文档介绍：CHAPTER
6
Stereochemistry
SOLUTIONS TO EXERCISES
1. Though all 19 chiral amino acids are homochiral (in other words, have the same side chain
orientation, as shown below), the priority ordering is different for cysteine. In all other
natural amino acids, the side chain has a lower priority than the carboxylate. In cysteine,
however, the sulfur atom gives the side chain a higher priority, such that the stereochemical
assignment switches to R. (Sulfur has a higher atomic number than oxygen.)
3 R 2 CH2SH
12 1 3
H2NCO2H H2NCO2H
SR
2. A sugar whose standard Fischer projection (with the carbonyl group at or near the top) has a
hydroxy group to the right off the bottom stereocenter is designated as D.
CHO
OH OH
H H HO H
OH OH
O OH HO H
HO HO
O ≡ D
HO OH HO H OH
H H H H
H H H H OH
CH2OH
74
Stereochemistry 75
CHO
OH OH
OH OH H OH
H H H H
HO H
≡ D
HO OHOOH HO OH HO H
H H H H
H H H OH
OH O CH2OH
CHO
OH OH
OH OH H OH
H H H H
H OH
H H ≡ L
HO O HO OH HO H
H H
OH O HO H
HO H HO H CH2OH
CHO
HO H O H HO H
O HO
H H H H ≡ HO H D
H OH H OH H OH
HO OH HO OH
CH2OH
CHO
HO HO
H H H OH
O OH
H H H H ≡ H OH D
H OH
H OH H O
HO OH HO OH
CH2OH
3. To determine Z and E labels, we need to determine the priority order at both ends of the
stereogenic alkene.
O O
E. In the parison, we find no distinction until the β
21OEt position: C vs. O.
H
1 2
E. In the parison, we must go out three bonds to the γ
21 position to find a difference. The attached atoms are both C,
and each of these is attached three times to C. In the next step,
H we find only H's on the t-butyl CH3 groups while the end C in
1 2 the 2-propenyl group is attached to two H's and one C. Note
that the double bond gives us an "extra" C on each end.
76 Chapter 6
E. In the parison, Br has a higher atomic number
1 2 (35) than Se (34).
PhSe Br
2 1
4. Assigning priorities at both ends of the