geneous and small amounts of mamegakinone 117 and
diosindigo A 128 could be isolated from the crude product.
These presumably resulted from the oxidative dimerisation
of the intermediate naphthalenetriol 8 and of its monomethyl
ether 9, respectively. The use of tetrahydrofuran in place of
methanol ensured a homogeneous reaction mixture for the
reduction and an 81% yield of 7-methyljuglone 2 resulted.
Tin() chloride in hydrochloric acid is known9 to effect the
reductive removal of chlorine attached to the quinonoid ring of
a naphthoquinone but such a reaction involving the benzenoid
ring was unexpected. It is significant that replacement of a
chlorine atom in the peri position by hydrogen would lead to
the relief of steric strain.10 We suggest that the chloronaphtho-
quinone 1 is first reduced by the tin() chloride to the naphtho-
quinol 10. In the presence of acid a proton is supplied to the
peri carbon atom carrying the chlorine as in structure 13. Loss
of a proton and expulsion of chloride ion followed by further
proton loss, as shown in the structures 14 and 15, would give
7-methyljuglone 2.
light petroleum. After evaporation of the solvent the resulting
solid was separated by column chromatography, using mixtures
of light petroleum and benzene, into two main fractions, A
and B. The faster-moving band A yielded a solid (450 mg),
which was triturated with ether to remove the chloronaphtho-
quinone 1. The red, insoluble residue (48 mg) after TLC on
silica gel–oxalic acid using dichloromethane and crystallisation
from aqueous methanol gave a mixture (10 mg) of the chloro-
anthraquinone 6 and helminthosporin 5 (Found: M+, 304.0144
and 270.0535. Calc. for C15H935ClO5: M, 304.0139. Calc. for
C15H10O5: M, 270.0528). Repeated column chromatography
using tetrachloromethane–benzene (9 : 1) finally gave pure
helminthosporin 5 as red needles (7 mg), mp 227–228 ЊC (lit.,12
227 ЊC), from ethanol, identical (IR, UV, mixed mp) with an
authentic specimen. A further sample of the above insoluble
residue (150 mg) was boiled under reflux with iodomethane
(l ml), potassium carbonate (l g) and acetone (10 ml) for 3 h.
Repeated TLC of the product gave 8-chloro-1,4,5-trimethoxy-7-
methyl-9,10-anthraquinone 7, which crystallised from light
petroleum as yellow needles, mp 246–248 ЊC (Found: M+,
346.0604. C18H1535ClO5 requires M, 346.0607); νmax/cmϪ1 1666
(CO); λmax/nm 225, 261, 350 and 428 (log ε 4.49, 4.42, 3.40 and
3.79); δH 2.45 (3H, s, ArMe), 3.92 (6H, s, OMe, C-4 and C-5),
3.94 (3H, s, OMe, C-1), 7.04 (1H, s, 6-H) and 7.13 (2H, s, 2-H
and 3-H); m/z 346 (81%, M+), 33l (100, M Ϫ Me), 329 (24,
M Ϫ OH), 316 (12, M Ϫ CH2O) and 302 (12, 331 Ϫ CHO).
The slower-moving component B from the original column
chromatography afforded a yellow solid (2.6 g), a portion
(150 mg) of which was heated under reflux with iodomethane
(1 g), potassium carbonate (1 g), and acetone (10 ml) for 3 h.
The product, after evaporation and subsequent TLC on silica
gel using dichloromethane, methyl (E)-β-(5-chloro-2-methoxy-
4-methylbenzoyl)acrylate 4, was obtained as yellow needles
(107 mg), mp 94–95 ЊC from light petroleum (bp 80–100 ЊC)
(Found: M+, 268.0502. C13H1335ClO4 requires M, 268.0503);
νmax/cmϪ1 1720 (conj. ester CO) and 1664 (conj. CO); λmax/nm
229, 244infl, 276sh and 345 (log ε 4.33, 4.08, 3.75 and 3.56);
δH 2.42 (3H, s, ArMe), 3.82 (3H, s, ArOMe), 3.90 (3H, s,
CO2Me), 6.76 and 7.80 (2H, ABq, JAB 16, trans-CH᎐CH-), 6.86
᎐
and 7.68 (each 1H, s, 3- and 6-H); m/z 268 (M+, 30%), 237 (14,
M Ϫ MeO), 209 (52, 237 Ϫ CO), 183 (100, 209 Ϫ C2H2).
8-Chloro-5-hydroxy-7-methyl-1,4-naphthoquinone 1 and its
conversion into 7-methyljuglone 2†
The ether-soluble material from the above fraction A after
column chromatography on silica gel using benzene–light
petroleum (1 : 9) gave the chlorohydroxynaphthoquinone 1
(350 mg), which crystallised as red needles, mp 160–161 ЊC (lit.,2
159–161 ЊC) from chloroform or aqueous methanol; νmax/cmϪ1
1663 and 1646 (quinone CO); λmax/nm 256.5, 350infl, 422infl,
435 and 450infl (log ε 4.09, 3.08, 3.55, 3.61 and 3.55); δH 2.47
(3H, s, ArMe), 6.88 (2H, s, 2-H and 3-H), 7.18 (1H, s, ArH) and
12.75 (1H, s, ArOH). The corresponding acetate crystallised
from light petroleum as yellow needles, mp 220 ЊC (Found:
C, 58.8; H, 3.5; Cl, 13.4. C13H9ClO4 requires C, 58.9; H, 3.4;
Cl, 13.4%); νmax/cmϪ1 1776 (aryl acetate CO) and 1664 (quinone
CO); λmax/nm 239 and 360 (log ε 4.19 and 3.49); δH 2.38 (3H,
s, ArOCOMe), 2.46 (3H, s, ArMe), 6.73 and 6.85 (2H, ABq,
JAB 10, 2-H and 3-H) and 7.23 (1H, s, ArH).
A solution of the chloroquinone 1 (1 g) in THF (100 ml)
was added dropwise to a solution of tin() chloride (5 g) in
4 M hydrochloric acid (350 ml) and THF (100 ml) at 60 ЊC.
The mixture was kept at 60 ЊC for 3 h, cooled and filtered
into a solution of iron() chloride (25 g) in water (200 ml).
The resulting yellow solid was collected and crystallised from
aqueous methanol or chloroform–light petroleum to give 7-
methyljuglone as orange needles (685 mg, 81%), mp 126.5–
Experimental
IR spectra were measured for potassium bromide discs and
UV–visible spectra were obtained for ethanolic solutions using
Perkin-Elmer Infracord 237 and 137UV spectrophotometers,
respectively. Molar absorptivity (ε) values are given in units
of dm3 molϪ1 cmϪ1. H NMR spectra were measured for solu-
1
tions in deuterochloroform with tetramethylsilane as internal
standard using a Varian HA 100 spectrometer. Electron-impact
mass spectra were measured on an AEI MS-70 instrument at
70 eV. TLC was performed on silica gel (Merck GF254) plates
prepared using either distilled water or aqueous 3% oxalic acid
with chloroform for development. For column chromatography
the silica gel was washed with 2 M hydrochloric acid before use.
‘Light petroleum’ refers to the fraction with bp 60–80 ЊC.
Reaction of maleic anhydride with 4-chloro-3-methylphenol
Repetition of the Friedel–Crafts reaction between the chloro-
phenol11 (4 g) and maleic anhydride (8 g) in molten aluminium
chloride–sodium chloride exactly as originally described2 gave
a crude product that was dried and extracted (Soxhlet) with
† Carried out with the assistance of T. J. Lillie and B.C. Maiti.
J. Chem. Soc., Perkin Trans. 1, 2001, 1318–1320
1319