Please read directions carefully and answer the following questions completely.
You may write directly on the problem set, but be sure to staple the pages
together. This problem set is due in class at the beginning of lecture by 9:05 AM
on the date indicated above.
Name:
Section TF:
Section Day/Time:
Collaborators:
Chem 20 pset 3 due Wednesday 2/17/16
Question 1
Fill in each box with the starting materials, reagents, or products of the indicated transformation. Some
transformations may require multiple steps.
1. EtMgBr, Et2O
2.
O
(a)
Ph
Ph
Ph
OH
Ph
3. H+ workup
O
(b)
OH
H+
H
Me
(c)
Me
O
O
OH
O
OH
O
O
Me
Me
HO
CN
1. Me2CuLi
(d)
Ph
Ph
H3O+
O
(e)
2. H+ workup
O
O
(f)
O
O
OEt
H
Chem 20 pset 3 due Wednesday 2/17/16
Question 2
Many carbohydrates exist in solution as hemiacetals. Ribose, the sugar found in RNA, also exists as a
hemiacetal. In fact, ribose can exist in two different hemiacetal forms: a 6-membered ring and a 5-membered
ring. The two forms interconvert via the straight-chain aldehyde.
(a) Provide a complete curved-arrow mechanism for the acid-catalyzed transformation of the 6-membered
hemiacetal into the straight-chain aldehyde.
O
OH
OH
O
+
H3O (cat.)
HO
HO
OH
OH
OH
OH
straight-chain aldehyde
6-membered hemiacetal
(b) Provide a complete curved-arrow mechanism for the acid-catalyzed transformation of the open-chain
aldehyde into the 5-membered hemiacetal (which is the form of ribose found in RNA).
OH
O
O
HO
HO
OH
straight-chain aldehyde
(redrawn)
H3O+ (cat.)
OH
HO
HO
OH
5-membered hemiacetal
Chem 20 pset 3 due Wednesday 2/17/16
Question 3
Provide a complete curved-arrow mechanism for the following transformation. Hint: the first step is an
acid-base reaction.
OH
NaBH4, NaOH
OH
+ NaCN
CN
H2O
Chem 20 pset 3 due Wednesday 2/17/16
Question 4
Consider the following mechanism:
HO
OH
NaOH
O
HO
H2O
O
HO
(a) Here is a demonstration of a curved-arrow mechanism that is not considered reasonable. Circle anything
that you see that is incorrect and give a brief explanation about why it is incorrect.
HO
HO
OH
NaOH
O
H2O
O
HO
H
O
we use this symbol to
mean "equivalent to"
O
OH
OH
O
(b) Provide a correct complete curved-arrow mechanism for the transformation.
OH
Chem 20 pset 3 due Wednesday 2/17/16
Question 5
Provide a multistep synthesis for each of the desired products. In addition to the given starting materials,
you may use any other organic and inorganic reagents. Please do not "laundry list" the synthesis (i.e.:
simply writing all the reagents without showing each individual transformation). Show the products of each
individual step. For reactions that follow with H+ workup, you may show those as 1. rxn 2. H+ workup.
OH
can be done
in 4 steps
(a)
HO
S
Starting Materials
Desired Product
O
(b)
The best answer will require
9 or fewer steps.
HO
NH2
+
H
Li
Starting Materials
Desired Product
Chem 20 pset 3 due Wednesday 2/17/16
Question 6
The following proposed synthesis has flaws with all but one step.
Starting Compound
O
NaBH4,
O
O
CrO3,
OH
PrOH
H2O
H
1. Pr2CuLi
2. H+ w/u
OH
CrO3 . pyr
O
NaOEt (2 eq),
EtO
OEt
EtOH
Desired Product
(a) For the four individual steps that are incorrect, indicate what the actual product of the reaction would be.
(b) Starting with the given starting compound, propose a correct synthesis to create the desired final product.
Chem 20 pset 3 due Wednesday 2/17/16
Question 7
(a) We learned that sodium borohydride is an effective reducing agent for aldehydes and ketones, but not for
carboxylic acids and the derivatives of carboxylic acid.
Provide a molecular orbital theory argument for the following observations:
O
NaBH4, EtOH
O
NaBH4, EtOH
reaction K
reaction E
HO
H
no reaction
O
(b) Rank the following ketones in order of increasing tendency to form a stable hydrate. Write "most" for most
stable hydrate, "least" for stable hydrate. Give a brief explanation of your answer.
O
O
O
C
(H3C)3C
C(CH3)3
Chem 20 pset 3 due Wednesday 2/17/16
Question 8
You have the skills necessary to solve the following mechanism! Provide a complete curved-arrow
mechanism for all steps in the following transformation. While step 3 may initially seem unfamiliar, it requires
you to apply principles you have been developing.
1. H3O+
2. NaBH4, EtOH
O
O
O
O
3. H+ cat.
O
+
OH
OH
O
+
OH