Benzoanalogous congeners of streptazolin

Article abstract of DOI:10.1007/PL00013505

Streptazolin, a unique natural compound with antibiotic and antifungal activities, is based on a hexahydro-1H-1-pyrindine system containing an internal urethane unit and an exocyclic ethylidene side chain. The diene system, which represents an important structural feature for biological activity, is also responsible for the propensity of streptazolin to polymerize upon concentration from organic solutions. The formal annulation of an aromatic ring under preservation (and prolongation) of the diene system leads to congeners with enhanced stability but reduced antibiotic activity.

Full text of DOI:10.1007/PL00013505

Monatshefte fuÈr Chemie 129, 967±974 (1998)
Benzoanalogous Congeners of Streptazolin
Ã
Martin Kratzel and Alexander Weigl
Institute of Pharmaceutical Chemistry, University of Vienna, A-1090 Vienna, Austria
Summary. Streptazolin, a unique natural compound with antibiotic and antifungal activities, is based
on a hexahydro-1H-1-pyrindine system containing an internal urethane unit and an exocyclic
ethylidene side chain. The diene system, which represents an important structural feature for
biological activity, is also responsible for the propensity of streptazolin to polymerize upon
concentration from organic solutions. The formal annulation of an aromatic ring under preservation
(and prolongation) of the diene system leads to congeners with enhanced stability but reduced
antibiotic activity.
Keywords. Streptazolin; Benzoanalogues; Antibiotic activity; Antifungal activity.
Benzoanaloga des Streptazolins
Zusammenfassung. Das antibiotisch und antifungal wirksame Streptazolin besitzt eine fuÈr einen
È
Naturstoff auûergewohnliche Struktur. Diese basiert auf einem Hexahydro-1H-1-pyrindin-System,
das als weitere Strukturmerkmale eine endocyclische Urethangruppierung und eine exocyclische
È È
Ethylidenfunktionalitat aufweist. Das fur die biologische Aktivitat essentielle Diensystem zeichnet
È
jedoch auch fuÈr die Polymerisationsneigung der Verbindung verantwortlich, die zu unwirksamen
È
Polymeren fuÈhrt. Die formale Anellierung eines Aromaten unter Erhalt (und Verlangerung) des
È È
Diensystem liefert Derivate mit erhohter Stabilitat, aber reduzierter antibiotischer Wirkung.
Introduction
Streptazolin (1), isolated from cultures of Streptomyces viridochromogenes,
exhibits antimicrobial and antifungal activities [1, 2]. Due to the diene system, it
tends to polymerize which not only causes dif®culties during isolation and
puri®cation but also results in a loss of activity. This property seems to be the
reason for the fact that pharmacological data of this unique antibiotic have not yet
been published.
Enhanced stability and solubility can be achieved by conversion of streptazolin
and dihydrostreptazolin to their glucopyranosides [3]. We strive for the same aim
but try to realize it by means of the total synthesis of congeners which contain the
structural features apparently essential for biological activity and provide a simpler
access. We have recently reported on the synthesis of monocyclic mimicries
Ã
Corresponding author

968
M. Kratzel and A. Weigl
(2, R1 ˆ alkyl) of streptazolin which have satisfactory stability, however, limited
antibiotic activity [4].
In the following we describe the synthesis of bicyclic congeners of the general
structure 3 (R1 ˆ alkyl, R2 ˆ H, alkyl) which represent benzoanalogous compounds
in comparison with our monocyclic derivatives 2 and also with streptazolin 1 itself,
considering a scission of the cyclopentane ring.
Results and Discussion
The synthesis of 3 was devised according to our strategy to construct the exocyclic
double bond starting from a carboxylic acid (4) via the Weinreb amide 5 which can
be converted to ketone 6 by reductive alkylation [5]. The last step would consist in
a Wittig reaction yielding the diene 3 (Scheme 1).
In a previous work, we have extensively examined the reduction of 3-
substituted pyridine derivatives using sodium borohydride in the presence of
chloroformates [6]. However, application of these experimental conditions to 3-
substituted quinolines, e.g. 7, resulted in only low yields of an isomeric mixture of
reduction products 8 and 9 which was dominated by the 1,4-dihydro product 9.
Quaternization of 7 by N-alkylation e.g. N-benzylation, followed by reduction
using sodium borohydride also provided a mixture of 1,2-(10) and 1,4-
dihydroquinolines (11) which subsequently isomerized, contrary to the reduction
Scheme 1

Benzoanalogous Congeners of Streptazolin
969
Scheme 2
of comparable compounds described in Ref. [7], to the 1,4-dihydro product 11
(Scheme 2).
Derivatives with alkyl substituents at the 2-position made the synthesis of the
demanded 1,2-dihydroquinolines more applicable, since 6 (R2 ˆ Me) should be
readily accessible (without formation of the unwanted 1,4-dihydro isomer) by
reductive alkylation of amide 7 using an excess of CH3Li. Interestingly, 7 Á HCl
gave the fully aromatic ketone 12 in acceptable yields after addition of
triethylamine without formation of the 1,2-dihydro compound under the same
reaction conditions (Scheme 3). Wittig reaction of 12 with ethyl-triphenyl-
phosphonium bromide yielded a mixture of E- and Z-13. Contrary to the case of the
monocyclic dienes 2 we were able to separate the isomers. The assignment of E/Z-
isomers could be solved using 1H, 1H NOE experiments. Upon irradiation of H-2 of
E-13, a response at H-20 and at the 10-methyl group could be observed. Moreover,
by irradiation of H-20 the signals of H-2 and obviously of the 20-methyl group were
affected. However, in Z-13 a signi®cant NOE between H-2 and both methyl groups
of the double bond was detected but no in¯uence of H-20 on H-2. This is most
probably due to the conformational equilibrium of the alkenyl moiety.
To prepare 13 by an alternative approach, the tertiary alcohol 14 was prepared
by reaction of 12 with ethylmagnesium bromide. The side reaction yielding the
1,2-dihydroquinoline 15 with preserved carbonyl moiety (which is a homologous
1,2-dihydroquinoline and therefore also applicable for our purposes) could not be
Scheme 3

970
M. Kratzel and A. Weigl
Scheme 4
prevented. It is remarkable that dehydration of 14, which required heating in 6 N
HCl, exclusively afforded E-13 whose ethylidene stereochemistry corresponds to
the con®guration of natural streptazolin (Scheme 4).
The reaction sequence was terminated by the generation of the 1,2-
dihydroquinoline 16 which was obtained by treatment of 13 with CH3Li. In the
last step, the secondary amine 16 was acylated by various chloroformates to
generate molecules with different pharmacokinetic properties (Scheme 5).
The target compounds 3, which were also produced with various substituents at
the exocyclic double bond, proved to be stable in comparison with streptazolin.
The antibiotic activity, which was at ®rst evaluated for 3 (with R1 ˆ Et and
R2 ˆ Me) against Staphylococcus aureus, coagulase-negative Staphylococcus and
E. coli, including the antifungal activity against Candida albicans, is considerably
lower compared to the activity of the monocyclic derivatives 2.
In conclusion, this work demonstrates that the prolongation of the diene system
by an aromatic ring leads to compounds with satisfactory stability, but reduced
antibiotic activity. However, the outlined strategy for the synthesis of compounds 3
with the opportunity for numerous variations of the molecular structure will
3 (R1 ˆ alkyl, R2 ˆ CH3)
Scheme 5
Scheme 6

Benzoanalogous Congeners of Streptazolin
971
probably offer the access to compounds with optimized activity; this topic is
currently under investigation. In that way, the separation of both steps of reductive
alkylation (7 Á HCl ! 12, 13 ! 16) allows to introduce different substituents at C-
2 and C-10. Moreover, the synthesis of ketone 12 as a key compound can be
economized by bromo-lithium exchange of 17 and reaction with 18 (Scheme 6).
The yields are satisfactory, and the use of quinoline-3-carboxylic acid, a
conventional but expensive starting compound for the synthesis of 7, can be
avoided.
Experimental
Melting points were taken on a Ko¯er hot stage apparatus and are uncorrected. Solvents and
common reagents were obtained commercially and used as received, tetrahydrofuran and diethyl
ether were dried by distillation from sodium/benzophenone. Elementary analyses were performed by
Mag. J. Theiner, Institute of Physical Chemistry, University of Vienna. Their result were in
satisfactory agreement with the calculated values. IR spectra were recorded as KBr pellets using a
Perkin Elmer model 298 spectrophotometer. NMR spectra were determined on Bruker AC 80 and
Varian Unity-plus 300 instruments. All substances were measured in CDCl3. 1H NMR spectra were
recorded with (CH3)4Si as the internal reference, the chemical shifts of the 13C NMR spectra are
given in ppm relative to the resonance of CDCl3 (77.0 ppm). Mass spectra were recorded on a
Hewlett Packard GC-MS equipment (HP-5890A, HP-5970C, HP-59970). Flash chromatography was
performed on Merck silica gel 60.
N. Methoxy-N-methyl quinoline-3-carboxamide (7; C12H12N2O2)
Method A. 1.7 g quinoline-3-carboxylic acid (10 mmol) were heated under re¯ux with 15 ml SOCl2
for 30 min. Excess SOCl2 was removed under reduced pressure, and the residual acid chloride was
redissolved twice in toluene and evaporated. The resulting brown residue was dissolved in 50 ml
CH2Cl2, cooled to 0 ꢀC, and treated with 1.1 g N-methoxy-N-methylhydroxylamine hydrochloride
(11 mmol). After addition of 1.85 g pyridine (22 mmol), the cooling bath was removed and the
solution was stirred for 2 h at room temperature. After washing the mixture with water, saturated
aqueous NaHCO3, and brine, the organic layer was separated, dried (Na2SO4), and the solvent was
removed under reduced pressure to give a red oil. Puri®cation by ¯ash chromatography (ethyl
acetate) afforded 1.5 g 7 (70%) as an oil.
1H NMR (80 MHz): ꢀ ˆ 9.23 (1H, d, J ˆ 2.4 Hz, 2-H), 8.58 (1H, d, J ˆ 2.4 Hz, 4-H), 8.25±7.50
(4H, m, arom. H), 3.57 (3H, s, OCH3), 3.45 (3H, s, NCH3) ppm; 13C NMR (20.12 MHz): ꢀ ˆ 176.2
(C ˆ O), 149.0 (2-C), 148.3 (8a-C), 136.5 (4-C), 130.6, 128.9, 126.9, 128.4 (arom. CH), 126.8 (3-C),
126.6 (4a-C), 61.0 (OCH3), 33.1 (NCH3) ppm; IR (7 Á HCl, KBr): ꢁ ˆ 1650, 1630 cmꢁ1; MS:
‡
m/z ˆ 216 (M ).
Reduction of N-methoxy-N-methyl quinoline-3-carboxamide (7) with sodium borohydride
after 1-benzylation (10, 11; C19H20N2O2)
2.16 g 7 (10 mmol) were heated to 130 ꢀC with 1.3 ml benzyl bromide (11 mmol) without solvent
until crystals were formed (about 20 min). The residue was recrystallized from ethanol and then
dissolved in a mixture of methanol (10 ml) and water (15 ml). After cooling to 0 ꢀC, 420 mg NaBH4
(12 mmol) were added in small portions. The cooling was removed, the mixture was stirred for
30 min and ®nally extracted with 3Â10 ml diethyl ether. The combined organic extracts were washed
with brine, dried (Na2SO4), and evaporated in vacuo. Flash chromatography (diethyl ether) afforded
a mixture of 10 and 11 (2.6 g, 85%) which gradually isomerized to 11.

972
M. Kratzel and A. Weigl
1
Yellow oil; H NMR (80 MHz): ꢀ ˆ 7.48±6.62 (10H, m, arom. H, 2-H), 4.78 (2H, s, benzyl-H),
3.92 (2H, s, 4-H), 3.64 (3H, s, OCH3), 3.26 (3H, s, NCH3) ppm; IR (KBr): ꢁ ˆ 1650 cmꢁ1; MS:
‡
m/z ˆ 308 (M ).
3-Acetyl-quinoline (12; C11H9NO)
Method A, starting from 7. To a solution of 505 mg 7 Á HCl (2 mmol) in dry THF 0.3 ml (2 mmol)
triethylamine were added. The mixture was cooled to ꢁ78ꢀC. Then, 2.5 ml CH3Li (1.6 M; 4 mmol)
were slowly added. After 1 h at ꢁ78ꢀC the solution was allowed to warm to room temperature. The
reaction was quenched with saturated aqueous ammonium chloride and extracted with 3Â10 ml
diethyl ether. The combined organic extracts were washed with brine, dried (Na2SO4), and con-
centrated in vacuo. Recrystallization from diethyl ether afforded 12 (250 mg, 72%) as colorless
crystals.
Method B, starting from 3-bromoquinoline (17). 8.4 ml n-butyllithium (1.5 M; 12.5 mmol) were
cooled to ꢁ78ꢀC, and a solution of 1.4 ml 17 (10 mmol) in dry diethyl ether was added over 2 min.
After 15 min, a solution of 1.05 g 18 (10 mmol) in 10 ml dry diethyl ether was added. The reaction
mixture was allowed to warm to room temperature over 3 h and was then quenched with saturated
aqueous ammonium chloride and extracted with 3Â10 ml diethyl ether. The combined organic
extracts were washed with brine, dried (Na2SO4), and evaporated in vacuo. Recrystallization from
diethyl ether yielded 12 (820 mg, 48%) as colorless crystals.
M. p.: 132±134ꢀC; 1H NMR (80 MHz): ꢀ ˆ 9.42 (1H, d, J ˆ 2.0 Hz, 2-H), 8.70 (1H, d, J ˆ 2.0 Hz,
4-H), 8.20±7.50 (4H, m, arom. H), 2.74 (3H, s, COCH3) ppm; 13C NMR (20.12 MHz): ꢀ ˆ 196.6
(C ˆ O), 149.7 (8a-C), 149.1 (2-C), 137.2 (4-C), 131.9, 129.4, 129.3 (5-C, 7-C and 8-C), 129.1
(4a-C), 127.5 (6-C), 126.7 (3-C), 26.7 (COCH3) ppm; IR (KBr): ꢁ ˆ 1680 cmꢁ1; MS: m/z ˆ 171
‡
(M ).
3-(1-Methylprop-1-enyl)-quinoline (13; C13H13N)
Method A, starting from 12. To a suspension of 410 mg carefully dried and pulverized ethyl-
triphenyl-phosphonium bromide (1.1 mmol) in dry diethyl ether, 0.7 ml n-butyllithium (1.6 M;
1.1 mmol) were added under argon at 0ꢀC. Dissolution of the salt and formation of the red ylide
occurred within 15 min. Then, a solution of 170 mg 12 (1 mmol) in 8 ml dry diethyl ether was added
dropwise by means of a syringe at room temperature, and the reaction mixture was stirred under
re¯ux for 6 h. After cooling to room temperature it was ®ltrated to remove triphenylphosphane oxide.
The resulting ®ltrate was washed with water and brine, dried (Na2SO4), and concentrated in vacuo.
The product was puri®ed by column chromatography (diethyl ether: light petroleum ˆ 1:3) affording
Z-13 (70 mg; 38%) and E-13 (60 mg, 33%) as colorless oils.
Method B, starting from 1-methyl-1-(quinolin-3-yl)-propanol (14). 400 mg 14 (2 mmol) were
dissolved in 20 ml 6 N HCl and heated under re¯ux for 4 h. Under cooling with ice the solution was
carefully alkalized with NaOH and extracted with 3Â10 ml CH2Cl2. The combined organic layers
were washed with brine, dried (Na2SO4), and evaporated under reduced pressure. The residue was
puri®ed by ¯ash chromatography (diethyl ether) affording E-13 (240 mg, 65%) as a yellowish oil.
1
E-13: H NMR (80 MHz):ꢀ ˆ 9.02 (1H, br, 2-H), 8.15±7.37 (5H, m, arom. H), 6.07 (1H, q,
J ˆ 6.7 Hz, 20-H), 2.13 (3H, m, 10-CH3), 1.88 (3H, d, J ˆ 6.7 Hz, 30-H) ppm; 13C NMR (20.12 MHz):
ꢀ ˆ 149.3 (2-C), 146.8 (8a-C), 136.2 (10-C), 132.6 (3-C), 131.0, 129.0, 128.9, 127.9, 127.0 (arom.
CH), 128.6 (4a-C), 124.9 (20-C), 15.2, 14.4 (10-CH3, 30-C) ppm; IR (KBr): ꢁ ˆ 3050, 1500 cmꢁ1; MS:
‡
m/z ˆ 183 (M ).
1
Z-13: H NMR (80 MHz): ꢀ ˆ 8.81 (1H, br, 2-H), 8.20±7.40 (5H, m, arom. H), 5.78 (1H, q,
J ˆ 6.7 Hz, 20-H), 2.12 (3H, m, 10-CH3), 1.65 (3H, dq, J ˆ 6.7 Hz, 30-H) ppm; 13C NMR
(20.12 MHz): ꢀ ˆ 151.1 (2-C), 147.1 (8a-C), 134.9 (10-C), 133.6 (3-C), 134.1, 129.3, 128.9, 127.7,

Benzoanalogous Congeners of Streptazolin
973
126.6 (arom. CH), 128.0 (4a-C), 124.1 (20-C), 25.1, 14.9 (10-CH3, 30-C) ppm; IR (KBr): ꢁ ˆ 2880,
‡
1490 cmꢁ1; MS: m/z ˆ 183 (M ).
1-Methyl-1-(quinolin-3-yl)-propanol (14; C13H15NO) and 3-acetyl-3-ethyl-1,2-dihydroquinoline
(15; C13H15NO)
A solution of 690 mg 12 (4 mmol) in 10 ml diethyl ether was cooled to 0ꢀC. After addition of 1.7 ml
ethylmagnesium bromide (1.8 M; 3 mmol), the solution was allowed to warm to room temperature
over 2 h. Then, the reaction mixture was quenched with saturated aqueous NH4Cl and extracted with
3Â10 ml diethyl ether. The combined organic layers were washed with brine, dried (Na2SO4), and
evaporated in vacuo. The resulting green-yellow oil represented a mixture of alcohol 14 and ketone
15 which could be separated by dissolution in 20 ml CH2Cl2 and extraction with 3 Â 10 ml aqueous
citric acid (10%). Work-up of the organic layer afforded after drying (Na2SO4) and evaporation of
the solvent in vacuo 15 (230 mg, 28%) as a yellow oil.
1H NMR (80 MHz): ꢀ ˆ 7.49 (1H, d, J ˆ 5.3 Hz, 4-H), 7.28±6.65 (5H, m, arom. H, NH), 4.12
(1H, t, J ˆ 5.3 Hz, 2-H), 2.29 (3H, s, COCH3), 1.75±1.15 (2H, m, CH2CH3), 0.73 (3H, t, J ˆ 7.5 Hz,
CH2CH3) ppm; 13C NMR (20.12 MHz):ꢀ ˆ 194.8 (C ˆ O), 139.7 (4-C), 136.6 (8a-C), 129.5, 126.6,
123.4, 114.5 (arom. CH), 125.4 (3-C), 113.2 (4a-C), 35.9 (2-C), 30.7 (CH2CH3), 24.4 (COCH3) 9.4
‡
(CH2CH3) ppm; IR (KBr): ꢁ ˆ 1650, 1600 cmꢁ1; MS: m/z ˆ 201 (M ).
Alkalization of the combined aqueous layers with NaOH and subsequent extraction with
3Â10 ml CH2Cl2 yielded after drying (Na2SO4) and evaporation of the solvent under reduced
pressure 14 (280 mg, 35%) as a yellow oil.
1H NMR (80 MHz): ꢀ ˆ 8.85 (1H, d, J ˆ 2.1 Hz, 2-H), 8.22 (1H, d, J ˆ 2.1 Hz, 4-H), 8.07±7.30
(4H, m, arom. H), 4.18 (1H, br, OH), 1.88 (2H, q, J ˆ 8.0 Hz, CH2CH3), 1.60 (3H, s, 10-CH3),
0.78 (3H, t, J ˆ 8.0 Hz, CH2CH3) ppm; 13C NMR (20.12 MHz): ꢀ ˆ 148.8 (2-C), 146.6 (8a-C),
140.5 (3-C), 131.7 (4-C), 127.6 (4a-C), 129.0, 128.6, 127.8, 126.5 (arom. CH), 73.6 (10-C), 36.5
(CH2CH3), 29.4 (10-CH3), 8.2 (CH2CH3) ppm; IR (KBr): ꢁ ˆ 3100, 2500 cmꢁ1; MS: m/z ˆ 201
‡
(M ).
Formation of 2-methyl-3-(1-methylprop-1-enyl)-1,2-dihydroquinoline (16; C14H17N)
and subsequent N-acylation to derivatives 3 (general procedure)
A ¯ame-dried ¯ask, containing 10 ml CH3Li (1.4 M; 14 mmol) was cooled to 0ꢀC. After addition of
2.1 g 13 (11 mmol), the mixture was stirred at room temperature for 2 h, quenched with saturated
aqueous NH4Cl, and extracted with 3Â20 ml diethyl ether. The combined organic layers were
washed with brine, dried (Na2SO4), and concentrated in vacuo. The residue was used without further
puri®cation. The 2-methyl-1,2-quinoline derivative (10 mmol) was dissolved in 20 ml CH2Cl2 and
cooled to 0ꢀC. After addition of 1.5 ml triethylamine (11 mmol), the solution was treated with the
selected chloroformate (10.5 mmol) and stirred for 2±6 h (TLC control). Water was added, and the
aqueous layer was extracted with 3Â20 ml CH2Cl2. The combined extracts were washed with 1 N
HCl, 2 N Na2CO3, and brine, dried (Na2SO4), and concentrated under reduced pressure. Reductive
alkylation of 420 mg 13 (2 mmol) with CH3Li and subsequent acylation with ethyl chloroformate
yielded 3 (R1 ˆ Et, R2 ˆ Me; C17H21NO2; 350 mg, 65%) as a yellow oil.
1H NMR (80 MHz): ꢀ ˆ 7.74±6.50 (5H, m, arom. H, 4-H), 5.92 (1H, q, J ˆ 6.8 Hz, 20-H), 5.75
(1H, q, J ˆ 6.6 Hz, 2-H), 4.21 (2H, q, J ˆ 7.2 Hz, OCH2CH3), 1.90 (3H, s, 10-CH3), 1.80 (3H, d,
J ˆ 6.8 Hz, 30-H), 1.30 (3H, t, J ˆ 7.2 Hz, OCH2CH3), 1.15 (3H, d, J ˆ 6.5 Hz, 2-CH3) ppm; 13C
NMR (20.12 MHz): ꢀ ˆ 155.5 (C ˆ O), 144.2 (10-C), 134.7 (8a C), 134.4 (4-C), 131.9 (3-C), 128.2
(4a-C), 129.2, 128.6, 127.8, 126.5 (arom. CH), 117.8 (20-C), 61.1 (OCH2CH3), 48.6 (2-C), 17.3
(2-CH3), 14.7 (OCH2CH3), 14.3 (10-CH3), 13.7 (30-C) ppm; IR (KBr): ꢁ ˆ 1715 cmꢁ1; MS:
‡
m/z ˆ 271 (M ).

974
M. Kratzel and A. Weigl: Benzoanalogous Congeners of Streptazolin
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È
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Received January 7, 1998. Accepted (revised) March 16, 1998