Plant-growth inhibitory activity of heliannuol derivatives

Article abstract of DOI:10.1016/j.phytochem.2004.03.035

In addition to (+)-, (-)- and (±)-heliannuol E, growth-inhibitory activities of five synthetic chromans and four tetrahydrobenzo[b]oxepins were examined against oat and cress. All heliannuol E isomers exhibited similar biological activities against cress,

Full text of DOI:10.1016/j.phytochem.2004.03.035

PHYTOCHEMISTRY
Phytochemistry 65 (2004) 1405–1411
www.elsevier.com/locate/phytochem
Plant-growth inhibitory activity of heliannuol derivatives
a
Fuminao Doi , Taiga Ohara , Takahisa Ogamino , Takeshi Sugai ,
a
a
a
b
Keiko Higashinakasu , Kosumi Yamada , Hideyuki Shigemori , Koji Hasegawa ,
b
b
b
a,
*
Shigeru Nishiyama
a
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223-8522, Japan
b
Institute of Applied Biochemistry, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-0044, Japan
Received 10 November 2003; received in revised form 22 March 2004
Abstract
In addition to (+)-, ())- and ðꢀÞ-heliannuol E, growth-inhibitory activities of five synthetic chromans and four tetra-
hydrobenzo[b]oxepins were examined against oat and cress. All heliannuol E isomers exhibited similar biological activities against
cress, whereas when tested against oat roots, the unnatural optical isomer (+) showed no inhibitory activity. Four brominated
chromans and two tetrahydrobenzo[b]oxepin derivatives also showed apparent inhibition against both cress and oat.
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Helianthus annuus L.; Compositae; Heliannuol; Plant-growth inhibition; Allelochemicals
1. Introduction
2003) have been reported utilising phenol derivatives in
a biomimetic way or benzopyrone derivatives as inter-
mediates. In this context, we have independently per-
formed the total synthesis of (+)-, ())- and (ꢀ)-
heliannuol E 1 by employing the electrochemically
generated spirodienone, with concomitant rearrange-
ment to the corresponding chroman framework as key-
steps (Doi et al., 2003a,b). During the investigation, a
number of heliannuol-related compounds (1–10) were
obtained as by-products or synthetic intermediates. We
describe herein their growth-inhibition of oat (Avena
sativa L.) and cress (Lepidium sativum L.) (Fig. 2).
Heliannuols are a family of allelochemicals of sun-
flower origin (Helianthus annuus L.) (Macias et al., 1993,
1994, 1999a,b, 2002) (Fig. 1). Their biosynthesis might
be through bisabolene-type terpene precursors, which
undergo oxidation reactions to introduce oxygen-func-
tionalities, followed by cyclization including ring-con-
traction, leading to heliannuols (Macias et al., 1994).
Their herbicidal activities against a wide range of
monocotyledons and dicotyledons were reported (Ma-
cias et al., 1994, 1999b), although detailed activities of
heliannuol E 1 have not been described (Macias et al.,
1999a). Such chemicals would be effective candidates for
herbicide from the viewpoint of environmental con-
cerns. Synthetic studies on heliannuols A (Grimm et al.,
1994; Takabatake et al., 2000; Tuhina et al., 2002;
Kishuku et al., 2003), C (Kamei et al., 2003), D (Vyvyan
and Looper, 2000; Takabatake et al., 2000; Tuhina
et al., 2002; Macias et al., 2003), E 1 (Sato et al., 2001)
and its methyl derivative (Sabui and Venkatesmaran,
2. Results and discussion
2.1. Synthesis
In addition to (+)-, ())-, ðꢀÞ-heliannuol E compounds
1, 3 and 4 (Doi et al., 2003a,b), 2, 5–10 were synthesized
to understand structure–activity relationships. As can be
seen in Schemes 1 and 2, their synthesis involved the
electrochemical spirodienone-construction, followed by
rearrangement (Mori et al., 2001) as the key process.
Constant current electrolysis (CCE) of 11, produced by a
* Corresponding author. Tel.: +81-45-566-1717; fax: +81-45-566-
1717.
E-mail address: nisiyama@chem.keio.ac.jp (S. Nishiyama).
0031-9422/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.phytochem.2004.03.035

1406
F. Doi et al. / Phytochemistry 65 (2004) 1405–1411
Me
yield of below 20% was observed under CCE conditions.
The subsequent rearrangement reaction provided 5 and 6
in 36% and 14% yields, respectively.
HO
Me
HO
Me
8
6
Me
OH
OH
O
O
Compounds 7–10 were synthesized to investigate the
activities of the tetrahydrobenzo[b]oxepin framework,
shared by heliannuols B, C and D (Fig. 2). The simple
benzo[b]oxepin derivative 7 was produced by the above-
mentioned procedure from 16 (Baird and Winstein,
1962) through the spirodienone 17. Upon comparison of
two methods to produce 17, anodic oxidation provided
a better yield (40%) than the 36% of PIFA oxidation.
To obtain the modified derivatives 8–10, 18 produced
by TBS protection of 4-bromo-2-methylphenol (Klar-
mann et al., 1933) was reacted with 6-methyl-5-hepten-2-
one to give 19 (68%). The coupling product was converted
into 21 through olefin 20. Anodic oxidation of 21 gave the
spiro derivative 22 in 62% yield, which was alternatively
produced by PIFA, although the yield using the chemical
oxidant (27%) was lower than that of the anodic oxida-
tion, as in the case of 17. Treatment of 22 with BF3 ꢁ OEt2
gave diastereomeric 8 in 77% yield, which was successively
converted by hydrogenolysis into 9 and 10 in 23% and
65% yields, respectively.
Me
Me
Me
Heliannuol E (1)
Heliannuol A
Me
HO
HO
Me
7
OH
7
Me
O
Me
OH
O
Me
Me
Me
Heliannuol B
Me
Heliannuol C
HO
7
Me
O
Me
OH
Me
Heliannuol D
Fig. 1.
standard procedure from 4-hydroxy-3-methylbenzalde-
hyde through a cinnamic acid derivative, provided the
spiro compound 12 in low yield, owing to a considerable
amount of by-products including preowned polymers.
This difficulty was circumvented by introduction of a
bromine substituent at the ortho-position of a phenol
group (Doi et al., 2003a) to accomplish our total syn-
thesis of 1. Exposure of 12 to BF3 ꢁ OEt2 effected a 1,2-
shift reaction, leading to 2 as the sole product in 62%
yield, while a bromo derivative of 11 afforded a 5:1
mixture of 3 and 4 (Doi et al., 2003a). To synthesize 5
and 6, partially masked 11 was oxidized to give 13 in 81%
yield, which was submitted to the Wittig reaction. The
resultant olefin was oxidized to give diol 14, while sub-
sequent removal of the protecting group followed by
bromination gave 15. Upon oxidation with [bis(triflu-
oroacetoxy)iodo]benzene (PIFA), the corresponding
spiro derivative was obtained in 69% yield, whereas a
2.2. Bioassay
The chroman and benzo[b]oxepin derivatives, 2–10
along with (+)-, ())- and ðꢀÞ-heliannuol E 1, were sub-
jected to assessment of growth-inhibition against the
shoot- and root-growth of cress (L. sativum L.) and oat
seedlings (A. sativa L.). The doses required for 50% in-
hibition, as interpolated from the dose–response curve,
are shown in Table 1. Natural ())-1 exhibited inhibitory
activity at 10ꢂ3 M concentration against both cress and
oat, while unnatural (+)- and ðꢀÞ-1 showed no effect
HO
Me
HO
Me
HO
HO
Me
*
Me
OH
Me
OH
*
Me
OH
O
O
Me
O
O
Me
Me
Me
(-)-1
(+)-1
( )-1
2
Br
Me
Br
Me
HO
Me
HO
Br
HO
Me
HO
Br
Me
OH
Me
OH
O
O
O
O
3
Me
Me
4
5
6
Me
Me
Br
Me
Br
HO
Br
HO
Me
HO
Br
O
O
O
Me
OH
Me
OH
Me
Me
7
8 mix
Me
Me
Me
HO
HO
Me
O
Me
OH
O
Me
OH
9
10
Me
Me
Fig. 2.

F. Doi et al. / Phytochemistry 65 (2004) 1405–1411
1407
Me
an important role in the inhibitory effect against oat. This
regio-recognition was apparent contrast to that of 3 and
4, which showed similar inhibition.
Me
HO
Me
O
ab
HO
OH
O
O
2
12
In conclusion, heliannuol-derivatives were synthe-
sized by employing the rearrangement of spirodienones
under Lewis acid conditions. The synthetic chromans
(2–6) and the tetrahydrobenzo[b]oxepins (7–10), along
with (+)-, ())- and ꢀ-1, were submitted to growth in-
hibition assay against oat and cress. Bromine-substi-
11
c
Me
Me
Me
HO
Br
BnO
BnO
e
d
Me
OH
Me
OH
Me
Me
CHO
HO
HO
tuted derivatives showed inhibitory activities at 10ꢂ3
–
13
14
15
10ꢂ4 M concentration, which were better than those of
natural ())-1. Compound 10 was observed to possess the
strongest activity against oat and cress.
Br
Me
HO
Me
HO
Br
f
Me
OH
Me
OH
O
O
Me
Me
5
6
3. Experimental
Scheme 1. Synthesis of 2, 5 and 6. Reagents: (a) CCE. (b) BF3 ꢁ OEt2.
(c) (i) K2CO3, BnBr; (ii) SO3 ꢁ pyr, DMSO, Et3N. (d) (i) Ph3PCHMe2I,
n-BuLi; (ii) OsO4, NMO. (e) (i) H2, l0% Pd–C; (ii) PyrHBr3. (f) (i) CCE
or PIFA; (ii) BF3 ꢁ OEt2.
3.1. General
IR spectra were recorded on a JASCO Model A-202
spectrophotometer. 1H NMR and 13C NMR spectra
were obtained on JEOL JNM EX-270 and JEOL JNM
GX-400 spectrometers in CDCl3 using tetramethylsilane
as an internal standard. HRMS were obtained on a
Hitachi M-80 B GC–MS spectrometer operating at the
ionization energy of 70 eV. Prep. and analytical TLC
against roots of oat. The absolute configuration of 1
seems to be recognized by oat. Inhibitory activity of 2,
carrying the most simple chroman structure, were com-
parable to those of 1. The brominated derivatives 3–8,
and the seven-membered ring derivatives 9 and 10, ex-
hibited remarkable inhibition against both hypocotyl and
root of cress, as well as oat, with the exception of 6 and 9.
However, introduction of bromine substituents (7 and 8)
give rise to inhibitory activity, although activity of the
each isomer of 8 was unclear. Strong activity against both
cress and oat was found in the case of 10, which has the
same carbon framework as that of heliannuol D. No
activity of the regio-isomer of the aryl methyl group 9 was
observed, indicating that the methyl substituent may play
were carried out on silica gel plates (Kieselgel 60 PF254
,
E. Merck AG, Germany) using UV light and/or 5%
molybdophosphoric acid in ethanol for detection.
Kanto Chemical Silica 60N (spherical, neutral, 63–210
lm) was used for cc. Anodic oxidation was performed
under CCE conditions using a glassy carbon beaker as
an anode and a platinum wire as a cathode in the
presence of appropriate supporting salts.
3.1.1. 7-Methyl-1-oxaspiro[4.5]deca-6,9-dien-8-one (12)
A solution of 11 (53 mg, 0.32 mmol) in acetone (160 ml)
in the presence of n-Bu4NClO4 (5.5 g) was electrolyzed
(1.2–1.6 V vs SCE, 2 F/mol). The reaction mixture was
evaporated and the residue was chromatographed on a
silica gel column (hexane:EtOAc, l/l) to give 12 (l0.6 mg,
20%) as an oil. IR: dmax 2952, 1671, 1643 cmꢂ1; 1H NMR:
d 1.87 (3H, d, J ¼ 1:0 Hz), 2.04 (2H, t, J ¼ 7:3 Hz), 2.15
(2H, tt, J ¼ 6:3, 7.3 Hz), 4.07 (2H, t, J ¼ 6:3 Hz), 6.13
(1H, d, J ¼ 9:8 Hz), 6.58 (1H, dd, J ¼ 1; 3:4 Hz), 6.78 (1H,
dd, J ¼ 3:4, 9.8 Hz); 13C NMR: d 15.7, 26.9, 36.9, 69.0,
77.8, 127.1, 133.7, 144.9, 149.3, 186.2. Found: m=z
164.0816. Calc. for C10Hl2O2: Mþ, 164.0836.
Br
Br
OH
O
HO
HO
a
b
Br
Br
O
O
7
16
17
Br
TBSO
Me
TBSO
Me
TBSO
OH
Me
HO
Me
c
d
e
Me
Br
Me
OH
Me
OH
Me
Me
18
19
20
21
Me
Me
O
Br
Me
O
Br
Me
HO
Br
O
HO
Me
f
g
Me
OH
Me
22
Me
OH
O
Me
Me
Me
OH
8
mix
Me
Me
Me
Me
HO
HO
Me
h
O
O
Me
OH
Me
OH
3.1.2. 7-Methylchroman-6-ol (2)
Me
Me
10
9
To a solution of 12 (l0.6 mg, 0.065 mmol) in CH2C12
(1 ml) was added BF3 ꢁ OEt2 (0.025 ml, 0.2 mmol); the
mixture was reacted at ambient temp. for l.5 h. The
resulting mixture was diluted with EtOAc, washed with
H2O and brine, and dried (Na2SO4). After evaporation,
Scheme 2. Reagents: (a) (i) Br2; (ii) CCE or PIFA. (b) BF3 ꢁ OEt2. (c) n-
BuLi, then 6 methylhept-5-en-2-one. (d) Et3SiH, BF3 ꢁ OEt2. (e) (i)
OsO4, NMO; (ii) TBAF; (iii) Br2. (f) CCE or PIFA. (g) BF3 ꢁ OEt2. (h)
H2, 10% Pd–C.

1408
F. Doi et al. / Phytochemistry 65 (2004) 1405–1411
Table 1
Inhibitory activity of heliannuol derivatives on the growth of cress and oat seedlings
EC50 (M)ꢃ
Compounds
Cress
Oat
Hypocotyl
Root
Coleoptile
Root
ꢃꢃ
())-1
(+)-1
1.2 ꢄ 10ꢂ3
1.3 ꢄ 10ꢂ3
1.7 ꢄ 10ꢂ3
1.4 ꢄ 10ꢂ3
8.5 ꢄ 10ꢂ4
8.4 ꢄ 10ꢂ4
7.1 ꢄ 10ꢂ4
6.5 ꢄ 10ꢂ4
6.1 ꢄ 10ꢂ4
4.1 ꢄ 10ꢂ4
8.5 ꢄ 10ꢂ4
1.1 ꢄ 10ꢂ4
2.0 ꢄ 10ꢂ3
6.2 ꢄ 10ꢂ3
1.0 ꢄ 10ꢂ2
1.0 ꢄ 10ꢂ2
7.2 ꢄ 10ꢂ3
1.5 ꢄ 10ꢂ3
1.4 ꢄ 10ꢂ3
6.3 ꢄ 10ꢂ4
6.5 ꢄ 10ꢂ4
4.8 ꢄ 10ꢂ4
4.7 ꢄ 10ꢂ4
1.0 ꢄ 10ꢂ3
1.1 ꢄ 10ꢂ3
>10ꢂ2
ꢃꢃ
ꢃꢃꢃ
(ꢀ)-1
5.0 ꢄ 10ꢂ3
ꢃꢃ
2
3.7 ꢄ 10ꢂ3
2.6 ꢄ 10ꢂ3
6.0 ꢄ 10ꢂ3
1.0 ꢄ 10ꢂ3
9.4 ꢄ 10ꢂ4
1.6 ꢄ 10ꢂ3
1.1 ꢄ 10ꢂ3
>10ꢂ2
3
1.1 ꢄ 10ꢂ3
2.3 ꢄ 10ꢂ3
2.4 ꢄ 10ꢂ3
>10ꢂ2
2.5 ꢄ 10ꢂ3
4.0 ꢄ 10ꢂ3
>10ꢂ2
1.7 ꢄ 10ꢂ3
4
5
6
7
8
9
10
2.3 ꢄ 10ꢂ3
* EC50 represents the concentration of sample which causes 50% inhibition of the shoot and root growth of cress or oat seedlings, respectively.
** The dose–response curves of cress and oat seedlings for sample were not linear when the percentage elongation was plotted against for the
logarithm of the dose, and EC50 was not estimated.
*** No inhibitory activity.
the residue was applied to a silica gel column (hex-
ane:EtOAc, 2/1) to give 2 (6.6 mg, 62%): mp 126–127 °C
(hexane–diethyl ether). IR: dmax 3315 cmꢂ1; 1H NMR: d
1.96 (2H, tt, J ¼ 5:4, 6.8 Hz), 2.17 (3H, s), 2.70 (2H, t,
J ¼ 6:8 Hz), 4.11 (2H, t, J ¼ 5:4 Hz), 6.47 (1H, s), 6.56
(1H, s); 13C NMR: d 15.6, 22.6, 24.6, 66.3, 115.3, 118.4,
120.2, 122.9, 147.1, 148.4. Found: m=z 164.0848. Calc.
for C10H12O2: Mþ, 164.0837.
3.1.4. 2-Methyl-5-(4-benzyloxy-3-methylphenyl)pentane-
2,3-diol (14)
To an ice-cooled solution of isopropyltriphenylphos-
phonium iodide (3.01 g, 7.0 mmol) in THF (20 ml) was
added n-BuLi (1.57 Msolution in hexane, 3.8 ml). After 30
min, a solution of 13 (505 mg, 2 mmol) in THF (5 ml) was
added, and the mixture was stirred for 5 min. The mixture
was diluted with EtOAc, washed with H2O and brine, and
then dried (Na2SO4). After evaporation, a crude product
was subjected to a silica gel cc (hexane:EtOAc, 40/1) to
give an olefin (87.3 mg, 16%) as an oil.
3.1.3. 3-(4-Benzyloxy-3-methylphenyl)propanal (13)
To an ice-cooled solution of 11 (0.72 g, 4.3 mmol) in
DMF (7.2 ml) were added K2CO3 (1.80 g, 13 mmol)
and BnBr (0.77 ml, 6.5 mmol); the mixture was stirred
at ambient temp. for 15 h. The reaction mixture was
diluted with hexane/EtOAc (1:1, 200 ml), washed with
H2O and brine and then dried (Na2SO4). After evap-
oration, a crude product was subject to silica gel cc
(hexane:EtOAc, 2/1) to give a benzyl ether (0.84 g,
76%).
A mixture of the benzyl ether (636 mg, 2.48 mmol),
SO3 ꢁ pyr (1.18 g, 7.4 mmol) and Et3N (2.06 ml, 15
mmol) in DMSO (10 ml) was stirred at ambient temp.
for 10 min. The reaction mixture was diluted with
EtOAc, washed with H2O and brine, and then dried
(Na2SO4). After evaporation, a crude product was ap-
plied to a silica gel column (hexane:EtOAc, 2/1) to give
13 (0.505 g. 81%) as an oil. IR: dmax 2921, 1724 cmꢂ1; 1H
NMR: d 2.73 (2H, t, J ¼ 7:3 Hz), 2.87 (2H, t, J ¼ 7:3
Hz), 5.05 (2H, s), 6.80 (1H, d, J ¼ 7:8 Hz), 6.95 (1H, dd,
J ¼ 2, 7.8 Hz), 6.99 (1H, brd, J ¼ 2 Hz), 7.31–7.44 (5H,
m), 9.80 (1H, s); 13C NMR: d 16.4, 27.4, 45.6, 69.9,
111.5, 126.2, 127.0, 127.2, 127.6, 128.4, 130.7, 132.1,
137.4, 155.3, 201.7. Found: m=z 254.1256. Calc. for
Cl7H20O2: Mþ, 254.1305.
To a solution of the olefin (13.5 mg, 0.048 mmol) in
acetone/H2O (l0:l, 1 ml) were added NMO (11.3 mg)
and OsO4 (0.04 M solution in t-BuOH, 0.05 ml); the
mixture was stirred at ambient temp. for 13 h. After the
addition of sat. aq. Na2SO3, the resultant mixture was
dilute with EtOAc, washed with 1 M HCl and brine, and
then dried (Na2SO4). After evaporation, the residue was
purified by silica gel cc (hexane:EtOAc, 3/1) to give 14
1
(11 mg 73%) as an oil. IR: dmax 3407, 2925 cmꢂ1; H
NMR: d 1.12 (3H, s), 1.19 (3H, s), 1.60–1.66 (H, m),
1.75 (1H, m), 2.27 (3H, s), 2.58 (1H, ddd, J ¼ 6:8, 9.3,
14.7 Hz), 2.83 (2H, ddd, J ¼ 4:9, 9.8, 14.7 Hz), 3.40 (1H,
brd, J ¼ 10:3 Hz), 5.06 (2H, s), 6.81 (1H, d, J ¼ 8:3 Hz),
6.99 (1H, dd, J ¼ 2, 8.3 Hz). 7.02 (1H, brd, J ¼ 2 Hz),
7.29–7.45 (5H, m); 13C NMR: d 16.5, 23.2, 26.6, 32.1,
33.8, 69.9, 73.l, 77.9, 111.4, 126.3, 127.0, 127.6, 128.4,
128.6, 130.9, 133.9, 137.5, 155.0. Found: m=z 314.1853.
Calc. for C20H26O3: Mþ, 314.1880.
3.1.5. 5-(3-Bromo-4-hydroxy-5-methylphenyl)-2-methyl-
pentane-2.3-diol (15)
A solution of 14 (49 mg, 0.16 mmol) in MeOH (3 ml)
in the presence of catalytic amounts of 10% Pd–C was

F. Doi et al. / Phytochemistry 65 (2004) 1405–1411
1409
stirred at ambient temp. for 2 h under a hydrogen at-
mosphere. After filtration, the filtrate was evaporated,
and the residue was purified by silica gel cc (hex-
ane:acetone, 2/1) to give a phenol (34.4 mg, 98%) as an
oil.
3.1.7. PIFA oxidation of (15)
To a solution of PIFA (640 mg, 1.5 mmol) in MeCN
(5 ml) was added a solution of 15 (214 mg, 0.71 mmol) in
MeCN (7 ml); the mixture was stirred at 0 °C for 15 min.
The resulting mixture was diluted with EtOAc, washed
with H2O and brine, and then dried (Na2SO4). After
removal of the solvent, the residue was subjected to a
silica gel cc to give the corresponding spiro compound
(147.5 mg, 69%).
To a solution of the phenol (34.4 mg, 0.15 mmol) in
CHCl3 (5 ml) was added PyrHBr3 (49 mg, 0.15 mmol) in
CHCl3 (5 ml)–pyridine (0.2 ml) at 0 °C. After addition
of H2O, the resulting mixture was diluted with CHCl3,
washed with 1 M HCl and brine, and dried (Na2SO4).
After evaporation, the residue was applied to a silica gel
column (hexane:EtOAc, 2/1) to give 15 (42 mg, 90%) as
an oil. IR: dmax 3399, 2974 cmꢂ1; 1H NMR: d 1.16 (3H,
s), 1.19 (3H, s), l.55–2. 05 (2H, m), 2.27 (3H, s), 2.54
(1H, ddd, J ¼ 7:3, 9.3, 13.7 Hz), 2.81 (2H, ddd, J ¼ 4.9,
9.7, 13.7 Hz), 3.37 (1H, brd, J ¼ 10:8 Hz), 6.92 (1H, s),
7.15 (1H, s); 13C NMR: d 16.7, 23.3, 26.6, 31.8, 33.6,
73.2, 77.6, 109.9, 125.6, 128.8, 130.5, 134.9, 148.4.
Found: m=z 302.0555. Calc. for Cl3H19O379Br: Mþ,
302.0517.
3.1.8.
one (17)
2,4-Dibromo-7-oxaspiro[5.5]undeca-1,4-dien-3-
To a solution of 16 (1.04 g, 6.2 mmol) in THF (83 ml)
at )78 °C was added Br2 (1.5 g, 9.4 mmol) in CHCl3 (l00
ml); the mixture was stirred at ambient temperature for
2 days. The resulting mixture was washed with H2O, aq.
Na2S2O3 and brine, and then dried (Na2SO4). After
evaporation, a crude product was applied to a silica gel
column (PhMe:EtOAc, 10/1) to give a dibromide (0.24
g, 12%), along with the corresponding monobromide
(0.81 g, 53%).
A solution of the dibromide (33 mg, 0.1 mmol) in
MeNO2 (l00 ml) in the presence of n-Bu4NClO4 (1.7 g)
was electrolyzed (3.3–3.5 V vs SCE, 6.2 F/mol). The
reaction mixture was evaporated, and the residue was
repeatedly purified by silica gel cc (hexane:EtOAc, 1/1,
then 3/1) to give 17 (13 mg, 40%) as an oil. IR: dmax
3.1.6.
5-Bromo-2-(1-hydroxyisopropyl)-7-methylchro-
man-6-ol (5) and 6-bromo-2-(1-hydroxyisopropyl)-8-
methylchroman-7-ol (6)
A solution of 15 (42 mg, 0.14 mmol) in dioxane-60%
aq. HClO4 (5/1, 117 ml) in the presence of LiClO4 (2 g)
was electrolyzed (1.6–1.9 V vs SCE, 10 F/mol). The re-
action mixture was neutralized by addition of sat. aq.
NaHCO3 and evaporated. The residue was dissolved in
CHCl3, washed with H2O and brine, and dried
(Na2SO4). After evaporation, a crude product was
subjected to silica gel cc (hexane:EtOAc, 2/1) to give a
spiro compound (7.6 mg, 18%) as an oil.
1
2940, 1684, 1595 cmꢂ1; H NMR: d 1.68–1.86 (6H, m),
3.88 (2H, t, J ¼ 5:4 Hz), 7.60 (2H, s); 13C NMR: d 19.5,
24.7, 34.1, 62.8, 74.2, 121.8, 149.0, 171.9. Found: m=z
319.9053. Calc. for Cl0H10O279Br2: Mþ, 319.9048.
3.1.9. PIFA oxidation of the dibromo derivative of (16)
To a solution of PIFA (333 mg, 0.78 mmol) in
MeCN (18 ml) was added a solution of the dibromide
(81 mg, 0.25 mmol) in MeCN (7 ml); the mixture was
stirred at 0 °C for 15 min. The resulting mixture was
diluted with EtOAc, washed with H2O, Na2S2O3, and
then dried (Na2SO4). After removal of the solvent,
the residue was subjected to a silica gel cc to give 17
(29.1 mg, 36%).
To a solution of the spiro compound (119.5 mg, 0.4
mmol) in CH2Cl2 (3 ml) was added BF3 ꢁ OEt2 (0.15 ml,
1.2 mmol); the mixture was washed with H2O, sat. aq.
NaHCO3 and dried (Na2SO4). After evaporation, the
residue was applied to a silica gel column (hex-
ane:EtOAc, 4/1) to give 5 (43.6 mg, 36%) and 6 (16.6 mg,
1
14%) as oils. 5: IR: dmax 3518, 2976 cmꢂ1; H NMR: d
1.27 (3H, s), 1.31(3H, s), 1.72 (1H, dddd, J ¼ 6:4, 11.7,
11.7, 15.1 Hz), 2.07 (1H, dddd, J ¼ 2, 2, 4.9, 15.1 Hz),
2.24 (3H, s), 2.64 (1H, ddd, J ¼ 6:4, 11.7, 17.1 Hz), 2.84
(1H, brdd, J ¼ 4:9, 17.1 Hz), 3.64 (1H, dd, J ¼ 2:0, 11.7
Hz), 6.64 (1H, s); 13C NMR: d 16.6, 22.4, 24.5, 25.9,
26.7, 71.7, 81.8, 111.9, 117.9, 119.1, 127.7, 144.4, 148.6.
Found: m=z 302.0383. Calc. for C13Hl7O381Br: Mþ,
302.0341. 6: IR: dmax 3408, 2976 cmꢂ1; 1H NMR: d 1 26
(3H, s), 1.31 (3H, s), 1.72 (1H, dddd, J ¼ 5:9, 11.7, 12.7,
13.7 Hz), 2.09 (1H, dddd, J ¼ 2, 2, 6.9, 13.7 Hz), 2.59
(1H, ddd, J ¼ 6:9, 12.7, 17.1 Hz), 2.75 (1H, ddd, J ¼ 2,
5.9, 17.1 Hz), 3.64 (1H, dd, J ¼ 2, 11.7 Hz), 6 88 (1H, s);
13C NMR: d 12.4, 22.2, 23.5 24.4, 25.9, 71.7, 81.7, 107.5,
116.4, 121.9, 123.7, 143.9, 148.8. Found: m=z 302.0348.
Calc. for Cl3Hl7O381Br: Mþ, 302.0341.
3.1.10. 6,8-Dibromo-2H,3H,4H,5H-benzo[f]oxepin-7-ol
(7)
A mixture of 17 (22.5 mg, 0.07 mmol) and BF3 ꢁ OEt2
(0.026 ml, 0.21 mmol) in CH2Cl2 (0.3 ml) was stirred at
ambient temp. for 3 h. The reaction mixture was diluted
with CHCl3, washed with H2O, sat. aq. NaHCO3 and
brine, and dried (Na2SO4). After evaporation, the resi-
due was purified by silica gel cc (hexane:EtOAc, 2/1) to
1
give 7 (12.7 mg, 56%) as an oil. IR: dmax 3487 cmꢂ1; H
NMR: d 1.70 (2H, m), 1.96 (2H, m), 3.04 (2H, t, J ¼ 5:6
Hz), 3.98 (2H, t, J ¼ 5:6 Hz), 5.76 (1H, s), 7.17 (1H, s);
13C NMR: d 24.8, 31.8, 32.9, 74.3, 105.6, 111.8, 124.2,

1410
F. Doi et al. / Phytochemistry 65 (2004) 1405–1411
136.4, 145.6, 154.2. Found: m=z 319.9044. Calc. for
(Na2SO4). After evaporation, the residue was dissolved
in CHCl3 (127 ml), and Br2 (1.25 g, 7.8 mmol) in CHCl3
(85 ml) was added. After being stirred at ambient tem-
perature for 15 min, the mixture was washed with H2O,
sat. aq. Na2S2O3 and brine. After being dried (Na2SO4),
the organic layer was evaporated and the residue was
purified by silica gel cc (hexane:EtOAc, 2/1) to give di-
astereomeric 21 (2.07 g, 98%) as an oil. IR: dmax 3392
C10H10O279Br2: Mþ, 319.9047.
3.1.11. 2-[4-(t-Butyldimethylsilyloxy)-3-methylphenyl]-
6-methylhept-5-en-2-ol (19)
To a solution of 18 (19.5 g, 65 mmol) in THF (600 ml)
at )78 °C was added n-BuLi (1.58 M solution in hexane,
41 ml) under Ar. After addition of 6-methylhept-5-en-2-
one (21.2 g, 0.17 mol) in THF (500 ml), the mixture was
stirred at the same temp. for 30 min. The resulting
mixture was diluted with EtOAc, washed with H2O, sat.
aq. NH4Cl and brine, and then dried (Na2SO4). After
evaporation, the residue was subjected to a silica gel cc
(hexane:EtOAc, 14/l) to give 19 (15.3 g, 68%) as an oil.
IR: dmax 3411 cmꢂ1; 1H NMR: d 0.21 (6H, s), 1.01 (9H,
s), 1.49 (3H, s), 1.51 (3H, s), 1.65 (3H, s), 1.76-1.95 (4H,
m), 2.21 (3H, s), 5.09 (1H, m), 6.71 (1H, d, J ¼ 8:3 Hz),
1
cmꢂ1; H NMR: d 1.08 (1.5H, s), 1.11 (1.5H, s), 1.14
(3H. s), 1.19 (1.5H, d, J ¼ 3:9 Hz), l.21 (1.5H, d, J ¼ 3:9
Hz), 1.32 (2H, m), 1.56 (1H, m), 1.82 (1H, m), 2.26
(1.5H, s), 2.27 (1.5H, s), 2.59 (1H, m), 3.28 (0.5H, dd,
J ¼ 2:7, l0.0 Hz), 3.35 (0.5H, dd, J ¼ 2.0, l0.3 Hz), 5.51
(1H, s), 6.88 (1H, m), 7.10 (1H, m); 13C NMR: d 16.9,
22.4, 22.8, 23.2, 26.6, 29.6, 29.9, 35.2, 35.6, 38.8, 39.2,
73.2, 78.3, 78.7, 109.9, 125.5, 127.3, 127.4, 128.88,
128.91, 140.1, 140.4, 148.3. Found: m=z 330.0798. Calc.
for Cl5H23O379Br: Mþ, 330.0829.
7.08 (1H, dd, J ¼ 2, 8.3 Hz), 7.18 (1H, d, J ¼ 2 Hz); 13
C
NMR: d )4.1, 17.2, 17.7, 18.3, 23.1, 25.75, 25.83, 30.4,
43.8, 74.7, 117.9, 122.9, 124.2, 127.4, 128,2, 131,9, 140.1,
152.2. Found: m=z 348.2498. Calc. for C21H36O2Si: Mþ,
348.2483.
3.1.14. 4-Bromo-8-(1-hydroxyisopropyl)-2,11-dimethyl-
7-oxaspiro[5.5]undeca-1,4-dien-3-one (22)
A solution of 21 (74.8 mg, 0.23 mmol) in MeNO2 (150
ml) in the presence of n-Bu4NClO4 (2.6 g, 7.7 mmol) was
electrolyzed under CCE conditions (3.4–3.6 V vs SCE,
6.6 F/mol). The reaction mixture was evaporated and
the residue was applied to a silica gel column (hex-
ane:EtOAc, 1/1), followed by prep TLC (hexane:EtOAc,
2/1, 3 times development) to give diastereomeric 22 (46
3.1.12. 2-[4-(t-Butyldimethylsilyloxy)-3-methylphenyl]-
6-methylhept-5-ene (20)
To a solution of 19 (2.54 g, 7.3 mmol) in CH2Cl2 (73 ml)
were added Et3SiH (1.4 ml, 8.8 mmol) and BF3 ꢁ OEt2 (0.9
ml, 7.2 mmol); the mixture was stirred at 0 °C for 15 min.
The resulting mixture was washed with H2O, sat. aq.
NaHCO3 and brine, and then dried (Na2SO4). After
evaporation, the residue was applied to a silica gel column
(hexane) to give an oily siloxy ether 20 (2.12 g, 87%). IR:
1
mg, 62%) as an oil. IR: dmax 3502, 2967, 1670 cmꢂ1; H
NMR: d 0.69–0.72 (1.5H, m), 1.06–1.08 (1.5H, m), 1.17–
1.18 (6H, m), 1.44–1.85 (5H, m), 1.96–1.99 (3H m), 2.38
(1H, brs), 3.53–3.69 (1H, m), 6.44 (0.25H, dd, J ¼ 1:6, 2.8
Hz), 6.68 (0.25H, dd, J ¼ I:6, 2.8 Hz), 7.06 (0.25H, dd,
J ¼ 1:2, 2.8 Hz), 7.07 (0.25H, d, J ¼ 2:8 Hz), 7.19
(0.25H, dd, J ¼ 1:6, 2.4 Hz), 7.30 (0.25H, d, J ¼ 2:4 Hz),
7.67 (0.25H, d, J ¼ 2:8 Hz), 7.83 (0.25H, d, J ¼ 2:8 Hz);
13C NMR: d 15.91, 15.93, 11.27, 16.33, 16.4, 16.5, 16.9,
17.1, 20.1, 20.3, 23.5, 23.6, 23.7, 23.8, 25.7, 25.8, 25,87,
25.93, 26.0, 26.1, 26.5, 26.6, 28.1, 28.2, 35.39, 35.45,
37.57, 37.62, 71.88, 71,89, 72.0, 72,1, 75.80, 75.82, 76.8,
77.5, 77.65, 77.69, 123.6, 124.4, 124.6, 126.3, 133.2,
133.3, 134.4, 135.8, 137.8, 142.3, 143.3, 145.6, 146.7,
147.2, 150.3, 151.1, 178.7, 178.8, 179.2. Found: m=z
328.0649. Calc. for Cl5H21O379Br: Mþ, 328.0672.
d
max 1595 cmꢂ1; 1H NMR: d 0.20 (6H, s), 1.01 (9H, s), 1.19
(3H, d, J ¼ 6:8Hz), 1.51 (3H, s), 1.55 (2H, m), 1.66 (3H, s),
1.86 (2H, m), 2.58 (1H, m), 5.09 (1H, m), 6.67 (1H, d,
J ¼ 7:6 Hz), 6.84 (1H, dd, J ¼ 1:4, 7.6 Hz), 6.92 (1H, d,
J ¼ 1:4 Hz); 13C NMR: d-4.1, 17.1, 17.7, 18.3, 22.6, 25.8,
25.9, 26.3, 38.71, 38.75, 77.2, 118.1, 124.7, 124.8, 129.4,
131.1, 139.9, 151.6. Found: m=z 332.2514. Calc. for
C21H36OSi: Mþ, 332.2533.
3.1.13. 6-(4-Hydroxy-3-methylphenyl)-2-methylheptane-
2,3-diol (21)
A mixture of 20 (1.51 g. 4.5 mmol), OsO4 (0.04 M t-
BuOH solution, 0.6 ml) and NMO (1.63 g, 14 mmol) in
aq. acetone (45 ml) was stirred at ambient temp. for 18
h. The resultant mixture was diluted with EtOAc, wa-
shed with sat. aq. Na2SO3, H2O and brine, and then
dried (Na2SO4). After evaporation, a crude product was
purified by silica gel cc (hexane:EtOAc, 2/1) to give a
diol as a diastereomeric mixture (1.63 g, 98%) as an oil.
A mixture of the diol (2.33 g, 6.4 mmol) and n-Bu4NF
(1.0M THF solution, 7 ml) in THF (63 ml) was stirred at
0 °C for 75 min. The resulting mixture was diluted with
EtOAc, washed with H2O and brine, and then dried
3.1.15. PIFA oxidation of (21)
Compound 21 (931 mg, 2.8 mmol) was oxidized with
PIFA (3.73 g, 8.67 mmol) in MeCN (280 ml) essentially
the same procedure as in the case of 17 to give 22 (250
mg, 27%).
3.1.16. 2-(1-Hydroxyisopropyl)-5,8-dimethyl-2H,3H,4H,
5H-benzo[f]oxepin-7-ol (9) and 2-(1-hydroxyisopro-
pyl)-5,6-dimethyl-2H,3H,4H,5H-benzo[f]oxepin-7-ol (10)
A mixture of 22 (117 mg, 0.36 mmol) and BF3 ꢁ OEt2
(0.134 ml, 1.1 mmol) in CH2Cl2 (1.4 ml) was stirred at

F. Doi et al. / Phytochemistry 65 (2004) 1405–1411
1411
ambient temp. for 1 h. The reaction mixture was diluted
with CHCl3, washed with H2O, sat. aq. NaHCO3 and
brine, and then dried (Na2SO4). After evaporation, the
residue was purified by silica gel cc (hexane:EtOAc, 4/1) to
give a diastereomeric mixture 8 (90.6 mg, 77%) as an oil.
A solution of 8 (90.6 mg, 0.28 mmol) in MeOH (5 ml)
in the presence of 10% Pd–C was stirred at ambient
temp. for 30 min under a hydrogen atmosphere. After
filtration, the filtrate was evaporated to give a residue,
which was subjected to a silica gel cc (PhH:EtOAc 3/1)
to give 9 (15.6 mg, 23%) and 10 (44.6 mg, 65%) as dia-
Ministry of Education, Culture, Sports, Science and
Technology, Japan.
References
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1
stereomeric mixtures. 9: IR: dmax 3369, 1589 cmꢂ1; H
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tive synthesis of heliannuol E; structural consideration of natural
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NMR: d 1.22–1.39 (9H, m), 1.60–2.20 (4H, m), 2.22 (3H,
m), 2.73 (1H, brs), 3.23–3.31 (1H, m) 3.35–3.45 (0.5H,
m), 3.84 (0.5H, dd, J ¼ 5:4, 11.3 Hz), 6.52 (1H, d,
J ¼ 8:8 Hz) 6.70 (1H, d, J ¼ 8:8 Hz); 13C NMR: d 11.8,
12.0, 17.0, 19.8, 24.1, 24.5, 24.7, 25.8, 26.3, 26.9, 27.8,
31.2, 32.7, 33.0, 49.08, 49.14, 72.6, 88.1, 90.5, 112.8,
112.9, 117.8, 119.1, 122.1, 140.2, 149.8, 152.7. Found:
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1
IR: dmax 3367, 1508 cmꢂ1; H NMR: d 1.25–1.31 (9H,
m), 1.67–2.09 (5H, m), 2.16 (1.5H, s), 2.18 (1.5H, s), 2.73
(1H, brs), 2.85–3.04 (1H, m), 3.24 (0.5H, dd, J ¼ 3:2, 10
Hz), 3.30 (0.5H, J ¼ 1:2, 11 Hz), 4.77 (1H, broad s), 6.54
(0.5H, s), 6.61 (0.5H, s), 6.73 (0.5H, s), 6.76 (0.5H, s);
13C NMR: d 15.4, 18.7, 20.3, 24.42, 24.43, 24.5, 25.6,
26.2, 26.3, 31.0, 31.8, 34.3, 34.4, 38.5, 72.5, 72.7, 89.7,
90.4, 112.6, 115.7, 122.0, 122.8, 123.5, 138.1, 149.5,
151.5. Found: m=z 250.1564. Calc. for C15H22O3: Mþ,
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3.2. Bioassay
Effects for coleoptile and root growth of oat (Avena
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1 ml of a test solution was added. After incubation at
25 °C for 3 days in the dark, length of coleoptiles and
roots of the seedlings was measured. Oat incubated
without test samples during the same period were used
as control.
Effects for hypocotyls and root growth of cress
(Lepidium sativum L.). Ten seeds of cress in the pres-
ence of a 0.5 ml solution of a test sample were incubated
at 23 °C in the dark. After 2 days, length of roots was
measured and hypocotyls were measured after incubated
one more day.
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Acknowledgements
This work was supported by a Grant-in-Aid for the
21st Century COE program ‘Keio Life Conjugate
Chemistry’, as well as Scientific Research C from the
Vyvyan, J.R., Looper, R.E., 2000. Total synthesis of (ꢀ)-heliannuol D,
an allelochemical from Helianthus annuus. Tetrahedron Lett. 41,
1151–11544.