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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 talking of diastereomers, syn and anti are used to describe groups on the same or opposite sites in zigzag projection, see Diastereomer#Syn / anti. syn and anti are always written small and italic, locants (if used) are placed in front of the word and separated by hyphens.
a torsion angle between ±90° and 180° is called anti (a) a torsion angle between 30° and 150° or between −30° and −150° is called clinal (c) a torsion angle between 0° and ±30° or ±150° and 180° is called periplanar (p) a torsion angle between 0° and ±30° is called synperiplanar (sp), also called syn-or cis-conformation
When all reagents are achiral, only simple diastereoselectivity (syn versus anti, see above) must be considered. Addition takes place via an S E ' mechanism involving concerted dissociation of tin and C-C bond formation at the γ position.
Syn describes groups on the same face while anti describes groups on opposite faces. The concept applies only to the Zigzag projection. The descriptors only describe relative stereochemistry rather than absolute stereochemistry. All isomers are same.
The prefix endo is reserved for the isomer with the substituent located closest, or "syn", to the longest bridge. The prefix exo is reserved for the isomer with the substituent located farthest, or "anti", to the longest bridge. Here "longest" and "shortest" refer to the number of atoms that comprise the bridge.
An example of modest stereoselectivity is the dehydrohalogenation of 2-iodobutane which yields 60% trans-2-butene and 20% cis-2-butene. [5] Since alkene geometric isomers are also classified as diastereomers, this reaction would also be called diastereoselective.
Overview of reaction with consideration of stereochemistry. Whether the anti-diastereomer or the syn-diastereomer is built depends largely on reaction conditions, substrates and Lewis acids. The archetypical reaction is that of the silyl enol ether of cyclohexanone, (CH 2) 5 CO, with benzaldehyde, C 6 H 5 CHO.