A nucleophilic addition ring closure [NARC]-based synthesis of (+) -nonactic acid

Article abstract of DOI:10.1039/b206656d

Nonactic acid 1 has been synthesized in 12 steps from readily available (S)-(-)-ethyl lactate in 20% overall yield. The key ("NARC") sequence in this method involved anti-aldol addition of acylsultam 3 with aldehyde 4 followed by intramolecular oxymercura

Full text of DOI:10.1039/b206656d

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A nucleophilic addition ring closure [NARC]-based synthesis of
(؉) -nonactic acid
Benjamin Fraser and Patrick Perlmutter*
1
School of Chemistry, Monash University, PO Box 23, Victoria, Australia 3800.
E-mail: patrick.perlmutter@sci.monash.edu.au; Fax: 61 3 9905 4597; Tel: 61 3 9905 4522
Received (in Cambridge, UK) 9th July 2002, Accepted 17th September 2002
First published as an Advance Article on the web 21st November 2002
Nonactic acid 1 has been synthesized in 12 steps from readily available (S)-(Ϫ)-ethyl lactate in 20% overall yield.
The key (“NARC”) sequence in this method involved anti-aldol addition of acylsultam 3 with aldehyde 4 followed
by intramolecular oxymercuration. The efficiency and selectivity of the anti-aldol reaction was found to be critically
dependent upon the ratio of Lewis acid to base. The intramolecular oxymercuration was also found to be highly
diastereoselective and was attributed to allylic control consistent with previous studies in our group.
Introduction
Several years ago we reported a synthesis of certain diastereo-
mers of methyl nonactate such as 2 below.1 The essence of the
approach involved a “NARC” sequence of syn-aldol addition
followed by intramolecular oxymercuration. Herein we report
the successful development of a set of conditions for executing
a highly anti-selective aldol addition of 3 with 4 which provides
the correct relative stereochemistry between C2 and C3 and
therefore forms the basis of a total synthesis of (ϩ)-nonactic
acid (1).2–16
Scheme 1 Reagents and conditions i) Et2BOTf, iPr2NEt, DCM,
Ϫ78 ЊC, 4 h, 88%. ii) Hg(OAc)2, DCM, 24 h, rt, 76%. iii) Bu3SnH,
AIBN, toluene, 2 h, rt, 98%.
aldol stereocontrol.24 There was a clear requirement for free
Lewis acid25 as employing three equivalents of diethylboron
triflate and Hünig’s base gave exclusively a syn-adduct.9 Further
increases in Lewis acid : base ratio >1.5 yielded anti-aldol
products but with poorer yields. Interestingly, no syn-addition
products were observed under these conditions. (This process
appears to be substrate specific as a similar aldol addition with
Results and discussion
benzaldehyde gave a mixture of diastereomeric adducts. Such
The two key processes in this synthesis were (i) an anti-selective
substrate dependence in Lewis acid-promoted anti-aldols has
aldol reaction17,18 and (ii) a diastereoselective intramolecular
been noted before by Heathcock and Walker22).
oxymercuration (Scheme 1).1,19 The anti-aldol addition was
Hence we believe the most likely transition states for this
reaction are those shown in Fig. 1. The presence of excess Lewis
acid ensures that open transition states are operating. Of these,
anti-periplanar transition state I is lower in energy than syn-
clinal II due to the latter’s unfavourable steric interaction
achieved via addition of the (Z)-boron enolate of acylsultam
3
20 to aldehyde 4.1 This enolate typically adds to aldehydes in a
syn-fashion (proceeding by a closed transition state) unless
an excess of Lewis acid is present (which has been proposed
to divert the reaction manifold though an open transition
state).21–23 We have found that through careful manipulation of
the ratio of Lewis acid to base the anti-aldol adduct 5 could be
obtained exclusively in 83% yield. The optimum ratio of Lewis
acid to base was found to be 3 : 2. Significantly, unlike most
previous reports,17,18,21–23 we were able to employ the same Lewis
acid (i.e. diethylboron triflate†) for enolate generation and anti-
† The IUPAC name for triflate is trifluoromethanesulfonate.
Fig. 1 Putative open transition states for the anti-aldol reaction.
2896
J. Chem. Soc., Perkin Trans. 1, 2002, 2896–2899
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b206656d

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between the auxiliary and the aldehyde chain. Transition state I
leads to the relative stereochemistry obtained.
Alternatively, following the method of Warm and Vogel,16
(ϩ)-methyl nonactate could be prepared by treatment of
benzoate 10 with 15% NaOMe in methanol. This gave
(ϩ)-methyl nonactate 11 in excellent yield (95%) without need
for purification.
The second key process, i.e. intramolecular oxymercuration,
was examined next. Reaction of 5 with mercury() acetate pro-
vided the desired isomer, 6a, in a 10 : 1 diastereoselectivity
(Scheme 1). Unlike previous studies1 we found that the stereo-
selectivity was invariant when run in more polar solvents such
as acetonitrile. Purification by recrystallisation afforded pure
6a in 76% yield. Its crystal structure is shown in Fig. 2‡. The
Experimental
General
Melting points were recorded on a Kofler hot stage apparatus
and are uncorrected. Elemental microanalyses were performed
by Chemical & Micro Analytical Services of the University of
Otago, New Zealand. Optical rotations were measured on a
PolAAr 2001 automatic polarimeter and are given in 10Ϫ1 deg
cm2 gϪ1. IR spectra were recorded on a 1600 Series Fourier
Transform spectrometer and refer to thin film of liquids (neat)
or paraffin (Nujol) mulls of solids between NaCl plates. Infra-
red band intensities of each frequency of absorption are
expressed as follows: s (strong), m (medium), w (weak) or b
1
(broad). H NMR spectra were recorded at 300 MHz on a
Bruker AM 300 spectrometer or Varian Mercury spectrometer.
Chemical shifts were recorded on the δ scale in parts per million
(ppm) in CDCl3. 13C NMR spectra were recorded at 75 MHz
on a Varian Mercury and at 75 MHz on a Bruker AM 300
spectrometer. Spectra were referenced using the solvent carbon
signal as an internal standard. Mass spectra (ESI) were
recorded using samples in MeOH and CH3CN on a Micromass
Platform QMS spectrometer. High resolution mass spectra
(HRMS) for accurate mass determinations were recorded on a
Bruker BioApex 47e FTMS. Silica gel used for flash chrom-
atography was 40–63 µm (230–400 mesh) silica gel 60 (Merck
No. 9385). Many of the reagents used were purchased from
commercial suppliers and used as supplied. Solvents were
dried by distillation from sodium–benzophenone ketyl before
use.
Fig. 2 X-Ray crystal structure of chloromercurio complex 6a (the
acylsultam moiety has been rotated away from the THF ring for
clarity).
diastereoselectivity of this ring closure is consistent with
our previous studies1,19 of the use of a remote allylic control
element in intramolecular oxymercurations. In these studies we
demonstrated that the remote allylic group (in this case the
OTBDPS group) controls the diastereoselectivity and the
reactions are essentially insensitive to the stereochemistry of
the incoming nucleophilic alcohol.
Relatively straightforward synthetic manipulations remained
to complete the synthesis of nonactic acid 1. Reductive
demercuration of 6a proceeded smoothly affording tetrahydro-
furan 7 in excellent yield (97%) (Scheme 1). Removal of the
chiral auxiliary under standard conditions followed by esterifi-
cation with diazomethane gave the methyl ester 8 in 62% yield
(Scheme 2). Desilylation of 8 with tetrabutylammonium fluor-
ide in THF afforded alcohol 9. A sequence of Mitsunobu inver-
sion and ester hydrolysis following a method reported by Lee
and Kim26 yielded (ϩ)-nonactic acid 1 in quantitative yield.
anti-Aldol adduct (5)
Freshly distilled triflic acid (0.885 mL, 10 mmol) was added to a
solution of 1 M triethylborane in hexanes (10 mL, 10 mmol)
and was allowed to stir for 0.5 h at room temperature. After this
time a yellow–orange colour developed and the solution
appeared homogeneous. The solution was then cooled to Ϫ5 ЊC
and a solution of the acylsultam 3 (910 mg, 3.33 mmol) in
dichloromethane (10 mL) was added dropwise. Freshly distilled
N,N-diisopropylethylamine (1.16 mL, 6.67 mmol) was then
added dropwise whilst maintaining the temperature of Ϫ5 ЊC.
The solution was allowed to stir for a further 0.5 h at Ϫ5 ЊC and
then cooled to Ϫ78 ЊC (dry ice–acetone bath). The aldehyde 4
(2.44g, 6.67 mmol) was then added dropwise over a period of
30 min whilst maintaining the temperature below Ϫ75 ЊC. The
solution was stirred for 1 h at Ϫ78 ЊC and then freshly distilled
N,N-diisopropylethylamine (1 mL, 5.75 mmol) was added
dropwise whilst maintaining the temperature below Ϫ75 ЊC.
Phosphate buffer (pH 7, 10 mL) was then added dropwise to the
solution again maintaining a temperature below Ϫ75 ЊC. After
stirring for 20 min at Ϫ78 ЊC the solution was removed from the
dry ice–acetone bath and allowed to warm to room temper-
ature. The organic layer was separated from the aqueous layer
and the aqueous layer extracted with ether (3 × 5 mL). The
combined organic extracts were then washed (sat. NH4Cl),
dried (MgSO4) and the solvent removed in vacuo to give a crude
yellow oil which was subjected to flash chromatography (20%
ethyl acetate–hexanes) yielding the title compound as a colour-
less oil (1.76 g, 83%). [α]2D3 ϩ42.1 (c 1.2, CHCl3). δH 0.97 (s, 3H,
CH3 sultam), 1.03 (s, 9H, OSiC(CH3)3), 1.14 (s, 3H, CH3 sul-
tam), 1.14 (d, J 4.4 Hz, 3H, OCHCH3), 1.16 (d, J 6.6 Hz, 3H,
CH3CH), 1.31–1.45 (m, 3H, CH2 and CH sultam), 1.60–1.72
(m, 2H, CH2 sultam), 1.82–1.95 (m, 4H, CH2, H4 and H5),
Scheme 2 Reagents and conditions i) a. H2O2, LiOH, THF–H2O 5 : 1,
7 h; b. CH2N2, ether, 0 ЊC, 1 h, 62% over two steps ii) TBAF, THF, 24 h,
rt, 79%. iii) PPh3, benzoic acid, DEAD, rt, 18 h, 82%. iv) 30% NaOH,
24 h, rt, 100%. v) 15% NaOMe, MeOH, 18 h, rt, 95%.
‡ CCDC reference number 189550. See http://www.rsc.org/suppdata/
p1/b2/b206656d/ for crystallographic files in .cif or other electronic
format.
J. Chem. Soc., Perkin Trans. 1, 2002, 2896–2899
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2.02–2.14 (m, 2H, CH2 sultam), 3.06 (quintet, J 6.6 Hz, 1H,
H2), 3.38–3.48 (m, 1H, H3), 3.41–3.54 (ABq, J 13.8 Hz, 2H,
CH2SO2), 3.88 (dd, J 5.1, 7.2 Hz, 1H, CHNSO2), 4.52–
4.61 (dquintet, J 6.6, 0.9 Hz, 1H, SiOCHCH3), 5.08–5.17
(m, 1H, H7), 5.47–5.54 (dd, J 8.4, 10.8 Hz, 1H, H8), 7.32–
7.45 (m, 6H, CHar), 7.64–7.7 (m, 4H, CHar). δC 14.5, 19.5,
20.2, 21.1, 24.0, 25.0, 26.8, 27.3, 33.3, 35.8, 38.8, 45.0, 45.8,
48.0 and 48.6, 53.4, 65.6, 66.2, 75.3, 127.5, 129.6, 134.4, 135.5,
135.9, 136.0, 175.3. IR ν/cmϪ1 3447b, 1654b (Found: C, 67.58;
H, 8.19; N, 2.33%; C36H51NO5SSi requires C, 67.78; H, 8.06; N,
2.20%).
(؉)-Methyl 8-epi-8-O-(tert-butyldiphenylsilyl)nonactate (8)
To a solution of the tetrahydrofuran 7 (1.2 g, 2.64 mmol) in 4 : 1
THF–water (15 mL) was added LiOH (160 mg, 6.7 mmol). The
solution was cooled to 0 ЊC and 30% H2O2 solution (1.6 mL)
was added dropwise maintaining the temperature at 0 ЊC. The
solution was then allowed to warm to room temperature and
stirred for 7 h. Sat. sodium sulfite (5 mL) was added and the
solution stirred for 0.5 h. The solution was then acidified with
1 M HCl to pH 1, diluted with ether (10 mL). The aqueous
phase was extracted with ether (3 × 5 mL). The combined
organic extracts were dried (MgSO4) and the solvent removed
in vacuo to give the crude carboxylic acid as a clear oil (∼0.9 g).
The crude oil was immediately dissolved up in ether (20 mL)
and treated with an excess of diazomethane (2 g Diazald®).
After allowing the excess diazomethane to dissipate the
solvent was removed in vacuo to give the crude methyl ester
which was purified by flash chromatography (0–20% ethyl
acetate–hexane solvent gradient) yielding the title compound
as a colourless oil (0.983g, 83%). [α]2D2 ϩ3.8 (c 1.032, CHCl3).
δH 1.04 (s, 9H, OSi(CH3)3, 1.07 (d, J 3.6 Hz, 3H, C2CH3), 1.08
(d, J 7.5 Hz, OCHCH3), 1.4–1.9 (m, 5H, CH2, H4, H5
and H7), 2.43 (quintet, J 7.2 Hz, 1H, H2), 3.63 (s, 3H, CH3
methyl ester), 3.93–4.01 (m, 3H, H3, H5 and H8), 7.32–7.46
(m, 6H, CHar), 7.64–7.70 (m, 4H, CHar). δC 13.6, 19.6, 23.8,
27.3, 28.7 and 31.5, 45.5, 51.9, 67.8, 80.3, 127.5, 129.6, 134.6,
136.0, 175.5. IR ν/cmϪ1 1749s. MS m/z 477.4 (Mϩ ϩ Na)
(Found: C, 71.47; H, 8.48%; C27H38O4Si requires C, 71.32; H,
8.42%).
Chloromercurio complex (6a)
To a solution of the aldol adduct (1.445 g, 2.27 mmol) 5 in
DCM (20 mL) was added mercury() acetate (1.1 g, 3.45
mmol). The solution was allowed to stir at room temperature
for 24 h. Brine (5 mL) was then added and the solution stirred
for a further 0.5 h. The organic layer was separated from the
aqueous layer and the aqueous layer extracted with further
portions of ether (3 × 7 mL). The combined organic extracts
were dried (MgSO4) and the solvent removed in vacuo to yield a
10 : 1 crude solid mixture of diastereomeric chloromercurio
complexes 6a and 6b. Recrystallisation of the crude solid from
ethyl acetate–hexanes gave the major diastereomer 6a in 76%
yield. Mp 170–172 ЊC. [α]2D3 Ϫ 61.8 (c 1.06, CHCl3). δH 0.96 (s,
3H, CH3 sultam), 1.05 (s, 9H, OSi(CH3)3, 1.09 (d, J 6.8 Hz, 3H,
C2CH3), 1.14 (d, J 6.0 Hz, OCHCH3), 1.27 (s, 3H, CH3 sultam),
1.32–1.45 (m, 3H, CH2 and CH sultam), 1.52–1.76 (m, 2H, CH2
sultam), 1.78–2.0 (m, 4H, 2 × CH2 sultam), 2.06–2.26 (m, 4H,
CH2, H4 and H5), 2.52 (dd, J 2.3, 9.5 Hz, 1H, CHHg), 3.03
(quintet, J 6.8 Hz, 1H, H2), 3.39–3.54 (ABq, J 13.8 Hz,
CH2SO2), 3.85–3.93 (dd, J 5.4, 5.1 Hz, 1H, CHNSO2), 3.93–
4.04 (m, 2H, H3 and H5), 4.11–4.17 (dq, J 2.3, 6.0 Hz, 1H,
SiOCHCH3), 7.34–7.48 (m, 6H, CHar), 7.61–7.69 (m, 4H,
CHar). δC 14.5, 19.5, 20.2, 22.6, 26.8, 27.5, 29.8, 32.1, 33.2, 39.2,
44.9, 45.9, 48.0 and 48.6, 53.6, 65.7, 70.6, 72.2, 80.5 and 81.5,
127.7, 129.93, 133.5, 136.1, 174.5. IR ν/cmϪ1 1684s (Found: C,
49.53; H, 5.77; N, 1.60%; C36H50NO5SSiHgCl requires C, 49.48;
H, 6.08; N, 1.66%).
(؉)-Methyl 8-epi-nonactate (9)
To a solution of the TBDPS ether 8 (0.446 g, 0.98 mmol) in
THF (17 mL) at room temperature was added tetrabutyl-
ammonium fluoride (4.6 mL, 4.6 mmol, 1 M solution in THF).
The solution was then allowed to stir at room temperature
for 24 h. The solution was then diluted with ether (50 mL),
dried (MgSO4)and the solvent removed in vacuo. The residue
was purified by flash chromatography (40% ethyl acetate–
hexanes) yielding the title compound as a clear oil (167 mg,
79%). [α]2D2 ϩ33.5 (c 1.0, CHCl3) lit.26 ϩ32.3. δH 1.11 (d, J 7.0
Hz, 3H, C2CH3), 1.16 (d, J 6.3 Hz, OCHCH3), 1.46–1.7 (m,
4H, CH2, H4, H5), 1.90–2.11 (m, 2H, H7), 2.48–2.58 (dq, J 7.0,
8.7 Hz, 1H, H2), 3.69 (s, 3H, CH3 methyl ester), 3.91–4.01 (m,
3H, H3, H5 and H8). δC 13.9, 23.7, 28.9 and 32.1, 44.9, 45.6,
52.1, 68.1, 80.5, 81.9. IR ν/cmϪ1 3518b, 1736s. MS m/z
239.1 (Mϩ). HRMS m/z 239.1247 calculated for C11H20O4Naϩ
239.1259.
Tetrahydrofuran (7)
To a solution of the chloromercurio complex 6a (1.8 g, 2.06
mmol) in toluene (36 mL) was added azoisobutyronitrile
(36 mg, 0.205 mmol) and tributylstannane (1.4 mL, 5.14
mmol). Mercury precipitated almost immediately and the solu-
tion was allowed to stir at room temperature for 2 h and then
heated to 40 ЊC for 1 h. Carbon tetrachloride (5 mL) was then
added and the solution stirred for 1 h at room temperature. The
solution was then decanted from the mercury and diluted with
25% dichloromethane–light petroleum (25 mL). The solution
was then washed with 5% potassium fluoride solution (3 ×
3 mL) and the organic layer dried (MgSO4) and evaporated
in vacuo giving a grey crude oil that was subjected to flash
chromatography (25% ethyl acetate–hexanes) yielding the
title compound as a colourless oil (1.28 g, 98%). Mp 171–172
ЊC. [α]2D2 Ϫ 7.2 (c 1.0, CHCl3). δH 0.93 (s, 3H, CH3 sultam),
1.04 (s, 9H, OSi(CH3)3), 1.08 (d, J 6 Hz, 3H, C2CH3), 1.09 (d,
J 6.3 Hz, OCHCH3), 1.16 (s, 3H, CH3 sultam), 1.30–1.45 (m,
3H, CH2 and CH sultam), 1.46–1.76 (m, 2H, CH2 sultam),
1.78–2.0 (m, 4H, 2 × CH2 sultam), 2.06–2.25 (m, 4H, CH2, H4
and H5), 3.05 (m, 1H, H2), 3.37–3.50 (ABq, J 13.8 Hz,
CH2SO2), 3.85–3.92 (dd, J 5.0, 7.8 Hz, 1H, CHNSO2), 3.92–
4.06 (m, 3H, H3, H5 and H8), 7.32–7.45 (m, 6H, CHar), 7.65–
7.70 (m, 4H, CHar). δC 13.8, 19.3, 19.9, 21.2, 24.1, 25.2, 26.6,
27.1, 29.2, 30.7, 32.9, 38.4, 44.7, 46.1, 47.7 and 48.2, 53.6,
65.2, 67.7, 81.6, 127.2, 129.3, 134.2, 135.8, 174.7. IR ν/cmϪ1
1698s, 1686s. MS m/z 660.3 (Mϩ ϩ Na) (Found: C, 67.43; H,
7.9; N 2.13%; C36H51NO5SSi requires C, 67.78; H, 8.06; N,
2.20%).
(؉)-Methyl 8-O-benzoylnonactate (10)
Following the procedure of Lee and Kim26 triphenylphosphine
(177 mg, 0.67 mmol), benzoic acid (87 mg, 0.67 mmol) and the
methyl ester 9 (72 mg, 0.3348 mmol) were dissolved in THF
(4 mL) at room temperature. Diethyl azodicarboxylate (0.108
mL, 0.67 mmol) was then added dropwise to the solution and
the initial yellow colour was lost indicating commencement of
reaction. The solution was then stirred at room temperature.
After 18 h the reaction was quenched with water (5 mL). The
aqueous phase was extracted with ether (3 × 3 mL) and the
combined organic extracts were dried (MgSO4) and the solvent
removed in vacuo. The resulting crude yellow oil was purified
(12.5% ethyl acetate–hexanes) to yield the title compound as a
colourless oil (87 mg, 82%). [α]2D1.5 Ϫ28.5Њ (c 1.2, CHCl3) lit.26
Ϫ 30.4. δH 1.09 (d, J 6.9 Hz, 3H, C2CH3), 1.36 (d, J 6.3 Hz,
OCHCH3), 1.53–2.02 (m, 5H, CH2, H4, H5 and H7), 2.47–2.56
(dq, J 6.9, 7.2 Hz, 1H, H2), 3.68 (s, 3H, CH3 methyl ester), 3.92–
4.02 (m, 2H, H3 and H5), 5.18–5.28 (m, 1H, H8), 7.40–7.57 (m,
3H, CHar), 8.00–8.03 (m, 2H, CHar). δC 13.6, 21.0, 28.7 and
31.6, 42.9, 45.6, 51.8, 70.1, 76.6, 80.4 128.3, 129.5, 130.8, 132.7,
165.9, 175.2.
2898
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2 M. J. Arco, M. H. Trammel and J. D. White, J. Org. Chem., 1976, 41,
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(؉)-Methyl nonactate (11)
Following the procedure of Warm and Vogel16 benzoate 10
(50 mg, 0.156 mmol) was dissolved in 10 mL of anhydrous
methanol. To this solution was added 15% NaOMe in methanol
(0.6 M, 2.2 mmol). The solution was allowed to stir for 18 h at
room temperature. The pH was then adjusted to 5 by addition
of 1 M HCl. Dichloromethane (5 mL) was then added and the
aqueous phase separated from the organic phase. The aqueous
phase was further extracted with dichloromethane (3 × 5 mL)
and the combined organic extracts were dried (MgSO4) and the
solvent removed in vacuo. The residual oil was purified by flash
chromatography yielding the title compound 11 as a colourless
oil in quantitative yield (34 mg). [α]2D4 ϩ15.3 (c 1.2, CHCl3) lit.16
ϩ16.1. δH 1.11 (d, J 6.9 Hz, 3H, C2CH3), 1.18 (d, J 6.3 Hz, 3H,
OCHCH3), 1.52–2.05 (m, 5H, CH2, H4, H5 and H7), 2.47–2.57
(dq, J 6.9, 8.4 Hz, 1H, H2), 2.93 (br s, 1H, OH), 3.67 (s, 3H,
CH3 methyl ester), 3.92–4.16 (m, 2H, H3 and H5). δC 13.8, 23.4,
29.0 and 30.7, 42.8, 45.5, 51.9, 65.3, 77.4 and 81.2, 175.2.
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(؉)-Nonactic acid (1)
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(ϩ)-Methyl nonactate 11 (34 mg, 0.157 mmol) was dissolved in
MeOH (1 mL) and to this solution was added methanolic
NaOH (0.8 mL, 2 M NaOH in MeOH). The solution was
allowed to stir for 24 h at room temperature and was then acid-
ified to pH 2 with 0.1 M HCl. Ether was added (1 mL)
and the aqueous phase was extracted with a further two
portions of ether (2 × 1 mL). The combined organic extracts
were dried (MgSO4) and the solvent removed in vacuo to yield
(ϩ)-nonactic acid in quantitative yield. [α]2D2 ϩ9.1 (c 1.2 CDCl3)
lit.10 ϩ9.0 (c 0.15). δH 1.16 (d, 3H, J 6.9 Hz), 1.22 (d, 3H, J 6.3
Hz), 1.52–1.56 (m, 4H), 1.91–2.21 (m, 2H), 2.45–2.55 (m, 1H),
3.95–4.27 (m, 3H), 4.80–5.60 (br s, 2H).
23 H. C. Heathcock and B. C. Raimundo, Synlett, 1995, 12, 1213–
1214.
24 W. Oppolzer and P. Lienard, Tetrahedron Lett., 1993, 34, 4321–4324.
25 D. A. Evans, J. M. Takacs, L. R. McGee, M. D. Ennis, D. J. Mathre
and D. J. Bartoli, J. Pure. Appl. Chem., 1981, 53, 1109.
26 J. Lee and B. Kim, Tetrahedron, 1996, 52, 571–588.
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