New cyclic aspartic acid and N-glucosyl asparagine mimetics

Article abstract of DOI:10.1055/s-2006-951541

Enantiopure tert-butyl 4-oxopipecolic esters were converted into cyclic analogues of bishomoaspartic acid and homoglutamic acid by Horner-Wadsworth- Emmons olefination and hydrogenation. Condensation of the side chain carboxylic group with anomeric glucos

Full text of DOI:10.1055/s-2006-951541

LETTER
3251
New Cyclic Aspartic Acid and N-Glucosyl Asparagine Mimetics
New
Cyclic
A
r
spartic
A
a
cid and
N
-
n
lucosyl Asp
c
aragine
M
a
imetics M. Cordero,* Paolo Fantini, Alberto Brandi*
Department of Organic Chemistry ‘Ugo Schiff’, Laboratory of Design, Synthesis and Study of Biologically Active Heterocycles, University
of Firenze, via della Lastruccia 13, 50019 Sesto Fiorentino (FI), Italy
Fax +39(055)4573531; E-mail: franca.cordero@unifi.it; E-mail: alberto.brandi@unifi.it
Received 6 September 2006
Dedicated to Professor Miha Tišler on the occasion of his 80th birthday
preparation of glycopeptides of biological interest. Here-
Abstract: Enantiopure tert-butyl 4-oxopipecolic esters were con-
in, we describe the synthesis of Na,Ca-protected 4-(carb-
verted into cyclic analogues of bishomoaspartic acid and homo-
oxymethyl)pipecolic acids starting from the 4-oxo-
derivatives. Furthermore, the (2S,4R) and (2R,4S) stereo-
glutamic acid by Horner–Wadsworth–Emmons olefination and
hydrogenation. Condensation of the side chain carboxylic group
with anomeric glucosylamine and suitable protection of the a-ami- isomers of the cyclic amino acids, that can also be con-
no acid moiety afforded the corresponding N-glucosyl asparagine or
glutamine mimetics as useful building blocks in glycopeptidomi-
metic synthesis.
sidered as rigid mimetics of aspartic and glutamic acids,
were successfully glycosylated with per-O-acetylated glu-
cosamine to generate the corresponding N4-b-D-gluco-
Key words: glycoamino acids, piperidines, stereoselectivity
pyranosyl asparagine analogues.
OH
N
OH
4-Hydroxy- and 4-oxopipecolic acids (Figure 1) are un-
usual amino acids that occur occasionally in nature. For
example, they are present in some depsipeptide antibiotics
belonging to the virginiamycin and actinomycin families.1
1) CHO
CO2H
4
2
·HCl
·HCl
+
NH
CO2H
N
CO2H
HCl
2)
Ph
Ph
Ph
These cyclic compounds represent valuable precursors of
conformationally constrained amino acids and peptidomi-
metics, since a variety of functionalized side chains can be
introduced on C-4 making use of the hydroxy or carbonyl
moiety.2
(2S,4R)-2
1
(2R,4S)-2
OH
N
OH
N
tBuBr
Cs2CO3
t
t
CO2 Bu
+
CO2 Bu
Cyclic amino acids such as pipecolic acid are able to sta-
bilize or promote turn conformations when incorporated
into polypeptides.3 Therefore, substituted pipecolic acids
can favorably expose side chain groups on the outermost
surface of the peptide, promoting the interaction with spe-
cific receptors.
Ph
(2S,4R)-3
Ph
(2R,4S)-3
Scheme 1
4-Hydroxypipecolic acid derivatives 2 were prepared in
two steps from enantiopure homoallylic amine 1 follow-
ing a slight modification of Beaulieu’s procedure
(Scheme 1).5 The crude strongly acidic mixture of 2 was
made alkaline with cesium carbonate and treated with
tert-butylbromide to obtain a diastereomeric mixture (2:3)
of tert-butyl pipecolic esters 3, which were easily separa-
ble by chromatography on silica gel. The absolute config-
uration of C-2 and C-4 stereocenters was established by
OH
O
N
H
CO2H
N
H
CO2H
4-hydroxypipecolic acid
4-oxopipecolic acid
Figure 1
1
comparison of H NMR spectra of diastereomers 3 pre-
Glycoproteins play a key role in many important biologi-
cal processes. The pendant carbohydrates which range
from monosaccharides to large complex glycans, can
strongly affect protein properties including folding, pro-
teolytic stability and recognition processes.4
pared by hydrolysis and tert-butyl esterification of the
corresponding separated bicyclic lactones.5
The reported process allows a rapid synthesis of tert-butyl
cis-4-hydroxypipecolic ester on multigram scale and is
particularly convenient when both enantiomers are re-
quired.
In this context, we are interested in synthesizing new cy-
clic analogues of N-glycoasparagine to be used in the
The oxidation of alcohols 3 with the N-methylmorpholine
N-oxide–tetrapropylammonium perruthenate (NMO–
TPAP) system6,7 smoothly afforded the corresponding
protected 4-oxopipecolic acids 4 in high yields. Ketones 4
SYNLETT 2006, No. 19, pp 3251–3254
Advanced online publication: 23.11.2006
DOI: 10.1055/s-2006-951541; Art ID: G25106ST
© Georg Thieme Verlag Stuttgart · New York
0
1
.1
2
.2
0
0
6

3252
F. M. Cordero et al.
LETTER
reacted with the sodium salt of methyl(dimethoxyphos- the equatorial C-4 substituent has been previously ob-
phoryl)acetate to give 1.6:1 mixtures of a,b-unsaturated served on related trans-4-substituted pipecolic ester de-
esters 5, which were directly hydrogenated. Selective hy- rivatives in solution.2j
drogenation of the C–C double bond in the presence of the
N-benzyl moiety catalyzed by PtO2 afforded the separable
cis- and trans-4-(2-methoxy-2-oxoethyl)pipecolic esters
6 in ca 4:1 ratio and 80% overall yield based on ketones 4
The relative configuration of (2R,4R)-6 was unambigu-
ously confirmed by single crystal X-ray structure analysis
that showed the chair conformation of the piperidine ring
with the axially oriented tert-butyl ester and the equatori-
(Scheme 2). Analogously to hydrogenation of related 4-
ally oriented 2-methoxy-2-oxoethyl moiety in the solid
alkylidene pipecolic ester derivatives,2g,j hydrogen adds
state.10
preferentially on the olefin face opposite to the C-2 sub-
Orthogonally protected amino acids 6 can be regarded as
stituent, favoring the formation of the cis stereoisomers.
conformationally constrained mimetics of Asp or Glu, and
the side chain carboxylic group can be used to insert a car-
bohydrate unit to obtain a glycoamino acid mimetic.
CO2Me
O
a
b
AcO
AcO
AcO
(2R,4S)-3
t
O
t
N
CO2 Bu
a
N
CO2 Bu
H
CO2R1
R2
(2S,4R)-6
N
Ph
(2R)-4
OAc
Ph
N
O
(2R)-5
(2S,4R)-7: R1 = tBu; R2 = (S)-CHMePh
CO2Me
CO2Me
b
(2S,4R)-8: R1 = H; R2 = Boc
c
Scheme 3 Reagents and conditions: (a) (i) NaOH (1 M); (ii)
2,3,4,6-tetra-O-acetyl-b-D-glucopyranosylamine, CDMT, NMM,
THF, 63%; (b) (i) H2, 10% Pd/C, MeOH; (ii) TFA; (iii) Boc2O,
DIPEA, MeOH, 34%.
+
t
t
N
CO2 Bu
N
CO2 Bu
9 : 2
Ph
Ph
(2R,4R)-6
(2R,4S)-6
The
cis-4-(2-methoxy-2-oxoethyl)pipecolic
esters
CO2Me
CO2Me
(2S,4R)-6 and (2R,4S)-6 were hydrolyzed under basic
conditions and the obtained salts were coupled with
2,3,4,6-tetra-O-acetyl-b-D-glucopyranosylamine11 in the
a, b, c
+
(2S,4R)-3
presence
of
2-chloro-4,6-dimethoxy-1,3,5-triazine
t
t
N
CO2 Bu
N
CO2 Bu
(CDMT) and 4-methylmorpholine (NMM) to yield
(2S,4R)-712 and (2R,4S)-7, respectively, in 63–69% yields
(Scheme 3).
4 : 1
Ph
(2S,4R)-6
Ph
(2S,4S)-6
Hydrogenolysis of the N-benzyl moiety followed by hy-
drolysis of the tert-butyl ester with trifluoroacetic acid
(TFA) afforded the glycoamino acids unprotected at the
N- and C-terminus. Finally, reaction with di-tert-butyl-
dicarbonate (Boc2O) in the presence of diisopropyl(eth-
yl)amine (DIPEA) provided the N-Boc glycoamino acids
(2S,4R)-813 and (2R,4S)-8 suitably protected for peptide
synthesis (Scheme 3).
Scheme 2 Reagents and conditions: (a) NMO, TPAP, 4 Å MS,
CH2Cl2, 91–92%; (b) (MeO)2P(O)CH2CO2Me, NaH, THF; (c) H2,
PtO2, EtOAc, 80–83%.
The relative configuration of cis- and trans-2,4-disubsti-
tuted piperidines 6 was determined through analysis of
their 1H NMR8 spectra.2a,j,9 In particular, the proton 2-H
(d = 3.27 and 2.79 ppm) in the major isomers displays
coupling constants of ca 11 Hz and 2.8 Hz to 3-Hax and
3-Heq, respectively, suggesting a diaxial relationship be-
tween 2-H and 3-Hax. The proton 4-H (d = ca 1.8 and 1.6
ppm) has coupling constants of ca 12 Hz to both 3-Hax and
5-Hax, consistent with an axial orientation.
In conclusion, the synthesis of new cyclic mimetics of
Asp and Glu has been accomplished starting from enan-
tiopure tert-butyl 4-oxopipecolic esters. The side chain
carboxylic group was coupled with glucopyranosylamine
to obtain conformationally restricted mimetics of N-gluc-
osyl-Asn and N-glucosyl-Gln. Standard protecting group
manipulations gave a building block suitable for peptide
synthesis.
In the minor isomers, the proton 2-H (d = 3.88 and 3.20
ppm) displays small coupling constants (£ 5.4 Hz) to the
3-H protons in accord with an equatorial orientation. The
proton 4-H (d = 1.9 ppm) has coupling constants of nearly
12 Hz to 5-Hax, consistent with an axial orientation.
The present work further demonstrates the value of 4-oxo-
and 4-hydroxypipecolic acid as precursors of conforma-
tionally constrained amino acids. The possibility of these
cyclic amino acids to induce turn conformations when in-
serted in peptides suggests the use of this class of amino
acids for the synthesis of biologically active peptides –
work that is currently in progress in our laboratories.
Accordingly, cis stereochemistry was assigned to
(2R,4S)-6 and (2S,4R)-6 and trans to (2R,4R)-6 and
(2S,4S)-6, assuming a chair-like conformation for the pip-
eridine ring. The preference of the trans diastereomer for
the conformation with the axial C-2 carboxyl group and
Synlett 2006, No. 19, 3251–3254 © Thieme Stuttgart · New York

LETTER
New Cyclic Aspartic Acid and N-Glucosyl Asparagine Mimetics
3253
32.6 (d, C-4), 31.6 (t, C-5), 28.0 (q, 3 × C, t-Bu), 9.0 (q,
CHCH3).
Acknowledgment
(2R,4R)-6: [a]D22 –0.55 (c = 1.02, CHCl3). 1H NMR (400
MHz): d = 7.25–7.38 (m, 4 H, Ph), 7.17–7.23 (m, 1 H, Ph),
4.00 (q, J = 6.7 Hz, 1 H, CHMe), 3.86–3.91 (m, 1 H, 2-H),
3.65 (s, 3 H, OCH3), 2.86 (dt, J = 2.3, 12.1 Hz, 1 H, 6-H),
2.45 (dm, J = 11.7 Hz, 1 H, 6-H), 2.11–2.27 (m, 3 H,
CH2CO2Me, 3-H), 1.86–1.98 (m, 1 H, 4-H), 1.46–1.56 (m, 2
H, 3-H, 5-H), 1.51 (s, 9 H, t-Bu), 1.25 (d, J = 6.7 Hz, 3 H,
CHCH3), 1.11 (dq, J = 4.5, 12.4 Hz, 1 H, 5-H). 13C NMR
(100 MHz): d = 173.0 (s, CO), 172.9 (s, CO), 147.2 (s, Ph),
128.2 (d, 2 × C, Ph), 127.0 (d, 2 × C, Ph), 126.6 (d, Ph), 80.6
(s, t-Bu), 61.7 (d, CHMe), 57.1 (d, C-2), 51.4 (q, OCH3),
45.5 (t, C-6), 41.2 (t, CH2CO2Me), 34.9 (t, C-3), 32.1 (t, C-
5), 29.6 (d, C-4), 28.3 (q, 3 × C, t-Bu), 22.0 (q, CHCH3).
(9) Sugg, E. E.; Griffin, J. F.; Portoghese, P. S. J. Org. Chem.
1985, 50, 5032.
The authors thank the Ministry of Instruction, University and
Research (MIUR, Italy) for financial support (projects PRIN
2002031849 and 2004031072), and Ente Cassa di Risparmio di
Firenze for granting funds to the Department of Organic Chemistry
for the acquisition of a 400 MHz NMR spectrometer. Dr. C. Faggi
is gratefully thanked for X-ray determination.
References and Notes
(1) (a) Mukhtar, T. A.; Wright, G. D. Chem. Rev. 2005, 105,
529. (b) Di Giambattista, M.; Nyssen, E.; Percher, A.;
Cocito, C. Biochemistry 1990, 29, 9203. (c) Reed, J. W.;
Purvis, M. B.; Kingston, D. G. I.; Blot, A.; Gossele, F. J.
Org. Chem. 1989, 54, 1161. (d) Kessler, H.; Kühn, M.;
Löschner, T. Liebigs Ann. Chem. 1986, 1. (e) Hollstein, U.
Chem. Rev. 1974, 74, 625. (f) Vanderhaeghe, H.;
(10) The X-ray CIF file for this structure has been deposited at the
Cambridge Crystallographic Data Centre and allocated with
the deposition number CCDC 607916. Copies of the data
can be obtained free of charge from CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (e-mail:
deposit@ccdc.cam.ac.uk; internet://www.ccdc.cam.ac.uk).
(11) Esteves, A. P.; Rodrigues, L. M.; Silva, M. E.; Gupta, S.;
Oliveira-Campos, A. M. F.; Machalicky, O.; Mendonça, A.
J. Tetrahedron 2005, 61, 8625.
(12) A slurry of diester (2S,4R)-6 (358 mg, 0.99 mmol) in THF
(0.5 mL) was treated dropwise with an aq 1 M NaOH
solution (1.6 mL, 1.6 mmol). The mixture was stirred at r.t.
for 3 h and concentrated. The residue was dissolved in THF
(1.0 mL) and NMM (0.109 mL, 0.99 mmol) and CDMT (174
mg, 0.99 mmol) were added. The reaction mixture was
stirred at r.t. for 30 min and then a solution of 2,3,4,6-tetra-
O-acetyl-b-D-glucopyranosylamine (321 mg, 0.99 mmol) in
THF (1.3 mL) was added. The resulting mixture was stirred
at r.t. overnight, diluted with EtOAc, and washed
Parmentier, G. J. Am. Chem. Soc. 1960, 82, 4414.
(2) (a) Machetti, F.; Cordero, F. M.; De Sarlo, F.; Papini, A. M.;
Alcaro, M. C.; Brandi, A. Eur. J. Org. Chem. 2004, 2928.
(b) Sata, N. U.; Kuwahara, R.; Murata, Y. Tetrahedron Lett.
2002, 43, 115. (c) Golubev, A. S.; Schedel, H.; Radics, G.;
Sieler, J.; Burger, K. Tetrahedron Lett. 2001, 42, 7941.
(d) Beaulieu, P. L.; Anderson, P. C.; Cameron, D. R.;
Croteau, G.; Gorys, V.; Grand-Maître, C.; Lamarre, D.;
Liard, F.; Paris, W.; Plamondon, L.; Soucy, F.; Thibeault,
D.; Wernic, D.; Yoakim, C. J. Med. Chem. 2000, 43, 1094.
(e) Ornstein, P. L.; Arnold, M. B.; Lunn, W. H. W.; Heinz,
L. J.; Leander, J. D.; Lodge, D.; Schoepp, D. D. Bioorg.
Med. Chem. Lett. 1998, 8, 389. (f) Bellier, B.; Da
Nascimento, S. H.; Gincel, E.; Roques, B. P.; Garbay, C.
Bioorg. Med. Chem. Lett. 1998, 8, 1419. (g) Skiles, J. W.;
Giannousis, P. P.; Fales, K. R. Bioorg. Med. Chem. Lett.
1996, 6, 963. (h) Pellicciari, R.; Natalini, B.; Luneia, R.;
Marinozzi, M.; Marinella, R.; Rosato, G. C.; Sadeghpour, B.
M.; Snyder, J. P.; Monahan, J. B.; Moroni, F. Med. Chem.
Res. 1992, 2, 491. (i) Hays, S. J.; Malone, T. C.; Johnson, G.
J. Org. Chem. 1991, 56, 4084. (j) Ornstein, P. L.; Schoepp,
D. D.; Arnold, M. B.; Leander, J. D.; Lodge, D.; Paschal, J.
W.; Elzey, T. J. Med. Chem. 1991, 34, 90.
sequentially with H2O and brine. The separated organic layer
was dried over Na2SO4, filtered, and concentrated. The
residue was purified by column chromatography on silica
gel (eluent: PE–EtOAc = 2:3) to afford (2S,4R)-7 (423 mg,
63%) as a white solid.
(2S,4R)-7: [a]D27 –32.65 (c = 0.525, CHCl3). 1H NMR (400
MHz): d = 7.23–7.34 (m, 3 H, Ph), 7.14–7.19 (m, 2 H, Ph),
6.18 (d, J = 9.3 Hz, 1 H, NH), 5.28 (t, J = 9.5 Hz, 1 H, 3¢-H),
5.19 (t, J = 9.4 Hz, 1 H, 1¢-H), 5.04 (t, J = 9.7 Hz, 1 H, 4¢-H),
4.87 (t, J = 9.6 Hz, 1 H, 2¢-H), 4.29 (dd, J = 4.2, 12.5 Hz, 1
H, 6¢-H), 4.04 (dd, J = 2.0, 12.5 Hz, 1 H, 6¢-H), 3.97 (q, J =
7.0 Hz, 1 H, CHMe), 3.78 (ddd, J = 2.0, 4.2, 10.1 Hz, 1 H,
5¢-H), 3.00 (dm, J = 11.4 Hz, 1 H, 6-H), 2.77 (dd, J = 2.9,
11.2 Hz, 1 H, 2-H), 2.07 (A part of an ABX system, J = 6.7,
14.6 Hz, 1 H, CHHCON), 2.06 (s, 3 H, CH3CO), 2.02 (s, 3
H, CH3CO), 2.00 (s, 3 H, CH3CO), 1.99 (s, 3 H, CH3CO),
1.96 (B part of an ABX system, J = 7.0, 14.6 Hz, 1 H,
CHHCON), 1.79 (dm, J = 12.4 Hz, 1 H, 3-H), 1.71 (tm, J =
11.5 Hz, 1 H, 6-H), 1.50–1.60 (m, 2 H, 4-H, 5-H), 1.53 (s, 9
H, t-Bu), 1.48 (d, J = 7.0 Hz, 3 H, CHCH3), 1.36 (q, J = 11.8
Hz, 1 H, 3-H), 1.19–1.30 (m, 1 H, 5-H). 13C NMR (100
MHz): d = 173.1 (s, CO), 171.6 (s, CO), 171.0 (s, CO), 170.6
(s, CO), 169.8 (s, CO), 169.5 (s, CO), 137.7 (s, Ph), 128.9 (d,
2 × C, Ph), 127.7 (d, 2 × C, Ph), 127.2 (d, Ph), 80.9 (s, t-Bu),
78.0 (d, C-1¢), 73.5 (d, C-5¢), 72.6 (d, C-3¢), 70.5 (d, C-2¢),
68.1 (d, C-4¢), 64.5 (d, C-2), 61.6 (t, C-6¢), 59.4 (d, CHMe),
43.8 (t, C-6), 43.2 (t, CH2CON), 36.3 (t, C-3), 32.6 (d, C-4),
31.3 (t, C-5), 28.1 (q, 3 × C, t-Bu), 20.7 (q, CH3CO), 20.6 (q,
CH3CO), 20.5 (q, 2 × C, CH3CO), 18.5 (q, CH3CH).
(13) A solution of (2S,4R)-7 (105 mg, 0.155 mmol) in MeOH (1.6
mL) and AcOH (18 mL, 0.310 mmol) was hydrogenated in
(3) Toniolo, C. Int. J. Peptide Protein Res. 1990, 35, 287.
(4) (a) Varki, A. Glycobiology 1993, 3, 97. (b) Dwek, R. A.
Chem. Rev. 1996, 96, 683. (c) Rudd, P. M.; Elliott, T.;
Cresswell, P.; Wilson, I. A.; Dwek, R. A. Science 2001, 291,
2370. (d) Pratt, M. R.; Bertozzi, C. R. Chem. Soc. Rev. 2005,
34, 58.
(5) Gillard, J.; Abraham, A.; Anderson, P. C.; Beaulieu, P. L.;
Bogri, T.; Bousquet, Y.; Grenier, L.; Guse, I.; Lavallée, P. J.
Org. Chem. 1996, 61, 2226.
(6) Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P.
Synthesis 1994, 639; and references cited therein.
(7) For examples of oxidation of 4-hydroxypipecolic acid
derivatives with other oxidants, see refs. 2c, 2g and 2j.
(8) (2R,4S)-6: [a]D24 6.45 (c = 0.93, CHCl3). 1H NMR (400
MHz): d = 7.49–7.52 (m, 2 H, Ph), 7.28–7.34 (m, 2 H, Ph),
7.19–7.24 (m, 1 H, Ph), 4.01 (q, J = 6.9 Hz, 1 H, CHMe),
3.65 (s, 3 H, OCH3), 3.27 (dd, J = 2.8, 10.9 Hz, 1 H, 2-H),
2.44 (dt, J = 3.5, 11.4 Hz, 1 H, 6-H), 2.22 (d, J = 7.0 Hz, 2
H, CH2CO2Me), 2.15 (dt, J = 2.5, 11.6 Hz, 1 H, 6-H), 1.92
(dm, J = 12.3 Hz, 1 H, 3-H), 1.76–1.88 (m, 1 H, 4-H), 1.43–
1.57 (m, 2 H, 3-H, 5-H), 1.48 (s, 9 H, t-Bu), 1.32 (d, J = 6.9
Hz, 3 H, CHCH3), 1.12 (dq, J = 3.8, 12.0 Hz, 1 H, 5-H). 13
C
NMR (100 MHz): d = 173.1 (s, CO), 172.9 (s, CO), 143.4 (s,
Ph), 127.9 (d, 2 × C, Ph), 127.8 (d, 2 × C, Ph), 126.6 (d, Ph),
80.9 (s, t-Bu), 64.3 (d, C-2), 57.3 (d, CHMe), 51.5 (q,
OCH3), 43.2 (t, C-6), 40.7 (t, CH2CO2Me), 36.2 (t, C-3),
Synlett 2006, No. 19, 3251–3254 © Thieme Stuttgart · New York

3254
F. M. Cordero et al.
LETTER
and stirred at r.t. for 1 h. After the organic layer was
separated, the aqueous layer was extracted with EtOAc (3 ×
1.4 mL). The combined organic phases were washed with
brine, dried over Na2SO4, filtered, and concentrated. The
crude product was purified by column chromatography on
silica gel (eluent: CH2Cl2–MeOH, 10:1) to afford (2S,4R)-8
(32.4 mg, 34%) as a white solid. Molecular mass
the presence of 10% Pd/C (9 mg) at r.t. and atmospheric
pressure overnight. The reaction mixture was filtered
through cotton wool and concentrated. The residue was
dissolved in the minimum amount of TFA at 0 °C and the
mixture was stirred at r.t. for 2.5 h and then concentrated.
The residue was dissolved in anhyd MeOH (217 mL) and
treated sequentially with DIPEA (67 mL, 0.386 mmol) and
Boc2O (28.1 mg, 0.129 mmol) at 0 °C. The reaction mixture
was stirred at r.t. overnight, and then concentrated. The
residue was dissolved in a mixture of EtOAc and CH2Cl2
(1:1, 2.8 mL), treated with 5% aq KHSO4 solution (1 mL)
determinations by electrospray ionization mass
spectrometry (ESI–MS) was used to identify the N-protected
amino acid (2S,4R)-8. MS (ESI): m/z = 639.3 [M + Na+].
Synlett 2006, No. 19, 3251–3254 © Thieme Stuttgart · New York