An enantiospecific route to (+)-(1R,3S)-cis-chrysanthemic acid from (-)-d-pantolactone

Article abstract of DOI:10.1055/s-0030-1258451

In this paper, a novel route for the synthesis of (+)-(1R,3S)-cis- chrysanthemic acid is described. The use of readily available (-)-d-pantolactone as a starting point, application of ring-closing metathesis to form the cyclopentene intermediate, and Haller-Bauer/Grob-type fragmentation to form the target compound are the highlights of the present synthesis. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0030-1258451

PAPER
1067
An Enantiospecific Route to (+)-(1R,3S)-cis-Chrysanthemic Acid from
(–)-D-Pantolactone1
E
nantiospecific Ro
t
ute to
c
is-Chrysant
lhemic A
cid K. Hajare,a,b Laxmikant S. Datrange,a Y. L. N. Murthy,b Debnath Bhuniya,a D. Srinivasa Reddy*a,c
a
Discovery Chemistry, Advinus Therapeutics Ltd., Quantum Towers, Plot No. 9, Rajiv Gandhi Infotech Park, Phase-I, Hinjewadi,
Pune 411057, India
b
c
Department of Organic Chemistry, Andhra University, Visakhapatnam 530003, India
Present address: National Chemical Laboratory (CSIR, India), Division of Organic Chemistry, Dr. Homi Bhabha Road, Pune 411008,
Maharashtra, India
Fax +91(20)66539620; E-mail: ds.reddy@ncl.res.in; E-mail: dsreddy88@yahoo.com
Received 3 December 2010; revised 31 January 2011
Retrosynthetically, the target compound (+)-(1R,3S)-cis-
chrysanthemic acid (1) could be prepared from Krief’s in-
termediate 3 using Haller–Bauer/Grob-type fragmenta-
tion. The cyclopentene intermediate 4 could be trans-
Abstract: In this paper, a novel route for the synthesis of (+)-
(1R,3S)-cis-chrysanthemic acid is described. The use of readily
available (–)-D-pantolactone as a starting point, application of ring-
closing metathesis to form the cyclopentene intermediate, and
Haller–Bauer/Grob-type fragmentation to form the target com- formed to 3 via cyclopropanation of the double bond and
pound are the highlights of the present synthesis.
conversion of hydroxy handle into a leaving group like to-
sylate. It was proposed that ring-closing metathesis
(RCM) could generate the key cyclopentenone intermedi-
ate 4 from its acyclic precursor 5, which in turn could be
prepared from commercially available (–)-D-pantolactone
using routine functional group transformations
(Scheme 1).
Key words: ring closure, stereoselective synthesis, Wittig reaction,
alkenes, metathesis
Chrysanthemic acids and their analogues/derivatives are
important starting materials for the preparation of a vari-
ety of pyrethroid insecticides.2 The high insecticidal activ-
ity of the pyrethroids coupled with their low mammalian
toxicity and biodegradability made them an attractive
class of compounds. Due to their high importance in hu-
man life, several syntheses of both cis- 1 and trans-chry-
santhemic acids 2 in racemic and enantiopure form have
been documented in the literature (Figure 1).3 The pio-
neering and continued work by Krief’s group needs spe-
cial mention in this regard. The creation of new and
efficient synthetic routes to important molecules like 1
and 2 is always exciting and rewarding. We have attempt-
ed an enantiospecific route for the synthesis of 1 starting
from commercially available (–)-D-pantolactone and the
results are disclosed in this paper.4 It is noteworthy to
mention that majority of efforts have been focused on the
synthesis of trans-chrysanthemic acid despite the impor-
tance of both compounds. In fact, the most important in-
secticides of this class are deltamethrin and cypermethrin
which have a cis relationship.5
O
O
O
OH
OTs
OR
1
3
4
O
O
HO
RO
O
5
D-(–)-pantolactone
Scheme 1
The details of this synthesis are outlined in Scheme 2. Our
synthesis began with the construction of the known acy-
clic precursor 6 following analogous procedures reported
in the literature from our group and others.4,6 Dess–Martin
oxidation and vinyl Grignard addition to the resulting al-
dehyde produced a diastereomeric mixture of alcohols 7
in excellent yields. Stereoselectivity at the newly formed
center is not important as it will be transformed into a car-
bonyl group. Having key acyclic precursor 7 in hand, we
subjected it to RCM using Grubbs’ second-generation
catalyst {[1,3-bis(2,4,6-trimethylphenyl)imidazolidin-2-
ylidene](benzylidene)dichloro(tricyclohexylphosphine)ru-
thenium} to give a hydroxycyclopentene derivative,
which upon Dess–Martin oxidation with DMP provided
cyclopentenone 8. Deprotection of the methoxymethyl
protecting group with aqueous sulfuric acid resulted in an
O
O
OH
OH
1
2
Figure 1 Structures of cis- and trans-chrysanthemic acids
SYNTHESIS 2011, No. 7, pp 1067–1070
x
x
.
x
x
.2
0
1
1
Advanced online publication: 01.03.2011
DOI: 10.1055/s-0030-1258451; Art ID: T50810SS
© Georg Thieme Verlag Stuttgart · New York

1068
A. K. Hajare et al.
PAPER
alcohol,7 which was further transformed (TsCl, py) to its
tosyl ester 9. The tosylate 9 reacted smoothly with isopro-
pylidenediphenylsulfurane (Ph2S=CMe2) generated in
situ to produce the bicyclo[3.1.0] derivative 3.3c,8 By fol-
lowing the protocol (KOH, DMSO, H2O) described by
Krief et al.,3d,e we were able to convert 3 into the target
molecule (+)-(1R,3S)-cis-chrysanthemic acid (1) in 56%
yield. The spectral data of our compound 1 was compared
with c in g/100 mL. Mass spectra were measured with ESI ioniza-
tion using Agilent MSD/VL spectrometer. HRMS data was record-
ed on MALDI-TOF using 2,5-dihydroxybenzoic acid as the solid
matrix. IR spectra were recorded on a Perkin-Elmer 100 FT-IR
spectrophotometer.
(S)-5-(Methoxymethoxy)-4,4-dimethylhepta-1,6-dien-3-ol (7)
To a stirred soln of (S)-3-(methoxymethoxy)-2,2-dimethylpent-4-
en-1-ol6 (6, 2.00 g, 11.48 mmol) in CH2Cl2 (25 mL) was added
DMP (7.31 g, 17.22 mmol) at 0 °C. After stirring for 1 h, the mix-
ture was diluted with CH2Cl2 (~30 mL) and filtered through a Celite
bed. The filtrate was washed with aq K2CO3 soln (20 mL), aq sat.
Na2S2O3 soln (20 mL), and brine (20 mL), dried (anhyd Na2SO4),
and concentrated to give (S)-3-(methoxymethoxy)-2,2-dimethyl-
pent-4-enal (1.97 g, 90%) as a colorless oil. The crude aldehyde was
used as such for further conversion.
1H NMR (400 MHz, CDCl3): d = 1.02 (s, 3 H), 1.10 (s, 3 H), 3.34
(s, 3 H), 4.14 (d, J = 8.4 Hz, 1 H), 4.47 (d, J = 6.8 Hz, 1 H), 4.69 (d,
J = 7.2 Hz, 1 H), 5.31 (d, J = 17.6 Hz, 1 H), 5.38 (d, J = 10.4 Hz, 1
H), 5.63–5.72 (m, 1 H), 9.60 (s, 1 H).
22.0
to the reported data and found to be identical, [a]D
20
+82.0 (c 0.1, CHCl3) {Lit.3c [a]D +83.0 (c 1.75,
CHCl3)}.3c Thus, we have completed the synthesis of 1 in
an enantiospecific manner.
O
O
a
b
OH
HO
OMOM
MOMO
OH
(–)-pantolactone
6
7
To a stirred soln of above aldehyde (1.71 g, 9.93 mmol) in anhyd
THF (15 mL) was added 1.0 M vinylmagnesium bromide in THF
(14.89 mL, 14.89 mmol) at 0 °C. After stirring for 1 h, the reaction
was quenched by the addition of aq NH4Cl soln (10 mL) and ex-
tracted with EtOAc (2 × 20 mL). The combined organic layers were
washed with brine (40 mL), dried (anhyd Na2SO4), concentrated,
and purified by column chromatography (8% EtOAc–hexane) to
give 7 (1.61 g, 81%) as a colorless oil and diastereomeric mixture.
O
O
c
d
e
f
3
1
OMOM
OTs
8
9
IR (neat): 1639, 3459 cm–1.
Scheme 2 Reaction conditions: (a) (i) DIPEA, MOMCl, CH2Cl2, 0
°C to r.t. 48 h, 90%; (ii) DIBAL-H, CH2Cl2, –78 °C, 2 h, 91%; (iii)
Ph3PCH3Br, BuLi, THF, 0 °C to r.t., 20 h, 81%; (b) (i) DMP, CH2Cl2,
0 °C 1 h, 90%; (ii) H2C=CHMgBr, THF, 0 °C, 1 h, 81%; (c) (i)
Grubb’s catalyst, CH2Cl2, r.t., 1 h, 83%; (ii) DMP, CH2Cl2, 0 °C, 1 h,
86%; (d) (i) 10% aq H2SO4, THF, reflux, 3 h, 80%; (ii) TsCl, py, 0 °C
to r.t., 15 h, 77%; (e) Ph2SCHMe2·BH4, LDA, CH2Cl2, DME, –78 °C,
30 min, 93%; (f) KOH, DMSO, H2O, 70 °C, 4 h, 56%.
1H NMR (400 MHz, CDCl3): d (major isomer) = 0.87 (s, 3 H), 0.90
(s, 3 H), 3.41 (s, 3 H), 3.98 (d, J = 8.0 Hz, 1 H), 4.12 (br s, 1 H), 4.54
(dd, J = 7.6, 6.8 Hz, 2 H), 4.68 (d, J = 6.8 Hz, 1 H), 5.19–5.35 (m,
4 H), 5.67–5.83 (m, 1 H), 5.87–5.95 (m, 1 H).
13C NMR (100 MHz, CDCl3): d = 19.9, 20.9, 40.9, 56.5, 77.9, 84.8,
94.6, 116.9, 120.3, 134.4, 137.4.
HRMS (MALDI-TOF): m/z [M + Na]+ calcd for C11H20O3Na:
223.1310; found: 223.1328.
In summary, we have utilized the chiral pool compound (–)-
D-pantolactone for the enantiospecific synthesis of (+)-
(1R,3S)-cis-chrysanthemic acid (1). In addition to the use
of a readily available chiral starting material, the applica-
tion of RCM to form a cyclopentene intermediate and
Krief’s Haller–Bauer/Grob-type fragmentation to form
the final compound are the highlights of the present syn-
thesis.
(S)-4-(Methoxymethoxy)-5,5-dimethylcyclopent-2-enone (8)
To a stirred soln of 7 (1.20 g, 5.99 mmol) in CH2Cl2 (15 mL) was
added Grubbs II catalyst (127 mg, 0.15 mmol) at r.t. After stirring
for 1 h, the mixture was diluted with CH2Cl2 (30 mL) and washed
with H2O (40 mL) and brine (40 mL), dried (Na2SO4), concentrated,
and purified by column chromatography to give (S)-4-(meth-
oxymethoxy)-5,5-dimethylcyclopent-2-enol (860 mg, 83%) as an
oil.
LCMS: m/z = 190.0 [M + H2O]+.
HRMS (MALDI-TOF): m/z [M + Na]+ calcd for C9H16O3Na:
All reactions were carried out in oven-dried glassware under a pos-
itive pressure of argon or N2 unless otherwise mentioned with mag-
netic stirring. Air-sensitive reagents and solutions were transferred
via syringe or cannula and were introduced to the apparatus via rub-
ber septa. All reagents, starting materials, and solvents (including
anhyd solvents) were obtained from commercial suppliers and used
as such without further purification. Reactions were monitored by
TLC with 0.25 mm (E. Merck) pre-coated silica gel plates (60 F254).
Visualization was accomplished with either UV light, or by immer-
sion in phosphomolybdic acid, or KMnO4 solns followed by heating
on a heat gun for ~15 s. Column chromatography was performed on
silica gel (60–120 mesh). Deuterated solvents (Cambridge Isotope
Laboratories) for NMR spectroscopic analyses were used as re-
ceived. All 1H and 13C NMR spectra were obtained using a Varian
400 MHz (400 MHz for 1H and 100 MHz for 13C) spectrometer. All
chemical shifts use the residual solvent peak as a reference standard.
Optical rotation was recorded on Rudolph Autopol-V digital pola-
rimeter at 589 nm (Na D-line) and optical rotation data was reported
195.0997; found: 195.1013.
To a cooled soln of (S)-4-(methoxymethoxy)-5,5-dimethylcyclo-
pent-2-enol (650 mg, 3.77 mmol) in CH2Cl2 (10 mL) was added
DMP (2.40 g, 5.66 mmol) at 0 °C. After 1 h, the mixture was diluted
with CH2Cl2 (20 mL) and filtered through a Celite bed. The filtrate
was washed with aq K2CO3 soln (20 mL), aq sat. Na2S2O3 soln (20
mL), and brine (20 mL), dried (anhyd Na2SO4), concentrated, and
purified by column chromatography (8% EtOAc–hexanes) to give
8 (550 mg, 86%) as a colorless oil.
[a]D22.5 +52.7 (c 1.0, CHCl3).
IR (neat): 1718 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.07 (s, 3 H), 1.18 (s, 3 H), 3.44
(s, 3 H), 4.45 (d, J = 2.0 Hz, 1 H), 4.78 (s, 2 H), 6.19 (dd, J = 6.0,
2.0 Hz, 1 H), 7.48 (dd, J = 6.0, 2.0 Hz, 1 H).
Synthesis 2011, No. 7, 1067–1070 © Thieme Stuttgart · New York

PAPER
Enantiospecific Route to cis-Chrysanthemic Acid
1069
13C NMR (100 MHz, CDCl3): d = 21.3, 23.4, 48.3, 56.0, 85.1, 97.1,
132.9, 160.1, 211.6.
13C NMR (100 MHz, CDCl3): d = 17.0, 19.0, 21.8, 22.1, 24.2, 26.8,
34.6, 38.6, 55.5, 83.7, 128.0, 130.1, 133.6, 145.3, 212.8.
LCMS: m/z = 171.1 [M + H]+.
LCMS: m/z = 323.2 [M + H]+.
HRMS (MALDI-TOF): m/z [M + Na]+ calcd for C9H14O3Na:
193.0841; found: 193.0848.
(1R,3S)-2,2-Dimethyl-3-(2-methylprop-1-enyl)cyclopropane-
carboxylic Acid (1)
To a cooled soln of KOH (52 mg, 0.93 mmol) in DMSO–H2O (4:1,
1 mL) was added tosylate 3 (50 mg, 0.15 mmol) at r.t. After stirring
at 70 °C for 4 h, the mixture was cooled to r.t. and acidified with
10% HCl (pH ~2). This was extracted with Et2O (4 × 10 mL). The
combined organic layers were washed with H2O (10 mL) and brine
(10 mL), dried (anhyd Na2SO4), concentrated, and purified by col-
umn chromatography to give 1 (14.5 mg, 56%) as a solid.
[a]D22.0 +82.0 (c 0.1, CHCl3) {Lit.3c [a]D20 +83.0 (c 1.75, CHCl3)}.
IR (neat): 1697, 3018 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.21 (s, 3 H), 1.25 (s, 3 H), 1.65
(d, J = 8.8 Hz, 1 H), 1.70 (s, 3 H), 1.75 (s, 3 H), 1.96 (dd, J = 8.8,
8.4 Hz, 1 H), 5.35 (dt, J = 8.8, 1.2 Hz, 1 H), 10.74 (br s, 1 H).
(S)-5,5-Dimethyl-4-oxocyclopent-2-enyl 4-Toluenesulfonate (9)
To a stirred soln of 8 (500 mg, 2.94 mmol) in THF (5 mL) was add-
ed 10% aq H2SO4 (5 mL) at r.t. and the mixture was refluxed for 3
h. The mixture was cooled to 0 °C and basified with aq sat. NaHCO3
soln and extracted with EtOAc (2 × 10 mL). The combined organic
layers were washed with H2O (10 mL) and brine (10 mL), dried (an-
hyd Na2SO4), concentrated, and purified by column chromatogra-
phy (9% EtOAc–hexanes) to give (S)-4-hydroxy-5,5-di-
methylcyclopent-2-enone (295 mg, 80%).
[a]D23.0 +80.2 (c 1.0, MeOH) {Lit.7 [a]D23.0 +87.2 (c 1.31, MeOH)}.
IR (neat): 1700, 3418 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.08 (s, 3 H), 1.17 (s, 3 H), 2.07
(d, J = 6.8 Hz, 1 H), 4.58–4.60 (m, 1 H), 6.20 (dd, J = 6.0, 1.2 Hz,
1 H), 7.46 (dd, J = 5.6, 2.0 Hz, 1 H).
13C NMR (100 MHz, CDCl3): d = 15.1, 18.7, 26.3, 27.8, 29.2, 31.3,
33.5, 118.2, 135.5, 177.2.
13C NMR (100 MHz, CDCl3): d = 20.5, 23.0, 48.7, 79.9, 132.7,
161.8, 212.9.
LCMS: m/z = 169.1 [M + H]+.
To a stirred soln of (S)-4-hydroxy-5,5-dimethylcyclopent-2-enone
(250 mg, 1.98 mmol) in pyridine (5 mL) was added TsCl (567 mg,
2.97 mmol) at 0 °C. After stirring at r.t. for 15 h, pyridine was evap-
orated under reduced pressure. The resulting residue was taken in
EtOAc (10 mL) and washed with 1.0 M HCl (10 mL), H2O (10 mL),
and brine (10 mL), dried (anhyd Na2SO4), concentrated, and puri-
fied by column chromatography (5% EtOAc–hexanes) to give 9
(425 mg, 77%) as a white solid; mp 71–72 °C.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis. Included
are copies of 1H and 13C NMR spectra of important intermediates 9,
8, 7, 6, 3, and synthesized cis-chrysanthemic acid 1.
Acknowledgment
[a]D21.7 +59.3 (c 1.0, CHCl3).
IR (neat): 1366, 1723 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.03 (s, 3 H), 1.04 (s, 3 H), 2.48
(s, 3 H), 5.19 (t, J = 1.6 Hz, 1 H), 6.27 (dd, J = 6.0, 1.2 Hz, 1 H),
7.28 (dd, J = 6.0, 2.4 Hz, 1 H), 7.40 (d, J = 8.0 Hz, 2 H), 7.85 (d,
J = 8.0 Hz, 2 H).
We are grateful to Drs. Rashmi Barbhaiya and Kasim Mookhtiar for
their support and encouragement. Help from analytical group in ge-
nerating spectral data is appreciated. The authors thank Venkat Pan-
changula and Ajeet Singh of NCL, Pune for the MALDI spectra.
References
13C NMR (100 MHz, CDCl3): d = 20.9, 21.9, 22.4, 47.4, 86.3, 128.0,
130.2, 133.3, 135.0, 145.6, 155.7, 208.7.
(1) Advinus Publication No.: ADV-A-010.
(2) (a) Itagaki, M.; Suenobu, K. Org. Process Res. Dev. 2007,
11, 509. (b) Jeanmart, S. Aust. J. Chem. 2003, 56, 559.
(c) Fishman, A.; Kellner, D.; Ioffe, D.; Shapiro, E. Org.
Process Res. Dev. 2000, 4, 77. (d) Naumann, K. Chemistry
of Plant Protection, Synthetic Pyrethroid Insecticides, Vol.
5; Springer: Berlin, 1990, and references cited therein.
(3) Selected references for previous syntheses: (a) Krief, A.;
Jeanmart, S.; Kremer, A. J. Org. Chem. 2008, 73, 9795.
(b) Krief, A.; Gondal, H. Y.; Kremer, A. Chem. Commun.
2008, 4753. (c) Krief, A.; Kremer, A. Synlett 2007, 607.
(d) Krief, A.; Swinnen, D. Tetrahedron Lett. 1996, 37,
7123. (e) Krief, A.; Surleraux, D.; Ropson, N. Tetrahedron:
Asymmetry 1993, 4, 289. (f) Lambs, L.; Singh, N. P.;
Biellmann, J. F. J. Org. Chem. 1992, 57, 6301. (g) Krief,
A.; Surleraux, D.; Robson, M. J. Synlett 1991, 276.
(h) Krief, A.; Surleraux, D.; Frauenrath, H. Tetrahedron
Lett. 1988, 29, 6157. (i) Mulhr, J.; Kappert, M. Angew.
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Spectra were compared with those of racemic 9 known in the
literature9 and found to be identical.
(1S,2S,5R)-3,3,6,6-Tetramethyl-4-oxobicyclo[3.1.0]hexan-2-yl
4-Toluenesulfonate (3)
A soln of isopropyldiphenylsulfonium tetrafluoroborate8 (182 mg,
0.57 mmol) and CH2Cl2 (42 mg, 0.03 mL, 0.49 mmol) in anhyd
DME (6 mL) was cooled to –78 °C and treated with LDA [prepared
freshly by the addition of 1.6 M BuLi in hexane (0.31 mL, 0.49
mmol) to i-Pr2NH (0.07 mL, 0.49 mmol) in DME (1 mL) at 0 °C].
After 45 min, a soln of 9 (107 mg, 0.38 mmol) in DME (1 mL) was
added and the mixture was stirred for 30 min. The reaction was
quenched by the addition of H2O (5 mL). The aqueous layer was
separated and extracted with EtOAc (2 × 10 mL). The combined or-
ganic layers were washed with brine (20 mL), dried (anhyd
Na2SO4), concentrated, and purified by column chromatography
(4% EtOAc–hexanes) to give 3 (115 mg, 93%) as a white solid; mp
103–104 °C (Lit.3h 106 °C).
[a]D22.0 +77.9 (c 0.8, CHCl3).
IR (neat): 1728 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.89 (s, 3 H), 1.00 (s, 3 H), 1.05
(s, 3 H), 1.07 (s, 3 H), 1.72 (d, J = 5.2 Hz, 1 H), 1.90 (d, J = 5.2 Hz,
1 H), 2.47 (s, 3 H), 4.50 (s, 1 H), 7.39 (d, J = 8.0 Hz, 2 H), 7.84 (d,
J = 8.0 Hz, 2 H).
(4) See earlier references from this group which use the
pantolactone chiral pool approach: (a) Hajare, A.; Datrange,
L.; Vyas, S.; Bhuniya, D.; Reddy, D. S. Tetrahedron Lett.
2010, 51, 5291. (b) Reddy, D. S.; Srinivas, G.; Rajesh, B.
M.; Kannan, M.; Rajale, T. V.; Iqbal, J. Tetrahedron Lett.
Synthesis 2011, No. 7, 1067–1070 © Thieme Stuttgart · New York

1070
A. K. Hajare et al.
PAPER
alternate route. Miyaoka, H.; Sagawa, S.; Nagaoka, H.;
Yamada, Y. Tetrahedron: Asymmetry 1995, 6, 587.
(8) Isopropyldiphenylsulfonium tetrafluoroborate was
synthesized according to the general procedure reported in
the literature: Matsuyama, H.; Nakamura, T.; Iyoda, M.
J. Org. Chem. 2000, 65, 4796.
2006, 47, 6373. (c) Hajare, A.; Ravikumar, V.; Khaleel, S.;
Bhuniya, D.; Reddy, D. S. J. Org. Chem. 2011, 76, 963.
(5) (a) Tombo, G. M. R.; Bellus, D. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1193. (b) Elliott, M. Pestic. Sci. 1989, 27,
337. (c) Ackermann, P.; Bourgeois, F.; Drabek, J. Pestic.
Sci. 1980, 11, 169.
(6) (a) Akbutina, F. A.; Sandretdinov, I. F.; Selezneva, N. K.;
Miftakhov, M. S. Mendeleev Commun. 2003, 3, 151.
(b) Pirrung, F. O. H.; Hiemstra, H.; Speckamp, W. N.;
Kaptein, B.; Schoemaker, H. E. Synthesis 1995, 458.
(7) Previously, the same alcohol was prepared from (–)-panto-
lactone in 10 steps with an overall yield of ~34% using an
(9) Dauvergne, J.; Happe, A. M.; Jadhav, V.; Justice, D.; Matos,
M.-C.; McCormack, P. J.; Pitts, M. R.; Roberts, S. M.;
Singh, S. K.; Snape, T. J.; Whittall, J. Tetrahedron 2004, 60,
2559.
Synthesis 2011, No. 7, 1067–1070 © Thieme Stuttgart · New York