A convenient synthesis of the cruciferous phytoalexins brassicanal A and brassilexin by mimicry of a fungal detoxification pathway

Article abstract of DOI:10.1039/a803485k

The cruciferous phytoalexin brassilexin 3 has been synthesized in four steps from indoline-2-thione via 3-(aminomethylene)indole-2-thione 2, a metabolic intermediate of the detoxification pathway of the phytoalexin cyclobrassinin 1; in addition, the phytoalexin brassicanal A 8 has been synthesized in two steps from 2-indolinethione.

Full text of DOI:10.1039/a803485k

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A convenient synthesis of the cruciferous phytoalexins brassicanal A and
brassilexin by mimicry of a fungal detoxification pathway
M. Soledade C. Pedras*† and Francis I. Okanga
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK, Canada S7N 5C9
The cruciferous phytoalexin brassilexin 3 has been synthe-
sized in four steps from indoline-2-thione via 3-(aminome-
thylene)indole-2-thione 2, a metabolic intermediate of the
detoxification pathway of the phytoalexin cyclobrassinin 1;
in addition, the phytoalexin brassicanal A 8 has been
synthesized in two steps from 2-indolinethione.
offering also the possibility of introducing carbon and/or
nitrogen isotopes into the indol-3-yl substituents in good yield.
Previous syntheses of brassilexin 3 from indole-3-carbaldehyde
via cyclobrassinin 1 (11% overall yield),11–13 or directly from
indole-3-carbaldehyde (11–30% overall yield),14 have been
reported. However, our route affords the best overall yield to
date while following the simplest process in terms of purifica-
tion and reaction conditions. Utilizing the route shown on
Scheme 2, brassilexin 3 was obtained from indolinethione 515 in
four steps (typically 50 mg scale reactions) in ca. 64% overall
yield. In addition, this route provided aldehyde 6, which on
reaction with CH2N2 quantitatively yielded brassicanal A 8,
another cruciferous phytoalexin,16 in ca. 92% yield. This
synthesis of brassicanal A 8 also presents a great improvement
to the previously reported procedure, which employed methyla-
tion of indolinethione 5 followed by Vilsmeier formylation of
the corresponding 2-thiomethyl ether 9 (Scheme 2, 39% overall
yield from 5).4
It is now well-recognized that the blackleg fungus [Phoma
lingam (Tode ex Fr.) Desm., perfect stage Leptosphaeria
maculans (Desm.) Ces. et de Not.], one of the most destructive
fungal pathogens of rapeseed (Brassica napus, B. rapa), can
overcome the plant’s induced chemical defenses, i.e. phytoalex-
ins, by enzymatic detoxification.1–6 Recently, we reported6 an
unprecedented detoxification of the cruciferous phytoalexin
cyclobrassinin 17 via the phytoalexins brassilexin 38 and
dioxibrassinin 49,10 by isolates of P. lingam‡ (Scheme 1). A
detailed analysis of the metabolites involved in that fungal
biotransformation of cyclobrassinin 1 indicated that 3-(amino-
methylene)indole-2-thione 2, or its thiol tautomer(s), was a
precursor of brassilexin 3 (Scheme 1).6 In continuation of that
work we have now accomplished the synthesis of brassilexin 3
by mimicry of that detoxification pathway, utilizing the key
intermediate 2 (Scheme 2). This route provides a very
convenient process for preparation of both compounds 2 and 3,
During previous biotransformation studies,6 we observed that
brassilexin 3 could be obtained from metabolite 2 after standing
in solution or on silica gel TLC plates. Interestingly, this
transformation was catalyzed by activated charcoal to afford
brassilexin almost quantitatively, demonstrating that metabolite
2 would be a synthetically useful brassilexin precursor if
available in reasonable yields. One apparent method for the
preparation of 2 was the reduction of amino derivatives of
3-(hydroxymethylene)indole-2-thione (or equivalent tautomer,
e.g. 6) similar to the preparation of 3-hydroxymethylene-
2-oxoindole derivatives.17 Thus, formylation of thione 515 with
EtO2CH afforded aldehyde 6, whose 1H and 13C NMR
spectroscopic data§ indicated that only the thiol tautomeric
form was present. Oximination of 6 under standard conditions18
yielded quantitatively oxime 7, which was readily reduced to
the desired intermediate 2 with NaBH3CN in the presence of
TiCl3.19 Finally, treatment of 2 with activated charcoal afforded
brassilexin in excellent yield.¶
It is worth noting that thione 2 was hypothesized to be a
biogenetic precursor of brassilexin 3 in planta.6 In addition,
although thiol 6 does not appear to have been prepared
previously, it was proposed as a precursor of brassicanal A in
planta.20 Our facile synthesis of intermediates 2 and 6 offers the
possibility of isotopic labelling and should facilitate future
biosynthetic studies on brassilexin 3 and brassicanal A 8.
Particularly because brassilexin 3 appears to be involved in the
blackleg disease resistance of several agriculturally important
cruciferous oilseeds (e.g. B. juncea, B. carinata),21 its bio-
synthesis has tremendous potential application and is presently
under investigation in our laboratory.
N
NH2
SMe
i
S
S
N
N
H
H
2
1
ii
i
H
N
N
SMe
HO
S
S
O
N
H
N
H
4
3
Scheme 1 Transformation of cyclobrassinin by the phytopathogenic fungus
Phoma lingam: i, ‘avirulent’ isolate Unity; ii, ‘virulent’ isolate BJ 2125
CHO
N
OH
i
ii
SH
S
SH
N
H
N
H
7
N
H
5
6
v
iii
iv
CHO
We gratefully acknowledge the financial support of the
Natural Sciences and Engineering Research Council of Canada
and the University of Saskatchewan.
2
SMe
SMe
N
H
N
H
9
3
8
Notes and References
Scheme 2 Reagents and conditions: i, NaH, HCO2Et, 25 °C, 92%; ii,
NH2OH·HCl, AcONa, EtOH, reflux, quant.; iii, TiCl3, NaBH3CN, MeOH,
25 °C, 85%; iv, activated charcoal, 25 °C, 82%; v, CH2N2, Et2O, 25 °C,
quant.
† E-mail: pedras@sask.usask.ca
‡ The fungal species Phoma lingam is subdivided in various groups (cf.
M. S. C. Pedras, J. L. Taylor and V. M. Morales, Phytochemistry, 1995, 38,
1215; J. L. Taylor, M. S. C. Pedras and V. M. Morales, Trends Microbiol.,
Chem. Commun., 1998
1565

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1995, 3, 202); the so-called ‘avirulent’ group, e.g. isolate Unity, is now
considered a species different from that of the ‘virulent’ group, e.g. BJ-125,
although no formal reclassification has been carried out.
7 First isolation and synthesis: M. Takasugi, N. Katsui and A. Shirata,
J. Chem. Soc., Chem. Commun. 1986, 1077.
8 First isolation: M. Devys, M. Barbier, I. Loiselet, T. Rouxel, A.
Sarniguet, A. Kollmann and J. Bousquet, Tetrahedron Lett., 1988, 29,
6447.
9 First isolation: K. Monde, K. Sasaki, A. Shirata and M. Takasugi,
Phytochemistry, 1991, 30, 2915.
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1994, 116, 6650.
11 M. Devys and M. Barbier, Synthesis, 1990, 214.
12 M. Devys and M. Barbier, J. Chem. Soc., Perkin Trans. 1, 1990,
2856.
13 M. Devys and M. Barbier, Z. Naturforsch., 1992, 47C, 318
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Bull., 1969, 17, 550.
§ Selected data for 6: dH(500 MHz, CD3OD) 9.49 (s, CHO), 8.15 (d, J 8.0,
1 H), 7.48 (d, J 8.0, 1 H), 7.37 (dd, J 8.0, 7.5, 1 H), 7.27 (dd, J 8.0, 7.5,
1 H); dH(500 MHz, [2H6]DMSO) 12.77 (br s, NH), 9.49 (s, CHO), 8.06 (d,
J 8.0, 1 H), 7.50 (d, J 8.0, 1 H), 7.35 (dd, J 8.0, 7.0, 1 H), 7.26 (dd, J 8.0,
7.0, 1 H); dC(125.5 MHz, [2H6]DMSO) 184.3, 138.5, 137.1, 125.2, 124.6,
122.9, 120.8, 119.8, 112.3; HRMS: found 177.0249 (calc. for C9H7NOS:
177.0248); m/z (EI) 177 (M+, 100%), 149 (18), 148 (24), 121 (15); m/z (CI)
178 (M+ + 1, 19%), 164 (32), 150 (24), 146 (100), 132 (50). For 7: dH(300
MHz, CD3CN): d 9.88 (br s, NH), 8.52 (br s, OH), 8.03 (d, J 8.0, 1 H), 7.95
(s, H-1’), 7.42 (d, J 8.0, 1 H), 7.31 (dd, J 8.0, 7.0, 1 H), 7.21 (dd, J 8.0, 7.0,
1 H); dC(75.5 MHz, CD3CN) 145.2, 138.9, 131.1, 126.0, 125.7, 123.5,
122.4, 117.2, 112.6; HRMS: found 174.0251 (calc. for C9H8N2OS 2 H2O:
174.0252); m/z (EI) 174 (M+ 2 H2O, 100%), 149 (51), 142 (24); m/z (CI)
175 (M+ + 12 H2O, 100%).
16 K. Monde, N. Katsui, A. Shirata and M. Takasugi, Chem. Lett., 1990,
209.
¶ All compounds gave satisfactory spectroscopic data.
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Chem. Soc., 1956, 78, 797.
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Received in Corvallis, OR, USA, 8th May, 1998; 8/03485K
1566
Chem. Commun., 1998