1. Lecture 1c
Asymmetric Synthesis

2. Assigned Reading
Hanson, J. J. Chem. Educ. 2001, 78(9), 1266 (including supplemental material).
Larrow, J.F.; Jacobsen, E.N. Org. Synth. 1998, 75, 1.
Cepanec, I. et al. Synth. Commun. 2001, 31(19), 2913.
Flessner, T.; Doye, S. J. Prakt. Chem. 1999, 341, 436.
McGarrigle, E.M.; Gilheany, D.G. Chem. Rev. 2005, 105(5), 1563.
Schurig, V.; Nowotny, H.P. Angew. Chem. Int. Ed. Engl. 1990, 29(9), 939.
Sharpless, B. Angew. Chem. Int. Ed. Engl. 2002, 41, 2024.
Katsuki, T. Coord. Chem. Rev. 1995, 140, 189.
Kunkely, H.; Vogler, A. Inorg. Chem. Comm. 2001, 4, 692.
Trost, B. PNAS 2004, 101, 5348
Yoon, J.W.; Soon, W.L.; Shin, W. Acta Cryst .1997, C53, 1685.
Yoon, J.W.; Yoon, T.; Soon, W.L.; Shin, W. Acta Cryst. 1999, C55, 1766.

3. Why Asymmetric Synthesis?
Chirality plays a key role in many biological systems i.e., DNA, amino acids, sugars, terpenes, etc.
Many commercial drugs are sold as single enantiomer drugs because often only one enantiomer (eutomer) exhibits the desired pharmaceutical activity while the other enantiomer is inactive or in many cases even harmful (distomer)
(*) These drugs are isomerized in vivo
Drug
R-enantiomer
S-enantiomer
Thalidomide
Morning sickness
Teratogenic*
Ibuprofen
Slow acting
Fast acting*
Prozac
Anti-depressant
Helps against migraine
Naproxen
Liver poison
Arthritis treatment
Methadone
Opioid analgesic
NMDA antagonist
Dopa
Biologically inactive
Parkinson’s disease

4. History of Asymmetric Synthesis I
1848: Louis Pasteur discovers the chirality of sodium ammonium tartrate
1894: Hermann Emil Fischer outlined the concept of asymmetric induction
1912: G. Bredig and P.S. Fiske conducted one of the first well documented enantioselective reactions (addition of hydrogen cyanide to benzaldehyde in the presence of quinine with 10 % e.e.)
1960ties: Monsanto uses transition metal complexes for catalytic hydrogenations i.e., Rh-DIPAMP for L-dopa (Parkinson disease, 95 % e.e.)
1980ties : R. Noyori developed hydrogenation catalyst using rhodium or ruthenium complexes of the BINAP ligand

5. History of Asymmetric Synthesis II
1980: T. Katsuki and K.B. Sharpless develop chiral epoxidation of allylic alcohols (90 % e.e., but moderate yields!)
They attribute the high selectivity to the in-situ formation of a chiral, dinuclear Ti-complexes
The alkene is tied to the reaction center by the allylic hydroxyl function
This places the peroxide function in close proximity to the alkene function
The reaction is usually carried out at low temperatures (-20 oC), is very sensitive towards water and require up to 18 hours to complete
The yields are moderate (77 % for the reaction above) due to the increased water solubility of the products

6. History of Asymmetric Synthesis III
Example: Sharpless epoxidation is used to prepare (+)-disparlure, a sex pheromone, that has been used to fight Gypsy moths through mating disruption (note that the (-) enantiomer is a deterrent and reduces trap captures)
The Sharpless epoxidation is also used to obtain intermediates in the preparation of methymycin and erythromycin (both macrolide antibiotic)
The Nobel Prize in Chemistry in 2001 was awarded to three of the pioneers in the field: K. B. Sharpless, R. Noyori, W. S. Knowles

7. How do Chemists control Chirality?
Chiral pool: optically active compounds that can be isolated from natural sources (i.e., amino acids, monosaccharides, terpenes, etc.) and can be used as reactants or as part of a chiral catalyst or a chiral auxiliary
The TADDOL, DIOP and the Chiraphos ligand have tartaric acid as chiral backbone
Enzymatic process: very high selectivity, but it needs suitable substrates and well controlled conditions
The Lipitor synthesis requires halohydrin dehalogenase, nitrilase, aldolase
The reduction of benzil using cryptococcus macerans leads to the formation of (R,R)-hydrobenzoin (dl:meso=95:5, 99 % e.e.)
Chiral reagent: it exploits differences in activation energies for alternative pathways
Chiral auxiliary: it is a chiral fragment that is temporarily added to the molecule to provide control during the key step of the reaction and is later removed from product

8. How do Chemists control Chirality?
By manipulating the energy differences in transition states (DDG‡)
Bottom line
The higher the energy difference in the transition states is the higher the selectivity will be at a given temperature
The lower the temperature, the more selective the reaction will be at a given difference in transition energy
T\DDG‡
4000 J
173
16.1
273
5.8
298
5.0
373
3.6

9. Chiral Reagent
Example: Enantioselective reduction of aromatic ketones using BINAL-H
The enantioselectivity for the reaction increases from R=Me (95 %) to R=n-Bu (100 %) but decreases for R=iso-Pr (71 %) and R=tert.-Bu (44 %) due to increased 1,3-diaxial interactions in the six-membered transition state

10. Chiral Auxiliary I Evans (1982): Oxazolidinones for chiral alkylations
Front view
Evans (1982): Oxazolidinones for chiral alkylations
The oxazolidinone is obtained from L-valine (via a reduction to form L-valinol, which is reacted with either urea or diethyl carbonate under MW conditions)
The iso-propyl group in the auxiliary generates steric hindrance for the approach from the same side in the enolate (the high-lighted atom is the one which is deprotonated)
Chiral auxiliaries
The auxiliary has to be close to reaction center, but not slow down the reaction significantly or change the structure in the transition state
The auxiliary should be easily removed without loss of chirality
It should be readily available for both enantiomers
Side view

11. Chiral Auxiliary II
In 1976, E. J. Corey and D. Enders developed the SAMP and RAMP approach that uses cyclic amino acid derivatives ((S)-proline for SAMP, (R)-glutamic acid for RAMP) and hydrazones to control the stereochemistry of the product.
Below is an example for the use of SAMP in an asymmetric alkylation reaction.
The condensation of SAMP with a ketone affords an E-hydrazone
The deprotonation with LDA leads to the enolate ion that undergoes alkylation from the backside
The chiral auxiliary is removed by ozonolysis

12. Chiral Catalyst
In Chem 30CL, a chiral catalyst is used to form a specific enantiomer of an epoxide