1. 17.14 Stereoselective Addition to Carbonyl Groups Nucleophilic addition to carbonyl groups sometimes leads to a mixture of stereoisomeric products. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 20% Example CH 3 H3CH3C O 80% OHOH H CH 3 H3CH3C OHOH H H3CH3C NaBH 4

3. This methyl group hinders approach of nucleophile from top. H 3 B—H – Preferred direction of approach is to less hindered (bottom) face of carbonyl group. Steric Hindrance to Approach of Reagent

4. Biological Reductions are Highly Stereoselective pyruvic acid  S-(+)-lactic acid O CH 3 CCO 2 H NADH H+H+ Enzyme is lactate dehydrogenase. CO 2 H HOHO H CH 3

5. Figure 17.10(a) One face of the substrate can bind to the enzyme better than the other.

6. Figure 17.10(b) Change in geometry from trigonal to tetrahedral is stereoselective. Bond formation occurs preferentially from one side rather than the other.

7. in aqueous solution RCH RCH RCOHOOH OH H2OH2OH2OH2OO 17.15 Oxidation of Aldehydes

8. K 2 Cr 2 O 7 H 2 SO 4 H2OH2O O O CH O O COH (75%) via O OH CH OH Example