Stereoselective approach to the pyrroloquinoline core of martinelline

Article abstract of DOI:10.1055/s-2002-33515

The synthesis of α-aminonitrile 4, a potent precursor to the martinellines is described. The preparation of the pyrroloquinoline core was realized in one step (90% yield) using a formal [4π+2π] cyclocondensation involving the cationic aza-diene 6 and the 1-(benzyloxy)carbonyl-2-pyrroline 5 as dienophile. The stereo-chemical outcome of the cyanation procedure performed on the tricyclic core thus obtained is discussed.

Full text of DOI:10.1055/s-2002-33515

1500
LETTER
Stereoselective Approach to the Pyrroloquinoline Core of Martinelline
Stereoselective
A
i
pproac
c
h
to the
P
y
h
a
e
Core of
M
r
artinell
d
ine Malassene,a Lourdes Sanchez-Bajo,a Loic Toupet,b Jean-Pierre Hurvois,*a Claude Moineta
a
Laboratoire d’Electrochimie, UMR 6509, Institut de Chimie, Université de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
b
Groupe Matière Condensée et Matériaux, Université de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
Fax +33(223)235967; E-mail: Jean-Pierre.Hurvois@univ-rennes1.fr
Received 1 July 2002
tion of the pyrroloquinoline ring system as a means for the
subsequent introduction of a requisite alkyl side chain in
a stereospecific fashion. Thus, we report here, how the
above-mentioned strategy has found application in the
Abstract: The synthesis of -aminonitrile 4, a potent precursor to
the martinellines is described. The preparation of the pyrroloquino-
line core was realized in one step (90% yield) using a formal
[4 +2 ] cyclocondensation involving the cationic aza-diene 6 and
the 1-(benzyloxy)carbonyl-2-pyrroline 5 as dienophile. The stereo- stereoselective preparation of 4, a potent precursor of the
chemical outcome of the cyanation procedure performed on the tri-
martinellines.
cyclic core thus obtained is discussed.
Key words: martinelline, stereoselective synthesis, nitriles, natural
products
NH
HN
1
N
O
Martinelline (1) and martinellic acid (2) (Scheme 1) have
been isolated by scientists at Merck from the roots of the
Amazonian plant Martinella Iquitosensis.1 These com-
pounds showed antibiotic activity against Gram-positive
and Gram-negative bacteria, affinity for several G-protein
coupled receptors, and are the first non peptidic bradyki-
nin receptors antagonists reported to date. Martinelline
and martinellic acid are also interesting from a chemical
standpoint in that they possess a tricyclic core, the pyr-
rolo[3,2-c]quinoline ring system, which has not been re-
ported previously in any natural product. The
development of new tools intended to the construction of
products analogous to martinelline is a process of pharma-
ceutical importance or may lead to new biochemical
probes. It is not surprising that the biological activities of
1 and 2 coupled with their intriguing molecular frame,
provided intense synthetic efforts2 which culminated in
the total synthesis of 2 by two independent research
groups.2k,l Therefore, there is a continuous demand for the
concise and stereoselective preparation of a precursor,
that could be utilized for the construction of martinelline
and/or structurally related compounds. In this context the
design of 4, a preformed building block which incorpo-
rates an -aminonitrile entity, seemed to us a promising
possibility. As underscored by a series of reviews, -ami-
nonitriles have found wide applications in the construc-
tion of a large number of alkaloids and synthetic products
of biological interest.3 Their dual reactivity has made
them ideal candidates for the diastereselective introduc-
tion of functionalized alkyl chains in the position to the
nitrogen atom.4,5 In addition, an enantioselective prepara-
tion of 3 (R1 = R2 = H), recently reported by Ennis and
coworkers2f has increased our interest in the anodic cyana-
9b
R
O
3a
H
H
N
N
N 4
H
NH
NH
NH2
1, R =
N
H
2, R = H
R2
R2
N
N
N
R1
CN
N
R1
3
4
Scheme 1
In considering the synthetic schemes that allow the prep-
aration of 3, we determined that the unprecedented cation-
ic aza-Diels–Alder cyclocondensation between the
endocyclic enamide 5 and the iminium cation 6 was obvi-
ously the more concise approach (Scheme 2).
The in situ formation of 6 arising from the Lewis acid ca-
talysed decomposition of either thioalkylamines
(X = SEt, R1 = Me)6a or 1-[(dialkylamino)methyl]benzo-
triazoles (X = Bt, R1 = Me)6b,c is now well documented,
but the use of readily available aminoacetal (8, X = OEt,
R1 = Et)7 or aminonitriles (7, X = CN, R1 = Et)8 as a valu-
able source of cationic 2-azabutadienes has received only
scant attention. In a series of experiments, aminonitrile 7
was treated in CH2Cl2 at –30 °C by a number of Lewis ac-
Synlett 2002, No. 9, Print: 02 09 2002.
Art Id.1437-2096,E;2002,0,09,1500,1504,ftx,en;G18302ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

LETTER
Stereoselective Approach to the Pyrroloquinoline Core of Martinelline
1501
Table 1 Reaction of 8 with Dienophiles
R2
5
6
N
Entry Dienophile
1
Product
t (h)
2
Yielda,b
(%)
3
N
R1
X
85
N
R1
N
7: X = CN
8: X = OEt
9
Scheme 2
2
2
2
75
80
TMS
TMS
ids including BF3 OEt2, TiCl4 or TMSOTf. In the pres-
ence of cyclopentadiene or allyltrimethylsilane as
dienophiles, and only in the case where TiCl4 was em-
ployed, the expected adducts 9 and 10 (cf. Table 1) were
obtained together with unidentified by-products. These
results are in sharp contrast with those obtained with the
vast majority of -aminonitriles which undergo facile cy-
anide ion loss under very mild conditions.9 Conversely,
aminoacetal 8 underwent cyclocondensation with the
same dienophiles but in the presence of BF3 Et2, in
CH2Cl2 at temperatures ranging from –30 °C to –80 °C.
This series of reactions showed that the intermolecular cy-
cloaddition between 6 and the aforementioned dieno-
philes was not temperature dependent and afforded results
which are collected in Table 1 (entries 1, 2). Interestingly,
adducts 9 and 10 were obtained as pure material as clearly
N
10
3
O
N
O
N
N
11
4
2
2
75
90
O
O
N
1
observed from the examination of their H NMR spec-
trum.
12
In contrast to other reports6 and studies10a performed in
our laboratory, 9 was obtained as a mixture (1:1) of dou-
ble-bond isomers. We reasoned that prior cleavage of the
aminoacetal function in 8 with a suitable oxophilic Lewis
acid, followed by a [4 +2 ] cyclocondensation between
the positively charged aza diene 6 and the desired dieno-
philes should be at the origin of the clean formation of tet-
rahydroquinolines 9 and 10. As a further extension of this
model study, we examined the reaction on readily acces-
sible polarized alkenes such as 1-vinylpyrrolidin-2-one
(entry 3) and 2,3-dihydrofuran (entry 4). In contrast to the
results described above, the reaction was more difficult
than initially anticipated. The formation of iminium cation
6 should be performed first, followed by the introduction
of a CH2Cl2 solution of the alkenes. By contrast, direct ad-
dition of BF3 OEt2 to a mixture of 8 and 2,3-dihydrofuran
at –80 °C led to a complex mixture, from which only 30%
of 1211 could be recovered after column chromatography
(diethyl ether/petroleum ether, 1:2). Following the two-
step procedure described above, the expected adducts 11
and 12 were obtained as pure material (Table 1, entries 3,
4). Both the 1H NMR spectra of 11 and 12 were well re-
solved and their interpretation was easy. For example, in
the spectrum of 12 a characteristic doublet system
(J = 5.35 Hz) attributed to 9b-H was found at = 4.54
ppm. This coupling constant was close to the value that
was reported for the cis-isomer (ref.12, J = 5.30 Hz). En-
5
Cbz
N
N
Cbz
N
3
a Reaction conditions: All reagents should be distilled prior to use: 8,
BF3 OEt2 (1 equiv), CH2Cl2, –80 °C (1 h), dienophile (1 equiv), –80
°C (2 h).
b Yields are of isolated products, all new compounds had satisfactory
elemental analysis, and the 1H, 13C NMR and mass spectra were con-
sistent with the proposed structures.
couraged by the successful preparation of the tricyclic ad-
duct 12 we turned our attention to the one-step
stereoselective synthesis of the heterocyclic core of mar-
tinelline using 1-(benzyloxy)carbonyl-2-pyrroline 5 as
substrate. This material was prepared according to the
method described by Kraus et al.13 After vacuum distilla-
tion of the crude reaction mixture, (105–110 °C, 4 10–2
Torr), the expected endocyclic enamide 5 was obtained
together with 1-(benzyloxy)carbonyl-pyrrolidine (15%)
which fortunately does not participate in the subsequent
reactions. As depicted above, the exposure of 5 to an
equimolar amount of 8 in the presence of BF3 OEt2
Synlett 2002, No. 9, 1500–1504 ISSN 0936-5214 © Thieme Stuttgart · New York

1502
R. Malassene et al.
LETTER
(1 equiv) afforded the expected cycloadduct 3 as a sole afford the expected -cyano adducts 14 and 4 in an 80%
product (90%).
and 85% yield, respectively (Scheme 3). Cyanation of
Hexahydrofuro[3,2-c]quinoline 12 provided a mixture of
diastereomers, which could be readily separated by col-
umn chromatography. Trans-14 (75%) eluted first, fol-
lowed by the less abundant and more polar cis derivative
The 1H and 13C NMR spectra of 3, recorded in C6D6, were
temperature dependent and, with the exception of few sys-
tems, exhibit coalescent signals. Heating the sample at
343 K induced decoalescence of these broad peaks and a
1
(25%). The H NMR spectra of both diastereomers were
13
sharpening of the signals. For example, in the C NMR
very similar. Therefore, their relative stereochemistry
could not be determined from the magnitude of the cou-
pling constants of the diagnostic protons 4-H, 3a-H and
9b-H. Thus, a series of NOE difference experiments,
(Scheme 4) was undertaken. The key finding that supports
the relative stereochemistry of trans-14 was an enhance-
ment (6%) on proton 4-H after saturation of the more
shielded proton 3-Ha ( = 2.05 ppm). This is consistent
with the fact that protons 4-H and 3-Ha are in close prox-
imity, placing 4-H in an equatorial disposition, hence, an
axial orientation for the nitrile group. Conversely, in the
case of cis-14, an enhancement of only 2% was recorded
between the same protons. These findings were definitive-
ly ascertained in the case of cis-14, which crystallized in
a mixture of ether and petroleum ether. An X-ray diffrac-
tion study15 performed on a single crystal obtained in this
way, revealed that the cyano group was axial and in a syn
configuration with respect to the tetrahydrofuran moiety.
It is noteworthy that after a three-month storage at 5 °C, a
partial interconversion (up to 25%) of the a priori more
constrained cis-14 adduct into the more energetically fa-
vored trans derivative was observed. Conversely, we
were please to find that 416 was isolated as a single diaste-
reomer (de 99%) and could be stored for months under
normal conditions without any appreciable decay of qual-
ity. The 1H NMR spectra of 4 was recorded in C6D6 at 343
K and, at that temperature, two characteristic doublet sig-
nals (J = 2.6 Hz and 7.5 Hz) attributed to 4-H and 9b-H re-
spectively, were found at = 3.28 and 5.50 ppm.
spectrum of 3, C-3a and C-9b were found at = 37.65 and
56.50 ppm respectively, whereas their counterparts in the
martinelline series were found at = 39.0 and 53.0 ppm
respectively. Removal of the CBz protecting group was
performed by catalytic hydrogenation of 3 with ammoni-
um formate in the presence of palladium to afford the
N-1 unprotected diamine 13 (Scheme 3) in 70% yield.
The 1H NMR spectrum of the hydrochloride salt of 13 was
performed in D2O, and the magnitude of the coupling con-
stant (J = 7.40 Hz) between 3a-H and 9b-H was in
keeping with that reported by Ennis and coworkers2f in
a similar system, which is indicative of a cis-fused
product.14
HN
90%
i
70%
ii
5 + 8
3
N
13
O
N
O
80%
12
+
iii
CN
N
CN
(trans/cis = 3:1)
trans-14
cis-14
1
Fortunately, in the H NMR spectrum of 4, recorded in
Cbz
CDCl3, the diagnostic proton H-4 was well resolved and
displayed a doublet system at = 4.24 ppm, which, upon
saturation of 3-Ha afforded an NOE enhancement of 7%,
providing evidence that the tricyclic adduct 4 was in a
trans conformation (Scheme 4).
N
85%
iii
3
N
CN
The major formation of the trans isomers (Scheme 2)
could be rationalized in terms of an axial approach (path
a) of the cyanide anion (under a stereocontrolled mode)17
on the prochiral iminium salts A (X = O) or B (X = CBz)
leading to the more energetically favoured chair-like tran-
sition state in which a maximum orbital overlap between
the developing nitrogen lone pair and the incoming nu-
cleophile is maintained.
4
Scheme 3 (i) BF3 OEt2 (1 equiv), CH2Cl2, –80 °C (1 h), 8 (1 equiv),
–80 °C (2 h). (ii) Pd-C (5%) HCOONH4/MeOH, 65 °C, 1 h. (iii) –2e,
–H+, NaCN (4 equiv), MeOH.
With the required substrates in hand we turned our atten-
tion to the direct conversion of the tricyclic adducts 12 and
3 into their corresponding -cyano derivatives. The com-
pounds were dissolved in methanol containing lithium ac-
etate (20 g/L) as supporting electrolyte, and sodium
cyanide (4 equiv) as trapping agent. The oxidation proce-
dure was performed at a glassy carbon electrode in a di-
vided batch cell at controlled potential (Ep = +0.75 V/
SCE). After the consumption of 2.0 F per mole of sub-
strate, the solvent was evaporated in vacuo and the crude
material was purified by a rapid filtration on silica gel to
In summary, we have developed a concise and stereose-
lective approach to the pyrroloquinoline core of the mar-
tinellines. The -aminonitrile function in 4, which exhibit
both potential and reversed iminium reactivity should en-
sure the stereoselective introduction of the requisite alkyl
side chain at C-4. Work is currently in progress in our
laboratory to confirm the likelihood of these hypotheses.
Synlett 2002, No. 9, 1500–1504 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Stereoselective Approach to the Pyrroloquinoline Core of Martinelline
1503
Thompson, N. Tetrahedron Lett. 2001, 42, 6417.
12%
H
N
9%
(j) Hadden, M.; Nieuwenhuyzen, M.; Potts, D.; Stevenson,
P. J.; Thompson, N. Tetrahedron 2001, 57, 5615. (k) Ma,
D.; Xia, C.; Jiang, J. Org. Lett. 2001, 3, 2189. (l) Snider, B.
B.; Ahn, Y.; O’Hare, S. M. Org. Lett. 2001, 3, 4217.
(m) He, Y.; Mahmud, H.; Wayland, B. R.; Rasika Dias, H.
V.; Lovely, C. J. Tetrahedron Lett. 2002, 43, 1171.
(3) For the chemistry of -aminonitriles see: (a) Enders, D.;
Shilvock, J. P. Chem. Soc. Rev. 2000, 29, 359. (b) Husson,
H.-P.; Royer, J. Chem. Soc. Rev. 1999, 28, 383.
H
H
H
O
N
H
O
N
H
H
H
H
H
N
6%
2%
(4) Polniaszek, R. P.; Belmont, S. E. J. Org. Chem. 1991, 56,
4868.
(5) Malassene, R.; Toupet, L.; Hurvois, J. P.; Moinet, C. Synlett
2002, 895.
(6) (a) Beifuss, U.; Ledderhose, S. J. Chem. Soc., Chem.
Commun. 1995, 2137. (b) Katritzky, A. R.; Rachwald, S.;
Rachwald, B. Tetrahedron 1996, 52, 15031; and references
cited therein. (c) Katritzky, A. R.; Nichols, D. A.; Qi, M.;
Yang, B. J. Heterocycl. Chem. 1997, 34, 1259.
trans-14
cis-14
N
H
H
CBz
N
N
H
H
H
7%
(7) Quintard, J. P.; Elissondo, B.; Jousseaume, B. Synthesis
1984, 495.
4
(8) Schneider, H. J.; Junker, A. Chem. Ber. 1986, 119, 2815.
(9) (a) Tamminen, T.; Jokela, R.; Tirkonnen, B.; Lounasmaa, M.
Tetrahedron 1989, 45, 2683. (b) Grierson, D. S.; Harris, M.;
Husson, H.-P. Tetrahedron 1983, 39, 3683.
12, 5
(10) (a) Posson, H.; Hurvois, J. P.; Moinet, C. Synlett 2000, 209.
(b) For aza-Diels–Alder reactions in aqueous media see:
Larsen, S. D.; Grieco, P. A. J. Am. Chem. Soc. 1985, 107,
1768. (c) Grieco, P. A.; Bahsas, A. Tetrahedron Lett. 1988,
29, 5855. (d) Waldman, H. Angew. Chem., Int. Ed. Engl.
1988, 2, 274.
a
a
4, trans-14
cis-14
H
H
X
(11) 5-Ethyl-2,3,3a,4,5,9b-hexahydro-furo-[3,2-
c]quinoline(12):
N
To a cooled (–80 °C) solution (10 mL, CH2Cl2) containing
the aminoacetal 8 (1.0 g, 5.58 mmol) was added dropwise
(by syringe) and under an argon atmosphere 0.870 g (6.17
mmol) of BF3 OEt2. The reaction mixture turned slightly
green and was stirred at that temperature for one hour. A
solution (CH2Cl2, 5 mL) of 2,3-dihydrofuran (0.470 g, 6.71
mmol) was added dropwise over a 5 min period and was
allowed to react at –80 °C with the iminium salt 6 for two
hours. The crude mixture was quenched with water, and
extracted with CH2Cl2 in the presence of Na2CO3. The
combined organic layers were dried over MgSO4 and
evaporated in vacuo to yield the crude material which was
purified by column chromatography (ether/petroleum ether,
1/2) to afford 12 (0.850 g, 75%) as a slightly yellow oil. 1H
NMR (300 MHz, CDCl3): =1.13 (t, J = 7.10 Hz, 3 H, CH2-
CH3), 1.69–1.80 (m, 1 H, 3-Ha), 2.17–2.29 (m, 1 H, 3-Hb),
2.37–2.48 (m, 1 H, 3a-H), 2.85 (t, J = 11.30 Hz, 1 H, 4-Ha),
3.00 (dd, J = 11.30 and 5.35 Hz, 1 H, 4-Hb), 3.27 (dq,
J = 14.30 and 7.15 Hz, 1 H, CH2-CH3), 3.50 (dq, J = 14.30
and 7.15 Hz, 1 H, CH2-CH3), 3.80 (td, J = 8.60 and 6.20 Hz,
1 H, 2-Ha), 3.94 (td, J = 8.45 and 5.90 Hz, 1 H, 2-Hb), 4.54
(d, J = 5.35 Hz, 1 H, 9b-H), 6.67–6.72 (m, 2 H), 7.15 (td,
H
b
CN-
b
A: X = O
B: X = CBz
Scheme 4
Acknowledgement
R. M. thanks the MENRT for a grant. Authors would like to thank
Dr. S. Sinbandhit (CRMPO) for the NOE difference experiments.
References
(1) Witherup, K. M.; Ransom, R. W.; Graham, A. C.; Bernard,
A. M.; Salvatore, M. J.; Lumma, W. C.; Anderson, P. S.;
Pitzenberger, S. M.; Varga, S. L. J. Am. Chem. Soc. 1995,
117, 6682.
J = 9.60 and 1.71 Hz, 1 H), 7.32 (dm, J = 7.00 Hz, 1 H). 13
C
(2) (a) Ho, T. C. T.; Jones, K. Tetrahedron 1997, 53, 8287.
(b) Gurjar, M. K.; Pal, S.; Rao, A. V. R. Heterocycles 1997,
45, 231. (c) Lovely, C. J.; Mahmud, H. Tetrahedron Lett.
1999, 40, 2079. (d) Batey, R. A.; Simoncic, P. D.; Lin, D.;
Smyj, R. P.; Lough, A. J. Chem. Commun. 1999, 651.
(e) Snider, B. B.; Ahn, Y.; Foxman, B. M. Tetrahedron Lett.
1999, 40, 3339. (f) Nieman, J. A.; Ennis, M. D. Org. Lett.
2000, 2, 1395. (g) Nyerges, M.; Fejes, I.; Toke, L.
Tetrahedron Lett. 2000, 41, 7951. (h) Franck, K. E.; Aubé,
J. J. Org. Chem. 2000, 65, 655. (i) Hadden, M.;
NMR (75 MHz, CDCl3): = 10.60, 30.01, 35.65, 45.29,
49.24, 65.00, 76.08, 111.59, 116.55, 121.28, 129.02, 131.74,
145.42. C13H17NO: Calcd C, 76.81; H, 8.43; N, 6.89; O,
7.87. Found: C, 76.38; H, 8.43; N, 7.13; O, 7.47.
(12) (a) Katritzky, A. R.; Rachwald, S.; Rachwald, B. J. Org.
Chem. 1995, 60, 2588. (b) Zhang, J.; Li, C. J. J. Org. Chem.
2002, 67, 3969.
(13) Kraus, G. A.; Neuenschwander, K. J. Org. Chem. 1981, 46,
4791.
Nieuwenhuyzen, M.; Osborne, D.; Stevenson, P. J.;
Synlett 2002, No. 9, 1500–1504 ISSN 0936-5214 © Thieme Stuttgart · New York

1504
R. Malassene et al.
LETTER
(14) A trans fused adduct displaying a coupling constant of 13.80
Hz between 3a-H and 9b-H has been recently isolated in 5%
yield and was reported by Lovely and co-workers in ref.2m
(15) Crystallographic data for the structure cis-14 reported in this
paper has been deposited at the Cambridge Crystallographic
Data Center as supplementary publication no: CCDC
186783. Copies of the data can be obtained free of charge on
application to The Director CCDC, 12 Union Road,
Cambridge CB2 IEZ, UK [fax: +44(1223)336033, e-mail:
deposit@ccdc.cam.ac.uk].
yellow oil. 1H NMR (300 MHz, C6D6, 343 K): = 0.89 (t,
J = 7.15 Hz, 3 H, CH2-CH3), 1.21–1.34 (m, 1 H, 3-Ha),
1.44–1.58 (m, 1 H, 3-Hb), 1.94–2.04 (m, 1 H, 3a-H), 2.75–
2.87 (dd, J = 15.0 and 7.15 Hz, 1 H, CH2-CH3), 2.90–3.11
(m, 2 H, CH2-CH3 and 2-Ha), 3.29 (d, J = 2.60, 1 H, 4-H),
3.33–3.36 (br, coal., 1 H, 2-Hb), 5.12 (AB, J = 13.40 Hz, 1
H, O-CH2-C6H5), 5.27 (AB, J = 13.40 Hz, 1 H, O-CH2-
C6H5), 5.50 (d, J = 7.50 Hz, 1 H, 9b-H, 6.41 (d, J = 8.30 Hz,
1 H), 6.72 (t, J = 8.70 Hz, 1 H), 6.98 (t, J = 8.80 Hz, 1 H),
7.05 (m, 5 H), 7.30–7.33 (m, 2 H). 13C NMR (75 MHz, C6D6,
343 K): = 11.99, 26.15, 40.23, 45.05, 45.10, 50.67, 54.70,
67.21, 112.60, 118.88, 119.96, 123.68, 127.72, 128.04,
128.73, 128.98, 131.15, 137.63, 141.34, 156.07.
(16) 4-Cyano-5-ethyl-2,3,3a,4,5,9b-hexahydro-pyrrolo[3,2-
c]quinolin-1-carboxylic Acid Benzyl Ester(4):
0.5 g (1.48 mmol) of 3 were dissolved in methanol (50 mL)
containing AcOLi (20 g/L) and 0.520 g (10.6 mmol) of
NaCN. The amine was oxidized at controlled potential
(Ep = +0.75 V/SCE) and after the consumption of 2.0 F per
mole of substrate, the electrolysis was stopped. Classical
work-up and chromatographic purification (diethyl ether/
petroleum ether, 1/2) afforded 4 (0.460 g, 85%) as a pale
C22H23N3O2: Calcd 361.1790. Found 361.1787 (MS).
(17) (a) Stevens, R. V. Acc. Chem. Res. 1984, 17, 289.
(b) Stevens, R. V.; Lee, A. W. M. J. Am. Chem. Soc. 1979,
101, 7032. (c) Deslongchamps, P. In Stereoelectronic
Effects in Organic Chemistry; Baldwin, J., Ed.; Pergamon
Press: Oxford, 1983.
Synlett 2002, No. 9, 1500–1504 ISSN 0936-5214 © Thieme Stuttgart · New York