Asymmetric centers addition reactions forming products

The aldol addition reaction, and the related crotyl metal additions (section 5.1), have figured prominently in the total synthesis of a number of complex natural products (reviews [48,140-142]). Figure 5.8 illustrates those mentioned in the preceding discussion, along with others selected from the recent literature, with the stereocenters formed by stereoselective aldol addition indicated ( ). For the Prelog-Djerassi lactone and ionomycin, recall (Figure 3.8) that most of the other stereo-centers were formed by asymmetric enolate alkylation. 

This gives control over the stereochemistry of the product, because 8.14 can be resolved, thanks to the presence of the optically active group (R ) on the Cp ring, in which case carrying out the addition with one enantiomer of the metal complex means that the new asymmetric center on the ligand is formed with very high asymmetric induction. This reaction therefore constitutes a chiral synthesis of the alkenes shown. 

Addition Reactions that Form a Product with One Asymmetric Center... 

Addition Reactions that Form Products with Two Asymmetric Centers... 

Addition Reactions that Form a Carbocation Intermediate When an addition reaction that results in a product with two new asymmetric centers forms a carbocation intermediate, four stereoisomers are formed. 

Each of the two asymmetric centers in the product of the following reaction is bonded to the same four substituents. Therefore, syn addition forms a meso compound. 

The Diels-Alder reaction is a syn addition reaction. One face of the diene adds to one face of the dienophile. Therefore, if the substituents in the dienophile are cis, then they will be cis in the product if the substituents in the dienophile are trans, then they will be trans in the product. Because each of the following reactions forms a product with two new asymmetric centers, a pair of enantiomers is formed (Section 6.15). The stereochemistry of the reaction will be discussed in greater detail in Section 28.4. 

We can now understand why an SnI reaction of an alkyl halide in which the leaving group is attached to an asymmetric center forms two stereoisomers addition of the nucleophile to one side of the planar carbocation intermediate forms one stereoisomer, and addition to the other side produces the other stereoisomer. Thus, the product is a pair of enantiomers. 

If a reactant that does not have an asymmetric center undergoes a radical substitution or a radical addition reaction that forms a product with an asymmetric center, then a racemic mixture will be obtained. 

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