TOPIC : STEREOSELECTIVE REACTIONS OF CYCLIC COMPOUNDS

Adnan Abudl QayyumTOPIC :

STEREOSELECTIVE REACTIONS OF CYCLIC COMPOUNDS

7/5/20122Stereoselectivity in Cyclic compounds

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Four Membered RingSaturated four membered ring: Puckered squares

Unsaturated four membered ring: Essentially flat

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In cyclobutane, the ring distorts from a planar conformation in order to reduce the eclipsing interactions, even though this reduces the bond angles further and so increases the bond angle strain. Cyclobutane adopts a puckered or wing-shaped conformation.7/5/20126Reactions of 4-membered rings:The formation of the lithium enolate is straightforward but it might be expected to be unstable because of a simple elimination reaction. It is not possible to make open-chain with -oxygen like this because they do undergo elimination.

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7/5/20129Five Membered RingSaturated five membered ring: Open envelope

Unsaturated four membered ring: Essentially flat

7/5/201210Reactions of 5 Membered RingReduction of 2-substituted cyclopentanones may not be very stereoselective, as you can see below

But by using a bulky reducing agent, the stereoselectivity increases

7/5/201211Epoxidation with a peroxy-acid occurs preferentially on the less hindered face.In the transition state, the peroxyacid prefers to be well away from R, even if R is only a methyl group.

7/5/201212The opposite stereoselectivity can be achieved by the bromination in water

7/5/201213Six - Membered Ring7/5/201214

Substituted Cyclohexane

7/5/201215Locking group : t-Butyl group , Declain

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Nucleophilic substitution

7/5/201218Substitution of an axial substitutent proceeds faster than the substitution of an equatorial substitutent.

7/5/201219Summary

7/5/201220When the stereogenic center is further away from the site of attack, the stereoselectivity may not be so good.

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