Concise enantioselective synthesis of abscisic acid and a new analogue

Article abstract of DOI:10.1039/b611880a

Short and high-yielding syntheses of enantiomerically pure (S)-(+) and (R)-(-)-abscisic acid are described. The syntheses proceed through key intermediates that preferentially recrystallise as single diastereoisomers for each enantiomer. This route allows the preparation of either enantiomer of abscisic acid in ca. 30% overall yield, and as demonstrated, gives access to an enantiomerically pure abscisic acid analogue. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b611880a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Concise enantioselective synthesis of abscisic acid and a new analogue†
Timothy R. Smith, Andrew J. Clark, Guy J. Clarkson, Paul C. Taylor and Andrew Marsh*
Received 17th August 2006, Accepted 27th September 2006
First published as an Advance Article on the web 12th October 2006
DOI: 10.1039/b611880a
Short and high-yielding syntheses of enantiomerically pure (S)-(+) and (R)-(−)-abscisic acid are
described. The syntheses proceed through key intermediates that preferentially recrystallise as single
diastereoisomers for each enantiomer. This route allows the preparation of either enantiomer of
abscisic acid in ca. 30% overall yield, and as demonstrated, gives access to an enantiomerically pure
abscisic acid analogue.
acid analogue that is identical to (S)-(+)-abscisic acid with the
Introduction
exception of an additional hydroxyl group on C6, Fig. 2. This new
Abscisic acid (Fig. 1) is an important phytohormone implicated
functionality will be used for conjugation of abscisic acid to other
in many aspects of plant growth and development. Its role in
biologically useful molecules such as biotin or fluorescein without
transpiration,1 seed germination,2 embryo maturation3 and a num-
disrupting pre-existing functional groups.
ber of stress responses4 has been well characterised. The sesquiter-
pene exists naturally in higher plants as the dextrorotatory
enantiomer 1 that has attracted considerable attention. Substantial
effort has been directed towards the elucidation of its signalling
pathways using affinity-based methods with enantiomerically pure
abscisic acid analogues to identify binding proteins.5 The recent
discovery of an abscisic acid receptor crucially depended on the
successful preparation of an active conjugate of the plant hormone
and demonstrates the need for reliable methods of preparing other
analogues.6
Fig. 2 (S)-(+)-6-Hydroxyabscisic acid (3).
Several formal syntheses of racemic8 and enantiomerically
pure abscisic acid9 have been reported, however they all fail
on at least one of the criteria required for efficiency, which
are measured as a low number of steps, the availability of
starting materials and high yields. Our new preparation of either
enantiomer proceeds in ca. 30% overall yield in six linear steps
from commercially available 4-oxoisophorone (4, Scheme 1),
also used as a starting material for other syntheses of abscisic
acid10 and analogues.11 Crucially, the side-chain components are
easily obtained which represents a significant improvement over
previous syntheses where starting materials are either no longer
commercially available,10a or require multi-step preparation from
Fig. 1 The structure of (S)-(+)-abscisic acid (1) with formal atom
numbering and (R)-(−)-abscisic acid (2).
Modification of the abscisic acid side-chain is an attractive
approach for conjugation to supports and labels, but must be
done without substantially altering either the carboxylic acid,
or alkene geometry as these have been shown to be essential
for biological activity.7 In previous syntheses of abscisic acid the
carbon skeleton of the side-chain has been added in one step,8b,c,10a
limiting the opportunity to introduce the additional functionality
required for our studies. Hence, following the work of Mayer9c and
Abrams11b we present a new synthetic route to enantiomerically
pure (S)-(+) and (R)-(−)-abscisic acid that also allows access to
enantiomerically pure analogues. We demonstrate the versatility
of this route by preparing a novel enantiomerically pure abscisic
Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL.
E-mail: a.marsh@warwick.ac.uk
† Electronic supplementary information (ESI) available: Copies of NMR
spectra for compounds 7, 9, 10, 16–19 and 3. 1H NMR spectra and GC-
MS data for compounds 7 and 14 indicating their level of diasteromeric
purity. The procedure for the synthesis of (R)-(−)-abscisic acid including
full compound characterisation data. See DOI: 10.1039/b611880a
Scheme 1 Reagents and conditons: (i) 5, TsOH, toluene, reflux 24 h (91%);
(ii) LDA, trimethylsilylacetylene, THF, −78 ◦C to rt, 1 h, then (iii) K2CO3,
MeOH, 2 h (71%).
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other compounds.9 In this new synthesis the [2(Z),4(E)] side chain
is constructed stereospecifically from trimethylsilylacetylene and
(Z)-3-iodobut-2-en-1-ol, prepared in one step by reduction of
R
commercially available but-2-yn-1-ol with Red-Alꢀ (sodium bis(2-
methoxyethoxy)aluminium hydride), followed by iodine quench.12
The desired stereochemistry at C1ꢀ is obtained without chromato-
graphic separation of diastereoisomers, thus preserving the overall
efficiency of this synthesis and making it an attractive alternative to
other syntheses.
Results and discussion
Previous work has seen the less hindered carbonyl of 4-
oxoisophorone (4) protected with (2S,3S)-(+)-2,3-butanediol (5)
to form the cyclic ketal 6.11b Alkynylation proceeded in a
diastereoselective fashion with the major product bearing the
same configuration as (S)-(+)-abscisic acid. Additionally, 4-
oxoisophorone, achirally protected at the C4ꢀ carbonyl, has been
shown to give a crystalline product after alkynylation with lithio
trimethylsilylacetylene followed by deprotection.10b We found that
when 6 was similarly processed and slowly recrystallised from
petroleum ether, 7 was obtained in 71% yield as lustrous white
needles that were of a single diastereoisomer (Scheme 1). X-
Ray crystallography‡ showed this key intermediate 7 to have the
same configuration at the new stereocentre as (S)-(+)-abscisic acid
(Fig. 3). The initial addition proceeds with dr = 3 : 1 estimated by
integration of NMR signals in the crude mixture at, for example
5.38 and 5.40 ppm. Upon a single recrystallisation from 40–
60◦ petroleum ether, no minor diastereomer was visible either
Fig. 3 The X-ray molecular structure of 7 with thermal ellipsoids at 50%
probability; note crystallographic numbering used differs from Fig. 1.‡
Oxidation of the primary alcohol 10 was performed cleanly
in two steps via aldehyde 11 prepared using tetrapropylammo-
nium perruthenate and 4-N-methylmorpholine-N-oxide as the co-
oxidant,13 followed by further oxidation using sodium chlorite with
added 2-methyl-2-butene as a chlorine scavenger.14 The cyclic ketal
protecting group was removed during the acidic work-up to give
(S)-(+)-abscisic acid (1) in ca. 30% overall yield with data identical
to both natural15 and previously synthesised material.9a–c
The enantiomer, (R)-(−)-abscisic acid (2) was synthesised
analogously from the enantiomer of 6 by selectively protect-
ing 4-oxoisophorone (4) with (2R,3R)-(−)-2,3-butanediol (12,
Scheme 3). The resultant (2R,3R)-(−)-2,3-butanediol ketal of 4-
oxoisophorone (13) imparts the opposite facial selectivity com-
pared to 6 during the alkynylation with the lithio anion of
trimethylsilylacetylene, giving key intermediate 14 after in situ
removal of the trimethylsilyl protecting group. In similar fashion
to the natural enantiomeric series, after a single recrystallisation a
dr ≥99 : 1 (estimated from GC-MS; see the ESI†) was achieved.
The trend for difference in yields (60% vs. 71% for the (S)-(+)-
enantiomer) for this key C–C bond formation with concomitant
deprotection was observed over several runs of this reaction.
Subsequent coupling of the side chain and oxidation proceeded
1
by H NMR or more rigorously by GC-MS. The dr after one
recrystallisation is hence estimated at ≥99 : 1 (see the ESI†).
Extension of the side chain by Sonogashira cross-coupling of
7 with (Z)-3-iodobut-2-en-1-ol (8) was achieved in 94% yield
(Scheme 2). The attempted reduction of acetylene 9 with LiAlH4
R
was unsuccessful, but, treatment with Red-Alꢀ gave the required
4(E)-alkene with highest yields obtained when following aqueous
quench, the reaction was stirred for 1 h before further work-up.
‡ CCDC reference number 618045. For crystallographic data in CIF
format see DOI: 10.1039/b611880a
◦
Scheme 2 Reagents and conditions: (i) 8, CuI, PdCl2(PPh3)2, toluene, 1.5 h (94%); (ii) Red-AlꢀR , THF, −78 C to rt, 3 h (88%); (iii) NMO, TPAP, 4 A
˚
mol. sieves, DCM, 20 min (95%); (iv) NaClO2, KH2PO4, 2-methyl-2-butene, tBuOH, H2O, 20 h, then (v) HCl (aq) (61%).
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and hydrolysis of the cyclic ketal the silyl protecting group was
removed using tetrabutylammonium fluoride to give (S)-(+)-6-
hydroxyabscisic acid (3).
Conclusion
We have developed short, efficient syntheses of enantiomerically
pure (S)-(+) and (R)-(−)-abscisic acid. This enhanced route enjoys
a number of improvements over previous methods including
commercially available and easily prepared starting materials,
and high yield of a key stereochemically defined intermediate.
Uniquely, this route also allows access to new side-chain analogues
that are finding application in the elucidation of abscisic acid’s
multiple biological activities.20
Scheme 3 Reagents and conditions: (i) 12, TsOH, toluene, reflux 24 h
(94%); (ii) LDA, trimethylsilylacetylene, THF, −78 ◦C to rt, 1 h, then (iii)
K2CO3, MeOH, 2 h (60%).
Experimental
with the same reagents and conditions, leading to (R)-(−)-abscisic
acid (2, see the ESI for details†).
Unless otherwise noted, all materials were obtained from commer-
cial sources and used without further purification. Toluene was
freshly distilled from sodium and THF from potassium. Column
chromatography was performed on Merck silica gel 60 H (230–
400 mesh). Melting points were recorded on a Gallenkamp melting
point apparatus and are uncorrected. Optical rotations (given in
10−1 deg cm2 g−1) were measured on an Optical Activity AA-1000
polarimeter. IR spectra were recorded on a Perkin-Elmer Paragon
1000 FT-IR instrument. 1H and 13C NMR spectra were recorded
using a Bruker DPX400 at 400 and 100 MHz respectively. NMR
spectra were recorded in the indicated solvent and chemical shifts
(d) are reported in ppm relative to residual non-deuterated solvent
as an internal standard. J-Values are given in Hz. Elemental
analysis was performed onan Exeter AnalyticalCE440 machineby
the Warwick Analytical Service. LSIMS and EI mass spectroscopy
was performed on a Micromass Autospec mass spectrometer. GC-
MS data was collected on a Varian 4000 GC-MS fitted with
The new analogue, (S)-(+)-6-hydroxyabscisic acid (3) was
prepared from the key diastereoisomerically-pure intermediate 7
and mono-protected vinyl iodide 15 (Scheme 4). Synthesis of 15
was achieved in two steps by desymmetrisation of but-2-yne-1,4-
diol with tert-butylchlorodiphenylsilane16 followed by reduction
R
with Red-Alꢀ and iodine quench.17 Sonogashira cross-coupling
of 7 with 15 was performed under a reducing atmosphere of
nitrogen diluted hydrogen to reduce Hay coupling,18 a problem
not observed when preparing abscisic acid itself. Reduction of
R
acetylene 16 with Red-Alꢀ proceeded in low yield despite efforts
at optimisation including varying reaction time, temperature and
reagent stoichiometry. Incompatibility between the silyl protect-
ing group and aluminium reducing agent appear likely since
deprotection and silyl migration have been observed previously
with this combination.19 Following oxidation of the C1 alcohol
Scheme 4 Reagents and conditions: (i) 15, CuI, PdCl2(PPh3)2, toluene, N2–H2 atm, 1 h (69%); (ii) Red-AlꢀR , THF, −78 ◦C to rt, 1 h (24%); (iii) NMO,
t
+
−
˚
TPAP, 4 A molecular sieves, DCM, 15 min (86%); (iv) NaClO2, KH2PO4, 2-methyl-2-butene, BuOH–H2O, 20 h, then (v) HCl (aq) (68%); (vi) Bu4N F ,
THF, 20 h (91%). TBDPS = tert-butyldiphenylsilane.
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an autosampler and a FactorFourTM Capillary column number
VF-5 ms operating a temperature programme from 50–250 ◦C
increased at 5 ◦C per min.
HRMS (EI): m/z requires (C15H22O3) 250.1569, found 250.1563,
48%, [M]+.
(2S,3S,8R)-8-(5-Hydroxy-3-methylpent-3-en-1-ynyl)-2,3,7,9,9-
pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-ol (9). Copper iodide
(86 mg, 15% mol) and PdCl2(PPh3)2 (211 mg, 10% mol), in
dry deoxygenated toluene (35 ml) under a nitrogen atmosphere
was treated with (Z)-3-iodobut-2-en-1-ol (8, 1.19 g, 6.0 mmol,
Preparation of (S)-(+)-abscisic acid
(2S,3S)-2,3,7,9,9-Pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-
one (6). 4-Oxoisophorone (4.22 g, 27.7 mmol, 1 eq.), 2S,3S-(+)-
2,3-butanediol (3.0 g, 33.3 mmol, 1.2 eq.) and p-toluenesulfonic
acid (160 mg) in toluene (40 ml) were heated under reflux for 24 h
using a Dean–Stark trap. The mixture was cooled and washed
with saturated NaHCO3 solution (30 ml). The organic layer was
separated and the aqueous layer extracted with diethyl ether (3 ×
30 ml). The combined organics were dried over mgSO4 and the
solvent removed under reduced pressure. The crude product was
purified by column chromatography (silica, 20% diethyl ether in
hexane) to yield the title product 6 as a pale yellow oil (5.66 g,
91%). Rf 0.51 (30% diethyl ether in hexane); [a]2D0 = −16.1 (c 0.97,
MeOH) (lit.,11b [a]D20 = −15.7); vmax(film) 1674, 1092, 969 cm−1; dH
(400 MHz, CDCl3) 1.17 (s, 3H), 1.22 (s, 3H), 1.27–1.30 (m, 6H),
1.79 (d, J = 1.5, 3H), 2.05 (dd, J = 1.5, 14.0, 1H), 2.13 (d, J = 14.0,
1H), 3.61–3.71 (m, 2H), 6.33 (t, J = 1.3, 1H); dC (100 MHz, CDCl3)
16.2 (CH3), 16.6 (CH3), 16.8 (CH3), 26.3 (CH3), 26.8 (CH3), 42.1
(CH2), 47.5 (C), 78.4 (2 × CH), 102.9 (C), 135.2 (C), 141.3 (CH),
204.4 (C); MS (EI): m/z requires (C13H21O3) 225, found 225, 100%,
[M + H]+.
2
eq.) and (2S,3S,8R)-8-(ethynyl)-2,3,7,9,9-pentamethyl-1,4-
dioxaspiro[4.5]dec-6-en-8-ol (7, 765 mg, 3.0 mmol, 1 eq.). Diiso-
propylamine (0.84 ml, 6.0 mmol, 2 eq.) was added dropwise and
the mixture stirred for 1.5 h. Saturated NH4Cl solution (30 ml)
was added and the organic layer separated. The aqueous layer was
extracted with diethyl ether (3 × 30 ml) and the combined organics
dried over MgSO4. The solvent was removed under reduced
pressure and the product purified by column chromatography
(silica, 30% EtOAc in hexane) to yield the title product 9 as a
pale yellow oil (899 mg, 94%). Rf 0.19 (40% EtOAc in hexane);
[a]2D0 = +168.2 (c 1.00, MeOH) (lit.,11b [a]D20 = +163.3); mmax(film)
3378, 1090, 982, 947 cm−1; dH (400 MHz, CDCl3) 1.10 (s, 3H),
1.15 (s, 3H), 1.24–1.26 (m, 6H), 1.83 (dd, J = 1.0, 14.0, 1H),
1.86 (s, 3H), 1.91 (s, 3H), 2.04 (d, J = 14.0, 1H), 2.20 (s, br,
1H), 3.53–3.63 (m, 2H), 4.26 (d, J = 6.8, 2H), 5.37 (s, 1H), 5.86–
5.89 (m, 1H); dC (100 MHz, CDCl3) 16.7 (CH3), 16.7 (CH3), 18.6
(CH3), 22.2 (CH3), 23.0 (CH3), 25.7 (CH3), 39.6 (C), 45.8 (CH2),
61.1 (CH2), 75.0 (C), 77.9 (CH), 78.0 (CH), 83.9 (C), 94.7 (C),
103.9 (C), 120.7 (C), 125.0 (CH), 136.1 (CH), 140.4 (C); HRMS
(LSIMS): m/z requires (C19H26O3) 302.1882, found 302.1865,
84%, [M-H2O]+.
(2S,3S,8R)-8-(Ethynyl)-2,3,7,9,9-pentamethyl-1,4-dioxaspiro-
[4.5]dec-6-en-8-ol (7). Trimethylsilylacetylene (6.14 g, 62.5 mmol,
2.5 eq.) was added slowly to a stirred solution of LDA (34.7 ml of a
1.8 M solution of LDA in THF–heptane–ethylbenzene, 2.5 eq.) in
THF (200 ml) at −78 ◦C under a nitrogen atmosphere. The mixture
was stirred for 15 min, then (2S,3S)-2,3,7,9,9-pentamethyl-1,4-
dioxaspiro[4.5]dec-6-en-8-one (6, 5.60 g, 25.0 mmol, 1 eq.) in
THF (30 ml) was introduced dropwise and the mixture allowed
to attain room temperature. After 1 h the reaction was quenched
with saturated NH4Cl (100 ml) and extracted with diethyl ether
(3 × 100 ml). The combined organic extracts were washed with
water (200 ml) then brine (200 ml) and dried over MgSO4. The
solvent was removed under reduced pressure to yield a pale yellow
residue that was dissolved in methanol (150 ml) and treated with
K2CO3 (20.0 g). The suspension was stirred vigorously for 2 h, then
concentrated under reduced pressure. Water (50 ml) was added
and the product extracted with diethyl ether (3 × 50 ml). The
combined organics were washed with water (100 ml) then brine
(100 ml) and dried over MgSO4. The solvent was removed under
reduced pressure and the residue (a mixture of diastereoisomers,
76 : 24, determined by NMR) recrystallised slowly from petroleum
ether (40–60◦) to give the title product 7 as white needles (4.42 g,
71%). (Found: C, 72.03;◦H, 8.89% C15H22O3 requires C, 71.97;
H, 8.86%); mp 140–141 C (petroleum ether); [a]2D0 = +122.4 (c
1.00, MeOH); mmax(solid) 3427, 3285, 1090, 991, 948 cm−1; dH
(400 MHz, CDCl3) 1.08 (s, 3H), 1.16 (s, 3H), 1.23–1.26 (m, 6H),
1.82 (dd, J = 1.2, 14.0, 1H), 1.90 (s, 3H), 1.94 (s, 1H), 2.06 (d,
J = 14.0, 1H), 2.49 (s, 1H), 3.53–3.63 (m, 2H), 5.38–5.39 (m,
1H); dC (100 MHz, CDCl3) 16.7 (CH3), 16.8 (CH3), 18.4 (CH3),
22.0 (CH3), 25.5 (CH3), 39.2 (C), 45.6 (CH2), 74.0 (CH), 74.5 (C),
77.9 (CH), 78.0 (CH), 84.2 (C), 103.8 (C), 125.4 (CH), 140.0 (C);
(2S,3S,8S)-8-(5-Hydroxy-3-methyl-2,4-pentadienyl)-2,3,7,9,9-
pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-ol (10). To a stirred
solution
of
(2S,3S,8R)-8-(5-hydroxy-3-methylpent-3-en-1-
ynyl)-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-ol (9,
635 mg,◦ 1.9 mmol, 1 eq.) in dry deoxygenated THF (15 ml)
R
at −78 C under a nitrogen atmosphere was added Red-Alꢀ
(1.19 ml of a 3.2 M solution in toluene, 3.8 mmol, 2 eq.) via
syringe. The reaction was allowed to warm to room temperature
and stirred for 3 h. It was then cooled in an ice bath and water
(15 ml) was carefully added dropwise and stirring continued at
R
room temperature for 1 h. (Caution: unreacted Red-Alꢀ reacts
violently with water.) The organic layer was separated and the
aqueous layer extracted with diethyl ether (3 × 15 ml). The
combined organics were washed with brine (50 ml) and dried
over MgSO4. The solvent was removed under reduced pressure
and the product purified by column chromatography (silica, 25%
hexane in diethyl ether) to yield the title alcohol 10 as a viscous
oil (538 mg, 88%). Rf 0.12 (25% hexane in diethyl ether); [a]D20
=
+199.5 (c 0.75, MeOH) (lit.,11b [a]D20 = +239.5); mmax(film) 3389,
1091, 978, 951 cm−1; dH (400 MHz, CDCl3) 0.90 (s, 3H), 1.09 (s,
3H), 1.23 (d, J = 5.5, 3H), 1.26 (d, J = 5.5, 3H), 1.67–1.71 (m,
4H), 1.85 (d, J = 0.7, 3H), 1.93 (d, J = 14.3, 1H), 3.52–3.62 (m,
2H), 4.23–4.35 (m, 2H), 5.43 (s, 1H), 5.58 (t, J = 7.3, 1H), 5.72 (d,
J = 15.7, 1H), 6.66 (d, J = 15.7, 1H); dC (100 MHz, CDCl3) 16.6
(2 × CH3), 18.0 (CH3), 20.7 (CH3), 23.2 (CH3), 24.9 (CH3), 39.3
(C), 46.3 (CH2), 58.2 (CH2), 77.6 (CH), 78.0 (CH), 79.4 (C), 104.0
(C), 125.5 (CH), 126.1 (CH), 128.5 (CH), 132.2 (CH), 134.8 (C),
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141.6 (C); HRMS (LSIMS): m/z requires (C19H31O4) 323.2222,
Preparation of (S)-(+)-6-hydroxyabscisic acid
found 323.2219, 35%, [M + H]+.
(2S,3S,8R)-8-(5-Hydroxy-3-[tert-butyldiphenylsiloxymethyl]-
pent-3-en-1-ynyl)-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-
en-8-ol (16). In flame-dried glassware, (Z)-2-iodo-1-(tert-
butyldiphenylsiloxy)but-2-en-4-ol (15, 1.6 g, 3.5 mmol, 1.1 eq.),
CuI (30 mg, 5% mol) and PdCl2(PPh3)2 (109 mg, 5% mol)
were mixed in dry degassed toluene (30 ml) under a ni-
(2S,3S,8S)-8-(3-Methylpenta-2,4-dienal)-2,3,7,9,9-pentamethyl-
1,4-dioxaspiro[4.5]dec-6-en-8-ol (11). (2S,3S,8S)-8-(5-Hydroxy-
3-methyl-2,4-pentadienyl)-2,3,7,9,9-pentamethyl-1,4-dioxaspiro-
[4.5]dec-6-en-8-ol (10, 460 mg, 1.43 mmol, 1 eq.) was dissolved in
DCM (15 ml) and treated with 4-N-methylmorpholine-N-oxide
trogen atmosphere. iPr2NH (1.3 ml, 9.4 mmol,
3 eq.)
˚
(250 mg, 2.1 mmol, 1.5 eq.), powdered 4 A molecular sieves
was added and the mixture purged with N2–H2 (1 : 1)
for five min. (2S,3S,8R)-8-(ethynyl)-2,3,7,9,9-pentamethyl-1,4-
dioxaspiro[4.5]dec-6-en-8-ol (7, 780 mg, 3.1 mmol, 1 eq.) in dry
degassed toluene (15 ml) was added dropwise over 10 min and
the mixture stirred for 1 h. Saturated NH4Cl solution (30 ml)
was added and the organic layer separated. The aqueous layer was
extracted with diethyl ether (3 × 30 ml) and the combined organics
dried over MgSO4. The solvent was removed under reduced
pressure and the product purified by column chromatography
(silica, 28% EtOAc in hexane) to yield the title product 16 as
a pale yellow oil (1.2 g, 69%). Rf 0.22 (40% EtOAc in hexane);
[a]2D0 = +97.9 (c 1.25, MeOH); mmax(neat) 3403, 1430, 1375, 1091,
700 cm−1; dH (400 MHz, CDCl3) 1.06 (s, 3H), 1.06 (s, 9H), 1.11 (s,
3H), 1.21–1.25 (m, 6H), 1.74 (br, 1H), 1.80 (dd, J = 1.1, 14.1, 1H),
1.86 (d, J = 1.5, 3H), 2.01 (d, J = 14.1, 1H), 3.53–3.60 (m, 2H),
4.16 (d, J = 1.5, 2H), 4.34 (d, J = 6.5, 2H), 5.35 (t, J = 1.5, 1H),
6.27 (tt, J = 1.8, 6.8, 1H), 7.36–7.45 (m, 6H), 7.66 (d, J = 7.3, 4H);
dC (100 MHz, CDCl3) 16.7 (CH3), 16.7 (CH3), 18.6 (CH3), 19.3
(C), 22.2 (3 × CH3), 25.8 (CH3), 26.8 (CH3), 39.6 (C), 45.7 (CH2),
60.9 (CH2), 65.5 (CH2), 75.1 (C), 77.7 (CH), 78.0 (CH), 81.2 (C),
96.1 (C), 103.9 (C), 124.2 (C), 125.1 (CH), 127.8 (4 × CH), 129.8
(2 × CH), 133.2 (2 × CH), 134.8 (CH), 135.5 (4 × CH), 140.2
(C); HRMS (LSIMS): m/z requires (C35H45O5Si) 573.3026, found
573.3036, 100%, [M–H]+.
(500 mg) and tetra-N-propyl ammonium perruthenate (TPAP)
(50 mg, 10% mol). The mixture was stirred under nitrogen for
20 min, then filtered through a short column of silica. The
column was washed with DCM and the filtrate and washings
combined. The solvent was removed under reduced pressure and
the product recrystallised from cyclohexane to yield the title
aldehyde 11 as off-white crystals (433 mg, 95%). (Found: C, 71.22;
H, 8.85%. C19H28O4 requires C, 71.22; H, 8.81%); mp 127–128 ◦C
(cyclohexane); [a]2D0 = +348.4 (c 1.14, MeOH); mmax(solid) 3450,
1656, 1629, 1092, 953, 729 cm−1; dH (400 MHz, CDCl3) 0.92 (s,
3H), 1.12 (s, 3H), 1.24 (d, J = 5.5, 3H), 1.27 (d, J = 5.5, 3H), 1.67
(d, J = 1.5, 3H), 1.75 (dd, J = 1.7, 14.6, 1H), 1.95 (d, J = 14.6,
1H), 2.07 (s, 3H), 3.53–3.63 (m, 2H), 5.47–5.48 (m, 1H), 5.86 (d,
J = 8.3, 1H), 6.12 (d, J = 15.6, 1H), 7.35 (d, J = 15.6, 1H), 10.22
(d, J = 8.3, 1H); dC (100 MHz, CDCl3) 16.6 (CH3), 16.6 (CH3),
17.8 (CH3), 21.6 (CH3), 23.2 (CH3), 25.1 (CH3), 39.4 (C), 46.3
(CH2), 77.7 (CH), 78.1 (CH), 79.3 (C), 103.6 (C), 124.8 (CH),
126.4 (CH), 128.9 (CH), 139.5 (CH), 140.4 (C), 154.1 (C), 190.6
(CH); HRMS (LSIMS): m/z requires (C19H29O4) 321.2066, found
321.2068, 100%, [M + H]+.
(S)-5-(1-Hydroxy-2,6,6-trimethyl-4-oxo-2-cyclohexen-1-yl)-3-
methyl-(2Z,4E)-pentadienoic acid (1), (S)-(+)-abscisic acid.
A
solution of KH2PO4 (850 mg, 6.2 mmol, 5 eq.) and sodium
chlorite (1.13 g, 12.5 mmol, 10 eq.) in water (10 ml) was added
dropwise over a period of 10 min to a stirred solution of
(2S,3S,8S)-8-(3-methylpenta-2,4-dienal)-2,3,7,9,9-pentamethyl-
1,4-dioxaspiro[4.5]dec-6-en-8-ol (11, 400 mg, 1.3 mmol, 1 eq.)
(2S,3S,8S)-8-(5-Hydroxy-3-[tert-butyldiphenylsiloxymethyl]-2,
4-pentadienyl)-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-
8-ol (17). (2S,3S,8R)-8-(5-Hydroxy-3-[tert-butyldiphenylsiloxy-
methyl]pent-3-en-1-ynyl)-2,3,7,9,9-pentamethyl-1,4-dioxaspiro-
[4.5]dec-6-en-8-ol (16, 800 mg, 1.4 mmol, 1◦eq.) was dissolved in
t
and 2-methyl-2-butene (8.0 ml) in BuOH (20 ml). The mixture
was stirred for 20 h, then concentrated under reduced pressure.
Methanol (10 ml) was added and the mixture carefully acidified
to pH 3 with concentrated HCl (aq). After 15 min the mixture
was extracted with EtOAc (3 × 20 ml) and the combined
organics washed in brine (50 ml) and dried over MgSO4. The
solvents were removed under reduced pressure and the residue
purified by column chromatography (silica, 30% EtOAc and 3%
AcOH in hexane). The solid obtained was recrystallised from
EtOAc–hexane to give (S)-(+)-abscisic acid as white crystals
(200 mg, 61%). (Found: C, 67.91; H, 7.63%. C15H20O4 requires C,
68.16; H, 7.63%); mp 159–161 ◦C (hexane) (lit.,9c mp 161–163 ◦C);
Rf 0.27 (40% EtOAc and 3% AcOH in hexane); [a]2D0 = +414.0 (c
1.00, EtOH) (lit.,9b [a]D20 = +415.3); mmax(solid) 3394, 1646, 1622,
1597, 1248 cm−1; dH (400 MHz, CDCl3) 1.04 (s, 3H), 1.12 (s, 3H),
1.94 (s, 3H), 2.05 (s, 3H), 2.30 (d, J = 17.3, 1H), 2.50 (d, J =
17.3, 1H), 5.77 (s, 1H), 5.98 (s, 1H), 6.18 (d, J = 16.0, 1H), 7.82
(d, J = 16.0, 1H); dC (100 MHz, CDCl3) 19.1 (CH3), 21.5 (CH3),
23.1 (CH3), 24.3 (CH3), 41.7 (C), 49.7 (CH2), 79.9 (C), 118.1
(CH), 127.1 (CH), 128.4 (CH), 136.9 (CH), 151.6 (C), 163.0 (C),
170.9 (C), 198.3 (C); HRMS (LSIMS): m/z requires (C15H20O4)
264.1362, found 264.1362, 100%, [M]+.
R
anhydrous THF (80 ml) and cooled to −78 C. Red-Alꢀ (1.75 ml
of a 3.2 M solution in toluene, 5.6 mmol, 4 eq.) was added via
syringe and the mixture allowed to attain room temperature. After
30 min the reaction was quenched by careful addition of potassium
sodium tartrate tetrahydrate (10 g) in water (40 ml). (Caution:
R
unreacted Red-Alꢀ reacts violently with water). The mixture was
stirred for 1 h, then the organic layer separated and the aqueous
layer extracted with EtOAc (3 × 50 ml). The combined organic
extracts were washed with brine (100 ml), dried over MgSO4 and
the solvent removed under reduced pressure. The product was
purified by column chromatography (silica, 30% EtOAc in hexane)
to yield the title product 17 as a viscous oil (190 mg, 24%). Rf 0.26
(40% EtOAc in hexane); [a]2D0 = +105.6 (c 1.00, MeOH); mmax(neat)
3424, 1093, 701 cm−1; dH (400 MHz, CDCl3) 0.84 (s, 3H), 1.06
t
(s, 3H), 1.07 (s, 9H, butyl), 1.21 (d, J = 5.5, 3H), 1.25 (d, J =
5.2, 3H), 1.63–1.67 (m, 4H), 1.88 (d, J = 14.3, 1H), 3.51–3.60 (m,
2H), 4.26–4.36 (m, 4H), 5.43 (t, J = 1.3, 1H), 5.67 (d, J = 16.3,
1H), 5.82–5.86 (m, 1H), 6.47 (d, J = 16.0, 1H), 7.36–7.45 (m, 6H),
7.67–7.69 (m, 4H); dC (100 MHz, CDCl3) 16.6 (2 × CH3), 18.1
(CH3), 19.3 (C), 23.2 (3 × CH3), 24.9 (CH3), 26.9 (CH3), 39.3 (C),
4190 | Org. Biomol. Chem., 2006, 4, 4186–4192
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46.2 (CH2), 58.5 (CH2), 64.9 (CH2), 77.6 (CH), 78.0 (CH), 79.4
(C), 103.9 (C), 123.9 (CH), 125.8 (CH), 127.6 (CH), 127.7 (4 ×
CH), 129.7 (2 × CH), 132.0 (CH), 133.5 (2 × C), 135.6 (4 × CH),
136.9 (C), 141.2 (C); HRMS (LSIMS): m/z requires (C35H49O5Si)
577.3349, found 577.3329, 22%, [M + H]+.
24.2 (CH3), 26.8 (CH3), 41.6 (CH2), 63.9 (CH2), 79.8 (C), 115.9
(CH), 125.8 (CH), 127.2 (CH), 127.9 (4 × CH), 130.1 (2 × CH),
132.7 (2 × C), 135.0 (CH), 135.5 (4 × CH), 152.9 (C), 162.4 (C),
171.5 (C), 198.1 (C); HRMS (LSIMS): m/z requires (C31H39O5Si)
519.2567, found 519.2567, 100%, [M + H]+.
(2S,3S,8S)-8-(3-[tert-Butyldiphenylsiloxymethyl]penta-2,4-di-
enal)-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-ol (18).
(2S,3S,8S)-8-(5-Hydroxy-3-[tert-butyldiphenylsiloxymethyl]-2,4-
pentadienyl)-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-
8-ol, (275 mg, 0.48 mmol, 1 eq.) was dissolved in DCM (15 ml)
and treated with tetra-N-propyl ammonium perruthenate (TPAP)
(17 mg, 10% mol), 4-N-methylmorpholine-N-oxide (96 mg,
(S)-(+)-6-Hydroxyabscisic acid or (S)-5-(1-hydroxy-2,6,6-tri-
methyl-4-oxo-2-cyclohexen-1-yl)-3-hydroxymethyl-(2Z,4E)-penta-
dienoic acid (3). (S)-5-(1-Hydroxy-2,6,6-trimethyl-4-oxo-2-
cyclohexen - 1 - yl) - 3 - (tert-butyldiphenylsiloxymethyl)-(2Z,4E)-
pentadienoic acid (19, 93 mg, 0.18 mmol, 1 eq.) in THF (10 ml)
was treated with tetrabutylammonium fluoride (0.45 ml of a 1 M
solution in THF, 0.45 mmol, 2.5 eq.) and stirred for 20 h. The
solvent was removed under reduced pressure and the product
separated by column chromatography (silica, 80% EtOAc, 3%
AcOH, 17% hexane) to yield the title product 3 as an off-white
solid (46 mg, 91%). Rf 0.30 (80% EtOAc, 3% AcOH, 17% hexane);
[a]2D0 = +430.0 (c 1.00, MeOH); mmax(solid) 2964, 1684, 1639, 1605,
1230, 978 cm−1, dH (400 MHz, CDCl3) 1.06 (s, 3H), 1.10 (s, 3H),
1.97 (d, J = 1.3, 3H), 2.22 (d, J = 17.1, 1H), 2.59 (d, J = 17.1,
1H), 4.41 (d, J = 1.2, 2H), 5.96 (s, 1H), 6.18 (s, 1H), 6.24 (d, J =
16.6, 1H), 7.63 (d, J = 16.6, 1H); dC (100 MHz, CDCl3) 19.6
(CH3), 23.6 (CH3), 24.7 (CH3), 42.9 (CH2), 50.7 (C), 63.2 (CH2),
80.6 (C), 117.4 (CH), 127.0 (CH), 127.6 (CH), 136.4 (CH), 153.1
(C), 166.5 (C), 169.9 (C),21 201.1 (C); HRMS (LSIMS): m/z
requires (C15H21O5) 281.1389, found 281.1386, 100%, [M + H]+.
˚
0.71 mmol, 1.5 eq.) and powdered 4 A molecular sieves (280 mg).
The mixture was stirred for 15 min, then filtered through a short
column of silica. The silica was washed with portions of DCM
and diethyl ether and the combined filtrates concentrated under
reduced pressure. The crude product was purified by column
chromatography (silica, 25% EtOAc in hexane) to yield the title
product 18 as a viscous oil (237 mg, 86%). Rf 0.15 (25% EtOAc
in hexane); [a]2D0 = +141.6 (c 1.00, MeOH); mmax(neat) 1662, 1428,
1093, 731, 701 cm−1; dH (400 MHz, CDCl3) 0.82 (s, 3H), 1.07 (s,
3H), 1.07 (s, 9H), 1.20–1.25 (m, 6H), 1.61 (s, 3H), 1.65–1.70 (m,
1H), 1.83 (d, J = 14.6, 1H), 3.50–3.59 (m, 2H), 4.41 (s, 2H), 5.43
(d, J = 1.2, 1H), 5.87 (d, J = 16.1, 1H), 6.37 (d, J = 8.3, 1H), 7.02
(d, J = 15.8, 1H), 7.37–7.46 (m, 6H), 7.64–7.66 (m, 4H), 10.21 (d,
J = 8.3, 1H); dC (100 MHz, CDCl3) 16.6 (CH3), 16.6 (CH3), 17.9
(CH3), 19.3 (C), 23.2 (3 × CH3), 25.0 (CH3), 26.8 (CH3), 39.3 (C),
46.1 (CH2), 63.7 (CH2), 77.6 (CH), 78.1 (CH), 79.3 (C), 103.5
(C), 122.4 (CH), 125.5 (CH), 126.6 (CH), 127.9 (4 × CH), 130.0
(2 × CH), 132.7 (2 × C), 135.5 (4 × CH), 138.5 (CH), 140.0 (C),
156.3 (C), 191.4 (C); HRMS (LSIMS): m/z requires (C35H47O5Si)
575.3193, found 575.3190, 100%, [M + H]+.
Crystal data for 7‡. C15H22O3, M = 250.33, colourless block,
0.60 × 0.25 × 0.01 mm, monoclinic, P2(1) (No 4), b = 95.392(3)◦,
˚
a = 7.6916(3), b = 11.7602(5), c = 8.2773(4)A, T = 298 K, l(Cu
Ka) = 0.154 mm , U = 745.41(6) A , Z 2, Dcal = 1.115 g cm−3,
l = 0.610, 4874 reflections measured, 2247 unique [Rint = 0.0301],
R [I>2r(I)] = 0.0370, wR [I>2r(I)] = 0.0879, GooF = 1.062.
−1
3
˚
(S)-5-(1-Hydroxy-2,6,6-trimethyl-4-oxo-2-cyclohexen-1-yl)-3-
(tert-butyldiphenylsiloxymethyl)-(2Z,4E)-pentadienoic acid (19).
A mixture of KH2PO4 (190 mg, 1.4 mmol, 5 eq.) and sodium
chlorite (253 mg, 2.8 mmol, 10 eq.) in water (7 ml) was added
dropwise over a period of ten min to a stirred solution of (2S,
3S,8S)-8-(3-[tert-butyldiphenylsiloxymethyl]penta-2,4-dienal)-2,
3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-ol (18, 160 mg,
0.28 mmol, 1 eq.) and 2-methyl-2-butene (4 ml) in tBuOH (10 ml).
The mixture was stirred for 20 h, then concentrated under reduced
pressure to half its initial volume. MeOH (10 ml) was added and
the mixture carefully acidified to pH 2 with 4 M HCl (aq). The
deprotection was monitored by thin layer chromatography (silica,
40% EtOAc, 3% AcOH, 57% hexane) and typically took 15 min,
after which water (20 ml) was added and the product extracted with
EtOAc (3 × 30 ml). The combined organic extracts were washed
with brine (60 ml), dried over MgSO4 and the solvent removed
under reduced pressure. The product was purified by column
chromatography (silica, 30% EtOAc, 3% AcOH, 67% hexane) to
yield the title product 19 as a viscous oil (98 mg, 68%). Rf 0.15
(30% EtOAc, 3% AcOH, 67% hexane); [a]2D0 = +394.0 (c 1.00,
MeOH); mmax(neat) 3499, 2960, 1654, 1427, 1228, 1110, 701 cm−1;
dH (400 MHz, CDCl3) 0.92 (s, 3H), 1.06 (s, 3H), 1.07 (s, 9H), 1.87
(d, J = 1.0, 3H), 2.21 (d, J = 17.1, 1H), 2.35 (d, J = 17.1, 1H),
4.41 (d, J = 1.5, 2H), 5.92 (d, J = 16.6, 1H), 5.93 (s, 1H), 6.22
(s, 1H), 7.38–7.45 (m, 6H), 7.57 (d, J = 16.6, 1H), 7.64–7.66 (m,
4H); dC (100 MHz, CDCl3) 19.0 (CH3), 19.2 (C), 23.1 (3 × CH3),
Acknowledgements
We thank the BBSRC for a BMS Committee Studentship (TRS),
Dr S. J. Dilly (Department of Chemistry, University of Warwick),
Professor R. Napier and Dr A. J. Thompson (Warwick HRI)
for helpful discussions. We acknowledge Dr A. J. Clarke and
Mr J. C. Bickerton for technical assistance with NMR and mass
spectrometry, respectively.
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The Royal Society of Chemistry 2006
©