Diastereoselective protonation of dienols: A formal approach to zaragozic acid C side chain

Article abstract of DOI:10.1016/S0957-4166(99)00102-0

The 2-methyl 5-phenylpropanal precursor of the side chain of zaragozic acid C has been prepared in 83% ee through the diastereoselective protonation of a photodienol generated in situ by irradiation of an α,β-unsaturated ester, bearing, as a chiral moiety

Full text of DOI:10.1016/S0957-4166(99)00102-0

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1061–1067
Diastereoselective protonation of dienols: a formal approach to
zaragozic acid C side chain
Sébastien Comesse and Olivier Piva ∗,†
Unité Mixte de Recherche ‘Réactions Sélectives et Applications’, 6519 CNRS et Université de Reims, Champagne-Ardenne,
BP 1039-51687 Reims Cedex 2, France
Received 27 January 1999; accepted 16 March 1999
Abstract
The 2-methyl 5-phenylpropanal precursor of the side chain of zaragozic acid C has been prepared in 83% ee
through the diastereoselective protonation of a photodienol generated in situ by irradiation of an α,β-unsaturated
ester, bearing, as a chiral moiety, the diacetone D-glucose group. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
Zaragozic acids (squalestatins) display inhibition of squalene synthase, an enzyme which catalyzes, in
the metabolic pathway for the production of cholesterol, the conversion of farnesyl pyrophosphate into
squalene.1 Due to their biological activities, they have been the subject of intensive studies.2 From a
structural point of view, they possess the same bicyclic core but differ by the length and substitution of
the side chains attached to carbon-1 or fixed on oxygen-6. In the case of the C-derivative, this latter chain
possesses one asymmetric center (Scheme 1).
Scheme 1.
∗
†
Corresponding author. E-mail: piva@copssg.univ-lyon1.fr
Present address: Laboratoire de Chimie Organique, Photochimie et Synthèse, UMR 5622, CNRS, Université de Lyon I,
43, Bd du 11 novembre 1918, F 69622, Villeurbanne, France.
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00102-0

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Its synthesis has already been achieved twice by the Claisen–Ireland rearrangement of an allylic ester
prepared from aldehyde 2. This chiral compound was obtained either by diastereoselective alkylation
using Evans’ oxazolidinone2b or from a chiral epoxyalcohol.3 We report herein on an alternative approach
based on the asymmetric protonation4 of an enol intermediate produced during photodeconjugation of an
optically active ester 6 (Scheme 2).
Scheme 2.
2. Results and discussion
The synthesis of the chiral α,β-unsaturated ester 6 (R*OH=diacetone-D-glucose), prepared in a
few steps is depicted in Scheme 4. TEMPO catalyzed oxidation of 3-phenylpropanol using N-
chlorosuccinimide (NCS) as co-oxidant5 afforded aldehyde 3 which was used without further purification
(Scheme 3). Condensation with triethyl phosphonopropionate using a Wittig–Horner reaction6 led to
ethyl α,β-unsaturated ester 4 (E:Z, 80:20) which was saponified to unsaturated acid 5 by treatment
with KOH in aqueous ethanol.7 Esterification to DAG ester 6 was conveniently achieved using DCC
activation.8
Scheme 3.
Irradiation at 254 nm of 6 was then performed at −45°C in methylene chloride in the presence of
only catalytic amounts of N,N-dimethylaminoethanol (0.1 equiv.) as already reported in previous work3
(Scheme 4). It is generally assumed that under UV activation, a photochemical E/Z isomerization first
takes place between the two α,β-unsaturated esters leading to a photochemical equilibrium. Then, the
Z-isomer (and only the Z) could undergo a photochemical sigmatropic hydrogen shift which affords a
photodienol with a defined configuration around the prochiral center C2. Finally, this intermediate can
be converted to the β,γ-unsaturated isomer by means of a proton source (typically the aminoalcohol).

S. Comesse, O. Piva / Tetrahedron: Asymmetry 10 (1999) 1061–1067
1063
Therefore, the E/Z ratio of the starting material has no influence on the path and the efficiency of the
process.
Scheme 4.
Compound 7 was thus isolated in good yield (89%) as an unseparable mixture of Z and E geometric
isomers (ratio=45:55). The C_C double bond was quantitatively hydrogenated over PtO2 in 4 h, without
over-reduction of the phenyl ring. According to NMR analyses performed in chloroform and in the
presence of a few drops of d6-benzene,9 a diastereoselectivity of 93% was measured. It is interesting to
note that this high value was apparently obtained whatever the configuration of the C3_C4 double bond
of the dienol. Finally, saturated ester 8 was converted into chiral aldehyde (R)-2 by reduction with LiAlH4
in ether, followed by a TPAP oxidation.10 The enantioselectivity of this compound was first estimated
1
to be near 90% by H NMR performed in the presence of Eu(hfc)3 as the shift reagent.11,12 In order to
find a much more confident value, chiral aldehyde 2 was transformed into chiral SAMP13 hydrazone 10
(Scheme 5). The racemic aldehyde 2, easily prepared in 3 steps from ester 4, was also transformed into
1
SAMP hydrazone 10. By comparing the two H NMR spectra, the selectivity for aldehyde 2 obtained
from the photochemical method was directly measured by integration of the methyl signal (one doublet
for each enantiomer) and reached a value of 83%. The R-configuration of the new stereogenic center
has been attributed by comparing the signs of optical rotation for 9 and 2 with those of the literature.3
Furthermore, the protonation occurred in accordance with a previously established model exhibiting
similar substrates.4
Scheme 5.
While 2 has already been converted into acid 13 in a three-step sequence, a formal synthesis of
zaragozic C acid side chain has been thus achieved in 83% ee using, as a key step, the highly
diastereoselective photodeconjugation of α,β-unsaturated ester 6 bearing as a chiral moiety the cheap
and readily available diacetone D-glucose (DAGOH).14 This approach to zaragozic C acid side chain,
therefore, represents a new application of asymmetric protonations as an alternative procedure to creating
stereogenic centers in the α position of carboxylic functionalities.

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S. Comesse, O. Piva / Tetrahedron: Asymmetry 10 (1999) 1061–1067
3. Experimental
The NMR spectra were recorded in CDCl3 or d6-benzene using a Bruker AC 250 instrument. FT-IR
spectra were carried out in CHCl3 on an IR MIDAC spectrometer. Mass spectra were obtained on a D-
300 JEOL apparatus at the UFR Pharmacy at the University of Reims. Optical rotations were measured
on a Perkin–Elmer 241 spectrometer. Elemental analyses were determined on a CHN 2400 Perkin–Elmer
apparatus. Flash chromatographies15 were performed on silica gel 60 (40–63 mesh).
3.1. 3-Phenylpropanal: 3
To a solution of 3-phenylpropanol (5.04 g, 37 mmol) in methylene chloride (350 ml) was successively
added TEMPO (0.58 g, 3.7 mmol), tetrabutylammonium chloride (1.03 g, 3.7 mmol) and an aqueous
solution (350 ml) of NaHCO3 (0.5 M) and K2CO3 (0.05 M). Under strong agitation and at room
temperature, NCS (9.88 g, 74.0 mmol) was added in one portion. After 15 h, the two phases were
separated and the aqueous solution was extracted with methylene chloride (2×200 ml). The organic
layers were washed with brine (2×100 ml), dried over MgSO4 and concentrated. The compound was
used without further purification. 1H NMR: 2.74 (dt, 2H, 1.4 and 7.3 Hz), 2.96 (t, 2H, 7.3 Hz), 7.17–7.32
(m, 5H), 9.81 (t, 1H, 1.4 Hz). 13C NMR: 28.1, 45.2, 126.3, 128.3, 128.6, 141.1, 201.5. IR ν: 3030, 2935,
2885, 1713, 1605, 1495, 1455, 1290, 1215, 1080, 1030.
3.2. Ethyl 2-methyl-5-phenyl pent-2-enoate: 4
To a suspension of sodium hydride (0.82 g, 20.6 mmol) in diethylether (160 ml) was added dropwise
ethyl 2-diethylphosphonopropionate (4.91 g, 20.6 mmol) in the same solvent (10 ml). After 1 h at
room temperature was slowly added a solution of crude 3-phenylpropanal 3 (2.52 g, 18.7 mmol) in
the same solvent (10 ml). The reaction mixture was stirred overnight at room temperature and carefully
hydrolyzed with brine. After extraction with ether (3×100 ml), the crude product was purified by flash
chromatography (eluent AcOEt:PE, 2:98) to give 4 (3.23 g, 14.8 mmol) as a mixture of geometric isomers
E:Z, 80:20. 80% (2 steps). 1H NMR: E-isomer: 1.30 (t, 3H, 7.25 Hz), 1.80 (s, 3H), 2.50 (q, 2H, 7.3 Hz),
2.76 (t, 2H, 7.3 Hz), 4.20 (q, 2H, 7.25 Hz), 6.83 (dt, 1H, 1.5 Hz and 7.30 Hz), 7.15–7.40 (m, 5H). Z-
isomer: 1.30 (t, 3H, 7.25 Hz), 1.80 (s, 3H), 2.50 (q, 2H, 7.3 Hz), 2.76 (t, 2H, 7.3 Hz), 4.20 (q, 2H, 7.25
Hz), 5.97 (dt, 1H, 1.5 Hz and 7.3 Hz), 7.15–7.40 (m, 5H). 13C NMR: E-isomer: 14.1, 30.4, 34.6, 60.2,
125.9, 128.2, 128.3, 140.7, 141.1, 141.3, 168.0. IR ν: 2935, 1715, 1655, 1455, 1265. MS: m/z (%): 218
(M+·, 100), 173 (59), 144 (32), 129 (21), 120 (33), 117 (39), 104 (35). Anal. calcd for C14H18O2: C,
77.03; H, 8.31. Found: C, 77.37; H, 8.86.
3.3. 2-Methyl-5-phenyl pent-2-enoic acid: 5
Ethyl ester 4 (3.20 g, 14.6 mmol) diluted in ethanol (5 ml) was added to a solution of potassium
hydroxide (1.23 g, 22.05 mmol) in ethanol (95 ml) and water (5 ml). The mixture was heated for 3 h at
reflux, cooled to room temperature, half-concentrated under vacuum and acidified with a 2 N sulfuric
acid solution. After extraction with hexanes, the organic phase was dried over MgSO4, filtered and
concentrated. The resulting oil was purified by flash chromatography over silica giving 5 (2.69 g, 14.1
1
mmol); 97%. H NMR: E-isomer: 1.77 (s, 3H), 2.50 (q, 2H, 7.25 Hz), 2.75 (t, 2H, 7.25 Hz), 6.95 (dt,
1H, 1.5 Hz and 7.30 Hz), 7.15–7.35 (m, 5H). Z-isomer: 1.77 (d, 3H, 1.1 Hz), 2.50 (q, 2H, 7.25 Hz), 2.75
(t, 2H, 7.25 Hz), 6.15 (dt, 1H, 1.1 Hz and 7.25 Hz), 7.10–7.35 (m, 5H). 13C NMR: E-isomer: 11.9, 30.7,

S. Comesse, O. Piva / Tetrahedron: Asymmetry 10 (1999) 1061–1067
1065
34.5, 126.1, 127.7, 128.4, 141.0, 143.8, 173.4. IR ν: 3030, 2930, 2554, 1695, 1645, 1425, 1285. MS: m/z
(%): 190 (M+·, 100), 172 (53), 144 (18), 127 (24), 126 (30), 117 (37), 115 (37), 104 (42), 143 (23). Anal.
calcd for C12H14O2: C, 75.76; H, 7.42. Found: C, 75.79; H, 7.90.
3.4. (1,2;5,6-Di-O-isopropyliden-α-D-glucofuranose-3-O-yl) 2-methyl-5-phenyl pent-2-enoate: 6
To a solution of acid 5 (1.75 g, 9.2 mmol) in methylene chloride were successively added DMAP (0.34
g, 2.8 mmol) and diacetone D-glucose (2.39 g, 9.2 mmol). The reaction mixture was cooled to 0°C and a
solution of dicyclohexylcarbodiimide (1.89 g, 9.2 mmol) in CH2Cl2 was added dropwise. After stirring
for 5 min at 0°C, the cooling bath was removed and the mixture stirred overnight at room temperature.
Urea was filtered off and the solvent removed by evaporation under reduced pressure. Compound 6 (3.76
g, 8.7 mmol) was obtained pure by flash chromatography (eluent hexanes:ethyl acetate, 90:10); 95%. 1H
NMR: E-isomer: 1.27 (s, 3H), 1.29 (s, 3H), 1.40 (s, 3H), 1.52 (s, 3H), 1.77 (s, 3H), 2.49 (q, 2H, 7.25 Hz),
2.75 (t, 2H, 2.75 Hz), 3.94–4.30 (m, 4H), 4.50 (d, 1H, 3.4 Hz), 5.27 (d, 1H, 2.7 Hz), 5.87 (d, 1H, 3.8 Hz),
6.80 (tq, 1H, 7.25 Hz and 1.5 Hz), 7.11–7.33 (m, 5H). 13C NMR: E-isomer: 12.3, 25.2, 26.2, 26.7, 30.6,
34.6, 67.2, 72.6, 76.3, 79.9, 83.3, 105.0, 109.2, 112.2, 126.1, 127.9, 128.3, 128.4, 140.9, 142.4, 166.5. IR
ν: 2990, 2935, 1720, 1655, 1455, 1380, 1265, 1165, 1030. MS: m/z (%): 432 (M+·, 77), 417 (85), 374
(19), 273 (22), 173 (91), 160 (18), 145 (87), 129 (27), 114 (53), 113 (75), 109 (100). [α]D23=−28.0 (c
0.5, CH2Cl2). Anal. calcd for C24H32O7: C, 66.65; H, 7.46. Found: C, 66.63; H, 7.72.
3.5. (1,2;5,6-Di-O-isopropyliden-α-D-glucofuranose-3-O-yl) 2-methyl-5-phenyl pent-3-enoate: 7
To a solution of DAG ester 6 (10 mmol) in methylene chloride (50 ml) was added N,N-
dimethylethanolamine and placed around a quartz Dewar in which a short wavelength OSRAM
lamp was disposed. The irradiation was carried out at −45°C. After total disappearance of the starting
material (thin-layer chromatography control), the solvent was removed by rota-evaporation. The
1
deconjugated ester was purified by flash chromatography (eluent AcOEt:petrol ether, 5:95); 89%. H
NMR: [E+Z mixture]: 1.20–1.42 (m, 9H), 1.39 (s, 3H), 1.50 (s, 3H), [3.37 (d, 6.5 Hz) and 3.45 (d, 6 Hz),
2H], [3.18 (dq) and 3.60 (dq), 1H], 3.90–4.25 (m, 4H), [4.42 (d, 1.4 Hz) and 4.46 (d, 1.4 Hz), 1H], [5.28
(d, 2.7 Hz) and 5.32 (d, 2.7 Hz), 1H], 5.48–5.70 (m, 2H), [5.85 (d, 3.7 Hz) and 5.87 (d, 3.8 Hz), 1H],
7.11–7.31 (m, 5H). 13C NMR [mixture E+Z]: [17.1, 17.7], 25.2, 26.2, 26.6, 26.7, [33.6, 38.7], [38.3,
42.8], [67.5, 76.4], [72.1, 72.2], 76.0, [80.2, 80.3], 83.4, 105.2, [109.2, 112.2], 126.9, 128.8, [128.9,
129.6], [130.5, 131.3], 139.9, 166.5. IR ν: 2995, 2940, 1745, 1605, 1455, 1380, 1215, 1155, 1080, 1030.
MS: m/z (%): 432 (M+·, 13), 417 (30), 374 (8), 273 (8), 145 (100), 129 (40), 117 (53), 145 (100), 129
(42), 117 (55). Anal. calcd for C24H32O7: C, 66.65; H, 7.46. Found: C, 66.70; H, 7.66.
3.6. (1,2;5,6-Di-O-isopropyliden-α-D-glucofuranose-3-O-yl) 2-methyl-5-phenyl pentanoate: 8
A solution of ester 7 (0.40 g, 0.92 mmol) in ether (15 ml) was hydrogenated over PtO2 with hydrogen
(balloon, 1 atm) for 3 h. After filtration over Celite and washing with ether, compound 8 (0.40 g, 0.92
mmol) was obtained; 99%. 1H NMR: 1.16 (d, 3H, 6.8 Hz), 1.22 (s, 3H), 1.30 (s, 3H), 1.37 (s, 3H), 1.51
(s, 3H), 1.55–1.80 (m, 4H), 2.49 (tq, 1H, 6.5 Hz and 6.8 Hz), 2.60 (t, 2H, 7.25 Hz), 3.90–4.12 (m, 4H),
4.17 (t, 1H, 3.8 Hz), 4.40 (d, 1H, 3.8 Hz), 5.27 (d, 1H, 1.9 Hz), 5.82 (d, 1H, 3.8 Hz), 7.11–7.32 (m, 5H).
13C NMR: E-isomer: 16.9, 25.1, 26.2, 26.7, 28.8, 33.2, 35.6, 39.5, 67.4, 72.3, 75.7, 80.1, 83.4, 105.1,
109.3, 112.2, 125.8, 128.3, 141.9, 175.0. IR ν: 2935, 2860, 1740, 1455, 1380, 1215, 1180, 1015. MS: m/z
(%): 434 (M+·, <5), 419 (100), 376 (98), 361 (10), 318 (12), 301 (17), 275 (35), 213 (17), 175 (53), 174

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(59), 147 (98), 127 (33), 113 (57). [α]D23=−27.8 (c 1.0, CH2Cl2). Anal. calcd for C24H34O7: C, 66.34;
H, 7.89. Found: C, 66.48; H, 7.89.
3.7. 2-Methyl-5-phenyl pentanol: 9
To a suspension of LAH (53 mg, 1.38 mmol) in ether (50 ml) was added a solution of 8 at 0°C in the
same solvent (5 ml). The resulting mixture was stirred for 4 h and carefully hydrolyzed with a saturated
aqueous solution of ammonium chloride (10 ml). The aqueous layer was then extracted with ether. The
combined organic phases were dried over MgSO4, filtered and concentrated. Alcohol 9 was purified by
1
flash chromatography (eluent hexanes:AcOEt, 10:90); 85%. H NMR: 0.95 (d, 3H, 6.5 Hz), 1.11–1.28
(m, 1H), 1.34–1.40 (s, OH), 1.41–1.56 (m, 1H), 1.60–1.80 (m, 3H), 2.60 (t, 2H, 7.2 Hz), 3.50 (dd, 2H,
6.5 and 10.3 Hz), 7.15–7.35 (m, 5H). 13C NMR: 16.5, 28.8, 32.8, 35.7, 36.2, 68.2, 125.7, 128.3, 128.4,
142.6. IR ν: 3350, 2930, 2860, 1604, 1455, 1380, 1030. MS: m/z (%): 178 (M+·, 56), 161 (30), 160 (96),
145 (19), 131 (57), 120 (33), 118 (56), 117 (91), 115 (30), 105 (100), 104 (68), 103 (41). [α]D21=+10.3
(c 1.0, CH2Cl2). Anal. calcd for C12H18O: C, 80.85; H, 10.17. Found: C, 80.51; H, 10.73.
3.8. 2-Methyl-5-phenyl pentanal: 2
To a solution of alcohol 9 (0.14 g, 0.75 mmol) in methylene chloride (1.6 mmol) was successively
added 4 Å MS (0.40 g) and NMO (0.27 g, 1.19 mmol). The resulting mixture was cooled to 0°C and TPAP
(0.024 g, 0.040 mmol) was added in one portion. The temperature was kept to 0°C for 5 min and then
allowed to reach room temperature. After 2 h, the crude mixture was filtered over a small pad of silica,
eluting with CH2Cl2. After concentration, the aldehyde 2 was purified by flash chromatography (eluent
hexanes:AcOEt, 99:1); 72%. Data in accordance with those of literature.3 1H NMR: 1.10 (d, 3H, 6.7 Hz),
1.15–1.50 (m, 2H), 1.60–1.80 (m, 2H), 2.28–2.42 (m, 1H), 2.65 (t, 2H, 7.2 Hz), 7.10–7.35 (m, 5H), 9.61
(d, 1H, 1.9 Hz). 13C NMR: 13.3, 28.7, 30.1, 35.8, 46.2, 125.9, 128.3, 141.9, 204.9. [α]D23=−11.4 (1.0,
CH2Cl2). Lit.4: [α]D23=−14.6 (c 23.0, heptane).
References
1. (a) Nicolaou, K. C.; Nadin, A. Angew. Chem., Int. Ed. Engl. 1996, 35, 1622–1656. (b) Hinterding, K.; Alonso Diaz, D.;
Waldmann, H. Angew. Chem., Int. Ed. Engl. 1998, 37, 688–749. (c) Koert, U. In Total Syntheses of Zaragozic Acid; Mulzer,
J.; Waldmann, H., Eds. Organic synthesis highlights III. Wiley-VCH: Weinheim, 1998; pp. 282–294.
2. (a) Evans, D. A.; Barrow, J. C.; Leighton, J. L.; Robichaud, A. J.; Sefkow, M. J. Am. Chem. Soc. 1994, 116, 12111–12112.
(b) Carreira, E. M.; Du Bois, J. J. Am. Chem. Soc. 1995, 117, 8106–8125. (c) Sato, H.; Nakamura, S-i.; Watanabe, N.;
Hashimoto, S-i. Synlett 1997, 451–454. (d) Armstrong, A.; Jones, L. H.; Barsanti, P. A. Tetrahedron Lett. 1998, 39,
3337–3340.
3. Santini, C.; Ball, R. G.; Berger, G. D. J. Org. Chem. 1994, 59, 2261–2266.
4. (a) Piva, O.; Pete, J. P. Tetrahedron: Asymmetry 1992, 3, 759–768. (b) Piva, O.; Caramelle, D. Tetrahedron: Asymmetry
1995, 6, 831–832. (c) Piva, O. J. Org. Chem. 1995, 60, 7879–7883. For recent reviews on asymmetric protonations: (a)
Fehr, C. Angew. Chem., Int. Ed. Engl. 1996, 35, 2566–2587. (b) Yangisawa, A.; Ishihara, K.; Yamamoto, H. Synlett 1997,
411–420.
5. (a) Einhorn, J.; Einhorn, C.; Ratajczak, F.; Pierre, J. L. J. Org. Chem. 1996, 61, 7452–7454. (b) de Nooy, A. E. J.; Besemer,
A. C.; van Bekkum, H. Synthesis 1996, 1153–1174.
6. (a) Etemad, G.; Seyden-Penne, J. Tetrahedron 1984, 40, 5153–5166. (b) Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989,
89, 863–927.
7. Freeman-Allen, C.; Kalm, M. J. In Org. Synth. Coll.; 1963; Vol. 4, pp. 608–611.
8. Neises, B.; Steglich, W. Angew. Chem., Int. Ed. Engl. 1978, 17, 522–524.

S. Comesse, O. Piva / Tetrahedron: Asymmetry 10 (1999) 1061–1067
1067
9. We warmly thank Dr. Valnot (CNRS, IRCOF-University of Rouen) for sharing with us this observation. See also:
Angibaud, P.; Chaumette, J. L.; Desmurs, J. R.; Duhamel, L.; Plé, G.; Valnot, J.-Y.; Duhamel, P. Tetrahedron: Asymmetry
1995, 6, 1919–1932.
10. (a) Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P. Synthesis 1994, 639–666. (b) Lenz, R.; Ley, S. V. J. Chem. Soc.,
Perkin Trans. 1 1997, 3291–3292.
11. Parker, D. Chem. Rev. 1991, 91, 1441–1457.
12. As a control experiment, NMR shift experiments were first conducted on racemic aldehyde 2, easily prepared in 3 steps
by reduction of ester 4.
13. (a) Enders, D.; Klatt, M. In Encyclopedia of Reagents for Organic Synthesis; Paquette, L. A., Ed. 1995; Vol. 1, pp. 178–182.
(b) Enders, D.; Mies, W. J. Chem. Soc., Chem. Commun. 1984, 1221–1223.
14. For a recent review on the use of sugars as chiral auxiliaries: Hultin, P. G.; Earle, M. A.; Sudharshan, M. Tetrahedron 1997,
53, 14823–14870.
15. Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923–2925.