Prochirality assignment

Fig. 41. Prochirality assignment of C-methyl groups in isopropyl O-methylmandeiate...

Preeclampsia, Viagra and, 164 Prelog, Vladimir, 181 Prepolymer, epoxy resins and, 673 Priestley, Joseph, 245 Primary alcohol, 600 Primary amine, 916 Primary carbon. 84 Primary hydrogen, 85 Primary structure (protein), 1038 Primer strand (DNA), 1108 pro-R prochiralitv center, 316 pro-S prochirality center, 316 Problems, how to work, 27 Procaine, structure of, 32 Prochirality, 315-317 assignment of, 315-316 naturally occurring molecules and, 316-317... 

More than a decade of experience on Sharpless asymmetric epoxidation has confirmed that the method allows a great structural diversity in allylic alcohols and no exceptions to the face-selectivity rules shown in Fig. 10.1 have been reported to date. The scheme can be used with absolute confidence to predict and assign absolute configurations to the epoxides obtained from prochiral allylic alcohols. However, when allylic alcohols have chiral substituents at C(l), C(2) and/or C(3), the assignment of stereochemistry to the newly introduced epoxide group must be done with considerably more care. 

The enantioselectivity principles portrayed in Figure 6A.1 have been followed without exception in all epoxidations of prochiral allylic alcohols reported to date, and one may use these principles to assign absolute configurations to the epoxy alcohols prepared by the method. On the other hand, epoxidation of allylic alcohols with chiral substituents at C-l, C-2, and/or C-3 does not always follow these principles, and assignment of absolute configuration to the products must be made with care. Even in the latter cases, reliable assignments usually can be made if the outcome (diastereomeric ratio) of epoxidation with both the (+) and (-)-tartrate ester ligands is compared. 

In this Section we shall use the ideas of prochirality in assignment of stereochemical configuration S8) (usually relative — especially meso vs. dl — rather than absolute configuration) and we shall also discuss assignment of prochirality symbol (i.e. recognition of which group is pro-R and which pro-S at a prochiral center). (Recognition of prochiral faces as Re or Si is usually obvious from the stereochemistry of the addition products thereto and will not be discussed here examples are found in Section 5.2). 

High resolution NMR studies, in combination with molecular mechanics, were performed to unequivocally assign the prochiral P-protons for the phenylalanine sidechain and to define the shape of the sweet receptor [49], The previous assignment [48] of the NMR signals for the P-protons of phenylalanine was shown to be in error. 

The original assignment of the absolute configuration (3S) for products 12 based on a comparison of the optical rotation value with a reported one for the phenyl derivative [27] must be considered uncertain. It would contradict the preferred attack of the nucleophile from the unshielded back side and the stereoselectivity observed in the reaction of A -galactosyl imines 7 with prochiral bis-silyl ketene acetals [28]. N-Galactosyl P-amino acids 13 are produced from bis-silylketene acetals in high yields and excellent diastereoselectivity (Scheme 10). The configuration at the P-position is R. 

Just as stereogenic centres can be described as ft or S, it is possible to assign labels to the enantiotopic groups at prochiral tetrahedral carbon atoms or the enantiotopic faces of prochiral trigonal carbon atoms. The basis of the system is the usual ft,S system for stereogenic centres, but j ro-ftand pro-S are used for groups and fte and S/for faces. 

When a compound exhibits 2D chirality it has two distinct, Prochiral , faces. These are designated Re or Si and are assigned by the familiar Cahn, Ingold, Prelog Sequence Rules . 

If you perform the replacement test to assign pro-R/pro-S prochirality, you will see that the right arm of citrate is pro-R and the product pictured on the right is formed. 

---