Additions to carbonyl groups can be diastereoselective even without rings

Additions to carbonyl groups can be diastereoselective even without rings 

We put Chapter 33 first because in rings conformation is well defined, and this averaging effect is held at bay. We are about to let it out again, but we will show you how it can be tamed to surprisingly good effect. 

In Chapter 32 we showed that homotopic and enantiotopic protons are identical by NMR. Similarly, homotopic faces or groups are always chemically identical. Enantiotopic faces are also chemically identical, provided that all the reagents in the reaction in question are achiral or racemic. In Chapter 45. we will consider what happens to enantiotopic faces when enantiomerica ly pure reagents are used. 

What happens if we bring the stereo genic centre closer to the carbonyl group than it was in the last example You might expect it to have a greater influence over the carbonyl group s reactions. And it does. Here is an example. 

There is three times as much of one of the two diastercoisomeric products as there is of the other, and the major (anti) diastereoisomer is the one in which the nucleophile has added to the front face of the carbonyl group as drawn here. We can make these same two diastereoisomers by addition of an organometallic to an aldehyde. For example, this Grignard reagent gives three times as much of the syn diastereoisomer as the anti diastereoisomer. The major product has changed, but the product still arises from attack on the front face of the carbonyl as shown. 

We have termed the major diastereoisomer anti because the two substituents (Me and OH) are on opposite sides of the chain as drawn. There is no formal definition of anti and syn they can only really be used in conjunction with a structural drawing. 

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ADDITIONS TO CARBONYL GROUPS CAN BE DIASTEREOSELECTIVE EVEN WITHOUT RINGS... 

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