Studies on the synthesis and biosynthesis of the fungal alkaloid necatorone

Article abstract of DOI:10.1016/j.tetlet.2013.07.118

An alternative pathway for the biosynthesis of the fungal alkaloid necatorone has been studied using fluorine-labeled 3-(2-carboxyphenylamino)-l- tyrosine. Although no incorporation of this compound could be detected in feeding experiments with young specimens of Lactarius necator, an analogous 3-aminotyrosine derivative could be converted synthetically into the oxopyridoacridine core structure of necatorone. In experiments aimed at the synthesis of aaptamine-type alkaloids, an unprecedented cyclization of a 3-aminotyrosine-methyl propiolate adduct to a methyl isoquinoline-3-carboxylate was observed. A mechanism is proposed, in which C3 of the propiolate delivers C1 of the isoquinoline nucleus.

Full text of DOI:10.1016/j.tetlet.2013.07.118

Tetrahedron Letters 54 (2013) 5445–5447
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Studies on the synthesis and biosynthesis of the fungal alkaloid
necatorone
a,
Jörn Carstens a, Markus R. Heinrich a,b, , Wolfgang Steglich
⇑
⇑
a Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München, Germany
b Department für Chemie und Pharmazie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Schuhstr. 19, 91052 Erlangen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
An alternative pathway for the biosynthesis of the fungal alkaloid necatorone has been studied using
Received 14 July 2013
Revised 18 July 2013
Accepted 23 July 2013
Available online 30 July 2013
fluorine-labeled 3-(2-carboxyphenylamino)-L-tyrosine. Although no incorporation of this compound
could be detected in feeding experiments with young specimens of Lactarius necator, an analogous 3-ami-
notyrosine derivative could be converted synthetically into the oxopyridoacridine core structure of neca-
torone. In experiments aimed at the synthesis of aaptamine-type alkaloids, an unprecedented cyclization
of a 3-aminotyrosine-methyl propiolate adduct to a methyl isoquinoline-3-carboxylate was observed. A
mechanism is proposed, in which C3 of the propiolate delivers C1 of the isoquinoline nucleus.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Alkaloids
Necatorone
Biosynthesis
Diphenyliodonium salts
Aminotyrosine
The fungal alkaloid necatorone (5)1,2 is formed by a biomimetic
Pictet–Spengler reaction between dopamine (1) and 2-aminobenz-
aldehyde (2),3 followed by dehydrogenation of the resulting tetra-
hydroisoquinoline intermediate 3, oxidative closure to give the
second pyridine ring, and introduction of the hydroxy group at
the 10-position (Scheme 1).
treated with iodine to afford aldehyde 10 (Scheme 3).8 Selective
oxidation of the latter was achieved with sodium chlorite in the
presence of sulfamic acid.9 Further oxidation of the resulting iodo-
benzoic acid 11 with potassium peroxodisulfate in benzene pro-
vided the iodonium salt 12.10–13
The 3-amino-L-tyrosine derivative 14, required for the N-aryla-
Evidence for this pathway (path A) was provided by the high
incorporation of 13C-labeled tyrosine, anthranilic acid, and amino
aldehyde 2 into alkaloid 5 in young fruit bodies of Lactarius necator
(= L. turpis), as well as by synthetic studies.4 In principle, a variant
of this sequence can be envisaged, in which the C–N bond between
tyrosine and 2-aminobenzaldehyde, or anthranilic acid, is formed
first, followed by an intramolecular Pictet–Spengler reaction of
the resulting amino aldehyde 4a (path B).5,6
tion reaction, was obtained by hydrogenation of the suitably pro-
HO
HO
NH2
CHO
HO
path A
NH
1
H2N
+
HO
H2N
In order to investigate whether diarylamines of type 4 are alter-
native biosynthetic intermediates for necatorone (5), we chose the
fluorinated derivative 6 for feeding experiments (Scheme 2). The
replacement of hydrogen by fluorine in this position should be tol-
erated by the mushroom, since 4-fluoroanthranilic acid (7) is well
accepted and yields 9-fluoronecatorone (8).7
The synthesis of diarylamine 6 started from commercially avail-
able 4-fluorobenzaldehyde (9), which was regioselectively lithiat-
ed in the presence of N,N,N’-trimethylethylenediamine and
2
3
?
path B
HN
CO2H
NH2
HO
HO
HO
O
N
?
R
N
10
OH
necatorone (5)
4a
: R = CHO
?
⇑
4b
: R = CO2H
Corresponding authors. Fax: +49 9133 8522585 (M.R.H.); +49 89 218077756
(W.S.).
E-mail addresses: markus.heinrich@fau.de (M.R. Heinrich), wos@cup.uni-
muenchen.de (W. Steglich).
Scheme 1. Established (A) and suggested alternative (B) biosynthetic pathway to
necatorone (5).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.118

5446
J. Carstens et al. / Tetrahedron Letters 54 (2013) 5445–5447
HO
O
CO2Me
NHBoc
CO2H
NH2
-
CO2
CO2H
N
14
I+
MeO
HO
CO2H
H2N
L. necator
CO2H
Cu(OAc)2
N
HN
HN
iPrOH-H2O
biosynthesis
in L. necator?
58-71%
OH
F
16
17
F
F
MeO2C
NH2
HCl
6
8
7
9-fluoronecatorone ( )
CO2Me
air
Pd/C
NaOMe
MeOH
Scheme 2. Fluorine-labeled compounds 6 and 7 used for the investigation of
SOCl2
AlCl3
N
biosynthetic pathways.
O
MeO
MeO
CH2Cl2
HN
HN
~ 50% (2 steps)
tected 3-nitrotyrosine 13 (Scheme 4).14,15 Coupling of 14 with the
iodonium salt 12 under copper(II) catalysis was the only procedure
found to give the desired product 15; several attempts with alter-
native organometallic methods failed.16,17 Complete deprotection
of diarylamine 15 to the desired amino acid 6 was achieved by
treatment with boron tribromide.18
18
19
CO2Me
N
(NH4)2Ce(NO3)6
O
CH3CN-H2O
88%
N
The possible role of 3-(2-carboxyphenylamino)-L-tyrosine (4b)
as precursor of necatorone (5) was studied by injecting an aqueous
solution of dihydrobromide 6 (20 mg each) into the caps of seven
young specimens of L. necator. After 5–7 days, the mushrooms
were harvested, and the main alkaloid necatorone (5) was isolated.
Neither 19F NMR spectroscopy nor mass spectrometry gave any
hint of the formation of 9-fluoronecatorone (8). It therefore ap-
pears unlikely that diarylamines of type 4 play a role in the biosyn-
thesis of necatorone (5) (path B, Scheme 1).
20
Scheme 5. Synthesis of oxopyridoacridine 20.
the 3-aminotyrosine derivative 14 yielded diarylamine 17. Double
cyclization and dehydrogenation to the pyridoacridine 19 were
achieved by employing Friedel–Crafts conditions and oxidation of
the resulting acridone 18 with air in the presence of Pd/C and so-
dium methanolate. Attempts to convert diarylamine 17 directly
into 19 by using polyphosphoric acid led to significantly lower
yields. Oxidative demethylation of 19 with cerium ammonium
nitrate19 cleanly gave the dideoxynecatorone derivative 20.
An unexpected result was obtained from attempts to use the 3-
aminotyrosine derivative 14 for the synthesis of heterocycles re-
lated to the marine alkaloid aaptamine (21) (Fig. 1).20–28
Attempts to synthesize derivatives of necatorone from diaryl-
amines of type
4 were more successful in the laboratory
(Scheme 5). Starting from the iodonium salt 16, coupling with
nBuLi
CHO
CHO
MeHN
NaClO2,
H2NSO3H
NMe2
I
I2
Michael addition of aminotyrosine 14 to methyl propiolate pro-
vided aminoacrylate 22 as a 2:5 mixture of E- and Z-isomers29
(Scheme 6). The cyclization of 22 was investigated under various
conditions, including such reagents as polyphosphoric acid,30 tri-
fluoroacetic anhydride/DMAP,31 and p-toluenesulfonic acid.
Unfortunately, cyclized products of the aaptamine type were
only obtained in very unsatisfactory yields from these
experiments.
acetone-H2O
85%
THF
48%
F
9
F
10
-
CO2H
CO2
K2S2O8
H2SO4
I
I+
benzene
68%
F
F
On the other hand, heating of ester 22 in phenol afforded a
product that, surprisingly, showed four aromatic singlets in the
1H NMR spectrum. 2D NMR techniques and mass spectrometry re-
vealed structure 23 for the new compound. This was confirmed by
conversion of product 23 into the known methyl 7-methoxyiso-
quinoline-3-carboxylate (24) via diazotation and reduction.32,33
The formation of isoquinoline 23 from 22 clearly begins by ther-
mal cleavage of the Boc group in molten phenol,34,35 followed by
transfer of the acrylate unit from the aromatic to the aliphatic ami-
no group to yield intermediate 25 (Scheme 7). Protonation of the
latter and cyclization of the derived iminium ion 26 to tetrahydro-
isoquinoline 27 is followed by a proton-catalyzed Grob fragmenta-
tion with concurrent loss of methyl acetate, thus explaining the
free C-1 position of the resulting isoquinoline 23.
11
12
Scheme 3. Synthesis of iodonium salt 12.
CO2Me
NHBoc
12
Cu(OAc)2
iPrOH-H2O
CO2Me
NHBoc
MeO
CO2H
HN
MeO
43%
R
F
H2, Pd/C
MeOH
97%
13: R = NO2
14
: R = NH2
15
CO2H
NH2 HBr
HO
CO2H
BBr3
MeO
HN
HBr
CH2Cl2
87%
N
MeO
HN
F
6
aaptamine (21)
Scheme 4. Synthesis of fluorine-labeled diarylamine 6.
Figure 1. Aaptamine (21) from the marine sponge Aaptos aaptos.6

J. Carstens et al. / Tetrahedron Letters 54 (2013) 5445–5447
5447
CO2Me
CO2Me
NHBoc
References and notes
HC CCO2Me
MeOH
NHBoc
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22
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22
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-
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isoquinoline 23. Additional experiments investigating the role of
propiolates as C1 building blocks and equivalents of formaldehyde
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We would like to thank Angus Liddon (Erasmus Exchange Pro-
gram, Glasgow University) for his work on the synthesis of the
fluorinated compounds, Mrs. Sabine Voss for skillful technical
assistance, Professor Dr. Peter Spiteller for helpful discussions,
and Dr. Kay Greenfield, Brisbane, for linguistic improvements.
Supplementary data
Supplementary data (experimental procedures and analytical
data) associated with this article can be found, in the online ver-
sion, at http://dx.doi.org/10.1016/j.tetlet.2013.07.118.