6954 Fegas et al.
Asian J. Chem.
38
In this work, a chiral stationary phase (CSP) based on
dry toluene, 1.5 mL of 3-triethoxysilyl isocyanate and 1 drop 70
of dibutyl tin dilaurate as catalyst were added. The mixture 71
was refluxed for 4 h. The solvent was evaporated and the 72
remaining raw materiel washed with dry diethyl ether. The 73
white solid (quinine derivative) was crystallized (99 %). The 74
39 tert-butyl carbamoyl quinine (tBuCQN) was used to separate
40 the enantiomer of amino acid derivative phthalylvalin and the
41 influence of different acids in mobile phases (acetic acid,
42 propionic acid, butanoic acid, hexanoic acid, heptanoic acid,
43 octanoic acid, nonanoic acid and dodecanoic acid). Overall
44 enantioselectivity was evaluated to gain more of an insight
45 into the chromatographic mechanism.
resulting product structure was confirmed by 1H NMR.
75
76
The work of Lindner et al.6,14-23 have shown the interest
3
46
47 that could represent quinine carbamate based stationary phase9-11
H
H
.
4
H
48 We are interested in making a grafting of quinine carbamate
49 in situ in a column filled with pure silica stationary phase. The
50 grafting method has been developed to be directly adaptable
51 to the grafting of quinine carbamate on monoliths silica-based
52 capillary for chromatography and electrochro-matography.
N
O
N
1
8
9
O
OCH3
N
53
In this work, a chiral stationary phase (CSP) based on
Fig. 4. Structure of tert-butyl carbamoylquinine (tBuCQN) (8S, 9R)
54 tert-butyl carbamoyl quinine (tBuCQN) was used to separate
55 the enantiomer of amino acid derivative phthalylvaline and
56 the influence of different acids in mobile phases. Overall
57 enantioselectivity was evaluated to gain more of an insight
58 into the chromatographic mechanism.
Synthesis of chiral stationary phase based on t-BuCQN:
The following protocol was applied: a column filled with parti- 77
cles of pure silica was dried by circulation of helium. 3 g of 78
3-mercaptopropyl trimethoxysilane were suspended in chloro- 79
form after addition of 3 g of o-(t-butylcarbamoyl)quinine and 80
200 mg of radical initiator azo-α,α′-bis-isobutyronitrile 81
(AIBN) in 100 mL methanol. The mixture was percolated into 82
the column for 15 h with a flow rate of 1 mL/min. The prepara- 83
tion was ended by washed with different polarities solvents. 84
The column of pure silica was a column type Lichrospher 85
60 (250 mm × 6 mm, 12 mm) (VWR, France). The chiral 86
phase obtained (Si-QN) was used to separation of amino acid 87
derivative phthalylvaline with a polar mobile phase a mixture 88
of methanol and acid CnH(2n+1)-COOH, n ranges from 2-18 89
(Table-1) with flow rate 1 mL/min and detection was carried 90
59
H
H
HC
H2C
H2C
N
HO
N
HO
H
H
H
H3CO
H3CO
N
N
Quinidine (3R, 4S, 8R, 9S)
pKa1 = 5.4, pKa2 = 10
Quinine (3R, 4S, 8S, 9R)
pKa1 = 4.1, pKa2 = 8.5
at 245 nm.
91
92
Fig. 2. Structure of quinine and quinidine
TABLE-1
NAME AND LABEL OF THE ACIDS
USED IN THE MOBILE PHASE
H3C
H3C
H
O
C2
C3
C4
C5
C6
C7
C8
Ethanoic acid
C9
Nonanoic acid
CH
C
O
Propanoic acid
Butanoic acid
Pentanoic acid
Hexanoic acid
Heptanoic acid
Octanoic acid
C10
C12
C14
C16
C18
Decanoic acid
N
OH
Dodecanoic acid
Tetradecanoic acid
Hexadecanoic acid
Octadecanoic acid
O
Fig. 3. Structure of phthalylvalin
RESULTS AND DISCUSSION
EXPERIMENTAL
The study of the retention on quinine carbamate stationary
phase was much more interesting. The mobile phase consists
in mixture of alcohol (methanol) and different acids.
Synthesis of o-(t-butylcarbamoyl)quinine: The synthe-
sis of carbamate strcture is prepared via isocyanate reaction: 3
g of quinine, as free base, were dissolved in dry toluene and
1.2 mL of t-butylisocyanate and 1 drop of dibutyl tin dilaurate
as catalyst were added. The mixture was refluxed for 4 h, the
solvent evaporated and the remaining raw material was washed
with n-hexane. The white solid was crystallized with cyclohexane
resulting o-(t-butylcarbamoyl) quinine in 80 % yield.
93
94
60
61
62
63
64
65
66
67
68
69
The concentration of acid was systematically modified in
order to highlight its influence on the retention temps (tr1, tr2),
retention factor (k) and the selectivity (α) (Tables 2-4). As
expected, the retention times of enantiomers depends on the
concentration of acid in the mobile phase. The concentration
factor directly influences the retention mechanism involved in
electrostatic interactions between the solute and the stationary
phase. The selectivity between the enantiomers is influenced
both by the nature of the acid and its concentration. The
95
96
97
98
99
100
101
102
103
Synthesis of t-BuCQN: We conducted a synthesis of
t-BuCQN according to protocol proposed by Lindner and
Lammerhofer1. 2 g of quinine carbamate were dissolved in
Fegas, Rachid
