Stereochemistry, total synthesis, and biological evaluation of the new plant hormone solanacol

Article abstract of DOI:10.1002/chem.201002817

The first total synthesis of solanacol, a member of the strigolactone family, features ring-closing metathesis, enzymatic kinetic resolution, and atom-transfer radical cyclization. This defines the stereostructure of the natural product. The best hormonal

Full text of DOI:10.1002/chem.201002817

COMMUNICATION
DOI: 10.1002/chem.201002817
Stereochemistry, Total Synthesis, and Biological Evaluation of the New Plant
Hormone Solanacol
Victor X. Chen,[a] FranÅois-Didier Boyer,*[a] Catherine Rameau,[c] Pascal Retailleau,[a]
Jean-Pierre Vors,[d] and Jean-Marie Beau*[a, b]
Dedicated to Professor Jean-Yves Lallemand on the occasion of his retirement
Strigolactones—a new group of hormones present in all
plants[1]—have been studied in pea, Arabidopsis, petunia,
and rice. These hormones are formed mainly in the lower
parts of the stem and roots and transported to the aerial
parts of the plant where they suppress shoot branching[2]
and are possibly involved in nodule formation[3] and root
growth.[4] The production would be inversely correlated to
the concentration of phosphate available for the plants.[5]
Strigolactones belong to a class of compounds that was first
identified in 1966 as stimulants of the seed germination of
the parasitic weeds Orobanche and Striga[6] and are pro-
duced in trace amounts with partial excretion in the rhizo-
sphere. These molecules were recently identified as stimu-
lants for spore germination and hyphal proliferation of ar-
buscular mycorrhizal fungi (AMF).[7] In the plant–AMF
symbioses, plants receive water and mineral nutrients from
their fungal partners, hence promoting optimal plant growth
conditions. Strigolactones are suggested to have additional
biological functions in rhizosphere communications and in
development; properties that will certainly be rapidly estab-
lished. The structural core of strigolactones is a tricyclic lac-
tone (ABC rings) connected through an enol ether linkage
to an a,b-unsaturated furanone moiety (D ring) (Scheme 1).
Strigolactones are thought to be derived from the carote-
noid pathway, as cleavage products of b-carotene.[8] Strigol
(1) and orobanchol (2) are two major strigolactones found
in nature. Solanacol (3)—the first natural strigolactone con-
taining a phenyl ring—was recently isolated from tobacco
and tomato root exudates.[9,10]
[a] V. X. Chen, Dr. F.-D. Boyer, Dr. P. Retailleau, Prof. J.-M. Beau
Centre de Recherche de Gif
Institut de Chimie des Substances Naturelles
CNRS, INRA, Avenue de la Terrasse
91198 Gif-sur-Yvette (France)
Fax : (+33)1-6907-7247
E-mail: boyer@versailles.inra.fr
Scheme 1. Structures of natural strigolactones.
jean-marie.beau@icsn.cnrs-gif.fr
[b] Prof. J.-M. Beau
Universitꢀ Paris Sud
Institut de Chimie Molꢀculaire et des Matꢀriaux
91405 Orsay (France)
Fax : (+33)1-6985-3715
The great importance of strigolactones in many areas of
plant chemical biology and their extremely low bioavailabil-
ity prompted us to develop a new strategy for the synthesis
of this class of compounds and to unambiguously confirm
the structure of 3 by means of MS/MS, 1H NMR and circular
dichroism (CD) data.[9–11] The covalent structure was recent-
ly reported by correcting the positions of the methyl groups
on aromatic ring A, but the relative stereochemistry at C2’
on ring D was left unresolved as well as the absolute config-
uration of the molecule.[11] Herein, we report the first total
E-mail: jean-marie.beau@u-psud.fr
[c] Dr. C. Rameau
IJPB UMR1318 INRA-AgroParisTech
RD10, 78126 Versailles Cedex (France)
[d] Dr. J.-P. Vors
Bayer SAS, 14-20 rue Pierre Baizet
BP 99163, 69263 Lyon Cedex 09 (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002817.
Chem. Eur. J. 2010, 16, 13941 – 13945
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13941

synthesis of 3 and preliminary biological activities. Our ret-
rosynthetic analysis is outlined in Scheme 2.
of 9 gave aldehyde 10, which was easily debrominated by
catalytic hydrogenolysis using hydrogen and Pd/C in the
presence of an excess of triethylamine. Triflation under stan-
dard conditions gave triflate 11, which was cross-coupled
with vinyltrifluoroborate[17] under Suzuki–Miyaura condi-
tions to give aldehyde 12, which was easily alkylated with a
vinylic Grignard reagent to give diene 8 in an 89% overall
yield.
Elaboration of the B ring was performed by using a ruthe-
nium-catalyzed RCM reaction of diene 8 to give the racemic
indenol (Æ)-13, which was acetylated to ester (Æ)-13Ac
(Scheme 4). Kinetic resolution of ester (Æ)-13Ac with im-
mobilized Candida antarctica lipase[18] was remarkably effi-
cient in producing the enantiomerically pure alcohol (R)-13
(>99% enantiomeric excess (ee)), as well as the enantio-
meric ester (S)-13Ac (>99% ee).[18,19]
Scheme 2. Retrosynthetic analysis for 3.
We envisaged tricyclic lactone 5, with the correct relative
stereochemistry, as a key precursor that would lead to 3 by
coupling to the D-ring precursor, bromofuranone 4,[12] using
well-reported procedures.[12d] The C4-hydroxyl group[13]
could be derived from trichloride 6 by stereoselective substi-
tution, with retention of configuration at the benzylic posi-
tion. Compound 6 could be synthesized from the enantio-
Scheme 4. Elaboration of B ring and enzymatic kinetic resolution of (Æ)-
13. Reagents and conditions: a) Grubbs I catalyst (5 mol%), CH2Cl2, RT,
12 h; b) Ac2O, pyr, THF, 08C, 15 min, 81% for two steps; c) immobilized
Candida antarctica lipase, CH3CN, H2O, 24 h, (R)-13 50%, (S)-13Ac
50%.
pure ester
7 by an atom-transfer radical cyclization
(ATRC),[14] which, in turn, would originate from a ring-clos-
ing metathesis (RCM)/kinetic resolution sequence on diene
8. This precursor would be formed from the cheap and com-
mercially available 3,4-dimethylphenol.
At this stage, the synthesis was pursued with (Æ)-7, as
Selective bromination at position 6 of 3,4-dimethylphenol,
which serves as a temporary protecting group at this posi-
tion as well as an attractive solution to deuterium labeling
of ring A for future biological studies,[15] was the basis for
the sequence to diene 8. Thus, bromination and alkylation
of the phenol with allylbromide and Lewis acid catalyzed
Claisen rearrangement were completely regioselective, pro-
viding bromide 9 in a 90% overall yield (see Scheme 3). Iso-
merization of the terminal C=C double bond and ozonolysis
well as the enantiopure compounds, obtained by trichloro-
AHCTUNGTREaGNNNU cetylation of 13 ((Cl3CCO)2O, pyr, THF, 08C, 15 min,
98%). Stereoselective lactonization to the ABC tricyclic
system 6 was obtained in 83% yield, through an ATRC cat-
alyzed by copper(I) coordinated to 4,4’-di-n-heptylbipyridine
(dHbipy; Scheme 5).[20,21] Sterically controlled halogen trans-
fer to benzylic radical i from the CuII complex generated in
the catalytic process proceeded stereoselectively anti to ring
C. The stereochemistry was unambiguously established by
X-ray analysis of trichloride 6 (see the Supporting Informa-
tion). Subsequent substitution with retention of configura-
tion of the benzylic chlorine atom in 6 by a hydroxyl group
was achieved with good stereocontrol (9:1 anti/syn to ring
C) under SN1 conditions, by heating a solution of 6 in water/
hexafluoroisopropanol.[22] Dechlorination of the gem-di-
chloro motif with zinc dust afforded tricyclic lactone 5 in a
91% overall yield from 6. Careful formylation of 5, followed
by coupling in the same pot with D-ring precursor 4 by con-
trolling the temperature, completed the synthesis to provide
compound 3, identified as (À)-solanacol (see below) and
(À)-2’-epi-solanacol (15) in a 76% combined yield. The two
compounds were separated by chromatography on silica gel.
The stereochemistry at C2’ relative to the other asymmet-
ric centers was unambiguously assessed by X-ray analysis of
compounds 3 (Figure 1) and 15, showing an R configuration
at C2’ in 3 and an S configuration at this center in 15. Fur-
Scheme 3. Formation of the B-ring precursor 8. Reagents and conditions:
a) Br2, CH2Cl2, 08C, 1 h, 90%; b) allylbromide, K2CO3, acetone, reflux,
3 h, quant.; c) cat. Et2AlCl, hexane, RT, 6 h, quant.; d) tBuOK, THF, RT,
48 h, 90%; e) 1) O3, CH2Cl2, À788C; 2) Me2S, À788C to RT, 12 h, 79%;
f) Pd/C, H2, NEt3, MeOH, RT, 2 h, quant.; g) pyridine (pyr), Tf2O,
CH2Cl2, 08C to RT, 1 h, 74%; h) CH2=CHBF3K, PdCl2, Cs2CO3, PPh3,
THF/H2O (9/1), 858C, 12 h, 98%; i) CH2=CHMgBr, THF, RT, 3 h, 91%.
1
thermore, comparison of the H NMR spectra of 3 and 15
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Chem. Eur. J. 2010, 16, 13941 – 13945

Synthesis and Biological Evaluation of Solanacol
COMMUNICATION
Scheme 5. Final steps in the synthesis of (À)-solanacol 3 and (À)-2’-epi-
solanacol 15. Reagents and conditions: a) ATRC, CuCl (5 mol%),
dHbipy (5 mol%), ClCH2CH2Cl, 908C, 6 h, 83%; b) H2O, hexafluoroiso-
propanol (HFIP), 908C, 30 min; c) Zn dust, NH4Cl, MeOH, 08C to
reflux, 2 h, 91% for two steps; d) 1) tBuOK, ethyl formate, THF, À78 to
À408C, 6 h; 2) 4, À608C to RT, 12 h, 3 38%; 15 38%; e) Ac2O, pyr, RT,
12 h, 92%.
Figure 2. Bud-outgrowth inhibition of the solanacol series. A) Size of bud
10 days after treatment [mm]; B)% of plants with a bud >5 mm.
number of plants per treatment)] repression in bud out-
growth was observed for the four new compounds and
GR24.[2a] Compound 3 was the least active compound in
comparison with 2’-epimer 15, acetates 14 and 16, and
GR24. At a concentration of 100 nm, a significant effect
(ANOVA test with P<0.05 and n=24) was only observed
for GR24 and acetate 14. Generally, acetates 14 and 16
were more active than their corresponding alcohols 3 and
15, with dose-dependent activity similar to that of GR24 for
compound 14. This may be attributed to instability in an
aqueous solution or lower lipophilicity of 3 than the corre-
sponding acetate 14 (see the Supporting Information),
making it difficult for 3 to reach its receptor. The higher ac-
tivity of 14 was confirmed by a higher expression than 3 in
the axillary bud at node 4 of the transcription factor
PsBRC1— the homologue from pea of the Arabidopsis
BRANCHED1 (BRC1) and the TEOSINTE BRANCHED1
(TB1) from maize—6 h after application of 14 (or GR24).[24]
Interestingly, and in sharp contrast, the acetylation of 1 or 2
resulted in significant reduction of both the germination-
stimulating activity of the parasitic weeds and the hyphal
branching activity of the AMF.[25,26]
Figure 1. X-ray structure of solanacol (Æ)-3.
with that of a natural sample of 3[9,10b] clearly showed that
the natural product corresponded to the more polar, syn-
thetic diastereomer 3 with diagnostic chemical shifts for H2’,
H4, and H6’ (see the Supporting Information). Comparison
of the CD spectrum of (À)-3 with that of the natural prod-
uct[9] conclusively defined the stereostructure of solanacol as
(À)-3.[23] This first total synthesis of solanacol 3 and 15 in-
volved 15 linear steps with a 21% overall yield.
The bud-outgrowth inhibition exerted by 3, 15, and ace-
tate derivatives 14 and 16 was evaluated in pea plants and
compared to the active synthetic strigolactone analogue
(Æ)-GR24.[2a] The solution to be tested (10 mL per plant)
was applied directly onto the axillary bud at node four of
ten-day-old rms1/ccd8 plants,[2a] (see the Supporting Infor-
mation). The results are summarized in Figure 2.
In summary, we have exploited an efficient RCM/enzy-
matic kinetic resolution/ATRC sequence of key transforma-
tions to construct the key ABC ring system in the first syn-
thesis of the aromatic strigolactone 3. This firmly established
the complete structure. The aromatic chemistry developed
At a concentration of 1 mm, a significant [ANOVA test
with P<0.05 and n=24 (P=probability for the F test, n=
Chem. Eur. J. 2010, 16, 13941 – 13945
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13943

F.-D. Boyer, J.-M. Beau et al.
(C6’), 141.9 (C4a or C8), 141.1 (C3’), 138.9 (C7 or C8a), 138.3 (C7 or
C8a), 136.4 (C4’), 135.6 (C4), 132.9 (C6), 122.6 (C5), 110.8 (C3), 100.9
(C2’), 84.3 (C8b), 80.2 (C4), 50.6 (C3a), 19.8 (C9), 15.8 (C10), 11.0 ppm
(C7’). IR (film): n˜ =3333, 1781, 1737,1675, 1329, 1183, 953 cmÀ1; MS: m/z
(%): 365 [M+Na]+ (100), 343 [M+H]+ (75); HRMS (ESI): m/z: calcd
for C19H18NaO6: 365.1001 [M+Na+]; ; found: 365.1015.
will also be useful for isotope labeling in future work in an
effort to search for and localize the strigolactone binding
proteins and quantify natural strigolactones. This approach
can also be applied to other natural strigolactones and ana-
logues. We have further demonstrated that 14 showed the
best hormonal activity in inhibiting bud outgrowth in the
pea model. More detailed structure–activity studies are
being developed in our laboratories.
CCDC-784236 and 784237 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Experimental Section
Detailed experimental procedures are provided in the Supporting Infor-
mation.
Acknowledgements
Preparation of 6: Commercial CuCl (5.0 mg, 0.05 mmol, 0.05 equiv) and
dHbipy[27] (19.0 mg, 0.05 mmol, 0.05 equiv) were dissolved in degassed di-
chloroethane (2 mL) and stirred at room temperature under argon. The
solution turned dark brown after 10 min and a solution of trichloroester
7 (328.0 mg, 1.07 mmol, 1 equiv) in degassed dichloroethane (4 mL) was
added. The mixture was heated at 908C for 6 h, cooled to RT, and the
solvent was evaporated under reduced pressure. The crude product was
directly separated by column chromatography (heptane/ethyl acetate
95:5). Product 6 was obtained as a white solid (271 mg, 0.89 mmol, 83%).
Mp: 135–1378C; [a]2D6 =À130.5 (c=1.03 in CHCl3); 1H NMR (300 MHz,
CDCl3): d=7.31 (d, J6,5 =7.9 Hz, 1H; H6), 7.20 (d, J5,6 =7.9 Hz, 1H; H5),
6.03 (d, J8b,3a =5.8 Hz, 1H; H8b), 5.40 (d, J4,3a =5.8 Hz, 1H; H4), 3.98 (t,
We are grateful to Bayer SAS (contract no. 08H0120RD) and the INRA
for the financial support of this study.
Keywords: hormones · kinetic resolution · radical reactions ·
strigolactones
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H9); 13C NMR (75 MHz, CDCl3): d=167.1 (C2), 140.3 (C4a), 139.1 (C7),
135.2 (C8), 133.9 (C8a), 133.8 (C6), 122.7 (C5), 82.3 (C8b), 78.9 (C3),
66.0 (C3a), 60.6 (C4), 19.6 (C9), 15.4 ppm (C10); IR (film): n˜ =1794,
1481, 1161, 955, 825 cmÀ1; elemental analysis calcd (%) for C13H11Cl3O:
C 51.10, H 3.63, O 10.47; found: C 51.05, H 3.56, O 10.32.
Preparation of 3 and 15: Potassium tert-butoxide (67.9 mg, 0.61 mmol,
2.2 equiv) was added to a mixture of lactone 5 (60.0 mg, 0.28 mmol,
1 equiv) and ethyl formate (0.23 mL, 2.80 mmol, 10 equiv) in THF
(1 mL) at À788C under argon. It was then warmed to À408C and stirred
for 6 h at this temperature. The mixture was cooled to À608C and (Æ)-4-
bromo-2-methyl-2-buten-4-olide (99.8 mg, 0.56 mmol, 2.05 equiv) was
gradually added. The mixture was warmed to room temperature. The re-
action was quenched with AcOH (1 mL) after 12 h at this temperature.
The solvent was evaporated and the crude product was purified by prepa-
rative TLC (heptane/ethyl acetate 50:50) to afford the two diastereomers
as two pure fractions (fraction 1=15: 36.0 mg, 0.11 mmol, 38%; frac-
tion 2=3: 36.0 mg, 0.11 mmol, 38%).
Compound 15: Colorless oil; [a]2D6 =À176.4 (c=1.7 in CHCl3); 1H NMR
(300 MHz, CDCl3): d=7.54 (d, J6’,3a =2.6 Hz, 1H; H6’), 7.25 (d, J6,5
7.7 Hz, 1H; H6), 7.17 (d, J5,6 =7.7 Hz, 1H; H5), 7.00 (t, J3’,2’ =1.5, J3’,7’
=
=
1.5 Hz, 1H; H3’), 6.24 (t, J2’,3’ =1.5, J2’,7’ =1.5 Hz, 1H; H2’), 6.15 (d,
J
J
8b,3a =7.5 Hz, 1H; H8b), 5.27 (d, J4,3a =5.8 Hz, 1H; H4), 3.81 (ddd,
3a,8b =7.5, J3a,4 =5.8 Hz, J3a,6’ =2.6 Hz, 1H; H3a), 2.37 (s, 3H; H10), 2.31
(s, 3H; H9), 2.08 (d, 1H; OH), 2.05 ppm (t, J7’,2’ =1.5, J7’,3’ =1.5 Hz, 3H;
H7’); 13C NMR (75 MHz, CDCl3): d=171.0 (C2), 170.2 (C5’), 151.5 (C6’),
141.9 (C4a or C8), 141.0 (C3’), 139.0 (C7 or C8a), 138.4 (C7 or C8a),
136.6 (C4’), 135.7 (C4), 132.9 (C6), 122.5 (C5), 110.9 (C3), 100.7 (C2’),
84.2 (C8b), 80.3 (C4), 50.8 (C3a), 19.8 (C9), 15.8 (C10), 11.1 ppm (C7’);
IR (film): n˜ =3437, 1780, 1740,1677, 1334, 1186, 1092, 1016, 957 cmÀ1
;
MS: m/z (%): 365 [M+Na]+ (100), 343 [M+H]+ (75); HRMS (ESI):
m/z: calcd for C19H18NaO6 [M+Na+]: 365.1001; found: 365.1010.
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(300 MHz, CDCl3): d=7.55 (d, J6’,3a =2.6 Hz, 1H; H6’), 7.23 (d, J6,5
7.7 Hz, 1H; H6), 7.16 (d, J5,6 =7.7 Hz, 1H; H5), 6.99 (t, J3’,2’ =1.5, J3’,7’
=
=
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J
J
8b,3a =7.5 Hz, 1H; H8b), 5.25 (d, J4,3a =5.8 Hz, 1H; H4), 3.81 (ddd,
3a,8b =7.5, J3a,4 =5.8 Hz, J3a,6’ =2.6 Hz, 1H; H3a), 2.37 (s, 3H; H10), 2.30
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Synthesis and Biological Evaluation of Solanacol
COMMUNICATION
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[15] This would be an alternative[16] for the synthesis of labeled strigolac-
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.
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[23] Noteworthy is that the relative configuration at C2’ is inverted com-
pared with most of the other strigolactones, such as
(Scheme 1).
1 or 2
[24] A. de Saint Germain, C. Rameau, unpublished results.
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[19] The absolute configuration of (+)-13Ac, [a]2D4 =+235.4 (c=1 in
CHCl3) and (À)-13, [a]2D6 =À223.8 (c=1 in CHCl3), were established
by analogy to (1S)-inden-1-acetate, [a]2D5 =+82.3 (c=0.21 in CHCl3)
and (1R)-inden-1-ol, [a]D31 =À225.5 (c=0.1 in CHCl3) obtained by
kinetic resolution using the same Candida antarctica lipase. M. Taka-
hashi, R. Koike, K. Ogasawara, Chem. Pharm. Bull. 1995, 43, 1585–
1587 and ref. [18b].
[27] K. Matyjaszewski, T. E. Patten, J. H. Xia, J. Am. Chem. Soc. 1997,
119, 674–680.
Received: September 30, 2010
Published online: November 24, 2010
Chem. Eur. J. 2010, 16, 13941 – 13945
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13945