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The concepts of syn and anti addition are used to characterize the different reactions of organic chemistry by reflecting the stereochemistry of the products in a reaction. The type of addition that occurs depends on multiple different factors of a reaction, and is defined by the final orientation of the substituents on the parent molecule .
When the single bond between the two centres is free to rotate, cis/trans descriptors become invalid. Two widely accepted prefixes used to distinguish diastereomers on sp³-hybridised bonds in an open-chain molecule are syn and anti. Masamune proposed the descriptors which work even if the groups are not attached to adjacent carbon atoms.
Anti-periplanar geometry will put a bonding orbital and an anti-bonding orbital approximately parallel to each other, or syn-periplanar. Figure 6 is another representation of 2-chloro-2,3-dimethylbutane (Figure 5), showing the C–H bonding orbital, σ C–H, and the C–Cl anti-bonding orbital, σ* C–Cl, syn-periplanar.
The disposition of the formed stereocenters is deemed syn or anti, depending if they are on the same or opposite sides of the main chain: Syn and anti products from an aldol (addition) reaction. The principal factor determining an aldol reaction's stereoselectivity is the enolizing metal counterion.
This image of a simple structural formula is ineligible for copyright and therefore in the public domain, because it consists entirely of information that is common property and contains no original authorship.
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For example, butane has three conformers relating to its two methyl (CH 3) groups: two gauche conformers, which have the methyls ±60° apart and are enantiomeric, and an anti conformer, where the four carbon centres are coplanar and the substituents are 180° apart (refer to free energy diagram of butane).
When a α-hydroxy aldehyde is used as a substrate in the synthesis of β-amino alcohols, a single diastereomer is generated. This reaction forms exclusively anti-product, confirmed by 1 H NMR spectroscopy. The product does not undergo racemization, and when enantiomerically pure α-hydroxy aldehydes are used, enantiomeric excess can be achieved.