17.1). Even by changing the non-chiral polysiloxane part of the chiral stationary phase used, the order of elution may significantly be changed [13]. The reliable assignment of the elution order in enantio-cGC implies the coinjection of structurally well defined references [11-13]. 

Although the term prochirality is frequently used, especially by biochemists, it suffers from a limitation which arises from a corresponding limitation in the definition of chirality. Molecules may display purely stereochemical differences without being chiral cis-tram isomers of olefins and certain achiral cis-trans isomers of cyclanes are examples. Thus (Fig. 2) (Z)- and ( )-1,2-dichloroethylene (4, 5) are achiral diastereomers, as are cis- and /rtww-1,3-dibromocyclobutanes (6, 7) being devoid of chirality these compounds have no chiral centers (or other chiral elements). Thus it is inappropriate to associate stereoisomerism with the occurrence of chiral... 

The two categories into which stereoisomers can be placed are absolutely distinctive in definition. Two stereoisomers that are nonsuperimposable mirror images are called enantiomers. The only other category is defined negatively. Stereoisomers that are not enantiomers are called diastereomers. 

Here we are primarily concerned with the fact that this ortho -adduct may occur in the form of two diastereomers. The diastereomers are formed as a 57 43 cis/trans-mixtme in the absence of A1C13, but a 95 5 cis/trans-mixture is obtained in the presence of A1C13. In the latter case, thus, one is dealing with a Diels-Alder reaction that exhibits a substantial simple diastereoselectivity (see Section 11.1.3 for a definition of the term). Here, the simple diastereoselectivity is due to kinetic rather than thermodynamic control, since the preferentially formed ds-disubstituted cyclohexene is less stable than its irans-isomer. 

The term meso (Greek, middle ) was used to describe an achiral member of a set of diastereomers, some of which are chiral. The optically inactive isomer seemed to be in the middle between the dextrorotatory and levorotatory isomers. The definition just given ( an achiral compound with chirality centers ) is nearly as complete, and more easily applied, especially when you remember that chirality centers are usually asymmetric carbon atoms. 

---