SSG 5.3 Presentation:
Chelation Control and Felkin-Anh
FELKIN-ANH STEREOSELECTIVITY:
In Carbonyl reactions:
No Stereoselectivity with
symmetrical compounds
No new stereocenters formed
With formation of new stereocenters:
Stereoselectivity
Usually:
Asymmetrical ketone
Aldehyde
Felkin-Anh:
Specifically:
a-chiral aldehydes
Ketones
NOT on ketenes
Transition states:
Diels-Alder:
pi-face to pi-face (cube visualization)
This diagram shows the typical
Diels-Alder transition states, endo and exo, where the dienophile approaches
from below and creates a very 3-D transition state.
Shumway, W.; Ham, S.; Whittlesey,
B.R.; Birney, D.M. J. Org. Chem. 2000. 65, 7731-7739.
Felkin-Anh:
Pseudo-pericyclic
orbitals
In this diagram, the planar
transition states are shown, where the R1 group is highest priority and
is either axial or equatorial.
Shumway, W.; Ham, S.; Whittlesey,
B.R.; Birney, D.M. J. Org. Chem. 2000. 65, 7731-7739.
Significance:
New quaternary stereocenter
formed
Transition state depends
on:
In this Newman projection, the
A group is the largest and most bulky. It is ANTI to the incoming
nucleophile. The next largest group is GAUCHE to the carbonyl (B).
