Many, if not most, naturally occurring or biologically active organic compounds contain chiral centers. These centers play a crucial role in determining the biological properties of a molecule, as the interaction of chiral compounds with enzymes, receptors, and other biological systems is often highly stereospecific. As a result, the ability to selectively generate chiral centers with precise geometries is a cornerstone of modern synthetic organic chemistry. Asymmetric synthesis enables chemists to construct these chiral centers efficiently, minimizing the formation of unwanted stereoisomers. This field has evolved into a sophisticated area of study, incorporating principles of stereoelectronics, reaction dynamics, and molecular recognition. From pharmaceutical development to the synthesis of agrochemicals and materials, asymmetric synthesis underpins the creation of molecules essential to medicine, industry, and research. It represents not only a technical challenge but also an opportunity to explore fundamental aspects of chemical reactivity and stereoselectivity.

Lets introduce some terms.

Stereoselective Reaction

A stereoselective reaction is one in which one stereoisomer is formed preferentially over others. This can be further categorized as diastereoselective or enantioselective, depending on whether the reaction selectively produces a diastereomer or an enantiomer.

Stereospecific Reaction

A stereospecific reaction is one in which two (or more) stereoisomers of the starting material produce distinct stereoisomers of the product. Each starting material stereoisomer gives rise to a specific product stereoisomer. These reactions can be further categorized as diastereospecific (though this term is less commonly used) or enantiospecific, depending on whether the specificity involves diastereomers or enantiomers. Importantly, all stereospecific reactions are inherently stereoselective, as stereospecificity is a more stringent criterion. A classic example of a stereospecific reaction is the aldol condensation. In this reaction, lithium or other Z-enolates preferentially form syn diastereomers of the product, while E-enolates yield anti-diastereomers.

Central Chirality

An atom, typically a carbon, bonded to four different groups in a way that it lacks a plane of symmetry, resulting in non-superimposable mirror images (enantiomers). It is a key structural feature that gives rise to molecular chirality. A chiral center is sometimes referred to as an asymmetric center or a stereogenic center, as it generates stereoisomerism by allowing the existence of two distinct spatial arrangements of its substituents. While the terms are often used interchangeably, it is worth noting that "stereogenic center" is broader, encompassing atoms that create any form of stereoisomerism, including diastereomers, whereas "chiral center" specifically refers to those that produce chirality and enantiomerism.

Axial Chirality

Chirality due to restricted rotation about four groups substituted on an axis, with the partners in each pair having to be different. The most common examples are in allenes and in
biaryls (with sizeable ortho substituents).

Helical chirality

refers to the type of molecular chirality arising from a helical or screw-like arrangement of atoms or groups in a molecule, which lacks a plane of symmetry and cannot be superimposed on its mirror image. This type of chirality is commonly observed in molecules with twisted, spiral, or helical structures, such as helicenes, certain polymers, and DNA

Helicene

Strategies for Asymetric Induction

Substrate Control

This is a form of stereoselective synthesis where a chiral centre already present
in the molecule is responsible for selective generation of a new chiral centre. These are stereoselective
reaction, and examples from 59-331 include Felkin-Ahn additions and (Cram) Chelate model additions to
carbonyls.

S^*  →  P^*

Auxiliary control

S+A^*  →  P^*-A^* → A^* + P^*

Reagent control

S + R^*  →  P^*

S^* + R^*  →  P^*  Double Stereodifferentiation

Catalyst control

S → P^*     (Cat* or Cat/L*)