Chapter 9 – Stereochemistry For the following questions questions MATCH each definition definition to a term from the list below. Place the letter of the term in the blank to the left of the definition. a. b. c. d. e.
racemates chirality center center chirality diastereomers enantiomers
f. g. h. i. j.
meso compounds optically active prochirality center optically inactive achiral
1. ____ describes organic molecules which which rotate plane-polarized plane-polarized light.
Answer: 2.
is the property of “handedness”; the property of an object that causes it to be nonsuperimposable on its mirror image. Answer:
3.
b
are molecules which contain chirality centers and a plane of symmetry. Answer:
6.
d
is an atom in a molecule that is bonded to four different atoms or groups of atoms. Answer:
5.
c
are stereoisomers that are not mirror images. Answer:
4.
g
f
_____ describes an sp an sp3-hybridized atom that can become a chirality center by changing one of its attached groups. Answer: h
7.
The specific specific rotation rotation of a compound compound is denoted by the symbol: symbol: a. R b. S c. α d. α D Answer:
68
d
Chapter 9: Stereochemistry
Place asterisks at all the chirality centers in each molecule below. 8.
COOH
O
HO
OH prostacyclin
Answer: COOH
O * * *
*
*
HO
OH
9.
CH CH3O
O N
H
H
CH3
HO H
codiene
Answer: CH CH3O
O H
*
*
*
*
H *
HO
N CH3
H Test Items for McMurry’s Organic Chemistry, Seventh Edition
69
10.
OH
NHCH NHCH3 ephedrine
Answer: OH *
*
NHCH NHCH3
O C6H5CHCNH
11.
S
NH2 N CH3
O cephalexin
CO2H
Answer: O * C6H5CHCNH
S * *
NH2
N CH3
O CO2H 12.
O H3C
HO
HO CCH CCH2OH CH3
H3C F O
70
betamethasone
Chapter 9: Stereochemistry
Answer: O H3C
HO H3C *
*
*
*
*
*
HO CCH CCH2OH * *
CH3
F
O
13.
COOH O O thromboxane A2
OH
Answer: * * O * O
*
COOH *
OH
Assign R Assign R,, S configurations configurations to each indicated chirality center in the molecules below. 14.
OH
15.
H
HO
COOH C
CH2 NH2
CH3
H2 N
H
HO norepinephrine
alanine
O 17.
16.
OH
HO2C H HO
H
H
H
18.
CH3
H2C CO2H CH2
tartaric acid
14. The configuration of this carbon atom is
dihydrocarvone
.
Answer: R 15. The configuration of this carbon atom is
Answer:
.
S
Test Items for McMurry’s Organic Chemistry, Seventh Edition
71
The configuration of this carbon atom is 16. The
.
Answer: R 17. The The configuration of this carbon atom is
Answer:
.
S
The configuration of this carbon atom is 18. The Answer:
.
S
Consider the structure of streptimidone to answer the following questions. O O
H
*
H3C
NH
OH *
O streptimidone
H
Assign R or or S configuration configuration to each chirality center indicated in streptimidone. 19. Assign R Answer: O O
S *
H3C
H
OH *
R
NH O
H
chirality centers, how many stereoisomers stereoisomers of streptimidone streptimidone are possible? 20. Based on the number of chirality Answer:
Streptimidone has two two chirality centers so there there are 22, or 4, possible stereoisomers.
streptimidone have a meso stereoisom meso stereoisomer? er? Explain your answer. 21. Will streptimidone Meso compounds are compounds that contain chirality centers, but possess a plane of Answer: No. Meso symmetry. Because the substituents on the chiral chiral carbons are different different in streptimidone it it is impossible for any stereoisomer to have a plane of symmetry.
72
Chapter 9: Stereochemistry
Label each pair of stereoisomers below as: a. b. c.
enantiomers diastereomers identical
Place the letter of the correct answer in the blank to the left of the pair of stereoisomers. stereoisomers. H
H3C 22.
H3C
OH
HO H Answer:
H HO
CH2OH
CH2OH
a H
CO2H
23. HO
Answer:
OH H
HO H2OC
H CH3
CH3
c HO2C
24.
HO2C
CO2H
CO2H
b
Answer:
H
25.
OH
HOH2 C
HO CHO
OHC
H OH HO H
Answer:
H CH2OH
H
OH HO H
b Br
Br 26.
H3C Answer:
Br
H3C
Br
c H
27.
CH2CH3
C CH3CH2
Answer:
CH3 Cl
H
C Cl
CH3
a
Test Items for McMurry’s Organic Chemistry, Seventh Edition
73
O
O H
28.
H
CH3
H H3C
H
H2C
CH2 CH2
Answer:
CH2
b
29. Draw a wedge-dash wedge-dash projection projection of (2R,3S (2R,3S )-dibromobutane. )-dibromobutane.
Answer: Br H H3C
CH3 H
Br
30. Draw a Newman projection of the most stable stable conformation of (2R,3S (2R,3S )-dibromobutane )-dibromobutane sighting down the C2-C3 bond.
Answer: Br H
CH CH3 H
CH3 Br
31. (2R,3S)-Dibromobutane (2R,3S)-Dibromobutane is:
a. b. c. d. Answer:
optically active. racemic. dextrorotatory. a meso compound. meso compound. d
formed by hydrogenation hydrogenation of (S (S )-4-methyl-1-hexene )-4-methyl-1-hexene is optically active while the one formed by 32. The alkane formed hydrogenation of (S (S )-3-methyl-1-pentene )-3-methyl-1-pentene is not. not. Explain. Answer:
74
Each starting starting alkene is optically active active because it contains a stereogenic stereogenic center—a carbon with four different groups bonded to to it. The alkane formed by the hydrogenation hydrogenation of (S (S )-4-methyl-1)-4-methyl-1hexene is optically because carbon carbon four is still chiral. The product of hydrogenation hydrogenation of (S (S )-3)-3methyl-1-pentene is not optically active because two of the groups on carbon three are the same now—they are ethyl groups.
Chapter 9: Stereochemistry
potent estrogenic hormone, hormone, has been isolated from from the urine of pregnant pregnant women. When 40 mg of 33. Estriol, a potent estriol is dissolved in 1.0 mL of dioxane and placed in a sample tube with 1 dm path length a rotation of +2.32° is observed. Calculate the specific specific rotation for estriol. estriol. Answer:
α
D =
+2.32° 1 dm × 0.040 g mL
= + 58°
34. Comp Compou ound nd A, C9 H16 , was found to be optically active. On catalytic reduction over a palladium pallad ium catal c atalyst, yst, 2
equivalents of hydrogen hydrogen were absorbed to yield yield compound B. Ozonolysis of A gave two compounds. One was identified as acetaldehyde, CH 3CHO . The other compound, C, C, was an optically optically active dialdehyde, C5H8O2 . Formulate the reactions reactions and draw structures structures for compounds A, B and C. Answer:
CH3 CH3CH2CH2CHCH2CH2CH2CH3 *
B
* CHCHCH2CH
CH3CH
A
CHCH3
CH3
H2 Pd/C
O3, then Zn, HOAc O CH3CH
O
O
* + HCCHCH2CH CH3
C Refer to the structure below to answer the following questions. CO2H H HOH HOH2C
(S)-(-)-Serine
C NH2
)-(−)-Serine: 35. (S )-(− a. b. c. d.
is dextrorotatory rotates plane-polarized plane-polarized light in a counterclockwise direction rotates plane-polarized light in a clockwise direction is racemic
Answer:
b
36. Draw the the enantiomer enantiomer of (S )-(− )-(−)-serine in a wedge-dash projection.
Answer:
CO2H C H2 N
H CH2OH
Test Items for McMurry’s Organic Chemistry, Seventh Edition
75
37. Give the complete complete name of the enantiomer enantiomer of (S (S )-(− )-(−)-serine.
Answer:
( R)-(+)-serine R)-(+)-serine
A natural product having [α [ α]D = +40.3° +40.3° has been isolated and purified 38. This information indicates that the natural product:
a. b. c. d. Answer:
is racemic. does not rotate plane-polarized light. light. is levorotatory. is dextrorotatory. d
39. Two structures have been proposed for this natural product. Circle the Circle the structure that is consistent with the information presented and briefly explain your choice.
CO2H HO
HO H HO H
OH
or
HO OH HO H
HO OH
HO H
Answer:
CO2H HO
OH
HO H HO H or
HO OH
HO OH
HO H
HO H
The information presented presented indicates that the natural natural product is optically optically active. To be optically active molecules must be chiral - that is, they must not have a plane of symmetry. symmetry. The cyclic structure, although is has chirality centers, has a plane of symmetry, indicated by the dashed line on the structure, and can, therefore, not be optically active. The circled structure has four chirality centers, and is not symmetric. symmetric. We would expect it to be optically optically active.
76
Chapter 9: Stereochemistry
catalyzes the hydration of the alkene functional group of aconitic acid to give two 40. The enzyme aconitase catalyzes products, citric acid and isocitric acid. Isocitric acid is optically active; citric acid is not optically active. Based on your knowledge of alkene hydration and optical activity, the structure of citric acid is: O
O
COH
HOCCH2 C
aconitase
C
HOC
citric acid
+
(optically active)
(optically inactive)
H
isocitric acid
O aconitic acid
a. HO2CCH2
OH H C
C H
CO2H
CO2H
c. HO2CCH2
b. HO2CCH2 C
CH2CO2H
HO CO2H
d. cannot be determined
H OH C
HO2C
Answer:
C CO2H
H
b
Identify the indicated hydrogens in the following molecules as pro-R as pro-R or or pro-S pro-S .
41.
H
H CO2
2OC
H
H succinate
Answer: ro-S H
H CO2
2OC
H
H pro-R
Test Items for McMurry’s Organic Chemistry, Seventh Edition
77
42.
H
H CO2 O
phenylpyruvate
Answer: ro-R
pro-S H
H CO2 O
phenylpyruvate
Identify the indicated faces in the following molecules as re or si or si..
43.
Ph C
O
2OC
phenylpyruvate
Answer: re Ph C
O
2OC
si
3OPOH2C
C
44.
O
HOH HOH2C
dihydroxyacetone phosphate
Answer: si 3OPOH2C
C
O
HOH HOH2C re 78
Chapter 9: Stereochemistry
45. Acetoacetate synthase catalyzes the addition of pyruvate to β-ketobutyrate to yield α-aceto-α -aceto-αhydroxybutyrate. If the addtion addtion occurs from from the si face si face of β-ketobutyrate, what is the stereochemistry of the product?
O
O
CH3CCO CCO2
CCO2 + CH3CH2CCO
O acetoacetate synthase thiamine pyrophosphate
CH3C
OH CCO CCO2
+
CO2
CH2CH3 α-aceto-α -aceto-α-hydroxybutyrate
si face results in the S enantiomer of α-aceto-α -aceto-α Answer: Addition of the aceto group to the si hydroxybutyrate.
addition
-
OOC
-
C
OOC
O
H3CH2C
Test Items for McMurry’s Organic Chemistry, Seventh Edition
H3CH2C
COCH3 S OH
79
