2) Cram Rule:1b If chelation cannot occur, 6 was
proposed to be the preferred reactive conformation
based on steric reasons (Scheme 3): the decisive
steric interaction to be avoided was thought to be
between the large substituent L and the carbonyl
group. Consequently, L is oriented anti to the carbonyl
group, placing S and M on different sides of
the carbonyl group. A nucleophile will now preferentially
attack from the side of the small substituent
S, leading to 7 as the major product, the so-called
Cram or Felkin-Anh product.6
The Cram rule proved to be a reliable tool to
explain the preferred diastereoselection in carbonyl
addition reactions if no polar substituents were
present on the R-stereocenter. However, if the R-stereocenter
contained acceptor groups such as chlorine
or trimethylsiloxyl, they took on the role of L even
though sterically more demanding substituents could
be present. This behavior could not be explained
under the assumptions made for 6. Conforth7 therefore
modified the Cram rule in that electron-withdrawing
substituents (EWG) assume the role of L in
order to minimize the dipole moment (Figure 1).
However, for various reasons 6, (or 8) did not seem
to be a good choice for representing the reactive
conformation for nucleophilic addition by a nonchelation
pathway: The steric bulk of the carbonyl
group was overestimated, resulting in an unfavorable
alignment of L and R in 6, especially in ketones (R
* H).