Absolute configuration priority rules

For compounds 7.42, the preferred substrate always had the 3D geometry shown here, which corresponded to an (R)- or -absolute configuration, depending on priorities assigned by the sequence rule, e.g., the (S,S)-enan-tiomer for X = CH2. The situation is not so clear-cut with compounds 7.43, with which contradictory results were obtained. 

Serine hydroxymethyl transferase catalyzes the decarboxylation reaction of a-amino-a-methylmalonic acid to give (J )-a-aminopropionic acid with retention of configuration [1]. The reaction of methylmalonyl-CoA catalyzed by malonyl-coenzyme A decarboxylase also proceeds with perfect retention of configuration, but the notation of the absolute configuration is reversed in accordance with the CIP-priority rule [2]. Of course, water is a good proton source and, if it comes in contact with these reactants, the product of decarboxylation should be a one-to-one mixture of the two enantiomers. Thus, the stereoselectivity of the reaction indicates that the reaction environment is highly hydro-phobic, so that no free water molecule attacks the intermediate. Even if some water molecules are present in the active site of the enzyme, they are entirely under the control of the enzyme. If this type of reaction can be realized using synthetic substrates, a new method will be developed for the preparation of optically active carboxylic acids that have a chiral center at the a-position. 

Using these rules we can assign the absolute configurations for adrenaline structures A and B. Placing the group of lowest priority behind the paper, in this case H. 

Fig. 9 Top-view on enantiomers and diastereoisomers of adsorbed glycine and alanine. The amino nitrogen and the two oxygen atoms form bonds to the surface and create a chiral footprint configuration. The surface-induced absolute configuration, as indexed with a superscripted i, is specified using CIP-rules and by giving an atom or group closer to the surface a higher priority...

In the determination of the absolute configuration of cysteine it is important to note that the sulfur atom has a higher priority than the oxygen atom. The priority order of the atoms and groups directly attached to the chirality centre by application of the CIP rules is N > C(S,H,H)) > C(0,0,(0)) > H. Thus L-cysteine has the R configuration in contrast to other L-amino acids which have the S configuration. 

In order to describe the absolute configuration of chiral allenes, the Cahn-Ingold-Prelog (CIP) rules are first used to ascertain the priorities at the each terminal carbon, as is done for alkenes at this point the similarity with alkenes ends. The method will be demonstrated with the trisubstituted allene 14a ... 

The absolute configuration system stems from the Cahn-Ingold-Prelog priority rules, which allow a precise description of a stereocenter without using any reference compound. In fact the basis is now the atomic number of the stereocenter substituents. 

R- and S- notation use the CIP priority rules for the assignment of the absolute configuration around a stereocenter. 

CH2CH2CH2OH), however, there is an oxygen. The CH(Me)CH2CH2CH2CH3 (C CH) chain does not contain a heteroatom. The rules dictate determination of priority at the first point of difference, not the second. The presence of the oxygen is, therefore, irrelevant and the C CH chain will have priority over the CCHH chain. The absolute configuration of 48 is S). 

For (-) castoramine (56) Rule 6 allows a priority determination that assigns the stereogenic nitrogen center as (R). Just as a nitrogen atom can be stereogenic, a phosphorus atom in a phosphine such as 53 can be considered chiral since the rate of inversion is negligible at temperatures up to = 130°C, as noted in Table 1.4. The lone pair electrons have the lowest priority by Rule 6 and the absolute configuration is (R). 

Determination of the absolute configuration for OC-6 type complex. Note that for the Pt complex on the left, the viewer should look from the iodide side before applying the priority rules. 

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