Intramolecular hydrogen bonding of o-hydroxyesters and related compounds evaluated by deuterium isotope effects on 13C chemical shifts and principal component analysis

Article abstract of DOI:10.1016/j.molstruc.2007.06.012

o-Hydroxyaromatic esters, β-hydroxyketoesters, Meldrums acids, other o-hydroxyesters and a few o-hydroxyacid anhydrides are investigated by deuterium isotope effects on ¹³C chemical shifts, ^xΔC(OD). Especially the ester carbons show unusual deuterium isotope effects. A principal component analysis based on bond lengths and distances around the hydrogen bond six-membered ring make it possible to predict isotope effects. The ratio ⁿΔC{double bond, long}O(OD)/²ΔC(OD) can be used to predict the extent of transmission via the hydrogen bond. Furthermore, deuterium isotope effects can be used to gauge possible tautomerism.

Full text of DOI:10.1016/j.molstruc.2007.06.012

Available online at www.sciencedirect.com
Journal of Molecular Structure 844–845 (2007) 300–307
www.elsevier.com/locate/molstruc
Intramolecular hydrogen bonding of o-hydroxyesters and related
compounds evaluated by deuterium isotope effects
on C chemical shifts and principal component analysis
13
q
*
Poul Erik Hansen , Fadhil S. Kamounah, Saif Ullah
Department of Science, Systems and Models, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark
Received 19 March 2007; received in revised form 12 June 2007; accepted 13 June 2007
Available online 26 June 2007
Abstract
o-Hydroxyaromatic esters, b-hydroxyketoesters, Meldrums acids, other o-hydroxyesters and a few o-hydroxyacid anhydrides are
investigated by deuterium isotope effects on 13C chemical shifts, xDC(OD). Especially the ester carbons show unusual deuterium isotope
effects. A principal component analysis based on bond lengths and distances around the hydrogen bond six-membered ring make it pos-
sible to predict isotope effects. The ratio nDC@O(OD)/2DC(OD) can be used to predict the extent of transmission via the hydrogen bond.
Furthermore, deuterium isotope effects can be used to gauge possible tautomerism.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Deuterium isotope effects on 13C chemical shift; PCA; o-Hydroxy esters
1. Introduction
A recent study has demonstrated that isotope effects in
o-hydroxy acyl aromatics can be quite large at the hydro-
gen bonded carbonyl carbon. It was shown that the isotope
effects can be transmitted effectively through the hydrogen
bond and this contribution is positive for ketones [6] but
negative for thioketones [7]. It is rather remarkable that
nDC@O(OD) can vary from ꢀ0.034 in 2,4-diacetyl-3,5-
dihydroxybenzene [8] to +0.30 ppm in 3,5-dinitro-2-
hydroxyacetophenone [6]. In the present study deuterium
isotope effects on 13C chemical shifts are studied in
o-hydroxy esters. Esters are chosen as they in many cases
have been shown not to be tautomeric. This allows a large
span of different compounds to be studied as seen in
Scheme 1. A broad range of compounds is investigated
covering compounds with electron withdrawing substitu-
ents (motif a of Fig. 1), electron donating groups (motif
b), compounds with steric or ring strain as well as com-
pounds with varying double bond order between the donor
(OH) and the acceptor group (COOR) as seen in the naph-
thalenes, anthracenes and pyridines (Scheme 1). Included
are also a couple of compounds with acid anhydride motifs
(37 and 38).
Can o-hydroxy esters or o-hydroxylactones be tauto-
meric? By o-hydroxy esters or lactones we mean enolic
forms of b-ketoesters, o-hydroxy esters of salicylic acids
and similar aromatic compounds, acylated Meldrums
acids, etc. as seen in Scheme 1. The question of tautomer-
ism of esters has been addressed on a number of occasions
treated either experimentally [1] or theoretically [2,3]. Deu-
terium isotope effects on 13C chemical shifts due to deute-
rium substitution at the OH position have been used
extensively to study tautomerism [4,5]. The characteristic
of tautomerism is that isotope effects can be of both signs
and the effects can be observed far away from the centre
of deuteriation [4,5]. The study of acylated Meldrums acids
have indicated, based on these facts, that they possibly
could be tautomeric [1].
q
On the occasion of Professor Lucjan Sobczyk’s 80th birthday.
Corresponding author. Tel.: +45 46742432; fax: +45 46743011.
*
E-mail address: poulerik@ruc.dk (P.E. Hansen).
0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.06.012

P.E. Hansen et al. / Journal of Molecular Structure 844–845 (2007) 300–307
301
Scheme 1. Deuterium isotope effects on chemical shifts are given only for 2DC(OD) and nDCAO(OD) for clarity. For the other positions these can be found in
the respective papers or for compounds investigated in this study under the compounds in Experimental. (a) Taken from Ref. [26]. (b) Taken from Ref. [10]. (c)
Taken from Ref. [27]. (d) Taken from Ref. [28]. (e) Taken from Ref. [1]. (f) Taken from Ref. [29]. (g) Taken from Ref. [30]. (h) Taken from Ref. [31]. (i) Taken
from Ref. [32]. (j) Recorded at ꢀ50 °C in CD2Cl2.

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P.E. Hansen et al. / Journal of Molecular Structure 844–845 (2007) 300–307
the enol form [1]. Whether they are tautomeric or not
is a question of debate. The structure of 12 has likewise
been determined recently to be on the exocyclic OH form
[10].
Structures are calculated using the DFT method. Bonds
lengths and distances are given in Table 1S, 5-dinitro-2-
salicylate (29) the nitro group at 3-position is twisted
slightly out of the ring plane. The ring of the five-mem-
bered lactone (9) is slightly non-planar.
Fig. 1. Structural motifs.
DFT calculations are performed to calculate structures,
For many hydrogen bonded systems RO. . .O and ROH
correlate quite well. This is not the case for the present
esters as seen in Fig. 2.
nuclear shieldings and changes in nuclear shieldings.
The observed isotope effects and OH chemical shifts are
analysed using principal component analysis (PCA) and
partial least squares regression (PLS) (Fig. 4).
3. PLS analysis
A PLS analysis is performed involving distances around
the hydrogen bond system (Fig. 3) and nDC@O(OD)
(Fig. 4). It also shows that the data roughly can be divided
into two groups, those of the olefinic and those of the aro-
matic compounds (Fig. 5). Furthermore, all the parameters
seem to be important (Fig. 6 and Table 2). The same result
is basically obtained from the analysis using the
nDC@O(OD)/2DC@O(OD) ratio (Fig. 7).
2. Results
Deuterium isotope effects on 13C chemical shifts are
measured in a number of new esters and are given in
Scheme 1 and Table 1 together with data from the litera-
ture including both olefinic and aromatic types. OH chem-
ical shifts are also given in Scheme 1. Deuterium isotope
effects on chemical shifts have been measured at different
degrees of deuterium enrichment to ensure the signs of
the isotope effects. The secondary deuterium isotope effects
are defined as DC(OD) = dC(OH) ꢀ d(OD). Isotope effects
have also been measured at different temperatures to make
sure that e.g. conformational equilibria do not play a role.
This is particularly important for compounds like 3-nitro
(30) and 3,5-dinitro (29) in which the OH group could pos-
sibly form hydrogen bonds to both the ester and the nitro
group leading to an equilibrium situation [9]. As the iso-
tope effects showed no temperature variation, this was
found not to be the case. It is seen that the ratio between
The PLS analysis gave a very good fit correlating the
n
distances shown in Fig. 2 and Table 1S with DC@O(OD)
(Fig. 6). It is seen that the dinitro derivative (29) and the
anhydride 37 fall outside the plot. In the correlations with
n
the ratio DC@O(OD)/2DC@O(OD) 38 was falling way off
the correlation (Fig. 7).
nDC@O(OD) and DC@O(OD) varies quite considerably
for the different compounds, but also that DC@O(OD)
2
n
can be very large for compounds of the acylated Meldrums
acids [1], etc. (Table 1 and Scheme 1).
The present analysis is based on structural parameters.
The acyl Meldrums acids (13–17) are found to exist on
Fig. 2. Plot of OÆÆÆO distances vs. OH bond lengths.
Table 1
nDC@O(OD)/2D(OD) ratios
Compound
nD/2D
Compound
nD/2D
Compound
nD/2D
1
2
0.350
0.414
0.415
0.477
0.472
0.375
0.374
0.325
0.351
0.395
0.499
0.938
0.684
14
15
16
17
18
19
20
21
22
23
24
25
26
0.723
0.693
0.710
0.743
0.663
0.632
0.432
0.475
0.470
0.442
0.456
0.443
0.433
27
28
29
30
31
32
33
34
35
36
37
38
0.407
0.385
0.617
0.548
0.413
0.430
0.465
0.383
0.418
0.414
0.867
0.393
3
4
5
6
7
8
9
10
11
12
13
Fig. 3. Notations of the hydrogen bond motif.

P.E. Hansen et al. / Journal of Molecular Structure 844–845 (2007) 300–307
303
3.1. Chemical shift calculations
The changes in the chemical shifts upon OH bond
stretching have been calculated. These are relevant for
the estimation of deuterium isotope effects on 13C chemical
shifts according to the Jameson equation [11]:
ꢀ
ꢁ
dr
hri ꢀ hrꢁi ¼
½hDr1i ꢀ hDr1iꢁꢂ
dr1
e
ꢀ
ꢁ
d2r
dr21
h
i
Fig. 6. Correlation loadings.
2
2
þ
hðDr1Þ i ꢀ hðDr1Þ iꢁ þ ꢃ ꢃ ꢃ
ð1Þ
Table 2
*
heavier isotope (marked with ).
Coefficients from Fig. 4
In our case we can concentrate on the ratio between
nDC@O(OD)/2DC(OD). In other words only the slope of
C@O and CAOH carbons are necessary. Calculations
show that the calculated ratio of the slopes are 1.15 for
the acylated Meldrum acids and even to 1.0 for the acid
anhydride compounds like 37 but also close to 1.0 for ethyl
Bond lengths
Coefficients
ROO
RHO
ROH
RC@O
CAO
C@C
ꢀ2.20Eꢀ02
ꢀ3.10Eꢀ02
4.94Eꢀ02
ꢀ3.95Eꢀ02
ꢀ5.69Eꢀ02
5.36Eꢀ03
n
salicylate (21). In the latter case the DC = O(OD) is over-
estimated as judged from the experimental values (Table 1).
CACO
1.10Eꢀ03
Fig. 4. Correlations based on nDC@O(OD) and distances around the hydrogen bond motif.
Fig. 5. Score plot.

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P.E. Hansen et al. / Journal of Molecular Structure 844–845 (2007) 300–307
Fig. 7. Correlations based on the ratio nDCAO(OD)/2D(OD) and distances around the hydrogen bond motif.
4. Discussion
Another interesting observation is the large variation in
the ratio between nDC@O(OD) and 2DC(OD).
2DC@O(OD) isotope effects have been used extensively to
judge the strength of the hydrogen bond [12]. By taking
According to the Jameson theory [11] isotope effects of
the CAx(OD) type depend on the change in the OH bond
length upon deuteriation multiplied by the change in the
nuclear shielding of the carbon, x, in question (see Eq.
(1)). One may therefore expect the effect at CAx to be pro-
portional to that of the two-bond isotope effect. This is
exactly what was found studying a large group of com-
pounds [30]. The exception was C@O carbon. For nitro
substituted o-hydroxyacyl aromatics it was furthermore
found that the nDC@O(OD) effect could become very large
[6].
n
the ratio DC@O(OD)/2DC(OD) the hydrogen bond prop-
erties are therefore included. The PLS analysis of bond
lengths and the nDC@O(OD)/2DC(OD) ratio gave a picture
not very different from that found for nDC@O(OD) as seen
in Fig. 7. However, the anhydride type (38) falls clearly
outside the plot.
The changes in the C@O chemical shifts for the different
compounds are rather limited. Mainly, the sterically hin-
dered ones like 3 and 12 show higher chemical shifts. How-
ever, the calculated variations in the 13C chemical shifts
upon variation of the OH bond lengths are quite different,
In the present series of compounds it can be seen that
n
2
the ratio between DC@O(OD) and DC(OD) varies quite
considerably, being close to 0.35 for simple esters like 1
and increasing to 0.74 for the acylated Meldrum
acids and even to 0.87 for the acid anhydride compounds
like 37 (Table 1). It is obvious for the latter two categories
that they have mesomerically electron-withdrawing
substituents at the C-2 carbon of the olefinic compounds
or at the C-3 or C-5 positions in the aromatic carbons.
In the latter case a similar pattern was seen for
o-hydroxynitroacylaromatics.
A PLS analysis based on bond lengths around the
hydrogen bond system and nDC@O(OD) sort the molecules
in a similar fashion as seen in Fig. 4. The correlation coef-
ficients are given in Table 2. However, it is obvious that
data for the dinitro (29) and 38 are falling outside the cor-
relation. The reason for the former is the rather short
CAOH distance calculated. The rather good fit could
˚
˚
for 12 44 ppm/A [10] and for 37 41 ppm/A. For the simple
˚
compounds like methyl salicylate (21) we find 21 ppm/A.
The isotope effects can be analysed in terms of bond
orders and related to that optimal pathways. Motif A will
n
give the largest positive DC@O(OD), whereas those of B
and C give small values close to zero. For the two-bond
isotope effect, 2DC@O(OD), the trend is different. The
strongly resonance assisted case will give large two-bond
isotope effects (motif C), whereas motif A will not. Taking
n
the ratio DC@O(OD)/2DC(OD) is therefore a very good
guide to the importance of the mesomeric forms and as
the transmission pathway through the hydrogen bond is
clearly dependent on orbital overlap, the ratio tells about
the orbital overlap directly. Ratios of isotope effect can
be calculated by following the Jameson scheme [11]. As
the variation in the OH bond length is the same for
n
n
clearly indicate that the DC@O(OD) isotope effects can
2DC(OD) and DC@O(OD), all we have to do is to calcu-
be explained based on distances and bond lengths and that
the present compounds are not tautomeric although this
was suggested a while back based on the large nDC@O(OD)
values, the isotope effect observed at the C-2 of the acylated
Meldrums acids as well as theoretical calculations of enols
[1–3].
late the change in the nuclear shielding upon bond stretch-
ing. For 12 the calculations showed that the effects at C-2
and C-6 are roughly the same magnitude leading to a ratio
of 1.15 compared to the experimentally determined one of
0.93. For 37 we calculate a ratio of 1.0 compared to the
experimentally one of 0.87. For methyl salicylate (21) a

P.E. Hansen et al. / Journal of Molecular Structure 844–845 (2007) 300–307
305
ratio of 1.0 was found, which is clearly too large compared
to 0.475 found experimentally. The large ratios are
obtained for non-tautomeric compounds. In other words
was heated to reflux for 15 h. After cooling to 4 °C, a lime
solid starts to crystallize out, filtered and washed with cold
methanol. The solid was recrystallized from methanol as a
pale lime crystals (0.56 g; 95%), mp 130 °C (lit. [13], mp
129–130 °C). 13C (CD2Cl2) d (nDCAx(OD)), 170.3 (0.102)
COOR, 156.1 (0.186) C-2, 137.8 (0.061) C-3, 132.4 C-4,
136.4 C-6, 119.1 C-5, 115.3 C-1, 54.1 OCH3, m/z 197.14.
n
it is possible to have large positive DC@O(OD) isotope
effects without invoking tautomerism. The key carbon in
this study is the COOR one. Furthermore, the overlap
depends strongly on the distance between heavy atom
acceptor and donor, ROO as well as the C@O and OH
bond lengths. Followingly, such isotope effects should also
be possible in intermolecular cases or in intramolecular
non-RAHB cases. In addition, as the esters are similar to
amides (in this study a few anhydride type compounds were
also included) we may assume that the findings described
above will cover acid derivatives in general. o-Hydroxya-
cylaromatics have already been investigated and show
much the same trends although the balance between
through bond and through hydrogen bonds is different.
6.2.2. Methyl 5-nitrosalicylate (28)
A solution of 5-nitrosalicylic acid (1.0 g; 5.45 mmol) in
absolute methanol (25 ml) was treated at room temperature
with concentrated sulfuric acid (0.8 ml) and heated as
described for compound 30. The crystallized white solid
was filtered and recrystallized from methanol as white crys-
tals (0.97 g; 90%), mp 118–119 °C (lit[14], mp 115–116 °C).
13C NMR (CD2Cl2) d (nDCAx(OD)) 169.8 (0.093) COOR,
166.4 (0.208), 140.0 (ꢀ0.02) C-5, 131.0 C-4, 127.0 (ꢀ0.016)
C-6, 119.0 (0.089) C-3, 112.4, (0.029) C-1, 54.0 OCH3, m/z
197.15.
5. Conclusions
o-Hydroxyaromatic esters, b-hydroxyketoesters, Mel-
drums acids, other o-hydroxyesters and a few o-hydroxy-
acid anhydrides are investigated by deuterium isotope
effects on 13C chemical shifts. The carbonyl carbons show
deuterium isotope effects that vary very much in magni-
tude. Large magnitudes are related to mesomeric substitu-
ents and to steric strain. The correlations can be explained
by bond length variations and do not require assumptions
of tautomerism. The isotope effects can be estimated using
bond lengths around the hydrogen bonded six-membered
6.2.3. Methyl 3,5-dinitrosalicylate (29)
A solution of 3,5-nitrosalicylic acid (1.0 g; 4.38 mmol) in
absolute methanol (25 ml) was treated at room temperature
with concentrated sulfuric acid (0.6 ml) and heated as
described for compound 30. The crystallized pale yellow
solid was filtered and recrystallized from methanol as pale
yellow needless (0.99 g; 94%), mp 129–130 °C (lit. [15], mp
129–130 °C). 13C NMR (CD2Cl2) d (nDCAx(OD)) 169.2
(0.142) COOR, 160.5 (0.214) C-2, 137.4 (0.057) C-3,
138.1 ( 0.044) C-5, 131.2 C-6, 127.8 C-4, 115.7 C-1, 54.1
OCH3, m/z 242.14.
n
ring. The ratio between DC@O(OD)/2DC(OD) is a good
measure of transmission of isotope effects across the hydro-
gen bond.
6.2.4. Methyl 3-hydroxyisonicotinate (36)
A
solution of 3-hydroxyisonicotinic acid (1.0 g;
6. Experimental
4.38 mmol) in absolute methanol (25 ml) was treated at
room temperature with concentrated sulfuric acid (1.6 ml)
and heated as described for compound 30. The mixture
was cooled to room temperature and poured into ice, and
the ice-cooled solution was treated with small portions of
solid NaHCO3 to pH 7.2, and the precipitated white solid
was filtered, washed several times with water and dried in
vacuum and then recrystallized from n-hexane as a white
crystals (0.88 g; 80%), mp 80–81 °C (lit. [16], 78.5–
80.5 °C). 13C NMR (CDCl3) d (nDCAx(OD)) 170.6
(0.077) COOR, 159.1 (0.186) C-3, 141.9 C-6, 130.1
C(0.032) C-1, 126.3 (0.077) C-2, 52.4 OCH3, m/z 153.12.
6.1. Chemicals
Compounds 2, 4, 10: 3-Hydroxy-2-anthracene carbox-
ylic acid potassium salt, 2-hydroxynicotinic acid, 3-hydrox-
yisonicotinic acid and 1,3-cyclohexanedione were
purchased from Aldrich Chemicals, Weinheim, Germany,
5-nitrosalicylic acid and 3,5-dinitrosalicylic acid from
Acros and 3-nitrosalicylic acid from Fluka.
6.2. Synthesis of o-hydroxy esters
All organic solvents and inorganic reagents were of ana-
lytical grade and used as received. Melting points were
determined on a Gallenkamp MF-370 apparatus and are
uncorrected. GC–MS measurements were performed on a
Hewlett Packard 5890 Series II instrument.
6.2.5. Methyl 6-hydroxy-2-oxo-6-cyclohexene-1-carboxylate
(11)
Modified methods of Kim [17] and Burger [18] were
used. A solution of 1,3-cyclohexanedione (1.12 g; 10 mmol)
and triethylamine (3.0 ml) in dry dichloromethane (20 ml)
was treated dropwise at room temperature with a solution
of methyl chloroformate (0.95 g; 10 mmol). The mixture
was left stirred at room temperature with stirring under a
nitrogen atmosphere for 20 h. The solvents were evapo-
rated in vacuum, and the solid residue was dissolved in
6.2.1. Methyl 3-nitrosalicylate (30)
A solution of 3-nitrosalicylic acid (0.55 g; 3.0 mmol) in
absolute methanol (25 ml) was treated at room temperature
with concentrated sulfuric acid (0.6 ml) and the mixture

306
P.E. Hansen et al. / Journal of Molecular Structure 844–845 (2007) 300–307
anhydrous acetonitrile (30 ml), followed by triethylamine
(3.0 ml) and catalytic amount (20 mg) of KCN. The mix-
ture was stirred at room temperature for 20 h, then the sol-
vents were evaporated in vacuum, and the residual pale
pink solids was dissolved in chloroform and washed with
1.0 N HCl, water and finally dried over anhydrous MgSO4.
Evaporation of solvent in vacuum affords a light amber oil
6.3. NMR
1
The H NMR (300 or 600 MHz) and 13C NMR (75 or
150 MHz) spectra of the dissolved samples were recorded
on a Varian Mercury 300 or an Inova 600 spectrometer,
using TMS as internal standard. Solvents were CD2Cl2
and CDCl3 for low temperature work.
(1.29 g; 76%). 13C NMR (CD2Cl2 at ꢀ50 °C)
d
(nDCAx(OD)) 193.8 C-6, 192.6 (0.558) C-2, 173.1 (0.283)
COOR, 105.1 (ꢀ0.056) C-1, 53.3 OCH3, 38.9 (C-6), 31.4
(0.138) C-3, 19.6 C-4, m/z 170.16.
6.4. Deuteriation
The compounds were dissolved in CH3OD with subse-
quent evaporation under vacuum.
6.2.6. Dimethyl (hydroxymethylene) malonate (5)
This compound was synthesized by the method of
Katagiri and coworkers [19].
6.5. Calculations
The crude white oil, which solidified on standing in cold,
was recrystallized from n-hexane as a colourless fine need-
less (82%), mp 44–45 °C [19] (lit. 43–45 °C). 1H NMR
DFT. The molecular geometries were optimised using
the Gaussian03 suite of programs [20] and Density Func-
tional Theory (DFT) (Beckes [21] exchange and Lee, Yang,
Parr [22] correlation term, B3LYP and basis set 6-31G(d,p)
was used. The nuclear shieldings were calculated using the
GIAO approach [23,24].
(CDCl3)
d
13.30 (d, J = 13.2 Hz) OH, 8.35 (d,
J = 12.9 Hz) H-3, 3.91. OCH3, 3.77 OCH3. Very similar
data are obtained in CD2Cl2. 13C NMR (CD2Cl2 at
ꢀ50 °C) d (nDCAx(OD)). 175.8(0.538) C-3, 171.9 (0.259)
COOR, 164.1 COOR, 100.5 (0.077), 53.2 OCH3, 52.2
OCH3, m/z 106.12.
PCA. The PCA and PLS analysis was done using The
Unscramler program [25].
Acknowledgements
6.2.7. Methyl 3-hydroxy-2-anthracenecarboxylate (35)
Potassium 3-hydroxy-2-anthracenecarboxylate (0.952 g;
3.45 mmol) was dissolved in a mixture of methanol
(80 ml) and toluene (20 ml) in the presence of conc.
H2SO4 (0.5 ml). The mixture was refluxed for 20 h under
nitrogen atmosphere, and then cooled to room tempera-
ture; a yellow solid was deposited on standing. The solid
was filtered and then purified by flash column chromatog-
raphy on silica gel, using chloroform as eluent. Evapora-
tion of solvent in vacuum affords a yellow solid, which
was recrystallized from chloroform–methanol as a yellow
flakes (0.64 g; 74%), mp 188–189 °C. 1H NMR
(300 MHz, CDCl3) d 10.28 (s), 8.17 (d, J = 0.6 Hz), 8.40
(s), 8.19 (s), 7.90 (d, t, J = 8.2 and J = 0.6 Hz), 7.40 (m),
4.03, 13C NMR(CDCl3) d (nDCAx(OD)) 170.6 (0.061)
COOR, 155.0 (0.146) C-3, 135.2 (0.012) C-4a, 134.9,
131.0, 129.5 C-1, 129.1, 128.1, 127.5, 126.8, 125.3, 124.0,
116.2 (0.036) C-2, 109.9 (0.072) C-4, 53.2 OCH3, m/z
252.25.
The authors thank Rita Buch for her excellent work and
Simon Bolvig for performing some of the early measure-
ments of unsaturated esters.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
2007.06.012.
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Compound 2: 13C NMR (CDCl3) d (nDCAx(OD)) 183.6
(0.389) C-2, 173.1 (0.161) COOR, 86.8 C-3, 50.0 (CH2)
33.8 (0.048) C-4, 18.2 CH3, 14.4 CH3.
Compound 4: 13C NMR (CDCl3) d (nDCAx(OD)) 172.8
(0.174) COOR, 171.0 (0.365) C-3. Remaining resonances
not identified unambiguously.
Compound 9: 13C NMR (CDCl3) d (nDCAx(OD)) 176.3
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C-4.
Compound 10: 13C NMR (CDCl3 at ꢀ20 °C) d
(nDCAx(OD)) 172.4 (0.147) COOR, 165.1 (0.372) C-2,
137.1 (C-4), 123.0 (0.086) C-3, 100.0 C-1 tentative, 60.1
CH2, 32.3, 24.8, 19.2, 14.2 CH3.

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