A novel biotransformation of benzofurans and related compounds catalysed by a chloroperoxidase

Article abstract of DOI:10.1016/S0040-4020(01)00837-7

The oxidation of 3-alkyl benzofurans, indoles, and a benzothiophene by the chloroperoxidase from Caldariomyces fumago has been investigated. Under conditions in which the catalase activity of chloroperoxidase was minimised in the presence of chloride and hydrogen peroxide, 3-methylbenzothiophene was oxidised at sulfur but the indoles (5-9) and benzofurans (1-4) gave 2,3-diols as initial products. In the case of N-unsubstituted indoles, these tautomerised to give the corresponding lactam. In contrast, the diols (predominantly trans) formed from the benzofurans were sufficiently stable for isolation and full characterisation. This novel reaction has the potential to be developed into a useful synthetic biotransformation.

Full text of DOI:10.1016/S0040-4020(01)00837-7

TETRAHEDRON
Pergamon
Tetrahedron 57 02001) 8581±8587
A novel biotransformation of benzofurans and related compounds
catalysed by a chloroperoxidase
Ricardo G. Alvarez,a Iain S. Hunter,b Colin J. Suckling,a,p Michael Thomasa and Ute Vitiniusa
aDepartment of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G11XL, UK
bDepartment of Pharmaceutical Sciences, Strathclyde Institute for Biomedical Sciences, University of Strathclyde, John Arbuthnott Building,
27 Taylor Street, Glasgow G4 0NR, Scotland, UK
Received 25 May 2001; revised 12 July 2001; accepted 8 August 2001
AbstractÐThe oxidation of 3-alkyl benzofurans, indoles, and a benzothiophene by the chloroperoxidase from Caldariomyces fumago has
been investigated. Under conditions in which the catalase activity of chloroperoxidase was minimised in the presence of chloride and
hydrogen peroxide, 3-methylbenzothiophene was oxidised at sulfur but the indoles 05±9) and benzofurans 01±4) gave 2,3-diols as initial
products. In the case of N-unsubstituted indoles, these tautomerised to give the corresponding lactam. In contrast, the diols 0predominantly
trans) formed from the benzofurans were suf®ciently stable for isolation and full characterisation. This novel reaction has the potential to be
developed into a useful synthetic biotransformation. q 2001 Elsevier Science Ltd. All rights reserved.
The versatility of chloroperoxidases has attracted signi®cant
interest in the ®eld of biotransformations.1,2 Although many
of the earliest examples can be understood by the reaction of
hypohalites generated by the enzymes from hydrogen per-
oxide and halide ion,3 a signi®cant number of reactions in
which non-halogenated, hydroxylated products are formed
have been described. In several of these, enantioselectivity
has been observed, thereby encouraging the further develop-
ment of chloroperoxidases in preparative biotransforma-
tions.4 Chloroperoxidases have also been characterised as
enzymes in the pathways leading to tetracyclines and other
halogenated natural products.5 Chloroperoxidases belong to
several classes; they may be haem-containing,6 have metals
such as vanadium at the active site,7a or have esterase
characteristics.7b Despite the structural and mechanistic
differences, the enzymes typically show preferences to
react with unhindered, electron rich systems such as
aromatic rings and disubstituted alkenes.
During a study of the metabolism of ergot alkaloids, van
Â
a chloroperoxidase 0Scheme 1). Ergot alkaloids are
2
Pee demonstrated an unusual double oxidation catalysed by
2
Scheme 1. a. Transformation reported by van Pee. b. Potential generalisation from that reaction.
Â
Keywords: biotransformations; chloroperoxidases; benzofurans; dihydroxylation.
Corresponding author. Tel.: 144-141-548-2271; fax: 144-141-548-4733; e-mail: c.j.suckling@strath.ac.uk
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020001)00837-7

8582
R. G. Alvarez et al. / Tetrahedron 57 12001) 8581±8587
Scheme 2. Compounds tested with the chloroperoxidase from C. fumago.
complex, polycyclic compounds but the reaction site can
reasonably be described as a cyclic enamine. Hence it can
be asked whether simpler cyclic enamines and other
analogous structures such as enol ethers might undergo a
similar double oxidation. In view of the ready accessibility
of enamines and enol ethers, a double oxidation in one-pot
would be a potentially useful reaction, leading potentially to
1,2-dicarbonyl compounds and their derivatives and, if
stereochemical control can be obtained, to carbohydrate
analogues. We now report the oxidation of electron rich
aromatic compounds such as benzofurans and indoles to
1,2-diols and some derivatives, using the fungal chloro-
peroxidase from Caldariomyces fumago in the presence of
hydrogen peroxide and chloride ion 0Scheme 2).
if the enzyme concentration is too great 0.100 units per
ml), hydrogen peroxide was rapidly destroyed. If the hydro-
gen peroxide concentration is too great 0[H2O2].3 mM),
the enzyme was quickly inactivated. The useful operating
range was found to be [H2O2] 0.06±0.6 mM and [CPO] 60
units per ml.
Under conditions where catalase activity was high, the
predominant products from benzofurans were heterocyclic
ring cleaved compounds such as the ketoformate 12
0Scheme 3). However, at mildly acidic pH, in the presence
of acetone and with careful control of both enzyme and
hydrogen peroxide concentration, it was possible to isolate
signi®cant quantities of 1,2-diol oxidation products of
benzofurans 0derivatives 1±4) by extraction of the reaction
mixture with ethyl acetate 0Scheme 3). When dichloro-
methane was used for extraction, the major product was
the lactone 11, presumably derived by acid-catalysed
dehydration of the diol, and the ketoformate 12. With indole
derivatives 5±7 as substrates, lactams, and a-hydroxyl-
actams were the main compounds isolated. 3-Methyl-
benzothiophene 010) not surprisingly underwent oxidation
1. Results and discussion
The primary problems to overcome were to ®nd reaction
conditions under which the catalase activity of the chloro-
peroxidase was not dominant and the enzyme was not
deactivated by excess hydrogen peroxide. We found that
Scheme 3. Oxidation of benzofurans by chloroperoxidase.

R. G. Alvarez et al. / Tetrahedron 57 12001) 8581±8587
8583
Scheme 4. Oxidation of indoles and thiophene analogues by chloroperoxidase.
Table 1. Percentage of diols and chloride obtained from benzofuran
derivatives. Conditions: 0.19 mmol of substrate dissolved in 4 ml phos-
phate buffer 0.1 M, pH 2.75, 1 ml of acetone, 6 mg sodium chloride, hydro-
gen peroxide added at 0.12 mmol h21, CPO 300 units h21, stirring at room
temperature
were puri®ed by chromatography and unambiguously char-
acterised by MS and NMR spectroscopy. In the case of the
benzofurans, evidence for the presence of the diols comes
from the resonance for the hemiacetal proton at dH 5.5±5.6
and carbon at dC 100±105 ppm. The observation of two
pairs of resonances in unequal amounts indicates a degree
of stereoselectivity in the oxidation. The lower ®eld reso-
nances were more intense than the higher ®eld resonance,
suggesting that the trans-diol isomer is predominant. Unlike
van Pee, who obtained an acetoxy amide as his double
oxidation product, the formation of a diol is a new reaction
of these systems.
Starting material
2,3-Diols
trans/cis Ratiob
2-Chloro 0%)
Yield 0%)a
1
2
3
4
017:18) 78
019:20) 64
021:22) 46
023:24) 50d
10:1
20:1
013) 9
014) 14
015) 26
016) 19
.50:1c
.50:1c
a
Calculated after chromatography puri®cation.
Calculated from 1H NMR spectra.
Compounds 22 and 24 were not detected by H NMR.
b
c
d
In order to investigate the con®guration of the products,
attempts were made to derivatise the diols, for example
with Mosher's acid. However, no derivatives were prepar-
able from even slightly hindered reagents, including benzo-
ate. Treatment of the diols with acetic anhydride and
pyridine did afford a derivative. In the case of the 3-methyl-
benzofuran 01), the diacetate 017-diAc) was recrystallised
and the con®guration determined by X-ray crystallography.
Unfortunately, the diacetate of 17 was found to be racemic
but was exclusively the trans isomer. The initial trans/cis
1
Combined yield based on derivatives.
at sulfur to form the sulfoxide 032) and sulfone 033)
0Scheme 4).
The general pattern of products obtained from the oxidation
of 3-substituted benzofurans was the same for the four
examples investigated 0Table 1). The products of oxidation
Scheme 5. Suggested mechanism for the reaction catalysed by CPO, and possible epimerization of the diols through a hydroxy-aldehyde intermediate.

8584
R. G. Alvarez et al. / Tetrahedron 57 12001) 8581±8587
ratio from the reaction 0Table 1), does not correspond to the
ratio after acetylation but before recrystallisation, indicating
an equilibrium between both isomeric hemiacetals during
the treatment with acetic anhydride±pyridine, probably
through a hydroxy-aldehyde 034) intermediate step 0Scheme
5). In the case of the 3-isopropylbenzofuran 03), it is note-
worthy that only the trans isomer could be isolated, as both
diol and diacetate, probably because of the larger size of the
isopropyl group compared with methyl. In the case of the
benzofuran 3-acetic-acid derivative 04), two of the isolated
products 025, 26) presumably derive from the initial diol
023, 24) that was formed by CPO-catalysed oxidation,
followed by dehydration or intramolecular nucleophilic
attack.
mechanism of the reaction catalysed by the chloroperoxi-
dase from C. fumago 0Scheme 5). Initially, an epoxide is
formed either inside the active site of the enzyme or out with
the active site by the addition of hypochlorite and elimina-
tion of hydrogen chloride. Acid catalysed opening of the
epoxide will give the trans-diol. In the case of benzofurans,
the diols are evidently suf®ciently stable for isolation but in
the case of indoles, dehydration to give the enol tautomer of
the lactams occurs. On the basis of this mechanism it can be
suggested that other cyclic enol ethers and enamines might
also be substrates. The chief limitation of the reactions
discovered so far is the lack of enantiomeric selectivity.
This is in contrast with the well-established arene dioxy-
genase chemistry that also extends to heterocyclic
substrates. It could be that other haloperoxidases might
have more favourable environments at the active site for
enantioselectivity and this is currently being investigated.
In the case of indoles 5±7, the products obtained were
more similar to those obtained by van Pee.2 After a short
reaction time, the lactams 027±29) were the main products
in our experiments. Longer reaction time gave mainly the
a-hydroxylactams 030±31). In all cases with indoles as
substrates, yields were lower than with benzofurans and
also the progress of the reaction was more dif®cult to
monitor; reaction conditions had to be checked more
often. The alcohol 2-01-H-indol-3-yl)-ethanol 08), and the
acid 1-H-indol-3-yl-acetic 09) were also tested as substrates,
but no compounds could be isolated from their biotransfor-
mation, unchanged starting material being recovered. This
indicates that this chloroperoxidase did not accept high
polarity functional groups in the side-chain.
2. Experimental
Compounds 1±5, 7 and 8 were synthesised. Compounds 6, 9
and 10 are commercially available. 1±3 were prepared
following Ref. 8, starting from 1-hydroxyacetophenone,
2-hydroxypropiophenone and 1-02-hydroxyphenyl)-2-
methylpropan-1-one, respectively. Compound
4 was
prepared following Ref. 9, starting from benzofuran-3-
one. Compound 5 was prepared from 6, by treatment with
methyl iodide/potassium carbonate. 7 was obtained from 9,
by treatment with diazomethane. Further reduction with
lithium aluminium hydride gave 8. Chloroperoxidase from
Caldariomyces fumago was bought from Sigma Co. and was
used without further puri®cation.
Since the products of interest are diols, the role of chloride
becomes signi®cant. The oxidation of 3-methylbenzofuran
01) was studied in the presence of varying concentrations of
chloride 0Table 2). If chloride was absent, no diol formation
was observed. At high concentrations of chloride, the reac-
tions proceeded easily, hardly affecting the ratio of diols:
chloro products. At low concentrations, however, the ratio
diols: chloro products increased, but with a considerable
decrease of the rate of the reaction. Thus the proportion of
diols was not strongly dependent upon chloride concentra-
tion, which is consistent with chloride's having a role as a
mediator in the reaction. In some cases, for example
3-isopropylbenzofuran, extra chloride had to be added,
because the reaction stopped when the chloride was
consumed. In control reactions with no enzyme present,
no chlorinated compounds were detected in 12 h. In the
case of indoles, the role of the chloride was similar, because
no reaction took place in the absence of chloride, although
no chlorinated derivatives were isolated from these
substrates.
Low and high resolution mass spectra were obtained on a
Jeol JMS-AX505HA mass spectrometer and on a Micro-
mass Autospec spectrometer. NMR spectra were obtained
on a Bruker AMX 400 spectrometer operating at 400 MHz
1
for H- and 100.6 MHz for 13C NMR. Column chromato-
graphy was performed with silica gel Prolabo 0200±400
mesh).
2.1. Oxidation by chloroperoxidaseÐgeneral method
300 units of CPO were added to a phosphoric acid buffer
solution 04 ml, 0.1 M, pH 2.75), containing acetone 01 ml),
potassium chloride 06 mg, giving 0.02 M solution) and
0.19 mmol of substrate. Hydrogen peroxide solution in
distilled water 01.20 M) was added at
a rate of
0.12 mmol h21 0100 ml h21) through a syringe pump, stir-
ring vigorously. Extra portions of CPO 0150 units) were
added every half hour. The reaction was followed by GC
and continued until starting material had disappeared.
Hydrogen peroxide concentration was checked regularly,
to avoid excess. The reaction was stopped by addition of
saturated aqueous sodium sulphite solution 00.1 ml). The
aqueous phase was extracted three times with ethyl acetate
03£10 ml). The organic layer was separated, dried over
magnesium sulfate and the solvent removed under reduced
pressure. The crude product was puri®ed by ¯ash chromato-
graphy, eluting with petroleum ether±ethyl acetate mixtures
of increasing polarity.
Taken together with the stereochemical outcome of the
reactions, a tentative suggestion can be made for the
Table 2. Product ratio from 3-methylbenzofuran oxidation at different chlo-
ride concentrations
Chloride concentration 00M) Diols:2-chloro ratio trans/cis Diol ratio
1.5
0.4
0.1
0.02
0.004
8:1
8:1
9:1
15:1
40:1
9
10
10
10
10

R. G. Alvarez et al. / Tetrahedron 57 12001) 8581±8587
8585
From 3-methylbenzofuran 01) 025 mg, 0.19 mmol) eluting
with petroleum ether, 2-chloro-3-methyl-benzofuran 013)
was obtained 03 mg, 9%). dH 0400 MHz): 2.21 03H, s,
Me), 7.23±7.30 02H, m, Ar±H), 7.39±7.45 02H, m,
Ar±H). dC 0100 MHz): 7.8 0C-10), 110.6 0C-3), 110.7
0C-8), 118.8 0C-6), 122.9 0C-5), 124.1 0C-7), 129.4 0C-4),
138.0 0C-2), 153.2 0C-9). Found [M]1 m/z 168.0174.
C9H7O37Cl requires 168.0156; found [M]1 m/z 166.0188.
C9H7O35Cl requires 166.0185. Elution with pet. ether/
EtOAc 07:3) gave 3-methyl-2,3-dihydrobenzofuran-2,3-
diol 017, 18) as a mixture 010:1) trans/cis, 024 mg, 78%,
oil) that could not be separated. dH 0400 MHz): 1.61 03H,
s, Me), 5.46 01H, s, H-2), 6.82 01H, d, Jˆ7.6 Hz, H-8), 6.95
01H, dt, Jˆ7.6 and 1.0 Hz, H-6), 7.25 01H, dt, Jˆ7.6 and
1.0 Hz, H-7), 7.32 01H, dd, Jˆ7.6 and 1.0 Hz, H-5) 0major
isomer). dC 0100 MHz): 23.8 0C-10), 75.9 0C-3), 105.4
0C-2), 110.8 0C-8), 121.5 0C-6), 123.7 0C-5), 130.3 0C-4),
130.8 0C-7), 156.9 0C-9). Found [M]1 m/z 166.0634.
C9H10O3 requires 166.0630. LRMS m/z 0rel. int.) 166
[M]1 035), 137 0100), 123 033), 121 036), 105 068), 91
039), 77 044).
30.5 0C-10), 78.9 0C-3), 101.5 0C-2), 110.7 0C-8), 121.4
0C-6), 124.1 0C-5), 129.2 0C-4), 130.7 0C-7), 157.3 0C-9).
Found [M]1 m/z 180.0786. C10H12O3 requires 180.0786.
LRMS m/z 0rel. int.) 180 [M]1 018), 157 027), 151 044),
133 033), 123 095), 131 0100), 105 030), 95 030).
Treatment of the diol 019, 20) 022 mg) with acetic
anhydride±pyridine overnight, at room temperature gave,
eluting with petroleum ether/EtOAc 09:1) trans-diacetate
of
3-ethyl-2,3-dihydrobenzofuran-2,3-diol
019-diAc)
015 mg, 46%, oil). dH 0400 MHz): 0.99 03H, t, Jˆ7.4 Hz,
H-11), 2.00, 2.14 03H each, s, Me), 2.09 and 2.87 01H each,
dq, Jˆ7.4 and 14.8 Hz, H-10), 6.72 01H, s, H-2), 6.93 01H,
d, Jˆ7.5 Hz, H-8), 7.01 01H, dt, Jˆ7.5 and 1.0 Hz, H-6),
7.30 01H, dt, Jˆ7.5 and 1.0 Hz, H-7), 7.59 01H, dd, Jˆ7.5
and 1.0 Hz, H-5). dC 0100 MHz): 8.6 0C-11), 23.4 0C-10),
20.9 and 21.7 0AcO±), 90.4 0C-3), 101.6 0C-2), 110.8 0C-8),
121.6 0C-6), 127.3 0C-5), 126.1 0C-4), 131.2 0C-7), 159.3
0C-9), 169.2 and 169.5 0AcO±). Found [M]1 m/z 264.0990.
C14H16O5 requires 264.0998. LRMS m/z 0rel. int.) 264 [M]1
05), 204 010), 179 016), 162 0100), 151 031), 147 065), 145
012), 144 039), 115 033). Further elution gave the cis-
derivative 020-diAc) 013 mg, 40%, oil). dH 0400 MHz):
0.87 03H, t, Jˆ7.5 Hz, H-11), 2.04, 2.14 03H each, s, Me),
2.03 and 2.41 01H each, dq, Jˆ7.5 and 15.0 Hz, H-10), 6.74
01H, s, H-2), 6.92 01H, d, Jˆ7.5 Hz, H-8), 7.05 01H, dt,
Jˆ7.5 and 1.0 Hz, H-6), 7.30 01H, dt, Jˆ7.5 and 1.0 Hz,
H-7), 7.36 01H, dd, Jˆ7.5 and 1.0 Hz, H-5). dC 0100 MHz):
6.9 0C-11), 20.8 and 21.0 0AcO±), 28.6 0C-10), 87.0 0C-3),
99.8 0C-2), 111.1 0C-8), 122.1 0C-6), 124.6 0C-5), 126.8
0C-4), 130.4 0C-7), 156.4 0C-9), 168.9 and 169.8 0AcO±).
Found [M]1 m/z 264.0995. C14H16O5 requires 264.0998.
LRMS m/z 0rel. int.) 264 [M]1 022), 204 011), 193 010),
179 037), 162 0100), 151 099), 147 070), 144 036), 134
029), 115 036), 91 024).
Treatment of the crude mixture 017, 18) 010 mg) with acetic
anhydride±pyridine overnight, at room temperature gave,
eluting with petroleum ether/EtOAc 09:1), trans-diacetate
of 3-methyl-2,3-dihydro-benzofuran-2,3-diol 017-diAc),
010 mg, 67%): mp 0pet. ether/EtOAc) 127±1288. dH
0400 MHz): 1.90, 2.00, 2.14 03H each, s, Me), 6.80 01H, s,
H-2), 6.94 01H, d, Jˆ7.5 Hz, H-8) 7.03 01H, dt, Jˆ7.5 and
1.0 Hz, H-6), 7.32 01H, dt, Jˆ7.5 and 1.0 Hz, H-7), 7.53
01H, dd, Jˆ7.5 and 1.0 Hz, H-5). dC 0100 MHz): 17.7
0C-10), 20.9 and 21.9 0AcO±), 87.4 0C-3), 101.7 0C-2),
110.8 0C-8), 121.9 0C-6), 126.1 0C-5), 126.8 0C-4), 131.3
0C-7), 158.9 0C-9), 169.2 and 169.9 0AcO±). Found [M]1
m/z 250.0837. C13H14O5 requires 250.0841. LRMS m/z 0rel.
int.) 250 [M]1 064), 207 024), 190 028), 180 058), 165 038),
148 073), 137 0100), 120 031). Further elution gave the cis-
derivative 018-diAc) which did not readily crystallise,
03.5 mg, 23%). dH 0400 MHz): 1.79, 2.04, 2.13 03H each,
s, Me), 6.73 01H, s, H-2), 6.92 01H, d, Jˆ7.5 Hz, H-8), 7.06
01H, dt, Jˆ7.5 and 1.0 Hz, H-6), 7.29 01H, dt, Jˆ7.5 and
1.0 Hz, H-7), 7.38 01H, dd, Jˆ7.5 and 1.0 Hz, H-5). dC
0100 MHz): 20.8 and 21.1 0AcO±), 23.9 0C-10), 84.0
0C-3), 99.9 0C-2), 111.1 0C-8), 122.5, 123.5 0C-5 and
C-6), 128.9 0C-4), 130.4 0C-7), 155.8 0C-9), 168.9, 169.9
0AcO±). Found [M]1 m/z 250.0848. C13H14O5 requires
250.0841. LRMS m/z 0rel. int.) 250 [M]1 049), 207 011),
179 034), 165 068), 148 082), 137 0100), 120 063), 91 027).
From 3-isopropylbenzofuran 03), 030 mg, 0.19 mmol)
eluting with petroleum ether, 2-chloro-3-isopropylbenzo-
furan 015), was obtained 09 mg, 26%, oil). dH 0400 MHz):
1.41 06H, t, Jˆ7.1 Hz, Me), 3.17 01H, m, H-10), 7.20±7.30
02H, m, Ar±H), 7.41 01H, m, Ar±H), 7.59 01H, m, Ar±H).
Elution with pet. ether/EtOAc 06:4) gave 3-isopropyl-2,3-
dihydro-benzofuran-2,3-diol 021), 017 mg, 46%, oil). dH
0400 MHz): 0.94 and 1.04 03H each, d, Jˆ6.9 Hz, Me),
2.26 01H, m, H-10), 2.50 01H, br s, ±OH), 4.59 01H, br s,
±OH), 5.62 01H, s, H-2), 6.83 01H, d, Jˆ7.5 Hz, H-8), 6.95
01H, dt, Jˆ7.5 and 1.0 Hz, H-6), 7.25 01H, dt, Jˆ7.5 and
1.0 Hz, H-7), 7.28 01H, dd, Jˆ7.5 and 1.0 Hz, H-5). dC
0100 MHz): 16.1 0C-12), 17.4 0C-11), 34.7 0C-10), 81.7
0C-3), 101.5 0C-2), 110.6 0C-8), 121.3 0C-6), 124.4 0C-5),
128.4 0C-4), 130.7 0C-7), 157.8 0C-9). Found [M]1 m/z
194.0948. C11H14O3 requires 194.0943. LRMS m/z 0rel.
int.) 194 [M]1 013), 165 022), 151 034), 133 073), 121
023), 105 024), 86 0100).
From 3-ethylbenzofuran 02) 028 mg, 0.19 mmol) eluting
with petroleum ether, 2-chloro-3-ethyl-benzofuran 014),
was obtained 05 mg, 14%). dH 0400 MHz): 1.28 03H, t,
Jˆ7.6 Hz, H-11), 2.69 02H, q, Jˆ7.6 Hz, H-10), 7.23±
7.29 02H, m, Ar±H), 7.41 01H, m, Ar±H), 7.49 01H, m,
Ar±H). Elution with pet. ether/EtOAc 07:3) gave 3-ethyl-
2,3-dihydrobenzofuran-2,3-diol 019, 20), as a mixture 020:1)
trans/cis 022 mg, 64%, oil) that could not be separated. dH
0400 MHz): 1.03 03H, t, Jˆ7.5 Hz, H-11), 1.93 and 2.05 01H
each, dq, Jˆ7.5 and 15.0 Hz, H-10), 2.64 01H, br s, ±OH),
4.15 01H, d, Jˆ8.0 Hz, ±OH), 5.57 01H, d, Jˆ8.0 Hz, H-2),
6.87 01H, d, Jˆ7.5 Hz, H-8), 6.99 01H, dt, Jˆ7.5 and
1.0 Hz, H-6), 7.29 01H, dt, Jˆ7.5 and 1.0 Hz, H-7), 7.33
01H, dd, Jˆ7.5 and 1.0 Hz, H-5). dC 0100 MHz): 7.6 0C-11),
Treatment of the diol 021) 010 mg) with acetic anhydride±
pyridine overnight, at room temperature gave, after
removing solvent under vacuum, trans-diacetate of 3-iso-
propyl-2,3-dihydro-benzofuran-2,3-diol 021-diAc), 012 mg,
84%, colourless crystals): mp 93±958 0pet. ether/EtOAc).
dH 0400 MHz): 0.69 and 0.99 03H each, d, Jˆ6.8 Hz, Me),
2.04 and 2.14 03H each, s, AcO±), 3.03 01H, m, H-10), 6.79
01H, s, H-2), 6.91 01H, d, Jˆ7.5 Hz, H-8), 7.04 01H, dt,

8586
R. G. Alvarez et al. / Tetrahedron 57 12001) 8581±8587
Jˆ7.5 and 1.0 Hz, H-6), 7.31 01H, dt, Jˆ7.5 and 1.0 Hz,
H-7), 7.34 01H, dd, Jˆ7.5 and 1.0 Hz, H-5).dC 0100 MHz):
15.6 0C-12), 16.4 0C-11), 20.8 and 21.1 0AcO±), 31.8
0C-10), 89.9 0C-3), 99.3 0C-2), 110.8 0C-8), 121.9 0C-6),
125.3 0C-5), 123.9 0C-4), 130.6 0C-7), 157.3 0C-9), 168.9
and 169.7 0AcO±). Found [M]1 m/z 278.1151. C15H18O5
requires 278.1154. LRMS m/z 0rel. int.) 278 [M]1 019),
235 09), 218 09), 193 059), 176 086), 165 027), 161 0100),
158 042), 133 045), 131 015).
20%, oil), a compound that was not present in the crude
reaction mixture. dH 0400 MHz): 2.53 01H, br s, ±OH),
3.18 and 3.24 01H each, d, Jˆ18.1 Hz, H-10), 6.22 01H, s,
H-2), 7.00 01H, d, Jˆ7.5 Hz, H-8), 7.12 01H, dt, Jˆ7.5 and
0.8 Hz, H-6), 7.40 01H, dt, Jˆ7.5 and 0.8 Hz, H-7), 7.42
01H, dd, Jˆ7.5 and 0.8 Hz, H-5). dC 0100 MHz): 41.0
0C-10), 83.2 0C-3), 111.7 0C-2), 112.0 0C-8), 123.6 0C-6),
124.2 0C-5), 127.5 0C-4), 132.4 0C-7), 158.0 0C-9), 171.4
0C-11). Found [M]1m/z 192.0403 C10H8O4 requires
192.0423. LRMS m/z 0rel. int.) 192 [M]1 011), 248 06),
164 010), 163 0100), 121 045), 91 011).
From methyl benzofuran-3-yl-acetate 04) 036.5 mg,
0.19 mmol), adding under rigorous control, hydrogen per-
oxide and CPO for 10 h, and treating the crude extract with
acetic anhydride±pyridine overnight at room temperature,
eluting with pet. ether/EtOAc 019:1), the 2-chloro derivative
016) was obtained 08 mg, 19%, oil). dH 0400 MHz): 3.69
02H, s, H-10), 3.73 03H, s, ±OMe), 7.25±7.32 02H, m,
Ar±H), 7.43 01H, m, Ar±H), 7.49 01H, m, Ar±H). dC
0100 MHz): 29.5 0C-10), 52.3 0±OMe), 108.4 0C-3), 111.0
0C-8), 119.2 0C-5), 123.4 and 124.6 0C-5, C-7), 128.2 0C-4),
139.6 0C-2), 153.4 0C-9), 170.1 0±COOMe). Found
[M]1m/z 226.0212. C11H9O337Cl requires 226.0211.
LRMS m/z 0rel. int.) 226 [M]1 014), 224 044), 167 033),
165 0100), 139 05), 137 015), 102 040), 101 069), 83 018),
75 040). Further elution gave starting material 04) 05 mg).
Elution with pet. ether/EtOAc 04:1) gave methyl 2-acetoxy-
benzofuran-3-ylideneacetate 025), 08 mg, 17%, oil). dH
0400 MHz): 2.14 03H, s, AcO±), 3.78 03H, s, ±OMe), 6.38
01H, d, Jˆ1.9 Hz, H-2), 6.99 01H, dd, Jˆ7.7 Hz, H-8), 7.04
01H, dt, Jˆ7.7 and 0.9 Hz, H-6), 7.40 01H, dt, Jˆ7.7 and
0.9 Hz, H-7), 7.51 01H, dd, Jˆ7.7 and 0.9 Hz, H-5), 7.62
01H, d, Jˆ1.9 Hz, H-10). Elution with pet. ether/EtOAc
06:4) gave methyl 2,3-diacetoxy, 2-3-dihydrobenzofuran-
3-yl-acetate 023-diAc), 016 mg, 27%, oil). dH 0400 MHz):
1.98 and 2.08 03H each, s, AcO±), 3.12 and 4.10 01H each,
d, Jˆ16.5 Hz, H-10), 3.72 03H, s, ±OMe), 6.87 01H, s, H-2),
6.96 01H, d, Jˆ7.6 Hz, H-8), 7.03 01H, dt, Jˆ7.6 and
1.0 Hz, H-6), 7.34 01H, dt, Jˆ7.6 and 1.0 Hz, H-7), 7.62
01H, dd, Jˆ7.6 and 1.0 Hz, H-5). dC 0100 MHz): 20.7 and
21.7 0AcO±), 35.8 0C-10), 52.0 0±OMe), 86.6 0C-3), 101.4
0C-2), 111.2 0C-8), 121.8 0C-6), 124.8 0C-4), 127.5 0C-5),
131.8 0C-7), 159.3 0C-9), 168.5, 169.5 and 169.8 0±CO±).
Found [M]1 m/z 308.0852. C15H16O7 requires 308.0896.
LRMS m/z 0rel. int.) 308 [M]1 030), 248 06), 237 016),
223 042), 206 093), 195 082), 163 039), 146 0100), 121
082). Elution with pet. ether/EtOAc 01:1), gave methyl
2-acetoxy-3-hydroxy-2-3-dihydrobenzofuran-3-yl-acetate
acid 023-Ac), 03 mg, 6%, oil). dH 0400 MHz): 2.08 03H, s,
AcO±), 3.00 and 3.14 01H each, d, Jˆ17.0 Hz, H-10), 3.80
03H, s, ±OMe), 6.64 01H, s, H-2), 6.96 01H, dd, Jˆ7.5 and
0.9 Hz, Ar±H), 7.04 01H, dt, Jˆ7.5 and 0.9 Hz, Ar±H),
7.31±7.35 02H, m, Ar-H). dC 0100 MHz): 20.7 0AcO±),
38.5 0C-10), 52.3 0±OMe), 80.0 0C-3), 103.7 0C-2), 111.2
0C-8), 122.3 and 123.7 0C-5, C-6), 127.9 0C-4), 131.3 0C-7),
158.4 0C-9), 168.8 and 172.9 0±CO±). Found [M]1m/z
266.0808. C13H14O6 requires 266.0790. LRMS m/z 0rel.
int.) 266 [M]1 010), 248 04), 223 020), 206 029), 195 037),
163 037), 146 053), 121 0100).
From 3-methylindole 06) 025 mg, 0.19 mmol), and stopping
the reaction after 4 h, eluting with petroleum ether/EtOAc
07:3), 3-methyl-1,3-dihydroindol-2-one 028)9 was obtained
013 mg, 47%), as a colourless solid: mp 100±102 8C 0pet.
ether/EtOAc) 0Lit.11 1208C). dH0400 MHz): 1.51 03H, d,
Jˆ7.7 Hz, H-10), 3.47 01H, q, Jˆ7.7 Hz, H-3), 6.91 01H,
d, Jˆ7.5 Hz, H-8), 7.04 01H, dt, Jˆ7.5 and 0.9 Hz, H-6),
7.18±7.24 02H, m, H-5 and H-7), 8.55 01H, br s, N±H). dC
0100 MHz): 15.2 0C-10), 41.0 0C-3), 109.6 0C-8), 122.4 and
123.8 0C-5, C-6), 127.8 0C-7), 131.2 0C-4), 141.1 0C-9),
181.2 0C-2). Found [M]1m/z 147.0683. C9H9NO requires
147.0684. LRMS m/z 0rel. int.) 147 [M]1 0100), 132 020),
119 061), 118 030), 91 010), 77 07).
When the reaction was continued for a longer time 010 h),
the main isolated compound was 3-hydroxy, 3-methyl-1,3-
dihydroindol-2-one 031),10 07 mg, 23%), as a white powder:
mp 147±1498C,11 0pet. ether/EtOAc) 0Lit.12 161±1628C).
dH 0400 MHz): 1.63 03H, s, H-9), 2.88 01H, br s, ±OH),
6.89 01H, d, Jˆ7.6 Hz, H-8), 7.10 01H, dt Jˆ7.6 and
0.9 Hz, H-6), 7.28 01H, dt, Jˆ7.5 and 0.9 Hz, H-7), 7.41
01H, dd, Jˆ7.5 and 0.9 Hz, H-5), 7.85 01H, br s, ±NH). dC
0100 MHz): 24.8 0C-10), 73.9 0C-3), 110.3 0C-8), 123.3 and
123.9 0C-5, C-6), 129.7 0C-7), 131.7 0C-4), 139.7 0C-9),
180.2 0±CO±). Found [M]1 m/z 163.0641. C9H9NO2
requires 163.0633. LRMS m/z 0rel. int.) 163 [M]1 08), 145
0100), 135 010), 120 020), 117 072), 90 056).
From 1,3-dimethylindole 05) 028 mg, 0.19 mmol), and stop-
ping the reaction after 4 h, eluting with petroleum ether/
EtOAc 07:3), 1,3-dimethyl-1,3-dihydroindol-2-one 027),13
was obtained 016 mg, 52%). dH 0400 MHz): 1.49 03H, d,
Jˆ7.6 Hz, H-10), 3.22 03H, s, ±NMe), 3.44 01H, q,
Jˆ7.6 Hz, H-3), 6.83 01H, d, Jˆ7.5 Hz, H-8), 7.06 01H,
dt, Jˆ7.5 and 0.9 Hz, H-6), 7.24 01H, d, Jˆ7.5 Hz, H-5),
7.28 01H, t, Jˆ7.5 Hz, H-7). dC 0100 MHz): 15.3 0C-10),
26.1 0C-3), 40.5 0±NMe), 107.9 0C-8), 122.4 and 123.4
0C-5, C-6), 127.8 0C-7), 130.7 0C-4), 144.0 0C-9), 178.7
0C-2). Found [M]1 m/z 161.0855. C10H11NO requires
161.0841. LRMS m/z 0rel. int.) 161 [M]1 0100), 146 054),
132 023), 118 079), 91 019), 77 016). Elution with pet. ether/
EtOAc 01:1) gave 3-hydroxy-1,3-dimethyl-1,3-dihydroin-
dol-2-one 030),14 03 mg, 9%) as a white powder: mp 140±
1428C 0pet. ether/EtOAc) 0Lit.14 148±1498C). dH
0400 MHz): 1.61 03H, s, H-10), 2.67 01H, br s, ±OH),
3.21 03H, s, ±NMe), 6.86 01H, d, Jˆ7.5 Hz, H-8), 7.12
01H, dt, Jˆ7.5 and 0.9 Hz, H-6), 7.34 01H, dt, Jˆ7.5 and
0.9 Hz, H-5), 7.42 01H, dd, Jˆ7.5 and 0.9 Hz, H-7). dC
0100 MHz): 24.8 0C-10), 26.2 0±NMe), 73.6 0C-3), 108.5
0C-8), 123.2 and 123.4 0C-5, C-6), 129.7 0C-7), 131.3 0C-4),
142.9 0C-9), 178.4 0C-2). Found [M]1 m/z 178.0770.
When the crude mixture was not treated with acetic
anhydride±pyridine, but puri®ed by column chromato-
graphy, 3a-hydroxy-3a,8a-dihydro-3H-1,8-dioxacyclo-
penta[a]inden-2-one 026) could also be isolated, 07 mg,

R. G. Alvarez et al. / Tetrahedron 57 12001) 8581±8587
8587
4. 0a) Dexter, A. F.; Hakner, F. J.; Campbell, R. A.; Hager, A. P.
J. Am. Chem. Soc. 1995, 117, 6412±6413. 0b) Hu, S.; Hager,
L. P. J. Am. Chem. Soc. 1999, 121, 872±873.
C10H11NO2 requires 177.0900. LRMS m/z 0rel. int.) 177
[M]1 0100), 162 055), 160 022), 149 028), 134 084), 132
027), 116 010), 106 013), 91 09).
5. Vandevelde, F.; Arends, I. W. C. E.; Sheldon, R. A. J. Inorg.
Biochem. 2000, 80, 81±89.
From methyl 1-H-indol-3-yl-acetate 07) 036 mg,
0.19 mmol), eluting with pet. ether/EtOAc 06:4), methyl
ester of 2-oxo-2,3-dihydro-1H-indol-3-yl-acetic acid 029)15
was obtained 023 mg, 59%). dH 0400 MHz): 2.82 01H, dd,
Jˆ17.0 and 8.1 Hz, H-10), 3.07 01H, dd, Jˆ17.0 and 4.5 Hz,
H-10), 3.68 03H, s, ±OMe), 3.80 01H, dd, Jˆ8.1 and 4.5 Hz,
H-3), 6.86 01H, d, Jˆ7.5 Hz, H-8), 7.00 01H, t, Jˆ7.5 Hz,
H-6), 7.19±7.24 02H, m, H-5 and H-7), 7.86 01H, br s,
±NH). Found [M]1 at m/z 205.0719. C11H11NO3 requires
205.0739. LRMS m/z 0rel. int.) 205 [M]1 045), 174 08), 173
011), 146 041), 145 0100), 132 012), 128 013), 117 037), 90
08).
6. Hudlicky, T.; Entwistle, D. A.; Pitzner, K. K.; Thorpe, A. J.
Chem. Rev. 1996, 96, 443.
7. 0a) Vilter, H. Phytochemistry 1984, 23, 1387±1390.
0b) Messerchsmidt, A.; Wever, R. Proc. Natl Acad. Sci. USA
1996, 93, 392±396. 0c) Itoh, N.; Kawanami, T.; Liu, J. Q.;
Dairi, T.; Miyakoshi, M.; Nitta, C.; Kimoto, Y. Biochim.
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1 1996, 1757±1765.
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Acknowledgements
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We acknowledge the ®nancial support from BBSRC 0ROPA
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