Stereoselective synthesis of β-amino-α-hydroxy(allyl)phosphinates and an application to the synthesis of a building block for phosphinyl peptides

Article abstract of DOI:10.1055/s-2002-33529

Hydrophosphinylation of N,N-dibenzyl-α-amino aldehydes with ethyl allylphosphinate in the presence of (S)-ALB afforded anti-β-amino-α-hydroxy(allyl)phosphinates in high diastereoselectivity. The hydrophosphinylation product was transformed to a potentiall

Full text of DOI:10.1055/s-2002-33529

LETTER
1471
Stereoselective Synthesis of -Amino- -hydroxy(allyl)phosphinates and an
Application to the Synthesis of a Building Block for Phosphinyl Peptides
Stereoselective
a
S
ynthesis
o
k
f
-Amino-
e
-hydroxy(a
h
llyl)phosphi
inates ro Yamagishi, Takanori Kusano, Tsutomu Yokomatsu,* Shiroshi Shibuya
School of Pharmacy, Tokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
Fax +81(426)763239; E-mail: yokomatu@ps.toyaku.ac.jp
Received 1 July 2002
In view of obtaining -amino- -hydroxyphosphinyl de-
rivatives (1/X = alkyl) in searching for potent biologically
active compounds, we envisaged the methodology based
upon using alkylated ethyl phosphinate as a nucleophile
Abstract: Hydrophosphinylation of N,N-dibenzyl- -amino alde-
hydes with ethyl allylphosphinate in the presence of (S)-ALB af-
forded anti- -amino- -hydroxy(allyl)phosphinates in high
diastereoselectivity. The hydrophosphinylation product was trans-
formed to a potentially useful transition state mimic for hydrolysis for the hydrophosphinylation. The
-amino- -hy-
of dipeptides.
droxy(allyl)phosphinates would be useful synthetic inter-
mediates toward a variety of their derivatives through
conversion of the double bond to the other functional
group. Here, we disclose a highly diastereoselective syn-
thesis of anti- -amino- -hydroxy(allyl)phosphinates
through hydrophosphinylation of N,N-dibenzyl- -amino
aldehydes with ethyl allylphosphinate.7 In addition, we
describe anti- -amino- -hydroxy(allyl)phosphinate was
a useful intermediate for the stereoselective synthesis of
-amino- -hydroxy(methoxycarbonylmethyl)phosphinic
acid, which would be applicable as a building block for
peptidic transition state analogue inhibitors of protease
(Scheme 1).8
Key words: phosphorus, amino aldehyde -amino- -hydroxy(al-
lyl)phosphinates, hydrophosphinylation, diastereoselectivity
Compounds containing phosphinic acid functional group
are currently an area of considerable importance due to
their interesting biological properties.1 The -amino- -
hydroxyphosphinic acids 1 serve as the key intermediates
for the synthesis of potent inhibitor of human renin and
HIV protease (Figure 1).2,3 The protease inhibitory activi-
ty is reported to be dependent upon the stereochemistry of
the asymmetric centers of the -amino alcohol moiety.
We recently found that the AlLibis(binaphthoxide)
(ALB)4-catalyzed hydrophosphinylation of N,N-dibenz-
yl- -amino aldehydes with ethyl phosphinate
[H2P(O)OEt] proceeds in a highly diastereoselective man-
ner.5 Both syn- and anti- -amino- -hydroxy-H-phosphi-
nates (1/X = H) were prepared selectively as N,N-dibenzyl
protecting form by tuning the chirality of the catalyst.
These hydrophosphinylation products seemed to be useful
intermediates for the synthesis of versatile -amino- -hy-
droxyphosphinyl derivatives (1/X = alkyl) by elongation
from the phosphorus atom. However, the alkylation of the
phosphinate functionality with aldehydes, acrylates and
allyl halides in the presence of TMSCl and Et3N6 resulted
Bn2N
O
O
Bn2N
R
P
+
HP
R
OEt
CHO
OEt
HO
H2N
R
O
P
COOMe
OH
HO
Scheme 1
in poor yields of the desired products due to the steric con- Treatment of N,N-dibenzyl- -amino aldehydes 2a–c9
gestion around the phosphorus atom arising from the with ethyl allylphosphinate in the presence of (R)-ALB
bulky dibenzyl protecting group on the nitrogen atom.5a,b
(20 mol%), generated from (R)-binaphthol and LiAlH4,
at 0 °C for 12 h afforded a mixture of syn-3a–c and anti-
3a–c in 48–74% yield (Table 1).10 While the diastereose-
lectivity for these reactions were varied slightly depend-
ing upon the substituents of 2a–c, poor diastereo-
selectivity were observed (entries 1, 3 and 5). When reac-
tions were conducted with (S)-ALB in place of (R)-ALB
under the same conditions, preferable formation of anti-
adducts (anti-3a–c) were observed (entries 2, 4 and 6).
The ratios of syn-3a–c and anti-3a–c were generally high
and determined to be up to 5:95. The results clearly
showed that the pair of 2a–c with (S)-ALB was matched
for inducing high diastereoselectivity.11 This trend was
consistent with the previous results of ALB-catalyzed hy-
H2N
R1
O
P
X
OR2
OH
1: X=H or alkyl
Figure 1
Synlett 2002, No. 9, Print: 02 09 2002.
Art Id.1437-2096,E;2002,0,09,1471,1474,ftx,en;U02902ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

1472
T. Yamagishi et al.
LETTER
(J4,5 = 5.2 Hz), but that for 6a resonated at 4.91 (J4,5
Table 1 Hydrophosphinylation of 2a–c with Ethyl Allylphosphi-
nate Catalyzed by ALB
=
8.8 Hz). The similar trend in the chemical shifts and the
vicinal coupling constants were also observed with 5b and
6b. The chemical shifts of H-5 and the vicinal coupling
constants of oxazolidin-2-ones have been used to assign
the relative stereochemistry, normally trans-isomers ap-
pearing at higher field than those of cis-isomers and cou-
pling constants J4,5 of cis-isomers being bigger than those
of trans-isomers.14 On the basis of the empirical rules,
5a,b and 6a,b were assigned to be trans and cis, respec-
tively.15
O
HP
OEt
Bn2N
R
O
Bn2N
R
O
NBn2
P
P
+
(S) or (R)-ALB
R
CHO
2a-c
OEt
OEt
(20 mol%)
HO
HO
syn-3a-c
anti-3a-c
a: R = CH2Ph; b: R = i-Bu; c: R = Me
Entrya
Substrate
ALB
syn:antib
50:50
7:93
Yield (%)c
1
2
3
4
5
6
2a
2a
2b
2b
2c
2c
(R)-ALB
(S)-ALB
(R)-ALB
(S)-ALB
(R)-ALB
(S)-ALB
74
63
71
51
48
52
syn-3a,b
anti-3a,b
1. H2, 20 % Pd(OH)2-C, EtOH
2. CDI, NMO, THF
3. a) TMSBr, CH2Cl2 b) MeOH
58:42
5:95
O
O
O
O
24:76
6:94
HN
R
HN
4
5
4
5
n-Pr
n-Pr
P
R
P
a All reactions were carried out in THF at 0 °C for 12 h.
b Determined by 31P NMR analysis of crude products.
c Combined yields of syn- and anti-isomers.
O
OH
O
OH
a: R= CH2Ph
b: R = i-Bu
5a,b
6a,b
Scheme 3
Table 2 The 1H NMR (400 MHz, CD3OD) Data of 5a,b and 6a,b.
drophosphinylation of N,N-dibenzyl- -amino aldehydes
with ethyl ethylphosphinate.5b
Compound
H-5 ( ppm)
4.46
J4,5 (Hz)
5.2
The hydrophosphinylation products syn-3a,b and anti-
3a,b, separable by column chromatography on silica gel,
were obtained as a mixture of diastereomers arising from
the chirality of the phosphorus atom. Treatment of anti-3b
with TMSBr followed by methanolysis afforded phos-
phinic acid 4 as a single product in 93% yield
(Scheme 2).12 In this compound, the asymmetric character
of the phosphorus atom is lost by the rapid exchange of
5a
5b
6a
6b
4.37
5.6
4.91
8.8
4.79
8.9
the acidic proton between the phosphoryl (P=O) and the The (S)-ALB-catalyzed hydrophosphinylation product
acidic (P–OH) sites.13
anti-3c, inseparable from the minor product syn-3c, was
obtained as a mixture of the diastereomers arising from
the chirality of the phosphorus atom (anti-3c-A, anti-3c-
B). The diastereomerically pure anti-3c-A (mp: 108–110
°C) was isolated from the mixture upon recrystallization
from hexane and ethyl acetate. The relative configuration
of anti-3c-A was determined unequivocally by X-ray
crystallographic analysis (Figure 2).16,17
Bn2N
i-Bu
O
1) TMSBr,
CH2Cl2
Bn2N
i-Bu
O
P
P
OEt
anti-3b
2) MeOH
(93 %)
OH
HO
HO
4
Scheme 2
The stereochemistry of syn-3a,b and anti-3a,b were con-
firmed after converting to the corresponding oxazolidin-
2-ones 5a,b and 6a,b, respectively (Scheme 3). Hydro-
genolysis of syn-3a,b followed by sequential N,O-carbo-
nylation with N,N-carbonyldiimidazole (CDI) in the
presence of N-methylmorpholine (NMO) and deesterifi-
cation with TMSBr and methanolysis gave the corre-
sponding oxazolidin-2-ones 5a,b. In an analogous
manner, 6a,b were obtained starting from anti-3a,b. The
vicinal stereochemistry of 5a,b and 6a,b were deduced by
comparison of their 1H NMR (400 MHz, CD3OD) spectra
(Table 2). The proton H-5 for 5a resonated at 4.46
Figure 2 ORTEP drawing of anti-3c-A
Synlett 2002, No. 9, 1471–1474 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Stereoselective Synthesis of -Amino- -hydroxy(allyl)phosphinates
1473
With anti-3b having the defined stereochemistry at the phosphinyl peptides. Further application of the present
asymmetric center of the amino alcohol moiety in hand, hydrophosphinylation methodology to the synthesis of
we next examined the oxidative transformation of the vi- biologically active compounds is under investigation.
nyl group to the carbomethoxy functional group, which
constitute the synthesis of -amino- -hydroxy(methoxy-
Acknowledgement
carbonylmethyl)phosphinic acid, a building block for
peptidic transition state analogue inhibitors of protease
(Scheme 4). Treatment of anti-3b with TESCl and imida-
zole afforded 7 in 87% yield. Oxidative cleavage of the
olefin moiety using with OsO4 -NaIO4 gave the dihydrox-
ylation product and the aldehyde 8 in 21% yield and 8%
yield, respectively. The poor yield of the oxidation prod-
uct seems likely to be associated with unfavorable coordi-
nation of OsO4 with the tertary amine inhibitting access of
the oxidant to the olefin under the conditions. Then, we
employed AD-mix- reagent as an oxidant; the cinchona
alkaloid ligand might be expected to prevent the unfavor-
able interaction between 7 and the OsO4.18 Dihydroxyla-
tion of 7 with AD-mix- reagent under the conditions of
Sharpless gave the corresponding dihydroxylation prod-
uct, without purification, which was treated with NaIO4 to
give aldehyde 8 in 72% yield (2 steps). Conversion of 8 to
the methyl ester 10 was achieved via 9 in a moderate over-
all yield by the reduction-oxidation sequence which in-
cluded exchange of the N,N-dibenzyl protecting group to
the Cbz group as shown in Scheme 4.19 Treatment of 10
with TMSBr followed by methanolysis afforded -amino-
-hydroxy(methoxycarbonylmethyl)phosphinic acid 11
in 90% yield.20
The authors wish to thank Mr. Haruhiko Fukaya (the analytical cen-
ter of this university) for the X-ray crystallographic analysis.
References
(1) For a review, see: Collinsová, M.; Jirácek, J. Curr. Med.
Chem. 2000, 7, 629.
(2) Patel, D. V.; Rielly-Gauvin, K.; Ryono, D. E.; Free, C. A.;
Rogers, W. L.; Smith, S. A.; DeForrest, J. M.; Oehl, R. S.;
Petrillo, E. W. Jr. J. Med. Chem. 1995, 38, 4557.
(3) Stowasser, B.; Budt, K.-H.; Jian-Qi, L.; Peyman, A.;
Ruppert, D. Tetrahedron Lett. 1992, 33, 6625.
(4) (a) Arai, T.; Bougauchi, M.; Sasai, H.; Shibasaki, M. J. Org.
Chem. 1996, 61, 2926. (b) Arai, T.; Sasai, H.; Aoe, K.;
Okamura, K.; Date, T.; Shibasaki, M. Angew. Chem., Int.
Ed. Engl. 1996, 35, 104. (c) Yamada, K.; Arai, T.; Sasai, H.;
Shibasaki, M. J. Org. Chem. 1998, 63, 3666. (d) Xu, Y.;
Ohori, K.; Ohshima, T.; Shibasaki, M. Tetrahedron 2002,
58, 2585.
(5) (a) Yamagishi, T.; Suemune, K.; Yokomatsu, T.; Shibuya, S.
Tetrahedron Lett. 2001, 42, 5033. (b) Yamagishi, T.;
Suemune, K.; Yokomatsu, T.; Shibuya, S. Tetrahedron
2002, 58, 2577. (c) For a catalytic asymmetric
hydrophosphinylation of prochiral aldehydes, see:
Yamagishi, T.; Suemune, K.; Yokomatsu, T.; Shibuya, S.
Tetrahedron 1999, 55, 12125.
(6) Thottathil, J. K.; Ryono, D. E.; Przybyla, C. A.; Moniot, J.
L.; Neubeck, R. Tetrahedron Lett. 1984, 25, 4741.
(7) Baylis, E. K. Tetrahedron Lett. 1995, 36, 9385.
(8) Patel, D. V.; Rielly-Gauvin, K.; Ryono, D. E. Tetrahedron
Lett. 1990, 31, 5591.
(9) The aldehydes 2a–c were prepared from the corresponding
L-amino acids according to the literature methods and used
without purification: Reetz, M. T. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1531.
(10) Genaral Procedure for the Hydrophosphinylation of
3a–c in the Presence of (S)-ALB. To a solution of ethyl
allylphosphinate (804 mg, 6.0 mmol) in THF (4 mL) was
added 0.1 M THF solution of (S)-ALB (8 mL, 0.8 mmol),
prepared from (S)-BINOL (458 mg, 1.6 mmol) and LiAlH4
(30.4 mg, 0.8 mmol) in situ, and a solution of 2a–c (4.0
mmol) in THF (8 mL) at 0 °C under stirring. After being
stirred for 12 h at the same temperature, the mixture was
diluted with H2O and extracted with CHCl3. The combined
extracts were washed with brine and dried over MgSO4.
Removal of solvent gave a residue, which was
Bn2N
O
Bn2N
i-Bu
O
b, c
P
P
CHO
i-Bu
OEt
OEt
XO
TESO
anti-3b: X = H
7: X = TES
8
a
CBz-HN
i-Bu
O
CBz-HN
O
P
g, h
d, e, f
P
COOMe
OH
i-Bu
OEt
OEt
TESO
HO
9
10
H2N
i-Bu
O
i, j
P
COOMe
OH
HO
11
chromatographed on silica gel (hexane–EtOAc = 2:1) to give
syn-3a–c and anti-3a–c.
Scheme 4 a) TESCl, imidazole, DMF (87%); b) AD-mix- ,
t-BuOH-H2O (1:1); c) NaIO4, H2O–MeOH–CHCl3 (72%, 2 steps);
d) LiBH4, Et2O (76%); e) H2, Pd(OH)2–C, EtOH; f) CBz-Cl,
NaHCO3, Et2O–H2O (72%, 2 steps); g) Jones reagent, acetone;
h) HCl, MeOH (31%, 2 steps); i) TMSBr, CH2Cl2; j) MeOH (90%,
2 steps)
syn-3a. This compound was obtained as a mixture of
diastereomers arising from the chirality of the phosphorus
atom in a ratio of 1:1.6. Mp 87–89 °C; 1H NMR (400 MHz,
CDCl3) 7.42–7.20 (15 H, m), 5.91–5.79 (0.5 H, m), 5.58–
5.46 (0.5 H, m), 5.28–5.21 (1 H, m), 4.93–4.88 (1 H, m),
4.16–4.12 (2 H, m), 3.84–3.59 (3 H, m), 3.53–3.43 (2 H, m),
3.04–2.94 (1 H, m), 2.82-2.60 (1 H, m), 2.44–2.34 (0.5 H,
m), 2.25–2.16 (0.5 H, m), 1.34 (3 H, t, J = 7.0 Hz); 31P NMR
(162 MHz, CDCl3) 52.35, 50.21; IR (KBr) 3215, 1046
cm–1; EIMS m/z 464 (MH+). Anal. calcd for C28H34NO3P: C,
72.55; H, 7.39. Found: C, 72.44; H, 7.25.
In conclusion, we have developed ALB-catalyzed hydro-
phosphinylation of N,N-dibenzyl- -amino aldehydes with
ethyl allylphosphinate to afford anti- -amino- -hy-
droxy(allyl)phosphinates, which were useful intermedi-
ates allowing for a conversion into a building block for
Synlett 2002, No. 9, 1471–1474 ISSN 0936-5214 © Thieme Stuttgart · New York

1474
T. Yamagishi et al.
LETTER
anti-3a. This compound was obtained as a mixture of
diastereomers arising from the chirality of the phosphorus
atom in a ratio of 1:1.1. Mp 168–170 °C; 1H NMR (400
MHz, CDCl3) 7.27–7.06 (15 H, m), 5.87–5.71 (1 H, m),
5.29–5.05 (2 H, m), 4.15–3.92 (3 H, m), 3.87 (2 H, d, J =
14.1 Hz), 3.59 (2 H, d, J = 14.1 Hz), 3.49–3.38 (1 H, m),
3.18–3.07 (2 H, m), 2.78–2.50 (2 H, m), 1.20 (3 H, t, J = 7.0
Hz), 1.18 (3 H, t, J = 7.0 Hz); 31P NMR (162 MHz, CDCl3)
49.18, 49.05; IR (KBr) 3259, 1033 cm–1; EIMS m/z 464
(MH+). Anal. calcd for C28H34NO3P: C, 72.55; H, 7.39.
Found: C, 72.30; H, 7.35.
syn-3b. This compound was obtained as a mixture of
diastereomers arising from the chirality of the phosphorus
atom in a ratio of 1:1. Oil; 1H NMR (400 MHz, CDCl3)
7.33–7.28 (10 H, m), 5.88–5.77 (0.5 H, m), 5.58–5.46 (0.5
H, m), 5.25–4.86 (2 H, m), 4.13–4.05 (2 H, m), 3.88 (2 H, d,
J = 13.1 Hz), 3.59 (2 H, d, J = 13.1 Hz), 3.53–3.46 (1 H, m),
3.28–3.14 (1 H, m), 2.77–2.56 (1 H, m), 2.39–2.11 (1 H, m),
2.11–2.04 (1 H, m), 1.83–1.67 (2 H, m), 1.31 (3 H, t, J = 7.0
Hz), 1.04–0.99 (6 H, m); 31P NMR (162 MHz, CDCl3)
51.75, 50.22; IR(neat) 3262, 1036 cm–1; EIMS m/z 430
(MH+). High resolution MS calcd for C25H37NO3P (MH+):
430.2511. Found: 430.2488.
(14) Dufour, M.; Jouin, P.; Poncet, J.; Pantaloni, A.; Castro, B. J.
Chem. Soc., Perkin Trans. 1 1986, 1895.
(15) The 1H NMR spectroscopic analysis has been successfully
applied to determine the relative stereochemistry of 5-
phosphonyloxazolidin-2-one, see: Yokomatsu, T.;
Yamagishi, T.; Shibuya, S. Tetrahedron: Asymmetry 1993,
4, 1401; see also ref. 2.
(16) X-Ray crystal data of anti-3c-A were collected by a Mac-
Science DIP Image plate diffractometer. The structure was
solved by a direct method using SIR-9723 and refined with a
full matrix least-squares method.24 Molecular formula =
C22H30NO3P, Mr = 387.46, orthorhombic, space group =
P212121, a = 8.454(5), b = 11.111(2), c = 23.484(10) Å, V =
2205.9(2) Å3, T = 296 K, Z = 4, Dx = 1.167 Mg m–3, (Mo-
K ) = 0.71073 Å, = 0.145 mm–1, R = 0.050 over 2591
independent reflections.
(17) Crystallographic data (excluding structure factors) for the X-
ray crystal structure analysis reported in this paper have been
deposited with the Cambridge Crystallographic Data Center
(CCDC) as supplementary publication No. CCDC 187211.
Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK [fax:+44(1223)336033 or e-mail:
anti-3b. This compound was obtained as a mixture of
diastereomers arising from the chirality of the phosphorus
atom in a ratio of 1:1.1. Mp 141–143 °C; 1H NMR (400
MHz, CDCl3) 7.35–7.20 (10 H, m), 5.91–5.76 (1 H, m),
5.30–5.15 (2 H, m), 4.22–3.91 (3 H, m), 3.88 (1 H, d, J =
13.6 Hz), 3.85 (1 H, d, J = 13.6 Hz), 3.53 (1 H, d, J = 13.6
Hz), 3.52 (1 H, d, J = 3.6 Hz), 3.22–3.11 (1 H, m), 2.82–2.39
(2 H, m), 1.80–1.70 (1 H, m), 1.40–1.29 (2 H, m), 1.24 (1.5
H, t, J = 7.0 Hz), 1.20 (1.5 H, t, J = 7.0 Hz), 0.91 (3 H, d, J
= 6.7 Hz), 0.58 (1.5 H, d, J = 6.5 Hz), 0.55 (1.5 H, d, J = 6.5
Hz); 31P NMR (162 MHz, CDCl3) 50.05, 49.44; IR (KBr)
3278, 1033 cm–1; EIMS m/z 430 (MH+). Anal. calcd for
C25H36NO3P: C, 69.91; H, 8.45. Found: C, 69.77; H, 8.28.
deposit@ccdc.cam.ac.uk].
(18) For a review on asymmetric dihydroxylation, see: Kolb, H.
C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem. Rev.
1994, 94, 2483.
(19) The PDC oxidation of 9 in DMF, followed by esterificaton
afforded 10 in 7% yield.
(20) 11. Amorphous; [ ]D26 = –7.21 (c 0.67, MeOH); 1H NMR
(400 MHz, CD3OD) 4.17 (1 H, dd, J = 5.3, 5.3 Hz), 3.73 (3
H, s), 3.69–3.54 (1 H, m), 3.23–3.00 (2 H, m), 1.87–1.59 (3
H, m), 1.00 (3 H, d, J = 5.9 Hz), 0.97 (3 H, d, J = 6.2 Hz); 31
P
NMR (162 MHz, CD3OD) 41.91; 13C NMR (100 MHz,
CD3OD) 168.4, 68.9 (d, JPC = 119.5 Hz), 59.1, 53.0, 38.8,
36.0 (d, JPC = 81.7 Hz), 25.9, 23.5, 21.8; IR(neat) 3314,
2956, 1729, 1115 cm–1; FABMS m/z 254 (MH+). High
resolution MS calcd for C9H21NO5P (MH+): 254.1157.
Found: 254.1164.
(11) For improving the syn-diastereoselectivity, we also
examined hydrophosphinylation of 2b using other types of
binaphthol-modified heterobimetallic complexes [(R)-
LPB21 and (R)-GaLB22]. However, the syn-selectivity was
not observed. The (R)-LPB catalyzed reaction showed
moderate anti-selectivity (syn-3b:anti-3b = 22:78). A poor
anti-selectivity (syn-3b:anti-3b = 44:51) was observed with
(R)-GaLB.
(21) Sasai, H.; Arai, S.; Tahara, Y.; Shibasaki, M. J. Org. Chem.
1995, 60, 6656.
(22) Iida, T.; Yamamoto, N.; Sasai, H.; Shibasaki, M. J. Am.
Chem. Soc. 1997, 119, 4783.
(23) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Spagna,
R. J. Appl. Cryst. 1999, 32, 115.
(12) McKenna, C. E.; Higa, M. T.; Cheung, N. H.; McKenna,
M.-C. Tetrahedron Lett. 1977, 18, 155.
(13) Albouy, D.; Brun, A.; Munoz, A.; Etemad-Moghadam, G. J.
Org. Chem. 1998, 63, 7223; and references cited therein.
(24) Sheldrick, G. M. SHELXL97, Program for the Refinement of
Crystal Structures; University of Göttingen: Germany,
1997.
Synlett 2002, No. 9, 1471–1474 ISSN 0936-5214 © Thieme Stuttgart · New York