Use Table 24-2 of Vollhardt and Schore to answer questions 1 and 2.
1. An aldopentose is oxidized by HNO3 to give an optically active diacid. Wohl
degradation of that same aldopentose gives an aldotetrose. This aldotetrose provides an
optically inactive diacid upon HNO3 oxidation. Identify the aldopentose.
Answer: Of the four D-aldopentoses, only arabinose and lyxose give optically active
diacids upon oxidation with nitric acid. Wohl degradation of lyxose gives threose, which
gives an optically active diacid with HNO3. Wohl degradation of Arabinose gives
erythrose, which gives a meso (optically inactive) diacid upon oxidation with HNO3. Thus,
the correct aldopentose is Arabinose.
2. An aldopentose is oxidized by HNO3 to give an optically inactive diacid. Kiliani-Fischer
synthesis on that same aldopentose gives two aldohexoses. Oxidation of the
aldohexoses with HNO3 gives two diacids, only one of which is optically active. Identify
the aldopentose.
Answer: Of the four D-aldopentoses, only ribose and xylose give optically active diacids
upon oxidation with nitric acid. Kiliani Fisher synthesis with Xylose gives Gulose and Idose,
both of which provide optically active acids upon oxidation. Kiliani Fisher synthesis with
Ribose gives Allose and Altrose. HNO3 oxidation of Altrose gives an optically active
diacid, but oxidation of allose gives a meso diacid. Therefore, the correct aldopentose is
Ribose.
3. Provide a mechanism for the following transformation
HO
HO
HO
HO
HO
HO
O
H+ , H2 O
O
HO
HO
H
O
HO
HO
O
OH
O
HO
HO
HO
OH
HO
+
H
HO
H
OH
cellobiose- a disaccharide
HO
:
H+
O
HO
HO
H
HO
HO
HO
O
HO
OH
O
HO
+
HO
HO
O
HO
HO
H
HO
HO
O
OH
HO
Attack on the oxonium ion can occur from either face
bottom face attack
HO
HO
HO
HO
O
:OH2
HO
HO
HO
HO
O
H
:OH2
HO
H
H
O
HO
HO
O
H
HO
OH
H
top face attack
HO
HO
HO
O
HO
HO
HO
H
HO
HO
:OH2
O
HO
H
O
:OH2
H
HO
HO
O
OH
HO
H
4. Provide a mechanism for interconversion of glucose with fructose and mannose.
Glucose to mannose is a simple epimerization reaction of the aldehyde enolate.
–
OH
O
O
H
HO
H
H
H–OH
CHO
OH
OH
H
HO
OH
OH
H
H
CH2OH
HO
HO
H
H
H
H
OH
OH
OH
H
OH
CH2OH
CH2OH
mannose
glucose
Glucose to fructose begins the same way, but is followed by proton tranfer to give a ketone enolate before
reprotonation
–
OH
O
O
H
OH
H
HO
proton transfer
H
OH
H
OH
HO
H
H
OH
H
OH
H
CH2OH
H
CH2OH
CH2OH
O
OH
OH
HO
HO–H
O
H
OH
HO
H
H
OH
OH
H
OH
CH2OH
CH2OH
fructose
glucose
5. Identify each of the stereocenters of fructose as either (R) or (S)
CH2OH
O
HO
S H
H
R OH
H
R OH
CH2OH
6. Provide a synthesis of the following tripeptide using Merrifield solid phase synthesis and
BOC protecting group strategies.
O
HO
O
N
H
H
N
O
N
H
Synthesis of BOC-protected amino acids:
Ph
Et3 N
HO
NH 2
similarly
HO
HO
NHBoc
O
tBuO
O
Ot Bu
O
HO
NH2
O
O
Ph
NHBoc
O
O
HO
O
HO
Merrifield resin
N
Boc
O
NHBoc
O
Cl
HO
N
H
F3CCO 2H
O
Cs 2CO3
O
NHBoc
O
NH2
O
DCC
pyridine
Ph
HO
NHBoc
O
O
O
2) DCC
pyridine
HO
O
O
N
H
O
N
H
H
N
HF
O
N
H
O
HO
O
O
F3CCO2H
2) F3CCO2H
N
Boc
O
O
O
NH2
N
H
H
N
O
N
H
O
+
NHBoc
N
H
F
7. Provide a mechanism for the formation of B, which occurs through the intermediacy of
structure A.
+
H3 C
D
Ph
Ph
O
A
hydrazone formation
(make sure that you
know how this step works!)
H
+ H2O
Ph
B
H
3,3 sigmatropic shift
Ph
HN N
N
H
NH2 NH
H N NH2
Ph
Ph
Ph
HN NH
H2 N: NH
NH2 NH
H+
H+
A
+
H
H+
H
Ph
Ph
NH3 +
N
H
B
Ph
+
N
H
NH3
N
H
NH2