the electron-withdrawing strength of the substituent at the β-position of the enamine does not seem unusual – the
benzofuran cyclization of the intermediate hydroquinone adduct depends directly on the electron deficiency at
the α-position of the enamine fragment – the formation of 6-hydroxyindoles has until now only been observed in
cases with variation of the structures of the initial enamines (but not quinones), e.g., in the transition from
N-alkyl- to N-aryleneamines or to enamines having strong electron acceptors such as cyano and particularly
nitro groups at the β-position [6]. In view of the fact that the Nenitzescu reaction very often takes place
ambiguously and the yield of the obtained 6-hydroxy derivative is low it is not possible to claim that the use of
the heterocyclic quinone 1 in reaction with the enamine 6 changes the direction of this reaction completely and
fundamentally and excludes the formation of the normal 5-hydroxyindoles for the Nenitzescu reaction. However,
the fact that the structure of the quinone can change this direction so dramatically is a new previously unknown
and unexpected phenomenon and requires further detailed investigation.
EXPERIMENTAL
1
The H NMR spectra were recorded on a Bruker AC-200 spectrometer (200 MHz). The 2D HMBC
NMR spectra (1H and 13C) were obtained on a Bruker DRX-500 spectrometer (at 500 and 125 MHz respectively)
using the manufacturer's standard procedures. The high-resolution mass spectra were obtained on a Finnigan
MAT TCQ 700 spectrometer (triple quadrupole) with direct injection of the sample into the ion source. The
purity of the obtained substances was monitored on Silufol UV-254 and Kieselgel 60 F-254 (Merck) in ethyl
acetate.
3-Benzoyl-5-hydroxy-7-methoxycarbonyl-2-methyl-9-oxofuro[2,3-f]quinoline (3a). A mixture of the
quinone 1 (0.23 g, 10 mmol), 2-p-anisidino-3-benzoyl-2-propene (2e) (0.27 g, 10 mmol), and glacial acetic acid
(4 ml) was heated to 60-70°C, kept at this temperature for 5 min, and then left at room temperature. The next day
the crystals that separated were filtered off, washed on the filter with petroleum ether, and dried, and 0.14 g
(37.1%) of the furoquinoline 3a was obtained; mp >300°C (DMF) (decomp.). High-resolution mass spectrum.
Found: m/z 377.0888 [M]+. С21H15NO6. Calculated: M = 377.359.
3-Acetyl-5-hydroxy-7-methoxycarbonyl-2-methyl-9-oxofuro[2,3-f]quinoline (3b). The compound
was obtained similarly to compound 3a from the quinone 1 and the enamine 2b with a yield of 59%; mp >300°C
(DMF) (decomp.). High-resolution mass spectrum. Found: m/z 315.077 [M]+. С16Н13NО6. Calculated:
M = 315.288.
1-Benzyl-3-ethoxycarbonyl-8-hydroxy-5-methoxycarbonyl-2-methyl-7-oxopyrrolo[2,3-h]quinoline
(7). The compound was obtained similarly to compound 3a from the quinone 1 and the enamine 6 with a yield of
32%. After recrystallization from dioxane the pure pyrroloquinoline 7 was isolated with a yield of 13%,
calculated on the initial quinone 1; mp 285-287°C. High-resolution mass spectrum. Found: m/z 434.1487 [M]+.
С24Н22N2O6. Calculated: M = 434.486.
5,9-Diacetoxy-3-benzoyl-7-methoxycarbonyl-2-methylfuro[2,3-f]quinoline (4a). To compound 3a
(0.38 g, 10 mmol) acetic anhydride (15 ml) and three drops of sulfuric acid were added. The reaction mixture
was heated until the precipitate had dissolved, kept at room temperature for 24 h, and poured into cold water
(150 ml). The precipitate was filtered off, washed with water on the filter, and dried. The yield of compound 4a
was 0.38 g (82%). The compound was purified by column chromatography on silica gel. The eluent was ethyl
acetate. The solvent was distilled, and compound 4a was obtained with a yield of 43%; mp 192-194°C (ethanol).
1
High-resolution mass spectrum. Found: m/z 419.099 [M]+. С23Н17NО7. Calculated: M = 357.32. H NMR
spectrum, δ, ppm: 2.40 (3Н, s, 5(9)-ОСОСН3); 2.51 (3Н, s, 9(5)-ОСОСН3); 2.49 (3Н, s, 2-СН3); 3.95 (3Н, s,
7-CООСН3); 7.43 (2Н, t, 3'-, 5'-H); 7.70 (1Н, t, 4'-H); 7.72 (1Н, s, 4-Н); 7.83 (2Н, d, 2'-, 6'-H); 8.11 (1Н, s,
8-Н).
20