Diastereoselective Reaction of Buta-1,3-diene with Chiral Derivatives of Glyoxylic Acid: Effective Route to Optically Pure 2-Substituted 3,6-Dihydro2H-pyrans

Article abstract of DOI:10.1055/s-2003-44372

The influence of Lewis acids on the diastereoselectivity of [4+2] cycloaddition of buta-1,3-diene (4) to N-glyoxyloyl-(2R)-bornane- 10,2-sultam (6a) and (R)-8-phenylmenthyl glyoxylate (6b) was investigated and found to have high levels of asymmetric induc

Full text of DOI:10.1055/s-2003-44372

PAPER
87
Diastereoselective Reaction of Buta-1,3-diene with Chiral Derivatives of
Glyoxylic Acid: Effective Route to Optically Pure 2-Substituted 3,6-Dihydro-
2H-pyrans
Synthesis of
Optically
a
Pure 2-Sub
g
stituted 3,6-D
o
ihydro-2
H
-
r
pyrans zata Kosior,a Monika Asztemborska,b Janusz Jurczak*a,c
a
b
c
Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Fax +48(22)8230944; E-mail: jjurczak@chem.uw.edu.pl
Received 12 September 2003; revised 15 October 2003
complications, prompted many researchers to search for
an effective method of synthesis of these compounds.3–7
The retrosynthetic analysis of compound 1 (Scheme 1)
leads to the chiral precursor 2; its preparation is a serious
Abstract: The influence of Lewis acids on the diastereoselectivity
of [4+2] cycloaddition of buta-1,3-diene (4) to N-glyoxyloyl-(2R)-
bornane-10,2-sultam (6a) and (R)-8-phenylmenthyl glyoxylate (6b)
was investigated and found to have high levels of asymmetric in-
duction. The highest asymmetric induction (ca. 100% de) was ob- synthetic challenge. The retrosynthetic analysis
tained for the reaction of 4 with 6b carried out in toluene and in the
presence of chelating Lewis acids (SnCl4, TiCl4).
(Scheme 1) indicates three independent pathways making
use of the hetero-Diels–Alder reaction. The first pathway
Key words: catalysis, chiral auxiliaries, Diels–Alder reactions,
Lewis acids, stereoselectivity
is based on the reaction between buta-1,3-diene (4) and
glycolaldehyde-derived nonactivated heterodienophile
5a, which fails to occur even under high pressure due to
low activities of reactants. The second pathway makes use
The biologically active derivatives of natural alkaloids of the same 1,3-diene 4 and prochiral glyoxylates 5b,
staurosporine and rebeccamycin – the macrocyclic bisin- leading to the product of desired stereochemistry only un-
dolylmaleimides of type 1 (Scheme 1) – are a group of re- der chiral catalysis and high pressure,8 although lower
versible inhibitors of protein kinase C (PKC), the than in the former pathway. Finally, the third stereo-
excessive activation of PKC being a cause of retinopathy selective pathway makes use of glyoxylic acid derivatives
and nephropathy in patients suffering from diabetes mel- 5c obtained from chiral alcohols or amines; due to the ac-
litus.1,2 The high selectivity of these derivatives (discov- tivated heterodienophile, this reaction proceeds in the
ered by Faul et al.3) with respect to PKC, as well as the presence of Lewis acid catalysts, under atmospheric pres-
very good therapeutic effect in the case of severe diabetic sure.7
Scheme 1
SYNTHESIS 2004, No. 1, pp 0087–0091
x
x
.
x
x
.2
0
0
3
Advanced online publication: 09.12.2003
DOI: 10.1055/s-2003-44372; Art ID: P07603SS
© Georg Thieme Verlag Stuttgart · New York

88
M. Kosior et al.
PAPER
Our recently published7 efficient synthesis of the optically the enantiomeric alcohols 8 (Scheme 2), which are sepa-
pure precursor 2 was based on this synthetic pathway. The rated on the gas chromatograph equipped with a chiral
first step in the synthesis of the derivative 2 was the highly column. We have developed conditions for the separation
diastereoselective hetero-Diels–Alder reaction of buta- of the alcohols 8 on a chiral column and assigned their ab-
1,3-diene (4) with the chiral heterodienophile 6a solute configurations due to separate reduction of the
(Scheme 2), which afforded a mixture of diastereoisomers chromatographically separated pure diastereoisomers
(2¢S)-7a and (2¢R)-7a. This, after separation, gave the de- (2¢S)-7a and (2¢R)-7a. Their absolute configuration was
sired pure isomer having the S-configuration at the newly confirmed by X-ray structural analysis7 of the monocrys-
formed stereogenic centre, in very good yield. The next tal of the diastereoisomer (2¢S)-7a being the major reac-
stage of the synthesis was reduction of the diastereoiso- tion product. Having devised a very precise analysis of the
mer (2¢S)-7a to the corresponding enantiomerically pure diastereoisomeric composition of cycloadducts, we start-
alcohol (2S)-8, whose optical purity was confirmed by ed the detailed study on the effect of solvents and temper-
GC. Following protection of the hydroxy group of alcohol ature on the reaction course and asymmetric induction in
(2S)-8 using trityl chloride, we obtained the derivative 3 order to obtain the best (from the synthetic point of view)
(Scheme 1). Its absolute configuration was confirmed by diastereoisomeric excess accompanied by good chemical
X-ray structural analysis.7 The derivative 3 was subjected yield.
to ozonolysis followed by reduction to the corresponding
diol, and then protected using mesyl chloride. The synthe-
sis gave the enantiomerically pure (>99% ee) precursor of
biologically active macrocyclic bisindolylmaleimides 2 in
The first investigated reaction was the [4+2] cycloaddi-
tion of diene 4 to N-glyoxyloyl-(2R)-bornane-10,2-sultam
(6a) as a heterodienophile (Scheme 2). In all the investi-
gated cases of Lewis acids (Table 1, entries 1, 3, 5, 9), the
15% chemical yield. This interesting method for the prep-
reaction proceeded in dichloromethane at room tempera-
aration of the chiral precursor 2 employing the diastereo-
ture and gave the major product having the S-configura-
selective version of hetero-Diels–Alder reaction between
tion at the newly generated stereogenic centre. The best
diastereoisomeric excess was obtained for the reaction
carried out in the presence of BF3·Et2O (entry 1) as a cat-
alyst, in a moderate chemical yield of 31%. Good asym-
the chiral derivative of glyoxylic acid, i.e. N-glyoxyloyl-
(2R)-bornane-10,2-sultam (6a) and the 1,3-diene of low
activity, i.e., buta-1,3-diene (4) seriously competes with
the other methods, among others, due to economical rea-
metric induction and good yield (75%) were obtained in
sons.
the same solvent using ZnBr2 (entry 3) as a catalyst.
This prompted us to perform further studies in this area,
The change of solvent to the less polar toluene was ac-
which included investigation of the course of this reaction
companied by dramatic decrease in asymmetric induction
under various conditions (the effects of solvent and tem-
(entries 2, 6, 10). In the case of strongly chelating Lewis
perature), as well as detailed analysis of the mixtures of
acids, SnCl4 and TiCl4, this change reversed the direction
the resulting diastereoisomers, and also a comparison of
of induction (entries 6, 10). Because of reduced reaction
the performance of two chiral auxiliaries used as inducing
rate, the experiments at low temperature were carried out
systems in the above-mentioned reaction.
only in the presence of SnCl4 and TiCl4 (entries 7, 8, 11,
In order to perform a thorough analysis of the composition 12). In these cases, lowering of temperature caused a sim-
of diastereoisomeric mixtures of the products of hetero- ilar reversal of direction of the asymmetric induction,
[4+2] cycloaddition, we used gas chromatography on a compared to the experiments performed at room tempera-
chiral column. This method of determining diastereo- ture in dichloromethane as a solvent (entries 7, 11). By
isomeric excess is more precise than 1H NMR spectrosco- changing the solvent from dichloromethane to toluene and
py of reaction mixtures after chromatographic decreasing the temperature, we achieved an increase in
purification. However, it requires an additional transfor- the asymmetric induction value at –20 °C. Unfortunately,
mation of the diastereoisomeric mixture into the pair of the direction of the induction R was undesired from the
Scheme 2
Synthesis 2004, No. 1, 87–91 © Thieme Stuttgart · New York

PAPER
Synthesis of Optically Pure 2-Substituted 3,6-Dihydro-2H-pyrans
89
Table 1 The Results of the Reaction of Heterodienophile 6a with
Diene 4 under Atmospheric Pressure
Table 2 The Results of the Reaction of Heterodienophile 6b with
Diene 4 at Room Temperature under Atmospheric Pressure
Entry Catalyst
Solvent Temp Yield
Diastereoiso-
meric Ratiob
(2¢S)-7a:(2¢R)-
7a
Entry Catalyst
Solvent
Yield (%)a Diastereoisomeric
Ratiob
(°C)
(%)a
(2¢S)-7b:(2¢R)-7b
1
2
3
4
5
6
7
8
9
BF3·Et2O
BF3·Et2O
AlCl3
CH2Cl2
toluene
CH2Cl2
CH2Cl2
toluene
CH2Cl2
toluene
CH2Cl2
toluene
45
80:20
1
2
BF3·Et2O
BF3·Et2O
ZnBr2
ZnBr2
SnCl4
SnCl4
SnCl4
SnCl4
TiCl4
CH2Cl2
toluene
CH2Cl2
toluene
CH2Cl2
toluene
28
25
28
25
28
25
31
81:19
72
73:27
ca. 100 63:37
65
87:13
3
75
38
62
94
86
92
98
89
46
44
80:20
84:16
58:42
46:54
45:55
38:62
60:40
47:53
48:52
42:58
ZnBr2
ZnBr2
SnCl4
43
91:9
4
17
84:16
5
88
ca. 100:0
ca. 100:0
95:5
6
SnCl4
ca. 100
65
7
CH2Cl2 –20
toluene –20
TiCl4
8
TiCl4
ca. 100
ca. 100:0
9
CH2Cl2
toluene
28
25
a Isolated yields.
b Determined by GC.
10
11
12
TiCl4
TiCl4
CH2Cl2 –20
toluene –20
TiCl4
In summary, we have optimised the first stage of the syn-
thesis of (S)-3-[2-{methylsulfonyl)oxy}ethoxy]-4-(triph-
enylmethoxy)butan-1-ol methanesulfonate (2) which is
the key intermediate for biologically important alkaloids
1. Our method, characterised by ca. 100% chemical yield
and ca. 100% asymmetric induction, improves signifi-
cantly the overall yield of the synthesis of precursor 2,
from 15 up to 75%, and is highly competitive to other
methods of preparation of this compound.
a Isolated yields.
b Determined by GC.
point of view of the planned synthesis of chiral precursor
2.
The unsatisfactory results obtained using (2R)-bornane-
10,2-sultam as a chiral auxiliary prompted us to search for
a more effective inducing system. Our attention was di-
rected to (R)-8-phenylmenthol which, in the form of the
corresponding derivative of glyoxylic acid, turned out to
be a better reactant for diastereoselective hetero-[4+2] cy-
cloaddition with buta-1,3-diene (4).
Melting points were determined on a Kofler hot-stage apparatus
with a microscope, and are uncorrected. All reported NMR spectra
were recorded with a Varian Unity plus spectrometer at 500 (1H
NMR) and 125 (13C NMR) MHz. Chemical shifts are reported as d
values relative to TMS peak defined at d = 0.00 (1H NMR) or
d = 0.0 (13C NMR). IR spectra were obtained on a Perkin-Elmer
1640 FTIR spectrometer. Mass spectra were obtained on an AMD
604 Intectra instrument using the EI technique, or on a Mariner Bio-
System unit using the ESI technique. Optical rotations were record-
ed using a Perkin-Elmer 241 polarimeter with a thermally jacketed
10 cm cell. Analytical TLC was carried out on commercially pre-
pared plates coated with 0.25 mm of Merck Kieselgel 60. Prepara-
tive flash silica gel chromatography was performed using Merck
Kieselgel 60 (230–400 mesh). Enantiomeric excesses of products
were determined by GC performed using a Hewlett-Packard 5890
unit equipped with a FID detector and a capillary chiral column b-
dex 225 (30 m × 0.25 mm I.D., Supelco, Bellefonte, USA). Chro-
matographic conditions: carrier gas: argon, 100 kPa; injection
temp.: 200 °C; detector temp: 250 °C. Chromatographic parameters
of enantioseparation of investigated compounds are as follows,
compound (S)-8: tR 15.13 min (temp 95 °C); compound (R)-8;
tR 16.16 min (temp 95 °C).
The hetero-Diels–Alder reaction of the diene 4 with (R)-
8-phenylmenthyl glyoxylate (6b) carried out both in
dichloromethane and in toluene, gave the major product
having S-configuration at the newly created stereogenic
centre (Table 2). In the case of the non-chelating Lewis
acids, toluene clearly decreased the asymmetric induction
(Table 2, entries 2, 5). In the cases of both chelating Lewis
acids, SnCl4 and TiCl4, which gave ca. 100% diastereo-
isomeric excess (entries 6, 7, 9), the change of the solvent
from dichloromethane to toluene caused only an increase
in the chemical yield to quantitative. Concluding this
stage of study, we solved the problem of the key transfor-
mation of the planned synthesis of the chiral precursor 2,
since we obtained, in quantitative yield and at ca. 100%
asymmetric induction, the product (2¢S)-7b, having the
absolute S-configuration at the newly created stereogenic
centre.
All commercially available chemicals were used as received unless
otherwise noted. Reagents grade solvents were dried and distilled
prior use. N-Glyoxyloyl-(2R)-bornane-10,2-sultam (6a)9 and (R)-8-
phenylmenthyl glyoxylate (6b)10 were prepared according to the lit-
erature procedures.
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90
M. Kosior et al.
PAPER
[4+2] Cycloaddition of Buta-1,3-diene to Heterodienophile 6a;
Lewis Acid Catalyzed Reaction; Diastereoisomers (2¢S)-7a and
(2¢R-7a); Typical Procedure
Diastereoisomer (2¢S)-7b
[a]D24 –23.0 (c = 1, CHCl3).
IR (film): 701, 765, 977, 1031, 1050, 1100, 1182, 1212, 1242, 1289,
1370, 1389, 1444, 1457, 1495, 1600, 1725, 2869, 2923, 2955, 3021,
3518 cm–1.
1H NMR (200 MHz, CDCl3–TMS, 25 °C): d = 0.87 (d, 3 H, J1 = 6.6
Hz), 1.20, 1.31 (2 s, 2 × 3 H), 0.89–2.12 (m, 10 H), 3.22 (dd, 1 H,
J1 = 9.2, J2 = 4.4 Hz), 4.04–4.35 (m, 2 H), 4.91 (td, 1 H, J1 = 10.8,
J2 = 4.4 Hz), 5.66 (s, 2 H), 7.07–7.29 (m, 5 H).
13C NMR (50 MHz, CDCl3–TMS, 25 °C): d = 22.0, 23.7, 26.5, 27.7,
29.2, 31.4, 34.7, 39.7, 41.7, 50.5, 65.5, 71,5, 74.6, 123.1, 125.1,
125.6, 126.2, 128.1, 152.1, 170.76.
To a solution of heterodienophile 6a (0.5 mmol) in anhyd CH2Cl2
or toluene (10 mL) was added the Lewis acid (0.5 mmol) and the
mixture was stirred for 20 min at r.t. Then the mixture was cooled
to –78 °C and buta-1,3-diene (4, 2.5 mmol) was added dropwise,
and the mixture was stirred for additional 24 h at r.t. or –20 °C. The
progress of the reaction was monitored by TLC. When the reaction
was complete, after usual work-up, the residue was chromato-
graphed on a silica gel column using a mixture of hexane–acetone–
EtOAc (2.5:3:1) to give two diastereoisomerically pure products
(2¢S)-7a and (2¢R)-7a, which were subjected to chemical correla-
tion.
MS (ESI-HR): m/z calcd for C22H30O3Na+: 365.2087, found:
365.2083.
Diastereoisomer (2¢S)-7a
Mp 189–191°C; [a]D20 –193.3 (c = 1, CHCl3).
Anal. Calcd for C22H30O3: C, 77.16; H, 8.83; O, 14.02. Found C,
77.22; H, 8.68.
IR (KBr): 1054, 1135, 1337, 1713, cm–1.
1H NMR (500 MHz, CDCl3–TMS, 27 °C): d = 0.97 (s, 3 H), 1.13 (s,
3 H), 1.34–1.47 (m, 2 H), 1.85–1.96 (m, 3 H), 1.99–2.04 (m, 1 H),
2.10–2.14 (dd, 1 H, J1 = 8, J2 = 14 Hz), 2.27–2.35 (m, 1 H), 2.41–
2.47 (m, 1 H, J1 = 1.5, J2 = 3.5, J3 = 15 Hz), 3.47 (AB, 2 H,
J1 = 13.5 Hz), 3.96 (dd, 1 H, J1 = 4.75, J2 = 7.75 Hz), 4.27–4.39 (m,
2 H), 4.71 (dd, 1 H, J1 = 3, J2 = 10.25 Hz), 5.74–5.77 (m, 1 H,
J1 = 1.5, J2 = 10.5 Hz), 5.81–5.85 (m, 1 H).
13C NMR (125 MHz, CDCl3–TMS, 27 °C): d = 19.9, 20.7, 26.4,
28.6, 32.7, 38.1, 44.5, 47.9, 48.7, 53.1, 65.0, 65.8, 72.8, 122.7,
126.1, 170.6.
Diastereoisomer (2¢R)-7b
[a]D24 +41.6 (c = 1.08, CHCl3).
1H NMR (200 MHz, CDCl3–TMS, 25 °C): d = 0.86 (d, 3 H, J1 = 6.4
Hz), 1.24, 1.34 (2 s, 2 × 3 H), 1.56 (s, 4 H), 1.89–2.20 (m, 4 H), 3.75
(dd, 1 H, J1 = 8.8, J2 = 4.8 Hz), 4.05–4.30 (m, 2 H), 4.95 (td, 1 H,
J1 = 10.8, J2 = 4.4 Hz), 5.65–5.81 (m, 2 H), 7.14–7.30 (m, 5 H).
MS (ESI-HR): m/z calcd for C22H30O3Na+: 365.2093; found:
365.2099.
Anal. Calcd for C22H30O3: C, 77.16; H, 8.83; O, 14.02. Found C,
76.81; H, 9.19.
MS (EI-LR): m/z (%) = 325 (M+, 1.6), 244 (10.9), 177 (15.0), 135
(23.4), 93 (15.5), 83 (100), 82 (32.8), 75 (48.1), 55 (51.6), 41 (10.8).
Chemical Correlation of Cycloadducts 7 to Alcohol 8
To a solution of LiAlH4 (0.5 equivalent) in distilled anhyd Et2O,
cooled to 0 °C was added a mixture of diastereoisomers (1 equiv)
dissolved in anhyd Et2O (or in the case of 7a in anhyd CH2Cl2) drop-
wise, and the reaction mixture was stirred for 2 h in r.t. After the
usual work-up, the crude product was analysed by GC to determine
the diastereoisomeric excess.
Anal. Calcd for C16H23NSO4: C, 59.05; H, 7.12; N, 4.30; S, 9.85.
Found: C, 58.79; H, 7.25; N, 4.14; S, 9.65%.
Diastereoisomer (2¢R)-7a
Mp 144–145 °C; [a]D20 –32 (c = 1, CHCl3).
IR (KBr): 1080, 1138, 1332, 1691 cm–1.
1H NMR (500 MHz, CDCl3–TMS, 27 °C): d = 0.99 (s, 3 H), 1.21 (s,
3 H), 1.32–1.43 (m, 2 H), 1.85–1.97 (m, 3 H), 2.09 (dd, 1 H, J1 = 8,
J2 = 13.5 Hz), 2.14–2.19 (m, 1 H), 2.24–2.31 (m, 1 H), 2.47–2.55
(m, 1 H), 3.47 (AB, 2 H, J1 = 13.5 Hz), 3.95 (dd, 1 H, J1 = 5,
J2 = 7.75 Hz), 4.23–4.33 (m, 2 H), 4.60 (dd, 1 H, J1 = 3.25, J2 = 10.5
Hz), 5.71–5.75 (m, 1 H, J1 = 10 Hz), 5.83–5.88 (m, 1 H).
13C NMR (125 MHz, CDCl3–TMS, 27 °C): d = 19.9, 21.3, 26.3,
26.4, 33.3, 38.6, 45.0, 47.8, 48.6, 53.3, 65.8, 66.1, 72,7, 123.2,
125.7, 170.2.
(2S)-3,6-Dihydro-2H-pyran-2ylmethanol [(2S)-8]
[a]D20 –154.5 (c = 0.95, CHCl3); [a]D20 –147.4 (c = 1.02, CDCl3);
>99% ee (determined by GC on chiral column b-dex 225).
IR (film): 463, 655, 792, 825, 954, 1025, 1057, 1087, 1183, 1247,
1388, 1429, 1641, 2836, 2889, 2928, 3037, 3393 cm–1.
1H NMR (500 MHz, CDCl3–TMS, 27 °C): d = 1.87–1.93 (m, 1 H),
2.06–2.14 (m, 1 H), 2.18–2.53 (m, 1 H, OH), 3.56–3.60 (m, 1 H),
3.65–3.69 (m, 1 H), 3.67–3.70 (m, 1 H), 4.22–4.24 (m, 2 H), 5.67–
5.76 (m, 1 H), 5.80–5.85 (m, 1 H).
13C NMR (125 MHz, CDCl3–TMS, 27 °C): d = 26.5, 65.6, 65.7,
74.1, 123.7, 126.2.
MS (ESI-HR): m/z calcd for C6H10O2Na+: 137.0573; found:
137.0579.
MS (EI-LR): m/z (%) = 325 (M+, 2.0), 244 (10.3), 177 (10.6), 135
(21.9), 93 (15.3), 83 (100), 82 (31.3), 55 (51.6), 41 (13.6).
Anal. Calcd for C16H23NSO4: C, 59.05; H, 7.12; N, 4.30; S, 9.85.
Found: C, 59.02; H, 7.16; N, 4.27; S, 9.70%.
[4+2] Cycloaddition of Buta-1,3-diene (4) to Heterodienophile
6b
(2R)-3,6-Dihydro-2H-pyran-2ylmethanol [(2R)-8]
[a]D15 +140.7 (c = 1, CDCl3); >99% ee (determined by GC on chiral
column b-dex 225).
1H NMR (500 MHz, CDCl3–TMS, 27 °C): d = 1.83–1.87 (m, 1 H),
1.92–1.97 (m, 1 H), 2.01–2.17 (m, 1 H, OH), 3.61–3.74 (m, 3 H),
4.20–4.26 (m, 2 H), 5.70–5.88 (m, 2 H).
The reaction was performed as in the former case, and after usual
work-up, the residue was chromatographed on a silica gel column
using a hexane–EtOAc (20:1) system to give two diastereoisomeri-
cally pure products (2¢S)-7b and (2¢R)-7b, which were subjected to
chemical correlation.
13C NMR (50 MHz, CDCl3–TMS, 25 °C): d = 27.18, 66.4, 74.7,
124.4, 126.9.
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Synthesis of Optically Pure 2-Substituted 3,6-Dihydro-2H-pyrans
91
(4) Faul, M. M.; Winneroski, L. L.; Krumrich, C. A.; Sullivan,
K. A.; Gilling, J. R.; Neel, D. A.; Rito, Ch. J.; Jirousek, M.
R. J. Org. Chem. 1998, 63, 1961.
(5) Cossy, J.; Bouzbouz, S.; Caille, J.-C. Tetrahedron:
Asymmetry 1999, 10, 3859.
Acknowledgement
Financial support from the Polish Committee for Scientific Rese-
arch (Grant 7 T09A 136 21) is gratefully acknowledged.
(6) Faul, M. M.; Krumrich, C. A. J. Org. Chem. 2001, 66, 2024.
(7) Kosior, M.; Malinowska, M.; Jóźwik, J.; Caille, J.-C.;
Jurczak, J. Tetrahedron: Asymmetry 2003, 14, 239.
(8) Kwiatkowski P., Kosior M., Asztemborska M., Jurczak J.;
unpublished results
(9) (a) Oppolzer, W.; Chapuis, C.; Bernardinelli, G. Helv. Chim.
Acta 1984, 67, 1397. (b) Bauer, T.; Jeżwski, A.; Chapuis,
C.; Jurczak, J. Tetrahedron: Asymmetry 1996, 7, 1385.
(10) (a) Fernandez, F.; Garcia, G.; Rodriguez, J. E. Synth.
Commun. 1990, 20, 2837. (b) Freund, R.; Mahboobi, S.;
Noack, K.; Schönholzer, P.; Bernauer, K. Helv. Chim. Acta
1990, 73, 439. (c) Freund, R.; Martinovic, S.; Bernauer, K.
Helv. Chim. Acta 1992, 75, 282.
References
(1) Jirousek, M. R.; Gilling, J. R.; Gonzalez, C. M.; Heath, W.
F.; McDonald, J. H. III; Neel, D. A.; Rito, C. J.; Singh, U.;
Stramm, L. E.; Melikian-Badalian, A.; Baevsky, M.; Ballas,
L. M.; Hall, S. E.; Winneroski, L. L.; Faul, M. M. J. Med.
Chem. 1996, 39, 2664.
(2) Jirousek, M. R.; Gilling, J. R.; Neel, D. A.; Rito, C. J.;
O’Bannon, D.; Heath, W. F.; McDonald, J. H. III; Faul, M.
M.; Winneroski, L. L.; Melikian-Badalian, A.; Baevsky, M.;
Ballas, L. M.; Hall, S. E. Bioorg. Med. Chem. Lett. 1995, 5,
2093.
(3) Engel, G. L.; Farid, N. A.; Faul, M. M.; Richardson, L. A.;
Winneroski, L. L. Int. J. Pharm. 2000, 198, 239.
Synthesis 2004, No. 1, 87–91 © Thieme Stuttgart · New York