Synthesis and isomerization of biindolinones from Collybia peronata and Tricholoma scalpturatum

Article abstract of DOI:10.1021/jo970388p

Peronatins A and B and 7,7'-dimethoxyperonatin B, originally isolated from the damaged fruiting bodies of Collybia peronata and Tricholoma scalpturatum, have been synthesized by oxidative dimerization of 2- alkylindoles. The conversion of peronatin A to peronatin B was shown to be catalyzed by Bronsted acids in chloroform solution and inhibited by triethylamine, implicating a retro-Mannich/Mannich isomerization pathway under these conditions. Attempts to identify or trap out radical or ionic intermediates were unsuccessful.

Full text of DOI:10.1021/jo970388p

4756
J . Org. Chem. 1997, 62, 4756-4762
Syn th esis a n d Isom er iza tion of Biin d olin on es fr om Collybia
per on a ta a n d Tr ich olom a sca lptu r a tu m
Shawn J . Stachel, Mark Nilges,† and David L. Van Vranken*
Department of Chemistry, The University of California, Irvine, California 92697-2025, and Illinois EPR
Research Center, 506, South Matthews Avenue, Urbana, Illinois 61801
Received March 3, 1997X
Peronatins A and B and 7,7′-dimethoxyperonatin B, originally isolated from the damaged fruiting
bodies of Collybia peronata and Tricholoma scalpturatum, have been synthesized by oxidative
dimerization of 2-alkylindoles. The conversion of peronatin A to peronatin B was shown to be
catalyzed by Brønsted acids in chloroform solution and inhibited by triethylamine, implicating a
retro-Mannich/Mannich isomerization pathway under these conditions. Attempts to identify or
trap out radical or ionic intermediates were unsuccessful.
Sch em e 1
1. In tr od u ction
Indole dimers, in the form of indigo, are among the
oldest known organic compounds of commercial impor-
tance, dating back to 2500 bc.1,2 The chemistry that
underlies indigo technology is surprisingly advanced and
exploits enzymatic hydrolysis, bioreduction, and free
radical carbon-carbon bond-forming reactions. The
mechanism for formation of indigo (5) under basic condi-
tions is in accord with Scheme 1.3 Indoxyl anion 2 can
react by single-electron transfer (SET) with molecular
oxygen to form a merostabilized indoxyl radical 3. Mero-
stabilization, known alternatively as captodative (push-
pull) stabilization, is anticipated when a radical center
is geminally substituted by both electron-donating (e.g.,
O, N, S) and electron-withdrawing groups (e.g., carbon-
yls).4,5 Dimerization of indoxyl radicals produces leu-
coindigo 4, a colorless, reduced form of indigo. A second
oxidation then leads to formation of the colored dye 5.
Sch em e 2
Tryptophan dimers are a second class of important
indole dimers. They are formed by exposure of tryp-
tophan derivatives to anhydrous acid.6-9 In contrast to
indigoids, the chemistry of tryptophan dimers is domi-
nated by ionic reactions. We have found that tryptophan
dimer 6 may be cleaved under acidic conditions with
concomitant amide bond hydrolysis (Scheme 2). This
cleavage probably involves a retro-Mannich fragmenta-
tion of intermediate 7.
† Illinois EPR Research Center.
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
(1) Pettit, F. H. America’s Indigo Blues: Resist-printed and Dyed
Textiles of the Eighteenth Century; Hastings House: New York, 1974.
(2) Leix, A. Ciba-Rev. 1938, 1, 422.
Recently, a series of leucoindigoid natural products
were obtained from the damaged fruiting bodies of two
fungi (Collybia peronata and Tricholoma scalpturatum).10
Four biindolinones were isolated: peronatin A (8a ),
peronatin B (8b), 7,7′-dimethoxyperonatin B (9b), and
7-hydroxy-7′-methoxyperonatin B (10). The isolation of
meso isomer 7,7′-dimethoxyperonatin A (9a ) was not
reported. Like indigo, these compounds are not found
in the intact plant but are believed to result from
enzymatic hydrolysis of an indole precursor and oxidative
coupling of the derived indoxyl radicals.10 Unlike indigo,
(3) Russell, G. A.; Kaupp, G. J . Am. Chem. Soc. 1969, 91, 3851.
(4) Baldock, R. W.; Hudson, P.; Katritzky, A. R.; Soti, F. J . Chem.
Soc., Perkin Trans. 1 1974, 1422.
(5) Viehe, H. G.; J anousek, Z.; Merenyi, R.; Stella, L. Acc. Chem.
Res. 1985, 18, 148.
(6) Hashizume, K.; Shimonishi, Y. Bull. Chem. Soc. J pn. 1981, 54,
3806-3810.
(7) Omori, Y.; Matsuda, Y.; Aimoto, S.; Shimonishi, Y.; Yamamoto,
M. Chem. Lett. 1976, 805.
(8) Stachel, S. J .; Habeeb, R. L.; Van Vranken, D. L. J . Am. Chem.
Soc. 1996, 118, 1225.
(9) Carter, D. S.; Van Vranken, D. L. Tetrahedron Lett. 1996, 37,
5629.
(10) Pang, Z.; Sterner, O. J . Nat. Prod. 1994, 57, 852.
S0022-3263(97)00388-5 CCC: $14.00 © 1997 American Chemical Society

Synthesis and Isomerization of Biindolinones
J . Org. Chem., Vol. 62, No. 14, 1997 4757
Sch em e 3
the leuco dimers 8, 9, and 10 are resistant to further
oxidation because of the 2,2′-methyl substituents.
14 give well-resolved EPR spectra attributable to the
monomeric radical species.11-13
Biindolinone 15 was also
proposed to cleave homolytically, but well-resolved EPR
signals were not observed.4 In contrast to the sym-
metrical dimers, unsymmetrical dimers 16, 17, and 18
dissociate to give either radicals or ionic intermediates,
depending on the polarity of the solvent.14-16 Information
regarding the mechanism of dimer cleavage was gleaned
from kinetic behavior in the case of dimers 11, 12, and
16. In no case has acid been shown to accelerate
homolytic cleavage of a symmetrical dimer. In fact,
dissociation of 11 has been shown to be in h ibited at
lower pH, presumably due to protonation of the nitro-
gen.17
Sterner had previously noted that peronatin A converts
slowly to peronatin B in deuteriochloroform.10 A ho-
molytic dissociation/recombination mechanism was pro-
ferred for this reaction since the intermediate radicals
should be stabilized by captodative substitution (Scheme
3). However, an acid-catalyzed retro-Mannich/Mannich
process (Scheme 3, ionic pathway) would also provide a
plausible mechanism since the isomerization was noted
in deuteriochloroform where DCl is a common contami-
nant. Since radical intermediates and ion pairs can
interconvert by disproportionation, the nature of the
monomeric intermediates does not necessarily reflect the
mechanism for dimer cleavage. Despite this complica-
tion, and because of our interest in Mannich/retro-
Mannich processes in 2,2′-indole dimers, we set out to
synthesize the peronatins via a biomimetic pathway and
investigate the isomerization of peronatin A to peronatin
B.
Specifically, we wished to synthesize the peronatins
and methoxyperonatins and answer three questions: (i)
Does 7,7′-dimethoxyperonatin A isomerize to 7,7′-dimeth-
oxyperonatin B? (ii) Can acids catalyze the isomerization
of peronatin A to peronatin B? (iii) Are persistent
radicals present during the isomerization of peronatin
A to peronatin B?
Steric congestion and captodative substitution are
known to facilitate homolytic carbon-carbon bond cleav-
age leading to persistent radicals. For example, at
temperatures above 80 °C, the symmetrical dimers 11-

4758 J . Org. Chem., Vol. 62, No. 14, 1997
Stachel et al.
Sch em e 4
Sch em e 6
Sch em e 5
reaction conditions. When hydrogen peroxide in acetic
acid was used, the only dimeric product that could be
isolated was (()-peronatin B. However, when the oxida-
tion is performed with hydrogen peroxide and triethy-
lamine, both peronatin A (45%) and (()-peronatin B (4%)
are formed.
Our next goal was to synthesize the 7,7′-dimethox-
yperonatins 9a and 9b and assess their relative reac-
tivities. The methoxyperonatins offer a unique means
of studying electronic effects on the isomerization of
peronatin A to peronatin B since the methoxy groups are
distal from the bond-forming/bond-cleaving centers. To
exploit the oxidation/dimerization in Scheme 4 for the
methoxyperonatins required access to 7-methoxy-2,4-
dimethylindole 22b. Unfortunately, o-alkoxytoluidines
such as 21b are notoriously poor substrates for the
Madelung reaction, so several alternative routes were
investigated for the synthesis of methoxyindoles. The
most direct route to 22b (addition of 2-propenylmagnsium
bromide to 4-methyl-2-nitroanisole) gave the desired
indole, albeit in 5% yield (eq 1). Poor yields are to be
2. Syn th esis of th e P er on a tin s a n d
7,7′-Dim eth oxyp er on a tin s
(1)
We sought to synthesize the peronatins using a bio-
mimetic coupling reaction involving indolin-2-one inter-
mediates 19a and 19b (Scheme 4). Oxidation of 2-sub-
stituted indoles with hydrogen peroxide is known to
afford biindolinones, presumably via indolin-2-ones and
their tautomeric indoxyls.18 At the outset, it was not
clear whether we would have access to both the peron-
atins by this route. While peronatin A was known to
isomerize to peronatin B, there was no evidence for the
converse isomerization.
2,4-Dimethylindole was prepared from 2,3-dimethy-
lacetanilide by a modification of the procedure of Baude.19
Madelung ring closure to give 2,4-dimethylindole was
accomplished with sodium amide at 250 °C and proceeded
in a gratifying 86% yield (Scheme 5). Oxidative dimer-
ization gave different product mixtures depending on the
expected from this reaction when the O-alkyl group is
not bulky, so we turned our efforts to alternative longer,
but more efficient, routes to the methoxyindole 22b. We
next sought to introduce oxygen into 2,4-dimethylindole
by way of the indoline (eq 2). Indole 22a was reduced
(2)
with triethylsilane in trifluoroacetic acid, and the indoline
nitrogen was then acetylated to provide a handle for a
directed thallation. Subsequent thallation of 23 with
thallium tris(trifluoroacetate) gave intractable mixtures,
so we next turned to syntheses of 7-chloro-2,4-dimeth-
ylindole by the Gassman route.20 Sommelet-Hauser
rearrangement of 24 afforded 7-chloroindole derivative
25 (Scheme 6). This method can not be used to prepare
7-alkoxyindoles directly because N-chloroanisidines are
unstable, even at -78 °C. Unfortunately, low yields in
the desulfurization to give indole 26 required recycling
of unreacted sulfide. This difficulty, combined with
(11) Bennett, R. W.; Wharry, D. L.; Koch, T. H. J . Am. Chem. Soc.
1980, 102, 2345.
(12) Himmelsbach, R. J .; Barone, A. D.; Kleyer, D. L.; Koch, T. H.
J . Org. Chem. 1983, 48, 2989.
(13) Van Hoecke, M.; Borghese, A.; Penelle, J .; Merenyi, R.; Viehe,
H. G. Tetrahedron Lett. 1986, 27.
(14) Maslak, P.; Narvaez, J . N. Ang. Chem., Int. Ed. Engl. 1990,
29, 283.
(15) Komatsu, K.; Aonuma, S.; Takeuchi, K.; Okamoto, K. J . Org.
Chem. 1989, 54, 2038.
(16) Arnett, E. M.; Molter, K. E.; Marchot, E. C.; Donovan, W. H.;
Smith, P. J . Am. Chem. Soc. 1987, 109, 3788.
(17) Koch, T. Personal communication.
(18) Piozzi, F.; Langella, M. R. Gazz. Chim. Ital. 1963, 1373.
(19) Noland, W. E.; Baude, F. J . J . Org. Chem. 1966, 1966, 3321.
(20) Gassman, P. G.; van Bergen, T. J .; Gilbert, D. P.; Cue, B. W.,
J r. J . Am. Chem. Soc. 1974, 96, 5495.

Synthesis and Isomerization of Biindolinones
J . Org. Chem., Vol. 62, No. 14, 1997 4759
Sch em e 7
F igu r e 1. Minimum energy conformation of (a) peronatin A
(8a ) and (b) peronatin B (8b).
amino hydrogens and carbonyl oxygen atoms, whereas a
similar arrangement in peronatin A leads to unfavorable
eclipsing interactions between methyl groups (Figure 1).
At the AM1 level, the lowest energy conformation of
peronatin B is 0.96 kcal/mol lower in energy than the
lowest energy conformation of the meso isomer peronatin
A. If the free energy of mixing for enantiomers is taken
into account (-0.41 kcal/mol at room temperature),21 an
equilibrium ratio of 10:1 in favor of peronatin B is
predicted, in qualitative agreement with all observations.
These results suggest that the product distribution
(mainly peronatin A) obtained from the oxidative dim er -
iza tion of 2,4-d im eth ylin d ole u n d er ba sic con d i-
tion s is u n d er k in etic con tr ol, w h ile a cid ic con d i-
tion s p r oceed u n d er th er m od yn a m ic con tr ol to
a ffor d th e m or e sta ble p r od u ct, p er on a tin B. The
relative stabilities of the 7,7′-dimethoxyperonatins 9a and
9b mirror the stabilities of 8a and 8b, respectively.
problems in the halide displacement, led us to return to
the Madelung cyclization.
We returned to the original Madelung route to the
methoxyindole 22b. The key aniline derivative 21b was
synthesized from 2-chloro-4,5-dimethylphenol (Scheme
7). Nitration under biphasic conditions followed by
methylation with methyl iodide gave the 2-nitroanisole
derivative 28. The chlorine group that was used to block
the 6 position of the phenol during the nitration was then
removed during reduction of the nitro group to afford
aniline 21b in excellent yield. As expected, Madelung
cyclization of 21b was inefficient, and yields for indole
22b were typically around 40%. Again, the oxidation/
dimerization reaction gave different results under acidic
and basic conditions. Acidic conditions (H2O2/acetic acid)
favor formation of (()-methoxyperonatin B, 9b, whereas
basic conditions favor formation of methoxyperonatin A.
The methoxyperonatins are much less stable than the
peronatins. Meso isomer 9a was so unstable that solu-
tions could not be stored without decomposition. It is
quite possible that this compound is formed upon damage
to the fruiting bodies of T. scalpturatum, but the instabil-
ity precluded isolation.
4. Isom er iza tion of P er on a tin A to P er on a tin B
In anhydrous chloroform, peronatin A isomerizes slowly
to peronatin B (vide supra). This isomerization is not
clean, and small amounts of unidentified products are
formed. A number of methods were explored to address
the nature of the intermediates that are present during
the isomerization process. While the rate of isomeriza-
tion increases with temperature, no EPR signals were
observed for degassed chloroform solutions of peronatin
A or B at temperatures up to 100 °C. Unfortunately,
numerous attempts to trap out or react with intermedi-
ates (radical or ionic) in the isomerization using TEMPO,
isatin,11 2,2-diphenylpicrylhydrazyl,12 and NaCNBH3
led either to complex mixtures of products or no reaction.
Thus, the identity of the intermediates in this reaction
remains elusive.
22
We h a ve fou n d th e isom er iza tion of p er on a tin A
to p er on a tin B in ch lor ofor m to be ca ta lyzed by
Br øn sted a cid s a n d in h ibited by a m in e ba ses. As
shown in Figure 2a, when a solution of peronatin A in
chloroform (filtered through anhydrous, activated alu-
mina) is allowed to stand in the dark under nitrogen, the
concentration of peronatin A decreases as the concentra-
tion of peronatin B increases. In contrast, when 10 mol
% camphorsulfonic acid is added to a solution of peronatin
A in chloroform, peronatin A rapidly and cleanly isomer-
izes to form peronatin B (Figure 2b). However, peronatin
B is not stable under these conditions and over many
hours decomposes to give an array of products. As the
peronatin B decomposes, the ratio of peronatin B to
peronatin A stays within the range of 10-20:1. If 1 equiv
of triethylamine is added to the solution, then neither
isomerization or decomposition is observed. Similarly,
3. Rela tive Sta bilities of P er on a tin A a n d
P er on a tin B
1
Previously reported H NMR data for the peronatins
suggested the importance of hydrogen-bonding in the
solution conformation of peronatin B, which is not
apparent in the less stable peronatin A. The inacces-
sibility of a similar hydrogen bonding geometry for
peronatin A was attributed to an unfavorable interaction
between the 2- and 2′-methyl substituents.10 To put this
observation on a more quantitative footing, Monte Carlo
searches were performed on the structures of peronatin
A and B using Macromodel with MM3* force fields
(CHCl3).
(21) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds;
J ohn Wiley & Sons: New York, 1994.
(22) Hassner, A.; Haddadin, M. J . J . Org. Chem. 1963, 28, 224.
Peronatin B possesses a global minimum energy
conformation that allows hydrogen bonding between

4760 J . Org. Chem., Vol. 62, No. 14, 1997
Stachel et al.
F igu r e 2. Isomerization of peronatin A to peronatin B expressed as a percentage of the original concentration of peronatin A
(-b-, peronatin A; -O-, peronatin B); (a) in chloroform, (b) in chloroform + 10 mol % camphorsulfonic acid (note the broken
time scale). Reaction progress was monitored by HPLC using o-nitrotoluene as an internal standard.
in THF, peronatin A is stable over 24 h. It is clear from
these results that acid accelerates both isomerization and
decomposition of the peronatins. At low concentrations
of acid, the rate of formation of peronatin B is comparable
to the rate of decomposition, whereas at high concentra-
tions of acid the rate for formation of peronatin B is much
faster than the rate of decomposition. In nearly stoichio-
metric quantities, BF3 catalyzes the isomerization of
peronatin A to peronatin B, but it also catalyzes decom-
position to a range of products. Similarly, TMSOTf
accelerates both isomerization and decomposition; how-
ever, triflic acid, produced in the reaction mixture, might
be responsible for some of the observed effects.
The electron-rich methoxyperonatin A displayed pat-
terns of isomerization and decomposition that were
similar to peronatin A, but the reactivity was much
higher. 7,7′-Dimethoxyperonatin A could not be stored
without the addition of triethylamine as a stabilizer. In
the context of an acid-catalyzed retro-Mannich C-C bond
cleavage, the decreased stability of 7,7′-dimethoxyper-
onatin A relative to peronatin A is consistent with the
Hammond postulate. The electron-donating o-methoxy
group should stabilize the R-ketoiminium intermediate23
(Scheme 3) and the energetically similar endothermic
transition state.
We clearly wanted to demonstrate the existence of
either radical or ionic species in the reaction mixture.
However, evidence for the existence of radical species,
ionic species, or both would not have distinguished
between a homolytic or heterolytic bond-cleavage mech-
anism. Kinetic evidence is the single best source of
information regarding the initial C-C bond cleavage, and
we have shown that the isomerization is acid catalyzed.
In theory, acid catalysis does not rule out a homolytic
cleavage mechanism. However, since acid-catalyzed ho-
molysis is unprecedented in captodative systems, it seems
reasonable to assume that a traditional retro-Mannich/
Mannich mechanism is operative.
in the isomerization of peronatin A to peronatin B could
not be obtained by trapping or EPR spectroscopy. On
the basis of these results, we are drawn toward the
conclusion that the acid-catalyzed dissociation of perona-
tin A proceeds by the ionic pathway shown in Scheme 3
and that the uncatalyzed isomerization is not important
under ambient conditions. The methoxyperonatins dis-
play a similar pattern of reactivity but are much less
stable than the peronatins.
6. Exp er im en ta l Section
All reactions were run under an atmosphere of dry nitrogen
unless otherwise indicated. Unless otherwise noted, organic
solvents were anhydrous and oxygen free. Melting points are
uncorrected.
(()-P er on a tin B, 8b. To a stirred solution of 2,4-dimeth-
ylindole (0.85 g, 5.86 mmol) in acetic acid (5.9 mL) was added
7 mL of aqueous hydrogen peroxide solution (30% w/w). The
reaction was stirred at room temperature for 2.5 h and then
adjusted to pH 7 with 10% NaOH followed by extraction with
ethyl acetate (3 × 50 mL). The combined organic layers were
dried over MgSO4 and concentrated in vacuo to afford the
crude product. The crude material was adsorbed onto silica
gel (3.0 g) and chromatographed on silica gel (5% ethyl acetate/
hexanes) to provide peronatin B as a yellow solid (0.76 g, 81%).
Spectroscopic data for synthetic peronatin B (IR, 13C NMR,
1H NMR, LRMS, and HRMS) matched the data reported by
Sterner.10
8b: mp 183-186 °C (EtOAc); Rf ) 0.48 20% ethyl acetate/
hexane. Anal. Calcd for C20H20N2O2: C, 74.96; H, 6.30; N,
8.75. Found: C, 74.87; H, 6.34; N, 8.66.
P er on a tin A, 8a . To a stirred solution of 2,4-dimethylin-
dole (0.667 g, 4.70 mmol) in THF (8.0 mL) was added
triethylamine (1.42 g, 14.1 mmol) followed by 5.5 mL of
aqueous hydrogen peroxide solution (30% w/w). The reaction
was stirred at room temperature for 10 days, after which time
it was diluted with H2O (10 mL) and extracted with ethyl
acetate (3 × 30 mL). The combined organic layers were dried
over MgSO4 and concentrated in vacuo to afford the crude
product. Chromatography on silica gel (5% ethyl acetate/
hexanes) provided peronatin A (0.34 g, 45%) in addition to
peronatin B (0.027 g, 4%) and recovered starting material
(0.128 g, 19%). Spectroscopic data for synthetic peronatin A
(IR, 13C NMR, 1H NMR, LRMS, and HRMS) matched the data
reported by Sterner.10
5. Con clu sion
In summary, we have synthesized peronatins A and B
and 7,7′-dimethoxyperonatins A and B using a biomi-
metic indoxyl dimerization. The isomerization of per-
onatin A to peronatin B is catalyzed by acid and inhibited
by base, but firm evidence regarding the intermediates
8a : mp 117-120 °C (CH2Cl2); Rf ) 0.31 in 20% ethyl acetate/
hexane. Anal. Calcd for C20H20N2O2: C, 74.46; H, 6.30; N,
8.75. Found: C, 75.03; H, 6.31; N, 8.74.
7-Meth oxy-2,4-d im eth ylin d ole, 22b. To a solution of
magnesium turnings (0.402 g, 16.53 mmol) in diethyl ether
(17 mL) was added dropwise a solution of 2-bromopropene in
diethyl ether (5 mL). The solution was stirred for 1 h until
all the magnesium was consumed. The freshly prepared
organomagnesium reagent was then added, via cannula, to a
flask containing 4-methyl-2-nitroanisole (0.92 g, 5.51 mmol)
in THF (55mL) at -40 °C. The solution was then stirred at
-40 °C for 1 h. The reaction was quenched by pouring over a
saturated aqueous solution of ammonium chloride (50 mL).
(23) Gas phase HF/6-31G*//HF/6-31G* calculations show the fol-
lowing isodesmic reaction to be exothermic by 1.94 kcal/mol.

Synthesis and Isomerization of Biindolinones
J . Org. Chem., Vol. 62, No. 14, 1997 4761
The solution was then extracted with diethyl ether (3 × 50
mL). The organic layer was dried over MgSO4 and concen-
trated in vacuo to afford the crude product. Chromatography
on silica gel (2% ethyl acetate/hexanes) provided 7-methoxy-
2,4-dimethylindole (0.0492 g, 5%) as a yellow oil.
22b: Rf ) 0.52 in 20% ethyl acetate/hexanes; IR (KBr) 3391,
2936, 1524, 1259, 1091, 792 cm-1; 1H NMR (500 MHz, CDCl3)
δ 8.31 (s, 1H), 6.89 (d, J ) 7.6 Hz, 1H), 6.61 (d, J ) 7.8 Hz,
1H), 6.31 (d, J ) 1.2 Hz, 1H), 4.03 (s, 3H), 2.57 (s, 3H), 2.49
(s, 3H); 13C NMR (125 MHz, CDCl3) δ 143.8, 134.0, 130.0,
125.7, 121.6, 119.3, 101.1, 99.3, 55.3, 18.0, 13.5; LRMS (+ CI)
175 (100), 165 (38), 160 (29), 150 (7), 144 (11), 132 (44); HRMS
(+CI) calcd for C11H13NO, 175.0997, found 175.0991. Anal.
Calcd for C11H13NO: C, 75.39; H, 7.48; N, 8.0. Found: C,
75.50; H, 7.56; N, 8.01.
(()-2,4-Dim et h ylin d olin e. To a stirred solution of 2,4-
dimethylindole (1.0 g, 6.90 mmol) in trifluoroacetic acid (25
mL) was added triethylsilane (11.02 g, 69.0 mmol). The
reaction was refluxed for 22 h, after which time the solvent
was evaporated in vacuo. A 1 N HCl solution (100 mL) was
added to the syrup and the resulting mixture extracted with
ethyl acetate (2 × 50 mL). The aqueous layer was basified
with 2N NaOH and extracted with ethyl acetate (3 × 50mL).
The combined organic layers were dried over MgSO4 and
concentrated in vacuo. The crude material was adsorbed on
silica gel (5.0 g) followed by silica gel chromatography (5%
ethyl acetate/hexanes) to provide 2,4-dimethylindoline as a
clear oil (0.62 g, 61%).
(s, 3H), 2.46 (s, 3H), 2.18 (s, 3H); 13C (125 MHz, CDCl3) δ 140.5,
132.0, 129.4, 128.8, 122.6, 120.5, 113.4, 105.7, 21.9, 18.2, 11.8;
LRMS (CI+) 225(100), 210 (61), 166 (11); HRMS (CI+) calcd
for C11H12ClNS 225.0379, found 225.0374. Anal. Calcd for
C11H12ClNS: C, 58.66; H, 5.37; N, 6.22. Found: C, 58.59; H,
5.39; N, 6.21.
7-Ch lor o-2,4-dim eth ylin dole, 26. To a solution of 7-chloro-
2,4-dimethyl-3-(methylthio)indole (1.02 g, 4.52 mmol) in ab-
solute ethanol (20 mL) was added freshly prepared Raney
nickel (7.0 g). The mixture was stirred at room temperature
for 24 h. The solution was then filtered through a pad of Celite
and concentrated in vacuo. Chromatography on silica gel (5%
dichloromethane/hexanes) provide 7-chloro-2,4-dimethylindole
(0.31 g, 38%) as a clear oil.
26: Rf ) 0.42 (33% dichloromethane/hexanes); IR (neat)
1
3424, 2907, 1231, 932, 790 cm-1; H NMR (500 MHz, CDCl3)
δ 7.98 (s, 1H), 6.98 (d, J ) 7.6 Hz, 1H), 6.76 (d, J ) 7.6 Hz,
1H), 6.21 (s, 1H), 2.44 (s, 3H); 13C (125 MHz, CDCl3) δ 135.2,
132.5, 130.0, 127.8, 120.5, 120.1, 113.0, 100.0, 18.3, 13.5; LRMS
(CI+) 179 (100), 144 (66), 115 (15), 90 (11), 72 (19); HRMS
(CI+) calcd for C10H10ClN 179.0502, found 179.0502. Anal.
Calcd for C10H10ClN: C, 67.02; H, 5.63; N, 7.82. Found: C,
67.11; H, 5.58; N, 7.77.
2-Ch lor o-6-n itr o-4,5-d im eth ylp h en ol, 27. To a stirred
solution of NaNO3 (5.43 g, 63.9 mmol) in 4.6 M HCl (129 mL)
was added a solution of 2-chloro-4,5-dimethylphenol (10.0 g,
63.9 mmol) dissolved in 3:2 diethyl ether:dichloromethane (387
mL). Acetic anhydride (0.72 g, 7.02 mmol) was added drop-
wise, and the biphasic reaction was stirred for 15 min, after
which time the reaction was essentially complete. The aque-
ous layer was removed and extracted with diethyl ether (50
mL). The combined organic layers were then rinsed with H2O
(50 mL). The organic layer was dried over Na2SO4 and
concentrated in vacuo to afford the crude product. Chroma-
tography on silica gel (5% ethyl acetate/hexane) provided
2-chloro-6-nitro-4,5-dimethylphenol as a yellow solid (11.0 g,
86%).
2,4-Dim eth ylin d olin e: Rf ) 0.52 (20% ethyl acetate/
hexanes); IR (neat) 3366, 2961, 1601, 1466, 1256, 767 cm-1
;
1H NMR (500 MHz, CDCl3) δ 6.91 (t, J ) 7.7 Hz, 1H), 6.51 (d,
J ) 7.6 Hz, 1H), 6.43 (d, J ) 7.6 Hz, 1H), 3.97 (dd, J ) 7.2,
15.3 Hz, 1H), 3.76 (s, br, 1H), 3.07 (dd, J ) 8.6, 15.3 Hz, 1H),
2.53 (dd, J ) 7.9, 15.5 Hz, 1H), 2.18 (s, 3H), 1.28 (d, J ) 6.4
Hz, 3H); 13C (125 MHz, CDCl3) δ 150.6, 134.2, 127.5, 127.2,
119.6, 106.6, 54.8, 36.5, 22.5, 18.7; LRMS (CI+) 147 (100), 132
(88), 117 (46); HRMS (CI+) calcd for C10H13N 147.1048, found
147.1051. Anal. Calcd for C10H13N: C, 81.57; H, 8.91; N, 9.52.
Found: C, 81.63; H, 8.94; N, 9.40.
27: mp 124-126 °C (EtOAc); Rf ) 0.41 20% ethyl acetate/
hexane; IR (KBr) 3432, 1528, 1378, 1181, 1048, 874, 756 cm-1
;
(()-N-Acetyl-2,4-d im eth ylin d olin e, 23. To a solution of
2,4-dimethylindoline (0.90 g, 6.08 mmol) in pyridine (20 mL)
was added acetic anhydride (0.96 g, 12.2 mmol). The reaction
was stirred at room temperature for 30 min, after which time
the solvent was evaporated in vacuo. The residue was then
dissolved in ethyl acetate (50 mL) and rinsed with 1 N HCl
(30 mL). The organic layer was dried over MgSO4 and
concentrated in vacuo. Chromatography on silica gel (1%
MeOH/CHCl3) provided N-acetyl-2,4-dimethylindoline (0.931
g, 81%) as a white solid.
1H NMR (300 MHz, CDCl3) δ 8.11 (s, 1H), 7.34 (s, 1H), 2.31
(s, 3H), 2.26 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 145.3, 139.1,
134.2, 130.8, 130.2, 119.3, 19.6, 15.8; LRMS (+CI) 201 (100),
196 (14), 184 (86), 171(7), 154 (36), 140 (6), 128 (9), 120 (15);
HRMS (+CI) calcd for C8H8NO3Cl, 201.0193, found 201.0188.
Anal. Calcd for C8H8NO3Cl: C, 47.76; H, 4.01; N, 6.97.
Found: C, 47.84; H, 3.97; N, 6.94.
2-Ch lor o-6-n itr o-4,5-d im eth yla n isole, 28. To a stirred
solution of 2-chloro-6-nitro-4,5-dimethylphenol (10.27 g, 50.98
mmol) in DMF (100 mL) was added potassium carbonate (7.04
g, 54.6 mmol). The solution immediately turned red in color.
Methyl iodide (7.96 g, 56.1 mmol) was then added dropwise,
and the reaction was stirred at room temperature for 4.5 h.
The reaction was quenched with H2O (500 mL) and extracted
using diethyl ether (3 × 150 mL). The combined organic layers
were washed again with H2O (200 mL). The organic layer was
dried over MgSO4 and concentrated in vacuo to afford the
crude product. Chromatography on silica gel (5% ethyl
acetate/hexanes) provided 2-chloro-6-nitro-4,5-dimethylanisole
as a yellow solid (10.98 g, 100%).
23: mp 54-56 °C (CHCl3); Rf ) 0.59 (5% dichloromethane/
chloroform); IR (neat) 2974, 1652, 1465, 1395, 1125, 990 cm-1
;
1H NMR (300 MHz, CDCl3) δ 7.86 (d, J ) 7.6 Hz, 1H), 7.05 (t,
J ) 7.4 Hz, 1H), 6.83 (d, J ) 7.6 Hz, 1H), 4.57 (m, 1H), 3.37
(m, 1H), 2.58 (d, J ) 15.9 Hz, 1H), 2.21 (s, 3H), 2.19 (s, 3H),
1.20 (d, J ) 6.0 Hz 3H); 13C (125 MHz, CDCl3) δ 167.8, 141.2,
133.9, 129.3, 126.9, 124.2, 114.3, 55.4, 34.6, 23.0, 21.7, 18.2;
LRMS (CI+) 189 (88), 147 (67), 132 (100); HRMS (CI+) calcd
for C12H15CNO 189.1154, found 189.1154. Anal. Calcd for
C12H15NO: C, 76.14; H, 7.99; N, 7.40. Found: C, 76.00; H,
8.05; N, 7.31.
7-Ch lor o-2,4-d im eth yl-3-(m eth ylth io)in d ole, 25. A so-
lution of 2-chloro-5-methylaniline (5.0 g, 35.3 mmol) in dichlo-
romethane (120 mL) was cooled to -78 °C in a dry ice/acetone
bath. tert-Butyl hypochlorite (3.76 g, 35.31 mmol) in dichlo-
romethane (15 mL) was then added, and the mixture was
stirred for 10 min. The bath temperature was raised to -65
°C, and (methylthio)acetone (3.68 g, 35.31 mmol) was added
in dichloromethane (15 mL). The solution was stirred for 1 h
followed by the addition of triethylamine (3.57 g, 35.3 mmol).
The reaction was quenched with H2O (50 mL). The aqueous
layer was removed, and the organic layer was dried over
MgSO4 followed by concentration in vacuo. Chromatography
on silica gel (5% ethyl acetate/hexanes) provided 7-chloro-2,4-
dimethyl-3-(methylthio)indole (7.4 g, 93%) as a red oil.
25: Rf ) 0.40 (5% dichloromethane/hexanes); IR (neat) 3424,
2916, 1725, 1234, 760 cm-1; 1H NMR (300 MHz, CDCl3) δ 8.33
(s, 1H), 6.97 (d, J ) 7.7 Hz, 1H), 6.74 (d, J ) 7.8 Hz, 1H), 2.83
28: mp 35-36 °C (EtOAc); Rf ) 0.62 in 20% ethyl acetate/
hexane; IR (KBr) 2945, 1540, 1285, 1064, 960, 910, 757, 735
1
cm-1; H NMR (300 MHz, CDCl3) δ 7.26 (s, 1H), 3.91 (s, 3H),
2.26 (s, 3H), 2.13 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 147.6,
145.4, 134.7, 132.2, 127.5, 125.1, 62.4, 19.3, 13.9; LRMS (+CI)
216 (61) [MH]+, 199 (100), 185 (10), 169 (7), 154 (7), 137 (10);
HRMS (+CI) calcd for C9H10NO3Cl, 215.0349, found 215.0314.
Anal. Calcd for C9H10NO3Cl: C, 50.22; H, 4.69; N, 6.51.
Found: C, 50.24; H, 4.68; N, 6.48.
2-Meth oxy-5,6-d im eth yla n ilin e, 21b. 2-chloro-6-nitro-
4,5-dimethylanisole (5.0 g, 23.20 mmol) was dissolved in
ethanol (100 mL) in a Parr flask. Acetic acid (5 mL) was added
followed by 10% Pd/C (1.0 g). The flask was then charged to
60 psi with hydrogen and shaken in a Parr apparatus at room
temperature 8 h. The reaction was then filtered through a
pad of Celite and concentrated in vacuo to afford the crude

4762 J . Org. Chem., Vol. 62, No. 14, 1997
Stachel et al.
product. Chromatography on silica gel (5% ethyl acetate/
hexanes) provided 2-methoxy-5,6-dimethyaniline as a clear oil
(2.61 g, 80%).
9b: mp 186 °C dec (CHCl3); Rf ) 0.33 in CHCl3. Anal. Calcd
for C22H24N2O4: C, 69.44; H,6.36; N, 7.37. Found: C, 69.27;
H, 6.45; N, 7.26.
21b: Rf ) 0.48 20% ethyl acetate/hexane; IR (KBr) 3468,
7,7′-Meth oxyp er on a tin A, 9a . To a stirred solution of
7-methoxy-2,4-dimethylindole (0.479 g, 2.74 mmol) in THF (11
mL) was added triethylamine (3.0 g, 21.5 mmol) followed by
8.0 mL of aqueous hydrogen peroxide solution (30% w/w). The
reaction was stirred at room temperature for 72 h, after which
time it was diluted with H2O (10 mL) and extracted with
chloroform (3 × 30 mL). Triethylamine (5 mL) was added to
the combined organic extracts to prevent premature isomer-
ization. The combined organic layers were dried over MgSO4
and concentrated in vacuo to afford the crude product.
Preparative TLC chromatography on silica gel (10% ethyl
acetate/hexane/1% triethylamine) provided the desired 7.7′-
methoxyperonatin A (0.068 g, 13%) as a yellow solid.
9a : Rf ) 0.21 20% ethyl acetate/hexane; 1H NMR (300 MHz,
CDCl3) δ 6.68 (d, J ) 7.9 Hz, 1H), 6.44 (d, J ) 7.8 Hz, 1H),
4.54 (s, 1H), 3.73 (s, 3H), 2.49 (s, 3H), 1.67 (s, 3H); 13C (500
MHz, CDCl3) δ 202.0, 150.4, 144.0, 131.1, 120.5, 116.8, 86.2,
55.6, 23.5, 17.3. The instability of this compound prevented
complete characterization.
Isom er iza tion Exp er im en ts, Gen er a l. Conversion of
peronatin A to peronatin B was monitored by analytical
reversed-phase HPLC. Stationary phase: Rainin C18 mi-
crosorb. Mobile phase: A, 50 mM triethylammonium acetate
buffer, pH 7.50; B, CH3CN. Gradient elution: 40-70% B over
30 min, 254 nm detection.
Isom er iza tion of 8a to 8b in CHCl3. Peronatin A (0.034
g, 0.10 mmol) was dissolved 5 mL of a chloroform solution
(filtered through anhydrous, activated, basic alumina) contain-
ing 0.02 M o-nitrotoluene as an internal standard. The
solution was allowed to stand in the dark under nitrogen.
Aliquots were removed, the chloroform was evaporated, and
the samples were redissolved in an equal volume of THF for
HPLC analysis.
Isom er iza tion of 8a to 8b in CHCl3 in th e P r esen ce of
Ba se. Peronatin A (0.034 g, 0.10 mmol) was dissolved 5 mL
of a chloroform solution (filtered through anhydrous, activated,
basic alumina) containing 0.02 M o-nitrotoluene as an internal
standard. To this solution was added triethylamine (0.01 g,
0.10 mmol). The solution was allowed to stand in the dark
under nitrogen. Aliquots were taken, the chloroform evapo-
rated, and the samples redissolved in an equal volume of THF
for HPLC analysis. No isomerization was observed over 24
h.
Isom er iza tion of 8a to 8b in CHCl3 in th e P r esen ce of
Acid . Peronatin A (0.017 g, 0.05 mmol) was dissolved 2.5 mL
of a chloroform solution (filtered through anhydrous, activated,
basic alumina) containing 0.02 M o-nitrotoluene as an internal
standard. To this solution was added a 0.01 M camphorsul-
fonic acid solution (0.4 mL 0.005 mmol) in chloroform. The
solution was allowed to stand in the dark under nitrogen.
Aliquots were taken, the chloroform was evaporated, and the
samples were redissolved in an equal volume of THF for HPLC
analysis.
Rea ction of 8b in CHCl3 in th e P r esen ce of Acid .
Peronatin B (0.017 g, 0.05 mmol) was dissolved 2.5 mL of a
chloroform solution (filtered through anhydrous, activated,
basic alumina) containing 0.02 M o-nitrotoluene as an internal
standard. To this solution was added a 0.01 M camphlorsul-
fonic acid solution (0.4 mL 0.005 mmol) in chloroform. The
solution was allowed to stand in the dark under nitrogen.
Aliquots were taken, the chloroform was evaporated, and the
samples were redissolved in an equal volume of THF for HPLC
analysis.
1
2937, 1609, 1241, 789, 761 cm-1; H NMR (500 MHz, CDCl3)
δ 6.59 (d, J ) 8.0 Hz, 1H), 6.56 (d, J ) 8.4 Hz, 1H), 3.82 (s,
3H), 2.21 (s, 3H), 2.07 (s, 3H); 13C NMR (125 MHz, CDCl3) δ
145.2, 133.9, 128.6, 120.8, 118.6, 107.2, 55.3, 19.6, 12.7; LRMS
(+CI) 151 (100), 136 (78); HRMS (+CI) calcd for C9H13NO
151.0997, found 151.0993. Anal. Calcd for C9H13NO: C,
71.48; H, 8.67; N, 9.27. Found: C, 71.53; H, 8.68; N, 9.32.
N-(2,3-Dim et h yl-6-m et h oxyp h en yl)a cet a m id e. To a
stirred solution of 2-methoxy-5,6-dimethylaniline (1.85 g, 12.33
mmol) in anhydrous THF (100 mL) at room temperature was
added triethylamine (1.87 g, 18.5 mmol) followed by acetyl
chloride (1.06 g, 13.57 mmol). The mixture was stirred at room
temperature for 3 h. The precipitated triethylamine hydro-
chloride was filtered off, and the filtrate was concentrated in
vacuo. The residue was then dissolved in ethyl acetate (100
mL) and washed with 1 N HCl (50 mL) and then H2O (50 mL).
The organic layer was dried over MgSO4 and concentrated in
vacuo to afford the crude product. Chromatography on silica
gel (2% MeOH/CHCl3) afforded N-(2,3-dimethyl-6-methox-
yphenyl)acetamide as a white solid (2.20 g, 92%).
N-(2,3-Dim eth yl-6-m eth oxyp h en yl)a ceta m id e: mp 146-
148 °C (CHCl3); Rf ) 0.66 in 10% MeOH/CHCl3; IR (KBr) 3248,
2919, 1663, 1478, 1087, 745 cm-1; 1H NMR (300 MHz, DMSO-
d6, 40 °C) δ 8.95 (s, 1H), 6.99 (d, J ) 8.3 Hz, 1H), 6.73 (d, J )
8.3 Hz, 1H), 3.69 (s, 3H), 2.16 (s, 3H), 1.99 (s, 3H); 13C NMR
(75 MHz, DMSO-d6, 40 °C) δ 167.9, 152.8, 134.9, 128.0, 127.6,
125.1, 108.3, 53.3, 22.5, 19.2 14.4; LRMS (+CI) 193 (100), 176
(13), 162 (6), 151 (80), 136 (80), 131 (5), 119 (7); HRMS (+CI)
calcd for C11H15NO2 193.1103, found 193.1102. Anal. Calcd
for C11H15NO2: C, 68.35; H, 7.83; N, 7.25. Found: C, 68.25;
H, 7.88; N, 7.22.
7-Meth oxy-2,4-d im eth ylin d ole, 22b. To a dry 250 mL
round-bottom flask containing N-(2,3-dimethyl-6-methoxyphe-
nyl)acetamide (1.30 g, 6.74 mmol) under N2 was added 95%
sodamide (0.73 g, 17.7 mmol). The mixture was heated slowly
to 250 °C. The brown melted material was then cooled to 100
°C followed by the addition of H2O (20 mL). The mixture was
then refluxed gently for 15 min. The mixture was partitioned
between water (50 mL) and diethyl ether (100 mL). The
aqueous layer was removed and extracted with diethyl ether
(3 × 100 mL). The combined organic layers were dried over
MgSO4 and concentrated in vacuo to afford the crude product.
The crude material was adsorbed on silica gel (10 g) and
chromatographed on silica gel (5% ethyl acetate/hexanes) to
provide 7-methoxy-2,4-dimethylindole as a yellow oil (0.479
g, 41%).
22b: Rf ) 0.52 20% ethyl acetate/hexanes; IR (KBr) 3391,
2936, 1524, 1259, 1091, 792 cm-1; 1H NMR (500 MHz, CDCl3)
δ 8.31 (s, 1H), 6.89 (d, J ) 7.6 Hz, 1H), 6.61 (d, J ) 7.8 Hz,
1H), 6.31 (d, J ) 1.2 Hz, 1H), 4.03 (s, 3H), 2.57 (s, 3H), 2.49
(s, 3H); 13C NMR (125 MHz, CDCl3) δ 143.8, 134.0, 130.0,
125.7, 121.6, 119.3, 101.1, 99.3, 55.3, 18.0, 13.5; LRMS (+CI)
175 (100), 165 (38), 160 (29), 150 (7), 144 (11), 132 (44); HRMS
(+CI) calcd for C11H13NO 175.0997, found 175.0991. Anal.
Calcd for C11H13NO: C, 75.39; H, 7.48; N, 8.0. Found: C,
75.50; H, 7.56; N, 8.01.
(()-7,7′-Meth oxyp er on a tin B, 9b. To a stirred solution
of 7-methoxy-2,4-dimethylindole (0.184 g, 1.02 mmol) in acetic
acid (1.0 mL) was added 1.2 mL of aqueous hydrogen peroxide
solution (30% w/w). The reaction was stirred at room tem-
perature for 30 min and then adjusted to pH 7 with 10% NaOH
followed by extraction with chloroform (3 × 20 mL). The
combined organic layers were dried over MgSO4 and concen-
trated in vacuo to afford the crude product. The crude material
was adsorbed onto silica gel (3.0 g) and chromatographed on
silica gel (50% chloroform/hexane) to provide 7,7′-methoxyper-
onatin B as a yellow solid (0.144 g, 74%). Spectroscopic data
for synthetic 7,7′-dimethoxyperonatin B (IR, 13C NMR, 1H
NMR, LRMS, and HRMS) matched the data reported by
Sterner.1
Ack n ow led gm en t. We are grateful to Professor Tad
Koch for helpful discussions. This material is based
upon work supported by the Camille and Henry Dreyfus
New Faculty Awards Program and the National Science
Foundation under Grant No. CHE-9523521.
J O970388P