6-Pyridylnicotine and bis-6,6'-nicotine - new chiral 2,2'-bipyridines

Article abstract of DOI:10.1055/s-1998-1997

Starting from (-)-nicotine, two new 2,2'-bipyridine ligands 6 and 7 with chiral pyrrolidine side chains have been synthesized. The stoichiometric complexation by palladium(II) or mercury(II) ions gives the bipyridyl complexes 8 and 9.

Full text of DOI:10.1055/s-1998-1997

4
2
Short Papers
SYNTHESIS
6
-Pyridylnicotine and Bis-6,6′-nicotine – New Chiral 2,2′-Bipyridines
*
Boris Schmidt, Vera Neitemeier
Institut für Organische Chemie der Universität Hannover. Schneiderberg 1b. D-30167 Hannover, Germany
Fax +49(511)762301 l; E-mail: boris,schmidt@mbox.oci.uni-hannover.de
Received 30 May 1997; revised 31 July 1997
Starting from (–)-nicotine, two new 2,2’-bipyridine ligands 6 and 7 give PYNIC (6) and BINIC (7) as depicted. Such syn-
with chiral pyrrolidine side chains have been synthesized. The
stoichiometric complexation by palladium(II) or mercury(II) ions
gives the bipyridyl complexes 8 and 9.
theses of bipyridyls from halopyridines and metallated
pyridines by palladium(0) catalysed cross-coupling
reactions1
4,15,20
are well established. (–)-Nicotine was
13
converted into its pyridine N-oxide using H O and ace-
2
2
Over the last few years, the classical metal bipyridyl com-
tic acid for oxidation of both nitrogens and subsequent
chemoselective reduction of the pyrrolidine N-oxide by
plexes have been developed into really interesting aggre-
gates,1–5 while new polybipyridyls have given access to
NaHSO . The crystalline pyridine N-oxide was treated
3
the supramolecular systems of J.-M. Lehn and others. The
alteration of pyridyl units, spacers and metals resulted in
many new and fascinating supramolecules. In particular,
optically active transition metal complexes of 2,2′-bipy-
ridyls are of high interest, both for reversible optical data
storage and enantioselective organic transformations. In
spite of their potential for asymmetric induction in C–C
with 10 equivalents of phosphoryl chloride at room tem-
perature, leading to two isomeric chloronicotines 3 (27%)
and 4 (20%). However, addition of 3.4 equivalents of
diisopropylamine14 suppresses the formation of the un-
desired 2-chloronicotine beyond observation (<1%), rais-
ing the yield of the desired isomer to 38%. Replacement
of diisopropylamine by other secondary amines, such as
piperidine and pyrrolidine, met with no success. The for-
mation of the sterically more demanding 3 is explained by
directing effects of the pyrrolidine nitrogen. An undefined
6
bond formation or oxidation processes, the number of
publications describing enantiomerically pure species re-
mained comparatively few. The most recent examples7
were published by S. Sakaki, who is interested in cop-
per(I) based chiral photosensitizers, and A. von Zelewsky,
who coined the term: super chiragens. Using the complex-
ammonium phosphoryl species holds the chloride in
place, ready for the Boekelheide1
5, 16
reaction at the 2-po-
8
sition. The competition of secondary amines for phospho-
ryl chloride makes the substitution at the more accessible
es as ligands approach a number of bipyridyls bearing
additional amino functions allow controlled synthesis of
heteropolynuclear complexes, resulting in both aestheti-
cally pleasing assemblies and supramolecules with inter-
esting redox and photochemical properties, In particular,
helical assemblies attract attention, because bistable, non-
6
-position the major reaction pathway. Palladium-cata-
lysed coupling of the chloronicotine 4 and 2-(tributylstan-
nyl)pyridine19 (5) gave the 2,2′-bipyridine 6 (PYNIC) in
5
2% yield with an enantiomeric excess greater than 90%
1
9
as determined by H NMR spectroscopy of the 1:1 salts
with D(–)-tartaric acid and L-(+)-tartaric acid in MeOH-
d4. Additional copper oxide (2 equivalents) raised the
racemic helices may be useful as optical switches in infor-
mation technology. The application as optical device is
based on the helicity and its inversion by circular po-
larised light (CPL).
For these purposes we envisioned ligands with mini-
mised, yet chiral, amino bearing side-chains. Moreover,
we were interested in obtaining epibatidine1
0-12
analogues
for pharmacological testing. To avoid the obstacles of
lengthy synthesis we focused on readily available natural
starting materials and chose 6-chloronicotine18 (4) for our
ligand design, as the chirality and the first pyridyl unit are
already implemented.
The synthetic task was thus reduced to the regioselective
attachment of a second 2-pyridyl unit or a dimerisation to

January 1998
SYNTHESIS
43
yield to 62%. The formal dimerisation17 of (–)-nicotine to
BINIC (7) was achieved by heating 4 in the presence of
palladium and hexabutylditin in 58% yield, which was not
improved by additional copper oxide. A potential epimer-
–)-6-Chloronicotine (4), (–)-2-Chloronicotine18 (3):
(
A solution of nicotine N-oxide (2) (356 mg, 2.00 mmol) in anhyd
CH Cl (3 mL) was treated with POCl (1.0 mL, 10.92 mmol) under
N at 0°C. Additional POCl (1.0 mL, 10.92 mmol) and (iPr) NH
(
2
2
3
2
3
2
0.98 mL, 6.93 mmol) was added over 2 h, Stirring was continued at
isation of the benzylic position by palladium(II) mediated r.t. for 1 h, prior to solvent removal. The oily residue was poured on
ice water, adjusted to pH 9 by solid Na CO (ca. 30 g) and extracted
by EtOAc (3 x 30 mL). The dried, concentrated organic phases were
processes can be excluded for the formation of BINIC (7),
as no meso compound was observed.
2
3
purified by gradient column chromatography (PE/CHCl ) to give 4 as
3
a pink liquid (143.1 mg, 37%); R = 0.64 (CHCl /EtOH 1:1). In the ab-
ƒ
3
sence of (iPr) NH both isomers were isolated in a ratio 3/4 1.4:1. Sep-
2
aration on deactivated Alox (1% H O, PE/CHCl 10:1) gave pure 4
2
3
(
78 mg, 0.40 mmol, 20%) and 3 (106 mg, 0.54 mmol, 27%),
: [a] 3–56.2 (c = 1.47, CHCl3).
2
4
D
1
H NMR (200 MHz. CDCl , TMS, 25 °C): d = 1.57–2.05 (m, 3H),
3
2
.16 (s, 3H, NCH ), 2.03–2.40 (m, 2H), 3.08 (t, J = 8 Hz, l¢-H), 3.23
3
(
ddd, 1H, J = 2 Hz. J = 8 Hz, J = 1 Hz, H-4¢), 7.30 (d, 1H, J = 8 Hz,
4
3
4
H-5), 7.69 (dd, J = 2 Hz, J = 8 Hz, H-4), 8.31 (d, J = 2 Hz, H-2).
1
3
C NMR (50 MHz, CDCl , TMS, 25 °C, APT): d = 22.6 (t, 3¢), 35.3
3
(
1
t, 2¢), 40.3 (q, CH3), 56.9 (t. 4¢), 69.0 (d,1¢), 124.3 (d, 5), 137.9 (d, 4),
38.1 (s, 3), 149.3 (d, 2), 150.0 (s, 6).
Scheme 3
–1
FT-IR: n = 2972s, 2948s, 2876m, 2780s, 1456s, 1104s cm .
MS: m/z (%) = 198 (M [Cl], 8), 196 (M [Cl], 20), 167 (M+ 35[Cl]-
+
37
+ 35
NCH , 12), 84 (C H N, 100).
3
5 10
HRMS: m/z 196.0695137 ([C H N235Cl] calcd 196.0767193).
10 13
The pale yellow palladium dichloro complex 8 (76%) was
obtained in moderate yield by mixing stoichiometric
amounts of dihalide and ligand in acetonitrile, and precip-
itation followed by recrystallisation. The maximum in the
3
:
1
H NMR (200 MHz, CDCl , TMS, 25 °C): d = 1.42–l.95 (m), 1.75–
3
2
.95 (m, 2H). 2.12 (s, 3H, Me), 2.2–2.45 (m, 2H). 3.32 (ddd, J = 8.5
Hz. J = 6 Hz, J = 1.5 Hz, H-1¢), 3.55 (bt, J = 8 Hz, H-4¢), 7,25 (ddd,
3
3
UV spectrum of the free ligand at 242 nm (log e = 5,458) 3J = 8 Hz, J = 4,5 Hz, J = 0.25 Hz, H-5), 7.97 (ddd, J = 8 Hz, J =
3
4
is shifted upon complexation by palladium(II) to 308 and 2 Hz, J = 0.5 Hz, H-4), 8.25 (dd, J = 4.5 Hz, J = 2 Hz, H-6).
13
C NMR (100 MHz, CDCl , TMS, 25 °C): d = 22.7 (t, 3¢), 33.2 (t, 2¢),
3
3
03 nm (log e = 6.151, 6.090) [l
Pd(MeCN) Cl in
max 2 2
4
0.5 (q, CH ), 56.7 (t, 4¢), 66.2 (d. 1¢), 122.8 (d, 5), 136.7 (d, 4), 147.4
3
MeCN: 230 nm (log e = 5.387)]. The off-white mercury
dichloro complex 9 was obtained by the same method, al-
(
d, 6), 150.7 (s, 2).
–1
FT-IR: n = 2972s, 2784s, 1564s, 1408s, 11 16s, 1072s, 804s cm .
though in poorer yield (54%). The metal complexes of MS: m/z (%) = 198 (M [Cl], 4), 196 (M [Cl], 14), 84 (C H N,
+
37
+ 35
5
10
1
00).
PYNIC (6) and BINIC (7) with a variety of cations are
currently under further investigation.
HRMS: m/z 196.0710596 ([C H N235Cl] calcd 196.0767193).
10 13
Melting points (uncorrected): open glass capillaries. Büchi apparatus. (–)-6-(2-Pyridyl)nicotine (6):
1
13
H and C NMR spectra: Bruker WP 200, Bruker AM 400 at 200 6-Chloronicotine (392 mg, 2.00 mmol), Pd(MeCN) Cl (51.9 mg,
2
2
(
50.3) MHz and 400 (100,58) MHz. Chemica1 shifts are reported as 0.200 mmol), dppe (79.7 mg, 0.200 mmol), LiCl (127 m,
19
d values (ppm) downfield from TMS. IR spectra: Perkin Elmer 1710 3.00 mmol), CuO (636 mg, 8.00 mmol). 2-(tributylstannyl)pyridine
–
1
FT and Bruker IFS 25, recorded in n
cm ). Mass spectroscopy: (1.47 g, 4,00 mmol) and anhyd DMF (6 mL) were heated under N to
2
max.(
Finnigan MAT 312, VG Autospec (FAB. HRMS). Elemental anal- 100°C for 3 d. The solvent was removed in vacuo. The mixture was
ysis: Elementar Vario EL, Heraeus Elementaranalysator CHN Rapid. diluted by CH Cl (30 mL) and extracted by 1 N HCl (3 ´ 10 mL).
UV/VIS spectra: Beckmann 3600, Optical activity: Perkin–Elmer The aqueous extracts were treated with NaOH (1.6 g, 40 mmol) and
2
2
2
6
41, l = 589 nm. Column chromatography: Baker silica gel 60 (40– extracted by CH Cl2 (5 ´ 20 mL). The dried (MgSO ) and con-
2 4
0 mm) and Sephadex LH 20, using Et O, EtOAc, PE (light petro- centrated extracts left a crude oil, which was purified by gradient col-
2
leum, bp 40–60°C), or CHCl . TLC was carried out using aluminium umn chromatography on deactivated Alox N (PE/CHCl 1:1, CHCl /
3
3
3
20
sheets precoated with silica gel 60 F254 (0.2 mm, E, Merck). Spots EtOH 5:1) to give 6 as yellow oil (292 mg, 62%); [a] –2.98 (c =
D
were visualised by UV and/or spraying with an acidic, ethanolic so- 0.265, EtOH). The enantiomeric excess was higher than 90% as de-
1
lution of p-anisaldehyde, KMnO in acetone or an ethanolic solution termined by H NMR spectroscopy of the 1:1 salts with D(–)-tartaric
4
of ninhydrin, followed by heating. DMF, DMSO, CHCl and CH Cl
were stored over 3Å molecular sieves.
acid and L-(+)-tartaric acid in MeOH-d
4.
3
2
2
1
H NMR (400 MHz, CDCl , TMS, 25 °C): d = 0.9 (m), 1.7–1.9 (m,
3
2
H), 2.0–2.1 (m), 2.2–2.4 (m), 3.2 (t), 3.3 (t), 7.3 (ddd, J = 1.1 Hz, J
(
–)-Nicotine N-Oxide (2):
= 4.8 Hz, J = 7.9 Hz. H-5¢), 7.81 (ddd, J = 1.8 Hz, J = 7.5 Hz, J = 7.9
13
Prepared on a 42 mmol scale according to the literature replacing Hz, H-4¢). 7.81 (dd. J = 2.0 Hz, J = 8.1 Hz, H-4), 8.37 (d, J = 7.9 Hz,
gaseous SO by 40% NaHSO solution (15 mL). HOAc (18 mL) and H-3), 8.38 (ddd, J = 1.1 Hz, J = 1. 1 Hz, J = 7.9 Hz, H-3), 8.60 (d, J =
2
3
H O (10 mL) at 0°C. The reduction was stopped after 30 min by evap- 1.8 Hz, H-6), 8.68 (ddd, J = 0.9 Hz, J = 1.8 Hz, J = 4.8 Hz, H-6¢).
2
13
oration under reduced pressure. Solid Na CO was added until pH 9.
C NMR (100 MHz, CDCl , TMS, 25 °C): d = 22.6 (t, 4²), 35.2 (t,
3
2
3
Extraction with CHCl gave a red oil after drying and concentration, 3²), 40.4 (q, l², N-CH ), 57.0 (t, 5²), 68.7 (d, 2²), 120.9 (d, 3), 121.0
3
3
which was purified by column filtration on silica gel (EtOH/CHCl3 (d, 6¢), 123.4 (d, 4¢), 135.8 (d, 4), 136.8 (d, 5¢), 138.9 (s, 5), 149.1 (d,
1
:3) to give a red crystalline mass (5.16 g, 29.0 mmol, 69%): mp 5– 3¢), 149.8 (d, 6), 155.2 (s, 2), 156.2 (s, 2¢).
0°C.
FT-IR: n = 2972s, 2780m, 1588m, 1460s, 1252m cm .
-
1
1
1
+
+
+
H NMR (200 MHz, CDCl , TMS, 25 °C): d = 1.6–2.0 (m, 3H), 2.15– MS: m/z (%) = 239 (M , 19), 224 (M -CH , 2), 210 (M -NCH , 18),
3
3
3
2
2
.40 (m, 5H), 3.09 (t, 1H, J = 8 Hz), 3.16–3.27 (m, 1H), 7.20–7.30 (m, 84 [(C H N), 100].
5 10
H), 8.12 (dt, 1H, J = 5.5 Hz, J = 2 Hz). 8.15–8.20 (m, H-2).
HRMS: m/z 239.1422100 ([C H N ], calcd 239.1422378).
15 17 3
–1
FT-IR: n = 2972s, 2784m, 1604s, 1440s, 1268s, 1136s cm .
UV-VIS (MeCN): l
(log e) = 280 (5.607), 242 (5.458), 233
max
+
MS: m/z (%) = 179 (2), 178 (12, M ), 85 (100),
(5.482) nm.

4
4
Short Papers
SYNTHESIS
Anal. calcd for C H N (239.323): C 75.28, H 7.16, N 17.56; found: 8.06 (dd, J = 2 Hz, J = 8.23 Hz, H-4), 8.19 (ddd, J = 0.5 Hz, J =
15 17 3
C 74.47, H 7.16, N 16.8.
0.5 Hz, J = 8.2 Hz, H-3). 8.21 (ddd, J = 0.9 Hz, J = 0.9 Hz, J = 8.0 Hz,
H-3¢), 8.67 (d, J = 1.5 Hz, H-6), 8.74 (ddd, J = 0.9 Hz, J = 1.7 Hz, J
= 5.2 Hz. H-6¢).
(
(
–)-Bis-6,6¢-nicotine (7):
–)-6-Chloronicotine (686 mg, 3.50 mmol), Pd(PPh3)4 (202 mg, FT-IR (KBr): n = 2964m, 2872w, 2776m, 1592s, 1472s, 1436s,
–1
0
0
2
.175 mmol), hexabutylditin (875 mL, 1.75 mmol), Et N (100 mL. 764s cm .
3
202
35
.72 mmol) and DMF (10 mL) were heated under N to 100°C for MS (95°C): m/z
=
511 (C H N Hg Cl , 4%, 461
15 17 3 2
2
202
35
d. The mixture was diluted by CH Cl (50 mL) and extracted by 1N (C H N
Hg Cl, 6%).
2
2
14 14 3
m
HCl (3 ´ 10 mL). The aqueous extracts were treated by NaOH (1.6 g, UV-VIS (MeCN): l
(log e) = 281 (5.777), 240 (5.993), 236
ax
4
0 mmol) and extracted by CH Cl (5 ´ 30 mL). The dried (MgSO ) (5.938), 210 (5,789) nm.
2 2 4
02
and concentrated extracts were purified by column chromatography Anal. calcd for C152 H N HgCl (510.82): C 35.27, H 3.35, N 8.23;
17
3
2
(
CHCl ) on 10 g silica gel deactivated by 30% NH (1 mL) to give 7 found C 34.87, H 3.26, N 8.04.
3 3
21.5
as colorless needles (329 mg, 58%); mp 122°C; [a]
–22.2 (c =
D
0
,69, EtOH).
1
H NMR (400 MHz, CDCl , TMS, 25 °C): 1.7–1.9 (m. 4H), 2.0–2.1
3
Financial assistance from the Deutsche Forschungsgemeinschaft and
the DAAD/SI is gratefully acknowledged.
(m, 2H), 2.2 (s, 6H, CH ), 2.2–2.3 (m, 2H), 2.3–2.4 (m), 3.2 (t, 2H),
3
3
.3 (t, 2H), 7.85 (dd, 2H, J = 2.0 Hz, J = 8.3 Hz, H-4/4¢), 8.35 (dd, 2H,
J = 8.3 Hz, J = 0.3 Hz, H-5/5¢), 8.6 (d, 2H, J = 1.8 Hz, H-2/2¢).
1
3
C NMR (100 MHz, CDCl , TMS, 25 °C): 27.7 (t, 4¢), 35.3 (t, 3¢),
3
4
4
0.6 (q, 1, N-CH ), 57.4 (t, 5¢), 69.2 (d, 2¢), 121.6 (d, 3), 136.6 (d,
), 139.5 (s, 5), 149.8 (d, 6), 156.3 (s, 2).
3
(1) Hanan, G. S.; Lehn, J.-M.; Kyritsakas, N.; Fischer, J. J. Chem.
Soc., Chem. Commun. 1995, 765.
-
1
FT-IR: n = 2972s, 2780s, 1596m, 1464s, 1252m cm .
FAB-MS: m/z = 323 (C H N ).
UV-VIS (CH Cl ): l (log e) = 290 (5.990), 244 (5.827) nm.
Anal. calcd for C H N (322.45): C 74.498. H 8.127, N 17.375;
(2) Goulle, V.; Harriman, A.; Lehn, J.-M. J. Chem. Soc., Chem.
Commun, 1993, 1034.
1
5 18 3
2
2
max
(3) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim,
1995.
20 26 4
found: C 74.17, H 8.12, N 17.20.
(4) Pfeil, A.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1992,
[(–)-6-(2-Pyridyl)nicotine]palladium Dichloride (8):
838.
Pd(MeCN) Cl (20.0 mg, 77 mmol) was dissolved in anhyd MeCN
(5) Sleiman, H.; Baxter, P.; Lehn, J.-M.; Rissanen, K. J. Chem.
Soc., Chem. Commun, 1995, 715.
2
2
(
3 mL). A solution of 6-(2-pyridyl)nicotine (170 mmol) in cyclohex-
ane (1.20 mL) was layered carefully over the MeCN phase. After an
initial growing period of 30 min, the mixture was stirred for 1 h. The
yellow precipitate was digested by MeCN (2 ´ 3 mL) at 50°C for 2 h.
The solvent was decanted; prolonged drying in vacuo gave the pale
(6) Chen, C.; Tagami, K.; Kishi, Y. J. Org. Chem. 1996, 60, 5386.
(7) Sakaki, S.; Ishikura, H.; Kuraki, K.; Tanaka, K.; Satoh, T.; Arai,
T.; Hamada, T. J. Chem. Soc. Dalton. Trans. 1997, 1815.
Fletcher, N. C.; Keene, F. R.; Ziegeler, M.; Stoeckli-Evans, H.;
Viebrock, H.; von Zelewsky, A. Helv. Chim. Acta 1996, 79,
1192.
(8) Denti, G.; Serroni, S.; Campagna, S.; Juris, A.; Ciano, M.;
Balzani, V. In Perspectives in Coordination Chemistry; Willi-
ams, A. F.; Floriani, C.; Merbach, A. E. Eds.; VCH: Basel,
1992, p 153.
(9) Huck, N. P. M.; Jager, W. F.; de Lange, B.; Feringa, B. L. Sci-
ence 1996, 273, 1686.
(10) Spande, T. F.; Garraffo, H. M.; Edwards, M. W.; Yeh, H. J. C.;
Pannell, L.;Daly, J. W. J. Am. Chem. Soc. 1992, 114, 3475.
(11) Dehmlow, E. V. J. Prakt. Chem. 1995, 337, 167.
yellow complex 8 (41 mg, 76%).
1
H NMR (400 MHz, DMSO-d , TMS, 25 °C): d = 0.7–0.8 (m), 0.9–
6
1
6
.05 (m, 2H), 1.2 (s), 1.28 (s, 3H), 1.35–l.5 (m, 2H), 2.3–2.4 (m),
.93 (ddd, J = 1.1 Hz, J = 4 Hz, J = 7.2 Hz, H-5¢), 7.41 (dd, J =
l.7 Hz, J = 8.1 Hz, H-4), 7.48 (ddd, J = 1.7 Hz, J = 7.9 Hz, J = 7.9 Hz,
H-4¢), 7.68 (d, J = 7.9 Hz, H-3), 7.70 (bd, H-3¢), 8.24 (d, 2H, J = 4 Hz,
H-6/6¢).
–1
FT-IR (KBr): n = 2940m, 2872m, 2780m, 1464s, 1444s cm .
106
FAB-MS: m/z = 380 (C H N Pd Cl).
15
17
3
35
UV-VIS (MeCN): l
(log e) = 318 (6.205), 308 (6.151), 303
max
(
6.090), 262 (6.248), 252 (6.235) nm.
Anal. calcd for C H N PdCl (416.83): C 43.24, H 4.11, N 10.09; (12) Müller, C. E. Pharmazie in unserer Zeit 1996, 25, 85.
15
17
3
2
found C 41.99, H 4.12, N 9.18.
(13) Taylor, E. C.; Boyer, N. E. J. Org. Chem. 1959, 24, 27.
14) Zoltewicz. J. A.; Cruskie Jr M. P. Tetrahedron 1995, 51, 11401.
(15) Zoltewicz, J. A.; Cruskie Jr M. P. Tetrahedron 1995, 51, 3103.
HgCl (43 mg, 0.158 mmol) was dissolved in abs MeCN (2.0 mL) and (16) Fontenas, C.; Bejan, E.; Ait Haddou, H; Balavoine, G. G. A.
(
[(–)-6-(2-Pyridyl)nicotine]mercury Dichloride (9):
2
added to 6-(2-pyridyl)nicotine (74 mg, 0.31 mmol) under Ar resulting
in the immediate formation of an off-white precipitate. The suspen- (17) The direct coupling of nicotine, using sodium or Raney nickel,
sion was stirred at 50°C for 18 h and cooled to r.t. to complete precip-
is subject of current investigations.
itation. The solvent was decanted and the residue was washed by (18) Gol’dfarb, Y. L.; Godovikowa, S. N. Bull. Acad. Sci. USSR,
MeCN (3 ´ 4 mL), dried in vacuo to give 9 (42 mg, 54%).
Div. Chem. Sci. 1961, 331.
H NMR (200 MHz, CDCl , TMS, 25 °C): d = 1.7–2.1 (m, 4H), 2.2 (19) McWhinnie, W. R.; Poller, R. C.; Thevarasa, M. J. Organomet.
Synth. Commun. 1995, 25, 629.
1
3
(s, 3H), 2.2–2.4 (m), 3.3 (t, 2H), 7.63 (ddd, J = 1.2 Hz, J = 5.2 Hz, J
Chem. 1968, 11, 499.
=
7.5 Hz, H-5¢), 8.03 (ddd, J = 1.7 Hz, J = 7.5 Hz, J = 8.32 Hz, H-4¢), (20) Kalinin, V. N. Synthesis 1992, 413.