Anellation and ring transformations of push-pull-functionalized deoxypyranosiduloses

Article abstract of DOI:10.1515/znb-2009-0613

Reaction of (E)-3-aminomethylene-α-D-erythro-hexopyranosid-2-ulose 5 with substituted 5-aminopyrazoles afforded the pyrano-anellated pyrazolo[1,5-a]pyrimidines 8. The treatment of the corresponding (E)-2-aminomethylene-α-D-erythro-hexopyranosid-3-ulose 6

Full text of DOI:10.1515/znb-2009-0613

Anellation and Ring Transformations of Push-pull-functionalized
Deoxypyranosiduloses
Marcus Kordian, Holger Feist, and Klaus Peseke
Institut Chemie, Universita¨t Rostock, Albert-Einstein-Str. 3a, 18051 Rostock, Germany
Reprint requests to Dr. H. Feist. Fax: +49(381)4986412. E-mail: holger.feist@uni-rostock.de
Z. Naturforsch. 2009, 64b, 676 – 682; received March 11, 2009
Dedicated to Professor Gerhard Maas on the occasion of his 60th birthday
Reaction of (E)-3-aminomethylene-α-D-erythro-hexopyranosid-2-ulose 5 with substituted 5-
aminopyrazoles afforded the pyrano-anellated pyrazolo[1,5-a]pyrimidines 8. The treatment of the
corresponding (E)-2-aminomethylene-α-D-erythro-hexopyranosid-3-ulose 6 with 5-aminopyrazoles
and (benzimidazol-2-yl)acetonitrile yielded in a ring transformation process the D-erythronoyl-pyraz-
olo[1,5-a]pyrimidine-3-carbonic acid derivatives 10 and D-erythronoyl-pyrido[1,2-a]benzimidazole-
4-carbonitrile (12), respectively.
Key words: Nucleoside Analogs, Push-pull Alkenes, Enaminones, Pyrazolo[1,5-a]pyrimidines,
Pyrido[1,2-a]benzimidazole, Ring Transformations
Introduction
group and the anomeric position of glycals. Substitu-
tion of the ring oxygen atom resulted in the ring cleav-
age of the cyclic carbohydrates.
Anellated cyclic monosaccharide derivatives have
attracted great interest in organic synthesis [1 – 3]
because of their biological importance, for exam-
ple as antibiotics and cancerostatics [4 – 7], or as in-
hibitors of different glycosidases [8, 9]. Furthermore,
the natural products bengazol A and (–)-biopterin
are representatives of biologically interesting acyclic
C-nucleosides [10, 11]. Natural as well as synthetic
acyclic nucleoside analogs [12– 17] have shown
antiviral activities against herpes virus [18], HIV
and SIV (simian immunodeficiency virus) [19], and
vaccinia [20]. The interesting properties of these
compounds encouraged scientists to develop ap-
proaches to the synthesis of other anellated nucle-
oside analogs using the push-pull functionalization
Bredereck et al. reported the reaction of ketones
with acetals of amides [30]. In this way, ulose 1
was reacted with N,N-dimethylformamide dimethyl-
acetal and bis(dimethylamino)tert-butoxymethane, re-
spectively, in tetrahydrofuran to furnish the (E)-methyl
4,6-O-benzylidene-3-deoxy-3-dimethylaminomethyl-
ene-α-D-erythro-hexopyranosid-2-ulose (5) with an
integrated push-pull alkene unit (Scheme 1). The re-
action of compound 5 with hydrazine hydrate and
amidines involved the substitution of the dimethyl-
amino group, and the attack on the carbonyl group
yielding a pyrano[3,4-c]pyrazole and the correspond-
ing pyrimido anellated pyranosides, respectively [23].
Similarly, the treatment of 3-ulose 2 with N,N-
of methyl 4,6-O-benzylidene-3-deoxy-α-D-erythro- dimethylformamide dimethylacetal afforded the
hexopyranosid-2-ulose(1) and methyl 4,6-O-benzylid- (E)-methyl 4,6-O-benzylidene-2-deoxy-2-dimethyl-
ene-2-deoxy-α-D-erythro-hexopyranosid-3-ulose (2), aminomethylene-α-D-erythro-hexopyranosid-3-ulose
respectively [21 – 26] (Scheme 1).
(6). Reaction of 6 with methylhydrazine and amidines
Acyclo-C-nucleoside analogs were prepared by yielded pyrano[4,3-c]pyrazole and pyrano[4,3-d]-
ring transformation reactions of push-pull function- pyrimidines, respectively [31]. On the other hand,
alized 2-formylglycals 3 and 4 with hydrazines, the hexopyranosidulose 6 is comparable with the
amidines, 2-aminobenzimidazole, 3-amino-2H-1,2,4- 2-formylglycals 3 and 4 because the displacement of
triazole, and 5(3)-amino-pyrazole-4-carboxylates[27 – the dimethylamino and methoxy group by binucleo-
29]. The mechanism of this ring transformation in- philes should afford the same products in a ring
volved the attack of the dinucleophiles at the formyl transformation process.
c
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M. Kordian et al. · Anellation and Ring Transformations of Push-pull-functionalized Deoxypyranosiduloses
677
Scheme 1. Push-pull-functionalized
uloses 5, 6 and 2-formylglycals 3, 4.
Scheme 2. Synthesis of pyranoanellated
pyrazolo[1,5-a]pyrimidines 8a, b.
In this paper we report the reaction of the uloses as a base for abstracting the protons of the attacking
5 and 6 with aminopyrazoles and (benzimidazol-2- nucleophile.
In the 13C NMR spectra of compounds 8 the ex-
yl)acetonitrile which was carried out in order to syn-
thesize new nucleoside analogs by anellation and ring
transformation reactions, respectively.
pected signals of the carboxylate and cyano groups, re-
spectively, were visible. Furthermore, an upfield shift
to δ = 138.2 and 138.4 for C-6a in 8a and 8b, respec-
tively (the corresponding chemical shift of the C-2 in
compound 5 is δ = 187.9), and the absence of signals
for the dimethylamino group proved the cyclization
through substitution of the dimethylamino group and
Results and Discussion
Pyrazolo[1,5-a]pyrimidine-C-nucleosides have
shown activities against various cancer cell lines and
were used also for other biological tests [32]. In order
to synthesize the corresponding pyranoanellated pyr-
azolo[1,5-a]pyrimidines 8 we utilized substituted 5(3)-
amino-pyrazole-4-carbonic acid derivatives 7 [33 – 35]
as binucleophiles in the reaction with the hexopyran-
1
condensation with the carbonyl group. The H NMR
spectra of 8a and 8b showed the signals for 12-H at
δ = 8.74 and 8.53, respectively, typical for hydrogen
bound to an sp2 carbon atom.
Heterocyclization of ulose 6 with the substituted
osid-2-ulose 5 (Scheme 2). Sodium hydride was used 5(3)-amino-pyrazole-4-carbonic acid derivatives 7 in
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M. Kordian et al. · Anellation and Ring Transformations of Push-pull-functionalized Deoxypyranosiduloses
Scheme 3. Reaction of ulose 6 with
aminopyrazoles 7.
dimethylformamide at −15 ◦C in the presence of tively, with coupling constants 3JOH−5ꢀ = 5.80 Hz con-
potassium carbonate/18-crown-6 yielded not the corre- firmed the structure 10. Furthermore, the absence of
sponding pyranoanellated pyrazolo[1,5-a]pyrimidines the signal for the anomeric methoxy group in the 13C
9 but, in a ring transformation process, the pyrazolo and 1H NMR spectra confirmed the postulated reaction
[1,5-a]pyrimidines 10 with an open-chain monosac- course. In addition, the carbonyl bands in the IR spec-
charidic unit (Scheme 3). The first step of the reac- tra and corresponding signals in the 13C NMR spectra
tion appears to be the attack of the exocyclic pyra- of compounds 10a, b at δ = 191.9 and 191.6, respec-
zole amino group at C-1ꢀ of 6 to yield the intermediate tively, proved that the carbonyl group of the hexopyr-
hexopyranosidulose I which reacts easily under elimi- anosidulose 6 was not involved in the cyclization reac-
nation of methanol to furnish the glycal II. The endo- tion.
cyclic double bond of this unsaturated monosaccharide
is push-pull-functionalized because of the substitution
with the electron donating ring oxygen atom and the
electron attracting imino function. Therefore, the in-
tramolecular substitution of the ring oxygen by attack
of the ring NH group of the pyrazole unit can yield the
From the literature, nucleoside analogs with an im-
idazo[1,2-a]pyridine skeleton are known and show
antiviral activity [36, 37]. Starting with compound 6
we tried to prepare the corresponding polycyclic C-
nucleoside analog by reaction with (benzimidazol-2-
yl)acetonitrile in N,N-dimethylformamide in the pres-
ence of potassium carbonate as a base. However, the
pyrazolo[1,5-a]pyrimidines 10.
1
The presence of OH signals in the H NMR spec- expected anellation reaction of (benzimidazol-2-yl)-
tra of 10a,b as doublets at δ = 5.58 and 5.55, respec- acetonitrile including condensation with the carbonyl
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M. Kordian et al. · Anellation and Ring Transformations of Push-pull-functionalized Deoxypyranosiduloses
679
Scheme 4. Reaction of ulose 6 with
(benzimidazol-2-yl)acetonitrile.
group of 6 and substitution of the dimethylamino group Experimental Section
to furnish the polycyclic compound 11 did not take
place. The signal of the carbonyl group was retained
at δ = 191 in the 13C NMR spectrum of the product.
Therefore, we had to postulate a reaction course which
could first afford the hexopyranosidulose III through
substitution of the dimethylamino group (Scheme 4).
This compound then could undergo elimination of
General
Solvents were distilled and if necessary dried using stan-
dard procedures. TLC was carried out on silica gel 60 GF254
(Merck) with detection by UV light (λ = 254 nm) and/or by
charring with 10 % sulfuric acid in methanol. Silica gel 60
(63 – 200 mesh) (Merck) was used for column chromatog-
methanol to yield the branched-chain glycal IV pos- raphy. Melting points were determined by using a Boetius
melting point apparatus and are corrected. Specific rota-
tions were determined with a Gyromat-HP instrument (Dr.
Kernchen Ltd.). IR spectra were recorded with a Nicolet
205 FT-IR spectrometer. 1H NMR spectra (250.13 MHz and
300.13 MHz, respectively) and 13C NMR spectra (62.9 MHz
and 75.5 MHz, respectively) were recorded on Bruker instru-
ments AC 250 and ARX 300, with CDCl3 or [D6]DMSO
as solvent. The calibration of spectra was carried out on
solvent signals [δ (1H, CDCl3) = 7.25; δ (13C, CDCl3) =
sessing a butadiene unit with push-pull functional-
ity (butadiene with the electron donating ring oxygen
atom and the electron attracting imino function). In-
termediate IV was not isolable because of the imme-
diately following intramolecular nucleophilic substitu-
tion by attack of the NH group at C-1 of the sugar ring
resulting in a ring transformation reaction to afford the
pyrido[1,2-a]benzimidazole-carbonitrile 12.
Structure 12 was supported by the OH signal at
1
77.0; [D6]DMSO: δ (1H) = 2.50; δ (13C) = 39.7]. The H
1
δ = 5.6 (3JOH−5 = 5.8 Hz) in the H NMR spec-
and 13C NMR signals were assigned by DEPT and two-
dimensional 1H,1H-COSY and 1H,13C correlation spectra.
The mass spectra were recorded on an AMD 402/3 spec-
trometer (AMD Intectra GmbH). Elemental analyses were
performed on a Leco CHNS-932 instrument.
ꢀ
trum. Furthermore, the couplings found over four
bonds between 1-H and 3-H due to a typical W-
coupling (1.2 Hz) confirmed the successful ring trans-
formation.
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M. Kordian et al. · Anellation and Ring Transformations of Push-pull-functionalized Deoxypyranosiduloses
Ethyl (2R,4aR,6S,12bS)-4a,12b-dihydro-6-methoxy -9-meth- Ethyl 6-(2,4-di-O-benzylidene-D-erythronoyl)-2-methyl-
ylsulfanyl-2-phenyl-4H,6H-[1,3]-dioxino[4ꢀ,5ꢀ:5,6]pyrano-
[3,4-d]pyrazolo[1,5-a]pyrimidine-8-carboxylate (8a)
sulfanyl-pyrazolo[1,5-a]pyrimidine-3-carboxylate (10a)
A solution of◦6 (319 mg, 1.0 mmol) in DMF (5 mL) was
cooled to −15 C. While stirring under argon atmosphere
potassium carbonate (830 mg, 6 mmol), 18-crown-6 (1.32 g,
5 mmol) and 7a (402 mg,◦2.0 mmol) were added. The suspen-
sion was stirred at −15 C until complete conversion (TLC
control). Then the reaction mixture was added to an ice/water
mixture and the product precipitated from the solution. The
solid was filtered and the aqueous phase extracted several
times with dichloromethane. The organic phases were com-
bined and dried with Na2SO4. After removal of the solvent
the combined solids were purified by column chromatogra-
phy (toluene/ethyl acetate = 1 : 1). For further purification
the product was recrystallized from ethyl acetate. – Yield
A solution of compound 5 (319 mg, 1.0 mmol) in
abs. THF (30 mL) was cooled to 0 ◦C. At this temper-
ature ethyl 5(3)-amino-3(5)-methylsulfanyl-1H-pyrazole-4-
carboxylate (7a, 402 mg, 2.0 mmol) and NaH (60 %; 150 mg,
3.75 mmol) were added. The solution was allowed to warm
◦
to 22 C and stirred until completion of the reaction (TLC-
control). Then ice water (100 mL) was added, the aque-
ous phase was extracted with dichloromethane (3 × 40 mL)
and dried with Na2SO4. Removal of the solvent under re-
duced pressure and purification of the residue by column
chromatography (toluene/ethyl acetate = 3 : 1) gave the pure
compound 8a. – Yield 180 mg (40 %, amorphous powder). –
[α]2D1 = +42.6 (c = 1.0, CHCl3). – Rf = 0.7 (toluene/ethyl ac-
etate = 1 : 1). – 1H NMR (250 MHz, CDCl3): δ = 8.74 (s,
1 H, 12-H), 7.49 – 7.33 (5 H, Ph), 5.81 (s, 1 H, 2-H), 5.73 (s,
112 mg (22 %, solid). – M. p. 211 – 213.5 ◦C. – [α]D22
=
−12.4◦ (c = 1.8, DMSO). – Rf = 0.3 (toluene/ethyl acetate =
1 : 1). – 1H NMR (250 MHz, [D6]DMSO): δ = 9.97 (d,
3
4
4
1 H, 6-H), 4.78 (d, 1 H, J12b−4a = 9.2 Hz, 12b-H), 4.44 –
1 H, J7−5 = 2.1 Hz, 7-H), 9.20 (d, 1 H, J5−7 = 2.1 Hz,
4.23 (m, 4 H, 4a-H, 6-H, CH2CH3), 3.93 (t, 1 H, 2J4ax−4eq
=
5-H), 7.49 – 7.37 (5 H, Ph), 5.87 (s, 1 H, 2ꢀ-H), 5.58 (d, 1 H,
3J4ax−4a = 10.1 Hz, 4ax-H), 3.69 (s, 3 H, OMe), 2.57 (s,
3 H, SMe), 1.36 (t, 3 H, 3J = 7.0 Hz, CH2CH3). – 13C NMR
(125.7 MHz, CDCl3): δ = 162.7 (C=O), 160.4 (C-9), 149.0
(C-7a), 148.7 (C-12), 138.2 (C-6a), 136.7, 129.4, 128.4,
126.2 (Ph), 115.9 (C-12a), 102.3 (C-2), 100.4 (C-6), 94.3
(C-8), 73.4 (C-12b), 68.9 (C-4), 64.4 (C-4a), 60.5 (CH2
CH3), 57.9 (OCH3), 14.5 (SMe), 13.4 (CH2CH3). – MS
(EI, 70 eV): m/z (%) = 457 (100) [M]+. – C22H23N3O6S
(457.50): calcd. C 57.76, H 5.07, N 9.18, S 7.01; found
C 57.66, H 5.01, N 9.07, S 6.74.
JOH−5 = 5.8 Hz, OH), 5.28 (d, 1 H, 3J4 −5 = 9.1 Hz, 4ꢀ-H),
3
ꢀ
ꢀ
ꢀ
3
4.31 (q, 2 H, JCH CH = 7.0 Hz, CH2CH3), 4.25 (dd, 1 H,
2
2
33
J6 ax−6 eq = 10.7 Hz, J5−6 eq = 5.2 Hz, 6ꢀeq-H), 3.94 (m,
ꢀ
ꢀ
ꢀ
1 H, 5ꢀ-H), 3.73 (t, 1 H, J6 ax−6 eq
= J5 −6 ax = 10.7 Hz,
2
3
ꢀ
ꢀ
ꢀ
ꢀ
6ꢀax-H), 2.60 (s, 3 H, SMe), 1.32 (t, 3 H, 3JCH CH = 7.0 Hz,
CH2CH3). – 13C NMR (75.5 MHz, [D6]DMSO):3δ = 191.9
(C=O), 162.4 (COOEt), 161.9 (C-2), 149.2 (C-5), 137.8
(C-7), 129.3, 128.5, 126.6 (Ph), 118.5 (C-3a), 100.4 (C-2ꢀ),
80.7 (C-4ꢀ), 71.1 (C-6ꢀ), 62.1 (C-5ꢀ), 60.3 (CH2CH3), 14.8
(SMe), 13.4 (CH2CH3). – MS (EI, 70 eV): m/z (%) = 443
(1) [M]+. – C21H21N3O6S (443.47): calcd. C 56.87, H 4.77,
N 9.48, S 7.23; found C 56.56, H 4.80, N 9.38, S 7.68.
2
(2R,4aR,6S,12bS)-9-Amino-4a,12b-dihydro-6-methoxy-2-
phenyl-4H,6H-[1,3]dioxino[4ꢀ,5ꢀ:5,6]pyrano[3,4-d]pyr-
azolo[1,5-a]pyrimidine-8-carbonitrile (8b)
2-(p-Anisidino)-6-(2,4-di-O-benzylidene-D-erythronoyl)
pyrazolo[1,5-a]pyrimidine-3-carbonitrile (10b)
Compound 5 (319 mg, 1.0 mmol) was reacted with 3,5-
diamino-1H-pyrazole-4-carbonitrile (7b, 181 mg, 2.0 mmol)
as described for the preparation of 8a. After removal of the
solvent the residue was purified by column chromatography
(toluene/ethyl acetate = 1 : 1). – Yield 265 mg (70 %, col-
Compound 6 (319 mg, 1.0 mmol) was reacted with
5(3)-amino-3(5)-(p-anisidino)-1H-pyrazol-4-carbonitrile
(7c, 458 mg, 2.0 mmol) as described for the preparation of
10a. After removal of the solvent the residue was purified
by column chromatography (ethyl acetate/chloroform =
5 : 1). – Yield 87 mg (18 %, solid). – M. p. > 360 ◦C. –
[α]2D4 = −24.5◦ (c = 0.8, DMSO). – Rf = 0.5 (ethyl acetate/
chloroform = 5 : 1). – 1H NMR (250 MHz, [D6]DMSO): δ =
◦
orless needles). – M. p. 303 C. – [α]2D1 = +52.3 (c = 1.0,
DMSO). – Rf = 0.4 (toluene/ethyl acetate = 1 : 1). – 1H NMR
(250 MHz, [D6]DMSO): δ = 8.53 (s, 1 H, 12-H), 7.55 – 7.38
(5 H, Ph), 6.90 (s, 2 H, NH2), 5.95 (s, 1 H, 2-H), 5.84 (s,
1 H, 6-H), 4.98 (d, 1 H, 3J12b−4a = 8.9 Hz, 12b-H), 4.39 (dd,
2
3
4
1 H, J4eq−4ax = 8.9 Hz, J4eq−4a = 3.4 Hz, 4eq-H), 4.12 –
9.76 (d, 1 H, J7−5 = 2.1 Hz, 7-H), 9.61 (s, 1 H, NH), 9.41
3.98 (m, 2 H, 4a-H, 4ax-H), 3.58 (s, 3 H, OMe). – 13C NMR (d, 1 H, 4J5−7 = 2.1 Hz, 5-H), 7.62 (d, 2 H, Ar), 7.43 – 7.34
(62.9 MHz, [D6]DMSO): δ = 161.8 (C-9), 150.9 (C-7a), (5 H, Ph), 6.91 (d, 2 H, Ar), 5.81 (s, 1 H, 2ꢀ-H), 5.55 (d, 1 H,
3
ꢀ
ꢀ
ꢀ
147.1 (C-12), 138.4 (C-6a), 137.4, 129.3, 128.4, 126.5 (Ph),
JOH−5 = 5.8 Hz, OH), 5.22 (d, 1 H, 3J4 −5 = 9.2 Hz, 4ꢀ-H),
2 3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
116.1 (C-12a), 113.9 (CN), 101.3 (C-2), 93.3 (C-6), 72.5 4.22 (dd, 1 H, J6 ax−6 eq = 10.7 Hz, J5 −6 eq = 5.2 Hz,
(C-12b), 67.9 (C-4), 66.8 (C-8), 64.1 (C-4a), 56.8 (OCH3). – 6ꢀeq-H), 3.93 (m, 1 H, 5ꢀ-H), 3.73 (s, 4 H, OMe, 6ꢀax-H). –
MS (EI 70 eV): m/z (%) = 379 (36) [M]+. – C19H17N5O4 13C NMR (75.5 MHz, [D6]DMSO): δ = 191.6 (C=O),
(379.37): calcd. C 60.15, H 4.52, N 18.46; found C 60.59, 158.9 (C-2), 155.3 (C-6), 152.1 (C-5), 139.8 (C-7), 137.9,
H 5.02, N 17.89.
133.4, 129.6, 128.5, 126.6, 121.3, 114.4 (Ar, Ph), 118.8
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681
3
(C-3a), 113.2 (CN), 100.5 (C-2ꢀ), 80.7 (C-4ꢀ), 71.1 (C-6ꢀ), 1 H, J9−8 = 8.6 Hz, 9-H), 7.57 (m, 2 H, 7-H, 8-H), 7.45 –
3
70.3 (C-3), 62.2 (C-5ꢀ), 55.6 (OMe). – MS (EI 70 eV): 7.30 (5 H, Ph), 5.87 (s, 1 H, 2ꢀ-H), 5.56 (d, 1 H, JOH−5
=
ꢀ
m/z (%) = 471 (30) [M]+. – C25H21N5O5 (471.46): calcd. 5.8 Hz, OH), 5.31 (d, 1 H, J4 −5 = 9.2 Hz, 4ꢀ-H), 4.26
3
ꢀ
ꢀ
2
ꢀ
ꢀ
ꢀ
2
ꢀ
C 63.69, H 4.49, N 14.85; found C 63.45, H 4.31, N 14.01.
(dd, 1 H, J6 ax−6 eq = 10.4 Hz, 3J5 −6 eq = 5.2 Hz, 6ꢀeq-H),
3
3.97 (m, 1 H, 5ꢀ-H), 3.73 (t, 1 H, J6 ax−6 eq
= J5 −6 ax =
ꢀ
ꢀ
ꢀ
ꢀ
10.4 Hz, 6ꢀax-H). – 13C NMR (62.9 MHz, [D6]DMSO): δ =
91.2 (C=O), 145.3 (C-4a), 144.7 (C-5a), 137.7 (i-Ph), 137.1
(C-7), 136.8 (C-1), 129.8 (C-3), 129.2, 128.4, 126.4 (Ph),
127.8 (C-7), 123.6 (C-2), 120.1 (C-6), 119.4 (C-9a), 115.2
(CN), 113.5 (C-9), 100.4 (C-4), 100.3 (C-2ꢀ), 80.2 (C-4ꢀ),
71.0 (C-6ꢀ), 61.9 (C-5ꢀ). – MS (EI 70 eV): m/z (%) = 399
(44) [M]+. – C23H17N3O4 (399.40): calcd. C 69.17, H 4.29,
N 10.52; found C 68.78, H 4.02, N 10.13.
2-(2,4-Di-O-benzylidene-D-erythronoyl)pyrido[1,2-a]-
benzimidazole-4-carbonitrile (12)
Compound 6 (319 mg, 1.0 mmol) was reacted with (benz-
imidazol-3-yl)acetonitrile (320 mg, 2 mmol) as described
for the preparation of 10a. After removal of the solvent the
residue was purified by column chromatography (toluene/
ethyl acetate = 1 : 2). – Yield 63 mg (16 %, yellow solid). –
◦
M. p. 226 C. – [α]D25 = −75.9 (c = 1.5, CHCl3). – Rf =
Acknowledgement
0.5 (toluene/ethyl acetate = 1 : 2). – 1H NMR (250 MHz,
[D6]DMSO): δ = 10.10 (s, 1 H, 1-H), 8.69 (d, 1 H, 4J3−1
=
We thank the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for financial support.
3
1.2 Hz, 3-H), 8.53 (d, 1 H, J6−7 = 8.9 Hz, 6-H), 7.92 (d,
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