Books
in black and white
Main menu
Home About us Share a book
Books
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics
Ads

The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
Previous << 1 .. 196 197 198 199 200 201 < 202 > 203 204 205 206 207 208 .. 240 >> Next

organic phase, the amino acid is coordinated as an additional ligand.
298
Ogoshi et al.
Wavelength (nm)
(b)
Wavelength (nm)
Figure 10. Circular dichroism spectra of (a) 56-L(D)-Leu-OMe complex and
(b) 57-L-Leu-OMe complex in CHCI3.
D. CHIRAL RECOGNITION OF AMINO-ACID DERIVATIVES
Investigation of chiral recognition47 provides information on how the
host-guest system is ordered and fixed, since fluctuation would lead to
reduced enantioselectivity. Several chiral porphyrins bearing three
independent recognition groups with different interaction natures were
prepared as hosts for amino-acid esters. Chiral host 59 has a zinc ion as
a Lewis acid site, phenolic OH groups as hydrogen-bonding (hydrogen-
donor) sites, and carbomethoxy groups as steric-interaction / hydrogen-
bonding (hydrogen acceptor) sites.18 Due to the C2 symmetry of the
molecule, host 59 exists as two enantiomers, which were separated by
chiral HPLC. This receptor showed binding for amino-acid esters with
moderate enantiomeric excesses in the range of 33-47% at 15 C in CHCI3
(AAG = 0.27-2.47 kJ / mol). The chiral recognition energy (AAGdl) 's
defined as the difference in the binding free energy between the two
enantiomeric guests; AAIGdV = IAG(l) - AG(d)I. The enantiomeric excess
increased with increase of the hydrogen-bonding energy between an OH
group of 59 and the C-O group of the guest, demonstrating that chiral
discrimination originates from restriction of rotational freedom of the
guest via
ditopic binding. Interestingly, the D/L-enantioselectivity was
predictable from the nature of the ligand-receptor interactions. One
enantiomer of 59, (+ )-59, systematically preferred the L-enantiomer of
most of amino-acid esters except for serine benzyl ester. Provided that,
in the D-amino-acid ester-host complexes, the side-chain group of amino-
acid esters and the CH2COOCH5 group are in van der Waals contact, this
enantioselectivity can be readily explained by assuming that steric
repulsion exists between the CH2COOCH3 group in (+ )-59 and the side-
chain groups of most amino-acid esters, while attractive hydrogen bonding
exists between the CH2COOCH3 group and the serine OH group (Figure 4).
All these results indicate that the Zn- NH? coordination and OH- 0=C
hydrogen bonding fix the ligand orientation, and l- and D-amino-acid
esters were differentiated through the different interactions between the
CH2COOCH3 group of 59 and the side chain of amino-acid esters.
Double bridging of porphyrins with a dissymmetric difunctional
molecule gave achiral (78) and chiral (63) porphyrins as shown in Scheme
l.81 These porphyrins have three interaction sites: zinc as a Lewis acid,
amide NH as a hydrogen-bonding site and the aromatic moiety as a steric-
repulsion / attraction site; they work as hosts for chiral
46 / Porphyrins and Metalloporphyrins as Receptor Models
299
Scheme 1
Z I ..........-
C2
achiral
chiral
recognition of amino-acid esters. Owing to its C2 symmetry, porphyrin 63
has intrinsic chirality. For each of the enantiomers of 63, the binding
constants for various amino-acid esters were determined by UV-Vis
titrations in _->. The chiral recognition energies are summarized in
Table 3. At 264 in , the ratio of the binding constants of
phenylalanine methyl ester by (he host and by the enantiomeric host was
41.6. which affords the value of
AAGni, of 8.18 kJ /mol ( 95%). Relatively high enantios-electivity was
obtained which was ascribed to the three-point interactions, namely
coordination of the amino group to zinc, hydrogen bonding between the
amide NH group of the bridge to the carbonyl, and steric repulsion
between the bridging moiety and amino-acid esters.

Inoue et al. reported that one of the enantiomers of N-
(carbobenzyloxy)amino acids was extracted to a chloroform
Table 3. Chiral Recognition Energy for Porphyrin Complexation Entry Host
Guest AAGDL/kJ-mol 1
1 59 isoleucine methyl ester 2.47
> 59 leudne methyl ester 2.20
3 59 valine methyl ester I "> j
4 59 proline methyl ester 1.97
5 59 phenylalanine methyl ester 1.67
6 59 alanine methyl ester 0.27
7 59 leucine ben/.yl ester 1.93
8 59 serine hen/yl ester 1.80
9 59 1 -phenylethyl-amine 0.07
10 6.3 valine methyl ester 4.89
11 63 leucine methyl ester 5.28
12 63 phenylalanine methyl ester 4,75
13 63 alanine methyl ester 3.43
14 63 phenylglycine methyl ester 0.54
15 71 N-C'bz alaninatc 5.64
16 71 N-Cbz-N-methylalaninate 0.00
17 71 N-Clv. vahnate 7.74
18 71 N-Boc-valiiiate 7.17
19 71 N-acetyl-valinate 4.23
20 71 N-Cb/-norvalinate 5.64
Previous << 1 .. 196 197 198 199 200 201 < 202 > 203 204 205 206 207 208 .. 240 >> Next