Improved synthesis of an ascaroside pheromone controlling dauer larva development in caenorhabditis elegans

Article abstract of DOI:10.1055/s-0029-1216967

Using an efficient Wacker oxidation as a key step, we describe a significantly improved synthesis of the dauer-promoting ascaroside 2 for biological studies of the novel sterol ring methylase STRM-1.

Full text of DOI:10.1055/s-0029-1216967

3488
PAPER
Improved Synthesis of an Ascaroside Pheromone Controlling Dauer Larva
Development in Caenorhabditis elegans
S
ynthesis of an A
e
scaroside
P
n
heromone é Martin,a Tina Schäfer,a Gabriele Theumer,a Eugeni V. Entchev,b Teymuras V. Kurzchalia,b
Hans-Joachim Knölker*a
a
Department Chemie, Technische Universität Dresden, Bergstraße 66, 01069 Dresden, Germany
Fax +49(351)46337030; E-mail: hans-joachim.knoelker@tu-dresden.de
Max-Planck-Institut für Molekulare Zellbiologie und Genetik, Pfotenhauerstraße 108, 01307 Dresden, Germany
b
Received 22 June 2009
C. elegans enters the so-called diapause generating dauer
Abstract: Using an efficient Wacker oxidation as a key step, we de-
scribe a significantly improved synthesis of the dauer-promoting as-
caroside 2 for biological studies of the novel sterol ring methylase
STRM-1.
larvae.2
Recently, several pheromones that induce dauer larva for-
mation of C. elegans have been isolated and synthe-
Key words: carbohydrates, natural products, pheromones, stereo- sized.6–8 As common structural feature, these pheromones
selective synthesis, Wacker reactions
represent glycosides of the dideoxy sugar ascarylose
(Figure 1). In 2005, Paik et al. described the isolation and
synthesis of the first compound in this series and called it
daumone (1).6 In the same year, O’Doherty and Guo re-
The life cycle of the nematode Caenorhabditis elegans
ported the second synthesis of daumone (1) using a palla-
dium-catalyzed glycosylation.7 This pheromone initiates
diapause entry of C. elegans. More recently, the identifi-
cation and synthesis of a series of different ascarosides 2–
4 has been reported (Figure 1).8 It has been demonstrated,
that the ascarosides 2–4 exhibit a significantly stronger
dauer-promoting activity than daumone (1) itself.8 More-
over, the blend of ascarosides 2–4 regulates formation of
dauer larvae and attraction of male larvae. At concentra-
tions (pM) more than 10,000 times lower than required for
dauer induction (nM–mM), ascarosides 2–4 function as
male attractants. At higher concentrations (nM–mM), the
male attractant effect is lost and they deter hermaphro-
dites.8
has been extensively studied over the last decade. Under
unfavorable conditions, such as scarcity of food or over-
crowding, C. elegans enters diapause and forms so-called
dauer larvae.1 The dauer larva represents a stress-resistant
and non-aging arrested developmental stage. Steroidal
hormones, the dafachronic (cholesten-26-oic) acids, are
required to bypass dauer larva formation and to enter re-
productive development.2 In the course of our studies,3 we
achieved efficient and stereoselective syntheses of the
(25R)-cholesten-26-oic acids4 as well as the more active
(25S)-cholesten-26-oic acids.5 In the presence of these
ligands, the hormonal receptor DAF-12 is inactivated and
the worms enter reproductive development. In the absence
of dafachronic acid ligands, DAF-12 is activated and
OH
OH
O
O
O
O
O
O
O
HO
HO
HO
OH
OH
daumone (1)
OH
ascarylose
2
O
O
O
OH
HO
HO
O
O
O
O
O
HO
OH
HO
HO
OH
3
4
Figure 1 Ascarylose, daumone (1), and the ascarosides 2–4
SYNTHESIS 2009, No. 20, pp 3488–3492
x
x.
x
x
.
2
0
0
9
Advanced online publication: 21.08.2009
DOI: 10.1055/s-0029-1216967; Art ID: Z13209SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Synthesis of an Ascaroside Pheromone
3489
OH
c
OH
OBz
O
a
b
O
O
O
O
BzO
HO
BzO
BzO
BzO
HO
BzO
BzO
OBz
OH
OBz
OBz
7
L-rhamnose (5)
6
8
OH
O
O
d
e
f
O
O
O
BzO
BzO
BzO
OBz
OBz
OBz
9
10
11
Scheme 1 Improved transformation of L-rhamnose (5) into 2,4-di-O-benzoylascarylose (11). Reagents and conditions: (a) BzCl, pyridine, 0
°C to r.t., 16 h, 95%; (b) NH3(g), MeOH–THF (3:7), 0 °C to r.t., 12 h, 87%; (c) PCC, 4 Å molecular sieves, CH2Cl2, 0 °C to r.t., 4 h, 85%; (d)
Et3N–CHCl3 (1:4), r.t., 16 h, 81%; (e) 10% Pd/C, H2, EtOAc, r.t., 24 h, 96%; (f) disiamylborane, THF, 0 °C to r.t., 20 h, 94%, b/a = 4.6:1.
Recently, we identified the novel sterol methylating en- (Scheme 3). Key steps of our approach are glycosidation
zyme STRM-1, which regulates the in vivo concentration of the anomeric hydroxy group with commercial (2R)-
of dafachronic acids by methylating the biosynthetic pre- hex-5-en-2-ol followed by Wacker oxidation of the termi-
cursors of dafachronic acids.9 Thus, strm-1 mutant worms nal double bond to generate the methyl ketone.
have elevated in vivo concentrations of dafachronic acids.
In order to study the biological function of STRM-1 in the
Glycosidation of 11 with (2R)-hex-5-en-2-ol was
achieved in the presence of boron trifluoride–diethyl ether
dauer-formation process, we prepared the ascaroside 2 to
test the response to this pheromone in strm-1 mutant
worms.9 The ascaroside 2 has been obtained previously in
10 steps and 13% overall yield starting from L-rhamnose
(5) (Schemes 1 and 2).8a Conversion of L-rhamnose (5)
into the dideoxysugar ascarylose was described in 1977
by Monneret et al.10,11 2,4-Di-O-benzoylascarylose (11),
the crucial intermediate for the ascarosides 1–4, was pre-
pared from L-rhamnose (5) as originally reported in 1979
(Scheme 1).6,11
complex, following a well-known literature protocol.6,13,14
Treatment of 11 with (2R)-hex-5-en-2-ol in the presence
of 3.0 equivalents of boron trifluoride–diethyl ether af-
forded compound 15 in only 61% yield. However, using
1.5 equivalents of boron trifluoride–diethyl ether led to a
significant improvement of the yield for compound 15
(93%). Wacker oxidation of 15 in a mixture of N,N-di-
methylformamide and water (7:1) using catalytic amounts
of palladium(II) acetate and copper(I) chloride in the pres-
ence of air afforded the methyl ketone 14 in 89% yield.15
Treatment of 5 with benzoyl chloride in pyridine led to the
fully protected rhamnose 6 (Scheme 1). Selective depro-
tection of the anomeric hydroxy group with ammonia to
the lactol 7 followed by pyridinium chlorochromate oxi-
dation afforded the lactone 8. By using dry triethylamine
and dry chloroform for the elimination of benzoic acid
from 8, we have been able to improve the yield of the un-
saturated lactone 9 from 65%6 to 81%. Increasing the re-
action time for the catalytic hydrogenation of 9 also
provided a better access to the lactone 10 (24 h, 96% yield
instead of 3 h, 85% yield).6 Reduction of the lactone with
disiamylborane provided 2,4-di-O-benzoylascarylose
(11) (6 steps, 51% overall yield) as an anomeric mixture
(b/a 4.6:1).6,12 Compound 11 is the central intermediate
for the synthesis of daumone (1) and the ascarosides 2–
4.6,8
NH
O
CCl3
a
b
O
11
BzO
OBz
12
O
c
OH
O
BzO
OBz
13
O
d
Clardy et al. used a four-step sequence for the transforma-
tion of 11 into the ascaroside 2 (Scheme 2).8a Conversion
into the trichloroacetimidate 12, followed by stereoselec-
tive glycosidation with (2R,5R)-hexane-2,5-diol to 13, py-
ridinium dichromate oxidation to 14, and final
saponification afforded the ascaroside 2 in 36% overall
yield based on 11.8a
O
2
O
BzO
OBz
14
Scheme 2 Previously reported synthesis of the ascaroside 2.8a
Reagents and conditions: (a) cat. DBU, Cl3CC≡N, CH2Cl2, r.t., 30
min, 81%; (b) cat. TMSOTf, (2R,5R)-hexane-2,5-diol, CH2Cl2, 0 °C,
30 min, 64%; (c) PDC, 4 Å molecular sieves, CH2Cl2, r.t., 2 h, 89%;
(d) 1 M KOH in MeOH, r.t., 3 h, 78%.
We have developed an optimized route for the synthesis
of the ascaroside 2 from 2,4-di-O-benzoylascarylose (11)
Synthesis 2009, No. 20, 3488–3492 © Thieme Stuttgart · New York

3490
R. Martin et al.
PAPER
Finally, saponification of 14 with potassium hydroxide in work:8a 4 steps, 36% overall yield). The highly efficient
methanol provided quantitatively the ascaroside 2.
Wacker oxidation at the olefinic side chain of intermedi-
ate 15 represents the key step of our approach. Moreover,
two key steps of the synthesis of 11 starting from L-rham-
nose (5) have been significantly improved. Thus, the ap-
proach to the ascaroside 2 starting from L-rhamnose (5)
has been improved from 10 steps and 13% overall yield6,8a
to 9 steps and 42% overall yield. The procedure described
in the present work provides access to the ascarosides 1–
4 in gram quantities. Moreover, the olefinic side chain at
the stage of intermediate 15 opens up the access to further
ascaroside derivatives, e.g. via olefin metathesis, cycload-
dition, hydroboration, or dihydroxylation. Using the asca-
roside 2, we have developed efficient conditions to induce
dauer larva formation.9 The assay for diapause shows that
STRM-1 activity plays a major role in regulating dauer
larva formation. Moreover, investigation of other mutant
worms using the present assay would potentially lead to
the identification of further genes involved in regulation
of dauer larva formation.
O
a
O
11
BzO
OBz
15
O
b
O
O
BzO
OBz
14
O
c
O
O
HO
All reactions were carried out using anhyd solvents in oven-dried
glassware under an argon atmosphere. THF, EtOAc, and CH2Cl2
were dried using a solvent purification system (MBraun-SPS). Py-
ridine was obtained from Fluka (H2O content less than 50 ppm).
Et3N was heated under reflux with CaH2 for 48 h and stored over 3
Å molecular sieves. Anhyd CHCl3 was obtained by passing through
a column of alumina and stored over 3 Å molecular sieves. All other
chemicals were used as received from commercial sources. Flash
chromatography was performed using silica gel from Acros Organ-
ics (0.063–0.200 mm). TLC was performed with TLC plates from
Merck (60 F254) using Ce(SO4)2/phosphomolybdic acid reagent for
visualization. PE = petroleum ether. Specific rotation values were
measured on a Perkin Elmer 341 polarimeter. IR spectra were re-
corded on a Thermo Nicolet Avatar 360 FT-IR spectrophotometer
using the ATR method. NMR spectra were recorded on Bruker
Avance II 300, DRX 500 and Avance III 600 NMR spectrometers
with the deuterated solvent as internal standard. ESI-MS spectra
OH
2
Scheme 3 Efficient three-step synthesis of the ascaroside 2 from in-
termediate 11. Reagents and conditions: (a) BF3·OEt2, (2R)-hex-5-en-
2-ol, CH2Cl2, 0 °C to r.t., 16 h, then Et3N, 20 min, 93%; (b) cat.
Pd(OAc)2, cat. CuCl, DMF–H2O (7:1), air, r.t., 18 h, 89%; (c) 1 M
KOH in MeOH, r.t., 3 h, 99%.
Clardy et al. described an assay for testing the biological
activity of ascarosides.8a This original protocol results in
large variation of the number of dauer larvae which are
formed. Reproducible results in dauer larva formation
were obtained only at high concentrations of the ascaro-
side 2 and daumone (1).9 Thus, we have developed robust
assay conditions for effective induction of dauer larva for- were recorded on an Esquire LC with an ion trap detector from
Bruker. Positive and negative ions were detected.
mation in wild-type worms (increase of the percentage of
dauer larvae which have been formed: 73% to 89%).9 Ex-
2,4-Di-O-benzoyl-3,6-dideoxy-L-erythro-hex-2-enono-1,5-lac-
tone (9)
Anhyd Et3N (75 mL) was added to a soln of 8 (9.6 g, 20.2 mmol) in
posing strm-1 mutant worms to these conditions resulted
in 0% to 10% induction of dauer larva formation. Our ob-
servations indicated that strm-1 mutant worms have sig-
nificantly reduced ability of dauer larva formation. Under
conditions supporting normal growth, the concentration
of pheromone in the surrounding medium is directly pro-
portional to the number of worms. High pheromone con-
centrations are the signal for worms to enter diapause in
order to avoid overcrowding and starvation. In conse-
quence of their low sensitivity to natural pheromones,
strm-1 mutant worms generate dauer larvae only under su-
per-crowded conditions leading to very high concentra-
tions of pheromones. Our findings emphasize that STRM-
1 is a major component in regulation of dauer larva forma-
tion.
anhyd CHCl3 (300 mL) and the mixture was stirred at r.t. for 16 h.
After addition of H2O (100 mL), the layers were separated. The or-
ganic layer was washed with H2O (2 × 100 mL) and dried (MgSO4).
Evaporation of the solvent and purification of the residue by flash
chromatography (silica gel, toluene–EtOAc, 10:1) afforded 9 as a
light yellow oil; yield: 5.81 g (81%).
1H NMR (500 MHz, CDCl3): d = 1.64 (d, J = 6.7 Hz, 3 H), 4.95 (m,
1 H), 5.69 (t, J = 4.7 Hz, 1 H), 6.70 (d, J = 4.7 Hz, 1 H), 7.47 (t,
J = 7.6 Hz, 2 H), 7.48 (t, J = 7.6 Hz, 2 H), 7.61 (t, J = 7.6 Hz, 1 H),
7.63 (t, J = 7.6 Hz, 1 H), 8.06 (d, J = 7.6 Hz, 2 H), 8.11 (d, J = 7.6
Hz, 2 H).
13C NMR (DEPT, 75 MHz, CDCl3): d = 18.33 (CH3), 68.52 (CH),
77.35 (CH), 125.48 (CH), 127.89 (C), 128.62 (2 CH), 128.65 (2
CH), 128.71 (C), 129.90 (2 CH), 130.41 (2 CH), 133.85 (CH),
134.18 (CH), 140.78 (C), 157.91 (C=O), 164.24 (C=O), 165.45
(C=O).
In conclusion, the present synthesis provides the ascaro-
side 2 in only three steps and an excellent overall yield of
82% based on 2,4-di-O-benzoylascarylose (11) (previous
MS (ESI): m/z = 353.1 [(M + H)+], 370.1 [(M + NH4)+], 375.0 [(M
+ Na)+].
Synthesis 2009, No. 20, 3488–3492 © Thieme Stuttgart · New York

PAPER
Synthesis of an Ascaroside Pheromone
3491
2,4-Di-O-benzoyl-3,6-dideoxy-L-arabino-hexono-1,5-lactone
(10)
70.53 (CH), 71.06 (CH), 71.44 (CH), 93.55 (CH), 128.44 (4 CH),
129.61 (2 CH), 129.75 (C), 129.85 (2 CH), 129.92 (C), 133.21
(CH), 133.28 (CH), 165.64 (C=O), 165.78 (C=O), 208.42 (C=O).
A soln of 9 (5.99 g, 17.0 mmol) in EtOAc (100 mL) was added to a
Schlenk flask loaded with 10% Pd/C (397 mg). The resulting mix-
ture was stirred under an H2 atmosphere at r.t. for 24 h and then fil-
tered through a short pad of Celite (EtOAc, 250 mL). Removal of
the solvent and purification of the residue by flash chromatography
(silica gel, toluene–EtOAc, 10:1) provided 10 as a light yellow syr-
up which slowly solidified on standing; yield: 5.76 g (96%).
MS (ESI): m/z = 339.1 [(M – C6H11O2)+], 472.1 [(M + NH4)+],
477.2 [(M + Na)+].
(–)-(5R)-5-(3,6-Dideoxy-a-L-arabino-hexopyranosyl)hexan-2-
one (2)
A 1 M soln KOH in MeOH (11 mL) was added to 14 (520 mg, 1.14
mmol) and the resulting mixture was stirred at r.t. for 3 h. After ad-
dition of solid NaHCO3 (370 mg), the solvent was evaporated. The
residue was purified by flash chromatography (silica gel, CH2Cl2–
MeOH, 12:1) to afford the ascaroside 2 as a colourless oil; yield:
280 mg (99%).
MS (ESI): m/z = 355.1 [(M + H)+], 372.1 [(M + NH4)+], 377.1 [(M
+ Na)+].
Further spectroscopic data are available in the literature.6
(2R)-1-(Hex-5-en-2-yl)-2,4-di-O-benzoyl-3,6-dideoxy-a-L-arabi-
no-hexopyranoside (15)
[a]D23 –68.0 (c 1.17, CH2Cl2) [Lit.8a [a]D23 –78.4 (c 1.2, CH2Cl2)].
BF3·OEt2 (0.15 mL, 884 mmol) was slowly added to a mixture of
(2R)-hex-5-en-2-ol (100 mL, 83 mg, 827 mmol), 11 (201 mg, 564
mmol) and powdered 4Å molecular sieves (24 mg) in CH2Cl2 (4 mL)
at 0 °C. The mixture was stirred at r.t. for 16 h. After addition of
Et3N (0.5 mL), the mixture was stirred for a further 20 min. Remov-
al of the solvent and purification of the residue by flash chromatog-
raphy (silica gel, PE–Et2O, 20:1) provided 15 as a light yellow oil;
yield: 230 mg (93%).
IR (ATR): 3405, 2969, 2930, 1709, 1652, 1376, 1261, 1202, 1126,
1099, 1024, 982, 950, 882, 852, 838, 782, 734 cm–1.
1H NMR (500 MHz, acetone-d6): d = 1.13 (d, J = 6.1 Hz, 3 H), 1.21
(d, J = 5.8 Hz, 3 H), 1.68–1.82 (m, 3 H), 1.97 (dt, J = 13.0, 3.1 Hz,
1 H), 2.14 (s, 3 H), 2.62 (t, J = 7.4 Hz, 2 H), 3.56–3.61 (m, 2 H),
3.73 (m, 1 H), 3.79–3.83 (m, 1 H), 4.66 (s, 1 H).
1H NMR (600 MHz, CD3OD): d = 1.20 (d, J = 6.1 Hz, 3 H), 1.29 (d,
J = 6.0 Hz, 3 H), 1.75–1.88 (m, 3 H), 2.02 (ddt, J = 13.1, 0.6, 3.4
Hz, 1 H), 2.23 (s, 3 H), 2.69 (dt, J = 2.5, 7.4 Hz, 2 H), 3.59 (ddd,
J = 11.0, 9.4, 4.4 Hz, 1 H), 3.63 (dq, J = 9.4, 6.0 Hz, 1 H), 3.78 (m,
1 H), 3.84–3.88 (m, 1 H), 4.71 (s, 1 H).
13C NMR (DEPT, 125 MHz, acetone-d6): d = 18.22 (CH3), 19.10
(CH3), 29.74 (CH3), 31.80 (CH2), 36.36 (CH2), 39.94 (CH2), 67.68
(CH), 69.44 (CH), 70.48 (CH), 70.92 (CH), 97.00 (CH), 207.88
(C=O).
IR (ATR): 3065, 2975, 2934, 1720, 1685, 1652, 1602, 1451, 1377,
1315, 1262, 1215, 1176, 1151, 1100, 1065, 1022, 998, 944, 910,
708, 685 cm–1.
1H NMR (500 MHz, CDCl3): d = 1.20 (d, J = 6.1 Hz, 3 H), 1.27 (d,
J = 6.3 Hz, 3 H), 1.57–1.64 (m, 1 H), 1.72–1.79 (m, 1 H), 2.16–2.27
(m, 3 H), 2.41 (dt, J = 13.5, 3.7 Hz, 1 H), 3.87 (sext, J = 6.1 Hz, 1
H), 4.12 (dq, J = 3.6, 6.3 Hz, 1 H), 4.95 (s, 1 H), 5.00 (ddd, J = 10.2,
3.0, 1.2 Hz, 1 H), 5.08 (m, 1 H), 5.14 (m, 1 H), 5.18 (ddd, J = 14.5,
10.0, 4.6 Hz, 1 H), 5.87 (m, 1 H), 7.44–7.48 (m, 4 H), 7.56–7.60 (m,
2 H), 8.04 (m, 2 H), 8.11 (m, 2 H).
13C NMR (DEPT, 125 MHz, CDCl3): d = 17.85 (CH3), 19.08 (CH3),
29.71 (CH2), 29.96 (CH2), 36.33 (CH2), 66.99 (CH), 70.62 (CH),
71.21 (CH), 71.95 (CH), 93.69 (CH), 114.75 (CH2), 128.43 (4 CH),
129.61 (2 CH), 129.83 (C), 129.85 (2 CH), 129.97 (C), 133.17
(CH), 133.24 (CH), 138.39 (CH), 165.66 (C=O), 165.79 (C=O).
13C NMR (DEPT, 150 MHz, CD3OD): d = 18.06 (CH3), 19.08
(CH3), 29.85 (CH3), 32.10 (CH2), 35.96 (CH2), 40.44 (CH2), 68.28
(CH), 69.85 (CH), 71.29 (CH), 71.51 (CH), 97.32 (CH), 211.45
(C=O).
MS (ESI): m/z = 247.2 [(M + H)+], 264.1 [(M + NH4)+], 269.1 [(M
+ Na)+].
MS (ESI): m/z = 339.1 [(M – C6H11O)+], 456.2 [(M + NH4)+], 461.2
References
[(M + Na)+].
(1) Golden, J. W.; Riddle, D. L. Dev. Biol. 1984, 102, 368.
(2) (a) Motola, D. L.; Cummins, C. L.; Rottiers, V.; Sharma, K.
V.; Li, T.; Li, Y.; Suino-Powell, K.; Xu, H. E.; Auchus, R.
J.; Antebi, A.; Mangelsdorf, D. J. Cell 2006, 124, 1209.
(b) Rottiers, V.; Motola, D. L.; Gerisch, B.; Cummins, C. L.;
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D. L.; Cummins, C. L.; Lehrach, H.; Mangelsdorf, D. J.;
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(3) (a) Matyash, V.; Entchev, E. V.; Mende, F.; Wilsch-
Bräuninger, M.; Thiele, C.; Schmidt, A. W.; Knölker, H.-J.;
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(b) Schmidt, A. W.; Doert, T.; Goutal, S.; Gruner, M.;
Mende, F.; Kurzchalia, T. V.; Knölker, H.-J. Eur. J. Org.
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(5R)-5-(2,4-Di-O-benzoyl-3,6-dideoxy-a-L-arabino-hexopyra-
nosyl)hexan-2-one (14)
Pd(OAc)2 (8 mg, 36 mmol) and CuCl (4 mg, 40 mmol) were added
to a soln of 15 (90 mg, 205 mmol) in DMF (7 mL) and H2O (1 mL).
The mixture was stirred in an open flask at r.t. for 18 h. After addi-
tion of sat. NH4Cl soln (20 mL) and Et2O (20 mL) the layers were
separated. The aqueous layer was extracted with Et2O (2 × 20 mL)
and the combined organic layers were dried (MgSO4). The solvent
was removed and the residue was purified by flash chromatography
(silica gel, PE–Et2O, 3:1 to 2:1) to provide 14 as a light yellow oil;
yield: 83 mg (89%).
IR (ATR): 2975, 2929, 1714, 1686, 1652, 1635, 1602, 1451, 1316,
1263, 1216, 1176, 1153, 1100, 1066, 1021, 942, 893, 853, 808, 709,
686 cm–1.
1H NMR (500 MHz, CDCl3): d = 1.20 (d, J = 6.1 Hz, 3 H), 1.28 (d,
J = 6.3 Hz, 3 H), 1.84–1.89 (m, 2 H), 2.14–2.20 (m, 1 H), 2.20 (s, 3
H), 2.41 (dt, J = 13.5, 3.7 Hz, 1 H), 2.60 (t, J = 7.5 Hz, 2 H), 3.88
(sext, J = 6.1 Hz, 1 H), 4.05 (dq, J = 3.6, 6.3 Hz, 1 H), 4.93 (s, 1 H),
5.12 (m, 1 H), 5.17 (ddd, J = 14.5, 9.9, 4.6 Hz, 1 H), 7.44–7.48 (m,
4 H), 7.56–7.59 (m, 2 H), 8.03–8.05 (m, 2 H), 8.09–8.11 (m, 2 H).
(4) (a) Martin, R.; Schmidt, A. W.; Theumer, G.; Kurzchalia, T.
V.; Knölker, H.-J. Synlett 2008, 1965. (b) Martin, R.;
Schmidt, A. W.; Theumer, G.; Krause, T.; Entchev, E. V.;
Kurzchalia, T. V.; Knölker, H.-J. Org. Biomol. Chem. 2009,
7, 909.
13C NMR (DEPT, 125 MHz, CDCl3): d = 17.85 (CH3), 18.86 (CH3),
29.66 (CH2), 29.95 (CH3), 30.82 (CH2), 39.65 (CH2), 67.11 (CH),
Synthesis 2009, No. 20, 3488–3492 © Thieme Stuttgart · New York

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R. Martin et al.
PAPER
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