Table 1 Experimental 13C DdRS (ppm) of the MPA, 1-NMA, 2-NMA and 9-AMA derivatives, and calculateda 13C DdRS (ppm) of the lowest
energy conformersb of MPA esters of corynanthine
DdRS
C(6)
C(5)
C(2)
C(3)
C(7)
C(8)
CO
OMe
MPAc
ꢀ0.60
ꢀ0.45
ꢀ0.17
ꢀ0.17
ꢀ0.31
ꢀ0.41
ꢀ1.03
ꢀ0.13
ꢀ0.34
ꢀ0.41
ꢀ0.34
ꢀ1.05
ꢀ1.57
ꢀ0.45
+0.74
+0.78
+1.21
+1.35
+0.67
+0.31
+0.44
+0.52
+0.70
+0.96
+0.23
+0.24
+0.19
+0.20
+0.67
+1.05
+0.49
+0.31
+0.99
+0.65
+0.90
+0.59
+0.82
+0.41
+0.10
+0.16
+0.21
+0.32
+0.34
+0.09
+0.12
+0.12
+0.22
+0.14
MPAd
Exp. MPA
Exp. 2-NMA
Exp. 1-NMA
Exp. 9-AMA
a
b
c
DFT-GIAO/B3LYP (PCM)/6-31+G(d) using CHCl3 parameters. DdRS calcd (ppm) = sS ꢀ sR. Considering only the lowest energy
d
conformers sp. An estimated relative population of 70/30 (sp/ap) was used for calculation.y
C(2) DdRS is +0.55 ppm. Changing the carbonyl disposition in the
sp conformation, the calculated value (for a 70/30 sp/ap ratio) is
+0.34 ppm.
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´
T. J. Wenzel, Discrimination of Chiral Compounds Using NMR
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Fig. 3 1H and 13C DdRS superimposed values of corynanthine MPA
and 9-AMA esters vs. distance measured from the auxiliary [C(10) of
phenyl ring] to C(6), C(5) and C(4) of corynanthine.
5 I. Louzao, J. M. Seco, E. Quinoa
2006, 1422.
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and R. Riguera, Chem. Commun.,
From a practical point of view, this means that the
configurational assignment of compounds with no protons
on L1/L2 can now be carried out using 13C NMR. Naturally, if
both 13C and 1H are present, the use of the two nuclei increases
the number of data points and therefore the reliability of the
assignment.
6 H.-O. Kalinovsky, S. Berger and S. Braun, Carbon-13 NMR
Spectroscopy, John Wiley and Sons, Salisbury, 1986, p. 92.
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Chem., 1996, 61, 8569; B. M. Trost, J. L. Belletire, S. Godleski,
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and R. Riguera, J. Org. Chem., 1999, 64,
and R. Riguera,
We thank Ministerio de Ciencia e Innovacion (CTQ2008-
´
4669; J. M. Seco, Sh. K. Latypov, E. Quinoa
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J. Org. Chem., 1997, 62, 7569; B. M. Trost, R. C. Bunt and
S. R. Pulley, J. Org. Chem., 1994, 59, 4202.
01110/BQU and CTQ2009-08632/BQU) and Xunta de Galicia
(PGIDIT09CSA029209PR) for financial support. We are also
9 M. J. Ferreiro, Sh. K. Latypov, E. Quinoa
Chem., 2000, 65, 2658; M. J. Ferreiro, Sh. K. Latypov, E. Quinoa
and R. Riguera, Tetrahedron: Asymmetry, 1997, 8, 1015.
´
and R. Riguera, J. Org.
grateful to the Centro de Supercomputacion de Galicia for
´
´
their assistance with the computational work, to Yamakawa
Chemical Industry Co. Ltd. (Japan) for their gift of (R)- and
(S)-MPA, to Bruker Espanola S.A. for its contribution as
Observant Development Entity (EPO), to Marcelo A. Munoz
(Universidad Austral de Chile) and to Pedro Joseph-Nathan
10 S. Porto, J. M. Seco, A. Ortiz, E. Quinoa
Org. Lett., 2007, 9, 5015.
11 I. Louzao, R. Garcıa, J. M. Seco, E. Quinoa
Org. Lett., 2009, 11, 53.
12 F. Freire, J. M. Seco, E. Quinoa
2005, 70, 3778; J. M. Seco, M. Martino, E. Quinoa
Org. Lett., 2000, 2, 3261.
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and R. Riguera,
´
´
and R. Riguera,
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and R. Riguera, J. Org. Chem.,
(Centro de Investigacion y de Estudios Avanzados del
´
Instituto Politecnico Nacional, Mexico) for helpful discussions
´
and R. Riguera,
´
13 V. Leiro, F. Freire, E. Quinoa
´
2005, 5554.
and R. Riguera, Chem. Commun.,
concerning NMR calculations. I. L. thanks MEC/MICINN
for a FPU fellowship.
14 M. J. Frisch, et al., Gaussian03, revision E.01, Gaussian, Inc.,
Wallingford, CT, 2004. For full reference see ESIw.
15 J. Boyd and N. R. Skrynnikov, J. Am. Chem. Soc., 2002, 124, 1832.
Notes and references
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z For the assignment of sec,sec-aminoalcohols, 13C DdRS data should
be complemented with 1H DdRS data. Also, in the case of carboxylic
acids, trans-2-phenylcyclohexanol (TPC) should be avoided as
CDA. Other functional groups (prim,sec-1,2-amino alcohols,16
prim,sec-diols17 and prim,sec,sec-1,2,3-triols,18 as well as b-chiral
primary alcohols19) have also been investigated but unfortunately
DdRS values lower than 0.1 ppm, too close to the experimental error
to allow a reliable assignment, are obtained.
17 F. Freire, J. M. Seco, E. Quinoa
´
2005, 11, 5509.
18 F. Freire, E. Lallana, E. Quinoa
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2009, 15, 11963; E. Lallana, F. Freire, E. Quinoa and R. Riguera,
Org. Lett., 2006, 8, 4449.
19 F. Freire, J. M. Seco, E. Quinoa
2007, 1456; Sh. K. Latypov, M. J. Ferreiro, E. Quinoa
R. Riguera, J. Am. Chem. Soc., 1998, 120, 4741; M. J.
Ferreiro, Sh. K. Latypov, E. Quinoa and R. Riguera, Tetrahedron:
Asymmetry, 1996, 7, 2195.
´
and R. Riguera, Chem. Commun.,
´
and
y The sign of C(2) is highly influenced by the acetyl group and
gives variable signs depending on the conformer being considered.
When the methyl ester is replaced by a methyl group, the calculated
´
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7903–7905 7905