Diastereoselective transformations of enol esters derived from acetylenes and chiral carboxylic acids

Article abstract of DOI:10.1055/s-0030-1260416

Markovnikov-type enol esters are synthesized selectively from N-protected amino acids by ruthenium-mediated coupling with the appropriate acetylenes. Subsequent hydrogenation of the enol esters over Adams' catalyst gives the corresponding saturated products in moderate to good diastereoselectivities. The enol esters undergo reaction with m-chloroperoxybenzoic acid to yield -acyl-oxy ketones, as the products of rearrangement, instead of the expected epoxides. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0030-1260416

PAPER
1809
Diastereoselective Transformations of Enol Esters Derived from Acetylenes
and Chiral Carboxylic Acids
D
iastereo
w
selective Transfo
o
r
mations of
En
nol Esters a M. Kalinowska,a Joanna Szawkało,a Krzysztof K. Krawczyk,a Jan K. Maurin,b,c Zbigniew Czarnocki*a
a
Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
Fax +48(22)8225996; E-mail: czarnoz@chem.uw.edu.pl
b
c
National Medicines Institute, Chełmska 30/34, 00-750 Warsaw, Poland
Institute of Atomic Energy, 05-400 Otwock-Świerk, Poland
Received 8 March 2011; revised 31 March 2011
examined by Sharpless who found it to be synthetically
useful despite some limitations. Several electron-deficient
olefins, such as a,b-unsaturated esters and amides, did not
undergo this aziridination reaction.6
Abstract: Markovnikov-type enol esters are synthesized selective-
ly from N-protected amino acids by ruthenium-mediated coupling
with the appropriate acetylenes. Subsequent hydrogenation of the
enol esters over Adams’ catalyst gives the corresponding saturated
products in moderate to good diastereoselectivities. The enol esters
undergo reaction with m-chloroperoxybenzoic acid to yield a-acyl-
oxy ketones, as the products of rearrangement, instead of the ex-
pected epoxides.
Another reaction of enol esters involves oxidation of the
double bond to form epoxides. Following rearrangement
promoted by Lewis acids7 or high temperatures,8 a-acyl-
oxy ketones or aldehydes can be obtained.
Key words: enol esters, diastereoselectivity, hydrogenation, epoxi-
Significant research has been devoted toward the effective
and selective synthesis of enol esters. The direct function-
alization of acetylenes with various carboxylic acids in the
presence of a catalyst has been studied intensively for this
purpose. Initial attempts employed stoichiometric
amounts of mercury salts.9 Later, the introduction of
iridium10 and rhenium11 complexes allowed considerable
reduction of the catalyst loading. However, ruthenium12
or ruthenium–phosphine catalysts (for example, triruthe-
nium dodecacarbonyl [Ru3(CO)12] and bis(cycloocta-
dienyl)ruthenium–tri-n-butylphosphine) were the most
common.13 In 2003, Moise et al. reported on the applica-
tion of titanium–ruthenium heterobimetallic complexes
for the synthesis of the Markovnikov enol formates.14
dation, ruthenium
The presence of a carbon–carbon double bond in enol es-
ters allows their application in many different types of re-
actions in sustainable organic chemistry. Enol esters can
be utilized effectively in [2+2], [2+4] and 1,3-dipolar cy-
cloadditions,1 and in cyclopropanation reactions.2
In 2003, Reetz and Goossen reported that complexation of
a rhodium catalyst with 1,1¢-bi(2-naphthol) (BINOL) de-
rived monodentate phosphites afforded efficient catalysts
for the asymmetric hydrogenation of enol esters.3 Many
other catalysts (for example, palladium, ruthenium, rhod-
ium) and ligands have been applied in this hydrogenation
giving products with high enantioselectivity (ee >90%).4
In our investigations, we synthesized the Markovnikov-
type enol esters using the method reported by Goossen,15
starting from protected amino acid derivatives and termi-
nal or internal alkynes, with the catalyst generated in situ
from dichloro(p-cymene)ruthenium(II) dimer [Ru(p-
cymene)Cl2]2 as the ruthenium precursor, and tri(2-fur-
yl)phosphine [(Fur)3P] as the ligand (Scheme 1). Herein,
we describe a procedure for the synthesis of chiral second-
ary alcohols via reduction and hydrolysis of enol esters
derived from acetylenes and chiral carboxylic acids.
On the other hand, multicomponent reactions have be-
come popular for the synthesis of complex structures. The
Mannich-type reaction of a primary amine, an aldehyde
and isopropenyl acetate under mild conditions, to yield a
secondary or tertiary amine,5 is a good example.
The bromine-catalyzed aziridination of olefins and ring-
opening of aziridines has produced building blocks suit-
able for the synthesis of important nitrogen-containing
compounds. In the case of enol esters, this reaction was
O
[Ru(p-cymene)Cl2]2
(Fur)3P, Na2CO3
toluene, 65 °C
R1
R2
*R
OH
O
R1
O
O
R1
+
+
R2
+
*R
O
R1
R2
*R
O
*R
O
R2
(Z)
(E)
Markovnikov product
anti-Markovnikov products
Scheme 1 Synthesis of enol esters
SYNTHESIS 2011, No. 11, pp 1809–1813
x
x.
x
x
.
2
0
1
1
Advanced online publication: 29.04.2011
DOI: 10.1055/s-0030-1260416; Art ID: T25711SS
© Georg Thieme Verlag Stuttgart · New York

1810
I. M. Kalinowska et al.
PAPER
Table 1 Synthesis of Enol Esters (Markovnikov Products) According to Scheme 1a
Entry
*R
R1
R2
Product
Time (d)
Yield (%)b
[a]D25 (c, CHCl3)
+11.6 (c 1.08)
–12.0 (c 0.88)
– 6.0 (c 1.01)
–14.0 (c 1.05)
– 6.6 (c 0.99)
1
2
3
4
5
N-Cbz-L-Trp
N-Cbz-D-Trp
N-Pht-L-Ala
N-Pht-L-Ala
N-Pht-L-Ala
H
C4H9
C4H9
Ph
1
2
3
4
5
6
5
99
91
93
69
4
H
H
6
CH2OMe
Ph
CH2OMe
Ph
5
10
a Reaction conditions: *RCOOH (1.6 mmol), R1CCR2 (2.1 mmol), Na2CO3 (0.04 mmol), (Fur)3P (0.02 mmol), [Ru(p-cymene)Cl2]2 (0.01
mmol), toluene, 65 °C.
b Yield of isolated product after column chromatography.
Table 2 Hydrogenation of Compounds 1 and 2a
We obtained several enol esters 1–5 starting from protect-
ed amino acids and the results are presented in Table 1.
The selectivity in favor of the Markovnikov product was
>99% in all cases. The use of L- and D-tryptophan gave, as
expected, comparable results, although the increase in the
yield as a result of the extended reaction time was associ-
ated with lower optical purity of the product (Table 1, en-
tries 1 and 2).
Entry Solvent Substrate Time Yield 6a/6bc [a]D25 (c, CHCl3)
(h)
20
20
5
(%) of 6b
1
2
3
4
i-PrOH
AcOH
AcOH
i-PrOH
1
1
1
2
84
80
80
78
78:22 +16.0 (c 0.99) (6a)
77:23 +16.0 (c 0.98) (6a)
80:20 +16.0 (c 1.13) (6a)
24:76 –15.9 (c 0.82) (6b)
In order to take advantage of the chiral auxiliary present
in the enol ester, we next performed hydrogenation of the
double bond using an achiral catalyst. Several N-protected
amino acids were tested, but only with the tryptophan-
containing products 1 and 2 were the diastereoselectivities
of the reductions found to be promising (Scheme 2).
5
a Reaction conditions: enol ester (0.71 mmol), PtO2 (0.071 mmol), H2,
i-PrOH or AcOH.
b Combined yield of 6a and 6b.
c Ratio determined by HPLC.
We established that Adams’ catalyst (PtO2) was more ef-
fective than palladium on carbon (Pd/C) or rhodium on
carbon (Rh/C), however, changing the solvent or lowering
the reaction temperature did not improve the diastereose-
lectivity. Table 2 contains selected data for the hydroge-
nation step.
Next, aliphatic esters 6 were hydrolyzed to yield the re-
spective enantiomerically enriched hexan-2-ols. When
the mixture of esters derived from L-tryptophan derivative
1 was hydrolyzed, (S)-(+)-hexan-2-ol was obtained in
69% ee, whereas D-tryptophan derivative 2 gave (R)-(–)-
hexan-2-ol in 67% ee. These results were consistent with
the HPLC analysis of the diastereomeric mixture after the
hydrogenation. Isolation of ester 6a by column chroma-
tography and subsequent hydrolysis enabled the forma-
tion of enantiomerically pure (S)-(+)-hexan-2-ol. The
absolute configuration of compound 6a was confirmed
unambiguously by X-ray crystal structure analysis
(Figure 1).
Figure 1 ORTEP diagram of 6a16
In order to extend the synthetic utility of chiral enol esters
to other transformations, we also studied the epoxidation
of 3 and 4 with m-chloroperoxybenzoic acid (Scheme 3).
Surprisingly, even under mild conditions (0 °C), we ob-
served rearrangement of the initially formed epoxide into
the corresponding a-acyloxy ketones 7 and 8; generally,
O
O
O
H2/PtO2
AcOH
(or i-PrOH)
r.t.
O
O
O
NH
NH
NH
O
O
O
C4H9
+
O
C4H9
O
O
C4H9
NH
NH
NH
6b
1
6a
Scheme 2 Hydrogenation of compound 1
Synthesis 2011, No. 11, 1809–1813 © Thieme Stuttgart · New York

PAPER
Diastereoselective Transformations of Enol Esters
1811
MCPBA, CHCl3
0 °C
after
rearrangement
O
O
O
O
O
O
O
O
O
N
N
N
O
R2
R1
R2
O
*
R2
O
O
O
R1
R1
3 R1 = H, R2 = Ph
4 R1 = R2 = CH2OMe
7
R1 = H, R2 = Ph 55%, [α]D25 +26.4 (c 1.0)
8a R1 =
8b R1 =
CH2OMe, R2 = CH2OMe
CH2OMe, R2 = CH2OMe
60%, 8a/8b = 5:3
Scheme 3 Synthesis of a-acyloxy ketones 7 and 8
Table 3 The Observed and Simulated Chemical Shifts (d, ppm) for Protons Near the Newly Formed Stereocenter in Diastereomers 8a and 8b
Compound 8
Observed 1H NMR data
Simulated 1H NMR data18 for diastereomers 8a and 8b
Fragment
Proton
HA
Major component Minor component Dd
8a (S,R)
4.07
8b (S,S)
4.04
Dd
3.87
3.70
5.44
3.83
3.60
5.40
0.04
0.10
0.04
0.03
0.03
0.16
O
HC
O
HA
C
HB
3.81
3.78
HB
OCH3
HC
4.92
4.76
this type of rearrangement is observed at higher tempera- spectra were computed within the latter. The simulated
tures.8 The structure of compound 7 was confirmed by X- spectra for 8a and 8b resembled those obtained in a previ-
ray crystal structure analysis (Figure 2). The epoxidation ous simulation approach18 thus supporting the stereo-
of enol ester 4 led to the formation of diastereomeric com- chemical assignments.
pounds 8a and 8b in the ratio 5:3 and in a combined 60%
yield.
In summary, we have described new applications of enol
esters in synthetic organic chemistry. The introduction of
the carboxylic acid moiety allows diastereofacial func-
tionalization of the double bond which may find applica-
tions in procedures aimed at preparing enantiomerically
enriched products.
Anhydrous solvents were used for all reactions. TLC analyses were
performed on silica gel plates (Merck Silica Gel 60 F254) and made
visual using a UV lamp and I2 vapor. Melting points were deter-
mined on a Boetius hot-plate microscope and are uncorrected. Col-
umn chromatography was carried out at atmospheric pressure using
Merck Silica Gel 60 (230–400 mesh) or Al2O3 using mixtures of
hexane–i-PrOH or hexane–Et2O as eluents. HPLC analyses were
performed using Knauer (model 64) instrumentation (4 mm × 250
Figure 2 ORTEP diagram of 7;17 two independent molecules diffe-
ring slightly in geometry are present in the asymmetric part of the unit
cell of the crystal
Unfortunately, despite numerous attempts, we were un-
able to obtain a crystal suitable for X-ray analysis, and mm Lichrosorb Si60 column) with Eurochrom 2000 software. The
NMR spectra were obtained on a Varian Unity Plus spectrometer at
therefore the stereochemical assignments of 8a and 8b
200 MHz or 500 MHz for 1H NMR and at 50 MHz or 125 MHz for
1
were based solely on analysis and simulation18 of the H
13C NMR. The spectra were recorded in CDCl3 and chemical shifts
NMR spectra. As shown in Table 3, a similar deshielding
(d) are reported in ppm relative to TMS. Mass spectra were obtained
trend for protons HA, HB and HC, being located in the vi-
using Quattro LC Micromass and LCT Micromass TOF HiRes ap-
cinity of the newly formed stereocenter, was observed al-
lowing tentative assignment of the stereochemistry of 8a
as S,R. This hypothesis seemed to be further supported by
quantum mechanical calculations using Spartan’08 soft-
ware. Initially, the four most stable conformers for both
diastereomers were determined using molecular mechan-
ics. Those conformers were then minimized using density
functional theory (DFT) calculations within the 6-31G*
basis set. The resulting geometries were used as an input
for further density functional theory calculations, employ-
paratus. Optical rotations were measured using a Perkin-Elmer 241
polarimeter. Single crystal X-ray measurements were carried out on
an Oxford Diffraction Xcalibur R k-axis diffractometer with a CCD
Ruby detector. In all cases, CuKa characteristic radiation was ap-
plied. After initial corrections and data reduction, reflection intensi-
ties were used to solve and refine consecutively the structures using
SHELXS9719 and SHELXL9720 programs. Further absorption cor-
rections were applied in the final refinement steps.
Enol Esters 1–5; General Procedure
Protected amino acid derivative (1.6 mmol) and Na2CO3 (4.2 mg,
0.04 mmol) were suspended in anhyd toluene (9 mL). A soln of
1
ing a more adequate cc-pVDZ basis set. The H NMR
Synthesis 2011, No. 11, 1809–1813 © Thieme Stuttgart · New York

1812
I. M. Kalinowska et al.
PAPER
(Fur)3P (4.6 mg, 0.02 mmol), [Ru(p-cymene)Cl2]2 (6.1 mg, 0.01
mmol) and the acetylene (2.1 mmol) in anhyd toluene (2 mL) was
added. The mixture was heated at 65 °C until TLC indicated com-
plete conversion of the amino acid derivative. The solvent was
evaporated and the residue dissolved in CHCl3 (10 mL). The organ-
ic layer was washed with sat. aq NaHCO3 soln (3 × 4 mL) and dried
over anhyd MgSO4. The solvent was removed and the product pu-
rified by column chromatography over silica gel (eluent: hexane–i-
PrOH).
MS (ESI+): m/z (%) = 445.2 (100) [M + Na]+, 867.4 (20) [2M +
Na]+.
(S)-(+)-Hexan-2-ol
Compound 6a (0.046 g, 0.11 mmol) was added to a mixture of H2O
(2 mL), EtOH (2 mL) and KOH (0.4 g). The resulting mixture was
heated at reflux temperature until ester 6a had been consumed
(TLC) and was then extracted with CHCl3 (2 × 1 mL). The com-
bined organic phase was dried over anhyd MgSO4 and evaporated
to afford (S)-(+)-hexan-2-ol.
Hex-1-en-2-yl N-Carbobenzyloxytryptophanate (1)
Yellow solid; yield: 1.33 g (99%); mp 58–61 °C; [a]D +11.6 (c
1.08, CHCl3).
Colorless liquid; yield: 6 mg (54%); ee >98%; [a]D25 +11.8 (c 0.56,
25
CHCl3) {Lit.21 [a]D +12.0 (c 0.2, CHCl3)}. All other analytical
25
data were consistent with those reported in the literature.21
1H NMR (500 MHz, CDCl3): d = 8.11 (br s, 1 H), 7.58 (d, J = 7.8
Hz, 1 H), 7.37–7.28 (m, 5 H), 7.19 (t, J = 7.1 Hz, 1 H), 7.10 (t,
J = 7.5 Hz, 1 H), 7.00–6.99 (m, 1 H), 5.33 (d, J = 8.3 Hz, 1 H), 5.12
(d, J = 12.5 Hz, 1 H), 5.08 (d, J = 12.5 Hz, 1 H), 4.80–4.77 (m, 1 H),
4.68 (br s, 1 H), 4.60 (br s, 1 H), 3.39–3.31 (m, 2 H), 2.09–2.04 (m,
2 H), 1.62 (s, 1 H), 1.37–1.25 (m, 4 H), 0.86 (t, J = 7.3 Hz, 3 H).
13C NMR (125 MHz, CDCl3): d = 170.3, 156.4, 155.7, 136.3, 136.1,
128.5, 128.2, 128.1, 127.6, 122.8, 122.3, 119.8, 118.8, 111.2, 109.8,
101.3, 66.9, 54.7, 32.8, 28.4, 27.9, 22.0, 13.8.
2-Oxo-2-phenylethyl (2S)-2-(1,3-Dioxo-1,3-dihydro-2H-isoin-
dol-2-yl)propanoate (7)
Enol ester 3 (0.4 g, 1.3 mmol) was dissolved in CHCl3 (2 mL) and
added dropwise to a soln of MCPBA (0.210 g, 2.6 mmol) in CHCl3
(8 mL) at 0 °C. The mixture was stirred at r.t. for 3 d. The organic
layer was washed with 10% aq NaHCO3 soln (3 × 4 mL), dried over
MgSO4 and evaporated. The residue was purified by column chro-
matography on silica gel (hexane–i-PrOH, 20:1).
White solid; yield: 0.24 g (55%); mp 133–135 °C; [a]D25 +26.4 (c
1.0, CHCl3).
1H NMR (500 MHz, CDCl3): d = 7.89–7.85 (m, 4 H), 7.76–7.72 (m,
2 H), 7.57 (t, J = 7.6 Hz, 1 H), 7.46 (t, J = 7.8 Hz, 2 H), 5.44 (d,
J = 16.5 Hz, 1 H), 5.33 (d, J = 16.5 Hz, 1 H), 5.21 (q, J = 7.3 Hz, 1
H), 1.79 (d, J = 7.3 Hz, 3 H).
MS (ESI+): m/z (%) = 443.2 (100) [M + Na]+, 863.4 (15) [2M +
Na]+.
1,4-Dimethoxybut-2-en-2-yl (2S)-2-(1,3-Dioxo-1,3-dihydro-2H-
isoindol-2-yl)propanoate (4)
Yellow oil; yield: 1.15 g (69%); [a]D25 –14.0 (c 1.05, CHCl3).
1H NMR (200 MHz, CDCl3): d = 7.91–7.83 (m, 2 H), 7.79–7.72 (m,
2 H), 5.59 (t, J = 7.3 Hz, 1 H), 5.09 (q, J = 7.1 Hz, 1 H), 4.14–3.92
(m, 4 H), 3.33 (s, 3 H), 3.29 (s, 3 H), 1.72 (d, J = 7.1 Hz, 3 H).
13C NMR (50 MHz, CDCl3): d = 168.5, 167.4, 148.0, 134.4, 132.1,
123.2, 119.6, 67.7, 66.7, 58.4, 58.1, 47.5, 15.5.
13C NMR (125 MHz, CDCl3): d = 191.3, 169.3, 167.3, 134.2, 134.1,
133.9, 132.0, 128.9, 127.8, 123.6, 66.9, 47.5, 15.3.
MS (ESI+): m/z (%) = 360.1 (100) [M + Na]+.
(2R)-1,4-Dimethoxy-3-oxobutan-2-yl (2S)-2-(1,3-Dioxo-1,3-di-
hydro-2H-isoindol-2-yl)propanoate (8a) and (2S)-1,4-
Dimethoxy-3-oxobutan-2-yl (2S)-2-(1,3-Dioxo-1,3-dihydro-2H-
isoindol-2-yl)propanoate (8b)
MS (ESI+): m/z (%) = 356.1 (100) [M + Na]+, 372.1 (10) [M + K]+,
689.2 (27) [2M + Na]+.
Enol ester 4 (0.3 g, 0.92 mmol) was dissolved in CHCl3 (8 mL) and
added dropwise to a soln of MCPBA (0.32 g, 1.9 mmol) in CHCl3
(2 mL) at 0 °C. The mixture was stirred at r.t. for 5 d. The organic
layer was washed with 10% aq NaHCO3 soln (3 × 4 mL) and dried
over MgSO4. Residual MCPBA was removed by column chroma-
tography over basic Al2O3 (eluent: CHCl3). The solvent was evapo-
rated and the residue subjected to column chromatography over
silica gel (eluent: hexane–Et2O, 1:1) to afford a-acyloxy ketones 8a
and 8b as a 5:3 mixture of diastereomers in 60% combined yield.
Hydrogenation of Enol Esters 1–5; General Procedure
The enol ester (0.71 mmol) was dissolved in i-PrOH (or AcOH) (6
mL) and PtO2 (16.2 mg, 0.071 mmol) was suspended in the mixture.
The suspension was stirred under H2 for 20 h (or 5 h) at r.t. The cat-
alyst was removed by filtration and the solvent evaporated. The res-
idue was dissolved in CHCl3 (6 mL), washed with a small amount
of sat. aq NaHCO3 soln (2 mL) and dried over anhyd MgSO4. The
solvent was evaporated and the residue purified by silica gel column
chromatography (eluent: hexane–i-PrOH).
White solid; yield: 0.19 g (60%); mp 121–129 °C.
(1S)-1-Methylpentyl N-Carbobenzyloxy-l-tryptophanate (6a);
Table 2, Entry 3
Compound 1 (0.4 g, 0.95 mmol) was dissolved in AcOH (6 mL) and
PtO2 (21.6 mg, 0.095 mmol) was suspended in the mixture. The
mixture was stirred under H2 for 5 h at r.t. Product 6a was purified
by silica gel column chromatography (eluent: hexane–i-PrOH, 9:1)
and subsequently recrystallized from Et2O–hexane.
1H NMR (500 MHz, CDCl3): d (major component, 8a) = 7.89–7.87
(m, 2 H), 7.76–7.75 (m, 2 H), 5.44 (dd, J1 = 4.9, J2 = 3.4 Hz, 1 H),
5.13 (q, J = 7.3 Hz, 1 H), 4.15 (d, J = 18.0 Hz, 1 H), 4.09 (d,
J = 18.0 Hz, 1 H), 3.87 (dd, J1 = 10.8, J2 = 4.9 Hz, 1 H), 3.70 (dd,
J1 = 10.8, J2 = 3.4 Hz, 1 H), 3.35 (s, 3 H), 3.32 (s, 3 H), 1.77 (d,
J = 7.3 Hz, 3 H).
1H NMR (500 MHz, CDCl3): d (minor component, 8b) = 7.87–7.86
(m, 2 H), 7.75–7.73 (m, 2 H), 5.40 (dd, J1 = 4.9, J2 = 2.8 Hz, 1 H),
5.11 (q, J = 7.3 Hz, 1 H), 4.22 (d, J = 17.5 Hz, 1 H), 4.19 (d,
J = 17.5 Hz, 1 H), 3.83 (dd, J1 = 11.3, J2 = 4.3 Hz, 1 H), 3.60 (dd,
J1 = 10.7, J2 = 3.5 Hz, 1 H), 3.42 (s, 3 H), 3.22 (s, 3 H), 1.76 (d,
J = 7.3 Hz, 3 H).
White solid; yield: 0.257 g (64%); mp 104–106 °C; [a]D25 +16.0 (c
1.13, CHCl3).
1H NMR (500 MHz, CDCl3): d = 8.04 (br s, 1 H), 7.56 (d, J = 8.3
Hz, 1 H), 7.37–7.27 (m, 6 H), 7.19–7.16 (m, 1 H), 7.10–7.06 (m, 1
H), 5.32–5.28 (m, 1 H), 5.13–5.05 (m, 2 H), 4.89–4.83 (m, 1 H),
4.70–4.67 (m, 1 H), 3.34–3.23 (m, 2 H), 1.60–1.58 (m, 1 H), 1.54–
1.46 (m, 2 H), 1.43–1.36 (m, 1 H), 1.24–1.23 (m, 3 H), 1.14–1.10
(m, 3 H), 0.89–0.83 (m, 3 H).
13C NMR (125 MHz, CDCl3): d = 171.7, 155.8, 136.4, 136.1, 128.5,
128.2, 122.7, 122.3, 119.7, 118.9, 111.1, 110.2, 72.7, 66.9, 65.7,
54.7, 35.4, 31.7, 27.3, 22.4, 19.7, 13.9.
13C NMR (125 MHz, CDCl3): d (diastereomeric mixture) = 202.4,
202.2, 169.1, 167.3, 167.2, 134.3, 134.2, 131.8, 123.6, 123.5, 77.6,
76.3, 76.2, 77.1, 71.0, 59.5, 59.4, 59.2, 47.5, 47.4, 15.2.
MS (ESI+): m/z (%) = 372.0 (100) [M + Na]+, 388.0 (3) [M + K]+,
721.1 (6) [2M + Na]+.
Synthesis 2011, No. 11, 1809–1813 © Thieme Stuttgart · New York

PAPER
Diastereoselective Transformations of Enol Esters
1813
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Acknowledgment
We thank Dr. Andrzej Leniewski for his valuable contribution in the
analysis of NMR spectra, and Mrs. Krystyna Wojtasiewicz for cry-
stallization of compounds 6a and 7.
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Synthesis 2011, No. 11, 1809–1813 © Thieme Stuttgart · New York