|SHIELD is a rendering of a space-filling model of the co-catalyst Lewis
acid with the ( S )-VAPOL ligand, bound as described in the text. The twisted
(and chiral) black portion which wraps around the system is the VAPOL ligand.
It prevents access by an enolate reagent to the aldehyde, which is bound
to the chiral co-catalyst but blocked from our view. The orange bodies
are a representation of bulky substituents on the VAPOL, which assures
that its chiral ( S ) form is not easily interconverted to the opposite
( R ) form.
||ACCESS is a view of the other side of the co-catalyst complex
with the reactant aldehyde The aldehyde is close to the center of
the figure, with the C=O fragment exposd. The black VAPOL can be
seen on the periphery but does not block access to the carbonyl carbon
from this approach.