Page 224 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

204 CH 3
K OsO (OH) 4 CH 3
2
2
OH
(CH ) CHCH C CH (DHQD) –PHAL (CH ) CHCH
CHAPTER 2 3 2 2 2 2 3 2 2
CH 2 OH
Stereochemistry, K Fe(CN)
Conformation, 3 6 S-enantiomer
and Stereoselectivity
2.6. Double Stereodifferentiation: Reinforcing and Competing
Stereoselectivity

Up to this point, we have considered cases in which there is a single major
influence on the stereo- or enantioselectivity of a reaction. We saw examples of
reactant control of facial selectivity, such as 1,2- and 1,3-asymmetric induction in
carbonyl addition reactions. In the preceeding section, we considered several examples
in which the chirality of the catalyst controls the stereochemistry of achiral reagents.
Now let us consider cases where there may be two or more independent influences on
stereoselectivity, known as double stereodifferentiation. 179 For example, if a reaction
were to occur between two carbonyl compounds, each having an -stereocenter, one
carbonyl compound would have an inherent preference for R or S product. The other
would have also have an inherent preference. These preferences would be expressed
even toward achiral reagents.

achiral X achiral X
R
R
A* P A prefers P R B* P B prefers P S
S
S
chiral A prefers P S chiral B prefers P R
If A and B were then to react with one another to create a product with a new chiral
∗
∗
center, there would be a two (enantiomeric) matched pairs and two (enantiomeric)
mismatched pairs.

A R + B R P R mismatched
A R + B S P R matched

A S + B R P S matched

A S + B S P R mismatched

In the matched cases the preference of one compound is reinforced by the other. In the
mismatched case, they are competing. The concept of matched and mismatched TSs

179
C. H. Heathcock and C. T. White, J. Am. Chem. Soc., 101, 7076 (1979); S. Masamune, W. Choy,
J. S. Petersen, and L. R. Sita, Angew. Chem. Int. Ed. Engl., 24, 1 (1985).